Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Zhang, Ziyue
Master of Applied Science - MASc
University of British Columbia
Polylactide (PLA) is a biodegradable thermoplastic polyester produced from renewable biomass sources. The low crystallinity and the low associated crystallization kinetics have a negative impact on its mechanical and barrier properties. In this Master thesis, the effect of flow on the PLA crystallization is studied. The flow-induced crystallization (FIC) of two PLA with different microstructures (different L-lactic acid content) is studied using simple shear, uniaxial extension and capillary flow experiments. In simple shear and capillary flow, increase in shear rate and decrease in temperature was found to enhance the crystallization kinetics particularly for Weissenberg numbers (based on the reptation relaxation time, Wi) greater than 1. On the other hand, in uniaxial extensional flow, once a critical Hencky strain is achieved, crystallization starts independently of strain rate and temperature. The amount of mechanical work per unit volume imposed/dissipated onto the polymers during flow to initialize crystallization was also calculated in capillary flow. The critical mechanical work for the onset of flow-induced crystallization was found to be independent of temperature and degree of molecular chain stretch (Wi) as Wi becomes greater than 1. Lastly, the PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization.
Flow-induced crystallization of polylactides in capillary flow by Ziyue Zhang B.A.Sc, The University of British Columbia, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2021 © Ziyue Zhang, 2021 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Flow-induced crystallization of polylactides in capillary flow submitted by Ziyue Zhang in partial fulfillment of the requirements for the degree of Master of Applied Science in Chemical and Biological Engineering Examining Committee: Dr. Savvas G. Hatzikiriakos, Chemical and Biological Engineering, UBC Supervisor E. Johan Foster, Chemical and Biological Engineering, UBC Supervisory Committee Member Dana Grecov, Mechanical Engineering, UBC Supervisory Committee Member iii Abstract Polylactide (PLA) is a biodegradable thermoplastic polyester produced from renewable biomass sources. The low crystallinity and the low associated crystallization kinetics have a negative impact on its mechanical and barrier properties. In this Master thesis, the effect of flow on the PLA crystallization is studied. The flow-induced crystallization (FIC) of two PLA with different microstructures (different L-lactic acid content) is studied using simple shear, uniaxial extension and capillary flow experiments. In simple shear and capillary flow, increase in shear rate and decrease in temperature was found to enhance the crystallization kinetics particularly for Weissenberg numbers (based on the reptation relaxation time, Wi) greater than 1. On the other hand, in uniaxial extensional flow, once a critical Hencky strain is achieved, crystallization starts independently of strain rate and temperature. The amount of mechanical work per unit volume imposed/dissipated onto the polymers during flow to initialize crystallization was also calculated in capillary flow. The critical mechanical work for the onset of flow-induced crystallization was found to be independent of temperature and degree of molecular chain stretch (Wi) as Wi becomes greater than 1. Lastly, the PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization. iv Lay Summary Polylactide (PLA) is a bioplastic that can be economically produced from renewable resources such as corn, sugarcane and wheats. Compared with traditional oil-based plastic such as polypropylene, PLA is more environmentally friendly due to its biodegradability and now it has been used as a substitution for oil-based plastic to make bottle, films and non-woven fibers. However, due to its low crystallinity, it has poor mechanical properties and barrier properties. These factors put an obstacle for PLA to be used in more durable application. To increase these properties, the crystallinity and crystalline kinetics of PLA should be enhanced during processing. This study investigates quantitively how parameters like temperature, flow rate and flow types affect the crystallization process and thus the mechanical properties of the final products. v Preface All the presented data in this thesis was obtained by the author of this Master thesis under the supervision of Professors Savvas G. Hatzikiriakos at the University of British Columbia. Several apparatuses were used in order to obtain data under various flow and thermal conditions. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Symbols ........................................................................................................................... xiii List of Greek Symbols .............................................................................................................. xvii List of Abbreviations ................................................................................................................. xix Acknowledgements .................................................................................................................... xxi Dedication .................................................................................................................................. xxii Chapter 1: Introduction ............................................................................................................... 1 1.1 Polylactide................................................................................................................... 1 1.2 Flow-induced crystallization ....................................................................................... 2 1.3 Research Objectives .................................................................................................... 6 Chapter 2: Materials, Experimental Equipment, and Methodology ........................................... 8 2.1 Materials ..................................................................................................................... 8 2.2 Differential Scanning Calorimeter (DSC) .................................................................. 9 2.3 Shear rheometry .......................................................................................................... 9 2.4 Extensional rheometry .............................................................................................. 16 Chapter 3: Theory of Flow induced crystallization .................................................................. 20 vii 3.1 Weissenberg number, Wi, based on the reputation relaxation time ......................... 20 3.2 The Deborah number, De, number ........................................................................... 20 3.3 Weissenberg number, WiE, based on the extensional rate ........................................ 21 3.4 The Mechanical Work, W dissipated ........................................................................ 21 Chapter 4: Results and Discussion ............................................................................................ 24 4.1 Thermal behavior ...................................................................................................... 24 4.2 Linear viscoelastic properties ................................................................................... 26 4.3 Reptation times ......................................................................................................... 29 4.4 Shear-induced/enhanced isothermal crystallization .................................................. 30 4.5 Uniaxial Extensional flow-induced isothermal crystallization ................................. 35 4.6 Capillary flow-induced isothermal crystallization .................................................... 40 4.7 Mechanical work and crystallization ........................................................................ 49 Chapter 5: Conclusions and Recommendations ....................................................................... 51 Bibliography .................................................................................................................................53 Appendix .......................................................................................................................................58 viii List of Tables Table 2.1: L-lactide content, number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) of the two PLA samples. ........................................ 8 Table 2.2: Schematics of sliding plate rheometer and rotational rheometer ................................. 11 Table 2.3: Schematics of different classes of extensional flow based on stretch direction. ......... 16 Table 4.1: Percent crystallinity of PLA 801 and PLA 804 for three cycles ................................. 26 Table 4.2: Maxwell model parameters (G, λ) for sample PLA 801 master curve at T=180°C and PLA 804 master curve at T=160°C ............................................................................................... 28 Table 4.3: The reptation and Rouse times of the two PLAs of Mw=180 kg/mol ......................... 29 Table 4.4: Weissenberg number for PLA 801 and PLA 804 at various temperature and shear rate....................................................................................................................................................... 44 ix List of Figures Figure 1.1: Schematic illustration of the structures of lactic acid, lactide and PLA, and the synthesis process for PLA ............................................................................................................... 2 Figure 1.2: Schematic illustration of the four flow regimes of flow-enhanced and -induced crystallization of polymers .............................................................................................................. 5 Figure 2.1: Schematic of capillary rheometer ............................................................................... 15 Figure 2.2: The schematic illustration of SER fixture that can be mounted on a rheometer platform ......................................................................................................................................... 17 Figure 4.1: The DSC thermograph for PLA 801. The glass transition temperature, crystallization temperature and melting peak are 62℃, 144℃ and 177oC .......................................................... 25 Figure 4.2: The DSC thermograph for PLA 804. The glass transition temperature in the first and third cycles are 61℃ and 63℃. The melting peak in the first cycle is 156℃ .............................. 25 Figure 4.3: a) The master curves of the linear viscoelastic moduli, G’, G” and complex viscosity of PLA 801 at the reference temperature of 180℃ (b) The master curves of the linear viscoelastic moduli, G’, G” and complex viscosity of PLA 804 at the reference temperature of 160℃ ......... 28 Figure 4.4: The horizontal shift factor, aT for a) PLA 801 and b) PLA 804 at the reference temperature of 180 ℃ and 160℃ .................................................................................................. 28 Figure 4.5: Master curves at 160°C predicted by Maxwell model and TMA tube-based model for a) PLA 801 and b) PLA 804 ......................................................................................................... 30 Figure 4.6: The normalized shear stress growth coefficient η+ of PLA 801 at temperatures of 150℃, 140℃, 130℃, and 120℃ at shear rate of 0.01 s-1. The sudden increase of this coefficient marks the onset of crystallization ................................................................................................. 31 x Figure 4.7: The shear stress growth coefficient η+/η0 of PLA 801 at shear rate ranging from 0.005 s-1 to 0.3s-1 and crystallization temperature at a) 150℃, b) 140℃, c) 130℃, and d) 120℃. The sudden increase of this coefficient marks the onset of crystallization ................................... 32 Figure 4.8: Induction time variation with shear rate at different temperatures for PLA 801 ....... 33 Figure 4.9: Deborah number versus Weissenberg number for shear induced crystallization at various shear rates and temperatures ............................................................................................ 35 Figure 4.10: Tensile stress growth coefficient as a function of time measured at various Hencky strain rate at 130℃, 120℃, 110℃, and 100℃ for PLA 801 ....................................................... 36 Figure 4.11: Tensile stress growth coefficient as a function of time measured at various Hencky strain rate at 130℃, 120℃, 110℃, and 100℃ for PLA 804 ....................................................... 37 Figure 4.12: Induction time as a function of Hencky strain rate at various temperatures for PLA 801................................................................................................................................................. 38 Figure 4.13: Induction time as a function of Hencky strain rate at various temperatures for PLA 804................................................................................................................................................. 39 Figure 4.14: Induction time as a function of Hencky strain at various temperatures for a) PLA 801 and b) PLA 804 ...................................................................................................................... 40 Figure 4.15: Wall shear stress versus wall shear rate for PLA 801 at 190℃ from capillary experiments compared with the LVE data from a rotational rheometer suggesting the validity of the Cox-Merz rule and the absence of any crystallization ............................................................ 41 Figure 4.16: Wall shear stress versus time (pressure transients) at various apparent shear rates for PLA 801 at a)172℃, b) 167℃, c) 161℃ and d) 156 ℃ .............................................................. 43 Figure 4.17: Crystallization induction time versus wall shear rate at various crystallization temperature for PLA 801 .............................................................................................................. 43 xi Figure 4.18: Wall shear stress versus wall shear rate for PLA 804 at 170℃ from capillary experiments compared with the LVE data from a rotational rheometer suggesting the validity of the Cox-Merz rule and the absence of any crystallization ............................................................ 45 Figure 4.19: Wall shear stress versus time (pressure transients) at various apparent shear rates for PLA 804 at a) 150℃, b) 145℃, c) 140℃ and d) 135 ℃ ............................................................. 46 Figure 4.20: Crystallization induction time versus apparent shear rate at various crystallization temperatures for PLA 804 ............................................................................................................. 47 Figure 4.21: Induction time versus shear rate for PLA 801 in both simple shear and capillary flow ............................................................................................................................................... 48 Figure 4.22: Schematic illustration of the four flow regimes of crystallization of polymers with flow regions for experiments highlighted in blue ......................................................................... 49 Figure 4.23: Mechanical work required to induce crystallization versus various Weissenberg number for flow-induced crystallization in shear and capillary flow for PLA 801 and PLA 804 50 Figure A.1: DSC thermograph of PLA 801 in the first cycle, where ∆𝐻𝐻𝐻𝐻 and ∆𝐻𝐻𝐻𝐻 are 46.25 J/g and 2.64 J/g ................................................................................................................................... 58 Figure A.2: DSC thermograph of PLA 801 in the second cycle, where ∆𝐻𝐻𝐻𝐻 and ∆𝐻𝐻𝐻𝐻 are 20.10 J/g and 16.61 J/g ........................................................................................................................... 58 Figure A.3: DSC thermograph of PLA 801 in the third cycle, where ∆𝐻𝐻𝐻𝐻 and ∆𝐻𝐻𝐻𝐻 are 20.04 J/g and 17.43 J/g ................................................................................................................................. 59 Figure A.4: DSC thermograph of PLA 804 in the first cycle, where ∆𝐻𝐻𝐻𝐻 is 26.29 J/g and no cold crystallization process presented................................................................................................... 59 Figure A.5: DSC thermograph of PLA 804 in the second cycle, no cold crystallization and melting process presented ............................................................................................................. 60 xii Figure A.6: DSC thermograph of PLA 804 in the third cycle, no cold crystallization and melting process presented .......................................................................................................................... 60 xiii List of Symbols 𝐴𝐴 cross-sectional area of the sample in SER 𝐴𝐴0 initial cross-sectional area of the sample in SER 𝐷𝐷 capillary die diameter 𝐷𝐷𝑏𝑏 barrel diameter 𝐷𝐷𝐷𝐷 Deborah number 𝐸𝐸𝑎𝑎 activation energy 𝐹𝐹 tangential force 𝐹𝐹𝐹𝐹 frictional force 𝐺𝐺′ storage modulus 𝐺𝐺′′ loss modulus 𝐺𝐺𝑁𝑁0 plateau modulus 𝑔𝑔𝑖𝑖 modulus of Maxwell nodes xiv ℎ gap between the two plates of the parallel plate geometry ∆𝐻𝐻𝑚𝑚 absolute value for heat of fusion ∆𝐻𝐻𝑚𝑚 absolute value for heat of fusion ∆𝐻𝐻𝑚𝑚° absolute value for heat of crystallization at equilibrium temperature 𝐿𝐿 length of capillary die 𝐿𝐿0 initial length of test specimen in SER 𝑀𝑀 torque 𝑀𝑀𝑒𝑒 entanglement molecular weight 𝑀𝑀𝑛𝑛 number average molecular weight 𝑀𝑀𝑤𝑤 weight average molecular weight 𝑛𝑛 Avrami index, power law exponent ∆𝑃𝑃 pressure drop in capillary tube 𝑄𝑄 volumetric flow rate xv 𝑅𝑅 radius 𝑟𝑟 distance from the center of the plate 𝑇𝑇 temperature 𝑡𝑡𝑅𝑅 average residence time in capillary die 𝑇𝑇𝑐𝑐 crystallization temperature 𝑇𝑇𝑔𝑔 glass transiti1on temperature 𝑡𝑡𝑖𝑖𝑛𝑛𝑖𝑖 crystallization induction time 𝑇𝑇𝑚𝑚 melting temperature 𝑇𝑇𝑚𝑚° equilibrium melting temperature 𝑉𝑉 volume of the barrel 𝑣𝑣 piston moving speed 𝑊𝑊 mechanical work 𝑊𝑊𝑊𝑊 Weissenberg number xvi X percent crystallinity 𝑍𝑍𝐸𝐸 number of entanglements xvii List of Greek Symbols 𝛾𝛾 strain ?̇?𝛾𝐴𝐴 apparent shear rate ?̇?𝛾 shear rate ?̇?𝛾𝑤𝑤 shear rate at the wall of the capillary die ?̇?𝛾𝑅𝑅 shear rate at the edge of the parallel-plate geometry 𝜀𝜀?̇?𝐻 Hencky strain rate 𝜀𝜀𝐻𝐻 Hencky strain 𝜂𝜂+ shear stress growth coefficient 𝜂𝜂0 steady state viscosity 𝜂𝜂∗ complex viscosity 𝜂𝜂𝐸𝐸+ tensile stress growth coefficient 𝜆𝜆𝑖𝑖 relaxation time of Maxwell nodes xviii 𝜏𝜏𝑒𝑒 equilibration time 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟 reptation time 𝜏𝜏𝑅𝑅 Rouse time 𝜎𝜎 shear stress 𝜎𝜎𝑤𝑤 wall shear stress 𝜛𝜛 angular frequency xix List of Abbreviations 𝐷𝐷𝐷𝐷𝐷𝐷 Differential Scanning Calorimetry 𝐹𝐹𝐹𝐹𝐷𝐷 Flow-Induced Crystallization 𝑃𝑃𝑃𝑃 Polybutylene 𝑃𝑃𝐷𝐷𝐹𝐹 Polydispersity Index 𝑃𝑃𝐸𝐸 Polyethylene 𝑃𝑃𝐿𝐿𝐴𝐴 Polylactide 𝑃𝑃𝐿𝐿𝐿𝐿𝐴𝐴 Poly-L-lactide 𝑃𝑃𝐷𝐷𝐿𝐿𝐴𝐴 Poly-D-lactide 𝑃𝑃𝐷𝐷𝐿𝐿𝐿𝐿𝐴𝐴 Poly-DL-lactide 𝑃𝑃𝑃𝑃𝑀𝑀 Polarized Optical Microscopy 𝑃𝑃𝑃𝑃 Polypropylene 𝐷𝐷𝐸𝐸𝑅𝑅 Sentmanat Extensional Rheometer xx 𝑇𝑇𝑀𝑀𝐴𝐴 Time Marching Algorithm xxi Acknowledgements I would like to express my deep and sincere gratitude to my research supervisor, Prof. Savvas G Hatizikiriakos, for providing invaluable guidance throughout this research and walking me into the world of rheology. I am extremely grateful for this advice and patience during our discussions on research work and thesis preparation. It was a great privilege and honor to work and study under his guidance. I would also like to all members in the Rheology Group, for their advice and encouragement. From my bottom of my heart, I thank my friends Zhaohong Hu, Hui Zhu, Bingxing Zhou, Tingting Yi and Qinyu Zhu for accompanying me during the quarantine period and helping me get through the difficult times. The most profound and loving thanks to my family who gave me the greatest love, support and encouragement. xxii Dedication Dedicated to my parents and brother, for always supporting me. 1 Chapter 1: Introduction This chapter first presents an introduction to some basic properties of polylactide and the idea of flow-induced crystallization. Some important previous studies on FIC of PLA are then reviewed followed by introducing the research objectives of this thesis. 1.1 Polylactide Polylactide or polylactic acid (PLA) is a thermoplastic polyester that can be economically produced from renewable biomass resources such as corn and sugar beets [1]. This bioplastic has become popular for its biocompatibility and biodegradability and has already been used in medical and packaging applications as an alternative for petroleum plastics. Its properties of low toughness, heat resistance and thermal stability, however, prevent PLA from becoming a commodity product of extensive use in durable applications [2]. The mechanical and thermal weakness of PLA are mainly caused by its low crystallinity, the degree of which is largely controlled by the monomers (lactic acid or lactides) used for polymerization [3]. Low molecular weight PLA or prepolymer is synthesized through condensation of lactic acid. Due to the chiral nature of the lactic acid, there are two enantiomers, L-(+)- lactic acid and D-(-)-lactic acid. The prepolymer is then depolymerized into lactide that has three stereoisomers, L-LA, D-LA and meso-LA [4]. Ring opening polymerization of these lactides lead to the synthesis of high molecular weight PLLA, PDLA, which are semicrystalline and PDLLA which is mostly amorphous [4]. The structures of lactic acid, lactide and PLA are shown in Figure 1.1. The final crystallinity of the polymer product is determined by polymer processing where flow imposes a critical phase transformation to the molten polymer [5], which manifests itself upon exiting from the die during cooling. In this study, 2 flow-induced crystallization of two PLAs of different microstructure is studied in an attempt to identify ways (thermal and flow conditions) to increase the crystallinity aiming in improving its mechanical properties. Figure 1.1: Schematic illustration of the structures of lactic acid, lactide and PLA, and the synthesis process for PLA 1.2 Flow-induced crystallization Polymer crystallization is a process that the atoms and molecules form highly ordered solid structure known as crystal from either a supercooled polymer melt or a supersaturated solvent [6]. In this thesis, we focus on crystallization from the polymers melt phase as the temperature decreases under the influence of flow. The crystallization process consists of two main steps. The first is nucleation which is the formation of a crystalline phase called nuclei, and the second, crystal growth which is the increase in the size of the crystal structure. Nuclei in varying size (point-nuclei or row nuclei) grow from precursors that are defined as the polymer chains above a “critical orientation molecular weight” that can initiate oriented structures that are stable even at 3 temperatures higher than the nominal melting temperature [7]. For semicrystalline linear polymers, the crystalline structure is usually in the form of spherulite with the schematic shown in Figure 1.2. The precursor for the structure is called point-precursor since its size is very small and cannot be detected by the current experimental technique [7]. In the crystalline region, the polymer chains fold together and form ordered regions called crystalline lamellae. The crystalline region is interrupted by the amorphous region where molecular bends and kinks present and hinder the growth of lamellae [8]. The radial growth direction of lamellae in spherulites is caused by the lack of thermal gradient [8]. Another form of the crystal is shish-kebab crystal structure with the schematic shown in Figure 1.2. When the precursor is very long (fibril precursor), up to a few micrometers, fibrils (shish) will emerge on which lamellae (kebabs) will grow [7]. The flow conditions for the emergency of fibril precursor and shish-kebab structure are discussed later. Flow-induced crystallization is an important process in polymer processing such as injection molding i.e. manufacturing of preforms for bottles [8-14] where the complex external flow (shear and extension) act on the melt state of the polymer, enhancing the crystallization kinetics and generating oriented crystallites and other more complex structures such as shish [5]. The final structure and properties of the material are affected by a number of parameters including the nature of the polymer, molecular weight and its distribution, molecular stiffness, level of long chain branching, and several processing parameters [9-18]. The latter include processing temperature, volumetric flow rate, extrusion pressure and mechanical energy imposed into the polymer during extrusion before crystallization. These parameters may affect molecular orientation, rearrangement of molecular segments and thus crystal formation, crystal size and its distribution as well as their orientation [15-18]. 4 Four different flow regions (Figure 1.2, normalized time for the onset of crystallization over the time for crystallization under quiescent conditions as a function of shear rate) are identified to characterize the effect of shear flow on the crystallization of polymer melts [19]. When the shear rate is less than 1/𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, where 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟 is the reptation time (characteristic relaxation time needed for a polymeric molecule to terminally relax upon cessation of flow), the kinetics of crystallization can be enhanced (faster crystallization compared to that in the absence of flow referred to as quiescent crystallization), however, the degree of crystallization remains the same. As the shear rate becomes larger than 1/𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟 but less than1/𝜏𝜏𝑅𝑅 , where 𝜏𝜏𝑅𝑅 is the Rouse time (𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟=3𝑍𝑍𝐸𝐸𝜏𝜏𝑅𝑅 where 𝑍𝑍𝐸𝐸 is the number of entanglements (𝑍𝑍𝐸𝐸=𝑀𝑀𝑤𝑤/𝑀𝑀𝑒𝑒), Mw is the weight average molecular weight and Me is the molecular weight between entanglement), more point-like nuclei will be formed due to the chain orientation towards the flow direction. In this flow regime, typically spherulite morphologies are formed which grow in size with time. This regime is referred to as flow-induced crystallization. 5 Figure 1.2: Schematic illustration of the four flow regimes of flow-enhanced and -induced crystallization of polymers Shear rate at this stage is not enough to generate conformational change of the polymer chain. Further increase of shear rate to higher than 1/𝜏𝜏𝑅𝑅 leads to chain stretch and the formation of “row” nuclei. In this region where the shear rate is larger than 1/𝜏𝜏𝑅𝑅 the anisotropic shishi-kekab crystalline will be formed due to the strong chain orientation and conformation change [19]. These crystalline structures enhance the mechanical properties of polymers in general due to their elongational morphology and stretched structure. To date, the effect of FIC on PLLA crystallization has NOT been widely researched. Zhong et all. [20] determined the critical shear rate for enhancement of crystallization and performed both isothermal and non-isothermal pre-shear induced crystallization experiments at 135℃ on PLA 4032D in the first and second flow regions. In the second flow region, they found that the flow significantly increases the nucleation process and the growth of crystalline [20]. Xu et all. [21] performed isothermal pre-shear (100 s-1) induced crystallization on PLA 4032D at various 6 temperatures and observed the formation of shish-kebab using polarized optical microscopy (POM) and scanning electron microscopy (SEM). Jalali et all. investigated the effect of isothermal continuous low shear flow and pre-shear strong flow at 140 ℃ on the crystallization kinetics of linear and branched PLLA. Linear PLLA was found to have a critical transition shear rate (region I to region II) about 100 times larger than that of branched PLLA [22]. They also found that PLLA with a high molecular weight (160 kg/mol) have a lower thermodynamic barrier for crystallization and thus a higher critical shear rate than PLLA with a low molecular weight (36 kg/mol) [23]. Although previous research has covered PLA crystallization by all flow fields, however, crystallization experiments were confined to limited range of temperatures and/or shear rate. Moreover, these experiments utilize a short period pre-shear flow to initiate the growth of nuclei and monitor the quiescent crystallization under oscillatory flow afterwards. The effect of long-term shearing to the crystallization process was not fully investigated, especially for the relationship between shear at various flow regions and the crystallization kinetics. 1.3 Research Objectives In this Master thesis, we study the effect of flow on PLA crystallization over a wider range of temperature and shear/extensional rates by utilizing a parallel-plate rheometer equipped with the SER extensional rheometer and most importantly a capillary rheometer capable of extending the shear rate range at high levels [24]. It is noted that capillary flow is more relevant to polymer processing and it is viewed as a rheological tool that bridges the gap between fundamental rheology and processing. Flow induced crystallization in capillary flow have been widely researched on many polymers such as polypropylene (PP) [16,24,25], polyethylene (PE) [25,26] and polybutylene (PB) 7 [27], however, its use for PLA is limited in the literature. The main objective of this work is to study comprehensively the flow induced crystallization of two different PLLA. Relevant experiments are performed in simple shear (parallel-plate), uniaxial extension and capillary flow for two polylactides to assess their crystallization performance under strong flow conditions. The experiments are analysed in terms of the Weissenberg number (𝑊𝑊𝑊𝑊 = ?̇?𝛾𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟), Deborah number (𝐷𝐷𝐷𝐷 = 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟/𝑡𝑡𝑅𝑅 where tR is the average residence time in the capillary die) that describe the degree of molecular orientation, and mechanical work (W) essentially the energy dissipated on the polymer melt during flow in order to calculate the critical mechanical work for the onset of crystallization in capillary flow. The ultimate objective is to identify flow and thermal conditions that enhance and induce crystallization and parameters that affect these phenomena in terms of the Wi, De and mechanical work. 8 Chapter 2: Materials, Experimental Equipment, and Methodology This chapter provides the basic molecular and thermal characteristics of the two PLAs used in the experiments. The methodology for the rheological measurement associated with the apparatus used in the experiments are also discussed here. 2.1 Materials Two commercial PLA labelled as PLA 801 and PLA 804 were studied in this work. These are two PLLA with different L-lactide contents and were obtained from Futerro (Belgium). The L-lactide content and the number average molecular weight (Mn) for the two PLAs are shown in Table 2.1. The polydispersity index (PDI) and the weight average molecular weight (Mw) of the samples are determined by fitting the linear viscoelastic modulus G', G" to the TMA tube-based model [28]. Table 2.1: L-lactide content, number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) of the two PLA samples. Sample L-lactide content (%) Mn (kg/mol) Mw (kg/mol) PDI PLA 801 99.5 180 360 2 PLA 804 96 180 330 1.83 Samples received in the form of pellets were dried at 70℃ overnight in a vacuum oven to remove moisture. The granular pellets were then melted (190℃ for PLA 801 and 180℃ for PLA 804) and compressed into disk-like films in a compression moulding unit producing samples with thickness between 0.4 to 0.6mm. These plaques were used to make samples for shear flow experiments in the parallel-plate geometry and for extensional experiments using the SER fixture. 9 For the capillary experiments, the dried pellets were packed slowly into the capillary barrel to allow enough time for melting. 2.2 Differential Scanning Calorimeter (DSC) In this study, the thermal behavior of the PLA was determined using the differential scanning calorimeter (DSC) Q1000 from TA instrument. Each sample weighted 1-2 mg was cut from the dried PLA pellets and sealed in an aluminum pan. Three heating and cooling cycles were applied to the sample and the glass transition temperature (𝑇𝑇𝑔𝑔), crystallization temperature (𝑇𝑇𝑐𝑐) and melting point (𝑇𝑇𝑚𝑚) were determined using the TA Instrument Universal Analysis software. The first heat-cool cycle was used to eliminate thermal history. In each cycle, the temperature increased from 25℃ to 200℃ at a rate of 10°C/min. The sample was kept at 200℃ for 5 min before cooling down to 25℃. 2.3 Shear rheometry Rheometry is referred to the experimental techniques used to determine rheological properties of materials which are the relationship between stresses and strain/strain rate [29]. A rheometer is an instrument that creates a flow (shear / extensional), and enforces stress / deformation and measures deformation /stress on a material. In shear rheometry, two different types of rheometers are used. Namely, drag flow rheometers where the fluid is deformed by moving one of the plates and capillary rheometer where the flow is generated by applying a pressure gradient (moving piston in a cylinder). These two types of flows are discussed next. 10 2.3.1 Drag flow rheometer In drag flow rheometers, the shear flow is created by a drag force. Commercial drag flow rheometer includes sliding parallel plate and rotational plate rheometers. Although in sliding parallel plate rheometer (Table 2.2), there is no effects of pressure on the measurement and there are ways to eliminate the edge effects, the rotational rheometer is used extensively since it is more suitable and convenient for measuring the viscoelastic behavior of the sample in various states. In rotational rheometer (Table 2.2), the angular velocity, 𝜔𝜔(t), is imposed and the torque, M, is measured. These two quantities are converted to strain/strain rate and stress for the sample, a relationship known as the flow curve [30]. These rheometers can be operated in two modes: controlled stress or controlled strain. In the first one, the torque is fixed, while the angular velocity (shear rate and shear strain) is measured with time. In the present study, the rheometer was operated in the strain-controlled mode, where angular velocity (shear rate) is fixed to create a constant shear flow and the torque is measured with time monitoring the change of stress. Different geometries such as parallel plate, cone-and -plate and concentric cylinders can be used as fixtures with the main body rheometer in order to characterize materials in different states. The schematics of sliding plate rheometer and rotational rheometer with different geometries are all shown in Table 2.2. 11 Table 2.2: Schematics of sliding plate rheometer and rotational rheometer Rheometer Geometry Schematic Sliding plate Rotational Parallel plate Cone-and-plate Concentric cylinder 12 In this study, we use the parallel-plate geometry to generate simple shear flow and oscillatory flow. Samples are placed between the two parallel plates. The upper plate rotates at a specified angular velocity 𝜔𝜔(t) while the bottom plate stays stationary. The shear rate increases linearly from the center to the edge of the plate as follows: ?̇?𝛾(𝑟𝑟) = 𝑟𝑟𝜔𝜔ℎ (2.1) where r is the distance from the center of the plate and h is the gap size. The shear rate at edge is: ?̇?𝛾𝑅𝑅(𝑟𝑟) = 𝑅𝑅𝜔𝜔ℎ (2.2) The shear strain is also dependent on the position with the equation shown below: 𝛾𝛾(𝑟𝑟) = 𝑟𝑟𝑟𝑟ℎ (2.3) where 𝑟𝑟 is the displacement in the 𝑟𝑟-direction. The torque M as a result of the upper plate movement is measured and is converted to shear stress using the following equation: 𝜏𝜏12 = 𝑀𝑀2𝜋𝜋𝑅𝑅3 [3 + 𝑑𝑑𝑑𝑑𝑛𝑛𝑀𝑀𝑑𝑑𝑑𝑑𝑛𝑛?̇?𝛾𝑅𝑅] (2.4) The linear viscoelasticity of the polymers was measured using the rotational rheometer MCR 501 from Anton Paar with 25 mm parallel plate geometry. First, strain sweep test with a constant frequency (10 rad/s) was performed first to determine the linear viscoelastic regime. Frequency sweep tests were performed using 1% shear strain that is within the linear viscoelastic regime for all tested temperatures. This test determined the linear viscoelastic moduli of the polymers, namely the loss modulus, G" (the viscous contribution to the shear stress) and storage 13 modulus, G' (the elastic contribution to the shear stress). The tests were performed over a large temperature range from 140℃ to 220°C for PLA 801 and from 110℃ to 170℃ for PLA 804. All the tests were performed in nitrogen environment to avoid sample degradation. The time-Temperature Superposition (tTS) [31, 32, 33] was applied to construct the master curve for PLA 801 at a reference temperature of 180°C and for PLA 804 at a reference temperature of 160℃. Simple shear induced isothermal crystallization experiments were carried out using the MCR 501 rheometer equipped with Peltier system to precisely control the temperature and the temperature change. Due to the small shear rates used (<0.5s-1) that were applied to the sample, 25mm parallel plate geometry was used for all tests. For PLA 801, all samples were loaded at 190℃ and held for 10 min to erase any thermal/deformation history. Samples were then cooled at a cooling rate of 10 K/min to the desired crystallization temperature before continuous shear was applied. Once the temperature was reached, a precisely controlled constant continuous shear rate was applied and the crystallization process was monitored through changes (typically sudden increase) of the shear stress growth coefficient. 2.3.2 Capillary rheometer In a capillary rheometer, a pressure-driven flow is generated by a moving of piston that compresses the molten polymer in a cylindrical chamber with diameters 𝐷𝐷𝑏𝑏 to a capillary die with a certain length L, diameter D and contraction angle 2𝛼𝛼. The schematic of the capillary rheometer is shown in Figure 2.1. Just before entering the capillary die, there is a mix of shear flow that is along the wall, and mainly extensional flow at the core region of the entry (fluid acceleration). In a typical capillary experiment, the moving speed of the piston 𝑉𝑉 is controlled at a constant value 14 that generates a constant wall shear rate ?̇?𝛾𝑤𝑤 in the wall of the capillary die. The magnitude of the wall shear rate ?̇?𝛾𝑤𝑤 for a Newtonian fluid can be calculated as: ?̇?𝛾𝑤𝑤 = 32𝑄𝑄𝜋𝜋𝐷𝐷3̇ (2.5) The conversion between the wall shear rate and piston speed for a Newtonian fluid is: ?̇?𝛾𝑤𝑤 = 8𝐷𝐷𝑏𝑏2𝑉𝑉𝐷𝐷3 (2.6) Since most polymer melts are non-Newtonian fluid, the wall shear rate calculated using equation 2.6 is only the apparent shear rate ?̇?𝛾𝐴𝐴. The actual wall shear stress can be obtained by applying Rabinowitch correction: 𝑏𝑏 = 𝑑𝑑(log ?̇?𝛾𝐴𝐴)𝑑𝑑(log𝜎𝜎𝑤𝑤) (2.7) ?̇?𝛾𝑤𝑤 = (3 + 𝑏𝑏)4 ?̇?𝛾𝐴𝐴 (2.8) As the flow is fully-developed over the length of the die, the value for the apparent shear stress is related with the pressure drop ∆𝑃𝑃: 𝜎𝜎𝐴𝐴 = ∆𝑃𝑃4𝐿𝐿/𝐷𝐷 (2.9) Before flow becomes fully-developed in the capillary die, the entry pressure drop ∆𝑃𝑃𝑒𝑒𝑛𝑛𝑖𝑖 is also present and should be accounted for. To correct the total pressure for the entrance pressure effects, the Bagley correction can be applied: 𝜎𝜎𝑤𝑤 = ∆𝑃𝑃 − ∆𝑃𝑃𝑒𝑒𝑛𝑛𝑖𝑖4𝐿𝐿/𝐷𝐷 (2.10) 15 As the length of the die increases or the die diameter decreases, the effect of entry pressure on the wall shear stress weakens. As L/D ratio is larger than 20, the effect of entry pressure is negligible for the present PLA experiments [34]. The crystallization behavior of PLA at high shear rates was investigated using a pressure driven capillary rheometer (Instron Model 4465) using a capillary die having a length-to-diameter ratio L/D=33 in order to minimize the effect of ends pressure. The experimental procedure was as follows: the barrel was initially filled with the appropriate number of dried pellets at a high temperature at which the PLA is in the molten state (190℃ for PLA801 and 170°C for PLA 804). Consequently, the temperature was lowered to the temperature of the test, which started once the temperature was stabilized. The cooling rate was around 1 K/min. The shear rates used were in the range of 5 s-1 to a maximum of 1000 s-1 to identify the temperature and shear rate at which crystallization starts. This can be seen from the unbound increase of pressure which is due to crystallization which leads to blockage of the die entry to the capillary. Figure 2.1: Schematic of capillary rheometer 16 2.4 Extensional rheometry Extensional flow is found to be dominate in many polymer processing techniques such as fiber spinning, blow molding and film manufacturing. It creates a deformation that stretches the sample along the streamlines. Based on the stretching direction, extensional flow is usually divided into uniaxial extension, biaxial extension and planar extension. The schematic of each class of extensional flow is shown in Table 2.3. Table 2.3: Schematics of different classes of extensional flow based on stretch direction. Simple schematic Uniaxial extension Biaxial extension Planar extension In this study, we focus on the flow induced crystallization in uniaxial extensional flow. The commercial rheometer, Sentmanat Extensional Rheometer (SER) fixture was used by mounting this fixture on the MCR 501 modular compact rheometer. The schematic illustration of the SER is shown in Figure 2.2. 17 Figure 2.2: The schematic illustration of SER fixture that can be mounted on a rheometer platform To control the sample temperature, the fixture is placed in a convection oven where nitrogen flow is circulated to prevent sample degradation. The sample is placed on the two windup drums by fixing the end of the sample to the metal clamps on each drum. The rotation of the drive shaft leads to an equal but opposite rotation of the windup drums that creates a stretch to the rectangular sample along the horizontal streamline. The polymer sample will wound up onto the drums. During the extension, the width and thickness of the sample decrease until it finally breaks. As the windup drum rotates at a constant rate of 𝜔𝜔, the Hencky strain rate applied to the sample is: 𝜀𝜀?̇?𝐻(𝑟𝑟) = 2𝑅𝑅𝜔𝜔𝐿𝐿0 (2.11) where R is the radius of the windup drums, and 𝐿𝐿0 is the fixed, unsupported length of the specimen sample being stretched, which is equal to the centerline distance between the master and slave drums. 18 To resist the stretch, the sample gives a tangential force, F to the surface of the drums and is measured by the sensor through torque signal, T. The relation between the tangential force and torque is [35]: 𝑇𝑇 = 2(𝐹𝐹 + 𝐹𝐹𝐹𝐹)𝑅𝑅 (2.12) where 𝐹𝐹𝐹𝐹 is the frictional contribution from the bearings and intermeshing gears which is less than 1% and therefore negligible. The torque can be simplified to: 𝑇𝑇 = 2𝐹𝐹𝑅𝑅 (2.13) For a constant Hencky strain rate, if there is no deviation between the nominal and actual strain rates, the cross-sectional area A(t) of the stretched sample changes exponentially with time and is expressed as: 𝐴𝐴(𝑡𝑡) = 𝐴𝐴0exp [−𝜀𝜀?̇?𝐻t] (2.14) where 𝐴𝐴0 is the initial cross-sectional area of the unstretched sample. The transient extensional viscosity (also referred to as stress growth coefficient), can be calculated from: 𝜂𝜂𝐸𝐸+(𝑡𝑡) = 𝐹𝐹(𝑡𝑡)𝐴𝐴(𝑡𝑡)𝜀𝜀?̇?𝐻 (2.15) where 𝐹𝐹(𝑡𝑡) is the instantaneous tensile force at time t. In this study, MCR 501 equipped with extensional fixture SER (Figure 2.2) was used to study the crystallization behavior of PLA under extension. Rectangular PLA 801 and 804 samples with thickness of about 0.5 mm were placed onto the SER geometry at 190℃ and 170℃ respectively to erase the thermal history for a few minutes. Consequently, the temperature was lowered to the desired temperature for stretching. The experiments started when the temperature was decreased to the desired temperature and tensile growth coefficient was monitored with time at a constant Hencky strain rate. Since PLA samples are easily prone to sagging, we minimized 19 the soak time of the materials at 190℃ and 170℃. The sagging behavior can be also observed in the results (discussed in chapter 4). 20 Chapter 3: Theory of Flow induced crystallization A number of important parameters should be determined in flow-induced crystallization experiments to identify the possibilities and favourite conditions for molecular alignment/morphological changes leading to crystallization and/or crystal formation. These are the following: 3.1 Weissenberg number, Wi, based on the reputation relaxation time 𝑊𝑊𝑊𝑊 = 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, 𝑇𝑇|?̇?𝛾| (3.1) where 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, 𝑇𝑇 is the reptation time at temperature T, and 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, 0 is the reptation time at the reference temperature 𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟, with the temperature-dependency of the reptation time is given by: 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟,𝑇𝑇 = 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, 0 ∗ 𝐷𝐷𝑒𝑒𝑒𝑒 �𝐸𝐸𝑎𝑎𝑅𝑅 �1𝑇𝑇 − 1𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟�� (3.2) and|?̇?𝜸| is the magnitude of the shear rate, where 𝐸𝐸𝑎𝑎 is the flow activation energy. For Wi > 3ZE (ZE is the number of entanglements defined as the ratio of molecular weight divided by the entanglement molecular weight of the polymer), the flow becomes strong to extend and maintain the chains within their confined tubes stretched, something that facilitates crystallization related phenomena (i.e. formation of shish-kebab morphologies) [36, 37]. 3.2 The Deborah number, De, number 𝐷𝐷𝐷𝐷 = 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟,𝑇𝑇𝑡𝑡𝑟𝑟𝑒𝑒𝑟𝑟 (3.3) 21 where the residence time, tres can be calculated at any point by performing an integration along the streamline (arc length integration) from the entry to the point where the calculation is performed. To facilitate morphological changes for crystal formation the chains should be remain stretched for times longer than the reptation time. Thus, the case De <1 favour crystallization. 3.3 Weissenberg number, WiE, based on the extensional rate 𝑊𝑊𝑊𝑊𝐸𝐸 = 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟, 𝑇𝑇|?̇?𝜺| (3.4) where|?̇?𝜺| is the magnitude of the extensional rate. For WiE > 1 conditions favour crystallization. 3.4 The Mechanical Work, W dissipated The mechanical work dissipated on a polymer during flow is given by: 𝑊𝑊 = � 𝝈𝝈: ?̇?𝜸𝑑𝑑𝑡𝑡𝑟𝑟𝑠𝑠0 (3.5) where 𝝈𝝈 is the shear stress tensor, ?̇?𝜸 is the strain tensor and ts is the shearing time (residence time). Typically, there is a critical value of the mechanical work that accelerates the crystallization kinetics. Physically this (mechanical work) can be viewed as a combination of equations for the Wi and that for the De. The Wi should be greater than 3ZE for sufficient period of time (De<1) to reach a critical energy for the onset of crystallization provided that the thermodynamic barrier reaches a certain threshold. The above equation can be transformed in the Eulerian frame and it can be written as: 𝑊𝑊 = � 𝜎𝜎|?̇?𝛾𝑙𝑙||𝑣𝑣|𝑟𝑟0 𝑑𝑑𝑑𝑑 (3.6) Below the mechanical work is derived for capillary and simple shear flows. 22 3.4.1 Capillary Flow If we assume a fully developed flow in a capillary then the shear rate changes from the center to the wall according to the following function [30]: ?̇?𝛾 = ?̇?𝛾𝑊𝑊 � 𝑟𝑟𝐷𝐷/2�1/𝑛𝑛 (3.7) The average shear rate then becomes: ?̇?𝛾𝑎𝑎𝑎𝑎𝑒𝑒𝑟𝑟𝑎𝑎𝑔𝑔𝑒𝑒 = ?̇?𝛾𝑊𝑊1𝑛𝑛 + 1 (3.8) The Mechanical work at a single point for flow in a capillary is: 𝝈𝝈: ?̇?𝜸 = 𝜂𝜂?̇?𝛾2 (3.9) Using a power law expression for the viscosity: 𝜂𝜂 = 𝐾𝐾?̇?𝛾𝑛𝑛−1 (3.10) then W becomes: 𝑊𝑊 = 𝐾𝐾?̇?𝛾1+𝑛𝑛 (3.11) The average mechanical work in a cross section is: ?̄?𝑊 = ∫ ∫ 𝑊𝑊𝑟𝑟𝑑𝑑𝑟𝑟𝑑𝑑𝑟𝑟𝐷𝐷/202𝜋𝜋0𝜋𝜋𝐷𝐷2/4 = ∫ ∫ 𝐾𝐾?̇?𝛾1+𝑛𝑛𝑟𝑟𝑑𝑑𝑟𝑟𝑑𝑑𝑟𝑟𝐷𝐷/202𝜋𝜋0 𝜋𝜋𝐷𝐷2/4 = 2𝑛𝑛1 + 3𝑛𝑛𝐾𝐾?̇?𝛾𝑤𝑤𝑛𝑛+1 𝑊𝑊 = 2𝑛𝑛1 + 3𝑛𝑛 �1 + 𝑛𝑛𝑛𝑛 �𝑛𝑛+1 𝐾𝐾?̇?𝛾𝑎𝑎𝑎𝑎𝑒𝑒𝑟𝑟𝑎𝑎𝑔𝑔𝑒𝑒𝑛𝑛+1 (3.12) Finally, the total average mechanical work integrated along the length of the capillary will be: 𝑊𝑊𝑇𝑇 = ?̄?𝑊𝜏𝜏𝑞𝑞 = ?̄?𝑊 𝑉𝑉𝑄𝑄 = 2𝑛𝑛3𝑛𝑛 + 1𝑉𝑉𝑄𝑄 �1 + 𝑛𝑛𝑛𝑛 �𝑛𝑛+1 𝐾𝐾?̇?𝛾𝑎𝑎𝑎𝑎𝑒𝑒𝑟𝑟𝑎𝑎𝑔𝑔𝑒𝑒𝑛𝑛+1 (3.13) 23 or in terms in terms of the wall shear rate as: 𝑊𝑊 = 𝐾𝐾?̇?𝛾𝑤𝑤1+𝑛𝑛(3𝑛𝑛 + 1)/2𝑛𝑛 𝑉𝑉𝑄𝑄 (3.14) 3.4.2 Simple Shear For simple shear flow in parallel plate, the shear rate is a function of the radius: ?̇?𝛾(𝑟𝑟) = ?̇?𝛾𝑅𝑅 𝑟𝑟𝑅𝑅 (3.15) where ?̇?𝛾𝑅𝑅 is the shear rate at the boundary of the plate. The mechanical work can be easily obtained that: ?̄?𝑊 = ∫ ∫ 𝐾𝐾?̇?𝛾1+𝑛𝑛𝑟𝑟𝑑𝑑𝑟𝑟𝑑𝑑𝑟𝑟𝐷𝐷/202𝜋𝜋0𝜋𝜋𝐷𝐷2/4 𝑡𝑡𝑟𝑟ℎ𝑒𝑒𝑎𝑎𝑟𝑟𝑖𝑖𝑛𝑛𝑔𝑔 = 23 + 𝑛𝑛𝐾𝐾?̇?𝛾𝑅𝑅𝑛𝑛+1𝑡𝑡𝑟𝑟ℎ𝑒𝑒𝑎𝑎𝑟𝑟𝑖𝑖𝑛𝑛𝑔𝑔 (3.16) where tshearing is the total time of shearing. Using this expression up to the point that shear stress increases 50% above its steady-state value (an indication of the crystallization process), one may calculate the critical mechanical work for the onset of crystallization. 24 Chapter 4: Results and Discussion 4.1 Thermal behavior Figure 4.1 and 4.2 show the DSC thermograph for PLA 801 and PLA 804. The glass transition temperature (𝑇𝑇𝑔𝑔 ), crystallization temperature (𝑇𝑇𝑐𝑐 ) and the melting peak (𝑇𝑇𝑚𝑚 ) were calculated through the TA Instrument Universal Analysis software. The glass transition temperatures (62℃ and 63℃) for the two PLAs are similar. In Figure 4.1 for PLA 801, after the glass transition temperature, an exothermic smooth crystallization peak at around 144℃ closely connects the melting peak at 177℃. These indicate that PLA 801 is a semi-crystalline polymer. In Figure 4.2, PLA 804 shows an endothermic melting peak at 156℃ in the first heat-cool cycle, however, the melting peak disappears in the second and third cycles. These indicate that PLA 804 is crystallizable but due to the fast-cooling rate after the first cycle, not enough time is given for PLA 804 to be recrystallized in the second and third cycles. The sluggish crystallization behavior in PLA 804 is due to a higher D- lactide content that prevents the isotactic configuration of the polymer. 25 Figure 4.1: The DSC thermograph for PLA 801. The glass transition temperature, crystallization temperature and melting peak are 62℃, 144℃ and 177oC . Figure 4.2: The DSC thermograph for PLA 804. The glass transition temperature in the first and third cycles are 61℃ and 63℃. The melting peak in the first cycle is 156℃ The percent crystallinity (𝑋𝑋) of the sample for each heat-cool cycle is calculated using equations shown below: 𝑋𝑋 = ∆𝐻𝐻𝑚𝑚 − ∆𝐻𝐻𝑐𝑐∆𝐻𝐻𝑚𝑚° ∗ 100% (4.1) 26 where ∆𝐻𝐻𝑚𝑚 and ∆𝐻𝐻𝑐𝑐 are the absolute value of heat of fusion and cold crystallization, ∆𝐻𝐻𝑚𝑚° is the heat of fusion at equilibrium temperature. The value for ∆𝐻𝐻𝑚𝑚° and equilibrium melting temperature (𝑇𝑇𝑚𝑚° ) are 91 ± 3 J/g and 480K [47]. ∆𝐻𝐻𝑚𝑚 and ∆𝐻𝐻𝑐𝑐 are calculated by integrating the area between the heat flow curve and baseline using the Universal Analysis software by TA Instruments. Integration and value for ∆𝐻𝐻𝑚𝑚 and ∆𝐻𝐻𝑐𝑐 are shown in Appendix A. Relative crystallinity of PLA 801 and PLA 804 for each cycle are shown in Table 4.1. Table 4.1: Percent crystallinity of PLA 801 and PLA 804 for three cycles 𝑋𝑋 (%) Cycle 1 Cycle 2 Cycle 3 PLA 801 47.92 3.84 2.87 PLA 804 29.89 0 0 Comparing the relative crystallinity for each cycle, PLA 804 has a low crystallinity compared with PLA 801 due to higher D-lactic acid content. The crystallization kinetics for both PLAs are extremely slow by observing the crystallinity in cycle 2 and 3 that represent situations after removing the effect of previous thermal history. Therefore, flow becomes a must to accelerate the crystallization kinetics and to improve the crystallinity. 4.2 Linear viscoelastic properties Using the time-temperature superposition, the master linear viscoelastic moduli, G' (storage modulus), G" (loss modulus) and complex viscosity, |𝜂𝜂∗| of PLA 801 and PLA 804 at the reference temperatures of 180℃ and 160℃ were calculated and are depicted in Figure 4.3. The black symbols represent the isothermal frequency sweep results after applying the time-Temperature superposition (tTS) principle that also determines the horizontal shift factors (aT). 27 The vertical shift factors (bT) are taken to be 1. The continuous red lines represent the master curve calculated from Maxwell model (see below). The horizontal shift factors 𝑎𝑎𝑇𝑇, calculated from the application of tTS are plotted with the inverse of temperature (Figure 4.4) to calculate the activation energies (Ea) according to Arrhenius equation shown below. The flow activation energy for PLA 801 and PLA 804 are 88±4.4 kJ/gmol and 124±6.5 kJ/gmol respectively. These values agree with previously reported values for various PLAs, ranging from 80 kJ/mol to 177.65 kJ/mol [4, 38, 39]. 𝑑𝑑𝑙𝑙𝑔𝑔𝑎𝑎𝑇𝑇 = 𝐸𝐸𝑎𝑎𝑅𝑅 �1𝑇𝑇 − 1𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟� (4.2) The relaxation spectrum of both polymers was calculated using the Maxwell model described by the following equations below. 𝐺𝐺′(𝜔𝜔) = �𝑔𝑔𝑖𝑖 (𝜔𝜔𝜆𝜆𝑖𝑖)21 + (𝜔𝜔𝜆𝜆𝑖𝑖)2𝑁𝑁𝑖𝑖=1 (4.3) 𝐺𝐺′′(𝜔𝜔) = �𝑔𝑔𝑖𝑖 𝜔𝜔𝜆𝜆𝑖𝑖1 + (𝜔𝜔𝜆𝜆𝑖𝑖)2𝑁𝑁𝑖𝑖=1 (4.4) where 𝜔𝜔 is the frequency of oscillation, 𝑔𝑔𝑖𝑖 are relaxation modulus corresponding to relaxation time 𝜆𝜆𝑖𝑖, N is the number of relaxation modes. Results for 6 modes of 𝑔𝑔𝑖𝑖 and 𝜆𝜆𝑖𝑖 for the two PLAs are shown in Table 4.2. The G' and G" calculated using the Maxwell model parameters nicely fit the experiment data and are shown in Figure 4.3 in continuous red lines. 28 Figure 4.3: a) The master curves of the linear viscoelastic moduli, G’, G” and complex viscosity of PLA 801 at the reference temperature of 180℃ (b) The master curves of the linear viscoelastic moduli, G’, G” and complex viscosity of PLA 804 at the reference temperature of 160℃ Figure 4.4: The horizontal shift factor, 𝐚𝐚𝐓𝐓 for a) PLA 801 and b) PLA 804 at the reference temperature of 180 ℃ and 160℃ 29 Table 4.2: Maxwell model parameters (g, λ) for sample PLA 801 master curve at T=180°C and PLA 804 master curve at T=160°C PLA 801 PLA 804 180℃ 160℃ 𝑔𝑔𝑖𝑖 (Pa) 𝜆𝜆𝑖𝑖 (s) 𝑔𝑔𝑖𝑖 (Pa) 𝜆𝜆𝑖𝑖 (s) 4.12E+05 6.52E-04 4.23E+05 1.51E-04 1.89E+05 4.51E-03 3.38E+05 1.56E-03 7.36E+04 2.07E-02 2.10E+05 1.09E-02 1.30E+04 8.99E-02 6.88E+04 6.51E-02 8.07E+02 4.73E-01 7.00E+03 4.05E-01 7.27E+00 8.26E+00 1.74E+02 3.58E+00 4.3 Reptation times The TMA modified tube-based model [28] was employed to determine the equilibration time of the polymers (𝜏𝜏𝑒𝑒) as well as the reptation and Rouse times which are functions of the equilibration time (Eqns 4.4 and 4.5) in Figure 4.5. The plateau modulus 𝐺𝐺𝑁𝑁𝑜𝑜 for the two polymers was found to be 1.1 MPa, which is within the range reported by Dorgan et al. of 1.0 MPa ± 0.2 [40]. The value for molecular weight between two entanglements (Me) was found to be 4400 g/mol according to Othman et al. [38]. The best set of PDI and 𝜏𝜏𝑒𝑒 are 2*10-8 s and 1.83 for PLA 801 and PLA 804 and 3 ∗ 10−8s at 160 °C. The difference in this equilibration time (although quite similar in magnitude) might be related to the different chirality of the backbone. Fittings for the model are shown in Figure 4.5. The reptation times 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟 and Rouse times 𝜏𝜏𝑅𝑅 can be calculated using the equations shown below [41]. 𝜏𝜏𝑅𝑅 = 𝜏𝜏𝑒𝑒𝑍𝑍2 = 𝜏𝜏𝑒𝑒(𝑀𝑀𝑤𝑤/𝑀𝑀𝑒𝑒)2 (4.5) 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟 = 3𝜏𝜏𝑒𝑒𝑍𝑍3 = 3𝜏𝜏𝑒𝑒(𝑀𝑀𝑤𝑤/𝑀𝑀𝑒𝑒)3 (4.6) 30 These characteristics times are summarized in Table 4.3. Figure 4.3 shows the complex viscosities for the two PLAs at the reference temperature of 160°C. From this Figure 4.3, the terminal zone has been reached and the zero-shear viscosity could be deduced from the graph. PLA 801 which has a larger reptation time has a higher zero-shear viscosity compared with PLA 804 at the same temperature. Table 4.3: The reptation and Rouse times of the two PLAs of Mw=180 kg/mol Polymer at 𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟=160°C Reptation time, 𝜏𝜏𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟at 𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟 Rouse time, 𝜏𝜏𝑅𝑅 (s) at 𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟 PLA801 0.049 1.83e-4 PLA804 0.038 1.69e-4 Figure 4.5: Master curves at 160°C predicted by Maxwell model and TMA tube-based model for a) PLA 801 and b) PLA 804 4.4 Shear-induced/enhanced isothermal crystallization The effect of crystallization temperature and shear rate on shear-induced crystallization is discussed in this section in terms of the evolution of shear stress growth coefficient, 𝜂𝜂+ or normalized shear stress growth coefficient, 𝜂𝜂+/𝜂𝜂0 where 𝜂𝜂0 is the steady state viscosity at a certain temperature and shear rate. In Figure 4.6, the shear stress growth coefficient was measured as a 31 function of time to observe the effect of temperature on the onset of crystallization at fixed shear rate 0.01 s-1. Crystallization occurs after a certain time that causes a sudden increase in the shear stress growth coefficient, 𝜂𝜂+. At higher temperatures, the steady-state before crystallization is longer than that at a lower temperature. In other words, the induction time for the onset of crystallization decreases significantly at lower temperatures. Figure 4.6: The normalized shear stress growth coefficient η+ of PLA 801 at temperatures of 150℃, 140℃, 130℃, and 120℃ at shear rate of 0.01 s-1. The sudden increase of this coefficient marks the onset of crystallization The effect of shear rate on the onset of crystallization is examined in Figure 4.7 at various temperatures. At 140℃ and 130℃, the growth curve of normalized shear growth coefficients at 0.005s-1 and 0.01s-1 overlaps, implying 0.01s-1 is the critical shear rate, below which the crystallization kinetics are not affected by the shear flow. Above 0.01s-1, increasing the shear rate 32 significantly affects (accelerates) the onset of crystallization at 150℃, 140℃ and 130℃. At 120℃, the time difference of the onset of crystallization between each shear rate reduces. Figure 4.7: The shear stress growth coefficient η+/η0 of PLA 801 at shear rate ranging from 0.005 s-1 to 0.3s-1 and crystallization temperature at a) 150℃, b) 140℃, c) 130℃, and d) 120℃. The sudden increase of this coefficient marks the onset of crystallization The induction time for the onset of crystallization at each shear rate is depicted in Figure 4.8. The time required to observe the onset of crystallization (induction time, 𝑡𝑡𝑖𝑖𝑛𝑛𝑖𝑖) is defined as the time at which 𝜂𝜂+ becomes 50% greater than its steady state value. In general, the induction time follows the following trend; when the shear rate is larger than a critical value (0.01 s-1 for this sample), increasing shear rate reduces the induction time at 33 all temperatures as shown in Figure 4.8. When shear rate is less than 0.01 s-1, the induction time is independent of shear rate and the crystallization kinetics is similar to that obtained in quiescent crystallization. Figure 4.8: Induction time variation with shear rate at different temperatures for PLA 801 Using the reptation times obtained from the TMA modified tube-based model, the Wi and De for each shear-induced crystallization experiment are calculated and are plotted in Figure 4.9. Limited effect of flow was exerted on the crystallization kinetics since the maximum Wi is smaller than 0.1. In other words, the crystallization observed in these experiments are temperature driven. As seem from the DSC, the crystallization temperature is 144℃ and the melting point 177℃. Therefore, the samples will crystallize eventually given enough time since the temperatures at which were exposed are below 𝑇𝑇𝑐𝑐, except the T of 150℃. These experiments show clearly the acceleration of crystallization kinetics and they can be referred to as flow-enhanced crystallization and not as flow-induced crystallization. 34 In quiescent crystallization, the shortest crystallization half-time was found at temperature around 110℃ for PLA with D-lactic acid content less than 2% and Mn less than 157 kg/mol [42]. According to Hoffman-Lauritzen theory [43,44], the fastest crystallization kinetics is associated with the competition between the surface nucleation rate (negatively related to the temperature) and the chain deposition rate on the formed crystal (related to the chain mobility). The minimum crystallization half-time for PLA 801 will occur at temperature higher than 110℃ since the large molecular weight reduces the chain mobility. When the temperature further reduces to 120℃ which is near the fastest crystallization temperature, the rate of nucleation increase due to the small shear flow is negligible compared to the increase due to the temperature change. This is probably the reason that the induction time is almost independent of shear rate at 120℃. As shown in Figure 4.9, the relation between De and Wi at a certain temperature can be nicely fitted using a power-law trendline. Given a certain shear rate (fixed Wi), the shearing time has to be larger than a critical value for the onset of crystallization. Therefore, only below the drawn trendlines will flow-enhanced crystallization occur. Based on this figure, we can approximate whether or not crystallization occur at a certain temperature, shear rate (Wi) and shearing time (De). 35 Figure 4.9: Deborah number versus Weissenberg number for shear induced crystallization at various shear rates and temperatures 4.5 Uniaxial Extensional flow-induced isothermal crystallization Figure 4.10 plots the tensile stress coefficient of PLA 801 in steady uniaxial extension at various Hencky strain rates measured at 100℃, 110℃, 120℃ and 130℃. At 100℃ to 130℃ temperature range, strain softening was observed at low shear rates (shear rates that do not induce crystallization, it is an effect of sagging) mainly because of the linear molecular architecture of PLA chain [22]. Strain softening behavior was also observed in PLA 7001D between 160-170℃ [38] due to the weakness of PLA of its low heat deflection temperature. The onset of crystallization was first observed at the Hencky strain rate of 4 s-1, and 130℃ with the tensile stress growth coefficient (extensional viscosity) first shown a decrease followed by a sharp increase. The dip before crystallization is due to necking of the sample caused by the transition from liquid-like to solid-like. As the crystallization temperature further decreases to 100℃, necking at high Hencky 36 strain rate (8s-1) is barely noticeable due to the fast onset of crystallization. At the same temperature, increasing the Hencky strain rate leads to decrease in the onset time of crystallization. As the crystallization temperature decreases, the smallest Hencky strain rate that gives rise to crystallization decrease from 4 s-1 to 0.1 s-1 from 130℃ to 100℃. Figure 4.10: Tensile stress growth coefficient as a function of time measured at various Hencky strain rate at 130℃, 120℃, 110℃, and 100℃ for PLA 801 Figures 4.11 plots the tensile stress coefficient of PLA 804 in steady uniaxial extension at various Hencky strain rates measured at 100℃, 110℃, 120℃ and 130℃. The highest temperature that gives rise to crystallization (with maximum strain rate equals to 8 s-1) is similar to PLA 801 which is 130℃. At this temperature, at strain rate equals to 1.0 s-1, the initiation of crystallization 37 was observed with a small amplitude of increase in viscosity after necking. At strain rate equals to 8 s-1, crystallization was more pronounced. Decreasing the crystallization further to 110℃ and 100℃, the crystallization strain rate decreases to 0.3 s-1 and 0.1 s-1. Figure 4.11: Tensile stress growth coefficient as a function of time measured at various Hencky strain rate at 130℃, 120℃, 110℃, and 100℃ for PLA 804 The induction time is next considered to quantitively characterize the effect of temperature and strain rate on crystallization. This is defined as the time that the extensional viscosity increases sharply after necking. Figure 4.12 and 4.13 show the effect of temperature and strain rate on induction time of crystallization for PLA 801 and PLA 804. The induction time for a certain shear rate did not show any dependence on temperature which is different from what was observed in 38 shear flow. As the strain rate increases, the effect of temperature further reduces for PLA 801 due to the increase of nuclei density. The overlap temperatures between the flow-induced crystallization in shear and in extensional flow are 130℃ and 120℃. The induction times in shear flow are above 100s over the shear-rate range from 0.1s-1 to 1s-1 (inset of Figure 4.12, replotted here for the sake of convenience to the reader), which is more than 10 times larger than those in extensional flow. This suggests that extensional flow is a much stronger flow than shear in accelerating the crystallization kinetics. However, it becomes difficult to build strong extensional flows in processing dies to impose/induce such phenomena. Figure 4.12: Induction time as a function of Hencky strain rate at various temperatures for PLA 801 39 Figure 4.13: Induction time as a function of Hencky strain rate at various temperatures for PLA 804 As seen in Figure 4.14, the induction time heavily depends on the Hencky strain as expected and reported in the past for HDPE and PB [45,46]. The average induction strains for the initialization of crystallization at all four temperatures are 1.08 ± 0.2 and 1.13 ± 0.25 for PLA 801 and PLA 804 (as shown in Figure 4.13) which are smaller than the previous report, from 2.0 to 2.5 [45,46]. Reports on extensional-induced crystallization on PLA is limited. A critical strain of 1.5 was found for a branched PLA to initialize extensional-induced crystallization [22] which is comparable to the present results. 40 Figure 4.14: Induction time as a function of Hencky strain at various temperatures for a) PLA 801 and b) PLA 804 4.6 Capillary flow-induced isothermal crystallization The capillary experiments were compared to the linear viscoelastic results on the assumption of the validity of the Cox-Merz rule in the absence of crystallization. The Rabinovitch correction was applied to the capillary results to fairly compare with the LVE, in other words the apparent shear rate was corrected to obtain the wall shear rate. As noted above, the Bagley correction (ends pressure) was neglected in view of the use of a long capillary die. Figures 4.15 shows the wall shear stress, 𝜎𝜎𝑤𝑤 (defined as 𝜎𝜎𝑤𝑤 ≡ 𝛥𝛥𝑒𝑒/4(𝐿𝐿/𝐷𝐷) where 𝛥𝛥𝑒𝑒 is the measure pressure drop along the capillary) versus wall shear rate at 190℃ for PLA801 respectivey. The capillary results are in agreement with LVE results obtained from rotational rheometer for these temperatures, which suggests the reliability of the capillary measurements and furthermore that no crystallization occurs at the corresponding temperature and shear rate within the timeframes considered in these experiments. In other words, at a given flow rate, the pressure drop was steady for a long period of time, indicating no crystallization. 41 Figure 4.15: Wall shear stress versus wall shear rate for PLA 801 at 190℃ from capillary experiments compared with the LVE data from a rotational rheometer suggesting the validity of the Cox-Merz rule and the absence of any crystallization However, starting at 172℃, crystallization was observed at relative low shear rates (>20s-1). PLA crystallization in capillary rheometer does not show a similar behavior as that in polypropylene (PP) where when crystallization happens, a constant shear stress that is larger than the steady state value can be observed [45]. Instead, in PLA, the shear stress (pressure drop) keeps increasing apparently unboundedly. This is possibly to the occurrence of gradual crystallization that is blocking gradually the entrance to the capillary die. Figures 4.16 depicts transients of shear stress (pressure drop) with time at fixed shear rates from 172℃ to 156℃. The complex modulus (LVE data) at a constant frequency from LVE are shown as dotted line. These lines indicate the shear stress expected in the absence of crystallization, essentially the values of shear stresses which would have been attained without the occurrence of crystallization. 42 Crystallization at 172 ℃ was first observed at 10 s-1 where the corresponding stress curve shows an increasing trend with time without reaching a steady state value (Figure 4.16a). As the shear rate increases, the increasing trend becomes steeper. In Figure 4.16b where the crystallization experiments are performed at 167℃ (5 ℃ lower), the smallest shear rate that gives rise to crystallization decreases from 10 s-1 to 5 s-1. All the curves show a parabolic increase after rapidly tending to reach a steady state value. The increase becomes steeper and steeper with increase of the apparent shear rate. When the crystallization temperature further decreases as shown in Figures 4.16c and 4.16d, the stress curves show a fast linear increase with time leading catastrophically to the die blockage. The slopes of these lines become steeper with increase of shear rate. The induction time for crystallization is defined here as the time when the shear stress reaches 1.5 times its steady state complex modulus as shown by the dotted lines in Figure 4.16 (a similar definition was used in the crystallization experiments in simple shear flow). Figure 4.17 shows the induction time as a function of shear rate at various temperatures. The induction time significantly reduces when the crystallization temperature decreases from 172℃ to 167℃ and from 167℃ to 161℃. Moreover, the induction time decreases with increasing of shear rate. At 161℃ and 156℃, the induction times are close and there is a slightly increase in the induction times from 161℃ to 156℃. 43 Figure 4.16: Wall shear stress versus time (pressure transients) at various apparent shear rates for PLA 801 at a)172℃, b) 167℃, c) 161℃ and d) 156 ℃ Figure 4.17: Crystallization induction time versus wall shear rate at various crystallization temperature for PLA 801 44 The Wi was also calculated for the capillary experiment are listed in Table 4.4. Wi ranges from 0.24 to 1.3 at 161℃ and 156℃. The temperature that gives the minimum induction time in the shear rate and temperature range above is 156℃ which is higher than 120℃ that was observed for flow-enhanced crystallization in simple shear flow, due to the increase of nucleation rate brought by the much stronger shear flow. The flow effect compensates the increase of nucleation density brought by reducing crystallization temperature. Table 4.4: Weissenberg number for PLA 801 and PLA 804 at various temperature and shear rate PLA 801 PLA 804 Temperature (℃) Shear rate (s-1) Wi Temperature (℃) Shear rate (s-1) Wi 172 47 1.16 150 51 3.88 22 0.55 24 1.82 11 0.26 145 56 6.10 167 48 1.54 24 2.64 23 0.74 12 1.29 11 0.35 6 0.63 5 0.17 140 59 9.20 161 23 1.06 26 4.07 11 0.50 12 1.84 5 0.24 6 0.92 156 24 1.44 135 29 6.40 11 0.68 12 2.74 5 0.32 6 1.30 45 Figure 4.18: Wall shear stress versus wall shear rate for PLA 804 at 170℃ from capillary experiments compared with the LVE data from a rotational rheometer suggesting the validity of the Cox-Merz rule and the absence of any crystallization Similar results are presented for PLA 804 in Figures 4.18 and 4.19. Crystallization starts at 150℃ and 10 s-1 where the increase of stress after reaching steady state value is small although with the tendency to increase. Increasing the shear rate to 40 s-1 greatly enhances the crystallization rate. Similar crystallization behavior of PLA 801 can be observed in PLA 804 when the crystallization temperature decreases. 46 Figure 4.19: Wall shear stress versus time (pressure transients) at various apparent shear rates for PLA 804 at a) 150℃, b) 145℃, c) 140℃ and d) 135 ℃ Figure 4.20 shows how induction time changes with shear rate at various crystallization temperature. Temperature has a less of an effect on induction time for tests at high shear rates (40 s-1 and 20 s-1) than those at low shear rates (10 s-1). From Table 4.3, Wi for all the high shear rate trials is large than 2 indicating the effect of flow (increasing the nucleation rate) is strong enough that the effect brought by temperature (increasing nucleation density) is negligible compared to flow. Crystallization at140℃ gives the smallest induction times. 47 Figure 4.20: Crystallization induction time versus apparent shear rate at various crystallization temperatures for PLA 804 Figure 4.21 summarizes the results of induction times in both simple shear flow and capillary flow for PLA 801. Although the temperatures presented in the two apparatuses are not completely the same, the general trend for induction times are in agreement. Open symbols are from simple shear and closed symbols from capillary testing. The crystallization induction time decreases by more than 50 times by increasing shear rate from 0.01s-1 to 20 s-1 at around 150℃. 48 Figure 4.21: Induction time versus shear rate for PLA 801 in both simple shear and capillary flow Overall, the flow regions for simple shear and capillary experiments range from first to the second regions where crystals are found in spherulite structure as shown in Figure 4.22. To reach the third or fourth region where shish-kebab structure could be found, shear rate needs to reach 5000 s-1 at 160℃ which is almost impossible in lab scale experiment. 49 Figure 4.22: Schematic illustration of the four flow regimes of crystallization of polymers with flow regions for experiments highlighted in blue 4.7 Mechanical work and crystallization Figure 4.23 shows the mechanical work required to induce crystallization at various Weissenberg number for shear-induced crystallization and crystallization in capillary flow for PLA 801 and PLA 804. Mechanical work for each capillary and rotational trial are calculated using equations 3.16 and 3.18 derived in the chapter 3. Since the Weissenberg number describes the degree of chain orientation of the polymer that experience a constant stretch history, Figure 4.23 below represents how temperature affects the work required to induce crystallization at different level of chain orientation. For PLA 801 in shear-induced crystallization where Wi is lower than 0.1, work is highly dependent on temperature and thus crystallization observed is temperature driven and resembles that of the quiescent crystallization. As the temperature increases, higher chain mobility (shorter reptation time) hinders the formation of nuclei and thus, more work is required at higher 50 crystallization temperature to achieve the same degree of chain orientation. In capillary flow induced crystallization, work is found to be independent of temperature for Wi>1. For PLA 804, most of the trials shown in Figure 4.23 have Wi larger than 1 and W is independent of temperature. Thus, these experiments are in the flow-induced crystallization region. As Wi is larger than 1, W is independent of both temperature and Wi. This indicates that in capillary flow-induced crystallization region, the required amount of work done on the polymer to achieve different degree of molecular stretch is the same. Comparing the work at Wi higher than 1 for the two PLAs, less work is required to achieve the same degree of chain orientation for PLA 801 than that for PLA 804 due to the higher D-lactide content in PLA 804 that disturbs the isotactic molecule configuration. Figure 4.23: Mechanical work required to induce crystallization versus various Weissenberg number for flow-induced crystallization in shear and capillary flow for PLA 801 and PLA 804 51 Chapter 5: Conclusions and Recommendations In summary, simple shear, extensional and capillary flow-induced/enhanced crystallization was studied for two PLLA samples with different L-lactide content over a range of temperature and strain rate. Mechanical work required to induce crystallization are introduced to combine the effect of strain rate and flow time on the onset of crystallization. Flow-enhanced crystallization is observed in simple shear flow with Weissenberg number smaller than 1. Above a critical shear rate 0.05s-1, the induction time for crystallization is found to decrease with increasing shear rate and decreasing temperature. Below the critical shear rate, the effect of flow is negligible and the crystallization kinetics was comparable to that of quiescent crystallization. The amount of mechanical work required increases to achieve a higher degree of chain orientation and decreases as temperature decreases. Different from what we observe in simple shear flow, induction times for crystallization in extensional flow depend on a critical Hencky strain rate and were found to be independent of temperature. Induction times decreases with increasing strain rate. A critical strain, around 1.1 was found for the two PLLA samples, above which crystallization could be observed. The crystallization induction times decreases by more than 40 folds by increasing the shear rate to 10 s-1 in capillary flow. Work is independent of temperature for all flow regions and become independent of Wi in flow-induced crystallization region. Several recommendations are proposed for the future work: • The change in the final crystallinity as the flow region migrates from flow-enhanced crystallization to flow-induced crystallization needs to be validated using DSC. 52 • A way to compare the mechanical works for capillary flow and simple shear flow is still not available due to the different mechanisms for generating capillary flow and simple shear flow. • A proper isothermal crystallization model such as suspension model could be proposed in order to capture the crystallization behavior and provide model parameters for process optimization. • The samples are easily prone to sagging in SER fixture at temperatures close to sample melting temperature. Adding an oil bath to the SER fixture to overcome the gravity may be a good option to get more accurate experimental data. 53 Bibliography 1. R.E. Drumright, P.R. Gruber, and D.E. Henton, “Polylactic Acid Technology”, Advanced Materials 12, 1841 (2000). 2. V. Nagarajan, A.K. Mohanty, and M. Misra, “Perspective on Polylactic Acid (PLA) based Sustainable Materials for Durable Applications: Focus on Toughness and Heat Resistance”, ACS Sustainable Chemistry & Engineering 4, 2899 (2016). 3. G. Liu, X. Zhang, and D. Wang, “Tailoring Crystallization: Towards High-Performance Poly(lactic acid)”, Advanced Materials 26, 6905 (2014). 4. N. Othman, C. Xu, P. Mehrkhodavandi, and S.G. Hatzikiriakos, “Thermorheological and mechanical behavior of polylactide and its enantiomeric diblock copolymers and blends” Polymer 53, 2443 (2012). 5. THERMAL PROPERTIES OF BIO-BASED POLYMERS (SPRINGER NATURE, Place of Publication Not Identified, 2020). 6. A. KELLER,” Morphology of crystallizing polymers” Nature 169, 913 (1952). 7. G.W.M. Peters, L. Balzano, and R.J.A. Steenbakkers, Handbook of Polymer Crystallization 399 (2013). 8. C.E. Carraher and R.B. Seymour, Seymour/Carraher's Polymer Chemistry (M. Dekker, New York, 2003). 9. E. Piorkowska and G. C. Rutledge, Handbook of polymer crystallization (John Wiley & Sons, 2013). 10. H. Janeschitz-Kriegl and E. Ratajski, “Some fundamental aspects of the kinetics of flow-induced crystallization of polymers,” Colloid and Polymer Science 288, 1525–1537 (2010). 54 11. P. C. Roozemond, M. van Drongelen, Z. Ma, M. A. Hulsen, and G. W. Peters, “Modeling flow-induced crystallization in isotactic polypropylene at high shear rates,” Journal of rheology 59, 613–642 (2015). 12. F. J. M. F. Custdio, R. J. A. Steenbakkers, P. D. Anderson, G. W. M. Peters, and H. E. H. Meijer, “Model development and validation of crystallization behavior in injection molding prototype flows,” Macromolecular Theory and Simulations 18, 469–494 (2009). 13. P. C. Roozemond, T. B. van Erp, and G. W. Peters, “Flow-induced crystallization of isotactic polypropylene: Modeling formation of multiple crystal phases and morphologies,” Polymer 89, 69–80 (2016). 14. R. S. Graham, “Understanding flow-induced crystallization in polymers: A perspective on the role of molecular simulations,” Journal of Rheology 63, 203–214 (2019). 15. Wang et al., “Confined crystallization of PEO in Nanolayered Films Impacting Structure and Oxygen Permeability”, Macromolecules, 42, 7055-7066, (2009) 16. M. Derakhshandeh, B. Jazrawi, G. Hatzikiriakos, A.K. Doufas and S.G. Hatzikiriakos, “Flow Induced Crystallization of Polypropylenes in Capillary Flow,” Rheol. Acta, 54, 207-221 (2015). 17. M. Derakhshandeh, G. Mozaffari, A.K. Doufas and S.G. Hatzikiriakos, “Quiescent Crystallization of Polypropylenes: Experiments and Modelling,” Journal of Polymer Science Part B: Polymer Physics, 52, 1259-1275 (2014). 18. M. Derakhshandeh, A.K. Doufas and S.G. Hatzikiriakos, “Quiescent and Shear-Induced Crystallization of Polypropylenes,” Rheologica Acta, 53, 519-535 (2014). 19. J.V. Meerveld, G.W.M. Peters, and M. Hütter, “Towards a rheological classification of flow induced crystallization experiments of polymer melts” Rheologica Acta 44, 119 (2004). 55 20. Y. Zhong, H. Fang, Y. Zhang, Z. Wang, J. Yang, and Z. Wang,” Rheologically Determined Critical Shear Rates for Shear-Induced Nucleation Rate Enhancements of Poly(lactic acid)” ACS Sustainable Chemistry & Engineering 1, 663 (2013). 21. H. Xu, L. Xie, and M. Hakkarainen, “Beyond a Model of Polymer Processing-Triggered Shear: Reconciling Shish-Kebab Formation and Control of Chain Degradation in Sheared Poly(L‑lactic acid)”ACS Sustainable Chemistry & Engineering 3, 1443 (2015). 22. A. Jalali, M.A. Huneault, M. Nofar, “Effect of branching on flow-induced crystallization of poly (lactic acid)” P.C. Lee, and C.B. Park, European Polymer Journal 119, 410 (2019). 23. A. Jalali, S. Shahbikian, M.A. Huneault, and S. Elkoun, “Effect of molecular weight on the shear-induced crystallization of poly(lactic acid)” Polymer 112, 393 (2017). 24. M. Farah and R.E.S. Bretas, “Characterization of i‐PP shear‐induced crystallization layers developed in a slit die”, Journal of Applied Polymer Science 91, 3528 (2004). 25. L. Scelsi and M.R. Mackley, “Rheo-optic flow-induced crystallisation of polypropylene and polyethylene within confined entry–exit flow geometries”, Rheologica Acta 47, 895 (2008). 26. J.N. Ness and J.Z. Liang, “A study of rheological properties and crystallization behavior for HDPE melts during extrusion”, Journal of Applied Polymer Science 48, 557 (1993). 27. C. Hadinata, C. Gabriel, M. Ruellmann, N. Kao, and H.M. Laun,” Shear-induced crystallization of PB-1 up to processing-relevant shear rates”, Rheologica Acta 45, 539 (2006). 28. E.van Ruymbeke, R. Keuning, and C. Bailly, “Prediction of linear viscoelastic properties for polydisperse mixtures of entangled star and linear polymers: Modified tube-based model and comparison with experimental results”, Journal of Non-Newtonian Fluid Mechanics 128, 7 (2005). 56 29. A.J. Malkin and A.I. Isayev, Rheology Concepts, Methods, and Applications (ChemTec Publishing, Toronto, 2017). 30. C.W. Macosko, Rheology Principles, Measurements, and Applications (VCH, New York, NY, 1994). 31. R.D. Andrews and A.V. Tobolsky, “Elastoviscous properties of polyisobutylene. IV. Relaxation time spectrum and calculation of bulk viscosity” Journal of Polymer Science 7, 221 (1951). 32. S.L.C. E., Physical Aging in Amorphous Polymers and Other Materials (Elsevier, Amst., 1978). 33. J.D. Ferry, Viscoelastic Properties of Polymers (John Wiley & Sons, New York, 1980). 34. Z. Zhang and S.G. Hatzikiriakos, “Entry pressure correlations in capillary flow” Physics of Fluids 32, 073106 (2020). 35. Xpansion Instruments - SER Principle of Operation. 36. E.E. Bischoff White, H.H. Winter, J.P. Rothstein, “Extensional FIC of i-PP,” Rheol. Acta, 51, 303-314 (2012). 37. O.O. Mykhayluk et al., “Control of structural morphology in Shear induced crystallization of polymers,” Macromolecules, 43, 2389-2405 (2010). 38. N. Othman, A. Acosta-Ramírez, P. Mehrkhodavandi, J.R. Dorgan, and S.G. Hatzikiriakos, “Solution and melt viscoelastic properties of controlled microstructure poly(lactide)”, Journal of Rheology 55, 987 (2011). 39. N. Othman, B. Jazrawi, P. Mehrkhodavandi, and S.G. Hatzikiriakos, “Wall slip and melt fracture of poly(lactides)”, Rheologica Acta 51, 357 (2011). 57 40. J.R. Dorgan, J. Janzen, M.P. Clayton, S.B. Hait, and D.M. Knauss, “Melt rheology of variable L-content poly(lactic acid)”Journal of Rheology 49, 607 (2005). 41. S. T. Milner, T. C. B. McLeish, “Reptation and Contour-Length Fluctuations in Melts of Linear Polymers”, Phys. Rev. Lett 81,725 (1998). 42. S. Saeidlou, M.A. Huneault, H. Li, and C.B. Park, “Poly(lactic acid) crystallization”, Progress in Polymer Science 37, 1657 (2012). 43. J.I. Lauritzen and J.D. Hoffman, “Extension of theory of growth of chainfolded polymer crystals to large undercoolings”, Journal of Applied Physics 44, 4340 (1973). 44. J.D. Hoffman, G.T. Davis, and J.I. Lauritzen, “The rate of crystallization of linear polymers with chain folding”, Treatise on Solid State Chemistry 497 (1976). 45. M. Derakhshandeh and S.G. Hatzikiriakos, “Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension”, Rheologica Acta 51, 315 (2011). 46. C. Hadinata, D. Boos, C. Gabriel, E. Wassner, M. Rüllmann, N. Kao, and M. Laun, “Elongation-induced crystallization of a high molecular weight isotactic polybutene-1 melt compared to shear-induced crystallization”, Journal of Rheology 51, 195 (2007). 47. Pyda, M., ed. ATHAS Data Bank, 2008, http://athas.prz.rzeszow.pl/Default.aspx?op=db. 58 Appendix The DSC thermograph showing the baselines and heats for exothermic peak of cold crystallization and endothermic melting peak and of PLA 801 and PLA 804 for the three cycles. Figure A.1: DSC thermograph of PLA 801 in the first cycle, where ∆𝑯𝑯𝒎𝒎 and ∆𝑯𝑯𝒄𝒄 are 46.25 J/g and 2.64 J/g Figure A.2: DSC thermograph of PLA 801 in the second cycle, where ∆𝑯𝑯𝒎𝒎 and ∆𝑯𝑯𝒄𝒄 are 20.10 J/g and 16.61 J/g 59 Figure A.3: DSC thermograph of PLA 801 in the third cycle, where ∆𝑯𝑯𝒎𝒎 and ∆𝑯𝑯𝒄𝒄 are 20.04 J/g and 17.43 J/g Figure A.4: DSC thermograph of PLA 804 in the first cycle, where ∆𝑯𝑯𝒎𝒎 is 26.29 J/g and no cold crystallization process presented 60 Figure A.5: DSC thermograph of PLA 804 in the second cycle, no cold crystallization and melting process presented Figure A.6: DSC thermograph of PLA 804 in the third cycle, no cold crystallization and melting process presented
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Flow-induced crystallization of polylactides in capillary flow
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