Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Kim, Diana
Master of Science - MSc
University of British Columbia
The rising environmental awareness has prompted investigation of biopolymers as alternative materials to the petroleum-based and non-biodegradable plastics. One such biopolymer is lignin, the second most abundant biopolymer on Earth. Although more than 50 million tons of lignin is extracted from the wood pulping industry annually, less than 2% is used for value-added products.1 Despite its many inherent advantages, lignin has several structural or mechanical limitations to be utilized in its pristine form. Chemical modification with other biopolymers, such as poly(lactide) (PLA), is a promising way to overcome the poor processability of this underutilized material. In this thesis, lignin-graft-PLLA and lignin-graft-PDLA copolymers were synthesized and transformed into nanofibers through electrospinning. The nanofibers generated with either lignin-graft-PLLA or lignin-graft-PDLA showed XRD patterns corresponded to the homocrystallites of isotactic PLA, whereas the nanofibers generated with 1:1 v/v solution blend of lignin-graft-PLLA/lignin-graft-PDLA showed stereocomplexation of the PLA chains. The stereocomplexation resulted in a different fiber morphology by more dense chain packing and enhanced Young’s modulus and tensile strength, although sacrificing elongation at break. All lignin-graft-PLA-based nanofibers showed good filtration performances as lignin-based aerosol filters. However, stereocomplexation resulted in brittle nanofibers and higher air blockage through the filter, which was disadvantageous to be used as a mask material.
INVESTIGATING THE ROLE OF POLYMER TACTICITY ON THE PROPERTIES AND FILTRATION EFFICIENCY OF POLY(LACTIDE)-MODIFIED LIGNIN NANOFIBERS by Diana Kim B.Sc., The University of British Columbia, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2021 © Diana Kim, 2021 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis entitled: Investigating the Role of Polymer Tacticity on the Properties and Filtration Efficiency of Poly(lactide)-Modified Lignin Nanofibers submitted by Diana Kim in partial fulfillment of the requirements for the degree of Master of Science in Chemistry Examining Committee: Prof. Parisa Mehrkhodavandi, Chemistry, UBC Supervisor Prof. Frank Ko, Materials Engineering, UBC Supervisory Committee Member Prof. Laurel Schafer, Chemistry, UBC Supervisory Committee Member iii Abstract The rising environmental awareness has prompted investigation of biopolymers as alternative materials to the petroleum-based and non-biodegradable plastics. One such biopolymer is lignin, the second most abundant biopolymer on Earth. Although more than 50 million tons of lignin is extracted from the wood pulping industry annually, less than 2% is used for value-added products.1 Despite its many inherent advantages, lignin has several structural or mechanical limitations to be utilized in its pristine form. Chemical modification with other biopolymers, such as poly(lactide) (PLA), is a promising way to overcome the poor processability of this underutilized material. In this thesis, lignin-graft-PLLA and lignin-graft-PDLA copolymers were synthesized and transformed into nanofibers through electrospinning. The nanofibers generated with either lignin-graft-PLLA or lignin-graft-PDLA showed XRD patterns corresponded to the homocrystallites of isotactic PLA, whereas the nanofibers generated with 1:1 v/v solution blend of lignin-graft-PLLA/lignin-graft-PDLA showed stereocomplexation of the PLA chains. The stereocomplexation resulted in a different fiber morphology by more dense chain packing and enhanced Young’s modulus and tensile strength, although sacrificing elongation at break. All lignin-graft-PLA-based nanofibers showed good filtration performances as lignin-based aerosol filters. However, stereocomplexation resulted in brittle nanofibers and higher air blockage through the filter, which was disadvantageous to be used as a mask material. iv Lay Summary The rising environmental awareness has prompted investigation of biopolymers as alternative materials to the petroleum-based and non-biodegradable plastics. One such polymer is lignin, the second most abundant biopolymer on Earth. Despite its many inherent advantages, lignin has several structural or mechanical limitations to be utilized in its pristine form. The aim of this thesis is to expand the methodology to modify the properties of lignin-based products, with the combination of multiple research areas including chemistry, materials engineering, and mechanical engineering, allowing lignin to be used for sustainable aerosol filters. v Preface All design of experiments, synthesis, and characterization in Chapter 2 were conducted by myself. The XRD data of nanofiber mats were obtained by X-ray specialist, Anita Lam. The SEM images were obtained by either Maddie Eghtesad supervised by Prof. Frank Ko in Materials Engineering department or SEM technician, Heli Eunike. The filtration testing of the six nanofiber mats in Chapter 3 was carried out by Thomas Bement who was supervised by Prof. Steven Rogak in Mechanical Engineering department. The instrumental setup for filtration testing followed the procedure from the previous literature by Rogak and co-workers.2 I completed the synthesis and characterization of the polymers and nanofiber mats, as well as analysis of the filtration data. The chapters were written by myself with significant commentary from Prof. Parisa Mehrkhodavandi and Prof. Frank Ko. This work is being prepared for publication and will be submitted shortly: Kim, D.; Bahi, A.; Liu, L.; Bement, T.; Rogak, S.; Renneckar, S.; Ko, F.; Mehrkhodavandi, P. "Poly(lactide)-modified lignin nanofibers: Investigating the role of polymer tacticity on fiber properties and filtration efficiency" In preparation. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Tables .............................................................................................................................. viii List of Figures .................................................................................................................................x List of Symbols ........................................................................................................................... xiv List of Abbreviations ...................................................................................................................xv Acknowledgements .................................................................................................................... xvi Dedication ................................................................................................................................. xviii Chapter 1: General Introduction. ................................................................................................1 1.1 Biopolymers .................................................................................................................... 1 1.1.1 Lignin ...................................................................................................................... 2 1.1.2 Poly(lactide) ............................................................................................................ 6 1.1.2.1 Synthesis of PLA ................................................................................................ 7 1.1.2.2 Tacticity and stereocomplexation of PLA .......................................................... 8 1.2 Nanofibers ..................................................................................................................... 10 1.2.1 Electrospinning process ........................................................................................ 11 1.2.2 Application of nanofibers ..................................................................................... 14 1.2.2.1 Application of lignin nanofibers ....................................................................... 14 1.2.2.2 Application of PLA nanofibers ......................................................................... 15 vii 1.3 Scope of thesis .............................................................................................................. 16 Chapter 2: Effect of PLA tacticity on non-woven lignin-graft-PLA nanofibers. ...................18 2.1 Introduction ................................................................................................................... 18 2.1.1 Lignin-graft-PLA copolymers .............................................................................. 18 2.1.2 Stereocomplexation of PLA in nanofibers ............................................................ 20 2.2 Results and discussion .................................................................................................. 23 2.2.1 Synthesis of lignin-graft-PLAs ............................................................................. 23 2.2.2 Characterization of lignin-graft-PLAs .................................................................. 37 2.2.3 Generation of non-woven nanofiber mats via electrospinning ............................. 41 2.2.4 Effect of tacticity of lignin-graft-PLAs on nanofiber morphology ...................... 42 2.2.5 Effect of tacticity of lignin-graft-PLAs on thermal and mechanical properties of nanofiber mat ........................................................................................................................ 44 2.3 Conclusion .................................................................................................................... 50 Chapter 3: Application of non-woven lignin-graft-PLA nanofibers in aerosol filtration. ....56 3.1 Introduction ................................................................................................................... 56 3.2 Results and discussion .................................................................................................. 60 3.2.1 Characterization of morphology of lignin-graft-PLA nanofibers ......................... 60 3.2.2 Filtration test of non-woven lignin-graft-PLA nanofibers .................................... 64 3.3 Conclusions ................................................................................................................... 74 3.4 Experimental ................................................................................................................. 75 Chapter 4: Conclusion and future directions ............................................................................78 Bibliography .................................................................................................................................81 Appendix .......................................................................................................................................91 viii List of Tables Table 1.1 Lignin and monolignol composition in different types of plants.23-31 ............................ 4 Table 2.1 Hydroxy group content of organosolv lignin and kraft lignin determined from 31P{1H} NMR spectra in Figure 2.4.a ......................................................................................................... 25 Table 2.2 Graft copolymerization of lignin with rac-lactide (rac-LA), L-lactide (L-LA), and D-lactide (D-LA) using DBU.a ......................................................................................................... 27 Table 2.3 Onset and inflection temperatures and % residual mass of lignin-graft-PLLA depending on mol % lig-OH obtained from TGA. ......................................................................................... 39 Table 2.4 Glass transition and melting temperatures of the isotactic graft copolymers and their stereocomplex mixture. ................................................................................................................. 40 Table 2.5 Optimized parameters for electrospinning of the graft copolymers. ............................ 41 Table 2.6 Average diameters of nanofibers on three graft copolymer nanofiber mats. ............... 44 Table 2.7 Onset and inflection temperatures, residual mass, and % lignin content of the three graft copolymer nanofibers compared to pristine PLA and lignin derived from TGA thermographs. . 45 Table 2.8 Glass transition and melting temperatures of the nanofibers derived from DSC thermograms. ................................................................................................................................ 46 Table 2.9 Mechanical properties of lignin-graft-PLA nanofibers obtained from the uniaxial tensile testing. ........................................................................................................................................... 49 Table 3.1 Nonwoven nanofiber mat samples for filtration testing. .............................................. 60 Table 3.2 Average diameter of nanofibers on nanofiber mats NF1 to NF6. ................................ 62 Table 3.3 Average surface pore size and surface porosity of the nanofiber mats NF1 to NF6. .. 64 Table 3.4 Mechanical properties of the six nanofiber mats, derived from stress-strain curve. ... 70 ix Table 3.5 The total filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop........................................................................................................................................................ 72 Table 3.6 The filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop for 2.75 µm aerodynamic diameter NaCl aerosols. ............................................................................ 74 x List of Figures Figure 1.1 Example of chemical structure of a lignin molecule. ................................................... 3 Figure 1.2 Simplified structure of lignocellulose in the secondary cell wall of plants. ................. 3 Figure 1.3 Monolignols and corresponding phenolic units in lignin. ............................................ 4 Figure 1.4 Four major extraction processes of lignin. ................................................................... 5 Figure 1.5 Tacticity of PLA depending on the lactide isomer. ...................................................... 9 Figure 1.6 Electrospinning setup used in the experiment. ........................................................... 11 Figure 1.7 Effect of solution concentration on average diameter and morphology of PLLA nanofibers at constant electrospinning parameters and solvent system. ....................................... 12 Figure 1.8 Orientation of nanofibers. Random (left) and aligned (right). ................................... 13 Figure 2.1 Graft-to route for the synthesis of lignin graft copolymers. Monomers are polymerized first prior to reaction with lignin. .................................................................................................. 19 Figure 2.2 Graft-from route for the synthesis of lignin graft copolymers. Lignin is used as an alcohol macro-initiator. ................................................................................................................. 20 Figure 2.3 Electrospinning of PLLA and PDLA to form stereocomplex in nanofibers. ............. 21 Figure 2.4 31P{1H} NMR spectra (chloroform-d : pyridine 1.6:1 v/v, 25°C, 300MHz) of phosphitylated (a) organosolv lignin and (b) kraft lignin used in this study. ............................... 24 Figure 2.5 Synthesis of lignin-graft-PLAs through ring-opening polymerization with DBU. .... 27 Figure 2.6 1H NMR spectra of lignin-graft-PLA copolymers with 14, 7, 2 mol% lignin OH (a-c), PLA homopolymer (d), and the pristine chloroform soluble portion of lignin (e) (chloroform-d, 25°C, 400MHz). ............................................................................................................................ 28 Figure 2.7 GPC traces of lignin-graft-PLLA (a), lignin-graft-PDLA (b), and lignin-graft-PDLLA (c) copolymers with 2, 7, and 14 mol% lignin-OH. ...................................................................... 30 xi Figure 2.8 31P{1H} NMR spectra (chloroform-d : Pyridine 1.6:1 v/v, 25°C, 400MHz) of phosphitylated lignin-graft-PLLA copolymers with 14 (a), 7 (b), 2 (c) mol% lignin OH, and pristine lignin (d). .......................................................................................................................... 31 Figure 2.9 31P{1H} NMR spectra (chloroform-d : Pyridine 1.6:1 v/v, 25°C, 400MHz) of phosphitylated lignin (a) and lignin-graft-PDLLA copolymer with large excess of lignin (b). ... 32 Figure 2.10 Experimental PLA segment MW (red circle) (NMR) vs mol % of lignin and theoretical segment MW (black square, Mn,theo = MLA ´ ([LA]/([OH]lignin + [DBU])) for graft copolymers. The difference in slope between the theoretical and experimental segment MWs was used to determine that 29% of the OH groups on lignin was activated during polymerization (corrected theoretical segment MW, blue triangle). ..................................................................... 33 Figure 2.11 MALDI-ToF spectra of cyclic PLA (no lignin incorporated) (a), and lignin-graft-PDLLA (addition of LA to DBU and lignin) (b). ......................................................................... 34 Figure 2.12 Mechanism of polymerization of lactide with DBU in the absence of an alcohol initiator, through deprotonation of an acylated DBU to a ketene aminal (a) and through zwitterionic propagation (b).53 .......................................................................................................................... 35 Figure 2.13 Expected mechanism of graft copolymerization of lignin and lactide with DBU, based on the proposed mechanism with polyethylene glycol as an alcohol-initiator.53 ......................... 36 Figure 2.14 1H{1H} NMR spectra (chloroform-d, 25°C, 400MHz) of lignin-graft-PDLLA (a), lignin-graft-PLLA (b), lignin-graft-PDLA (c).133 ........................................................................ 37 Figure 2.15 Thermogravimetric Analysis (TGA) curves of lignin-graft-PLLA with different lignin loading in the reaction. .................................................................................................................. 38 xii Figure 2.16 DSC curves of lignin-graft-PLLA (a), lignin-graft-PDLA (b), and SC-lignin-graft-PLA (1:1 blend of lignin-graft-PLLA/lignin-graft-PDLA) (c). Both graft copolymers were synthesized at the same condition, with 14 mol% lignin-OH. ...................................................... 40 Figure 2.17 20X and 5000X magnified images of NF1, NF2, and NF3 taken from optical microscope (left) and SEM (right). ............................................................................................... 43 Figure 2.18 TGA curves of nanofiber mats NF1, NF2, and NF3 compared to pristine linear PLLA and lignin. ..................................................................................................................................... 45 Figure 2.19 DSC thermograms of NF1, NF2, and NF3 nanofibers. ............................................ 46 Figure 2.20 XRD spectra of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3) nanofibers. ........................................................................................................ 48 Figure 2.21 Average stress vs. strain curve of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3) nanofiber mats. .............................................................. 49 Figure 3.1 Concentration of particles outside and inside the mask after filtration through nanofibers. ..................................................................................................................................... 57 Figure 3.2 Different filtration mechanisms of particles with fibers. ............................................ 58 Figure 3.3 SEM images of nanofiber samples (NF1 to NF6) under 5000X magnification. ........ 61 Figure 3.4 Diameter distribution of the nanofiber mats NF1 to NF6. Based on 100 nanofibers throughout the different regions up to 1000 nm for NF1 to NF5 and up to 2000 nm for NF6. .... 63 Figure 3.5 Experimental setup for the filtration test and pressure drop measurement using NaCl aerosols.2 ....................................................................................................................................... 65 Figure 3.6 Pressure drop dependent on the average surface pore size (a), the surface porosity (b), and the average fiber diameter of the nanofiber mats (c). ............................................................ 67 xiii Figure 3.7 Total filtration efficiency dependent on the average pore size (a), the porosity (b), and the average fiber diameter of the nanofiber mats (c). ................................................................... 68 Figure 3.8 Filtration efficiency of the nanofiber mats NF1 to NF6 depending on NaCl aerosol aerodynamic diameter. Values reported in Table A6. .................................................................. 69 Figure 3.9 Average stress vs. strain curve of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3), lignin-graft-PDLLA (NF4), and PLLA (NF5) nanofiber mats........................................................................................................................................................ 70 Figure 3.10 Morphology of different regions of nanofiber mats with defects (red circles) and non-uniformity of NF3 and NF4 at 1000X magnification (top), and NF6 at 200X magnification (bottom). ........................................................................................................................................ 71 Figure 3.11 The total filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop. Quality factor isolines are labeled in kPa-1. ......................................................................... 72 Figure 3.12 The filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop for 2.75 µm aerodynamic diameter NaCl aerosols. Quality factor isolines are labeled in kPa–1. ...... 73 xiv List of Symbols [OH]lig Hydroxy concentration in lignin (mmol g–1) D Dispersity Mn, GPC Number-averaged molecular weight obtained by GPC (g mol–1) Mw, GPC Weight-averaged molecular weight obtained by GPC (g mol–1) Nin Number of particles inside filter (after filtration) Nout Number of particles outside filter (before filtration) P Penetration PM2.5 Particulate matter (2.5 microns or less in width) Q Quality factor (Pa–1) Tcc Cold crystallization temperature (ºC) Tg Glass transition temperature (ºC) THC Melting temperature for homocrystalline isotactic PLA (ºC) Tm Melting temperature (ºC) TSC Melting temperature for stereocomplex isotactic PLA (ºC) wt% Weight per cent ∆Hcc Enthalpy of cold crystallization (J g–1) ∆HHC Enthalpy of fusion for homocrystallization of isotactic PLA (J g–1) ∆HSC Enthalpy of fusion for stereocomplexation of isotactic PLA (J g–1) ∆p Pressure drop (Pa) η Filtration efficiency 𝜒! Chi-square value xv List of Abbreviations CDCl3 Chloroform-d CHCl3 Chloroform DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene DCM Dichloromethane D-LA D-lactide DMF N,N-Dimethylformamide DSC Differential scanning calorimetry GPC Gel permeation chromatography Lignin-graft-PDLA Lignin-graft-poly(D-lactide) copolymer Lignin-graft-PDLLA Lignin-graft-poly(D,L-lactide) copolymer Lignin-graft-PLA Lignin-graft-poly(lactide) copolymer Lignin-graft-PLLA Lignin-graft-poly(L-lactide) copolymer L-LA L-lactide MALDI-ToF Matrix-assisted laser desorption ionization time of flight N2 Nitrogen gas NMR Nuclear magnetic resonance OH Hydroxyl group PDLA Poly(D-lactide) PDLLA Poly(D,L-lactide); racemic poly(lactide) PEO Poly(ethylene oxide) PLA Poly(lactide) PLLA Poly(L-lactide) rac-LA rac-lactide (racemic) ROP Ring opening polymerization SC-lignin-graft-PLA 1:1 stereocomplex blend of lignin-graft-PLLA and lignin-graft-PDLA SC-PLA 1:1 stereocomplex blend of PLLA and PDLA SEM Scanning electron microscopy TBD 1,5,7-Triazabicyclo[4.4.0]dec-5-ene TGA Thermogravimetric analysis THF Tetrahydrofuran TMDP 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane WAXS Wide-angle X-ray scattering XRD X-ray diffraction xvi Acknowledgements The last few years were the most meaningful and enlightening years of my life. I have learned much more during my master’s degree than I could have ever imagined, not only in terms of research, but also in terms of life skills. There are many people that have guided me in my journey. First and foremost, I am truly grateful for my supervisor, Prof. Parisa Mehrkhodavandi, for encouraging and supporting me both academically and mentally throughout my undergraduate and graduate studies. I am lucky to have encountered her in my life and have had her as my supervisor, and I will never forget her patience, even when through the times I stumbled. She is my role model, and I would not have been able to come this far without her. Thank you for providing me opportunities to explore the world of research and for being my friend and family through the difficult times. I would like to also thank Prof. Frank Ko for introducing me to the interesting new field of nanofibers. I truly appreciate his continuous advice and great assistance in widening my perspectives on different fields of research. As well, I would like to thank Prof. Laurel Schafer for encouraging me to think critically and for being so open and positive to all my new ideas in the lignin-based filtration project. Thanks to Dr. Addie Bahi for always being there to answer my questions and providing me with new ideas and information on nanofibers. I appreciate his kindness in helping me adjust to a new laboratory environment. Also, I would like to thank Dr. Taesik Chae for always being willing to provide me with academic and life advice. I would like to thank my past/present lab mates and fellow graduate students, Hyuk Joon Jung, Hassan Baalbaki, Kudzanai Nyamayaro, Chatura Goonesinghe, Afagh Habibzadeh, Dr. Carlos Diaz, Shazia Camillla Soobratte, Hootan Roshandel, Kimia Hosseini, Joseph Chang, Julia Shu, xvii and Youngjung Cho, for creating a loving and fun laboratory environment and never hesitating to reach out to help each other. I especially thank Hyuk Joon Jung and Hootan Roshandel for being my mentors during my first year and consistently encouraging me through the challenging times. I would like to also thank my friends I truly love deep down in my heart, Jenny Lee and Benjamin Oh, for always being there and being supportive no matter what we encounter in our lives, for over 12 years. Special thanks to my parents James Kim and Grace Kang, and my sister Solar Kim for love and support throughout my years of education. Thank you for believing me unconditionally. xviii Dedication To my beloved family.1 Chapter 1: General Introduction. Increased demand for petroleum-based non-degradable plastics has brought about increased environmental and health concerns. Almost 300 million tons of non-renewable and petroleum-based polymers are produced annually, and is predicted to grow by 12% annually to compensate the demand for plastics.3, 4 The most prevalent commodity polymers are polyvinylchloride, polypropylene, polystyrene, polyethylene terephthalate, and polyethylene.3, 5 Currently, only 1% of the plastic manufactured annually are biomaterials.3 The rising environmental awareness has prompted investigation of biopolymers as alternative materials that have capability of undergoing hydrolytic or enzymatic degradation.6, 7 1.1 Biopolymers The term ‘biopolymer’ refers to several classes of polymers, which can be grouped based on 1) source of the raw materials and 2) degradability of the polymer.8, 9 As a broad definition, biopolymers are bio-based, biodegradable, or both and can degrade or decompose microbiologically to the ultimate products of water and carbon dioxide and are recycled or recovered in the nature.3, 10, 11 Naturally produced bio-based polymers such as cellulose, hemicellulose and lignin come from wood sources, and biopolymers such as DNA can be found in living organisms.6 Production of bioplastics has been increasing. In 2018, the European Bioplastics reported that the worldwide production capacity is expected to increase from 2.06 million tons in 2017 to 2.62 million tons in 2023.3 Bio-derived and biodegradable polymers, such as polyhydroxyalkanoates and poly(lactide) (PLA) play the major role in this increment. Despite the increased interest, biopolymers have several limitations that inhibit features that are inevitably required for everyday 2 applications.3 For example, many biodegradable polymers exhibit relatively lower mechanical properties. Despite having a disadvantage in that regard, this chemical nature such as bearing suspectable linkages e.g. ester bonds, enables them to undergo hydrolysis or other bacterial/enzymatic degradation.9, 12, 13 Thus, there should be a balance between the mechanical properties for industrial usage and biodegradability. Bioderived biodegradable polymers have advantages of being readily available from renewable sources and some are available at low costs in large quantities to meet industrial demands.6 As such, they have a variety of applications including, but not limited to, packaging, biomedical applications, electronics, catalysis, adhesives and coatings, and energy storage.14-18 In this thesis, the aim is to develop sustainable aerosol filters to tackle the pollution resulting from non-degradable filters. Lignin, an abundant and underutilized biopolymer, was selected as the material to fabricate the filters through electrospinning. However, due to lignin’s poor mechanical properties and electrospinnability in its pristine form, another biopolymer, poly(lactide) or PLA, was introduced to improve these properties. 1.1.1 Lignin Lignin is the second most abundant bio-derived material after cellulose and is found in 25-30% of non-fossil-based organic molecules within the plant cell wall.6, 19 It comprises ~ 17 to 30% of the secondary cell wall of vascular plants. The composition and structure varies dramatically between wood source and even the age of wood (Figure 1.1).19 Its main function is to provide structural support to the plant, providing enough rigidity to allow them to grow upright and to promote efficient water, metabolites, and nutrient transport in xylem or in sclerenchyma.6, 20 Lignin serves as the binder, similar to the resin in composite materials. 3 Figure 1.1 Example of chemical structure of a lignin molecule. In the cell wall, cellulose fibers are surrounded by hemicellulose to form a micelle. These micelles are combined to form microfibrils, which then further form macrofibrils.19 Lignin functions as a putty, filling the interspaces in between the micelles and microfibrils (Figure 1.2). Figure 1.2 Simplified structure of lignocellulose in the secondary cell wall of plants. Lignin is the only primary plant material with polyphenolic structure and is hydrophobic in nature.6, 21, 22 It is primarily based on three different monomers, or monolignols, that are phenyl propanoids having C3-substituted phenols (Figure 1.3). The composition of the monomers and the OOOOOOOHOOHOOHOOOOHOOOHHOOOHOHOOHOHOOOH4 type of linkages differ between hardwood and softwood, where the b-O-4 linkages are the most favorable for both (Table 1.1). Figure 1.3 Monolignols and corresponding phenolic units in lignin. Table 1.1 Lignin and monolignol composition in different types of plants.23-31 Entry Wood type Lignin (wt%) p-Coumaryl alcohol (wt%) Coniferyl alcohol (wt%) Sinapyl alcohol (wt%) 1 Softwooda 21-33 <5 >90 0-10 2 Hardwoodb 18-25 0-8 25-50 46-75 3 Grass 17-24 5-35 25-80 20-55 aGymnosperm trees. bAngiosperm trees. As a by-product of the pulping process, lignin has been treated as waste material and has been incinerated as fuel.32 Although more than 50 million tons of lignin is extracted from the wood pulping industry annually, less than 2% is used for value-added products.1 A growing realization OHOHOHOHOHOHH3CO H3CO OCH3OH OH OHH3CO H3CO OCH3p-coumaryl alcohol coniferyl alcohol sinapyl alcoholp-coumaryl unit guaiacyl unit syringyl unit5 of the environmental and health concerns of petroleum-based and non-degradable plastics has revolutionized the effort to utilize lignin in more sustainable products.33-37 To extract lignin from wood, chemical modification of its structure is inevitable since it is partially covalently bound to cellulose through benzyl ether, benzyl ester, and phenyl glycoside linkages.19, 38, 39 Thus, lignin’s definite molecular structure is unknown due to difficulty in extraction in its pristine form, as well as its random combination of monomers which further provides difficulty in characterizations and modifications. There are two major ways to extract lignin from the wood: sulfur-involving process and sulfur-free process (Figure 1.4).40 Figure 1.4 Four major extraction processes of lignin. Increase in paper demand is leading towards more production of lignin as the waste product along with hemicellulose because they exhibit yellowness in colour, which is an undesired property of paper.19 Out of the four processes introduced, kraft lignin is majorly produced as a by-6 product of cellulose extraction under basic conditions, involving decomposition of lignin with sodium hydroxide and sodium sulfide.19 Despite their abundance, presence of sulfur in lignin extracted through kraft pulping may limit their utilization in many areas. The organosolv process, on the other hand, involves extraction of cellulose through solubilizing lignin and hemicellulose by organic solvents including ethanol, water, acetone, and weak acid. The absence of sulfur and better solubility compared to kraft lignin are the main advantages of organosolv lignin. 1.1.2 Poly(lactide) Poly(lactide), or PLA, is a thermoplastic aliphatic polyester generated from a bio-derived resource such as corn and sugar beets.41, 42 With advanced production procedures for lactide monomer by Bischoff, PLA was first developed as low molecular weight oligomers by Carothers and coworkers in 1932.42 Until the year 2000, its usage, due to high production cost, was mainly limited to biomedical applications such as sutures.43 The development of new low-cost procedures to synthesize PLA resulted in 300×106 lb per year commercial-scale production in North America in 2002.44 With its low toxicity, good biocompatibility, and being produced from renewable resources, PLA has been gaining more attention as a promising material to reduce plastic waste. The degradation process has two major steps. First, with acceleration by acids or bases, PLA undergoes hydrolysis into lower molecular weight oligomers at a certain temperature and moisture levels.41 Through biodegradation, these oligomers can be decomposed to produce water, carbon dioxide, and hummus.41, 45 Moreover, not only its degradation products are non-toxic to the human body at low composition, but PLA also hydrolyzes in the human body to be incorporated into tricarboxylic acid, which is then excreted. The Food and Drug Administration has approved PLA to be used in direct contact with biological fluids, which opens up its versatile applications in areas 7 such as food packaging and biomedical applications.42 Furthermore, PLA production requires less energy compared to other petroleum-based plastics, by 25 to 55%.42 1.1.2.1 Synthesis of PLA Direct polycondensation of lactic acid is one of the methods to produce PLA yet has a lot of significant limitations. Polycondensation of lactic acid involves dehydration, where an excessive amount of water is produced as a by-product. Difficulties come from complete removal of water, as well as inability to control the stereoregularity and molecular weight due to high water content in the polymer mixture, unless a specific catalyst is used.46 Ring opening polymerization (ROP) of lactide is the most widely used method to produce PLA.46 The driving force of the ROP is enthalpy coming from the ring strain of lactide (DH = 22.1 kJ/mol for 1 mol/L of solution of lactide at room temperature) which overcomes the formation of polymer that is entropically unfavoured.42, 47 There are three major mechanisms for ROP: cationic, anionic, and coordination-insertion. In an activated cationic polymerization, the lactide is first activated by acid and is protonated, which enables alcohol to initiate the polymerization via nucleophilic attack.42, 48 Anionic ROP of lactide can also exhibit high activities compared to the coordination-insertion mechanism.42 A lot of transesterification and racemization can occur, as shown in the study by Kricheldorf and co-workers.42, 49 In a metal-catalyzed coordination-insertion ROP, the nature of the metal centre and its ligand can contribute to the dispersity, rate of polymerization, and stereoselectivity of the polymer chain. The first three-step coordination-insertion mechanism for ROP of lactide was proposed by Dittrich and Schulz in 1971.42, 50 Coordination-insertion mechanism follows 1) coordination of lactide to 8 Lewis-acidic metal centre, 2) acyl-oxygen bond cleavage leading to ring-opening of lactide, and 3) isolation of PLA polymer chain through hydrolysis.42, 51 One of the most widely used catalyst in bulk polymerization of lactide is tin(II) bis-2-ethylhexanoic acid (known as tin octanoate) due to high catalytic activity, high conversion with the low rate of racemization of polymer, and simple procedure which comes from its capability to dissolve molten lactide to produce a high molecular weight PLA.41 However, it often requires high temperature, resulting in typical polymerization conditions in the range of 180 to 210 °C for a fast reaction.41 For more precise control of molecular weight and low dispersity, indium or aluminum complexes can be used. Mehrkhodavandi and co-workers have developed a series of dinuclear indium catalysts for the ROP of lactide. A highly active chiral Indium catalyst that is capable of going under living ROP of lactide via coordination-insertion mechanism was reported to give high conversion and low dispersity PLA.52 Organocatlysts are also widely used to polymerize lactide, especially in some copolymerization reactions that may cause catalyst poisoning if an organometallic catalyst is used. 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), are used to generate linear or cyclic PLA either with or without an alcohol initiator. The ratio of DBU and alcohol initiator can lead to different PLA topology, as shown in the study by Won and co-workers.53 1.1.2.2 Tacticity and stereocomplexation of PLA PLA chains can exhibit different tacticity (Figure 1.5). Amorphous PLA (PDLLA) is generated through a random combination of lactide enantiomers. Different tacticity of PLA chains leads to distinct crystal structures. For instance, poly(L-lactide) (PLLA) has three crystal structure phases, 9 a, b, and g.42 Among the three phases, a-phase is found the most in melt crystallization, solution crystallization, and fiber spinning.42 In the a-phase, isotactic PLA exhibits 103 helix.42, 54 It has an orthorhombic unit cell with a=1.06nm, b=0.595nm, and c=2.78nm. b-phase can form when tensile stress is applied on the a-phase structure, which was first shown in the study by Gogolewski and co-workers, and shows left-handed 31 helix for PLLA with a difference in unit-cell parameters. This further results in a lower melting temperature and higher stability compared to a-phase.42, 55, 56 g-phase was observed at a temperature of 140 °C, after being grown epitaxially on hexamethylbenzene.42, 57 Figure 1.5 Tacticity of PLA depending on the lactide isomer. One of the reasons that PLA was selected to be incorporated into lignin nanofibers in this thesis was due to its ability to form stereocomplexes. Stereocomplex of PLA can be formed by blending PLLA and PDLA. It exhibits a different crystal structure from that of isotactic PLLA or PDLA homopolymers. By forming a stereocomplex, higher melting temperature, Tm, of 220-230 °C can OOOOOOOOD-lactideL-lactideSSRR OOOOOOOOnR R RROOOOOOOOnS S SSOOOOOOOOnisotacticatacticisotactic1:1 mixturerac-lactidecrystallinecrystallineamorphousPLLAPDLAPDLLA10 be obtained. Moreover, stereocomplex of PLA is known to have relatively high Young’s modulus (2.5-4.5 GPa) and ultimate strength (100-400 MPa) in melt-spun fibers, as well as greater tensile strength when made into a film, compared to isotactic PLLA or PDLA films.42 Stronger hydrogen bonding and dipole-dipole interactions in stereocomplex PLA are derived from the greater number of interactions occurring in a shorter distance between the atoms, in which the average distances are 2.00 Å for hydrogen bonds and 3.00 Å or below for dipole-dipole interactions.42 There are 9 times more hydrogen bonds compared to dipole-dipole interactions in the stereocomplex blend. 1.2 Nanofibers Nanofibers are defined as fibers with a diameter equal to or less than 100 nanometers. However, fibers with a diameter below 1000 nm are generally accepted as nanofibers. Due to the high specific surface area and surface area-to-volume ratio, it often has advantages in various properties compared to bulk materials.58 Specifically, high porosity of nanofibers makes them excellent for filtration applications, being able to block small particles while allowing air molecules to go through.2 Fiber diameter plays a significant role in nanofiber properties. For instance, by reducing the fiber diameters to the nanoscale, a specific surface area of 1000 m2/g level can be reached, resulting in a dramatic increase in chemical or biological reactivities.58 It is expected that by reducing the fiber diameter from 10 µm to 10 nm, a million times increase in flexibility can be achieved.58 11 1.2.1 Electrospinning process Electrospinning is an efficient process to produce nanofibers with small diameters, which is desirable for filtering out nano- to micro-sized aerosols. Instead of mechanical forces, it involves electrostatic forces to generate fibers, which differentiates it from the conventional fiber spinning techniques such as melt, wet, or dry spinning.58 The ultrafine nanofibers as fine as 9 nm have been produced previously.59 In the electrospinning process, a polymer solution is loaded into a syringe where the syringe pump speed adjusts the formation of a droplet at the tip of the needle (Figure 1.6). High voltage source is used to create the electric field between the capillary tip, usually a needle, and a collecting drum.58 Voltage has to be strong enough to overcome the surface tension of the droplet, allowing the traveling of the solution towards the target to generate nanofibers. The polymer solution droplet turns into a conical shape by the electric field, which is known as the Talyor cone.58 Rapid evaporation of solvent occurs as the fibers are elongated in the Taylor cone creating unstable airflow, and the resulting nanofibers are ejected towards the metal collector or target drum. Figure 1.6 Electrospinning setup used in the experiment. 12 Polymer solution viscosity is one of the most important factors in determining nanofiber formation. The Mark-Houwink-Sakurada equation shows the relationship between intrinsic viscosity ([𝜂]) and molecular weight (M), where K and a are constants for a specific polymer-solvent system at a given temperature (Equation 1.1).58 Berry number (Be) is another parameter that shows the degree of chain entanglement, where C is the polymer concentration (Equation 1.2).58 Higher Berry number results in larger fiber diameter (d) and fewer beads, where a and c are function of molecular weight and crystallinity, respectively (Equation 1.3, Figure 1.7).60-64 [𝜂] = KM" (1.1) Be = [𝜂]C (1.2) d = a Be c (1.3) Figure 1.7 Effect of solution concentration on average diameter and morphology of PLLA nanofibers at constant electrospinning parameters and solvent system. 13 Solvent boiling point is another factor to be considered in electrospinning as the fiber formation involves rapid evaporation of the solvent. A low boiling point is usually desirable for the deposition of polymer in a solid state and to prevent fiber fusion or film formation. However, when the solvent evaporates too rapidly, it can cause clogging of the spinneret. To prevent this, mixed solvent systems are often used to balance the evaporation rate.65 Temperature and humidity affect the fiber diameter as well, with a more uniform diameter and better processability at high temperatures. Larger fiber diameter is achieved at higher relative humidity due to higher electrostatic charge density on the fiber surface and a change in solvent’s evaporation rate.58 Not only the polymer solution but also the instrumental parameters have significant effects on nanofibers.58 For example, the speed and rotating direction of the collecting drum can affect the orientation of fibers, in which the mechanical properties depend on the direction of deformation within the same nanofiber mat (Figure 1.8). Figure 1.8 Orientation of nanofibers. Random (left) and aligned (right). 14 1.2.2 Application of nanofibers To date, both the nanofibers made of lignin or PLA have been actively studied for a variety of applications ranging from filters, wearable devices, scaffolds, to packaging, to replace other bulk materials.66, 67 1.2.2.1 Application of lignin nanofibers Nanofibers made of lignin are used in many places as composite materials due to their advantages of being affordable, sustainable, and having inherent antioxidant properties. However, lignin has poor fiber spinnability and is often blended or chemically modified to make graft copolymers with other polymers. Ko and co-workers have demonstrated the effect of a dendrimer, polyamidoamine dendritic polymer (PAMAM) in lignin blend nanofibers.68 Although the addition of 2 wt% PAMAM resulted in a significant decrease in viscosity in lignin-PAMAM solution which resulted in droplets instead of fibers, the addition of 1 wt% PAMAM in the blend helped to reduce the fiber diameter compared to that of pristine lignin nanofibers, also enabling better thermal stability. Furthermore, it was proposed that the change in viscosity over 5 days was different in its trace compared to that of pristine lignin, showing the interaction between PAMAM and lignin with more chain entanglement. The resulting nanofiber mat showed potential to be used as filtration membranes. Inherent antimicrobial abilities were also observed with lignin-based nanofibers. Lee and co-workers produced nanofibers with 85 wt% lignin content to polyvinyl alcohol and observed 99.9% bacterial reduction against gram-positive organisms, whereas no reduction was observed for gram-negative bacteria.69 When no lignin was added to the spinning solution, the nanofibers showed 0% 15 bacterial reduction, confirming that the phenolic component of lignin is known to damage the cell membranes of the microorganisms and cause lysis of such bacteria.70 Yoon, Kim, and Yarin, and co-workers studied lignin/polycaprolactone nanofibers’ ability to protect wood against environmentally hazardous fungi.71 After two weeks of incubation, the sapwood samples covered with lignin/polycaprolactone nanofiber mat was perfectly protected against fungal infection showing good inhibition of penetration due to its small pore size working as a filter, which was significantly different from that of control wood without nanofiber mat coverage which showed spore growth along with discoloration. 1.2.2.2 Application of PLA nanofibers PLA nanofibers have been used in many areas, especially in biomedical applications, and have been preferred over other biopolymers. Good biocompatibility of being able to undergo scission in the human body, degrading into lactic acid as a natural intermediate in carbohydrate metabolism, having a degradation rate close to that of healing time of bone tissues, and having thermal and mechanical properties similar to that of bone tissues allow PLA to be an excellent material to be used in medical applications.72-80 PLA nanofibers have been investigated to be used in drug delivery applications. For instance, Javar and co-workers investigated multilayer nanofibers incorporating PLLA to be used as scaffolds for bone tissue engineering and drug delivery.81 PLLA/pluronic P123 composite scaffolds were generated through electrospinning, where the drug was loaded in the middle layer. It was found that single and multi-layer scaffolds exhibited different drug release profiles, showing better osteogenesis and controlled release of drugs and bioactive molecules for the multi-layer scaffolds compared to single-layer counterparts. 16 Blending the drug in the nanofiber spinning solution of PLA was also found to be effective in generating antimicrobial nanofiber mats, as shown by the study by Duygulu and co-workers.82 Importance of nanofiber diameter was investigated, and strong antimicrobial activity against Escherichia coli bacteria was observed with 8% wt/v drug blended PLA nanofibers, whereas no inhibition of microbial growth was observed for the control group. PLA nanofibers were developed to be used as bio-derived and biodegradable air/water filters. Ren, Wang, and co-workers developed electrospun PLA nanofibers to test their filtration efficiency towards <2.5 µm particles.83 Electrostatic charges generated from isotactic PLLA or PDLA nanofibers reached 99.3% filtration efficiency, which is higher than that of the control 3M commercial filter. Higher efficiency than that of commercial filter was observed even after 6 hours of filtration time. 1.3 Scope of thesis The aim of this thesis is to develop sustainable aerosol filters out of lignin, by tuning its mechanical properties with PLA tacticity. Chapter 2 investigates the role of tacticity and stereocomplexation of PLA on mechanical properties of lignin-based nanofibers. An organosolv lignin is modified through graft copolymerization with PLA to enhance their compatibility in fiber spinning. The homocrystallization and stereocomplexation of lignin-graft-PLLA and lignin-graft-PDLA are attempted in electrospinning process involving a rapid evaporation of solvent, and their effect on the fiber properties and morphology are discussed. Expanding the methodology to tune the properties of lignin-based nanofibers will allow lignin to be used in a wider array of applications including filtration. Chapter 3 investigates the potential of the fabricated nanofibers to be used as aerosol filters. The morphology of the lignin-graft-PLA nanofibers are observed in 17 more depth. The effect of stereocomplexation and resulting mechanical properties of nanofibers on filtration performances are investigated. 18 Chapter 2: Effect of PLA tacticity on non-woven lignin-graft-PLA nanofibers. 2.1 Introduction 2.1.1 Lignin-graft-PLA copolymers Lignin and its composites have been used as fillers and dispersing agents in other polymers to improve flow properties and durability.84, 85 In addition to incorporation in films and foams, nanofibers of lignin have been synthesized to develop lightweight and highly porous material with a high surface-to-volume ratio.86-90 However, the major limitation of the lignin composite materials come from their poor processability and mechanical properties.91 This is due to the insufficient compatibility of lignin with other biopolymers at high lignin loadings.6, 92, 93 One way to overcome the incompatibility of lignin with another biopolymer is through graft copolymerization. Studies on the graft copolymerization of lignin with poly(caprolactone), poly(lactic acid) (PLA), acrylamide/maleic anhydride, poly(acrylonitrile), chitosan, and cellulose have been reported.87, 94-97 Previously, the Mehrkhodavandi group has reported a thermorheological analysis of lignin-graft-PLA and lignin-graft-PLA/PLA blends.98 We were interested in the study by Sattely and co-workers who used graft copolymerization of lignin and PLA to overcome the challenge of their different solubilities in pristine forms.33 Since this pioneering study, others have also developed methodologies for the synthesis lignin-graft-PLA copolymers through ring-opening polymerization of rac-lactide (rac-LA), L-lactide (L-LA), and D-lactide (D-LA), yielding corresponding PDLLA, PLLA, and PDLA grafts.33, 86, 94, 98-108 Many studies used them as compatibilizers to toughen PLA by improving the dispersion of lignin.98-100, 102-104, 109 These lignin-graft-PLA/PLA composite materials were suitable for UV-blocking, active food packaging, and biomedical applications.33, 86, 100, 102, 107-109 19 Lignin-graft-PLA copolymers can be synthesized through two major approaches: graft-from and graft-to.101, 110 Although graft-to approaches have advantages that the PLA arm length is known prior to its reaction with lignin, only a few examples are known due to its relatively more complicated procedures and difficulties of long PLA chains to access the reaction sites on the complex structure of lignin compared to the graft-from methods (Figure 2.1).98 Kim and co-workers graft copolymerized methanol-soluble kraft lignin with PLA through acylation substitution of PLA chain end with thionyl chloride.111 A higher molecular weight of the graft copolymers was produced when lower molecular weight PLA was used. The authors suggested that smaller PLA chains have higher chances of successfully grafting onto lignin hydroxyl groups due to less steric hindrance. Higher tensile strengths and Young’s modulus were observed for the graft copolymers compared to the linear PLA. Figure 2.1 Graft-to route for the synthesis of lignin graft copolymers. Monomers are polymerized first prior to reaction with lignin. The graft-from method is a more common approach to produce the lignin-graft-PLA copolymers (Figure 2.2). Organocatalysts, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), are 20 often used for this method. Lignin is used as an alcohol macro-initiator and therefore becomes the chain-end of the PLA. Liu and co-workers produced lignin-graft-PLLA with high molecular weight PLLA chain arms by decreasing the number of hydroxyl groups available through partly alkylating the hydroxyl groups prior to the graft copolymerization.99 Figure 2.2 Graft-from route for the synthesis of lignin graft copolymers. Lignin is used as an alcohol macro-initiator. 2.1.2 Stereocomplexation of PLA in nanofibers In this chapter, stereocomplexation between PLA chain arms of lignin-graft-PLA in nanofibers is investigated. Thus, it is important to first understand how PLA homopolymers form stereocomplexes in electrospun nanofibers. To generate PLA-based nanofibers with enhanced mechanical properties, stereocomplexation was attempted in previous studies (Figure 2.3).112-115 The presence of the stereocomplex crystallites in nanofibers exhibit enhanced mechanical properties such as Young’s modulus and tensile strength. However, elongation at break was sacrificed in most cases, with some exceptions.116 Moreover, the stereocomplexation of PLA 21 enhanced heat resistance and induced different morphology and longer degradation period, which may be appealing for many applications.112 These distinct properties of the stereocomplex PLA nanofibers originate from a different crystal lattice from that of the homocrystallites, as introduced in Chapter 1. Figure 2.3 Electrospinning of PLLA and PDLA to form stereocomplex in nanofibers. Stereocomplexation in nanofibers can be achieved through blending PLLA and PDLA in the solution or melt, thus allowing interdiffusion between PLLA and PDLA. Tsuji and co-workers first attempted the electrospinning of PLLA/PDLA blend for generating nanofibers with stereocomplex crystals.117 The authors suggested that high voltage applied during the electrospinning can act as an “electrically induced high shearing force,” and therefore can cause the orientation of the polymer chains along the electric field to bring the PLLA and PDLA chains in close proximity to each other. The stereocomplexation in the as-spun nanofibers containing PDLA and PLLA was found in a number of previous studies. Srithep and co-workers successfully induced stereocomplexation with PLLA/PDLA blends through electrospinning, which was clearly proven by the stereocomplex crystal lattice peaks on the X-ray diffraction (XRD).118 In the study by Manolova and co-workers, O OOOnO OOOnPLLAPDLAElectrospinningstereocomplex22 the presence of polybutylene succinate in a PDLA-b-polybutylene succinate copolymer in the PLLA matrix induced stereocomplexation between the PDLA segment of the copolymer and the PLLA matrix at room temperature.119 The same peak was not visible for the nanofibers made of PLLA or 𝛼, 𝜔-dihydroxy PDLA/PLLA blend, highlighting the role of polybutylene succinate to allow stereocomplexation. Despite these reports, depending on the electrospinning solution and parameters, inducing the stereocomplexation in the nanofibers can be challenging. In many studies, neither the crystal lattice for homocrystallites nor stereocomplexes were observed, indicating the amorphous nature of the PLLA/PDLA blend in the as-spun nanofibers. This is attributed to the rapid evaporation of the solvent during electrospinning, causing insufficient time for the polymer molecular chains to arrange. Annealing at a temperature higher than the glass transition temperature was necessary to induce crystallization.96, 112, 113, 119-123 The impact of PLA tacticity and stereocomplexation on lignin-graft-PLA and related composite films has been studied.104, 108, 109 The resulting stereocomplexed polymers have a more dense chain packing compared to homocrystalline PLLA or PDLA. In the studies by Si, Dai, and co-workers and He and co-workers, the stereocomplexation or homocrystallization between the isotactic PLA chains of the graft copolymer and the linear PLLA matrix were observed, resulting in higher Young’s modulus and tensile strength of the films.104, 108, 109 Amorphous lignin-graft-PDLLA/PLLA nanofibers have been synthesized by Kai, Loh, and co-workers and showed good miscibility of the graft copolymer into the PLLA matrix. Although these materials showed enhanced antioxidant properties and cell proliferation performances, the nanofibers did not show enhanced mechanical properties, unlike the case of bulk materials.86 23 From these previous studies, lignin-graft-PLA copolymers has shown its potential for not only introducing various properties to the PLLA homopolymer, but also opening up the ways to utilize lignin by enhancing its miscibility, mechanical properties, and electrospinnability. In this chapter, lignin-grafts with PLLA and PDLA are synthesized to generate and study electrospun nanofibers of these materials. The effect of PLA tacticity and stereocomplexation on the mechanical properties of the nanofibers are investigated. In particular, the interaction between two graft copolymers is explored, which may have been overlooked in the previous studies where only the interaction between the graft copolymer and the matrix were investigated. Thus, lignin-graft-PLLA or -PDLA copolymers are used not as the compatibilizer in a matrix, but as the only polymer to synthesize the nanofibers. 2.2 Results and discussion 2.2.1 Synthesis of lignin-graft-PLAs Prior to the graft copolymerization, the number of hydroxyl groups of the organosolv lignin used in this study was determined through 31P{1H} NMR spectroscopy after phosphitylation with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) (Figure 2.4, Table 2.1). The quantity of each hydroxyl group with different functional groups was calculated following the previous literature, based on the peak integration on the spectra.124-127 The aliphatic hydroxyl groups were the most prominent, which were shown as the broad peaks in the region from 146 to 149 ppm. 24 Figure 2.4 31P{1H} NMR spectra (chloroform-d : pyridine 1.6:1 v/v, 25°C, 300MHz) of phosphitylated (a) organosolv lignin and (b) kraft lignin used in this study. 25 Table 2.1 Hydroxy group content of organosolv lignin and kraft lignin determined from 31P{1H} NMR spectra in Figure 2.4.a Entry d (ppm) Structure Assignment Organosolv (mmol/g)b Kraft (mmol/g)b 1 146-149 Aliphatic OH 3.85 (0.02) 3.14 (0.11) 2 140.5-145 Syringyl(S) or 5-substituted OH 1.45 (0.05) 2.04 (0.13) 3 139-140.5 Guaiacyl(G) OH 1.17 (0.01) 2.39 (0.16) 4 138 p-Hydroxy(H) OH 0.29 (0) 0.31 (0.02) 5 135 Carboxylic Acid 0.23 (0) 0.47 (0.01) Total 7.00 (0.08) 8.35 (0.43) aCalculated from the integration of each region on 31P{1H} NMR spectra of phosphitylated lignin based on an internal standard at 151.9 ppm (N-hydroxy-5-norbornene-2,3-dicarboximide). Reference peak 132.2 ppm (phosphitylated water). bHydroxyl (OH) group content. Standard errors from three runs are in parentheses. PO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acidPO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acidPO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acidPO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acidPO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acidPO OORPO OOOMeMeOPO OOOMePO OOPO OOROPO OOOMealiphatic S G H 5-substituted carboxylic acid26 Moreover, the peaks at 143 ppm on the spectrum of organosolv lignin indicate the presence of syringyl units, and therefore suggests that its source is likely to be a hardwood, whereas the absence of this peak on the spectrum of kraft lignin suggests that it is likely from a softwood, which is further supported by the 13C{1H} NMR spectra (Figure A1). Peaks from 139.0 to 140.5 ppm on the spectra suggest that the organosolv lignin has a smaller number of guaiacyl units, which agrees with the literature that hardwood lignin tends to have a lesser amount of guaiacyl units compared to the softwood lignin.24 The total mmol per gram of each organosolv and kraft lignin samples were calculated to be 7.00 mmol g–1 and 8.35 mmol g–1, respectively, which are in the reasonable range to the previously reported ranges.128 The source of lignin, whether from hardwood or softwood, can affect the lignin properties including solubility, which may affect the reaction conditions when going under a copolymerization. Also, the presence of sulfur in kraft lignin can be a limiting factor for a variety of applications. Throughout the experiments, the organosolv lignin was used and will be referred to as lignin for convenience. With the number of hydroxyl groups determined, the lignin was used as an alcohol macro-initiator in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), L-lactide (L-LA), and D-lactide (D-LA) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to synthesize a series of graft copolymers (Figure 2.5).129 Excess lignin was first reacted with DBU before the addition of lactide to ensure activation of the lignin hydroxyl groups and to minimize undesired reactions, such as epimerization of lactide and formation of cyclic PLA, as much as possible.53 All reactions reached full conversion after 24 h (Table 2.2). 27 Figure 2.5 Synthesis of lignin-graft-PLAs through ring-opening polymerization with DBU. Table 2.2 Graft copolymerization of lignin with rac-lactide (rac-LA), L-lactide (L-LA), and D-lactide (D-LA) using DBU.a Entry [LA]:[DBU] mol % lignin-OH [LA]: [OH]lignin conv. (%)b PLA segment Mn, theo (g mol–1)c PLA segment Mn (g mol–1)d Mn, GPC (g mol–1)e Mw, GPC (g mol–1)e D rac-LA 1 52 0 N/A 99 N/A N/A 31080 62010 2.0 2 52 2 49.6 99 3650 12642 193400 270400 1.4 3 52 7 12.4 97 1441 6929 175500 209400 1.2 4 52 14 6.2 98 797 4014 120600 129500 1.2 L-LA 5 52 0 N/A 98 N/A N/A 32530 55070 1.7 6 52 2 49.6 98 3650 15171 194400 299200 1.5 7 52 7 12.4 98 1441 6212 166100 194900 1.2 8 52 14 6.2 97 797 4002 111800 138600 1.3 D-LA 9 52 0 N/A 98 N/A N/A 30080 63370 2.1 10 52 2 49.6 97 3650 13726 195300 275900 1.4 11 52 7 12.4 97 1441 5318 173400 217100 1.3 12 52 14 6.2 98 797 3446 100200 133400 1.3 aReactions were carried out in DCM at 25 °C for 24 h. bLactide conversion determined by 1H NMR spectroscopy. cTheoretical segment MW = MLA ´ ([LA]/([OH]lig + [DBU])) dCalculated from the integration of the polymer and chain end methine protons multiplied by the molecular weight of LA. eMolecular weights were determined by triple-detector GPC (THF 3.5 mg mL–1, flow rate = 1.0 mL min–1, dn/dc = 0.050 mL g–1). OOOONND-lactideDBUOOOOL-lactideOOOHOOOOOOOHOOOOBlendStereocomplexLignin-graft-PLLALignin-graft-PDLALigninLigninOHLignin DCM, r.t., 24 hDCM, r.t., 24 h+NNDBUOHLignin +nnOOO OHOOO nOOOHOOOOnLigninLignin28 With the constant [LA]:[DBU] ratio, 1H NMR spectra of the graft copolymers showed a gradual increase in the intensities of the peaks that are characteristic of the chloroform-soluble portion of the pure lignin, with the increase in [OH]lignin (Figure 2.6). The formation of PLA is confirmed by the methine proton peaks appearing at 5.2 ppm and methyl proton peaks appearing at 1.5 ppm. Moreover, chain-end methine protons of PLA at 4.3 ppm increased in their intensity as a higher amount lignin was loaded into the reaction. This observation supports the trend of the theoretical PLA segment molecular weight, where shorter PLA chains are expected as the lignin loading is increased, decreasing the ratio of the polymer-to-chain-end methine proton peak intensities. Using a different LA isomer did not have a significant effect on the polymer and PLA segment molecular weight (Table 2.2). Figure 2.6 1H NMR spectra of lignin-graft-PLA copolymers with 14, 7, 2 mol% lignin OH (a-c), PLA homopolymer (d), and the pristine chloroform soluble portion of lignin (e) (chloroform-d, 25°C, 400MHz). 29 Study of the resulting grafts by gel permeation chromatography (GPC) showed bimodal peaks more frequently when the lignin concentration was low (Figure 2.7). With higher lignin concentrations, the dispersity of the graft copolymers decreased, supporting the study by Won and co-workers that as [OH]:[DBU] increases, the dispersity decreases by suppressing the formation of ketene aminal chain-end via acylation.53 However, the molecular weights of the graft copolymers calculated through the GPC data were difficult to control between each batch, likely due to the tendency of randomly shaped lignin to aggregate in many solvents including tetrahydrofuran (THF), as well as the star-shaped topology that may affect the elution time.98 30 Figure 2.7 GPC traces of lignin-graft-PLLA (a), lignin-graft-PDLA (b), and lignin-graft-PDLLA (c) copolymers with 2, 7, and 14 mol% lignin-OH. Following the same procedure introduced in Figure 2.4, the graft copolymers were phosphitylated with TMDP to compare the 31P{1H} NMR spectra to that of pure lignin (Figure 2.8). The spectra of the graft copolymers showed the presence of the peaks corresponding to the phosphitylated lignin hydroxyl groups. A significant decrease in the integration of these peaks indicates a decrease in the hydroxyl groups of lignin by undergoing ring-opening polymerization, and that only a small amount of lignin is incorporated in the copolymer. This is supported by the appearance of a peak at 146 ppm which indicates the phosphitylated chain-end secondary alcohol (a) (b) (c) 31 of PLA. Based on the internal standard, the integration of the chain-end alcohol peak of PLA has increased as more lignin was loaded in the reaction, indicating shorter PLA chains attached to the lignin, supporting the 1H NMR data. Figure 2.8 31P{1H} NMR spectra (chloroform-d : Pyridine 1.6:1 v/v, 25°C, 400MHz) of phosphitylated lignin-graft-PLLA copolymers with 14 (a), 7 (b), 2 (c) mol% lignin OH, and pristine lignin (d). To better observe how the OH groups of lignin are affected through copolymerization reactions, a significant excess of lignin was reacted to generate the graft copolymers following the same reaction procedures. Preferred reactivity of aliphatic OH groups was clearly shown through 31P{1H} NMR spectrum of the graft, indicated by the significant decrease in the intensities of the peaks in the region of 146 to 149 ppm (Figure 2.9). Specifically, the integration ratio of aliphatic OH to the sum of phenolic OH and carboxylic acid peaks has decreased from 1.23 to 0.75. This can be attributed to higher nucleophilicity of aliphatic OH groups, and therefore relatively more efficient activation of aliphatic OH groups compared to other phenolic OH groups or carboxylic acids. The 32 more efficient reactivity of the aliphatic OH groups of lignin was also shown in previous reports.99, 130 Figure 2.9 31P{1H} NMR spectra (chloroform-d : Pyridine 1.6:1 v/v, 25°C, 400MHz) of phosphitylated lignin (a) and lignin-graft-PDLLA copolymer with large excess of lignin (b). The average molecular weight of the PLA segment attached to OH groups of the lignin was significantly higher than that of the theoretical molecular weight. The slope of the plot of molecular weight vs. [OH]lignin for the theoretical PLA segment molecular weights and the experimental PLA segment molecular weights were compared (Figure 2.10). Compared to the theoretical segment molecular weights, the slope of the experimental molecular weights suggested only 29% of the hydroxyl groups of lignin went under the graft copolymerization. This explains why phosphitylated hydroxyl groups of lignin remain present in the 31P{1H} NMR spectra after copolymerization (Figure 2.8). PLA OHInternal StandardAliphatic OHPhosphitylatedwaterPhenolic OH Carboxylic Acid(a)(b)33 Figure 2.10 Experimental PLA segment MW (red circle) (NMR) vs mol % of lignin and theoretical segment MW (black square, Mn,theo = MLA ´ ([LA]/([OH]lignin + [DBU])) for graft copolymers. The difference in slope between the theoretical and experimental segment MWs was used to determine that 29% of the OH groups on lignin was activated during polymerization (corrected theoretical segment MW, blue triangle). Chain-end analysis was done with matrix-assisted laser desorption/ionization-time-of-flight (MALDI-ToF) mass spectrometry on the synthesized graft copolymers (Figure 2.11). The order of addition of reactants resulted in different polymer topology: cyclic PLA homopolymer or star-shaped lignin-graft-PLA. The addition of DBU to the suspension of LA and lignin in DCM yielded cyclic PLA and did not show any other significant peaks in the MALDI-ToF spectrum, suggesting the failure of lignin incorporation. Synthesis of cyclic PLA in a DBU-catalyzed reaction without an alcohol initiator has been known through the previous studies.53, 131 Reacting the lignin with DBU before the addition of LA resulted in a more complex MALDI-ToF spectrum, as expected of the complicated and inconsistent structure of lignin. 34 Figure 2.11 MALDI-ToF spectra of cyclic PLA (no lignin incorporated) (a), and lignin-graft-PDLLA (addition of LA to DBU and lignin) (b). The difference in the polymer topology may be attributed to the two different possible polymerization mechanisms. In the absence of lignin, DBU can undergo nucleophilic attack pathways, which then propagates to form cyclic PLA (Figure 2.12).131, 132 DBU and lignin first needed to be reacted without the presence of LA to incorporate lignin (Figure 2.13). Even then, the peaks corresponding to cyclic PLA were present as a minor species on the MALDI-ToF spectrum of lignin-graft-PLA which may be due to the reaction of DBU and LA before the complete activation of lignin, as well as possible transesterifications (Figure 2.11b). Won and co-35 workers suggested that cyclization is likely to occur when approaching or in excess DBU compared to the alcohol macro-initiator.53 Given that lignin is insoluble in DCM and has an undefined structure, the possibility of the nucleophilic attack pathway to yield cyclic PLA cannot be ruled out, even in excess lignin. Figure 2.12 Mechanism of polymerization of lactide with DBU in the absence of an alcohol initiator, through deprotonation of an acylated DBU to a ketene aminal (a) and through zwitterionic propagation (b).53 NN OOOONNOOOONNNNHOOOO NNHNNOOOO NNHOOOOnORNN OOOONNOOOONNOOOONNOOOOOOOOnDBUDBUDBU(a)(b)OOOOOOOOOOOOnOOOOOOOOOOOOnDBU lactideDBU lactidecyclic PLAcyclic PLA36 Figure 2.13 Expected mechanism of graft copolymerization of lignin and lactide with DBU, based on the proposed mechanism with polyethylene glycol as an alcohol-initiator.53 The tacticity of the graft copolymers was confirmed through 1H{1H} NMR spectroscopy (Figure 2.14). The difference in the chemical shifts of spectra of lignin-graft-PDLLA, lignin-graft-PLLA, and lignin-graft-PDLA, suggests different stereochemistry of the PLA chains on the lignin. Unlike the presence of sis, sii, iis, iii, and isi tetrads for the methine region of the 1H{1H} NMR spectrum of atactic lignin-graft-PDLLA, the presence of one major peak at 5.17 ppm for the iii tetrads for the lignin-graft-PLLA and lignin-graft-PDLA suggests that the isotactic PLA chains were formed, indicating no epimerization has occurred during the polymerization. NNLigninHO NNLigninOHNNLigninOH OOOONNOHO OOOOOOOLigninnH AcHO O OOOOOOOLigninnNNHAcCascade of DBU deactivation37 Figure 2.14 1H{1H} NMR spectra (chloroform-d, 25°C, 400MHz) of lignin-graft-PDLLA (a), lignin-graft-PLLA (b), lignin-graft-PDLA (c).133 This is in contrary to the cases of the polymerization in the absence of lignin, where the PLAs showed atactic nature regardless of LA isomer (Figure A2). This may be due to the epimerization of lactide with DBU at room temperature that has been reported in the previous studies.134 2.2.2 Characterization of lignin-graft-PLAs Thermogravimetric analysis (TGA) of the graft copolymers with different lignin content did not show any notable dependency of the onset degradation temperature or the inflection point on the concentration of lignin in the reaction (Figure 2.15, Table 2.3). However, the onset temperatures of all four graft copolymers were lower than that of the pure PLLA and were higher than that of (b) (c) (a) 38 the pure lignin. This may be due to the broader degradation temperature range of lignin compared to PLA. Moreover, the inflection temperatures were lower for the graft copolymers compared to pure PLLA or lignin, suggesting that the graft copolymers reach a maximum thermal degradation rate earlier than their pristine materials. This can be explained by the higher free volume resulting from the star-shaped geometry, resulting in a higher surface area affected by the heating. The average percentage of lignin present in the graft copolymers was determined through ash content analysis, considering the residual mass of 1.20% of pure linear PLLA homopolymer and 38.98% of pure lignin at 600 °C. As expected, more lignin was incorporated in the graft copolymers with an increase in lignin loading in the reaction. Figure 2.15 Thermogravimetric Analysis (TGA) curves of lignin-graft-PLLA with different lignin loading in the reaction. 39 Table 2.3 Onset and inflection temperatures and % residual mass of lignin-graft-PLLA depending on mol % lig-OH obtained from TGA. Entry lig-OH (mol%) Onset (ºC) Inflection (ºC) Residual Mass (%) % lignina 1 2 258 (17) 295 (9) 3 (1) 6 (3) 2 7 269 (17) 312 (6) 5 (1) 10 (3) 3 14 259 (1) 299 (4) 7 (0) 17 (1) 4 19 255 (11) 302 (4) 9 (0) 21 (1) 5b PLLA 330 357 1 0 6b Lignin 246 355 39 100 aCalculated based on residual masses of PLLA and Lignin on entry 5 and 6. Differential scanning calorimetry (DSC) was used to probe stereocomplex formation within lignin-graft-PLLA and lignin-graft-PDLA (Figure 2.16, Table 2.4). Before blending, lignin-graft-PLLA and lignin-graft-PDLA showed cold crystallization temperature (Tcc) and homocrystallite melting temperature (THC), suggesting the crystalline regions are present between the PLA segments of the graft copolymers. After blending a one-to-one ratio of lignin-graft-PLLA and lignin-graft-PDLA to form stereocomplexed SC-lignin-graft-PLA, the DSC curve showed one endothermic peak for stereocomplex melting temperature (TSC) at 192.9 °C. This melting temperature was significantly increased by 60 °C compared to pre-blending, showing the main characteristic of PLA stereocomplexation. The DSC data supports that the stereocomplexation between PLLA and PDLA chains on lignin-graft-PLLA and lignin-graft-PDLA were possible at the PLA arm length of 4000 g/mol. 40 Figure 2.16 DSC curves of lignin-graft-PLLA (a), lignin-graft-PDLA (b), and SC-lignin-graft-PLA (1:1 blend of lignin-graft-PLLA/lignin-graft-PDLA) (c). Both graft copolymers were synthesized at the same condition, with 14 mol% lignin-OH. Table 2.4 Glass transition and melting temperatures of the isotactic graft copolymers and their stereocomplex mixture. Entry Polymer Tg (ºC) Tcc (ºC) ∆ Hcc (J/g)a THC (ºC) ∆ HHC (J/g)a TSC (ºC) ∆ HSC (J/g)a 1 lignin-graft-PLLA 58 112 4 155 28 - - 2 lignin-graft-PDLA 57 110 2 155 29 - - 3 SC-lignin-graft-PLAb 51 91 2 - - 193 44 aDerived from the area under the curve. b1:1 blend of lignin-graft-PLLA/lignin-graft-PDLA. When compared to the pure linear PLLA, PDLA, and stereocomplex blend of the two homopolymers (SC-PLA), the three lignin-graft-PLA copolymers exhibited lower Tcc, THC, and TSC (Figure A3, Table A2). The star-shaped geometry of the lignin graft copolymers overall exhibited homocrystallization and stereocomplexation yet with a smaller extent compared to the linear PLA homopolymers. 41 2.2.3 Generation of non-woven nanofiber mats via electrospinning A variety of parameters, including the spinning dope solution and instrumental settings, were tested to optimize the fiber formation through electrospinning. To make the spinning dope solution, lignin-graft-PLLAs and lignin-graft-PDLAs with different lignin ratios were tested in N,N-dimethylformamide (DMF) and dichloromethane (DCM). The graft copolymers with lignin content higher than 21% could not form any fibers, suggesting the star-shaped molecular shape of the graft copolymer and short PLA segments does not provide enough chain entanglement on the compact lignin structure, which results in poor fiber spinnability. Thus, among different lignin ratios, graft copolymers generated with 14 mol% OH-lignin having PLA segment molecular weights of 4000 g/mol were used for electrospinning. After adjusting the conditions, the parameters were optimized and were kept consistent for all nanofibers generated (Table 2.5). Table 2.5 Optimized parameters for electrospinning of the graft copolymers. Entry Parameter 1 Solid Content (%)a 23 2 Distance (cm)b 18 3 Target Speed (m/min)c 0.50 4 Traverse Speed (cm/min)d 0.00 5 Syringe Pump Rate (mm/min) 0.085 6 Voltage (kV) 12 7 Needle Gauge (G) 18 8 Syringe Size (mL) 6 9 Temperature (°C) ~25 10 Relative Humidity (%) 22 to 30 11 Solvent DCM:DMF 3:1 aWeight percent of polymer with respect to polymer mass + solvent mass. bFrom the needle tip to the target collector surface. cRotating speed of target drum. dTraverse speed of the targe drum. Two different nanofiber spinning solutions were made using lignin-graft-PLLA (NF1) and lignin-graft-PDLA (NF2). The third solution for inducing stereocomplexation (SC-lignin-graft-PLA) (NF3) was made by mixing the 1:1 v/v ratio of each lignin-graft-PLLA and lignin-graft-42 PDLA solutions. The areal densities of the three nanofiber mats were kept in the range of 5 to 6 g/m2 by tracking the areal density over electrospinning time, and the nanofibers were dried under vacuum for 48 hours after spinning. 2.2.4 Effect of tacticity of lignin-graft-PLAs on nanofiber morphology The morphology of the NF1, NF2, and NF3 nanofibers was investigated with scanning electron microscopy (SEM) (Figure 2.17). The nanofibers were bead-less and showed random orientation for all three samples. Furthermore, physical cross-linking, also known as inter-fiber fusing, of the nanofibers was observed for all three nanofiber mats.135 However, notably higher degree of the inter-fiber fusion was observed for NF3 compared to NF1 or NF2, making the nanofibers closer in proximity to each other and compressing down the nanofiber mat into a flatter structure (more evident in Figure A9). 43 Figure 2.17 20X and 5000X magnified images of NF1, NF2, and NF3 taken from optical microscope (left) and SEM (right). Fiber diameter distributions of each of the three nanofibers were calculated from the SEM images using the ImageJ software. A hundred nanofibers were sampled from SEM images of four different regions of each nanofiber mat, as the fibers deposited on the different regions of the mat may exhibit different morphologies. The difference between the average fiber diameters of three nanofiber mats were not significant by analysis of variation (Table 2.6). Nevertheless, when the lignin-graft-PLLA and lignin-graft-PDLA were blended one-to-one to produce the SC-lignin-graft-PLA nanofiber mat (NF3), the diameter distribution was clearly different from that of NF1 44 or NF2, with the number of fibers being more frequent at smaller diameters (see Figure 3.4 in the next chapter). The greater number of smaller fibers in NF3 can be attributed to the denser crystal packing of the polymer chains in stereocomplexes compared to the homocrystallites. This observation is in contradictory to the relationship in Equation 1.3 in Chapter 1, where larger fiber diameter is expected from higher crystallinity. However, the equation was derived from the study where polybutadiene and polystyrene were used to differentiate the crystallinity of the nanofibers, and the difference of molecular structures has not been considered in the equation. Thus, the effect of stereocomplexation of graft copolymers on fiber diameter may be different. Table 2.6 Average diameters of nanofibers on three graft copolymer nanofiber mats. Entry Nanofibers Average Diameter (nm) 1 NF1 381 (51) 2 NF2 325 (43) 3 NF3 307 (55) Standard errors in parenthesis, with 95% confidence interval (p = 0.05). 2.2.5 Effect of tacticity of lignin-graft-PLAs on thermal and mechanical properties of nanofiber mat TGA curves of the nanofibers were compared to that of pristine PLA and lignin (Figure 2.18, Table 2.7). NF1 and NF2 did not show identical thermal degradation profiles to each other, which may be due to inconsistency of the graft copolymer reactions in each batch and their non-uniform fiber deposition consequently. The difference in the fiber spinnability of the two copolymers was reflected in the onset temperature, inflection temperature, and % residual mass, yet they were not significant. All three nanofibers showed similar lignin content, as expected from the same amount of lignin loading in the polymerization reactions. The three nanofiber mats showed clearly 45 different thermal degradation behaviours compared to that of pristine PLLA or lignin. The residual mass on the nanofibers higher than that of pristine PLA indicates that the lignin is incorporated into the nanofibers. Figure 2.18 TGA curves of nanofiber mats NF1, NF2, and NF3 compared to pristine linear PLLA and lignin. Table 2.7 Onset and inflection temperatures, residual mass, and % lignin content of the three graft copolymer nanofibers compared to pristine PLA and lignin derived from TGA thermographs. Entry Polymer Onset (ºC) Inflection (ºC) Residual Mass (%) wt% lignin 1 NF1 252 (16) 307 (26) 4 (1) 8 (3) 2 NF2 260 (15) 331 (9) 6 (1) 13 (2) 3 NF3 238 (5) 294 (4) 4 (1) 8 (3) 4 PLLA 330 357 1 0 5 Lignin 246 355 39 100 Standard errors with 95% confidence interval in parenthesis (p = 0.05). DSC thermograms of the nanofibers clearly showed the effect of stereocomplexation (Figure 2.19, Table 2.8). NF1 and NF2 showed Tm (or THC) at 144 and 148 °C, respectively, where NF3 showed significantly increased Tm (or TSC) of 192 °C, indicating stereocomplexation. This 46 indicates that the electrospun nanofibers of the stereocomplexed graft copolymers exhibit similar phase transition behaviors to that of the bulk. On the other hand, the nanofibers NF1 and NF2 have lower melting temperatures compared to the bulk material, indicating the weaker packing of the homocrystallites after electrospinning. Figure 2.19 DSC thermograms of NF1, NF2, and NF3 nanofibers. Table 2.8 Glass transition and melting temperatures of the nanofibers derived from DSC thermograms. Entry Nanofibers Tg (ºC) Tcc (ºC) ∆Hcc (J/g)a THC (ºC) ∆HHC (J/g)a TSC (ºC) ∆HSC (J/g)a 1 NF1 56 98 15 148 37 - - 2 NF2 53 96 8 144 32 - - 3 NF3 56 98 15 - - 192 29 aDerived from the area under the curve. Monticelli and co-workers suggested that the DSC thermograms can significantly overestimate the homocrystallite and stereocomplex formation of PLA, since the fast heating rate may induce rearrangement of the polymer chains during the scanning.122 The appearance of stereocomplex melting temperature, TSC, on DSC thermograms even before the stereocomplexation in nanofibers 47 was also shown in other studies where no crystal lattice was observed on wide angle X-ray scattering (WAXS) or X-ray diffraction (XRD) analyses.96, 120 Thus, it is necessary to do a further analysis using either of these techniques. The three nanofibers NF1, NF2, and NF3 were characterized with XRD to investigate the presence of crystalline phases in the nanofibers without heating (Figure 2.20). Unlike many previous studies on the stereocomplexation in nanofibers where annealing was crucial to induce homocrystallization and stereocomplexation, the crystal lattice peaks were present in all three as-spun lignin-graft-PLA nanofibers.121 The peak at 16.7 ° was clearly present for both the NF1 and NF2 nanofibers, indicating the 200/110 plane of the 𝛼 crystal phase of isotactic PLA, exhibiting pseudo-orthorhombic unit cell with dimensions of a=1.06 nm, b=0.595 nm, and c=2.78 nm. Furthermore, the peaks at 11.9 ° and 20.7 ° were observed for NF3, indicating the 110 and 300/030 plane reflections of the PLA stereocomplex crystal lattice, confirming the successful crystallization of star-shaped lignin graft copolymers under the electrospinning conditions involving rapid evaporation of the solvent.112 48 Figure 2.20 XRD spectra of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3) nanofibers. NF3 showed significantly different tensile properties compared to that of NF1 and NF2 (Figure 2.21, Table 2.9). Stereocomplexation of the graft copolymers in NF3 enhanced Young’s modulus. The yield strength has also increased, indicating the stereocomplexation resulted in higher stiffness and better ability to withstand the tension before going under a plastic deformation. Yield strain did not show any significant change, meaning the plastic deformation starts beyond the same amount of elongation of the nanofiber mats. 49 Figure 2.21 Average stress vs. strain curve of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3) nanofiber mats. Table 2.9 Mechanical properties of lignin-graft-PLA nanofibers obtained from the uniaxial tensile testing. Entry Nanofiber Young’s Modulus (MPa)a Yield Strength (MPa)b Yield Strain (%)c Ultimate Tensile Strength (MPa)d Elongation at Break (%)e 1 NF1 212 (69) 3.3 (0.9) 1.9 (0.6) 3.6 (1.0) 6.5 (2.7) 2 NF2 298 (56) 3.2 (1.3) 1.4 (0.4) 3.7 (1.4) 6.7 (2.8) 3 NF3 469 (191) 6.2 (2.4) 1.6 (0.4) 6.6 (2.4) 3.8 (0.9) a Derived by the slope of the elastic region on the average stress-strain curve. b, c Derived from the highest stress point of the elastic region on the stress-strain curve. d The maximum stress achieved. e Strain at the breakage point. The ultimate tensile stress of NF3 is higher than that of NF1 or NF2. On the other hand, elongation at break has decreased significantly, suggesting that the SC-lignin-graft-PLA nanofiber mat is much more prone to breakage after elongation, and therefore is less stretchable. A decrease in the elongation at break for the SC-PLA (stereocomplex of linear PLLA and PDLA homopolymers) was shown in the previous study by Yu and co-workers.136 The brittle nature of the NF3 can be therefore explained by the effect of stereocomplexation between PDLA and PLLA 50 segments on the graft copolymer which results in stronger polymer packing. Moreover, as the physical cross-linking of the nanofibers is known to improve tensile strength with the decrease in elongation at break, the high degree of inter-fiber fusion of the nanofibers in NF3 may have also played a role in changing the mechanical properties. In conclusion, the stereocomplexation of the graft copolymers has introduced different nanofiber morphology and crystal structure, which together resulted in different tensile behavior from that of homocrystallites. 2.3 Conclusion A series of star-shaped lignin-graft-PLA copolymers was synthesized with L- and D-lactide. The copolymerization depended largely on the activation of lignin OH groups, to minimize the formation of cyclic PLA as the major species as well as epimerization. Definitive determination of [OH]lignin was an important factor in controlling the PLA segment molecular weight on the graft copolymers. The effect of stereocomplexation between lignin-graft-PLLA and lignin-graft-PDLA on the electrospun nanofibers was investigated. Even during the electrospinning process involving the rapid evaporation of solvent, stereocomplexation was confirmed through DSC and XRD analyses upon blending 1:1 ratio of lignin-graft-PLLA and lignin-graft-PDLA in solution. Under the same conditions, the nanofibers made of the stereocomplex blend of the graft copolymers had a greater number of smaller diameter fibers and a larger extent of inter-fiber fusion compared to lignin-graft-PLLA and lignin-graft-PDLA nanofibers. 51 The mechanical properties of the nanofibers were tested through the uniaxial tensile test. Stereocomplexation of the graft copolymers resulted in improved Young’s modulus and tensile strength, yet decreased the elongation at break, which comes from the combined effect of the change in morphology and crystal structure. In conclusion, lignin-graft-PLA copolymers with isotactic chains with PLA arm length of 4000 g/mol formed stereocomplexes in nanofibers via simple solution blending to tune the morphology and properties of electrospun nanofibers. Introducing the different crystal structures of PLA was a simple way to improve lignin-based nanofibers’ properties. 2.1 Experimental General Methods. All the air and moisture sensitive reactions were carried out in a Vacuum Atmosphere glovebox. NMR spectra were recorded on Bruker Avance 300 MHz and 400 MHz spectrometers. 1H NMR chemical shifts are reported in ppm versus the residual protons of CDCl3 δ 7.27. 13C{1H} NMR chemical shifts are reported in ppm versus the residual 13C of d6-DMSO δ 39.52. 31P{1H} NMR chemical shifts are reported in ppm versus 31P of the residual phosphitylated water δ 132.2. Average molar mass (Mn) and dispersity (Ð) were determined by a Malvern Panalytical OMNISEC multi-detector GPC/SEC system with light scattering, differential refractive index, viscometer, and diode-array-based UV/Vis detectors with THF as eluent. A flow rate was 1.0 mL/min and the column temperature was 25 °C. Polymer samples were dissolved in tetrahydrofuran (ca. 1.5 mg/1 mL) and filtered prior to the measurements. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometric analysis was performed on a Bruker Autoflex MALDI-TOF equipped with a nitrogen laser (337 nm). The accelerating 52 potential of the Bruker instrument was 19.5 kV. The polymer samples were dissolved in dichloromethane. Sodium trifluoroacetate in dichloromethane was used as the cationization agent. 2,5-Dihydroxybenzoic acid (DHB) in DCM was used as the matrix. Thermogravimetric analysis (TGA) data were obtained with NETZSCH TG 209 F1 Libra with Al2O3 crucible. The polymers were heated at a rate of 10K/min, under nitrogen with a flow rate of 20 mL/min. The temperature range was 30 to 600 ºC, and the degradation temperatures were directly calculated from the thermographs. Differential scanning calorimetry (DSC) data were obtained with NETZSCH DSC 214. The polymers were heated at a rate of 10K/min, under nitrogen with a flow rate of 20mL/min. The temperature range was 30 to 250 ºC, and Tg, Tc, Tm, and ∆H values were directly calculated from the thermographs. Electrospinning was performed using Kato-Tech Co. Ltd. NEU Nanofiber Electrospinning Unit, equipped with syringe pump, roller movement, and a voltage generator. The viscosity values for the polymers in DCM:DMF 3:1 solvent system was obtained by SEKONIK torsional oscillation-type viscometer. The concentration was increased by addition of polymer into the solution followed by thorough mixing before the measurement. Morphology of the electrospun nanofibrous mats was analyzed by Hitachi S3000N VP-SEM scanning electron microscope, using secondary electron detector. The nanofibers were sputter coated with gold using Edwards S150A Sputter Coater prior to imaging. Accelerating voltage was 10.00 kV. X-ray diffraction (XRD) analysis were carried out on a Bruker D8-Advance X-ray diffractometer equipped with Cu Kα1 and Kα2 radiation. Bragg-Brentano configuration and LynxEye silicon strip detector were used for the measurement. Slits were 1mm divergent, 8 mm anti-scatter, and 2.5º soller. The mechanical properties of the nanofibers were evaluated with KES-G1, Kato-Tech Co. Ltd. multipurpose micro-tensile tester. 5kg capacity force transducer was used. Nanofiber mats were cut into strips of 0.3 cm in width × 3 cm in length and were tested 10 times throughout different regions of each 53 mat. The deformation rate was set at 0.02 cm/s. The areal density was calculated for each sample. Stress-strain curve were plotted after conversion of obtained data according to the following equations.58 Engineering stress can be calculated by: σ#$%&'('& 2 )*%+3 = ,-". ())/3'.*4 (55)67%"8 9%:#'*; ()/5!) , tex = g/km Load(g) = Load cell setting × ∆V Areal Density 2 gm!3 = Sample Mass (g)Length (m) ×Width (m) 𝛔𝐄𝐧𝐠 (𝐌𝐏𝐚) = σ#$%&'('& 2 gtex3 × 9.81 × ρ$-8;5%7( gcm?) Strain can be calculated by: Deformation (cm) = Deformation rate × ∆t 𝐒𝐭𝐫𝐚𝐢𝐧 2𝐜𝐦𝐜𝐦3 = DeformationSpecimen Length Materials. DCM was purchased from Sigma Aldrich and were dried over CaH2, degassed through three freeze-pump-thaw cycles, and distillated under vacuum before use. CDCl3 was purchased from Cambridge Isotope Laboratories Inc. and used as received. MeOH, DMF, THF, acetic acid, TMDP, and DBU were purchased from Sigma Aldrich and was used as received. Indulin AT kraft lignin and organosolv lignin were provided by IngevityTM and Advanced BioCarbon 3D, respectively, and were dried at 50 °C under vacuum for 48 hours prior to use. Racemic, L-, D-lactide were purchased from Sigma Aldrich and/or provided by PURAC America Inc. and were recrystallized three times from hot dry toluene prior to use. Phosphitylation of lignin for Quantitative 31P{1H} NMR spectroscopic analysis. Phosphitylation of lignin and a series of lignin-graft-PLA copolymers were achieved according to 54 the previously reported method.33, 137 Chloroform-d (CDCl3) and pyridine (1.6:1.0 v/v) was used as a solvent mixture and was dried over molecular sieves prior to use. Relaxation reagent (chromium(III) acetylacetonate) and internal standard (N-hydroxy-5-norbornene-2,3-dicarboximide) were dissolved into the solution with a concentration of 5.6 and 9.0 mg/mL, respectively. 20 mg of dried polymer was dissolved in 400 μL solvent mixture, followed by the addition of 100 μL and 40 μL or internal standard and relaxation reagent, respectively. 50 μL of 2-chloro-4,4,5,5-tetramethyl-1,2,3-dioxaphospholane (TMDP) was added into the solution, mixed thoroughly for immediate NMR analysis. Representative graft-from synthesis of lignin-graft-PLA copolymers using DBU. Under an N2 atmosphere, a vial equipped with a magnetic stir bar was charged with lignin ([OH]lig = 7.0 mmol g–1). Lignin was suspended in 3 mL of dry DCM. While stirring, DBU (10 μL, 0.067 mmol) was added and was stirred for 5 minutes. The solution of rac-, L-, or D-lactide (500 mg, 3.47 mmol) in 2 mL dry DCM was added dropwise, and resulting mixture was stirred for 24 h at room temperature under N2. Conversion was determined using 1H NMR spectroscopy. The reaction was quenched with acetic acid (5 μL, 0.087 mmol) and was filtered through a 2 μm syringe filter. The collected filtrate was concentrated under reduced pressure. Polymers were then precipitated with excess cold methanol. The extraction was repeated at least three times to remove residual species, and dried under vacuum for 48 h to yield light to dark brown solid. Synthesis of linear PLLA using Sn(oct)2. A vial was equipped with a magnetic stir bar and charged with L- or D-lactide (1.0 g, 5 mmol) and stirred in 5 mL of toluene. Sn(oct)2 (2 μL, 0.02 mmol) was added and the volume made up to 15 mL. The reaction was stirred at 90 °C for 72 h under N2. Conversion was determined using 1H NMR spectroscopy. Solvent was removed under reduced pressure and the polymer was redissolved in minimal amount of DCM. The polymer was 55 then precipitated with excess cold methanol for at least three times to remove residual species and dried under vacuum for 48 h to yield white solid. Solution blending of isotactic lignin-graft-PLA copolymers for electrospinning. The two graft copolymers, lignin-graft-PLLA (3.6 g, Mn = 111800 g mol–1) and lignin-graft-PDLA (3.6g, Mn = 100200 g mol–1), were dissolved separately in 10 mL 3:1 v/v mixture of DCM:DMF. The copolymers were stirred for 1 h to yield clear brown solutions. Then, the equal volume of each of the two solutions were combined in a vial equipped with a magnetic stir bar which was immediately capped to prevent the evaporation of the solvent. After vigorous stirring for 5 minutes, the resulting blend solution was transferred into a syringe for electrospinning. 56 Chapter 3: Application of non-woven lignin-graft-PLA nanofibers in aerosol filtration. 3.1 Introduction Airborne particles with diameters smaller than 2.5 µm (PM2.5) are harmful to us as they can easily penetrate into the human respiratory system to cause serious diseases.138, 139 Especially during the pandemic, the aerosols that contain viruses are the key targets to remove via filtration to prevent infections. The respiratory particles are generated through air reopening mechanism, physical vibrations, and dispersion of saliva between the epiglottis and lips.2, 140 These particles are first emitted as droplets and are dried rapidly into aerosols in the size range of 0.01 µm to 1 mm with overlapping modes at 1 to 10 µm and 100 to 200 µm.2, 141 To develop an efficient filtration system to capture such small particles, and to reduce the environmental problems coming from the disposal of masks or filters, nanofibrous filters made of sustainable resources are gaining attention.142 3.1.1 Nanofibers for filtration Nanofibers provide one of the most efficient filtration systems as they exhibit a high surface-area-to-volume ratio providing lightweight and high porosity. Being close to the size of the aerosols, nanofibers have small pore sizes, providing a high chance of interaction with the particles, and can retain the particles after filtration.138, 143 Generally, bigger fiber diameter leads to larger average pore size and higher porosity.143 The filtration ability of a material can be expressed through three parameters: filtration efficiency, pressure drop, and quality factor which incorporates both filtration efficiency and 57 pressure drop. Filtration efficiency can be derived from the penetration (P), where Nin and Nout are the concentration of the particles inside the mask after the filtration and the concentration of particles in the non-filtered air outside the mask, respectively (Figure 3.1, Equation 3.1).2 Figure 3.1 Concentration of particles outside and inside the mask after filtration through nanofibers. P = @"#@$%& (3.1) η = 1 − P (3.2) The filtration efficiency (𝜂) is used to estimate how many particles are captured by the filter (Equation 3.2). For instance, the National Institute for Occupational Safety and Health (NIOSH) standardized N95 masks blocks greater than 95% of the particles coming through the filter, expressed with either 𝜂 > 0.95 or P < 0.05.2 The filtration efficiency of 80% for PM2.5 (particles smaller than 2.5 µm) were sufficient to bring the indoor air pollution to a satisfactory level.144 While the greater filtration efficiency is desired, the high blockage of the particles may mean the high blockage of the air molecules at the same time, resulting in a higher pressure drop. Smaller fiber diameter can enhance the filtration efficiency due to smaller pore sizes but can lead to a higher pressure drop consequently. The relationship between the filtration efficiency and the pressure drop depends on many factors other than the pore size, such as fiber morphology and packing density.143, 145 Thus, the trade-off between the filtration efficiency and pressure drop is important to consider especially in face masks which require good breathability. 58 The effectiveness of the mask material is expressed using a quality factor (Q), which takes into consideration both the filtration efficiency (𝜂) and the pressure drop (∆𝑝) (Equation 3.3). The higher filtration efficiency and lower pressure drop increase the quality factor. Q = − 8:(A)∆$ = − 8:(CDE)∆$ (3.3) Particles are filtered through three general mechanisms: diffusion, interception, and inertial impaction (Figure 3.2). The filtration mechanisms depend on the particle size. For the particles with diameters below 300 nm, the diffusion mechanism predominates. The diffusion comes from the Brownian motion where the fibers interact with the particles during their random movement. The particles with diameters above 300 nm are filtered through interception or inertial impaction, where interception is the interaction of the particle with the fiber along the airflow direction, and the inertial impaction occurs as the particle experiences a sudden change from the direction of the airflow.146 Figure 3.2 Different filtration mechanisms of particles with fibers. Face velocity can also impact the filtration mechanism (Equation 3.4). At a low face velocity, it allows more time for the aerosols to be removed by the electrostatic attraction or Brownian motion.2 At a higher face velocity, the effect from the inertial impaction is enhanced. Face velocity = "'7 (8-F('8*%7 #G7("&% "7%" (3.4) 59 Because of the different mechanisms for different particle sizes, there is a region where the particle size is too big for the diffusion mechanism and too small for the inertial impaction. Due to this reason, the particle diameter of ~300 nm is considered as the most penetrating particle size (MPPS), for which many previous studies have been targeting high filtration efficiency.2 However, because the number of the viruses inside the smaller aerosols also decreases, it is reasonable to shift the focus onto a bigger size range of 1 to 5 µm.2 To enhance the filtration efficiency without sacrificing the pressure drop, electrostatic charging of the filters can be applied to attract the particles through dipole-dipole interaction.2 Also, different nanofiber morphology can provide a higher quality factor by inducing the slip-flow effect, in which the nanofiber diameter is close to the mean free path of the air molecules. Kim and co-workers fabricated nanofibers with a branched morphology by adjusting the solution viscosity.138 By producing the branched morphology, the resulting single fiber diameters decreased to lower the pressure drop due to the slip-flow effect, still increasing the filtration efficiency by the area of the beads having a higher chance to capture particles. To develop a biopolymer-based filter, lignin-based nanofibers have been fabricated by Chang and co-workers, where its poor mechanical properties were the limitations to withstand the airflow during the filtration test.147 In this chapter, we aim to generate a sustainable air filter with a non-woven electrospun nanofiber mat made of lignin-graft-PLA copolymers. The enhanced mechanical properties of the lignin-based nanofibers through graft copolymerization with PLA, as shown in Chapter 2, can provide lignin-based nanofibers a wider usage in filtration applications. 60 3.2 Results and discussion 3.2.1 Characterization of morphology of lignin-graft-PLA nanofibers The fabricated non-woven lignin-graft-PLA nanofiber mats in Chapter 2, NF1, NF2, and NF3, were tested on their filtration performances as one of their potential applications to overcome the mechanical limitation of lignin nanofibrous filters. In addition, an atactic lignin-graft-PDLLA nanofiber mat with a smaller molecular weight was synthesized (NF4). For comparison, two nanofiber mats made of linear PLLA homopolymer (NF5) and 95% lignin/5% poly(ethylene oxide) (PEO) blend (NF6) were also synthesized (Table 3.1). Same instrumental parameters were used for all six nanofibers (Table 2.5), yet the solution concentration and solvent system were adjusted for NF5 and NF6 to fabricate bead-less nanofibers to minimize the number of variables. Table 3.1 Nonwoven nanofiber mat samples for filtration testing. Entry Sample number Material Solid content (%)a Concentration (g/mL) Solvent 1 NF1 lignin-graft-PLLA 23 0.36 3:1 DCM:DMF 2 NF2 lignin-graft-PDLA 23 0.36 3:1 DCM:DMF 3 NF3 SC-lignin-graft-PLAb 23 0.36 3:1 DCM:DMF 4 NF4 lignin-graft-PDLLAc 23 0.36 3:1 DCM:DMF 5 NF5 PLLA 12 0.16 3:1 DCM:DMF 6 NF6 Lignin 95%/ PEOd 5% 30 0.94 DMF aCalculated by weight % of polymer in the solution. b1:1 w/w blend of lignin-graft-PLLA and lignin-graft-PDLA. cMn = 80000 g/mol. dPEO Mn = 4000000 g/mol. Morphology of the nanofiber mats were characterized under scanning electron microscope (SEM) (Figure 3.3). Although all nanofiber mats were bead-less, they exhibited different nanofiber morphology. NF3 and NF4 showed a higher degree of inter-fiber fusion via physical crosslinking of the nanofibers, compared to NF1 and NF2. Also, NF6 made of 95% lignin showed a lot of broken fibers which may be due to 1) weak mechanical strength of lignin as the major component of the blend and 2) the use of different plasticizer (PEO) and solvent system (DMF) 61 which may have resulted in a change in the influence of the electrical field on the fiber stretching under the same instrumental parameters. Nanofibers NF1, NF2, and NF5 showed similar fiber morphology with continuous and bead-less nanofibers without breakage. The average fiber diameters of the six nanofiber mats were in the range from 300 nm to 1050 nm, where NF6 made of 95% lignin has a significantly larger fiber diameter of 1050 nm compared to the rest (Table 3.2, Figure 3.4). Figure 3.3 SEM images of nanofiber samples (NF1 to NF6) under 5000X magnification. 62 Table 3.2 Average diameter of nanofibers on nanofiber mats NF1 to NF6. Entry Sample Average diameter (𝛍m)a 1 NF1 0.38 (0.26) 2 NF2 0.33 (0.22) 3 NF3 0.31 (0.28) 4 NF4 0.30 (0.37) 5 NF5 0.41 (0.21) 6 NF6 1.05 (0.41) aThe averaged diameters of hundred nanofibers in four different regions on the mat. Standard errors in parenthesis at the confidence interval of 95%. 63 Figure 3.4 Diameter distribution of the nanofiber mats NF1 to NF6. Based on 100 nanofibers throughout the different regions up to 1000 nm for NF1 to NF5 and up to 2000 nm for NF6. NF1 NF2 NF3 NF4 NF5 NF6 64 The average surface pore size and the surface porosity of the six nanofiber mats were determined through the SEM images using the ImageJ software (Table 3.3). NF1 and NF2 made of lignin-graft-PLLA and lignin-graft-PDLA, respectively, had similar average surface pore size. After stereocomplexation of these two graft copolymers, NF3 made of SC-lignin-graft-PLA had a significantly smaller average surface pore size compared to those of NF1 and NF2. This can be attributed to the greater number of fibers with smaller diameter, resulted from the denser packing of the PLLA and PDLA chains on the graft copolymers (Figure 3.4). NF4 made of atactic lignin-graft-PDLLA showed an even smaller average surface pore size due to its greater number of smaller fibers coming from its lower molecular weight, compared to NF1 and NF2. NF5 and NF6 made of PLLA and 95% lignin, respectively, had larger surface pores compared to the graft copolymer nanofibers. The surface porosity of NF3 and NF4 were significantly smaller than that of NF5 and NF6. Table 3.3 Average surface pore size and surface porosity of the nanofiber mats NF1 to NF6. Entry Sample Average Surface Pore Size (𝛍m2) Surface Porosity (%)a 1 NF1 0.23 (0.06) 10 (3) 2 NF2 0.26 (0.09) 10 (3) 3 NF3 0.12 (0.02) 8 (1) 4 NF4 0.08 (0.01) 7 (1) 5 NF5 0.44 (0.05) 12 (1) 6 NF6 1.26 (0.57) 14 (6) aCalculated from the area of the pores / total area of the image. 3.2.2 Filtration test of non-woven lignin-graft-PLA nanofibers With the information of morphology of the nanofibers in hand, the filtration test was performed on the six nanofiber mats, with the identical procedure to the study by Rogak and co-workers (Figure 3.5).2 The nanofiber mats were first removed from the support material (polypropylene 65 microfibers) that was used to collect the nanofibers during the electrospinning and were cut into a circular shape with a diameter of 3.4 cm. Five measurements were done on each mat. The measurement was done with the nanofiber mat placed on top of the support material for security, yet the effect of the support material on the filtration efficiency and pressure drop was negligible. Due to its low mechanical properties, 95% lignin-based NF6 could not be removed from the support material after electrospinning, and therefore was measured without the removal of the support material. Figure 3.5 Experimental setup for the filtration test and pressure drop measurement using NaCl aerosols.2 The sodium chloride (NaCl) aerosol was first generated with a solution using the ultrasonic mesh nebulizer. The aerosol goes through the charge neutralizer followed by the two pathways: with the nanofiber mat (filter holder) and without (filter bypass). The pressure drop and size-dependent particle concentration were measured by a digital manometer and an optical particle 66 sizer, respectively. The flow rate was set by the downstream optical particle sizer, resulting in 4.9 cm/s face velocity to simulate normal to moderate breathing.2 This rate is lower than the NIOSH standard of 8-9 cm/s. However, the decrease in the face velocity tends to increase the effect of the electrostatic filtration and decreases the effect of the filtration through impaction. Thus, it is expected that the overall filtration efficiency of the particles would be conservative, at the particle size range of 0.3 to 3.2 µm used in the present work.2 The geometric particle size was converted into aerodynamic diameter based on the literature.2 The relationship between the surface pore size, the surface porosity, and the pressure drop is shown in Figure 3.6. An increase in the surface pore size resulted in a decrease in pressure drop, supporting the study by Ohzawa and co-workers, where the inverse proportionality between pressure drop and pore size was derived.148 The decrease in the pressure drop was also found with the increase in surface porosity (Figure 3.6b). Although the difference in the average diameter of NF1 to NF5 were not significant, similar trend was observed for its effect on pressure drop, between the five nanofiber mats and NF6 (Figure 3.6c).67 Figure 3.6 Pressure drop dependent on the average surface pore size (a), the surface porosity (b), and the average fiber diameter of the nanofiber mats (c). The effect of the average surface pore size, the surface porosity, and the average fiber diameter on the total filtration efficiency was plotted (Figure 3.7). A decrease in the filtration efficiency is expected with an increase in the average pore size, the porosity, or the average fiber diameter.143 This trend is clearly seen between NF6 and the rest. However, the other five nanofiber mats, NF1 to NF5, showed similar filtration efficiencies. This suggests that the surface pore size, surface (a) (b) (c) 68 porosity, and the average fiber diameter of the five nanofibers were not significantly different enough to make notable changes in filtration efficiency among the five nanofiber mats. Figure 3.7 Total filtration efficiency dependent on the average pore size (a), the porosity (b), and the average fiber diameter of the nanofiber mats (c). The particle aerodynamic diameter-dependent filtration efficiency of the six nanofiber mats was plotted (Figure 3.8). NF5 exhibited the highest filtration efficiency over the NaCl particle aerodynamic sizes, followed by NF1 and NF2. These three nanofiber mats showed filtration (a) (b) (c) 69 efficiency of higher than 83.2% over all particle sizes, showing great filtration performances at the low areal density of 5-6 g/m2, being suitable for providing sufficient indoor air quality.144 NF3 and NF4 also showed good average filtration efficiency of higher than 70.9% for all particle sizes. NF6, on the other hand, could achieve only 10.8 to 52.2% average filtration efficiency at the same areal density likely due to its large pore size. Overall, the lignin-graft-PLA nanofibers showed good filtration efficiencies close to that of linear PLLA homopolymer nanofibers NF5. However, a large variation in filtration efficiency was shown between the measurements for NF3 (Table A6). Figure 3.8 Filtration efficiency of the nanofiber mats NF1 to NF6 depending on NaCl aerosol aerodynamic diameter. Values reported in Table A6. The nonuniformity of NF3 can be explained by its brittle nature, where the nanofibers are broken upon elongation of only 3.8% resulting in defects as the nanofibers experience slight stretching while being removed from the support material (Figure 3.9, Table 3.4). Defects were also observed for NF4 and NF6 due to their brittleness, limiting their filtration efficiency to reach higher values (Figure 3.10). Thus, a material with ideal mechanical properties for mask will require 70 not only high tensile strengths, but also higher elongation at break. Another possibility is that the high degree of inter-fiber fusion of NF3 and NF4 may have limited their filtration efficiencies, as the flat structure of the nanofibers decreases the residence time of the particles lowering the chance of filtration by diffusion.149, 150 Figure 3.9 Average stress vs. strain curve of lignin-graft-PLLA (NF1), lignin-graft-PDLA (NF2), and SC-lignin-graft-PLA (NF3), lignin-graft-PDLLA (NF4), and PLLA (NF5) nanofiber mats. Table 3.4 Mechanical properties of the six nanofiber mats, derived from stress-strain curve. Entry Nanofiber Young’s Modulus (MPa)a Yield Strength (MPa)b Yield Strain (%)c Ultimate Tensile Strength (MPa)d Elongation at Break (%)e 1 NF1 212 (69) 3.3 (0.9) 1.9 (0.6) 3.6 (1.0) 6.5 (2.7) 2 NF2 298 (56) 3.2 (1.3) 1.4 (0.4) 3.7 (1.4) 6.7 (2.8) 3 NF3 469 (191) 6.2 (2.4) 1.6 (0.4) 6.6 (2.4) 3.8 (0.9) 4 NF4 665 (380) 7.5 (3.1) 1.4 (0.6) 8.3 (3.3) 2.8 (0.1) 5 NF5 66 (18) 1.9 (0.4) 2.9 (0.8) 3.2 (0.6) 20.5 (4.8) 6 NF6f - - - - - aDerived by the slope of the elastic region on the average stress-strain curve. b,cDerived from the highest stress point of the elastic region on the stress-strain curve. dThe maximum stress achieved. eStrain at the breakage point. fNF6 lacked mechanical properties to be handled for tensile testing. 71 Figure 3.10 Morphology of different regions of nanofiber mats with defects (red circles) and non-uniformity of NF3 and NF4 at 1000X magnification (top), and NF6 at 200X magnification (bottom). The total filtration efficiency was plotted with the pressure drop for the six nanofiber mats (Figure 3.11, Table 3.5). The highest quality factor was achieved for NF5, with 242 kPa–1 as expected of high filtration efficiency and low pressure drop. Also, a high quality factor of 87 kPa–1 was calculated for NF6, which is attributed to the extremely low pressure drop of 2 Pa, although the filtration efficiency was only 16 %. NF1 and NF2 had quality factors of 39 and 65 kPa–1, which are in the common range of electrospun non-woven nanofibrous filters that have been reported previously, suggesting that the lignin-graft-PLLA or lignin-graft-PDLA nanofibers have reached the same level of filtration performance even with the incorporation of lignin.83 The stereocomplex NF3 NF4 NF6 72 and atactic nanofibers NF3 and NF4, however, had the lowest quality factors, due to higher pressure drop compared to the other nanofibers. Figure 3.11 The total filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop. Quality factor isolines are labeled in kPa-1. Table 3.5 The total filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop. Entry Nanofibers Filtration efficiency (%) Pressure drop (Pa) Quality factor (kPa–1)a 1 NF1 89 (9) 55 (7) 39 (25) 2 NF2 86 (7) 34 (10) 65 (20) 3 NF3 76 (23) 103 (31) 16 (17) 4 NF4 80 (7) 161 (28) 10 (3) 5 NF5 91 (7) 13 (6) 242 (55) 6 NF6 16 (8) 2 (1) 87 (47) aDue to a large variability between the runs, the quality factors were calculated by the points’ best fit to quality factor isolines from equation Q = -ln(1- η)/∆p. The standard deviations with confidence interval of 95% are in parenthesis. The filtration efficiency for 2.75 µm aerodynamic diameter NaCl particles was plotted versus pressure drop, for a direct comparison to the study by Rogak and co-workers (Table 3.6, Figure 3.12).2 The highest quality factor of 420 kPa–1 was achieved for NF5, with the filtration efficiency of 98% at the low pressure drop of 13 kPa–1. NF6 also had a high quality factor due to its low 73 pressure drop of 2 Pa, despite low filtration efficiency of 38 %, suggesting that the quality factor can be overestimated at an extremely low pressure drop and cannot be used as the only parameter to evaluate the filtration performance. NF1 and NF2 could filter out above 91 % and 89 % of the particles at a low pressure drop of 55 to 34 Pa, respectively, showing excellent filtration performances. On the other hand, although NF3 and NF4 showed high filtration efficiency for 2.75 µm aerodynamic diameter NaCl particles, the quality factors were limited by their high pressure drop of 103 Pa and 161 Pa, respectively. Figure 3.12 The filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop for 2.75 𝛍m aerodynamic diameter NaCl aerosols. Quality factor isolines are labeled in kPa–1. 74 Table 3.6 The filtration efficiency of nanofibers NF1 to NF6 depending on the pressure drop for 2.75 𝛍m aerodynamic diameter NaCl aerosols. Entry Nanofibers Filtration efficiency (%) Pressure drop (Pa) Quality factor (kPa–1)a 1 NF1 91 (9) 55 (7) 42 (46) 2 NF2 89 (8) 34 (10) 71 (59) 3 NF3 82 (19) 103 (31) 21 (18) 4 NF4 89 (4) 161 (28) 15 (3) 5 NF5 98 (3) 13 (6) 420 (131) 6 NF6 38 (10) 2 (1) 237 (85) The standard deviations with a confidence interval of 95% are in parenthesis. a As the samples have high variability in quality factors, the quality factor was calculated from the best fit to the isolines of quality factors, Q = -ln(1- η)/∆p. 3.3 Conclusions The filtration performance of the bio-derived non-woven nanofiber mats fabricated with a series of lignin-graft-PLA copolymers with different tacticity and stereocomplexation were investigated. The nanofibers made with lignin-graft-PLLA and lignin-graft-PDLA copolymers showed total quality factors of greater than 58 and 94 kPa–1, respectively, showing great filtration performances with low pressure drop, making them promising candidates for sustainable air filters incorporating lignin. Moreover, stereocomplexation of the lignin-graft-PLLA and lignin-graft-PDLA in nanofibers resulted in higher pressure drop and lower filtration efficiency, due to a combination of multiple factors such as small fiber diameter, inter-fiber fusion, and defects. The brittle nature and flat structure of the nanofiber mats were the important factors in limiting the filtration performances. Stereocomplex nanofibers may be useful for other applications that do not require stretching as they may provide higher rigidity. Compared to 95% lignin nanofibers, graft copolymerization of lignin with isotactic PLA allowed the incorporation of lignin in mechanically stronger filters that can be handled without other support materials. Although the 95% lignin nanofiber mat (NF6) showed a high quality factor, it may have been overestimated as there is a large variability at low pressure drop. Also, 75 the poor mechanical properties of lignin may still influence the filtration performance, when aiming for higher filtration efficiency. Also, the graft copolymer nanofibers showed significantly higher filtration efficiency, close to that of PLLA homopolymer at the same areal density, suggesting only a small amount of material is needed for the similar performance compared to 95% lignin nanofibers. 3.4 Experimental General Methods. All the air and moisture sensitive reactions were carried out in a Vacuum Atmosphere glovebox. Electrospinning was performed using Kato-Tech Co. Ltd. NEU Nanofiber Electrospinning Unit, equipped with syringe pump, roller movement, and a voltage generator. Morphology of the electrospun nanofibrous mats was analyzed by Hitachi S3000N VP-SEM scanning electron microscope, using secondary electron detector. The nanofibers were sputter coated with gold using Edwards S150A Sputter Coater prior to imaging. Accelerating voltage was 10.00 kV. The mechanical properties of the nanofibers were evaluated with KES-G1, Kato-Tech Co. Ltd. multipurpose micro-tensile tester. 5kg capacity force transducer was used. Nanofiber mats were cut into strips of 0.3 cm in width × 3 cm in length and were tested 10 times throughout different regions of each mat. The deformation rate was set at 0.02 cm/s. The areal density was calculated for each sample. Stress-strain curve were plotted after conversion of obtained data according to the following equations.58 Engineering stress can be calculated by: σ'()*+,+* % gtex* = Load (g)/Width (mm)Areal Density (g/m-) Load(g) = Load cell setting × ∆V 76 Areal Density % gm-* = Sample Mass (g)Length (m) ×Width (m) 𝛔𝐄𝐧𝐠 (𝐌𝐏𝐚) = σ'()*+,+* % gtex* × 9.81 × ρ(1234)5( gcm6) Strain can be calculated by: Deformation (cm) = Deformation rate × ∆t 𝐒𝐭𝐫𝐚𝐢𝐧 %𝐜𝐦𝐜𝐦* = DeformationSpecimen Length Materials. DCM was purchased from Sigma Aldrich and were dried over CaH2, degassed through three freeze-pump-thaw cycles, and distillated under vacuum before use. MeOH, DMF, acetic acid, and DBU were purchased from Sigma Aldrich and was used as received. Organosolv lignin was provided by Advanced BioCarbon 3D, and was dried at 50 °C under vacuum for 48 hours prior to use. Racemic, L-, D-lactide were purchased from Sigma Aldrich and/or provided by PURAC America Inc. and were recrystallized three times from hot dry toluene prior to use. Linear PLLA was provided by NatureWorks® US (55400 g/mol, D = 1.62). PEO was purchased from Sigma Aldrich and was used as received. Polymerization of lignin-graft-PDLLA and electrospinning. Under a N2 atmosphere, a vial equipped with a magnetic stir bar was charged with lignin ([OH]lig = 7.0 mmol g–1). Lignin was suspended in 3 mL of dry DCM. While stirring, DBU (10 μL, 0.067 mmol) was added and was stirred for 5 minutes. The solution of rac-lactide (500 mg, 3.47 mmol) in 2 mL dry DCM was added dropwise, and resulting mixture was stirred for 24 h at room temperature under N2. Conversion was determined using 1H NMR spectroscopy. The reaction was quenched with acetic acid (5 μL, 0.087 mmol) and was filtered through a 2 μm syringe filter. The collected filtrate was concentrated under reduced pressure. Polymers were then precipitated with excess cold methanol. 77 The extraction was repeated at least three times to remove residual species, and dried under vacuum for 48 h to yield light to dark brown solid. The resulting polymer was dissolved in 3:1 DCM:DMF at 23% solid content and was transferred into a 6 mL syringe for electrospinning. Blending of lignin and PEO for electrospinning. In a dry scintillation vial (20 mL) charged with a stir bar, PEO was suspended in DMF. The mixture was heated at 50 °C for 1 h until PEO was completely dissolved into a clear solution. Pre-weighed lignin ([OH]lig = 7.0 mmol g–1) was added and was thoroughly mixed with vortex. The mixture was further stirred at 50 °C until homogeneous clear brown solution was obtained. The solution was transferred into a 6 mL syringe for electrospinning. Filtration testing of non-woven nanofiber mats. The nanofiber mats were cut into circular pieces (diameter = 3.4 cm, area = 0.0907 m2). Five measurements were done on each of the six nanofiber mats according to the study by Rogak and co-workers.2 The pressure drop was measured by a digital manometer (Fluke 922) and the particle detector by an optical particle sizer (TSI 3330), both measured with and without the filtration of aerosols. The sodium chloride aerosol was generated with a solution (20 g/L) using the ultrasonic mesh nebulizer (Sonar MedPro). The aerosol goes through the charge neutralizer and through the pathways: with nanofiber mat (filter holder) and without (filter bypass). The flow rate was set by the downstream optical particle sizer, resulting in 4.9 cm/s face velocity. Porosity and pore size measurement using ImageJ software. The threshold level of at least four SEM images with 5000X magnification on each nanofiber mat was adjusted. The area distribution and % area of pores were determined using the ‘analyze particles’ function. 78 Chapter 4: Conclusion and future directions The aim of this thesis was to modify the organosolv lignin into a more applicable material to be utilized in the fabrication of bio-derived and sustainable nanofibrous filters. To enhance and tune lignin’s mechanical properties, the concept of the tacticity of PLA was introduced in Chapter 2. The challenge of blending PLA and lignin was overcome by graft-copolymerization, and the homocrystallization and the stereocomplexation of the PLA segments of the graft copolymers were confirmed even in the electrospinning process involving a fast solvent evaporation. Furthermore, the different crystal structure of the PLA segments of the graft copolymers could change the tensile behaviour of the nanofibers. There have been many attempts to target the poor mechanical properties of lignin to date, and the idea of introducing the stereocomplexation between the two graft copolymers will add an important insight to expand the knowledge about the lignin-based nanofibers. I was interested to explore the filtration performances of the fabricated lignin-graft-PLA nanofiber mats as one of their possible applications. The role of PLA tacticity of the graft copolymers on the nanofiber morphology and the aerosol filtration performances were introduced in Chapter 3. The nanofiber mats made of lignin-graft-PLLA and lignin-graft-PDLA showed great filtration efficiency and low pressure drop, showing their potential as lignin-incorporating nanofibrous air filters. Although stereocomplexation of lignin-graft-PLLA and lignin-graft-PDLA in nanofiber mats resulted in defects upon elongation, having a small diameter and fused morphology can be still useful in other applications that do not require stretching of the fibers, such as window filters, anti-oxidant coatings, or cell proliferation applications, as they exhibit higher rigidity. 79 Further enhancement of the properties of these nanofibers may be possible by controlling the molecular weight of the PLA chain by selectively alkylating the lignin before the graft copolymerization. Also, changing the ratio of the lignin-graft-PLLA and lignin-graft-PDLA in the electrospinning solution to see their impact on the mechanical properties would be an interesting study to explore further, to balance the advantages brought by the homocrystallites and the stereocomplex. The diverse molecular structure is both an advantage and disadvantage of lignin. Because the properties of the nanofibers are highly dependent on the pristine materials, using different type or fraction of lignin will generate different nanofibers. If an application does not require sulfur-free material, using lignosulfonate, which tends to have the highest molecular weight of 5000 to 400000 g/mol compared to kraft lignin or organosolv lignin, may improve strength of the nanofibers through higher chain entanglement.151 Furthermore, study of piezoelectricity of the nanofibers synthesized with lignin-graft-PLA in the present work may be able to explain the filtration properties in more depth, as linear isotactic PLA homopolymer nanofibers generated through electrospinning are known for their good piezoelectric properties.83 By doing so may introduce the potential to apply the electrostatic mechanism in filtration for these lignin-based nanofibers. Si, Dai, and co-workers showed that PLLA/lignin-graft-PDLA films with stereocomplexation have great ultraviolet resistance and excellent biocompatibility.109 To the best of our knowledge, the electrospun nanofibers made with PLLA/lignin-graft-PDLA blend and their medical applications has not been reported to date. In combination with the lignin-graft-PLA nanofibers introduced in this thesis, electrospinning the PLLA/lignin-graft-PDLA blend into a nanofiber mat may introduce higher surface area-to-volume ratio, lightweight, and different morphology with 80 potential to enhance the biocompatibility. Also, the effect of using a cyclic PLLA matrix to blend with lignin-graft-PDLA to generate nanofibers can introduce different material properties from using linear PLLA matrix. The interaction between the linear PDLA segment on the graft copolymer and cyclic PLLA matrix can result in different stereocomplexation behaviours in nanofibers. The idea of introducing the stereocomplexation of PLA in nanofibers may be useful to utilize other biopolymers than lignin. For instance, electrospun nanofibers made of bio-based materials including chitosan, cellulose, hydroxyapatite, and PLA have been used to mimic the tissue matrix in biomedical applications, due to their high biocompatibility, biodegradation, and bioactive properties.152 Chitosan and hydroxyapatite both have been graft copolymerized with PLA in the previous studies, where the stereocomplexation between the isotactic PLA segment on the graft copolymer and the isotactic linear PLA matrix was observed to improve their mechanical properties.96, 153 Attempting the same methodology with cellulose-graft-PLA will add a useful insight to modify the mechanical properties of the cellulose-based materials without using non-biodegradable polymers. 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Table A1 Integration of 13C{1H} NMR peaks and assignments based on Figure A1. d (ppm) Assignmenta Organosolv Indulin AT Kraft Clusters 166-142 Aromatic C⎯O 2.53 2.18 142-124 Aromatic C⎯C 1.65 1.55 124-102 Aromatic C⎯H 1.93 2.38 90-58 Alkyl⎯O 6.50 3.06 Individual signals 166 PCAg 0 0 160 PCA4 0 0 156 H4 0 0 151-154 S3/5 (ether & condensed) 0.94 0 148-151 G3 (ether & condensed) 1.23 0.49 147-148 G4 (ether) & S3,5 (Non-ether) 0.36 1.25 145 G3,4 (non-ether) 0 0.45 135 S1/G1 0.90 0.24 130 PCA2/6 0.42 0.46 127-130 H2,6 0.33 0.55 92 125 PCA1 0 0.30 119 G6 0.10 1.05 116 PCA3,5 & H3,5 0.60 0.52 155.5-114.5 G5 0.02 0.32 114.5-113 PCAb 0 0.11 110-113 G2 0 0.38 103-106 S2,6 1.21 0 88-77 Cb in b⎯O⎯4, Ca in b⎯5&b⎯b 0 0 71-73 Ca in b⎯O⎯4, Cg in b⎯b 0.27 0.06 65 Cg PCA ester 0 0 63 Cg in b⎯5 0.01 0.09 58-61 Cg (in b⎯O⎯4 and primary OH) 0.36 0.41 55 MeO 5.85 2.50 29 a-b methylene groups 0.03 0.05 22 Acetic acid (CH3) 0 0 aG = guaiacyl, H = p-hydroxy, S = syringyl groups. PCA = p-coumaric acid. Figure A2 1H{1H} NMR spectra (CDCl3, 25°C, 400MHz) of PLA from DBU catalyzed polymerization with L-LA (left) and D-LA (right) without an alcohol initiator at the methine proton region.154 93 Figure A3 DSC curves of linear PLLA (a), PDLA (b), and SC-PLA (c). Table A2 Glass transition and melting temperatures of the PLLA, PDLA, and SC-PLA. Entry Polymer Tg (ºC) Tcc (ºC) ∆Hcc (J/g)a THC (ºC) ∆HHC (J/g)a TSC (ºC) ∆HSC (J/g)a 1 PLLA 65 86 13 177 53 - - 2 PDLA 66 87 11 178 61 - - 3 SC-PLAb 69 131 8 - - 225 52 aDerived from the area under the curve. b1:1 blend of PLLA and PDLA. Figure A4 DSC curves of cyclic PLAs generated using rac-LA (a), L-LA (b), D-LA (c), and the 1:1 blend of polymers generated with L-LA and D-LA (d), in the absence of lignin. Table A3 Tg, Tc, and Tm of cyclic PLAs generated in the absence of lignin. Derived from their DSC curves. Entry Monomer used Tg (ºC) Tcc (ºC) Tm (ºC) 1 rac-LA 46.2 - - 2 L-LA 53.1 - - 3 D-LA 54.6 - - 4 1:1 blend of polymers entry 2 and 3a 55.6 - - aPolymers of entry 2 and 3 were blended in solution and were re-extracted. 94 Figure A5 Relationship of solution viscosity depending on the concentration of lignin-graft-PLLA in 3:1 DCM:DMF. Table A4 Berry Number corresponding to the solid content of the lignin-graft-PLLA in 3:1 DCM:DMF. Entry Berry Numbera Concentration (g/mL) Solid Content 1 3.5 0.23 0.15 2 4.0 0.27 0.17 3 4.5 0.30 0.18 4 5.0 0.33 0.20 5 5.5 0.37 0.22 6 6.0 0.40 0.23 7 6.5 0.43 0.25 8 7.0 0.47 0.26 9 7.5 0.50 0.27 aCalculated from Be = [𝜂]C Figure A6 Areal density vs. electrospinning time for the NF1, NF2, and NF3. 95 Figure A7 Probability vs. log (diameter) curves of the nanofiber mats with Gaussian distribution fit (red curve). NF1 NF2 NF3 NF4 NF5 NF6 96 Table A5 Median, mean, standard deviation and Chi-square values for the six nanofiber mats. Entry Nanofibers Med[log(d)] Gaussian fit to log (d) 𝝌𝟐a m 𝛍 𝜹 1 NF1 2.5 2.5 0.18 3.83 2 NF2 2.4 2.4 0.18 3.03 3 NF3 2.4 2.3 0.23 4.38 4 NF4 2.2 2.2 0.26 9.49 5 NF5 2.5 2.5 0.21 6.22 6 NF6 3.0 3.0 0.15 5.42 aCalculated from 𝜒- = ∑ (19')5:); *2='' ,5)>?)@*3A)B()*C); *2='' ,5)>?)@*3)!)B()*C); *2='' ,5)>?)@*3,15 =22 *2='' . Figure A8 XRD spectra of nanofibers NF1 (a), NF2 (b), and NF3 (c) after heating. 97 Figure A9 SEM images of the nanofiber mats after NaCl particle filtration (5000X). NF1 NF2 NF3 NF4 NF5 NF6 98 Table A6 Filtration efficiency of the nanofiber mats NF1 to NF6 depending on NaCl aerosol aerodynamic diameter. Aerodynamic diameter (μm) Filtration efficiency (%) 0.50 0.62 0.80 0.96 1.19 1.48 1.91 2.32 2.76 3.40 4.22 5.25 1 NF1 87(10) 88(10) 89(9) 89(9) 90(9) 90(8) 91(8) 91(8) 91(8) 91(9) 92(10) 97(5) 2 NF2 83(7) 85(8) 86(8) 87(8) 88(8) 89(7) 89(7) 89(8) 89(8) 89(8) 86(11) 84(14) 3 NF3 71(26) 73(25) 76(23) 77(22) 78(22) 80(20) 81(20) 82(19) 82(19) 82(19) 84(19) 88(14) 4 NF4 74(7) 76(6) 79(6) 80(6) 82(6) 84(5) 85(5) 86(4) 88(4) 89(4) 90(6) 91(5) 5 NF5 87(8) 90(7) 93(6) 94(5) 95(5) 96(4) 97(4) 97(3) 97(3) 98(3) 96(4) 95(10) 6 NF6 11(4) 14(5) 18(7) 21(8) 23(10) 26(10) 28(12) 31(11) 35(10) 38(10) 36(22) 52(21) Filtration efficiency reported in %. Standard errors with 95% confidence interval are in parenthesis.
Thesis/Dissertation
eng
Chemistry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Mehrkhodavandi, Parisa
Investigating the role of polymer tacticity on the properties and filtration efficiency of poly(lactide)-modified lignin nanofibers
Text