Forestry, Faculty of
DSpace
UBCV
Song, Mingyao
Master of Science - MSc
University of British Columbia
This study aimed to improve the thermal management and mechanical performance of cellulose nanofibrils (CNF) aerogels. We expected that closed-pore structure of CNF aerogel can be gained by Pickering emulsion templating and solvent exchange method, and that paraffin can be encapsulated by CNF to fabricate CNF/Phase change material (PCM) aerogel with excellent stability, super thermal regulation performance, and flexibility. It was also expected that the addition of CNF to prepared emulsion can enhance the structure to further improve the thermal management and mechanical performance. First, CNF aerogels with closed internal pores were successfully fabricated by Pickering emulsion templating and solvent exchange techniques. CNF stabilized oil-in-water Pickering emulsion can be converted into closed pores by sequential solvent exchanging to acetone and tert-butanol, followed by freeze drying from tert-butanol to suppress the formation of large ice crystals. The closed pore was verified by both confocal microscopy and scanning electron microscopy images, and was confirmed to reduce thermal conductivity for aerogel. In addition to the ultra-low thermal conductivity, the CNF/emulsion composite aerogel also demonstrates high performance in properties such as low density, high mechanical properties, superb flexibility, and infrared shielding properties. In addition, robust CNF/PCM aerogels were successfully fabricated with CNF stabilized paraffin by Pickering emulsion templating technique. The CNF/PCM aerogels were ultralight with impressive mechanical properties, which were significantly enhanced by the addition of CNF to the as-formed PCM emulsion. The freeze-dried CNF/PCM aerogels were flexible, and the shape of CNF/PCM aerogel can be well maintained even loaded with a weight of over thousands of times its own weight. The exceptional thermal regulation performance was demonstrated by a series of heating and cooling tests. The CNF/PCM aerogels exhibited excellent shape stability with no leakage after 51 heating-cooling thermal cycles. The latent heat of CNF/PCM aerogel (4:6) can reach approximately 84.4% of that of pure paraffin. In brief, this work fabricated sustainable CNF aerogel with closed-pore structure for the first time, and also obtained CNF/PCM aerogel with high shape stability and energy density. The thermal management and mechanical performance of CNF aerogel were significantly improved.
CELLULOSE NANOFIBRILS AEROGEL WITH ENHANCED THERMAL MANAGEMENT PERFORMANCE by Mingyao Song A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2020 © Mingyao Song, 2020 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Cellulose Nanofibrils Aerogel with Enhanced Thermal Management Performance submitted by Mingyao Song in partial fulfillment of the requirements for the degree of Master of Science in Forestry Examining Committee: Feng Jiang (Assistant Professor, Forestry, The University of British Columbia) Supervisor Philip D. Evans (Professor, Forestry, The University of British Columbia) Supervisory Committee Member Scott Renneckar (Professor, Forestry, The University of British Columbia) Supervisory Committee Member Mohammad Arjmand (Assistant Professor, Mechanical, The University of British Columbia) Additional Examiner iii Abstract This study aimed to improve the thermal management and mechanical performance of cellulose nanofibrils (CNF) aerogels. We expected that closed-pore structure of CNF aerogel can be gained by Pickering emulsion templating and solvent exchange method, and that paraffin can be encapsulated by CNF to fabricate CNF/Phase change material (PCM) aerogel with excellent stability, super thermal regulation performance, and flexibility. It was also expected that the addition of CNF to prepared emulsion can enhance the structure to further improve the thermal management and mechanical performance. First, CNF aerogels with closed internal pores were successfully fabricated by Pickering emulsion templating and solvent exchange techniques. CNF stabilized oil-in-water Pickering emulsion can be converted into closed pores by sequential solvent exchanging to acetone and tert-butanol, followed by freeze drying from tert-butanol to suppress the formation of large ice crystals. The closed pore was verified by both confocal microscopy and scanning electron microscopy images, and was confirmed to reduce thermal conductivity for aerogel. In addition to the ultra-low thermal conductivity, the CNF/emulsion composite aerogel also demonstrates high performance in properties such as low density, high mechanical properties, superb flexibility, and infrared shielding properties. In addition, robust CNF/PCM aerogels were successfully fabricated with CNF stabilized paraffin by Pickering emulsion templating technique. The CNF/PCM aerogels were ultralight with impressive mechanical properties, which were significantly enhanced by the addition of CNF to the as-formed PCM emulsion. The freeze-dried CNF/PCM aerogels were flexible, and the shape of CNF/PCM aerogel can be well maintained even loaded with a weight of over thousands of times its own weight. The exceptional thermal regulation performance was demonstrated by a series of heating and cooling tests. The CNF/PCM aerogels exhibited excellent iv shape stability with no leakage after 51 heating-cooling thermal cycles. The latent heat of CNF/PCM aerogel (4:6) can reach approximately 84.4% of that of pure paraffin. In brief, this work fabricated sustainable CNF aerogel with closed-pore structure for the first time, and also obtained CNF/PCM aerogel with high shape stability and energy density. The thermal management and mechanical performance of CNF aerogel were significantly improved. v Lay Summary Sustainable material for thermal management applications in building and clothing is in pressing need due to the environment and energy crisis. The low density, high aspect ratio, natural abundance, and biodegradability of cellulose nanofibril make it promising for the fabrication of ultralight aerogels for thermal insulation. However, the fabrication methods, thermal management ability, and mechanical properties of CNF aerogel need to be further improved for practical applications. In this work, aerogels with closed-pore structure were obtained by CNF stabilized emulsion and subsequent removal of oil via acetone. Moreover, CNF/paraffin phase change aerogel was fabricated surrounding paraffin with CNF layer. The addition of CNF to emulsion also contributed to the excellent thermal regulation performance and enhanced mechanical properties of aerogels. These favorable properties make CNF aerogel promising for applications in smart textile, smart building, batteries, and electronic devices, where energy saving and load bearing are urgently demanded. vi Preface In Chapter 3, BET specific surface area test was performed by Dr. Hengfei Qin (Jiangsu University of Technology). SEM and TEM were performed by Prof. Feng Jiang. Jungang Jiang and Xueyong Ren provided advices to experiments and manuscript. I did most other design, experimental and analysis work presented in this chapter. Prof. Feng Jiang gave advices to revise the manuscript. This chapter has been submitted for publication (Mingyao Song, Jungang Jiang, Hengfei Qin, Xueyong Ren and Feng Jiang, 2020). In chapter 4, DSC and TGA test were performed by Fancheng Sun (Composites Knowledge Network, University of British Columbia). SEM was performed by Prof. Feng Jiang. Jungang Jiang helped to operate DLS. Jungang Jiang and Xueyong Ren provided advices to experiments and manuscript. I did most other experimental and analysis work presented in this chapter. Prof. Feng Jiang gave advices to modify the manuscript. This chapter has been prepared for publication (Mingyao Song, Jungang Jiang, Xueyong Ren and Feng Jiang, 2020). vii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables ................................................................................................................................ xi List of Figures .............................................................................................................................. xii List of Abbreviations ............................................................................................................... xviii Acknowledgements .................................................................................................................... xix Dedication .....................................................................................................................................xx Chapter 1: Introduction ............................................................................................................... 1 1.1 Cellulose and nanocellulose ........................................................................................ 2 1.1.1 Cellulose ............................................................................................................. 2 1.1.2 Nanocellulose ...................................................................................................... 4 1.1.3 Nanocellulose isolation methods ........................................................................ 7 1.2 Thermal insulation ...................................................................................................... 9 1.2.1 Thermal insulation principles ............................................................................. 9 1.2.2 Thermal insulation materials ............................................................................. 15 1.2.3 Thermal conductivity measurement .................................................................. 17 1.3 Aerogel ...................................................................................................................... 21 1.3.1 Properties .......................................................................................................... 21 1.3.2 Synthesis methods ............................................................................................. 22 1.3.3 Drying techniques ............................................................................................. 25 viii 1.4 Pickering Emulsion ................................................................................................... 28 1.4.1 Types and formation ......................................................................................... 28 1.4.2 Nanocellulose Pickering emulsion .................................................................... 31 1.5 Phase change materials ............................................................................................. 35 1.5.1 Properties .......................................................................................................... 35 1.5.2 Encapsulation methods ..................................................................................... 38 1.6 Hypotheses ................................................................................................................ 41 Chapter 2: Materials and methods ............................................................................................ 44 2.1 Materials ................................................................................................................... 44 2.2 Preparation of CNF ................................................................................................... 44 2.3 Synthesis of CNF/hexadecane emulsion and composite aerogel .............................. 45 2.4 Fabrication of CNF/paraffin emulsion and composite aerogel ................................. 45 2.5 Characterization ........................................................................................................ 46 2.5.1 Density, porosity, and volume shrinkage of acetone gels and aerogels ............ 46 2.5.2 Optical and laser scanning confocal microscope .............................................. 46 2.5.3 Transmission Electron Microscopy (TEM) ...................................................... 47 2.5.4 UV-vis spectroscopy ......................................................................................... 47 2.5.5 Scanning Electron Microscopy (SEM) ............................................................. 48 2.5.6 Mechanical testing ............................................................................................ 48 2.5.7 Brunauer–Emmett–Teller (BET) surface area measurement ............................ 48 2.5.8 Thermal conductivity ........................................................................................ 49 2.5.9 Infrared imaging................................................................................................ 50 2.5.10 Dynamic light scattering (DLS) ........................................................................ 50 ix 2.5.11 Thermal analysis ............................................................................................... 50 Chapter 3: Results and Discussion - flexible and super thermal-insulating cellulose nanofibrils aerogel with closed pores .......................................................................................................... 52 3.1 Introduction ............................................................................................................... 52 3.2 Morphologies of emulsion droplets and aerogel pores ............................................. 54 3.3 Solvent exchange and acetone gel ............................................................................ 58 3.4 Shrinkage and flexibility of aerogel .......................................................................... 63 3.5 Internal structure of aerogel ...................................................................................... 65 3.6 Specific surface area and pore characteristics of aerogel ......................................... 66 3.7 Mechanical properties of aerogel .............................................................................. 69 3.8 Thermal insulation performance of aerogel .............................................................. 72 3.9 Conclusions ............................................................................................................... 74 Chapter 4: Results and Discussion - phase change thermal regulation materials based on cellulose nanofiber-stabilized Pickering emulsion ................................................................... 76 4.1 Introduction ............................................................................................................... 76 4.2 CNF/paraffin emulsion ............................................................................................. 78 4.3 Densities of CNF/PCM aerogels ............................................................................... 80 4.4 Internal structure of CNF/PCM aerogels .................................................................. 81 4.5 Mechanical properties of CNF/PCM aerogels .......................................................... 82 4.6 Shape stability of CNF/PCM aerogels ...................................................................... 85 4.7 Thermal stability analysis of CNF/PCM aerogels .................................................... 87 4.8 Thermal reliability analysis of CNF/PCM aerogels .................................................. 88 4.9 Thermal regulation performance of CNF/PCM aerogels .......................................... 91 x 4.10 Conclusions ............................................................................................................... 96 Chapter 5: General conclusions and future work ...................................................................... 98 5.1 General conclusions .................................................................................................. 98 5.2 Future work ............................................................................................................... 98 Bibliography ...............................................................................................................................101 Appendices ..................................................................................................................................116 Appendix A. A typical TPS sensor: (a) sensor, (b) tip of the sensor.88 .............................. 116 Appendix B. (a) TPS 2500 S thermal conductivity analyzer and (b) schematic diagram of the testing setup.95 ..................................................................................................................... 116 Appendix C. (a) C-Therm TCi thermal conductivity analyzer and (b) schematic diagram of the testing setup.264 .............................................................................................................. 117 Appendix D. Scheme of temperature test for a model house in an oven. ........................... 117 Appendix E. Young’s modulus (dark color, error drawn) and yield strength (light color, error drawn) of acetone gel with varying emulsion: CNF ratios. ................................................ 118 Appendix F. Digital image of acetone gel after compression. ............................................ 118 Appendix G. Internal temperature change versus time curve of the model house covered without and with CNF/PCM4 (thickness: 5.6 mm, diameter: 55 mm) on a hotplate. ........ 119 xi List of Tables Table 1. The degrees of crystallinity of microfibrils of native celluloses20 .................................... 3 Table 2. Correlation of influence factors to total thermal conductivity ........................................ 14 Table 3. Thermophysical properties of typical thermal insulation materials ................................ 16 Table 4. Comparison of TPS and MTPS methods94, 96, 97 ............................................................. 20 Table 5. Correlation of influence factors to Pickering emulsion stability and droplet sizes (✔: positive correlation ✘ : negative correlation)181-184 ...................................................................... 34 Table 6. Thermophysical properties of common solid-liquid PCMs ............................................ 38 Table 7. Comparison of various encapsulation methods207, 210 ..................................................... 40 Table 8. The melting temperature (Tm), crystallization temperatures (Tc), and transition enthalpy (ΔH) of pure paraffin and CNF/PCM aerogels ............................................................................. 90 xii List of Figures Figure 1. The hierarchical structure of wood: from cellulose sources to cellulose molecules. Cell wall layers: middle lamella (ML), primary wall (P), outer (S1), middle (S2) and inner (S3) layers of secondary wall, and warty layer (W); Secondary wall structure: cellulose (C), hemicellulose (H), and lignin (L); Microfibril & elementary fibrils: amorphous domain (Am), crystalline domain (Cr), elementary fibril (EF), and microfibril (MF). Reproduced with permission.15 Copyright 2016, Elsevier. .......................................................................................................................................... 3 Figure 2. TEM images of (a) CNC and (b) CNF. Reproduced with permission.40 Copyright 2013, American Chemical Society. .......................................................................................................... 6 Figure 3. The anisotropic structure of cellulose Iβ crystallite. Reproduced with permission.40, 46 Copyright 2019, Elsevier. ............................................................................................................... 7 Figure 4. Schematic illustration of the TEMPO-mediated oxidation under two systems: (a) TEMPO/NaBr/NaClO at basic pH and (b) TEMPO/NaClO/NaClO2 at neutral or slightly acidic pH. Reproduced with permission.48 Copyright 2016, Elsevier. ...................................................... 9 Figure 5. Schematic illustration of heat transfer in aerogels. ....................................................... 10 Figure 6. Phase diagram illustrating different drying routes. Reproduced with permission.112, 140 Copyright 2003, Taylor & Francis. ............................................................................................... 25 Figure 7. Schematic diagram of different types of emulsions. Reproduced with permission.154 Copyright 2007, John Wiley and Sons. ........................................................................................ 29 Figure 8. Schematic illustration of emulsions stabilized by surfactant and nanoparticle. Reproduced with permission165. Copyright 2018, Elsevier. ......................................................... 30 xiii Figure 9. Comparison of sensible heat storage method and latent heat storage method. ΔHF: latent heat of fusion. TM: melting temperature. Reproduced with permission186. Copyright 2018, The Royal Society of Chemistry. ......................................................................................................... 36 Figure 10. Schematic illustration of (a) internal structure and (b) cross-section view of CNF aerogel with tangled nanofibrils; (c) internal structure and (d) cross-section view CNF/emulsion composite aerogel with both tangled nanofibrils and closed hollow microspheres. ...................................... 55 Figure 11. TEM image of CNF ..................................................................................................... 56 Figure 12. Confocal (a) and optical (b) microscopy images of emulsion droplets, inset in (a) shows the schematic illustration of internal structure of the emulsion droplet; (c) Diameter distribution of the emulsion droplets measured from optical microscopy image. ................................................ 56 Figure 13. SEM image of pores in CNF/emulsion aerogel sample (8:2 emulsion:CNF ratio) .... 57 Figure 14. Digital images and schematic illustration of CNF/emulsion mixed suspension with varying emulsion:CNF ratios (0:10 to 10:0). ................................................................................ 58 Figure 15. Digital images of CNF emulsion going through sequential solvent exchange by acetone in (top) upright and (bottom) inverted glass vials. Leftmost: CNF emulsion immediately after preparation; second to the left: CNF emulsion after overnight aging. The 3rd to 8th glass vials showed sequential solvent exchange until a transparent gel is formed. ....................................... 59 Figure 16. Photograph of CNF/Hexadecane Pickering emulsion after one year storage in a refrigerator. ................................................................................................................................... 59 Figure 17. Picture of CNF/emulsion sample during solvent exchange. ....................................... 60 Figure 18. Digital images of the acetone gel with varying emulsion:CNF ratios after complete solvent exchange. .......................................................................................................................... 61 xiv Figure 19. (a) UV-vis transmittance spectra and (b) transmittance at 650 nm of the CNF/emulsion acetone gels (thickness: 2.8 mm). ................................................................................................. 62 Figure 20. Laser scanning confocal microscopy images of different CNF/emulsion acetone gels........................................................................................................................................................ 63 Figure 21. (a) Digital images of different freeze-dried aerogels with varying emulsion: CNF ratios........................................................................................................................................................ 63 Figure 22. A large piece of (a) 8:2 aerogel (thickness: 6 mm) demonstrating great flexibility that can be (b) rolled and (c) recovered its shape. ............................................................................... 64 Figure 23. (a) Volume shrinkage of acetone gels and aerogels with varying emulsion (error drawn): CNF ratios; (b) Density (dark color, error drawn) and porosity (light color) of freeze-dried aerogels; The error bar represent the variability of both density and porosity. ............................ 65 Figure 24. SEM images of aerogels with varying emulsion: CNF ratios, (a-c): 0:10, (d-f): 2:8, (g-i) 8:2, and (j-l) 10:0. ...................................................................................................................... 66 Figure 25. Specific surface area and porosity of aerogel with varying emulsion: CNF ratios. (a) Nitrogen adsorption–desorption isotherms and (b) BJH pore size distribution. ........................... 67 Figure 26. (a) Specific surface area; (b) Pore volume of CNF/emulsion composite aerogels. ..... 68 Figure 27. High-resolution SEM images of (s) CNF-only and (b) emulsion-only aerogels. ........ 68 Figure 28. The mechanical properties of aerogel with varying emulsion: CNF ratios. (a) stress-strain curves, (b) Young’s modulus (dark color, error drawn) and yield strength (light color, error drawn). .......................................................................................................................................... 69 Figure 29. Specific Young’s modulus (dark color) and specific yield strength (light color). ...... 70 Figure 30. Comparison of the Young’s modulus vs. density of this work ( ) with various bio-aerogels without chemical crosslinking: MFC (microfibrillated cellulose) aerogel ( ),249 CNF xv aerogels ( 126, 250, 122), chitosan aerogel ( ),251 nanofibrillated cellulose emulsion aerogel (),252 electrospun cellulose aerogel ( ),253 Nano-CMC (carboxymethylcellulose) aerogel ( ).148 71 Figure 31. (a) Thermal conductivity of aerogel with varying emulsion:CNF ratios and (b) 4:6 aerogel sample (thickness: 3.8 mm) on human palm. ................................................................... 73 Figure 32. (a) Temperature-versus-time curve and (b) infrared thermal image of 8:2 aerogel (thickness: 6 mm) on hot plate (blue dash line represents room temperature; large red dots represent the surface temperature of aerogel on hot plate) ........................................................................... 74 Figure 33. (a) Temperature-versus-time curve and (b) infrared thermal image of 8:2 aerogel (thickness: 6 mm) on ice (large blue dots represent the surface temperature of aerogel on ice; red dash line represents the temperature of hot plate or ice). ............................................................. 74 Figure 34. Digital image of CNF/paraffin emulsion. .................................................................... 79 Figure 35. (a) Size distribution of emulsion droplets determined from DLS; (b) Optical microscopy image and (c) size distribution of emulsion droplets determined from optical microscopy images........................................................................................................................................................ 79 Figure 36. (a) Densities of freeze-dried CNF/PCM aerogels with varying emulsion to CNF ratios (1:9, 2:8, and 4:6); (b) Digital image of CNF/emulsion composite aerogel (4:6, thickness: 16 mm) standing on top of a flower. .......................................................................................................... 80 Figure 37. SEM images of the CNF/PCM aerogels with varying emulsion:CNF ratios, (a-c): 1:9, (d-f): 2:8, and (g-i) 4:6. ................................................................................................................. 82 Figure 38. (a) Stress-strain curves of CNF/PCM aerogels with varying emulsion:CNF ratios; (b) Thickness change of CNF/PCM composite aerogel with different paraffin emulsion fractions after a load of 500g weight on top, and the insect is the digital image of CNF/PCM4 composite aerogel (thickness: 15 mm) loaded with 500 g weight. ............................................................................. 83 xvi Figure 39. (a) Young’s modulus and yield strength of CNF/PCM aerogels with varying emulsion:CNF ratios (error drawn); (b) Specific Young’s modulus and specific yield strength (error drawn). ................................................................................................................................ 84 Figure 40. Bending test of PCM (thickness: 5 mm) prepared without additional CNF. .............. 85 Figure 41. Bending test of CNF/PCM1 (thickness: 5 mm) prepared with emulsion:CNF ratio 1:9........................................................................................................................................................ 85 Figure 42. Digital images demonstrating the shape stability of CNF/PCM4 aerogel and pure paraffin (thickness: 18 mm) after heating at different temperatures for 20 min. .......................... 86 Figure 43. Digital image of CNF/PCM4 composite aerogel (thickness: 14 mm) supporting a 500 g weight at 190 °C. .......................................................................................................................... 86 Figure 44. (a) (a) TGA (N2), (b) DTGA, and (c) DSC curves of pure paraffin and CNF/PCM composite aerogels with varying emulsion:CNF ratios. ............................................................... 88 Figure 45. DSC curves of pure paraffin and CNF/PCM aerogels with varying emulsion:CNF ratios........................................................................................................................................................ 89 Figure 46. (a) DSC curves of CNF/PCM4 composite aerogels after 51 heating and cooling cycles; (b) Digital image of CNF/PCM4 composite aerogel (thickness: 9 mm) after 20 heating and cooling thermal cycles. .............................................................................................................................. 91 Figure 47. Thermal conductivity of CNF/PCM aerogels with varying paraffin fraction. ............ 92 Figure 48. Temperature of CNF/PCM aerogels with varied paraffin fraction (thickness: 9.5 mm) versus time curves on hot plate of (a) 52°C and (b)195 °C. ......................................................... 93 Figure 49. Temperature change of pure CNF aerogel (thickness: 10 mm) and CNF/PCM aerogels (thickness: 10 mm) during cooling process (the first row from left to right: CNF/PCM1, 2, and 4; the second row: pure CNF aerogel). ............................................................................................. 94 xvii Figure 50. Temperature of CNF/PCM aerogels (thickness: 10 mm) versus time curves during cooling from 53°C. ....................................................................................................................... 94 Figure 51. (a) Model house covered with CNF/PCM4 (thickness: 5.6 mm) in lab oven; (b) Internal temperature change versus time curves of model house covered with and without CNF/PCM4. 95 Figure 52. Infrared thermal images of the human palm covered by prepared CNF/PCM4 (thickness: 5 mm). ......................................................................................................................... 96 xviii List of Abbreviations BNC Bacterial nanocellulose CNC Cellulose nanocrystals CNF Cellulose nanofibrils DI Deionized DLS Dynamic light scattering DSC Differential scanning calorimetry IR Infrared NBSK Northern bleached softwood Kraft PCM Phase change material SEM Scanning Electron Microscopy TEM Transmission Electron Microscopy TEMPO 2,2,6,6-Tetramethylpiperidine 1-oxyl TGA Thermogravimetric analysis TPS Transient plane source Wt. Weight xix Acknowledgements First, I would like to offer my enduring gratitude to my supervisor, Dr. Feng Jiang, for all of his support and guidance throughout the study and research. Dr. Feng Jiang has inspired me to continue my work in this field, and his penetrating questions taught me to question more deeply. I owe particular thanks to my committee members, Dr. Scott Renneckar, for his scientific expertise and mentorship; and Dr. Philip D. Evans, for his insightful suggestions and support. Special thanks go towards Jungang Jiang, who was particularly generous with his time, scientific advice, and support. I am also very thankful for the support and friendship of my past and current fellow lab members, Dr. Chen Xu, Dr. Hale Oguzlu, Dr. Hengfei Qin, and Dr. Xueyong Ren. I would also like to thank the staff in the Faculty of Forestry for their support. xx Dedication This study is dedicated to my family who provide continued love and support. 1 Chapter 1: Introduction In light of the current increasing concern toward energy and environment issues, it appears that sustainable aerogels are promising for thermal insulation and attract much attention.1-4 Functional aerogels are highly desirable for a wide variety of applications due to their extraordinary physicochemical properties. The ultralight, highly porous, and large surface area of the aerogels make them very promising for versatile applications such as acoustic and thermal insulating, personal care, chemical adsorbent, filtration, separation, energy, and catalysis.5-10 In this context, numerous research efforts have been oriented towards the fabrication of high performance functional aerogels, especially from materials that are abundant, potentially biocompatible, biodegradable, and renewable. Thermal insulation is one of the most important and attractive applications, which can reduce energy loss and hence reduce consumption of fossil fuels and CO2 emission. Cellulose nanofibril (CNF), as one of the sustainable materials, has favorable properties such as high aspect ratio, large surface area, low density, and abundant surface functionalities, which make it a promising candidate as thermal insulating materials.11 Superior thermal insulation properties, tunable mechanical properties and low densities are desirable for the practical use of thermal insulating materials, and has been actively investigated.12-14 Strategies used to improve the thermal management performance of CNF aerogels includes the control of pore morphology to reduce the thermal conductivity, and the inclusion of materials with enhanced thermal storage properties. 2 1.1 Cellulose and nanocellulose 1.1.1 Cellulose Cellulose is a sustainable material with broad applications, and is a desirable thermal insulating material due to its sustainability, abundance, as well as numerous functional groups for potential chemical modification.15 First isolated by a French chemist, Anselme Payen, in 1838,16 cellulose can be obtained from a variety of natural resources such as wood (hardwood and softwood), cereal straws (barley, oat, rice, corn, wheat), seed fibers (cotton, coir, milkweed, kapok), fruit fibers (coconut, oil palm), bast fibers (flax, ramie, hemp, jute, kenaf, nettle), leaf fibers (sisal, pineapple, date-palm, abaca, curaua), grasses (bagasse, bamboo, alfa, bagasse), marine animals (tunicate), algae, invertebrates, fungi, and bacteria.17, 18 Among them, wood pulp and cotton are two main sources for cellulose production in industry.19 The hierarchical structure of plants, from cell walls to cellulose molecules, is presented in Figure 1. The morphological hierarchy of cellulose mainly include elementary fibrils, microfibrils, and fibers, and their lateral dimensions are 1.5-3.5 nm, 10-30 nm, and on the order of over a few µm, respectively.20 In addition, cellulose is a semicrystalline polymer which has two different regions, i.e. highly ordered regions (crystalline) and disordered regions (amorphous). The ratio of crystalline to amorphous region has an influence on the accessibility and reactivity of cellulose. The degree of crystallinity of cellulose are shown in Table 1. 3 Figure 1. The hierarchical structure of wood: from cellulose sources to cellulose molecules. Cell wall layers: middle lamella (ML), primary wall (P), outer (S1), middle (S2) and inner (S3) layers of secondary wall, and warty layer (W); Secondary wall structure: cellulose (C), hemicellulose (H), and lignin (L); Microfibril & elementary fibrils: amorphous domain (Am), crystalline domain (Cr), elementary fibril (EF), and microfibril (MF). Reproduced with permission.15 Copyright 2016, Elsevier. Table 1. The degrees of crystallinity of microfibrils of native celluloses20 Cellulose source Algal cellulose Bacterial cellulose Cotton linters Ramie Flax Hemp Dissolving pulp Degree of crystallinity (%) >80 65-79 56-65 44-47 44(56) 44(59) 43-56 4 The structure of cellulose, repeating glucose units linked through covalent bonds, was first determined by Hermann Staudinger in 1920.21 The single molecular chain of cellulose consists of D-glucose units, which are condensed to form the linear chains via β(1→4)-glycosidic bonds. The molecular chains of cellulose can assemble together through hydrogen bonding between hydroxyl groups as well as Van der Waals force to form crystalline structure known as elementary fibrils.22 The properties of cellulose can be influenced by the degree of polymerization (DP), which varies for cellulose obtained from different sources and with different isolation methods. The DP ranges from 300 to 1700 for cellulose derived from wood pulp, around 800-10,000 for cotton, and 44,000 for Valonia.23-25 There are four major different polymorphs of cellulose, including cellulose I, II, III, and IV, and their properties are different dependent on the crystal structures. Native cellulose, existing in parallelly-packed strands, is known as Cellulose I, includes two allomorphs, Iα and Iβ.26 Iα is typically found in bacteria and algae and Iβ is found in higher plants.27 Cellulose II (regenerated state), existing in antiparallel-packed strands, can be obtained by using alkaline to treat Cellulose I in the solid state (mercerization process), and is more thermodynamically stable as the conversion from Cellulose I to Cellulose III is irreversible.26 Cellulose III is formed by treating Cellulose I or II with supercritical ammonia. Cellulose IV can be obtained by heating Cellulose III in glycerol at a high temperature (ca. 260 °C).28 1.1.2 Nanocellulose Cellulose is typically used to produce traditional forest products such as pulp, paper, paperboard and so on. As the growing environmental concern requires more efficient bioeconomy, it has become a pressing issue to develop high value bio-based materials to optimize the utilization 5 of forest resources. Due to sustainable and biocompatible properties and broad applications, nanocellulose is regarded as a good candidate for producing high value functional materials.17, 29 As a nanostructured cellulose with at least one dimension in nanometer range (1-100 nm), nanocellulose can be divided into three main types, cellulose nanofibrils (CNF, also called nanofibrillated cellulose or microfibrillated cellulose), cellulose nanocrystals (CNC, also known as cellulose nanowhiskers), and bacterial nanocellulose (BNC). Nanocellulose derived from native cellulose fiber by mechanical or chemical methods is typically referred to as CNFs or CNCs, while that obtained by the bacterial strain is called BNC.30-33 These organic nanostructured particles, consisting of a bundle of linear polymer chains of β (1−4) linked D-glucose units, have attractive chemical and mechanical properties due to their large specific surface area, high aspect ratio and relatively high crystallinity to fabricate hierarchical functional materials34. Through different drying methods (freeze-, supercritical-, or ambient-drying), different forms of nanostructured materials can be derived from nanocellulose, such as powder, fibers, film, hydrogel, and aerogel.35-38 Both CNCs and CNFs can be isolated from the cell walls of abundant plants, and can act as reinforcements for producing nanocomposites, but they significantly differ from each other in their morphologies. The clear distinction in size and morphology between CNCs and CNFs are presented in Figure 2. In general, the diameter and length of CNFs are approximately 5–100 nm and a few micrometers, respectively, while, for CNCs, the dimensions are 5-20 nm in diameter and several hundred nanometers in length.39, 40 Therefore, much higher aspect ratio is expected from CNFs (over 50) as compared to CNCs (less than 20). Due to its high aspect ratio, CNFs are more flexible and more easily entangled than CNCs, which makes CNFs form more stable 6 hydrogels and aerogels. Even at a lower solid content, the viscosity of CNFs suspension (1.8%) is still much higher than CNCs suspension (5.7%).40 Figure 2. TEM images of (a) CNC and (b) CNF. Reproduced with permission.40 Copyright 2013, American Chemical Society. CNF and CNC are amphiphilic as there are both hydrophilic and hydrophobic planes on the surface of nanocellulose. There are three different lattice planes for anisotropic nanocellulose crystallite, and the different polarities of these planes endow nanocellulose amphiphilic behavior.41, 42 The crystallite consists of cellulose chain sheets stacked by Van der Waals forces and CH-OH bonds, and these sheets are formed by OH-O hydrogen bonds of cellulose chains.43 The hydroxyl and carboxyl groups in the equatorial position are on the planes of (11̅0) and (110) and these planes are hydrophilic, while C-H bonds mainly in the axial position on the plane (200) and this plane is hydrophobic.41, 44, 45 Figure 3 illustrates the chain of anhydroglucopyranose (Figure 3a), the positions of hydroxyl groups and C-H bonds in a glucose unit (Figure 3b), and three kind of lattice planes in the crystallite (Figure 3c). 7 Figure 3. The anisotropic structure of cellulose Iβ crystallite. Reproduced with permission.40, 46 Copyright 2019, Elsevier. 1.1.3 Nanocellulose isolation methods CNFs and CNCs can be isolated or purified by mechanical, biological (enzymatic hydrolysis), and chemical pre-treatment (acid hydrolysis or oxidation). CNCs are generally produced by concentrated mineral acid hydrolysis of chemical pulp, such as 64% sulfuric acid, which can degrade the amorphous regions to obtain rod-shape nanocrystals and also introduce sulfate groups onto the surface of CNCs.47 The production of CNF generally requires a combination of processes, i.e. biological/chemical pretreatment (e.g. enzymatic hydrolysis, carboxylation, carboxymethylation, quaternization, sulfonation) to break inter-fibril hydrogen bonds, followed by 8 mechanical shearing (assisted by grinding, high-speed blending, homogenization, ultrasonication, and cryocrushing) to separate the individual CNFs.48 As degradation of the amorphous region is not the main mechanism for isolating CNFs, there are both crystalline and amorphous regions in such nanocellulose. Different treatments generally result in CNF with different dimensions ranging from 5 nm to tens of nanometers.48 More recently, it has become very popular to isolate CNFs from pulp through the method of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated regioselective oxidation (only oxidize primary alcohol groups) followed by mechanical blending in water for disintegration of fibrils. The TEMPO-mediated oxidation method imparts CNF large surface area, transparency and high fibrillation.11 In the presence of NaClO (a primary oxidant) and NaBr, the catalyst TEMPO is oxidized to provide a stable nitroxyl radical to convert the hydroxymethyl groups present on the surface of cellulose to carboxylic form.49-51 CNF prepared by the TEMPO-mediated oxidation contains negative carboxyl groups on the cellulose surface, which can inhibit the aggregation of fibrils by electrostatic repulsion. This method was used at room temperature in 2006 to produce CNF with a carboxyl content of up to 1.52 mmol·g-1, and the cellulose was further disintegrated by blending into nanofibrils with average diameter of 3-5 nm.52 The TEMPO oxidation method can be used for different cellulose sources such as bleached sulfite wood pulp, cotton, and tunicin.53 However, it was found that there was also depolymerization reaction for cellulose besides oxidation, which may be the result of β-elimination induced by C6 aldehyde groups under alkaline conditions or the result of anhydroglucose unit cleavage caused by hydroxyl radicals formed as side reactions.49, 54 The system of TEMPO/NaClO/NaClO2 was developed to avoid the depolymerization of cellulose and oxidize the primary alcohol groups on cellulose surface.55, 56 Different from the room temperature and essential basic reaction condition for the TEMPO/NaBr/NaClO system, the oxidation reaction in 9 the TEMPO/NaClO/NaClO2 system was carried out at a higher temperature (60 °C) under slightly acidic or neutral conditions and this reaction also required much longer time (up to 72 h). With the TEMPO/NaClO/NaClO2 method, most of the original degree of polymerization was successfully maintained. The mechanisms of two different TEMPO systems, TEMPO/NaBr/NaClO (aq) at basic pH and TEMPO/NaClO/NaClO2 (aq) at neutral or weakly acidic pH, is described in figure 4. Figure 4. Schematic illustration of the TEMPO-mediated oxidation under two systems: (a) TEMPO/NaBr/NaClO at basic pH and (b) TEMPO/NaClO/NaClO2 at neutral or slightly acidic pH. Reproduced with permission.48 Copyright 2016, Elsevier. 1.2 Thermal insulation 1.2.1 Thermal insulation principles Thermal insulation performance of aerogels is largely dependent on the heat transfer process that is determined by the inner structure of aerogels. Figure 5 illustrates four main forms of heat transfer in aerogels: thermal conduction in solid pore walls (red arrow, λs), thermal conduction through gas phase in pores (pink arrow, λg), thermal radiation between pore walls (orange arrow, λr), and thermal convection of gas within pores (yellow arrow, λgv). The total thermal conductivity 10 of aerogels λtotal is determined by their sum: λtotal = λs + λg +λr+ λgv.57-60 For aerogel, the nano- or micron-sized pores are filled with gas, the circulation of which can be restricted by the confined pore structure. However, heat can still transfer through pores via infrared radiation. Thus, the total effective thermal conductivity λeff strongly depends on three main forms of heat transfer in porous aerogel materials: heat conduction in solid pore wall λs, heat conduction in gas phase λg, and heat radiation in pores λr. Figure 5. Schematic illustration of heat transfer in aerogels. Total thermal conductivity can be influenced by many factors. The chemical composition, connectivity of the pore walls, density of nanoparticles, and porosity can control the heat 11 conduction in the solid pore walls,58 while pore morphology (size, shape, openness, etc.) has a significant effect on the heat conduction in gas phase, and finally mass-specific light absorption coefficient determines heat radiation in pores.58-60 The thermal conductivity of porous nanomaterial in solid pore walls is given in terms of derived valid phonon diffusion model: λ𝑠𝑠(T) = λ𝑠𝑠0 ‧ 𝜌𝜌𝜌𝜌0 ‧ 𝑣𝑣𝑣𝑣0 , where λs0 is the thermal conductivity of solid pore walls, v and v0 are the respective longitudinal sound velocities of the aerogel and the solid material, ρ and ρ0 are the respective density of the aerogel and the solid material.61-63 According to the above equation, the solid conductivity λs of a specific material can be tuned by the density of material. The thermal conductivity of porous nanomaterial through gas phase within the pores can be described as λ𝑔𝑔 =λ𝑔𝑔0‧ϕ1+α ‧ Kn , where both λg0 and α are the characteristics of the filled gas, representing thermal conductivity and a constant value (~2 for air), respectively, ϕ is the porosity of aerogel, and Kn is the Knudsen number (=lg/D, where lg is the mean-free path for gas molecules and D is the effective pore size).62 Although, as suggested by the equation, the thermal conductivity in the gas phase λg will increase with increased porosity due to more gas molecules, the porosity for highly thermal insulating materials should be as high as possible to minimize the thermal conductivity through the solid pore walls. Therefore, the porosity effect should dominate the thermal conductivity through the solid pore walls, instead of that through gas molecules. Based on this equation, it is clear that λg will decrease as the pore size decreases through the effect of Knudsen number. The thermal conductivity via radiation in the pores of porous nanomaterial is described as λ𝑟𝑟 =16‧𝑛𝑛2‧σ‧𝑇𝑇𝑟𝑟33‧𝜌𝜌‧ 𝐾𝐾𝑠𝑠/𝜌𝜌0 , where σ = 5.67 × 10-8 W/(m2 ‧K4) is the Stephan-Boltzmann constant, n is the refractive index (describing the propagation velocity of light in a medium; 1 for vacuum and around 1 for aerogels), ρ and ρ0 are the respective densities of the aerogel and the solid material, Tr is the average 12 temperature within the material, and Ks is the extinction coefficient of the solid materials (describing the ability to absorb light at a given wavelength, and higher concentration/density and deeper color resulting in lower light transmittance and higher Ks.64 According to the above equation, the significance of radiative conductivity of aerogels increases as temperature increases. At a constant temperature, λr can be decreased by higher specific extinction coefficient Ks/ρ0 and/or ρ. Infrared opacifier can be added to aerogels to tune the Ks for reduced thermal radiation. Specific extinction coefficient Ks/ρ0 at 300 K is 22.7 and 84.2 for silica aerogel without and with opacifier such as carbon black, respectively.62, 65, 66 As suggested by the respective equation of λs and λr, thermal conductivity λeff will not be linear with density ρ of aerogels, since an increasing density ρ will lead to an increase in λs but a decrease in λr, and λeff will first decrease and then increase with a minimum λeff at a certain density ρ. Thermal conductivity can be minimized by appropriate application of the aforementioned parameters, such as higher porosity, reduced pore size, use of infrared radiation opacifier, and their combinations. In summary, the thermal conductivity will be highly affected by both porosity and pore morphology (including the size, shape, openness, etc.). Higher porosity indicates more gas and less solid material, and gas commonly has lower conductivity than solid material. Higher porosity is desired in terms of reduced thermal conductivity; however, higher porosity also leads to weakened structure and mechanical properties that is highly dependent on the morphology of inner structure of aerogels.14 Materials with small pore size are usually optically opaque, which leads to less heat radiation. Pore size will also affect thermal conductivity of gas based on the Knudsen effect. According to Knudsen effect in the intra-granular pores of highly aerogel, when pore size becomes comparable or smaller than the mean free path (~68 nm for air at ambient pressure and room temperature) of gas molecules,67 the thermal conductivity can become close to 13 and even lower than that of ambient air, i.e., 0.026 W·m−1·K−1 at 25 ℃, ascribed to the restricted movement of gas molecules.68, 69 Another important factor for heat conduction is pore openness, i.e., open pores or closed pores. For insulation materials with open pores, the convection of gas may be important, and thus the thermal conductivity of open-pore materials is more easily affected by moisture content and temperature in comparison to closed-pore materials with partitioned air.70, 71 This effect will be even worse under highly humid conditions. Nano-sized and closed pores are therefore desirable for super-insulating materials. Thermal conductivity can also be affected by a number of other factors such as grain contact area, crystallinity, coefficient of linear thermal expansion, pressure of filled gas, density and conductivity of gas, direction of heat flux and so on,72 but these factors have either fixed values for a specific material or are not significant for highly aerogels. Thus, the overall goal of this research is to design nanocellulose-based thermal insulation materials with finely tuned pore morphology to obtain more effective thermal insulating properties, as well as enhanced mechanical toughness for practical applications. Table 2 summarizes the correlation of factors that influence the total thermal conductivity of aerogels. 14 Table 2. Correlation of influence factors to total thermal conductivity (✔: positive correlation ✘ : negative correlation) 70, 73, 74 Influence factors Correlation to Total Thermal Conductivity λ Pore size ✔ Owing to phonon ballistic effects; Uniform small pore size distribution and closed pore lead to lower λ Porosity ✘ Owing to phonon ballistic effects Density of aerogel ρ As ρ increases, λ decreases(✘ ) to an extent at first and then increases(✔) for ultralightweight materials Temperature ✔ Not significant when temperature is 0 ~ 50 ℃. Grain contact area/connectivity ✔ Crystallinity (vs. amorphous) ✔ Not significant for highly aerogels Specific heat capacity ✔ Coefficient of linear thermal expansion ✔ Not significant for highly aerogels Humidity ✔ When moisture content is greater than 5% ~ 10% Pressure of filled gas/Vacuum ✔ λ=0.002~0.008 W·m-1·K-1 Density and Conductivity of gas ✔ Direction of heat flux for anisotropic materials ✔ Parallel to fiber direction ✘ Vertical to fiber direction 15 1.2.2 Thermal insulation materials The role of thermal insulation materials is to reduce the thermal energy transfer between objects of different temperatures. In comparison with the amount of energy conserved by thermal insulation materials, the energy consumed for production of thermal insulation materials is much less. There are significant benefits from thermal insulation materials besides inhibiting the transmission of heat energy. They can reduce the amount of fossil fuel energy consumption and CO2 emissions. Conventional thermal insulation materials are primarily obtained from petrochemicals, e.g. polystyrene, which have significantly negative effect on the environment during the whole life cycle of the product, from production to disposal. Sustainable biomaterials are highly desirable for producing thermal insulation materials. The thermophysical properties of commercial thermal insulation materials are presented in Table 3. Foam glass has the highest thermal conductivity among these thermal insulation materials, but it has a high mechanical strength and can be used for applications that include a large load. Silica aerogels and vacuum insulation panels have the lowest thermal conductivities, and the ultralight silica aerogels can be used at a very high temperature. However, the brittleness of silica aerogels prevents their applications where load is expected. Conventional thermal insulation materials (such as glass wools, rock wool, polystyrene, and polyurethane) generally have low lost and good reliability, but their drawbacks include non-sustainability and high thermal conductivity. Aerogels and vacuum insulation panels are the most promising thermal insulation materials based on all thermophysical properties. With increasing commercialization and scaling-up of production of aerogels in combination with improved production techniques, the cost of aerogels may drop in the near future. 16 Table 3. Thermophysical properties of typical thermal insulation materials Insulation materials Thermal conductivity (mW·m−1·K−1) Density (kg·m-3) Maximum service temperature (°C) Compressive stress at 10% deformation (kPa) Cost (Can$ m–2) Ref. Silica aerogels 4-20 3-350 750 0-5000 831.44 75, 76 Vacuum insulation panels 4-8 65-300 90 45-120 375.44 75, 76 Polyurethane-polyisocyanurate foam (PUR-PIR) 19-30 25-100 120 100-500 89.68 75-77 Expanded polystyrene (EPS) 29-41 10-50 80 60-260 48.64 75-77 Extruded polystyrene (XPS) 25-35 20-80 75 150-700 69.92 75, 77, 78 Glass wool 30-46 8-150 500 15-80 65.36 76, 77, 79 Rock wool 33-46 13-240 750 15-80 54.72 77-79 Foam glass 38-61 100-200 >400 400-1600 199.12 75, 80 Cork 40-50 120-160 200 - - 81-83 Balsa wood 38.1-66.5 113-137 163 - - 84 17 1.2.3 Thermal conductivity measurement There are numerous methods to characterize thermal conductivity, e.g. transient plane source method, steady-state electrical heating method, radial heat flow method, parallel conductance method, pulsed power technique, laser flash diffusivity method, thot-wire method, and 3ω method 85. Generally, these methods can be classified into two categories, i.e. steady state method and transient method. The steady-state method performs a measurement by keeping the temperature of the material constant, having the drawbacks such as requiring well-engineered experimental setup, long waiting time for establishing steady-state, and contact resistance of temperature sensors.86 To overcome these drawbacks, the transient method was developed, which typically measure the thermal conductivity by time-dependent energy dissipation during heating up. The transient method does not require waiting for a steady-state situation to perform a measurement, and therefore can be performed more quickly, usually in several seconds. Two transient methods are commonly used to measure the thermal conductivity of bulk materials, i.e. traditional transient plane source (TPS) method and modified transient plane source (MTPS). TPS can be used to measure thermal diffusivity and thermal conductivity, while MTPS can be used to measure thermal effusivity and thermal conductivity. Both of them can be used to test various sample size and types such as solids, liquids, powders, and pastes.87-90 TPS method utilizes a thin metal disk or strip as sensor (Appendix Aa) to measure thermal transport properties.91 The sensor, in the shape of a double spiral (Appendix Ab), is comprised of ~ 10μm thick electrically conducting nickel foil which is sealed by two layers (0.013 -0.025 mm) of electrical insulation materials such as polyimide (Kapton) to keep the physical shape of the sensor.91 The nickel foil works as both a constant heat source and a temperature sensor. Before the measurement, the sensor is sandwiched between two identical testing samples and needs to have 18 good contact with the entire planar surfaces of samples. Other surfaces of samples are thermally insulated. Appendix B shows the schematic diagram of a typical instrument, TPS-2500 S, which can measure thermal conductivity and diffusivity directly. The test instrument and operational procedure for the aforementioned method is according to ASTM D7984 and ISO 22007-2.92, 93 During testing, a small constant current passing through the nickel foil is used as a heat source to heat up the metal foil by dissipating the heat on both sides of the sensor. Testing samples transfer some heat depending on the thermal transport characteristics of the samples, and the rest of the generated heat will induce a temperature increase of the sensor surface 94. Thus, the rate of the temperature rise is inversely proportional to the ability of the sample to absorb heat, and thermal conductivity is inversely proportional to the rate of temperature rise 94. The output of heating power and the total time of temperature increase varies and can be adjusted according to the characteristics, e.g. size, state, and thermal properties, of different samples to get more accurate data.85 The thermal properties of samples can be calculated by fitting the curve of temperature increase at the sensor surface versus time response.85 The accuracy of temperature measurement is generally ± 0.01 °C.95 There are several factors that may cause errors for TPS method, including the measured power input, thermal inertia of sensor, thermal resistance between sample and sensor, and electrical resistance change of sensor, and it is necessary to correct these errors to improve the accuracy of data.87, 88, 90 A variation of the above method is the MTPS method which is based on the traditional TPS method, and the main difference between them is that MTPS uses a single-side, interfacial, and heat reflectance sensor which is supported on a thermal insulated and electrical insulated backing.94 MTPS offers flexibility in testing samples of any size and state without destruction. 19 Appendix C shows a typical instrument for MTPS, C-Therm TCi thermal conductivity analyzer which can measure the thermal conductivity and thermal effusivity directly. Thermal conductivity analyzer based on MTPS method generally comprises a one-sided sensor that generates a constant current to heat up the sample by approximately 1–3 °C in several seconds. The one-dimensional heat flow is generated to the testing samples from both the spiral element of the sensor and the guard ring surrounding the sensor.94 During measurement, the voltage, factory-calibrated to temperature, will decrease on the sensor element when there is a temperature rise at the sensor surface, and the change of voltage is recorded before and during the transient.94 The thermal effusivity value of the tested sample is translated from the measured voltage data which is also utilized to determine thermal conductivity.94 Table 4 lists the comparison of two testing methods for measuring the thermal properties of aerogel samples. 20 Table 4. Comparison of TPS and MTPS methods94, 96, 97 Typical instruments C-Therm TCi (MTPS) TPS 2500S (TPS) Test Methods Modified Transient Plane Source (MTPS) Traditional Transient Plane Source (TPS) Sample preparation This method doesn’t require extensive sample preparation. One sample is needed for each test. Smooth surfaces of samples for good contact with sensor is required. Two identical samples with smooth surfaces for good contact with sensor are required for each test. Sample Configuration Any shape with a minimum diameter of 17 mm Minimum Thickness: Nominally 0.5 mm, dependent on thermal conductivity of material. 0.5 mm x 2 mm diameter or square for bulk testing. 0.1 mm x 10 mm diameter or square for slab testing. 5 mm x 3 mm diameter or square for one-dimensional testing Materials testing Capabilities Solids, Liquids, Powders, Pastes Solids, Liquids Testing Time 0.8 - 3 seconds 1- 1280 seconds. Thermal Conductivity Range 0 - 100 W·m-1·K-1 0.005 -1800 W·m-1·K-1 Thermal Effusivity/Diffusivity Thermal Effusivity: 5 to 40,000 Ws½·m-²K-1 Thermal Diffusivity: 0.1-o 1200 mm / s Temperature Range -50 – 200°C with option to extend to 500 °C -235 – 1000 °C Reproductivity Typically less than 1%. Typically less than 1%. Accuracy (thermal conductivity) Better than 5% Better than 5% 21 1.3 Aerogel 1.3.1 Properties Aerogel is an ultralight aerogel formed by replacing liquid medium in hydrogel with air while maintaining the porous structure without substantial volume shrinkage or collapsed network structure. The densities and specific surface areas of aerogel range from around 0.003 to 0.15 kg·m-3 and 500 to 1000 m2·g-1, respectively.98-101 Aerogels can be classified based on their composition: organic aerogel (synthetic polymer, cellulose, lignin, and chitosan), inorganic aerogel (carbon, silicon, alumina, titania, and zirconia), and inorganic-organic composite aerogel. Cellulose aerogel is a type of biodegradable, potentially biocompatible, and renewable new generation aerogel, which can be used for thermal insulation, biomedical scaffold, adsorption and separation, carrier of metal nanoparticles and metal oxides, and precursor of carbon aerogels.6-8, 10, 100, 102 Cellulose aerogel can be prepared with natural cellulose (nanocellulose and bacterial cellulose), regenerated cellulose, and cellulose derivatives.7, 100, 103-106 Aerogels generally have low thermal conductivity due to their highly porous structure, and their thermal conductivities can be lower than that of ambient air (0.026 W·m−1·K−1) due to Knudsen effect. 68, 69 Their ultralight property generally limit the thermal conduction via solid phase. Thermal conduction via air can be inhibited by Knudsen effect, i.e. the movement of air molecules in the porous structure is restricted for aerogels with pore size smaller or comparable to the mean free path of air molecule. The thermal conductivities of aerogels made from biobased materials such as cellulose, alginate, starch, and chitosan can be as low as 0.015 to 0.020 W·m−1·K−1, depending on structure, composition, and pore morphologies which can be controlled by fabrication conditions and posttreatments.10, 107-109 The extraordinary properties of aerogel make it a promising material for thermal insulation applications. However, its commercialization has 22 been hampered by deficiencies such as high production cost and relatively low mechanical properties. In recent years, rapid developments of aerogel, especially those made from biobased materials such as cellulose, provide potential for their use as thermal insulation material.98, 101, 110 1.3.2 Synthesis methods As precursor to aerogel, hydrogels can be synthesized by chemical or physical crosslinking methods.111 Aerogel fabricated through sol-gel process commonly includes three main steps: sol formation for solid network structure, gelation for structure reinforcement, and drying for removal of liquid medium with minimum shrinkage.112, 113 Resorcinol-formaldehyde (RF) in the presence of a catalyst (sodium carbonate or potassium carbonate), as a precursor mixure,114 forms RF aerogel via CO2 supercritical fluid drying after sol-gel polycondensation of resorcinol and formaldehyde.115 Chemical crosslinking is a method based on heating, ultraviolet, or high-energy irradiation of the mixture of monomer, initiator, solvent, and bi-functional or multi-functional cross-linkers to form a solid network hydrogel.116, 117 Initiators, crosslinkers, nonreacted monomer, and products from side reactions need to be removed after the preparation of hydrogel.111 The network junctions, generated from physical crosslinking such as entanglement, hydrogen bonding, ionic interaction, and hydrophobic interactions, are transient, whereas the junctions formed from chemical crosslinking are usually more stable.118 Cyclic freeze-thawing is a common physical crosslinking method, which entangles hydrogen-bonded polymer chains in semi-crystalline polymers after crystalline regions are formed by freezing.116 This method is mostly for water-soluble polymers that can form crystallites, e.g. poly (vinyl alcohol) (PVA), through densifying the macromolecular structure in aqueous solutions.116, 117 Hydrogel from cyclic freeze-thawing has high water content and good mechanical properties.117 23 CNF hydrogels can be prepared by a wide range of methods due to the propensity CNF to entangle and the availability of surface functionalities.119 CNF hydrogels can be readily formed by simple homogenization, addition of salt, and/or lowering pH of CNF suspension to cause gelation by surface charge screening.100, 101, 110, 119, 120 The first CNF aerogel was produced by freeze-drying an CNF aqueous suspension.121 It has been found that ice crystals from water in hydrogel can cause CNF to aggregates significantly. During freezing, ice crystals are formed and then grow slowly to large size, which consequently squeeze surrounding CNFs closer into a lamellar structure, which become interconnected pores after the sublimation of the ice crystals. CNF aggregation can be reduced by dispersing CNF in tert-butanol, and the aerogel based on such CNF suspension typically has a higher specific surface area (153-284 m2 g-1).122 The cooling rate and temperature during freezing can influence the structure of hydrogel.123 Compared to conventional freezing (-20 °C), smaller ice crystals can be formed through rapid cooling by extremely low-temperature coolant such as liquid nitrogen (-196 °C) or liquid propane (-173 °C),124 which can also increase porosity by inhibiting agglomeration of cellulose and achieving a uniform and smooth surface structure.125 During conventional freezing, more ice crystals are formed and they grow slowly to larger size, which consequently squeezes surrounding CNFs closer into a lamellar structure between ice crystals and after sublimation the ice crystals become interconnected pores.126 Pore morphology can also be tuned by directional freezing to achieve anisotropic thermal conductivities. Ice crystals grow vertically from bottom to top in the direction of temperature gradient to form aligned channels.123 However, such aligned channels are short (2~4 mm) owing to the randomly nucleated ice crystals on the cold finger surface.127 There is no such problem with bidirectional freezing, during which a polydimethylsiloxane (PDMS) wedge is put on the cold finger surface to obtain two temperature gradients, vertical and horizontal directions.128 Due to 24 low thermal conductivity of PDMS, the nucleated ice crystals only occur at the bottom end of the wedge in the beginning and grow upward, and then grow along the wedge rapidly.128 After ice crystals have covered all the surface of the wedge, the ice crystals will only grow upward in the mold.128 In recent years, some novel aerogels have been developed for specific applications. Spherical cellulose aerogel was successfully obtained by several steps: sulfuric acid hydrolysis of microcrystalline cellulose, physical gelation through inorganic salt solution, solvent exchange to tert-butanol, liquid nitrogen freezing, and freeze drying.129 The mechanical properties of aerogels can be improved by homogenous or special network structures to achieve strong hydrogels, such as topological hydrogels, Tetra-PEG hydrogels, double network hydrogels, and composite hydrogels.130-133 Wet spinning and 3D printing techniques can be used to form oriented fibrils to tailor mechanical properties.134, 135 With increasing research interest and investment in 3D printing techniques, aerogel has been produced by 3D bioprinting with CNFs hydrogel. The cross-linking capacity, pH sensitivity, and shear thinning of CNF aqueous suspension made the 3D printing feasible.48 Researchers from Chalmers University in Sweden mixed CNF into a hydrogel that contain 95-99% water and then printed the gel using a high-resolution 3D bio-printer.136 Compared to the high viscosity of CNF suspension gained from TEMPO-mediated oxidation, nanocellulose suspension obtained by the combination of carboxymethylation and periodate oxidation was more homogeneous and the nanofibrils were shorter (width <20 nm, length <200 nm).137 Such smaller dimensions reduced viscosity to make the nanocellulose suspension suitable as a flowable bioink, and allowed the 3D bio-printer to print aerogels. 25 1.3.3 Drying techniques Drying is one of the key steps that hamper the scale-up production of aerogels. It also has a significant influence on the properties of aerogels. During drying, the inhomogeneous and high capillary stress formed by the hydrophilic groups at gas-liquid interface of meniscus in pores can cause collapse and crack of the porous structure.100 Therefore, drying techniques have a significant influence on the morphology of aerogel. The crucial drying step in the preparation of aerogel from hydrogel can be implemented via three main drying techniques, which are the vacuum freeze drying (aerogel), supercritical drying (aerogel), and ambient pressure drying (xerogel).138, 139 The drying routes of different drying techniques are shown in figure 6. Figure 6. Phase diagram illustrating different drying routes. Reproduced with permission.112, 140 Copyright 2003, Taylor & Francis. 26 Freeze-drying is the sublimation of frozen solvent in the pores of hydrogel in a vacuum. Aerogel obtained by freeze-drying generally has sheet-like network structure with interconnected pores. The pore diameters in several to hundreds of micrometers are due to the pressure on pore walls from ice crystal growth during freezing.123, 141, 142 Ice crystals can be fast formed by rapid cooling through liquid nitrogen or liquid propane, which increases porosity by inhibiting the agglomeration of nanoparticles and results in a uniform and smooth surface structure.125 The sublimation rate during freeze drying can be influenced by gel size and shape, temperature, and composition, and the pore structure can be influenced by temperature and cooling rate.100 This technique is environment-friendly and there are suitable large devices available in food industry, while it has the disadvantages of long processing time, high electric energy consumption, and volume expansion.100 Supercritical drying is the process of removing capillarity by reaching supercritical conditions and there is no distinct liquid and gas interface in supercritical fluid.119 Supercritical CO2 drying leads to a cauliflower-like, agglomeration of small shaggy beads, cellulose aerogel.123 Carbon dioxide (CO2) is the most common liquid used for supercritical drying due to its low cost, high safety, and relatively low critical point (Tc=304K, Pc=7.4MPa).100 Compared to freeze drying, aerogels from supercritical drying present minimal shrinkage, higher porosity, smaller pore size and higher specific surface area, although it has the limitation in size due to available devices and it requires operation at high pressure.100 As the drawbacks of freeze drying and supercritical drying have limited the potential applications and scalable production of aerogels, an attractive drying technique, ambient drying, was introduced in 1995 to prepare silica aerogel with 98.5% porosity and was later employed to produce CNF aerogel membranes in 2015.103, 138 Ambient drying is the removal of liquid via evaporation at ambient pressure and at temperatures from room temperature to 200 °C.143 Ambient 27 drying doesn’t complicated and expensive equipment and high energy, whereas, in order to preserve the structure of hydrogel, it requires the skeleton of hydrogel to be either strong enough to resist capillary pressure or hydrophobic to reduce capillary effects prior to drying.120, 138 Non-uniform pore size may result in inhomogeneous distribution of capillary forces during ambient drying and consequently cause the destruction of the network. The requirements for ambient drying can be met by different approaches, e.g. solvent exchange, surface modification, freeze casting, and Pickering emulsion, and/or ionic bond with metal cation.29, 39, 142, 144-149 There are several solvents of lower capillary tension that can be used for ambient drying, including ethanol, acetone, hexane, and so on. For nanocellulose aerogel, capillary pressure induced during drying can also be reduced by surface modification with hydrophobic chemicals, e.g. anhydrides or trityl chloride.113, 150 The mechanical strength of fiber network can be enhanced by increasing the solid density such as the addition of carboxymethylated CNFs,145 and it can be further improved by forming hemiacetal cross-links in fiber joints after periodate oxidation.146 Cervin et al. prepared CNF foam with porosity of 98% and density of 0.03g cm-3 by ambient drying based on carboxymethylated and octylamine-coated CNF suspensions and Pickering emulsion technique.148 The foam stabilizer, anionic CNF particles, were modified by adsorbing cationic octylamine to prevent bubble coalescence.39, 148, 151 In recent years, instead of solvent exchange and surface modification, a novel and scalable method to fabricate aerogel was inspired by the wings of damselfly, which was reported to employ in situ carbon dioxide to support the pores in aerogel to oppose the capillary pressure from air-liquid interface during ambient drying.149 Another novel method for ambient drying is the combined impingement-infrared and through-air drying, which has relatively short drying time, high drying rate, and low shrinkage. The through-air drying did not affect the foam structure, and was suggested to be used in final drying stage.152 28 Ambient-drying has numerous advantages, such as no requirement for complicated and expensive equipment, cost-effective, and more energy efficient, whereas the requirements of high strength and reduction of intrinsic hydrophilicity, i.e., intermolecular hydrogen bonding and capillary pressure, still need to be met.138, 153 When pore size is nano-scale, the way to exchange to a solvent of lower surface tension may not be enough to reduce the high capillary pressure that increases as the pore size decreases, and therefore enhancement of the emulsion network structure may be necessary. The mechanical strength of hydrogel can be enhanced to prevent collapse through several ways before ambient drying process. Entanglements of nanofibrils caused during freezing can strengthen the network, and CNF aerogel with density of 0.018 g·cm–3 was prepared via a process of freezing/solvent exchange/ambient pressure drying without complicated devices or surface modification.153 The network structure can also be reinforced by the condensation of CNF dispersion via vacuum filtration, which induced dense interconnection between CNFs.138 In brief, the drying techniques have an important influence on the mechanical and thermal properties of nanocellulose based aerogel, so it is important to choose appropriate drying techniques for different applications. 1.4 Pickering Emulsion 1.4.1 Types and formation An emulsion is a thermodynamically unstable colloid dispersion system, in which there are two or more immiscible liquid phases. The dispersed phase consisting of small droplets is surrounded by the liquid continuous phase of a different composition. Classical emulsion is prepared by mixing two immiscible phases with amphiphilic surfactant molecules, which is used to decrease the surface tension. Based on the spatial distribution of the aqueous and oil phases, 29 emulsions can be classified into four types, including oil-in-water (o/w), water-in-oil (w/o), water-in-oil-in-water (w/o/w), and oil-in-water-in-oil (o/w/o) (Figure 7). Both w/o/w and o/w/o emulsions are also known as multiple emulsions, where w/o or o/w emulsions are further dispersed in a continuous water or oil phase. Figure 7. Schematic diagram of different types of emulsions. Reproduced with permission.154 Copyright 2007, John Wiley and Sons. The drawback of the classical emulsion is its low stability. A more stable emulsion system, Pickering emulsion, was recognized by Walter Ramsden in 1903.155 Pickering emulsion is stabilized by solid particles that have partial wettability to both of the two immiscible phases and can irreversibly adsorb onto the outer surface of droplets.96 The solid particles adsorb at the interface and form a stable emulsion system by providing a physical barrier between two phases. Various solid particles with different aspect ratio and morphologies have been used to stabilize Pickering emulsion, including fibrils, globules, and platelets (Figure 8). The solid particles at the interface can form a rigid layer to effectively inhibit Ostwald ripening and droplet expansion to resist coalescence by efficient steric stabilization.148, 156 Compared to classical emulsions, the Gibbs free energy for the desorption of solid particles in Pickering emulsion is considerably higher 30 according to reported thermodynamic calculations.157-160 The adsorption is strong when the contact angle of solid particles at the oil-water interface is in the range of 90°~165°, and it is strongest at 90°.161, 162 For optimum stability, the HLB (Hydrophilic-Lipophilic Balance) should be in the range of 9 ~ 15 for o/w emulsions, and 3~7 for w/o emulsions.163, 164 Figure 8. Schematic illustration of emulsions stabilized by surfactant and nanoparticle. Reproduced with permission165. Copyright 2018, Elsevier. Dispersion process or emulsification for the formation of emulsion is commonly implemented by high-pressure homogenization, high-shear stirring, or ultrasonic treatment. It has been reported that smaller emulsion droplet sizes can be produced by high-pressure homogenization, due to the more homogeneous flow and availability of sufficient energy in short time.166 Ultrasonic emulsification can also be used to form emulsion with reduced droplet sizes, but it is limited by the small-scale production capacity. 31 1.4.2 Nanocellulose Pickering emulsion Pickering emulsions prepared with nanocellulose as stabilizer have attracted considerable attention due to the excellent nanometric size, amphiphilic properties, and renewability.52, 55, 167 CNF prepared from TEMPO-mediated oxidation contains carboxyl groups converted from C6 primary hydroxyl groups of cellulose, and the length and width of these CNFs are in the range of 0.5 to 2 μm and 3 to 4 nm, respectively.52, 55, 167 The high aspect ratio of CNF allows them to cover more surface area of droplets in emulsion to physically entangle with each other and to form a large number of hydrogen bonds to create a long flexible network of higher strength and modulus and consequently to increase the viscosity of aqueous phase to counter the destabilization of emulsion.40, 168-170 The negative charges on the surface of TEMPO-mediated CNFs can inhibit aggregation of fibrils as well as coalescence of droplets in emulsion by electrostatic repulsion. The amphiphilic properties of CNFs make them an ideal candidate for the formation of Pickering emulsion. The concentration of CNF plays an important role in the formation and properties of Pickering emulsion, and the CNF Pickering emulsion generally has two different states, including stable emulsion and stable emulsion gel, depending on the CNF concentration. At lower CNF concentration, the droplets aggregate due to depletion flocculation, which is a process induced by osmotic pressure changes among droplets. When CNF concentration reaches the critical concentration, 0.15 wt. %, the emulsion (O/W ratio is 20/80 by volume) becomes stable without creaming after 1 day, and the size of droplets shows little change with increasing CNF content, indicating the stable adsorption of amphiphilic CNFs at the oil-water interface171 and the restricted movement of flocculated droplets ascribed to depletion stabilization. An increase in CNF concentration can induce higher aqueous viscosity and elasticity of the zones surrounding the 32 droplets and therefore allowing the flexible CNF to form a network in water to prevent the movement of flocculated droplets.172 The stability of the formed network is CNF concentration dependent. At lower concentration, the flocculated droplets and network system is still liquid and the emulsion can flow freely, indicating that the network system is weak. A strong network is essential to stabilize the emulsion droplets to form a gel, and this requires the presence of sufficient CNFs that are not adsorbed onto the droplet surfaces. As the CNF concentration further increases, the system becomes stronger to restrict more droplets, including both flocculated droplets and individual droplet, and therefore forms a more stable network system. The critical concentration of additional CNF is around 0.3 wt. %, corresponding CNF concentration of around 0.45 wt. % in total, to achieve stable emulsion gel.170 In addition to the concentration of CNF, the properties of CNF Pickering emulsion can also be influenced by other intrinsic factors. A high oil volume fraction may lead to a thicker creamed layer.173 Moderate charge density on the surface of solid particles can prevent the coalescence of droplets by electrostatic repulsion between charged surfaces of droplets, whereas strong lateral repulsion between charged particles can resist the adsorption of charged particles at the surface of droplets.173 The different ratios of hydrophilic to hydrophobic groups on the surface of CNF can impact the amphiphilic properties and the stability of emulsion. The properties of CNF Pickering emulsion can also be tailored by extrinsic factors. Stronger hydrogen bonds can be formed at lower pH (acidic) and therefore lead to a more spherical droplet shape enhancing the network structure and viscosity of emulsion.174 Considerably lower pH and higher ionic strength may cause flocculation of the electrostatically stabilized nanoparticles.175 The viscosity of emulsion can be increased by the addition of electrolyte, e.g. NaCl, which can weaken the electrostatic repulsion between charged particles and consequently lead to the aggregation of particles.176 However, high 33 viscosity can result in slower creaming and induce a high yield stress that indicates the dissipated energy for droplets dispersion.177 Droplets coalescence may occur and the three dimensional network of emulsion may break when the shear stress from turbulent flow is larger than the yield stress. An amphiphilic polymer such as partially hydrolyzed and flexible poly (vinyl alcohol) (PVA) added into nanocellulose suspension can affect the structures and mechanical properties of aerogel. The addition of amphiphilic polymer can act as co-stabilizer with nanocellulose to lower the interfacial tension and hence break oil droplets into smaller ones.117 For instance, PVA is compatible with cellulose nanofibrils due to the large amount of hydroxyl groups in PVA, and it is nontoxic and crystallizable.15, 32 PVA can hinder agglomeration of cellulose by limiting hydrogen bonds among cellulose to form a uniform structure,178 and the strong hydrogen bonding formed from hydroxyl groups present on PVA can improve the mechanical properties of network structure.179 The temperature during emulsion preparation also has an influence on emulsion stability and droplet morphology. A higher preparation temperature can induce lower surface tension and increase the likelihood of desorption of adsorbed particles at the interface. An increase in the temperature of emulsion preparation can induce more molecular thermal activity, which can decrease the cohesive forces of solvent molecules and consequently decrease the solvent surface tension.175,180 Table 5 summarizes the correlation of influence factors to the stability and droplet sizes of Pickering emulsion. Due to its unique characteristics such as closed pores upon removal of the oil phase and tunable pore sizes during emulsification, Pickering emulsion templating can be a potential approach to produce super-insulating materials.176 Compared to interconnected open pores in traditional aerogels, aerogels obtained by Pickering emulsion templating have more closed pores to build walls to seal air inside and inhibit the movement of air molecules. Therefore, Pickering 34 emulsion templating can be an effective way to control the thermal conductivity of aerogels for thermal insulation in the real world. Table 5. Correlation of influence factors to Pickering emulsion stability and droplet sizes (✔: positive correlation ✘ : negative correlation)181-184 Influence Factors Positive Correlation to Emulsion Stability Positive Correlation to Pore Size Lower pH Closer fibril-fibril interactions Irreversible fibril aggregation ✔ Higher ionic strength Same as above ✔ Surface area covered per mass of solid particles ✔ ✘ Homogenization time - ✘ Oil surface tension - ✔ Oil density - ✘ Volume ratio of oil to water ✔ ✔ Concentration of solid particles ✔ ✘ Preparation temperature - ✘ Amphiphilic properties of solid particles Contact angle of particles at the oil-water interface: 90°~165° - Surface charge density of solid particles Prevent coalescence of droplets by electrostatic repulsion - 35 1.5 Phase change materials 1.5.1 Properties With the rapid development of current society and increasing global population, the need for energy has reached an unprecedented level. The primary energy sources today are derived from fossil fuels, which are unsustainable and have adverse impacts on the environment and human health. Overcoming such energy and environment challenges will require the development of efficient energy storage systems with renewable and biocompatible materials to prevent energy waste and environmental pollution. Energy can be stored in the form of either sensible or latent heat. Sensible heat is the amount of energy needed to change temperature in a linear way and it is not related to phase, while latent heat is the energy absorbed and released during a phase transition, e.g. absorbing energy from solid to liquid with little or no temperature change during the phase transition. Although the sensible heat storage method, e.g. rock bed application, has been commonly applied for energy storage, the latent heat storage method has attracted much interest in recent years because of its potential in development of new energy storage systems. The latent heat storage method can be more energy and cost effective than the sensible heat storage method, as there is almost no temperature change during energy storage and release processes. The heat stored by sensible heat and latent heat is compared in Figure 9, showing significantly higher heat storage at the same temperature. For example, the latent heat for water is 334 kJ/kg at 0 ℃, which is much higher than the sensible heat (4.2 kJ/kg at 25 ℃) for water.185 It is apparent that the latent heat storage method is ideal for developing and constructing effective energy storage systems due to the superior ability to store and release energy. 36 Figure 9. Comparison of sensible heat storage method and latent heat storage method. ΔHF: latent heat of fusion. TM: melting temperature. Reproduced with permission186. Copyright 2018, The Royal Society of Chemistry. Phase change materials (PCMs) are substances with high latent heat, e.g. hydrated salts, paraffin waxes, glycols, and fatty acids, which can store and release a large amount of energy at a constant temperature during phase transition.183, 187 PCMs have attracted significant attention due to their excellent properties, e.g. high energy density, little volume change during phase transition, appropriate phase transition temperature, high thermal conductivity, high latent heat capacity, high specific heat capacity, low vapor pressure, sufficient crystallization rate, non-corrosiveness, long-term chemical stability, no sub-cooling during freezing, and similar temperature for melting and solidification.187, 188 PCMs have been widely studied for various applications such as solar energy systems, buildings, vehicles, electronic equipment, temperature-regulating textiles, and heating, ventilation, and air conditioning (HVAC) systems.183, 185, 187, 189, 190 PCMs can be classified by compositions as inorganic type (e.g. salt hydrate or metals), organic type (e.g. paraffin or fatty 37 acid), and combinations (eutectic of organic and non-organic compounds), as well as by the particular phase change state as liquid-gas, solid-gas, solid-solid, and solid-liquid PCMs.187 There are two major parts of organic solid-liquid PCMs, non-paraffin PCMs and paraffin PCMs, Organic PCMs are generally stable without common deficiencies such as supercooling, corrosiveness, and phase segregation during phase change.191 Table 6 summarizes the thermophysical properties of a variety of common solid-liquid common PCMs. Paraffin, an organic solid wax consisting of straight- or branched-chain saturated hydrocarbons, possesses numerous advantages, e.g. high latent heat, low vapor pressure, not flammable, safe, chemically stable, low cost, and noncorrosive, that make it the most common PCM. However, its drawbacks of low thermal conductivity (ca. 0.21-24 W·m-1·K-1) and high volume change (10-15%) during phase change process have also become apparent.192, 193 The non-paraffin organic compounds, e.g. fatty acids, alcohols, glycols, and esters, are also ideal candidates for PCMs, but they also have low thermal conductivities and are either flammable or unstable at high temperature. 38 Table 6. Thermophysical properties of common solid-liquid PCMs Types of PCMs Materials Melting point Latent heat (kJ/kg) Thermal conductivity (W·m-1·K-1, just above melting point) Density (g·cm-3, 20 ℃) Ref. Paraffin organic PCMs Paraffin C22-C45 58-60 189 0.2 0.92 194-196 Non-paraffinic organic PCMs Glycerin 18 199 0.285 1.25 194, 196 n-Octanoic acid 16 149 0.148 0.91 197, 198 Palmitic acid 61.3 198 0.162 0.989 197, 198 PEG E600 22 127.2 0.189 1.126 198 Inorganic salt hydrates CaCl2·6H2O 29.8 191 1.08 1..802 195, 196 Na2SO4·10H2O 32.4 248, 254 0.544 1.49 199, 200 CH3COONa·3H2O 58 226, 265 1.97 1.45 200, 201 Eutectics Mg(NO3)2·6H2O + MgCl2·6H2O 59 144 0.51 1.63 198 Mg-Zn (72:28) 342 155 67 2.85 202, 203 1.5.2 Encapsulation methods Although PCMs based on solid-liquid phase change have been very attractive for thermal energy storage applications, the widespread applications of PCMs have been hampered by the 39 critical problem of liquid leakage. It is not easy to maintain the thermal stability of PCMs if there is liquid leakage. Much effort has already been devoted to developing methods such as containers, matrix material, and encapsulation to prevent leakage for stable thermal performance.204-206 The most common method is encapsulation which fabricate capsules consisting of a core-shell structure. The core is the active PCM, and the shell consisting of natural and/or synthetic polymeric materials can protect the core and also improve the thermal and mechanical properties of PCMs. Generally, these methods are classified into three main groups according to the formation mechanism of micro/nano-capsules, including physio-mechanical methods, physio-chemical methods, and chemical methods. The physico-mechanical methods include spray drying, one-step method, and electrostatic encapsulation.207 The physico-chemical methods include supercritical CO2-assisted, and sol–gel encapsulation, and the chemical methods include in situ polymerization, suspension polymerization, interfacial polymerization, dispersion polymerization, and emulsion polymerization.187, 207-209 The thermal properties of PCMs mainly depend on the sizes and materials of capsules. The capsule sizes for encapsulation method typically range from 100 nm to thousands of micrometers.204, 210, 211 The capsule sizes for the physio-mechanical methods are mainly at micro scale, which are much larger than that of the chemical methods.212 The suspension or emulsion polymerization method involves diffusion of surfactants or solid particles onto the interface of two immiscible phases. The main step is to suspend or emulsify the hydrophobic oil (e.g. paraffin, disperse phase) in hydrophilic medium (e.g. water, continuous phase), where surfactants or solid particles (e.g. nanocellulose) exist, by high-speed homogenization or sonication. The surfactants or solid particles are adsorbed at the interface of oil and water phases to stabilize the oil droplets to form an oil in water emulsion. Another step is optional, which is usually the addition of hydrophobic monomers to oil before emulsification and then polymerization of the monomers in 40 a thermal, chemical or enzymatic way to obtain a crosslinked system.207, 208 The advantages and disadvantages of different encapsulation methods are listed in table 7. Table 7. Comparison of various encapsulation methods207, 210 Methods Benefits Disadvantages Physico-mechanical methods Spraying drying Low cost, easy to scale up, versatile High temperature, remaining uncoated particles, agglomeration One-step method Self-stabilization, easy to scale-up -- Electrostatic encapsulation Spherical microbeads Only for size <50 μm Physico-chemical methods Coacervation Good control of particle thickness and size, versatile Agglomeration Supercritical CO2-assisted Green and effective method Not commonly used Sol–gel encapsulation Inorganic shell with high thermal conductivity Still under research Chemical methods In situ polymerization Uniform coating Size range: 1-2000 μm Dispersion polymerization Micro-scale monodisperse polymer particles -- Interfacial polymerization Good properties in size, mechanical resistance, versatile Size range: 2-2000 μm Suspension polymerization Good heat control of reaction Most monomers are water insoluble, size range: 2-4000 μm Emulsion polymerization High MW polymer, fast Purify polymer from the surfactant The selection of the core and shell materials depend on the requirements of different application areas. The most common requirement for shell materials is that they should be cohesive 41 or chemically compatible with the core material; enhanced mechanical properties, high stability, and low cost are also desirable. Safety in use (not toxic) is also important for end-users of PCMs. The typical materials that can be used as the shell are polymethylmethacrylate, polystyrene, polyurea, urea formaldehyde, urea/methanol melamine formaldehyde, polyaniline, and so on, whereas the most desirable shell materials are renewable and bio-compatible materials such as nanocellulose.211, 213 The Pickering emulsion technique has been extensively studied for various applications as mentioned above, and it is a promising approach for producing aerogels.161, 214-217 The droplets (capsules) in Pickering emulsions are stabilized by irreversibly adsorbed solid particles that form rigid layer to effectively inhibit Ostwald ripening and droplet expansion to resist coalescence by efficient steric stabilization.148, 156, 158. The droplets in Pickering emulsion are closed (covered with solid particles), and the size and shell coverage can be tuned by the amount of solid particles, volume of oil, and power of emulsification process.176, 218-220 The amphiphilic properties and high aspect ratio of CNF make it an excellent candidate to enclose and stabilize the oil core to form oil in water emulsion by Pickering emulsion technique. The high aspect ratio allows CNFs to physically entangle with each other and further form a large number of hydrogen bonds to establish a strong network.40, 168, 169 CNFs prepared from TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation also contain negative carboxyl groups on their surface,49-51 which can inhibit aggregation of fibrils as well as coalescence of droplets in emulsion by electrostatic repulsion. 1.6 Hypotheses Aerogels prepared by traditional freezing-thawing and freeze-drying method generally have large interconnected pores which are formed from ice crystal during freezing. The thermal 42 conductivity of aerogels with large open pores can be easily affected by humidity and temperature of environment. Therefore, closed and small pores are desirable for super thermal regulation performance. Aerogels with closed pores made from sustainable materials have not been previously reported and their thermal regulation performance needs further investigation. Although droplets in Pickering emulsion are generally covered with nanoparticles, forming closed microspheres, they can be hardly preserved after freezing. Besides, PCMs can be used for the tuning of thermal regulation ability of aerogel. However, the deficiencies of PCMS such as shape change and liquid leakage during phase change and poor mechanical properties have hampered their practical applications. Pickering emulsion has a two-phase structure with closed droplets, which make it a good template to prepare aerogels with closed pores. CNF is sustainable and abundant, and it is a good candidate to prepare Pickering emulsion due to its amphiphilic and high aspect ratio properties. Also, the negative charge on CNF surface can inhibit the aggregation of nanofibrils and the coalescence of droplets by electrostatic repulsion. Therefore, this study will mainly use CNF stabilized Pickering emulsion to prepare aerogel. In this study, as both water and oil (hexadecane) can dissolve in acetone, it is hypothesized that aerogel with closed pore structure can be obtained from CNF/hexadecane oil-in-water Pickering emulsion templating and can be preserved by using solvent exchange method to remove oil in droplets, and the thermal insulation and mechanical properties are therefore expected to be improved. Moreover, it is expected that the shape stability and liquid leakage challenges can be overcome by encapsulating PCM with CNF to form a core-shell structure. Paraffin is selected as it is a safe, organic, and hydrophobic PCM with high latent heat. It is expected that CNF/paraffin Pickering emulsion can be used to prepare aerogel with high stability, exceptional thermal 43 regulation performance, and enhanced mechanical properties. It is also hypothesized that the addition of free CNF to Pickering emulsion can establish a new internal structure, which can tune the thermal regulation and mechanical properties of aerogels. 44 Chapter 2: Materials and methods 2.1 Materials Pulp was the Northern bleached softwood Kraft pulp (NBSK, Canfor Corporation). Sodium hypochlorite solution (NaClO, available chlorine 10-15 %), TEMPO (2,2,6,6-Tetramethylpiperidine 1-oxyl, C9H18NO, 98%), and Calcofluor White were purchased from Sigma-Aldrich. Sodium Bromide (NaBr, 99%, Acros Organics), hexadecane (Certified ACS 99%, Fisher Chemical), acetone (Certified ACS 99.8%, Fisher Chemical), paraffin, and Tert-Butyl Alcohol (>99%, Alfa Aesar) were purchased from Fisher Scientific. All chemicals were used as received without further purification. All water used was deionized (DI) water obtained from a Barnstead Mega-Pure System. 2.2 Preparation of CNF Cellulose nanofibrils was prepared by TEMPO (2,2,6,6-Tetramethylpiperidine 1-oxyl, 98%, Sigma-Aldrich) oxidization (5 mmol/g of NaClO) and mechanical blending (Vitamix 5200, 25 min) of northern (NBSK, Canfor Corporation) following previously described procedure.27 In the presence of NaOCl (a primary oxidant) and NaBr, the catalyst TEMPO is oxidized to provide a stable nitroxyl radical to convert the hydroxymethyl groups present on the surface of cellulose to carboxylic form. Typically, the first step was to add TEMPO and NaBr aqueous solution to the pulp fiber suspension and then regulation of the pH to around 10 by 0.5M NaOH solution. 5 mmol NaClO for every gram of cellulose was added dropwise for cellulose oxidation while keeping the pulp suspension stirred for around 3 h. The obtained pulp suspension was washed with DI water for 10 times within 7 days. To reach high concentration, the CNF suspension was centrifugated and the upper layer of water was removed. Finally, CNF of high concentration (above 0.8 wt %) 45 was obtained by mechanical treatment with Vitamix blender for around 10 min. High-speed shearing during this mechanical treatment generated a large amount of heat, and an ice bath was used to cool the suspension intermittently to avoid thermal degradation and fiber aggregation due to drying. In this research, all CNF suspensions were diluted to 0.8 wt. % for further use. 2.3 Synthesis of CNF/hexadecane emulsion and composite aerogel The oil-in-water Pickering emulsion was prepared by mixing hexadecane and 0.8 wt. % CNF aqueous suspension at 2:8 volume ratio and then sonicated (Fisherbrand model 705 sonic dismembrator) at 4 W/mL power using a titanium tip for 3 min. As-prepared CNF Pickering emulsion and aqueous CNF suspension (0.8 wt.%) were mixed at varying emulsion/CNF volume ratios of 0:10 (CNF-only), 2:8, 4:6, 8:2, and 10:0 (emulsion-only), and then vortexed for 30 s to form a homogeneous suspension. The mixed suspension was then stored overnight to fix the structure, followed by solvent-exchanging with acetone to gel the structure and extract hexadecane. Fresh acetone was constantly replaced for several days until a transparent acetone gel was obtained. The acetone gel was then solvent-exchanged with tert-butyl alcohol (TBA) for several days to ensure the complete removal of residual acetone. The TBA gel was frozen at – 20 ℃ in a freezer and then freeze-dried in a freeze dryer (Labconco Corporation, USA) for 3 days. 2.4 Fabrication of CNF/paraffin emulsion and composite aerogel Cellulose nanofibrils were isolated from Northern bleached softwood Kraft pulp (NBSK, Canfor Corporation) by TEMPO-mediated oxidation (5 mmol/g of NaClO) and high speed blending (Vitamix 5200).27 Solid paraffin was added into 0.8 wt. % CNF suspension (1:5 weight ratio) and then heated at 100 ºC to melt paraffin. The liquid CNF/paraffin mixture was sonicated 46 (Fisherbrand model 705 sonic dismembrator with sound enclosure) for 3 min by intermittent pulses using a titanium tip at 4 W/mL power to obtain oil-in-water Pickering emulsion. The as-formed emulsion was then mixed with 0.8 wt. % CNF suspension at varying volume ratios of 1:9 (CNF/PCM1), 2:8 (CNF/PCM2), and 4:6 (CNF/PCM4) by a Vortex mixer for 30 s. The obtained mixtures were cooled at room temperature to fix the structure, and then frozen at – 20 ℃, followed by freeze-drying in a freeze dryer (Labconco Corporation, USA) at −50 °C and 0.003 mbar for 72 h. 2.5 Characterization 2.5.1 Density, porosity, and volume shrinkage of acetone gels and aerogels The dimensions of acetone gels and composite aerogels were measured using a digital caliper at 0.01 mm resolution, and the mass of aerogels were measured using an analytical balance at 0.1 mg resolution. The volume shrinkage of acetone gels and composite aerogel was determined by comparing their respective volumes with the volume of starting emulsion volume. The density (ρa) of aerogel was calculated by the ratios of mass to volume. The porosity of aerogel was calculated as follows. Porosity (%) = (1-ρa/ρc )×100% (1) Where ρa is the density of cellulose bulk material taken as 1.6 g cm-3.221 2.5.2 Optical and laser scanning confocal microscope The CNF stabilized Pickering emulsions, including both CNF/hexadecane emulsion and CNF/paraffin emulsion, were diluted 50 times using DI water, and then a droplet of the diluted suspension was deposited onto a glass slide and visualized using a fluorescence light microscope 47 (Olympus BX53) at 60x magnification. The diameters of emulsion droplets were measured and calculated from over 300 droplets using Image J analysis software (Image Processing and Analysis in Java 1.8.0, National Institutes of Health). The acetone gels, CNF stabilized Pickering emulsion, and mixtures of CNF and PCM emulsion were stained with Calcofluor White, and then visualized on glass slides using a Confocal Multiphoton Microscope (Olympus FV1000) at 60x magnification. The pore diameters inside the acetone gels were measured and calculated from over 300 individual pores using Image J analysis software. 2.5.3 Transmission Electron Microscopy (TEM) A droplet (8 µL) of CNF suspension (0.01 wt. %) was deposited onto glow-discharged carbon-coated TEM grids (300-mesh copper, Formvar-carbon, Ted Pella Inc., Redding, CA), followed by blotting with a filter paper to remove excess suspension, and drying at ambient condition. The dried sample on TEM grid was then negatively stained using 2% uranyl acetate solution. The CNF was visualized using a transmission electron microscope (Hitachi H7600) operating at an accelerating voltage of 80 kV. The width of CNF was measured from over 300 individual nanofibrils using Image J analysis software. 2.5.4 UV-vis spectroscopy Acetone gels (~2.8 mm thick) were submerged in a quartz cuvette filled with acetone, and the transmittance of the gels was determined using a Cary 60 UV-vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) at wavelengths of 300-800 nm. Acetone solution was scanned as a baseline prior to scanning the acetone gels. 48 2.5.5 Scanning Electron Microscopy (SEM) Freeze-dried aerogels were cut using a sharp razor across their cross section to expose their internal structure. The samples were mounted on an aluminum stub by conductive carbon tape and then coated with 9 nm of gold using a Cressington High-Resolution Sputter Coater (Ted Pella Inc., Sweden). The internal structure of aerogels was visualized using a field emission scanning electron microscope (Hitachi S4700) at an accelerating voltage of 5 kV and a working distance of 5 mm. 2.5.6 Mechanical testing The mechanical properties of aerogels were measured using a compression test. Cylindrical aerogels (Chapter 3: diameter:25 mm, height: 10 mm; Chapter 4: diameter:20 mm, height: 10 mm) were compressed using an Instron 3345 Materials Testing System with a 2 kN load cell at 1 mm/min displacement rate until they reached 80% compressive strain. At least three samples were tested for each aerogel, and the Young's modulus and yield strength were calculated from the initial linear elastic region of the stress-strain curve. 2.5.7 Brunauer–Emmett–Teller (BET) surface area measurement The specific surface area and pore characteristics of aerogels were determined by nitrogen adsorption-desorption method using Quadrasorb SI (Quantachrome Instruments, US). The samples were degassed under vacuum at 60 °C for 12 h before measurement. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) theory from the linear region of experimental isotherms obtained at 77 K and relative pressure from 0.01 to 0.3. Pore size distributions were determined from the desorption curve of the isotherm using the Barrett-Joyner-Halenda (BJH) method. When relative pressure (P/P0) decreases, the adsorbed nitrogen will be 49 removed from pores, and the removed amount can be related to pore size by modified Kelvin equation.222 2.5.8 Thermal conductivity Thermal conductivity and diffusivity of the aerogels were determined by transient plane source (TPS) method using a thermal conductivity analyzer (TPS 2500 S) at ambient condition. The temperature sensor of the analyzer was sandwiched between two cylindrical samples (Chapter 3: diameter:23 mm, height: 10 mm; Chapter 4: diameter:20 mm, height: 10 mm) during the test. The thermal properties were determined from the curve of recorded temperature versus time. The specific heat capacity of the aerogel at 21.5 ℃ was measured using a TA DSC2A-00188R Instrument. The thermal energy transfer behaviours of CNF/PCM1, CNF/PCM2 and CNF/PCM4 aerogels (9.5 mm thick) were determined by placing the aerogel on top of a hot plate with temperature up to 195 °C. The surface temperature of CNF/PCM aerogels were recorded using an infrared thermal imager (Rohs HT-19) over a period of time. A model house was built using cardboard, and the outer surface was covered using CNF/PCM aerogel (5.6 mm thick). The model houses with and without CNF/PCM aerogel covers were placed in an oven with different temperatures (40 and 70 °C). The internal temperature of the model house was measured using a digital thermometer (Taylor TAP1710) probe inserted in the house (Appendix D). The infrared (IR) picture of the house inside oven was taken by removing the front panel. 50 2.5.9 Infrared imaging The infrared image and upper surface temperature of aerogel placed either on an 80 °C hot plate or -17 °C ice cube (a plastic wrap was placed between the aerogel and ice cube to avoid water absorption) was recorded using an infrared thermal imager (Rohs HT-19). The thermal energy transfer behaviors of CNF/PCM aerogels were recorded in a hot environment using an infrared thermal imager (Rohs HT-19) with a temperature range from room temperature to 195 °C. The imager focused on the top surface of samples to record temperature and time. The model house was put in an oven with different temperatures (40 and 70 °C) and a digital thermometer probe was put in the house through a small hole which was then sealed with CNF/PCM composite aerogel. 2.5.10 Dynamic light scattering (DLS) The DLS samples were prepared by diluting the emulsion 50 times with DI water. The average droplet sizes of emulsions was determined using a Malvern Zetasizer Nano Particle Analyzer (Malvern Zetasizer Nano ZS, Malvern, UK) at a temperature of 25 °C. 2.5.11 Thermal analysis The thermal properties of the CNF/PCM aerogels were explored using a differential scanning calorimetry (DSC) system (DSC2A-00188R, TA Instruments). The CNF/PCM aerogels (approximately 6 mg) were first heated at a 10 °C min−1 heat rate from 0 to 120 °C under a nitrogen atmosphere, held for 1 min, and then cooled to 0 °C at a 10 °C min−1 cooling rate. Such melting and freezing cycle was repeated 51 times. The latent heat and melting and freezing temperatures of samples were obtained from the DSC curves. 51 The thermal stability of the CNF/PCM aerogel was examined by thermogravimetric analysis (TGA5500-0061R) at a heating rate of 10 °C min−1 under nitrogen atmosphere from 25 to 600 °C. 52 Chapter 3: Results and Discussion - flexible and super thermal-insulating cellulose nanofibrils aerogel with closed pores 3.1 Introduction As an ultralight and highly porous three dimensional network structure, aerogel has attracted significant research interest with numerous potential applications spanning acoustic and thermal insulation,223 energy storage,224 air and water treatment,102, 225 and biomedical scaffold,226 as mentioned above. Although aerogel can be derived from various organic and inorganic precursors, nanocellulose, predominately cellulose nanofibrils (CNF), has shown great potential in aerogel fabrication owing to its low density, high mechanical performance, natural abundance, and biodegradability.34, 126, 227 Among all applications, thermal insulation is attractive and potentially applicable to building and clothing. CNF based thermal insulating aerogel has been reported with thermal conductivity as low as 18 mW·m-1·K-1 being achieved by different strategies, including supercritical drying of its liquid-crystalline phase,228 Pickering emulsion templated nanofibrillated cellulose,214 spray freeze-drying of nanofibrillated cellulose,229 and unidirectional freeze-casting of cellulose nanofibrils.223 Composite aerogels containing CNF showed further reduced thermal conductivity, including polymethylsilsesquioxane-cellulose nanofibrils aerogel (15.3 mW·m-1·K-1)230 and graphene oxide/CNF/sepiolite nanorods aerogel (15 mW·m-1·K-1)223 due to the reduced radiative contribution to thermal conductivity by the carbonaceous materials for the latter. However, adding secondary materials in the composite aerogel may sacrifice the biodegradability and sustainability of the aerogel. Therefore, further reducing the thermal conductivity of pure CNF aerogel is of great interest. 53 Based on the microstructure morphologies, CNF aerogels can be classified into two categories, including one with micron or even hundreds of micron wide pores formed by ice-crystals templating during direct freeze-drying,36, 126 and another with predominant mesopores from acid crosslinking and supercritical CO2 drying.228 Clear distinction in thermal conductivity has been observed regarding different pore morphologies, primarily due to the different contribution of trapped air. Although mesoporous aerogel with pore size smaller than the free path of air molecules (70 nm) has shown to be effective in limiting the thermal conduction through gas phase,228, 229 known as Knudsen effect,231 it has been shown that closed micropores with air partitions is more effective in suppressing the thermal diffusion of air, showing lowest thermal conductivity at the same solid volume fraction as compared to the mesoporous aerogels232. Thermal insulation foam with closed cell is typical for synthetic polymer such as expanded polystyrene233 or polyurethane.234 However, it remains a challenge to fabricate closed cell cellulose foam due to the fibrous morphology of cellulose and aqueous processing method. The claimed microscale closed pores formed by liquid nitrogen freezing232 are not exactly closed due to the formation of random ice crystals during freezing. In addition, CNF aerogel prepared by liquid nitrogen freezing demonstrates cracked structure due to the lack of sufficient hydrogen bonding.235 Therefore, strong CNF aerogel containing more partitioned microscale pores should be developed to improve the thermal insulation properties. Emulsion templating represents a facile method to fabricate porous polymer monoliths with both open and closed cells.236-238 As an amphiphilic one dimensional nanomaterial, nanocellulose, including both cellulose nanocrystals (CNCs) and CNFs, has been explored to stabilize Pickering emulsion.238-241 Nanocellulose aerogel can then be obtained by sublimating both oil and water phase, leading to highly aerogels.214, 242, 243 However, by direct freezing and freeze-drying, the CNF 54 emulsion aerogel showed heterogeneous pore structures and large open pores from ice crystal templating can still be observed,214 which might contribute to its relatively high thermal conductivity. As Pickering emulsion droplets are generally covered with nanoparticles, forming a closed microsphere, it is expected that CNF aerogel with closed pore structure can be obtained from emulsion templating. However, such structure has not been previously reported and the relationship with the thermal conductivity needs further investigation. We herein report a facile yet effective method to prepare CNF-based nanofibrils/emulsion composite aerogel with closed microscale pores, reduced thermal conductivity, and tailored mechanical properties based on Pickering emulsion templating technique. Oil-in-water Pickering emulsion with droplet size ranging from 1-5 µm was stabilized using 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) oxidized CNF. The emulsion was turned into transparent acetone gel by crosslinking and dissolving hexadecane phase into acetone, which was then exchanged with tert-butanol for freeze-drying. By acetone induced gelation and freeze-drying from tert-butanol, the microemulsion structures can be well preserved and the formation of large open pores by ice crystal templating can be suppressed. Composite CNF/emulsion aerogel can be formed by mixing CNF and emulsion at varying ratios, presenting tunable density, porosity, mechanical properties, and thermal conductivity. Due to the presence of closed microscale pores, the CNF emulsion aerogel showed ultralow thermal conductivity of 15.5 mW·m-1·K-1, while maintaining excellent mechanical properties and flexibility. 3.2 Morphologies of emulsion droplets and aerogel pores In this chapter, a super thermal-insulating CNF-based nanofibrils/emulsion composite aerogel was prepared. Compared to conventional CNF aerogel, where interconnected microscale pores are 55 formed among tangled nanofibrils (Figure 10a&b), the composite aerogel combined the tangled nanofibrils and closed hollow emulsion microsphere (Figure 10c&d). Four main forms of heat transfer in CNF/emulsion composite aerogel exist: (1) thermal conduction through solid pore walls, (2) thermal conduction through air in pores, (3) thermal radiation between pore walls, and (4) thermal convection of air within pores. The total thermal insulation performance of aerogel is determined by the sum of these four forms.57-60 Although convection of gas can generally be neglected for closed pores, the micro scale convection can still play a significant role in the thermal insulation of aerogel with open pores.244 Therefore, inclusion of closed microspheres is expected to reduce the thermal conductivity of composite aerogel due to the restricted gas convection in the closed microenvironment. Figure 10. Schematic illustration of (a) internal structure and (b) cross-section view of CNF aerogel with tangled nanofibrils; (c) internal structure and (d) cross-section view CNF/emulsion composite aerogel with both tangled nanofibrils and closed hollow microspheres. 56 TEMPO oxidized CNF with average diameters of 5.9 ± 1.7 nm and lengths ranging from 200-600 nm was used for oil-in-water Pickering emulsion stabilizer (Figure 11). Spherical microspheres can be observed from both confocal and optical microscopy (Figure 12a & b), with an average diameter of 1.29 µm determined from optical microscopy images (Figure 12c). Figure 11. TEM image of CNF Figure 12. Confocal (a) and optical (b) microscopy images of emulsion droplets, inset in (a) shows the schematic illustration of internal structure of the emulsion droplet; (c) Diameter distribution of the emulsion droplets measured from optical microscopy image. 57 The spherical shape of emulsion droplets was preserved in the freeze-dried aerogel, which showed some spherical beads in the SEM image (Figure 13). Closed microscale spheres can still be discerned under the imbedded structure (circled in yellow), although many of these spheres were cut open during sample preparation. It should be noted that the microspheres are bigger under SEM than those in the optical image, possibly due to the structure expansion during solvent exchange and freeze-drying. Figure 13. SEM image of pores in CNF/emulsion aerogel sample (8:2 emulsion:CNF ratio) The CNF stabilized Pickering emulsion could be directly mixed with CNF aqueous suspension at varying ratios to form CNF/emulsion mixed suspension (Figure 14), which was used as precursor for CNF/emulsion aerogel. In contrast to the transparent CNF suspension, the mixed CNF/emulsion suspension showed white creamy color due to the light scattering at the water/emulsion droplet interfaces. 58 Figure 14. Digital images and schematic illustration of CNF/emulsion mixed suspension with varying emulsion:CNF ratios (0:10 to 10:0). 3.3 Solvent exchange and acetone gel CNF/emulsion suspensions were subject to solvent exchange to acetone to induce gelation and remove hexadecane phase. Sequential solvent exchange by acetone after different time intervals can be observed in Figure 15. CNF stabilized Pickering emulsion appeared as white creamy viscous liquid that flowed freely in an inverted glass vial. However, after aging overnight, the CNF emulsion behaved like a gel that could be held to the bottom of the inverted glass vial (second vial from left). The emulsion gel was stable after storage for over one year in refrigerator (Figure 16). Such gelation behavior and long-term stability can be ascribed to the depletion stabilization effect, where both flocculated and individual oil droplets can be arrested by the free CNFs in the suspension.240, 245 59 Figure 15. Digital images of CNF emulsion going through sequential solvent exchange by acetone in (top) upright and (bottom) inverted glass vials. Leftmost: CNF emulsion immediately after preparation; second to the left: CNF emulsion after overnight aging. The 3rd to 8th glass vials showed sequential solvent exchange until a transparent gel is formed. Figure 16. Photograph of CNF/Hexadecane Pickering emulsion after one year storage in a refrigerator. During solvent exchange with acetone, the white and opaque gel gradually turned into transparent gel, indicating the removal of oil phase from the gel. By extracting oil and water using 60 acetone, the gel became saturated with acetone, leading to a transparent acetone gel. Diffusion of hexadecane into acetone can be clearly visualized from the partially solvent exchanged gel (Figure 17). The hairy edge at the center of the gel suggested gradual diffusion of hexadecane into acetone. After complete solvent exchange, a strong and transparent acetone gel can be obtained. Figure 17. Picture of CNF/emulsion sample during solvent exchange. Solvent exchange by acetone led to clear shrinkage of the gel, especially in the longitudinal direction (Figure 18).Volume shrinkage values indicated largest shrinkage (ca. 45.1%) for CNF-only acetone gel, which gradually decreased with increasing Pickering emulsion fractions, reaching to ca. 6.05% for the emulsion-only acetone gel (Figure 23a). As a bad dispersing solvent for CNFs, acetone is a known gelation medium for CNFs, leading to stronger inter-fibril hydrogen bonding by displacing water.246 Such severe shrinkage (40-50% of volume shrinkage) has been previously observed for cellulose nanocrystal247 and TEMPO oxidized cellulose nanofibrils aerogels prepared by ethanol or acetone induced gelation.248 I demonstrated that shrinkage of CNF 61 acetone gel can be restricted by inclusion of CNF emulsion, which may be explained by the effect of closely packed CNF on the emulsion droplet. The dense CNF outer layer and spherical morphology of emulsion droplets can better withstand acetone induced gelation force among CNFs as compared to the loose random network structure formed by CNF suspension. The CNF hydrogels prepared by freezing-thawing method are typically very brittle, while acetone gel in this work was super strong. The highest mechanical properties (Appendix E) were obtained for 2:8 CNF/emulsion acetone gel (302.9 kPa for Young’s modulus, and 23.5 kPa for yield strength). There was no fracture during compression for any of the acetone gels in this work and the gels can be compressed to a thin, transparent, and strong membrane (Appendix F). The aerogel obtained from this membrane after freeze drying was super-strong and can be recovered to a transparent gel again in acetone. Figure 18. Digital images of the acetone gel with varying emulsion:CNF ratios after complete solvent exchange. All acetone gels appeared transparent and their light transmittance curves and values are shown in Figure 19. The CNF-only acetone gel showed high transmittance of 90.3% at 650 nm, which decreased to 80.9% with the addition of 20% emulsion, possibly due to different scattering 62 effects at the interface of emulsion droplets and CNF networks, caused by the difference in local CNF concentration. Light transmittance gradually recovered with increasing emulsion content, and reached 94.6% for emulsion-only acetone gel. The high transmittance of emulsion-only acetone gel indicated homogeneous dispersion of emulsion droplets in the gel. In addition, the emulsion-only acetone gel showed smallest shrinkage, which may enhance light transmittance due to reduced shrinkage of pore structures. Figure 19. (a) UV-vis transmittance spectra and (b) transmittance at 650 nm of the CNF/emulsion acetone gels (thickness: 2.8 mm). The morphology and pore structures within the acetone gels were visualized from fluorescent confocal micrographs (Figure 20). In contrast to the uniform dark green color of CNF-only acetone gel, the CNF/emulsion gels showed clear round shaped pores derived from the emulsion droplets. The density of these spherical pores increased with increasing emulsion concentration, and the averaged pore sizes were determined to be 1.39 μm, which is very close to the emulsion droplets size as observed by optical microscopy image. This observation support the claim that emulsion droplets were not broken and were well preserved, which can prevented gel shrinkage during solvent exchange. 63 Figure 20. Laser scanning confocal microscopy images of different CNF/emulsion acetone gels. 3.4 Shrinkage and flexibility of aerogel The acetone gels were further solvent exchanged to tert-butanol and then freeze-dried, yielding white aerogel following the same volume shrinkage trend as the acetone gel (Figure 21). Tert-butanol was selected due to the smaller crystals formed as compared to ice crystal from water during freezing,36, 246 which can benefit preservation of the emulsion pore structures. Large dimension aerogel (80 mm in diameter and 6 mm in thickness) can also be prepared using this emulsion templating technique, which showed excellent flexibility it could be rolled and recovered to its original shape (Figure 22). Figure 21. (a) Digital images of different freeze-dried aerogels with varying emulsion: CNF ratios. 64 Figure 22. A large piece of (a) 8:2 aerogel (thickness: 6 mm) demonstrating great flexibility that can be (b) rolled and (c) recovered its shape. Freeze-drying leads to slightly higher volume shrinkage and the emulsion-only aerogel still showed the lowest volume shrinkage of 11.5% (Figure 23a). Corresponding to the different shrinkage percentage, the aerogels showed decreasing trend in densities with increasing emulsion contents (Figure 23b). The lowest density (11.4 mg·cm-3) was observed for emulsion-only aerogel with smallest shrinkage, while the highest density (22.0 mg·cm-3) was from CNF-only aerogel due to the significant shrinkage. Nevertheless, all aerogels were highly porous with porosity ranging from 98.6% for CNF-only aerogel to 99.3% for emulsion-only aerogel. Such low density and highly porous structures are ideal for thermal insulation applications, which will be further examined. 65 Figure 23. (a) Volume shrinkage of acetone gels and aerogels with varying emulsion (error drawn): CNF ratios; (b) Density (dark color, error drawn) and porosity (light color) of freeze-dried aerogels; The error bar represent the variability of both density and porosity. 3.5 Internal structure of aerogel Internal pore morphologies of the aerogels were visualized from SEM images (Figure 28). CNF-only aerogel showed a smooth and compact structure with interconnected pores framed among the assembled CNF networks (Figure 24a-c). Some spherical pores can be observed with the inclusion of 20% emulsion (Figure 24d-f). Further increasing the emulsion content led to hierarchical structure with more spherical microscale pores (Figure 24g-i). The presence of these spherical pores indicated that emulsion droplets were successfully imbedded in the aerogel. At higher emulsion contents such as 8:2 and 10:0, most pores appeared to be closed with intact bottom walls (Figure 24h & k). These walls were very dense and compact as shown at high magnification (Figure 24i & l). Some of the observed pores appeared larger under SEM images as compared to those in confocal micrographs, which can be due to the expansion and breakage of the emulsion droplets by the tert-butanol crystals during freezing. Such defects can be mitigated by using supercritical CO2 drying method to remove acetone from the acetone gel, without going through the crystal formation freezing stage. Nevertheless, the pores in the CNF/emulsion aerogels are 66 significantly smaller than those (over tens to hundreds of micrometers) of aerogels obtained by freezing-thawing method under similar CNF concentrations,36, 214, 249 as well as previously reported emulsion aerogel (1% CNF) prepared by direct freezing of emulsion in water suspension.214 Freezing aqueous CNF suspension or emulsion generally leads to the growth of large ice crystals that can break emulsion droplets and create interconnected pores with the removal of ice crystals templates, which was avoided by acetone and tert-butanol exchange in this work. Such small and closed microscale pores are expected to reduce thermal conductivity. Figure 24. SEM images of aerogels with varying emulsion: CNF ratios, (a-c): 0:10, (d-f): 2:8, (g-i) 8:2, and (j-l) 10:0. 3.6 Specific surface area and pore characteristics of aerogel The specific surface area and pore characteristics of all aerogels are shown in Figure 25. BET nitrogen adsorption-desorption curves for all aerogels showed mixed type I and type IV isotherms with clear hysteresis loop, indicating the presence of both micropores and mesopores. The CNF-67 only aerogel showed a steep H1 type hysteresis, indicating high pore size uniformity and connectivity. With increasing emulsion content, the hysteresis loop turned into H4 type, indicating narrow slit-like pores and presence of microporositites.225 As shown from the pore size distribution curve, the CNF-only aerogel contained mostly mesopores with pore size greater than 20 nm. In contrast, both emulsion: CNF (8:2) and emulsion-only aerogels showed clear peaks of micropores with respective pore sizes of 3.2 nm and 4.2 nm. Figure 25. Specific surface area and porosity of aerogel with varying emulsion: CNF ratios. (a) Nitrogen adsorption–desorption isotherms and (b) BJH pore size distribution. The specific surface area of the aerogels ranged from 96.5-135.4 m2·g-1, which are close to other CNF aerogels freeze-dried from tert-butanol.246 Tert-butanol is known to prevent hydrogen bonding between CNF due to the presence of non-polar tert-butyl group, thus leading to increased surface area.36, 225, 246 On the other hand, pore volume decreased with increasing emulsion content (Figure 26), possibly due to closer packing of CNF on the surface of the emulsion droplets. Such mesoporous and microporous morphologies can be clearly observed from high-resolution SEM images. 68 Figure 26. (a) Specific surface area; (b) Pore volume of CNF/emulsion composite aerogels. CNF-only aerogel showed mostly tens to hundreds of nm interconnected mesopores and macropores framed by assembled nanofiber networks (Figure 27), which is consistent with the BET isotherm and H1 type of hysteresis loop. In contrast, the emulsion-only aerogel showed densely packed CNF on the surface of emulsion droplets with very small micropores. Such micropores are consistent with the H4 type hysteresis loop and the appearance of a sharp peak centered at 4.2 nm in the pore size distribution curve. Figure 27. High-resolution SEM images of (s) CNF-only and (b) emulsion-only aerogels. 69 3.7 Mechanical properties of aerogel The mechanical properties of my aerogels are shown in Figure 28. The compressive stress-strain curves of all aerogels showed typical foam-like properties including three distinctive regimes: (1) initial elastic deformation stage at low strain; (2) a relative flat plastic deformation region beyond the yield point; and (3) the final densification phase at compressive strain over 60%.225As shown in the figure of Young’s modulus, with increasing emulsion ratios, both Young’s modulus and yield strength increased first and then decreased. The highest mechanical properties were obtained for 2:8 CNF/emulsion composite aerogel (308.7 kPa for Young’s modulus, and 29.1 kPa for yield strength), even though the density slightly decreased from 0.022 to 0.020 mg·cm-3 compared with the CNF-only aerogel. Figure 28. The mechanical properties of aerogel with varying emulsion: CNF ratios. (a) stress-strain curves, (b) Young’s modulus (dark color, error drawn) and yield strength (light color, error drawn). Given the strong dependence of mechanical properties on density, the specific modulus and specific yield strength were calculated for all aerogels (Figure 29). The CNF-only aerogel showed the lowest specific modulus and yield strength of 10.1 and 0.84 kPa·mg-1·cm-3, respectively. The CNF/emulsion composite aerogel (8:2 of emulsion: CNF ratio) showed the highest specific 70 modulus of 18.2 kPa·mg-1·cm-3 and specific yield strength of 1.6 kPa·mg-1·cm-3, almost double those for CNF-only aerogel. The high mechanical properties of the CNF/emulsion composite aerogel can be attributed to the enhanced bridging of the emulsion microspheres by the addition of CNFs, which is supported by the interconnected porous fibrous-like structure shown in SEM image. Sufficient free CNFs can assemble into a strong 3D network to trap the emulsion droplets by physical entanglement, hydrogen bonds, and Van der Waals interactions. The closely and densely packed CNFs on the surface of microspheres can further serve as strong load-bearing components, leading to a super-strong system. In contrast, the emulsion-only aerogel had relatively weaker mechanical properties due to the low interaction surface of its spherical morphology. Figure 29. Specific Young’s modulus (dark color) and specific yield strength (light color). The Young’s moduli of the CNF/emulsion composite aerogels are plotted against other bio-based aerogels with different densities, including CNF aerogel,126,250,122 MFC aerogel,249 chitosan 71 aerogel,251 nanofibrillated cellulose emulsion aerogel,252 electrospun cellulose aerogel,253 and Nano-CMC aerogel148: an exponential growth of Young’s modulus versus density can be observed for these bio-based aerogels (Figure 30). The CNF/emulsion aerogel fitted the curve of the MFC foam, with an exponent of 1.8.249 The mechanical properties of the current aerogels are apparently higher than the CNF aerogel, chitin aerogel, and emulsion cellulose aerogels, indicating much improved mechanical properties with emulsion templating technique. Figure 30. Comparison of the Young’s modulus vs. density of this work ( ) with various bio-aerogels without chemical crosslinking: MFC (microfibrillated cellulose) aerogel ( ),249 CNF aerogels ( 126, 250, 122), chitosan aerogel ( ),251 nanofibrillated cellulose emulsion aerogel ( ),252 electrospun cellulose aerogel ( ),253 Nano-CMC (carboxymethylcellulose) aerogel ().148 72 3.8 Thermal insulation performance of aerogel Ultralight aerogel has been widely used as super thermal-insulating material due to its low density and highly porous internal structure. As previously described, the overall thermal conductivity of an aerogel can be obtained from the thermal conduction of solid pore walls, thermal conduction of the gas phase, thermal convection of the gas phase and the radiative component of the solid materials.58 Thermal conduction of solid pore walls can be lowered by reducing the solid fraction, or the density of the aerogel,58 while gases thermal conductivity depends on pore size, with smaller pore size leading to lower gas thermal conductivity due to the Knudsen effect.59, 254 Gas phase thermal convection can be affected by the openness of the pore, and closed pores generally have negligible convection contribution from the gas phase.244 The thermal conductivity of CNF-only aerogel was measured as 21.4 mW·m-1·K-1 (Figure 31a), which is lower than that of ambient air at room temperature (25 mW·m-1·K-1).255 Such low thermal conductivity has been previously observed for other nanocellulose aerogel due to blockage of the free path of the air and reduced air diffusion through pores.232, 256 The thermal conductivity of CNF/emulsion composite aerogels decreased continuously with increased emulsion content, and reached the lowest value of 15.5 mW·m-1·K-1 for emulsion-only aerogel. The thermal conductivity of the 8:2 CNF/emulsion composite aerogel was found to be 15.7 mW·m-1·K-1, which is close to that of emulsion-only aerogel due to their similar closed pores morphologies as observed above from SEM images. The significantly reduced thermal conductivity of the CNF/emulsion composite aerogels can be ascribed to the following mechanisms. Firstly, the densities of the composite and emulsion-only aerogels are much lower than that of CNF aerogel, due to the restricted volume shrinkage by the strong emulsion droplets, which contributes to reduced solid wall thermal conduction. Secondly, although CNF-only aerogel showed much smaller pores that were close to the mean free path of 73 the air, the pores were all interconnected with assembled CNF networks, which cannot effectively restrict thermal conduction of gas, as gas molecules can move through the pores. Thirdly, the CNF/emulsion composite and emulsion-only aerogels showed closed emulsion templated microspheres, which can partition the aerogel, further reducing gas thermal convection through pores. In addition, close observation of the pore walls showed micro-/meso-pores with diameter of only a few nm, which could reduce the gas thermal conduction between pores due to the Knudsen effect. Heat shielding was also demonstrated by covering a human hand with a piece of aerogel, showing clear temperature difference between the bare hand and top surface of the aerogel, thus indicating IR shield effect (Figure 31). Figure 31. (a) Thermal conductivity of aerogel with varying emulsion:CNF ratios and (b) 4:6 aerogel sample (thickness: 3.8 mm) on human palm. The thermal insulating ability of the CNF/emulsion composite aerogel (8:2 emulsion: CNF ratio) was measured using an infrared thermal imager further demonstrates their effectiveness in blocking heat (Figure 32) and cold (Figure 33). With background temperature of 81 ℃, a 6 mm thick aerogel effectively blocked heat transmitting to the top surface, with a stabilized temperature of 40.1 ℃. Similarly, the aerogel can also was able to block the cold transmitting from the bottom 74 (-17 ℃) to the top surface, maintaining a stable 15.8 ℃ after 20 min in contact with ice. These results indicate that the CNF/emulsion composite aerogel had good thermal insulating performance. Figure 32. (a) Temperature-versus-time curve and (b) infrared thermal image of 8:2 aerogel (thickness: 6 mm) on hot plate (blue dash line represents room temperature; large red dots represent the surface temperature of aerogel on hot plate) Figure 33. (a) Temperature-versus-time curve and (b) infrared thermal image of 8:2 aerogel (thickness: 6 mm) on ice (large blue dots represent the surface temperature of aerogel on ice; red dash line represents the temperature of hot plate or ice). 3.9 Conclusions A strong, flexible, and super thermal-insulating CNF/emulsion composite aerogel with closed micropores was developed using emulsion templating and solvent exchange techniques. 75 Conventional cellulose aerogels are featured with either large open pores of a few hundred of µm wide or interconnected mesopores of tens to hundreds of nm wide framed among cellulose networks, leading to relatively high thermal conductivity. CNF/emulsion composite aerogel with closed pores was demonstrated for the first time by the strategical design of Pickering emulsion formation, solvent exchange, ice crystals suppression, and freeze-drying. CNF stabilized oil-in-water Pickering emulsion, with average droplet diameter of 1.3 µm, was used as a precursor for closed pores embedded in the aerogel. Due to the compact and closely assembled CNF on the surface, the emulsion droplets can serve as a strong support to restrict the shrinkage and increase the mechanical properties of the aerogels. Owing to such structural advantages, the typical CNF/emulsion composite aerogel (8:2 emulsion: CNF ratio) showed significantly improved performance as compared to the CNF-only aerogel, including approximately one half of the density (12.7 vs. 22.0 mg·cm-3), almost doubled specific modulus (18.2 vs 10.1 kPa·mg-1·cm-3), and much lower thermal conductivity (15.7 vs. 21.4 mW·m-1·K-1). The CNF/emulsion composite aerogel demonstrated great flexibility that can be rolled and recover from rolling. In addition, results showed that the aerogel can effectively block heat/cold transmission and also can serve as an IR shielding layer. Thus, the superior mechanical and thermal insulating properties of the CNF/emulsion composite aerogel prepared by this new method make it an ideal candidate for practical thermal insulation application. On the other hand, thermal management includes two main forms thermal insulation and energy storage. Thermal energy storage system such PCMs can also be used to tune the thermal regulation performance. However, the applications of PCMs have been hindered by their deficiencies such as large volume change and liquid leakage during phase transition and poor mechanical properties, which will be addressed in the following chapter. 76 Chapter 4: Results and Discussion - phase change thermal regulation materials based on cellulose nanofiber-stabilized Pickering emulsion 4.1 Introduction With the increasing concerns about energy crisis and environmental pollution, it is pressing for the development of efficient energy storage systems with sustainable materials. The latent heat storage method can store more heat energy than that of sensible heat storage method, 185 which can be a potential method for the development of effective energy storage systems. Phase change materials (PCMs) based on latent heat method has been attractive owing to theirs favorable properties for energy storage and saving applications such as buildings, textiles, and vehicles.183, 185, 187, 189, 190 The good thermal regulation performance of PCMs mainly depends on such properties as high shape stability, no liquid leakage, high latent heat, and excellent mechanical properties.191, 194-196 The extensive applications of PCMs have been hindered by the use of unsustainable materials and the absence of these favorable properties. The shape change and liquid leakage can be overcome by using the encapsulation technique to cover the core material, i.e. PCM, with shell material such as solid nanoparticles to fabricate capsules. Paraffin is an attractive core material due to its high latent heat, safe, chemically stable, low cost, and so on. Its disadvantages such as the shape change and liquid leakage during phase transition from solid to liquid still can be overcome by the aforementioned encapsulation technique.192, 193 Pickering emulsion templating can be an effective encapsulation technique for the stabilization of PCM such as paraffin to form stable droplets in emulsion, which is a green approach and has been extensively studied for various applications.161, 257, 258. The droplets in Pickering emulsions are generally stabilized by irreversibly adsorbed solid particles which form 77 rigid layer to effectively inhibit Ostwald ripening and droplet expansion by effective steric stabilization.148, 156, 158 The size and coverage of droplet can be tuned by the type and quantity of solid particles which can also be used to improve thermal and mechanical properties of PCM.176, 218-220 The shell materials should be cohesive or chemically compatible with the core material,211, 213 and natural materials with renewable and bio-compatible properties are desirable. The sustainability, amphiphilic property, and high aspect ratio of cellulose nanofiber (CNF) make it an ideal candidate to enclose and stabilize the oil core for the formation of oil in water Pickering emulsion. CNF prepared from TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation has high aspect ratio, which can physically entangle with each other and form a large number of hydrogen bonds to establish a strong network.40, 168, 169 The negative carboxyl groups on the surface of CNF can inhibit the aggregation of fibers and the coalescence of droplets in emulsion by electrostatic repulsion.49-51 These excellent properties CNF exhibits can provide high stability and enhanced mechanical properties for PCM. For thermal insulation application, CNF are generally used to fabricate ultralight aerogel with low thermal conductivity. The active core material of PCM typically has much higher thermal conductivity than CNF, whereas it can harvest thermal energy to benefit thermal insulation properties. It is expected that super thermal insulating ability and tailored mechanical properties can be achieved by PCM made from paraffin and CNF. In this study, we investigated the relationship between CNF and paraffin for Pickering-emulsion based CNF/PCM aerogels, and successfully obtained CNF/PCM aerogels with superior thermal regulation ability, enhanced mechanical properties, and high shape stability. 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) oxidized CNFs were used to encapsulate paraffin to form oil-in-water Pickering emulsion with most droplet sizes in the order of hundreds of nanometers. 78 The paraffin dispersion in CNF and capsules formation were formed in one step by Pickering emulsion technique. The CNFs were also used to reinforce the solid structure of PCM by mixing additional CNFs with as-formed emulsion. There was no liquid leakage during phase change process. The Young’s modulus of freeze-dried CNF/PCM aerogel can reach as high as around 1 MPa with density still in a low level for CNF/PCM aerogel with 4:6 volume ratio of emulsion to CNF suspension. The shape of CNF/PCM aerogel can be well maintained even loaded with a weight of over 5000 times its own weight. Due to the presence of small pores, the CNF/PCM aerogel exhibited superior thermal regulation abilities of both thermal energy storage and thermal insulation. The latent heat of CNF/PCM aerogel (4:6) can reach approximately 84.4% of that of pure paraffin, while maintaining exceptional mechanical properties. The mechanical properties and thermal regulation ability can be tuned by prepare CNF/PCM aerogel with varying ratios of emulsion to CNF. These excellent properties shape it a promising thermal regulation candidate for energy-efficient and load bearing smart textile, building and electronic applications. 4.2 CNF/paraffin emulsion Due to the amphiphilic nature of TEMPO oxidized CNF with the presence of hydrophilic plane along the equatorial direction and hydrophobic plane along the axial direction, it is used to stabilize oil-in-water Pickering emulsion using paraffin as the oil phase. A freshly prepared CNF/paraffin Pickering emulsion was an opaque and white creamy suspension due to the strong light scattering effect at water/emulsion droplets interfaces (Figure 34a). The emulsion was highly stable with no phase separation after cooling down to room temperature, and appeared as highly viscous suspension that could adhere hold to the bottom of an inverted glass vial (Figure 34b). 79 Figure 34. Digital image of CNF/paraffin emulsion. The emulsion droplet size distribution was determined by dynamic light scattering (DLS), and showed a bimodal size distribution with respective emulsion droplets diameters of 377 nm and 5 µm (Figure 35a). Spherical particles can be clearly observed from the optical micrograph (Figure 35b), showing a majority of emulsion droplets between 1-5 µm, with an average diameter of 2.36 µm (Figure 35c). It should be noted that the smaller emulsion droplets from DLS cannot be observed using optical microscopy as their diameters are less than the wavelength of visible light, and they can be discerned in the SEM images. Figure 35. (a) Size distribution of emulsion droplets determined from DLS; (b) Optical microscopy image and (c) size distribution of emulsion droplets determined from optical microscopy images. 80 4.3 Densities of CNF/PCM aerogels Although the prepared CNF/paraffin emulsion was very stable with a robust gel-like structure, freeze-dried CNF/paraffin emulsion was extremely brittle due to lack of sufficient interactions among the spherical emulsion droplets. In order to fabricate flexible PCM incorporated aerogels for practical applications, additional cellulose nanofibrils were mixed with the as-formed CNF/paraffin emulsion to prepare CNF/PCM composite aerogel. The CNF/PCM composite aerogel was expected to possess enhanced mechanical properties as a result of the addition of cellulose nanofibrils, which can form a strong 3D network structure by the combined effect of physical entanglements,170-172 Van der Waals forces, and hydrogen bonds. The freeze-dried CNF/PCM composite aerogels with different emulsion fractions showed similar foam-like structure (Figure 36a). The density of the CNF/PCM composite aerogel increased with increasing PCM content (Figure 36a), reaching the highest density of 78 mg·cm-3 with 40% PCM volume fraction. The low density of the composite aerogel can be demonstrated by placing it on top of a flower (Figure 36b). This paragraph indicates that a lightweight CNF/PCM composite aerogel can be formed by simply freeze-drying a mixture of CNF and PCM emulsion. Figure 36. (a) Densities of freeze-dried CNF/PCM aerogels with varying emulsion to CNF ratios (1:9, 2:8, and 4:6); (b) Digital image of CNF/emulsion composite aerogel (4:6, thickness: 16 mm) standing on top of a flower. 81 4.4 Internal structure of CNF/PCM aerogels The morphologies of the CNF/PCM composite aerogels were visualized from the cross-sectional SEM images, showing highly porous structures at all emulsion:CNF ratios (Figure 37). The CNF/PCM composite aerogel exhibited 3D-interconnected porous structure with large pores over 100 µm wide (Figure 37a,d, &g). These large interconnected pores were caused by the large ice crystals forming during the direct freezing of the emulsion. It can be easily observed that the concentration of CNF had a significant influence on the pore morphology of the aerogel, showing slightly increased pore size with growing emulsion fraction (Figure 37b, e, & h). As the CNF concentration was reduced as a constituent of the emulsion, an increasing emulsion fraction led to reduced CNF concentration, resulting in increased pore sizes. Under high magnification, it clearly showed that microspheres are attached to the surface of the cell walls. The cell walls are generally smooth and are composed of assembled CNFs during ice crystal formation, which is consistent with the CNF aerogel being formed by direct freezing and freeze-drying.126 Due to the strong entanglement and hydrogen bonding of the CNFs, all PCM emulsion droplets are firmly imbedded within the cell walls, while maintaining separation from each other. The intact association between the separated emulsion droplets and cell walls lead to flexible yet strong CNF/PCM composite aerogels. Increased numbers of microspheres can be clearly observed for the composite aerogel with more PCM fractions (Figure 37c, f, & i). The emulsion droplets remained spherical after freeze-drying, with similar sizes as observed from the optical microscopy images, indicating the PCM is well preserved within the microspheres. Other than these microspheres observed from both optical microscopy and SEM images, the high-resolution SEM image clearly showed the presence of numerous small particles of a few hundred nm in diameter (Figure 37i). The SEM 82 images confirmed the presence of small nanoparticles as shown in the DLS but these are invisible in optical microscopy images. Figure 37. SEM images of the CNF/PCM aerogels with varying emulsion:CNF ratios, (a-c): 1:9, (d-f): 2:8, and (g-i) 4:6. 4.5 Mechanical properties of CNF/PCM aerogels The mechanical properties of CNF/PCM composite aerogels were investigated by compression testing. Representative stress-strain curves for foam-like materials typically showed three stages, including initial linear elastic region, plastic deformation after yield point, and the final densification phase (Figure 38a). Due to the additional CNF networks, all aerogels could be compressed to 80% strain without breakage, indicating the original brittleness issue of PCM was 83 addressed. The load bearing performance of the CNF/emulsion composite aerogel is demonstrated by its ability to support a 500 g weight (Figure 38b). The CNF/PCM1, CNF/PCM2, and CNF/PCM4 composite aerogels can support loads of 5360, 3937, and 2645 times their own weight, respectively, with little or no shape deformation. Due to increased density at higher emulsion contents, the deformation in the thickness direction decreased from 5.76% for CNF/PCM1 composite aerogel to 0.1% for CNF/PCM4 composite aerogel (Figure 38b), suggesting strong network structure for practical applications. Figure 38. (a) Stress-strain curves of CNF/PCM aerogels with varying emulsion:CNF ratios; (b) Thickness change of CNF/PCM composite aerogel with different paraffin emulsion fractions after a load of 500g weight on top, and the insect is the digital image of CNF/PCM4 composite aerogel (thickness: 15 mm) loaded with 500 g weight. Both Young’s modulus and yield strength of CNF/PCM composite aerogels increased with increasing paraffin emulsion content (Figure 39a), which is consistent with increasing density. CNF/PCM4 composite aerogel showed highest Young’s modulus of 1055.1 kPa and yield strength of 30.6 kPa, which approximately double those for CNF/PCM1 composite aerogel (386.8 kPa Young’s modulus and 17.7 kPa yield strength). However, the CNF/PCM4 composite aerogel showed lowest specific Young’s modulus and specific yield strength, which can be explained by 84 the reduced interactions between CNF and emulsion droplets at higher emulsion content (Figure 39b). As observed from the SEM image, excess PCM emulsion droplets accumulated on the surface of CNF cell walls. The lack of interactions among the PCM microspheres could lead to weakened structures. The enhanced flexibility and structural integrity of the CNF/PCM composite aerogels are further demonstrated in Figure 40 & 41. Freeze-dried PCM emulsion was very brittle and can be easily broken into half by gentle hand-bending. In contrast, the CNF/PCM1 composite aerogel demonstrated great flexibility and could be bent to 180 º, and then recover its original shape when force was released (Figure 41). Such high flexibility is introduced by the additional CNF in the aerogel, which can form continuous cell wall structures to bind emulsion microsphere, preventing breakage of the structure. Figure 39. (a) Young’s modulus and yield strength of CNF/PCM aerogels with varying emulsion:CNF ratios (error drawn); (b) Specific Young’s modulus and specific yield strength (error drawn). 85 Figure 40. Bending test of PCM (thickness: 5 mm) prepared without additional CNF. Figure 41. Bending test of CNF/PCM1 (thickness: 5 mm) prepared with emulsion:CNF ratio 1:9. 4.6 Shape stability of CNF/PCM aerogels Shape change and leakage of PCM at high temperature has been one of the major concerns for its practical applications. Here I demonstrate shape and temperature stability of CNF/PCM composite aerogels at various varied temperature up to 190 °C (Figure 42). Pure cylindrical-shaped paraffin started melting at a temperature close to the phase change temperature (60 °C). Then the structure collapsed and eventually turned into liquid as temperature increasing temperature. In contrast, the CNF/PCM composite aerogel was much more stable under all investigated temperatures, showing little or no shape change even at 190 °C (Figure 43). The extremely high heating temperature, much higher than the melting point of paraffin, was selected for the stability test in order to counteract the thermal insulation effect of the aerogels, to ensure 86 the encapsulated paraffin microspheres reach their the phase change temperature.217 It should also be noted that the CNF/PCM composite aerogel can well maintain structure integrity even at 190 °C, and withstand 500 g weight (Figure 43). The excellent shape and structure stability of the CNF/PCM composite aerogel even at extremely high temperatures suggested the paraffin was well encapsulated in the CNF shells, and the densely packed CNF and thermal insulating properties of the aerogel prevented the leakage of the PCM. Figure 42. Digital images demonstrating the shape stability of CNF/PCM4 aerogel and pure paraffin (thickness: 18 mm) after heating at different temperatures for 20 min. Figure 43. Digital image of CNF/PCM4 composite aerogel (thickness: 14 mm) supporting a 500 g weight at 190 °C. 87 4.7 Thermal stability analysis of CNF/PCM aerogels The thermal stability and thermal reliability of CNF/PCM composite aerogels were examined by thermogravimetric analysis (TGA, Figure 44) and differential scanning calorimetry (DSC, Figure 45), respectively. In contrast to the flat curve of paraffin below 150 °C, the CNF/PCM composite aerogels showed slight weight loss possibly due to evaporation of absorbed water in the aerogel (Figure 44a). The weight loss at 150 °C for CNF/PCM1, CNF/PCM2, and CNF/PCM4 composite aerogels were 2.1%, 1.3%, and 0.96%, respectively, showing a reduced trend with increasing PCM fractions. Water absorption of the composite aerogel is primarily through the hydrophilic CNF, and therefore will decrease as the CNF content decrease. Correspondingly, DTGA curves were significantly different for paraffin and CNF/PCM aerogels (Figure 44b). Compared to paraffin, the Tmax of CNF/PCM aerogels reduced to 263.34-285.98 °C, which is due to the lower thermal degradation temperature of TEMPO-oxidized CNFs.259 The onset degradation temperature of CNF/PCM aerogel is around 220 °C, indicating its high thermal stability for high temperature applications. Due to the volatilization of paraffin, the weight of paraffin dropped to 0% at 335.91 °C. The char residues at 600 °C were 2.92%, 1.84%, and 1.11% for CNF/PCM1, CNF/PCM2, and CNF/PCM4 aerogels, showing a decreasing trend with increasing PCM composition, indicating the char residues are mainly derived from carbonized CNF. 88 Figure 44. (a) (a) TGA (N2), (b) DTGA, and (c) DSC curves of pure paraffin and CNF/PCM composite aerogels with varying emulsion:CNF ratios. 4.8 Thermal reliability analysis of CNF/PCM aerogels Thermal properties of paraffin and CNF/PCM, including the phase change temperature and latent heat of melting and crystallization, were investigated using DSC (Figure 45). Two peaks (endothermic during melting and exothermic during crystallization) can be observed for pure paraffin during both melting and crystallization, including the primary peak representing solid-liquid phase transition and a shoulder peak due to the solid-solid phase transition (Figure 45).260 Similar to pure paraffin, the CNF/PCM aerogel also had two peaks during each thermal process, with reduced peak intensity with decreasing PCM content. The melting temperature of pure paraffin was around 55.1 °C, which slightly increased to 55.4-56.2 °C for CNF/PCM aerogel, which can be due to the thermal insulation of the aerogel (Table 8). With poor thermal conductivity of the aerogel, it may need extra heat to melt the encapsulated paraffin. Due to the same thermal insulation effect, the crystallization temperature decreased from 50.4 °C for pure paraffin to 47.8-48.1 °C for CNF/PCM aerogels. As an important parameter to evaluate the performance of PCM, the latent heat (equal to transition enthalpy), became smaller with decreasing paraffin content for 89 both melting and crystallization processes. The enthalpy of pure paraffin was around 205.6 and 203.4 J·g-1 for melting and crystallization, respectively. Although a decline in enthalpy was found for CNF/PCM4 aerogel, the latent heat was still around 84.4% of that of pure paraffin, due to the encapsulation of large amount of paraffin in the aerogel. Even with the smallest paraffin content, the latent heat of CNF/PCM1 aerogel still reached around 57.1% of the latent heat of pure paraffin. Figure 45. DSC curves of pure paraffin and CNF/PCM aerogels with varying emulsion:CNF ratios.90 Table 8. The melting temperature (Tm), crystallization temperatures (Tc), and transition enthalpy (ΔH) of pure paraffin and CNF/PCM aerogels In order to examine the cyclic performance of the CNF/PCM aerogel, the CNF/PCM4 sample was subjected to 51 melting and crystallization cycles (Figure 46a). The important thermal parameters, melting temperature (Tm), crystallization temperatures (Tc), and latent heat (ΔH) of CNF/PCM4 aerogel almost did not change even after 50 cycles (Table 8), which indicates excellent encapsulation effect due to the strong CNF network. Such good thermal stability was also apparent when heating and cooling the CNF/PCM aerogels in an 80 °C oven for up to 20 cycles; the sample with the most PCM content (CNF/PCM4) showed no signs of shape change or liquid leakage (Figure 46b). This observation indicates that CNF/PCM aerogel has high energy density and excellent thermal stability that potentially makes it suitable for thermal energy storage applications. Samples Tm (℃) ΔHm (J g-1) Tc (℃) ΔHc (J g-1) Paraffin 55.1 205.6 50.4 203.4 PCM1 55.4 117.5 47.8 116.2 PCM2 56.2 152.4 47.2 150.0 PCM4 (1 cycle) 55.4 173.6 48.1 171.9 PCM4 (26 cycle) 55.0 175.2 48.0 172.3 PCM4 (51 cycle) 55.2 175.4 47.8 172.5 91 Figure 46. (a) DSC curves of CNF/PCM4 composite aerogels after 51 heating and cooling cycles; (b) Digital image of CNF/PCM4 composite aerogel (thickness: 9 mm) after 20 heating and cooling thermal cycles. 4.9 Thermal regulation performance of CNF/PCM aerogels For aerogels, the thermal insulating properties are primarily determined by the heat conduction and radiation via solid skeleton, and heat conduction and convection via the filled gas, which are significantly influenced by the inner structure and composition of aerogels. The white color of the aerogel indicated strong light scattering that can inhibit heat radiation.261 The presence of air as a poor heat conductor in the internal voids of the aerogel can also lead to a low thermal conductivity. A low thermal conductivity is highly desirable for thermal insulating applications, while a high thermal conductivity is typically required for fast thermal response to accelerate the rate of thermal energy storage and release. 262 In comparison with CNF, paraffin contributes higher thermal conductivity, and therefore the thermal conductivity of CNF/PCM aerogel was expected to increase with increasing paraffin content.263 However, the highly porous structure of prepared CNF/PCM aerogel inhibited the substantial increase of thermal conductivity. The thermal conductivities of CNF/PCM aerogels varied from 30.2 to 37.7 mW·m-1·K-1 (Figure 47). Although 92 the thermal conductivities of all CNF/PCM aerogels were still at a low level (higher than that of pure CNF aerogel) compared to paraffin and the thermal response to temperature change was not very fast accordingly, the porous structure provided excellent thermal insulation properties to CNF/PCM aerogel for practical applications Figure 47. Thermal conductivity of CNF/PCM aerogels with varying paraffin fraction. In an attempt to examine the thermal regulation ability of the prepared PCM during energy storage for practical applications, infrared thermal imaging was used to record the temperature changes of PCM composites during energy storage and release. Low heating temperature (52 ℃) was also used to test PCM without phase change (Figure 48a). Given the excellent thermal insulating ability of PCM, the hot plate was heated up to an extremely high temperature of 195 ℃ to reach the phase change temperature (Figure 48b). According to the temperature-time curves of PCM during the heating process, it can be clearly observed that the surface temperature increase of PCM was inversely proportional to paraffin content. This can be attributed to energy absorption 93 by the paraffin. The temperature differences between samples and heating source were approximately 25℃ and 140 ℃ for low and high heating temperature, respectively, indicating excellent thermal insulating performance of the prepared PCM. The original shapes of all composites were well maintained after testing and no liquid leakage was observed. Therefore the effectiveness of PCM at both high and low temperatures were therefore demonstrated. Figure 48. Temperature of CNF/PCM aerogels with varied paraffin fraction (thickness: 9.5 mm) versus time curves on hot plate of (a) 52°C and (b)195 °C. With the purpose of investigating the temperature change during energy release, aerogels were placed on a hot plate at 78 ℃ to heat up, and then moved away from the heating source to cool down when their surface temperature became stable (33 ℃ for CNF/PCM4). IR images of the cooling process were monitored showing clear thermal energy release for CNF/PCM aerogels (Figure 49). Pure CNF aerogel cooled down to room temperature immediately after being removed from the heat source. However, the surface temperature of CNF/PCM aerogel remained higher with greater PCM fractions, and gradually cooled to room temperature over time. The surface cooling profile of the CNF/PCM aerogels were monitored over time at a surface temperature (53 94 ℃) close to the phase change temperature (Figure 50), showing that the greater PCM fractions can slow down the cooling rate as more thermal energy was stored. Figure 49. Temperature change of pure CNF aerogel (thickness: 10 mm) and CNF/PCM aerogels (thickness: 10 mm) during cooling process (the first row from left to right: CNF/PCM1, 2, and 4; the second row: pure CNF aerogel). Figure 50. Temperature of CNF/PCM aerogels (thickness: 10 mm) versus time curves during cooling from 53°C. To demonstrate a potential application, a box was built to act as a model house (Figure 51), which was then covered with CNF/PCM4 aerogel to demonstrate the effectiveness of the aerogel in suppressing heating up the internal under high temperature. When placed in an oven at 95 °C, 95 the internal model house without CNF/PCM4 aerogel covers heated up immediately, whereas the one covered by CNF/PCM4 aerogel remained at relative low temperatures (Figure 51a). The internal temperature profiles of the model house with and without CNF/PCM4 aerogel covering are shown in Figure 51b. Clearly, the model house covered by CNF/PCM4 aerogel showed a much slower rate of temperature increase. Under 40 and 70 °C environment temperature, the internal temperature of the model house covered with CNF/PCM4 aerogel reached 38.6 and 66.6 °C, slightly lower than those without cover. Such thermal regulating performance is more obvious when the house was heated on one side by a hot plate (Appendix G), simulating the sun irradiation during summer, showing significantly lower internal temperature for the house covered with CNF/PCM aerogel as compared to the uncovered one. Figure 51. (a) Model house covered with CNF/PCM4 (thickness: 5.6 mm) in lab oven; (b) Internal temperature change versus time curves of model house covered with and without CNF/PCM4. Due to its thermal energy absorbing performance and high energy density, the CNF/PCM aerogel can act as an infrared shielding material to shield the human body. When a human palm was partially covered by the CNF/PCM4 aerogel, it completely blended into the background with similar temperature of 22.2 °C, and was invisible to an IR camera (Figure 52). In contrast, the 96 surface temperature of the uncovered half of the palm exhibited a clear infrared image with a temperature of 34.9 °C. All the aforementioned tests demonstrate the superb thermal regulation performance of the CNF/PCM aerogel, which shows potential as energy storage, thermal insulating, and IR shielding materials. Figure 52. Infrared thermal images of the human palm covered by prepared CNF/PCM4 (thickness: 5 mm). 4.10 Conclusions CNF/PCM aerogels with superb thermal regulation ability, enhanced mechanical properties, high shape stability, and excellent cyclic thermal performance were successfully prepared by the oil-in-water Pickering emulsion templating. The paraffin was encapsulated by CNF to prepare PCM microsphere with two main sizes of around 377 nm and 2.4 µm. The PCM was fragile after freeze drying. The mechanical strength and flexibility were significantly enhanced by the addition of CNF to as-formed emulsion, and CNF/PCM aerogels were obtained. While the CNF/PCM (1:9) aerogel has low density (28.9 mg·cm-3), it can support a load of over 5,000 times of its own weight with little shape change. The thermal conductivity was still at a low level (30.2 mW·m-1·K-1) 97 after the incorporation of paraffin. The exceptional thermal regulation performance of both thermal insulation and thermal energy storage were achieved owing to the small pores in the aerogel. The excellent shape stability can be maintained without liquid leakage even at temperature as high as 190 ºC. The latent heat of CNF/PCM aerogel (4:6) can reach 173.59 J·g-1 during phase change, which was 84.4% of that of pure paraffin. The outstanding thermal regulation performance and can still be well maintained after 50 thermal cycles. The easy preparation and exceptional thermal properties of CNF/PCM aerogel make it very promising for energy saving and load bearing in areas of smart textile, smart building, batteries, and electronic devices. 98 Chapter 5: General conclusions and future work 5.1 General conclusions In summary, the overall objective of this study was to improve the thermal management and mechanical performance of CNF aerogels. This objective was successfully achieved mainly based on Pickering emulsion templating technique, solvent exchange method, filling of PCM, and the addition of CNF into emulsion. The closed pore structure for CNF aerogel was successfully obtained for the first time, and the thermal and mechanical properties were improved. Also, the shape stability, liquid leakage, flexibility, and thermal regulation performance of PCM were significantly improved. Moreover, the addition of CNF to emulsion successfully improved the mechanical and thermal regulation performance of aerogels attributed to the enhanced bridging of the emulsion microspheres. The impressive thermal management ability and enhanced mechanical properties of CNF aerogel prepared in this study make it an ideal candidate for thermal insulation, thermal energy storage, energy saving, load bearing, and IR shielding in areas such as smart textile, smart building, and electronic devices. 5.2 Future work While the thermal management performance and mechanical properties of CNF aerogels have been significantly improved in this work, there is still some work that may need further investigation. First, the infrared shielding effect can be affected by the moisture in environment, as CNF is hydrophilic and can absorb water. The possible inefficiency of infrared shielding in environment with high humidity can be probably overcome by making the surface of CNF hydrophobic. The 99 hydrophobic surface can be obtained by the addition of Tetraethyl orthosilicate or Polydimethylsiloxane. This may also be beneficial to improve the mechanical properties. Second, the relationship between surface charge density of CNF and the droplet size and stability need further research. The charge density on CNF can influence the strength of electrostatic repulsion between droplets in emulsion as well as nanofibrils. Low charge density can lead to the aggregation of nanofibers and coalescent of droplets, while high charge density may result in thinner interface. This effect can have an influence on the porous structure as well as the thermal and mechanical performance of aerogel. Third, cellulose is amphiphilic but the hydrophilic and hydrophobic groups are not respectively on an individual lattice plane. The twisted conformation of the entire chain along the fiber-axis of cellulose crystal can probably result in water existing between the hydrophobic surface of CNF and oil at the interface. The influence of water at the interface on the size and stability of droplets in emulsion needs in-depth investigation. Fourth, the influence of drying technique, including ambient drying, freeze drying, and supercritical CO2 drying, on the porous structure, mechanical and thermal properties of aerogel can be investigated. Different drying methods produce materials with different pore morphology and structure of aerogel. A low-cost drying method such as ambient drying is also desirable. Fifth, although acetone in the transparent gel was exchanged to tert-butanol to avoid the formation of large crystals compared to ice crystals formed from water during freezing, expansion of the pores in gel can still exists. The possible expansion may affect the structure and properties of aerogel. As ethanol can have similar effect as acetone to get the transparent gel and aerogel with closed pores, the use of supercritical CO2 drying method to remove ethanol/acetone from the ethanol/acetone gel can be a feasible way to overcome the pore expansion problem. By 100 supercritical CO2 drying, the ethanol/acetone gel does not need to go through the freezing stage and therefore no crystals will be formed in the gel, which is beneficial for the preservation of pore morphology to obtain aerogel with more closed and smaller pores. Sixth, the emulsion-preparation temperature may affect the emulsion stability and droplet morphology. The droplet size may be controlled by different preparation temperatures. Increasing temperature for emulsion preparation will induce more molecular thermal activity, which will decrease the cohesive forces of solvent molecules. Also, higher temperature may increase the likelihood of desorption of adsorbed particles at the interface. 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Thesis/Dissertation
eng
Forestry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
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Cellulose nanofibrils aerogel with enhanced thermal management performance
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