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Yes / Orientation direction dependency of whitening activated at large strains was studied using four pre-oriented isotactic polypropylene (iPP) samples with different molecular weights stretched along different directions with respect to the pre-orientation (0°, 45°, and 90°) by means of in situ wide-, small-, and ultra-small-angle X-ray scattering techniques. A macroscopic fracture of iPP materials was also observed following the stress whitening at large strains. These two associated processes in pre-oriented iPP samples at elevated temperatures were found to be governed by not only the molecular weight of iPP but also the pre-orientation direction. For a certain pre-orientation direction of iPP, both the critical stress of cavitation induced-whitening and failure stress increased with increasing molecular weight. For one given molecular weight, the pre-oriented iPP showed the smallest critical stress for whitening and failure stress along the pre-orientation direction (0°) while the samples displayed larger values for the same behaviors when stretched at 45° or 90° with respect to the pre-orientation direction. Such behavior suggested that oriented amorphous networks, with different mechanical strengths, can be generated during the second deformation processes in these pre-oriented iPP samples. The evolution of inter-fibrillar tie chains in highly oriented amorphous networks was considered as the main factor controlling the response of the inner network to the external stress since the cavitation-induced whitening activated at large strains was caused by the failure of load bearing inter-fibrillar tie chains in the oriented amorphous network.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16879号 / 工博第3600号 / 新制||工||1544(附属図書館) / 29554 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 金谷 利治, 教授 長谷川 博一, 教授 辻井 敬亘 / 学位規則第4条第1項該当

Yes / Cavitation is an important phenomenon in solid-phase deformation of polymers, which either has potential adverse effects on physical properties or creates potential opportunities for new properties. In either case, it needs to be better understood to help achieve better control of cavitation and its effects. Cavitation associated with solid-phase deformation in a β-nucleated isotactic polypropylene was found to depend on the solid-phase deformation route employed. Compared with samples obtained by free tensile stretching, cavitation was suppressed in samples deformed via die-drawing, although an almost identical β-to α-phase transition was observed for both deformation routes. Even when die-drawn samples were subsequently deformed to large strains by free stretching, there was still no comparable cavitation compared with the single free tensile-stretching route. The die-drawing process appears to suppress cavitation by fundamentally diminishing the number of growable nuclei of cavities, rather than merely hindering the growth of cavities. A relationship between cavitation intensity and the fractions of lamellae along specific directions has been established. During subsequent free stretching of die-drawn samples, newly created cavities were suggested to be initiated within the crystalline layers. The reduction of the cavity nuclei in the die-drawing process originated from the stabilization of the connections between the crystalline blocks within the lamellae. / This work is supported by the National Natural Science Foundation of China (21704102 and 51525305), Newton Advanced Fellowship of the Royal Society, United Kingdom (NA 150222) and ExxonMobil.

No / Crystallization of an isotactic polypropylene (iPP) homopolymer and two propylene/ethylene random copolymers (RACO), induced by high-stress shear, was studied using in situ synchrotron wide-angle X-ray diffraction (WAXD) at 137 °C. The “depth sectioning” method (Fernandez-Ballester Journal of Rheology 2009, 53 (5), 1229−1254) was applied in order to isolate the contributions of different layers in the stress gradient direction and to relate specific structural evolution to the corresponding local stress. This approach gives quantitative results in terms of the specific length of fibrillar nuclei as a function of the applied stress. As expected, crystallization becomes faster with increasing stress—from the inner to the outer layer—for all three materials. Stress-induced crystallization in a RACO with 7.3 mol % ethylene content was triggered at only 1 °C below its nominal melting temperature. The comparison of iPP and RACO’s with 3.4 and 7.3 mol % ethylene monomer reveals the effect of ethylene defects on high-stress shear induced crystallization at 137 °C. It is found that, for a given applied stress, the specific nuclei length formed by flow increases with ethylene content—which is attributed to a greater high molecular weight tail. However, the linear growth rate is significantly reduced by the presence of ethylene comonomers and it is found that this effect dominates the overall crystallization kinetics. Finally, a time lag is found between development of parent lamellae and the emergence of daughter lamellae, consistent with the concept of daughter lamellae nucleated by homoepitaxy on the lateral faces of existing parent lamellae.

Yes / The morphology distribution of isotactic-polypropylene (iPP) shaped through a die during hot stretching process was investigated via wide-angle X-ray diffraction technique. The evolution of micro-structures in the outer layer (layer closer to the die wall) and the inner layer (layer in the center of die) of die-drawn iPP were both recorded. It turned out that the difference of morphology distribution between outer and inner layers changes with the distance from the die entrance to exit. In general, a larger difference between outer and inner layers could be found at the intermediate deformation region inside the die while such difference disappeared at both of the entrance and exit regions of die. These behaviors could be interpreted as a result of the existence of a heterogeneous distribution of force field inside the die, which was caused by the die geometry and inclination of the drawing force. This work showed that the heterogeneous force field inside the die could be revealed through analyzing the morphology of a die-drawn sample.

Yes / The isotactic polypropylene (iPP) usually shows a unique parent-daughter lamellae structure in which the parent and daughter lamellae are against each other with a near perpendicular angle (80° or 100°). Inducing a high fraction of oriented cross-hatched structure in iPP during processing is desirable for designing the bi-oriented iPP products. We processed a commercial iPP via tensile-stretching and die-drawing to evaluate the structural evolution of oriented parent-daughter lamellae. It turned out that the die-drawing process had an advantage in attaining a high fraction of oriented cross-hatched structure of iPP, as compared to the free tensile stretching. Besides, the presence of α-nucleating agents affected the formation of oriented parent-daughter lamellae in the die-drawn samples whereas such influence diminished in the free stretched ones. It was found that the confined deformation inside the die led to the well-preserved oriented cross-hatched structure in the die-drawn iPP. / This work was financially supported by the National Natural Science Foundation of China (Nos. 21704102, U1832186, and 51525305), Newton Advanced Fellowship of the Royal Society, United Kingdom (No. NA150222) and ExxonMobil Asia Pacific Research & Development Co., Ltd.

Yes / The structural formation and development of isotactic polypropylene (iPP) upon the micro-injection molding process was investigated at different mold temperatures and molecular weights utilizing a real-time synchrotron radiation small angle X-ray scattering (SAXS) technique combined with a customized micro-injection molding apparatus. Shish-kebab structure and parent-daughter lamellae were found to be formed during micro-injection molding for all iPP samples. In the case of kebab lamellae, a considerable growth in the long period and in the average thickness of lamellar crystallites and amorphous domains is observed at initial stages of crystallization for samples molded at varying temperatures. This effect is caused by the successive formation of thin lamellae in the outer layer and thick lamellae in the inner layer during the manufacturing process as evidenced by the spatial distribution of the crystalline lamellae across the thickness. In addition, the length of the shish formation increases remarkably at the onset of crystallization, the extent of which is dependent on the mold temperature. Despite the large changes of the lamellar stacks and the shish misorientation, the final length of the shish remains essentially unchanged when varying mold temperature. Since there is a critical orientation molecular weight above which the chains are stretched and oriented to form stable shish, the iPP sample with a low molar mass exhibits an overall decrease in the scattering intensity of SAXS patterns compared to the high molecular weight polypropylene. / This work is financially sponsored by the National Key R&D Program of China (2018YFB0704200), National Natural Science Foundation of China (21674119, 21790342 and 51525305), and Royal Society Newton Advanced Fellowship, United Kingdom (NA150222).

In this study, commercially available isotactic polypropylene (PP) and nylon-6 (PA6) blends and laminates were prepared, to develop a material with optimal water vapour and oxygen barrier properties. The effect of compatibilizers on phase dispersion has been investigated using three commercial Polybond's, PB3002, PB1001, and PB3009. Three compatibilizers prepared in-house were also used as, maleic anhydride(MA) grafted on PP, MA and butyl methacrylate(BMA) co-polymer grafted on PP, and BMA grafted on low density polyethylene. The effect of two silanes( methacrylate functional and vinyl functional) on PP were also investigated and also the plasticization of PA6 with formic acid. The results were compared with a commercial blend of PP and PA6, Orgalloy R-6000. Light microscopy with phase and fluorescence contrast has been used for morphological evaluation. Chemical changes were studied by Fourier Transform Infrared Spectroscopy and rheology by dynamic and steady state measurements. Barrier properties were determined gravimetrically for water vapour and organic solvents, and for oxygen by an Oxtran apparatus. The results have shown that phase dispersion can be more easily explained by molecular interactions than by the rheological parameters. The blend slip factor has been improved however by compatibilizers and consequently the phase dispersion, which had little effect on the barrier properties of the blends and indeed the laminates were more effective water vapour barriers. The availability of particular functional groups, which can interact with the permeant is the most important parameter, which can be affected by processing and blending conditions. The addition of hydrophobic functional groups into polypropylene was therefore the most effective method for enhancing the barrier properties of polypropylene. Cross-linking of the matrix polymer has improved the barrier properties to a lesser extent. It has also been shown, that PP solvent permeability (particularly di-chloromethane) can be improved, by silane addition.

The relationship between microstructure and mechanical properties of semicrystalline polymer materials has been a hot topic since many years in materials science and engineering. Isotactic polypropylene (iPP) is frequently used as a model material, due to its good mechanical properties and wide applications. In the past few years, numerous studies have been performed in the field of structural evolution during deformation. Previous results revealed that phase transition from crystal to mesophase happens in the crystal scale, lamellae orientation and fragmentation occurs in the lamellae scale, and even cavitation behavior exists in the larger scale. Although abundant work has been done, some problems remain under debate, for instance the relationship between lamellae deformation and cavitation behavior, the role of phase transition on the void formation, et al. In this study, well defined microstructure of iPP is obtained by annealing or adding nucleating agent. Afterward, the structural evolution under three types of mechanical load modes (including uniaxial stretching, creep, and stress relaxation) is in-situ monitored by synchrotron X-ray scattering. During uniaxial stretching, we revealed, for the first time, how lamellae deformation occurs in the time scales of elastic deformation, intra-lamellar slip, and melting-recrystallization, separated by three critical strains which were only rarely found to be influenced by annealing. Strain I (a Hencky strain value of 0.1) marks the end of elastic deformation and the onset of intra-lamellar slip. Strain II (a Hencky strain value of 0.45) signifies the start of the recrystallization process, from where the long period in the stretching direction begins to decrease from its maximum and the polymer chains in the crystal start to orient along the stretching direction. The energy required by melting arises from the friction between the fragmented lamellae. Strain III (a Hencky strain value of 0.95) denotes the end of the recrystallization process. Beyond the strain of 0.95, the long period and the crystal size remain nearly unchanged. During further stretching, the extension of the polymer chains anchored by lamellae triggers the strain hardening behavior. On the other hand, annealing significantly decreases the critical strain for voids formation and increases the voids number, but restricts the void size. For those samples annealed at a temperature lower than 90 oC, voids are formed between strain II and strain III. The voids are oriented in the stretching direction once they are formed. For those samples annealed at a temperature higher than 105 oC, voids are formed between strain I and strain II. The voids are initially oriented with their longitudinal axis perpendicular to the stretching direction and then transferred along stretching direction via voids coalescence. Additionally, the formation of voids influences neither the critical strains for lamellae deformation, nor the final long period, the orientation of polymer chains or the crystal size. β-iPP is a kind of metastable phase which can be induced only under special condition. By adjusting the morphology of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide (NJS) through self-assembly, the relative content of β-iPP (Kβ) is successfully controlled, under the condition that the weight content of NJS in the composite keeps at 0.3 wt. %. The microstructural evolution of the iPP/NJS composites with different Kβ during uniaxial stretching is studied. The results show that a higher Kβ could increase the number of the voids. However, the size of the voids is similar regardless of the NJS morphology. The β-α phase transition takes place after voids formation. During intralamellar and inter-lamellar slip, no obvious polymer chains orientation can be found for α-iPP. In the strain range of 0.1~0.6, the c-axis of the β-iPP crystal tends to orient perpendicular to the stretching direction due to lamellae twisting, which is a unique deformation mode of β-iPP lamellae. And the lamellae twisting are proposed to be responsible for the intense voids formation of the composite with higher Kβ. During creep, the evolution of the long period can be divided into four stages (primary creep, transition stage, secondary creep, and tertiary creep). This fits quite well with the macroscopic displacement and strain evolution. In primary creep, the long period along loading direction (L_p^∥) increases with time due to the stretching of amorphous phase, whereas the long period perpendicular to loading direction (L_p^⊥) decreases slightly. In secondary creep, strain increases linearly with time. Both L_p^∥ and L_p^⊥ exhibit the same tendency with strain. The increase of the long period is caused by lamellae thickening, which is a kind of cooperative motion of molecular chains with their neighbors onto the lamellae surface. The increasing rate of L_p^∥ is larger than that of L_p^⊥, indicating that the orientation of molecular chains along loading direction decreases the energy barrier for the cooperative motion. In tertiary creep, strain grows dramatically within a limited time. The lamellae are tilted and rotated, and then disaggregated. In addition, fibrillary structure is formed during lamellae breaking. The length of the fibrillary structure increases from 364 nm to 497 nm but its width stays at 102 nm as creep time increases. During stress relaxation, the local deformation behavior of the long period is affine with the macroscopic stress relaxation. However, the evolution of the crystal orientation and the void size lag behind the macroscopic stress relaxation. The decrease of the long period is mainly caused by the relaxation of the strained polymer chains in the amorphous phase. The retardation of the evolution of the crystal orientation is probably caused by the phase transition from stable α-iPP to metastable mesomorphic-iPP. By phase transition, the highly oriented α-iPP is transferred to weakly oriented mesomorphic-iPP. Due to the fact that the void is confined by the network of the strained polymer chains where lamellae blocks serve as the physical anchoring points, the phase transition contributes greatly to the viscoplastic deformation of the network. Consequently, the evolution of the voids size shows a similar trend with that of the phase transition. With this thesis, we gained a deeper insight into the relationship between structure and properties of semicrystalline polymers. The current study will not only benefit the understanding of polymer materials science but also serve as guidance for the processing of semicrystalline polymers for engineering applications.:1 Introduction 1 1.1 Isotactic polypropylene (iPP) 1 1.1.1 Chain structure of PP 1 1.1.2 Crystal forms of iPP 2 1.1.3 Lamellae of iPP 4 1.1.4 The morphology of the supra-structure of iPP 4 1.2 Structural evolution during deformation 5 1.2.1 Deformation process of semicrystalline polymers 5 1.2.2 Cavitation behavior of semicrystalline polymers 7 1.3 Synchrotron X-ray scattering 9 1.3.1 X-ray and its sources 9 1.3.2 The interaction between X-rays and objects 11 1.3.3 Wide angle X-ray scattering 12 1.3.4 Small angle X-ray scattering 13 2 Motivation and objectives 15 3 Samples preparation and basic characterization 17 3.1 Materials and samples preparation 17 3.1.1 Preparation of iPP films with single layer of spherulites and transcrystalline regions 17 3.1.2 Preparation of iPP plates crystallized with different thermal histories 17 3.1.3 Preparation of iPP/NJS plates with different morphologies of NJS 18 3.1.4 Preparation of microinjection molded iPP/NJS sample 18 3.2 Characterization 18 3.2.1 Differential scanning calorimetry (DSC) 18 3.2.2 Dynamic mechanical analysis (DMA) 19 3.2.3 Scanning electron microscopy (SEM) 19 3.2.4 Polarized optical microscopy (POM) 20 3.2.5 Rheology test 20 3.2.6 Gel Permeation Chromatography (GPC) 21 3.2.7 In situ synchrotron X-ray scattering measurements 21 3.2.8 X-ray scattering pattern processing and calculation 24 4 Microstructure characterization in a single iPP spherulite by synchrotron microfocus wide angle X-ray scattering 29 4.1 Introduction 30 4.2 The nucleation efficiency of the carbon fiber on iPP 31 4.3 Morphology of iPP spherulites and transcrystalline region 32 4.4 Defining of the position of the carbon fiber 33 4.5 Microstructure studies of the spherulite 34 4.5.1 Crystallinity in the spherulite 35 4.5.2 The ratio between “daughter” lamellae and “mother” lamellae in the spherulite 36 4.5.3 The orientation of the crystal axis in the spherulite 37 4.6 Conclusion 39 5 Influence of annealing on the mechanical αc-relaxation of iPP: a study from the intermediate phase perspective 41 5.1 Introduction 42 5.2 Crystal form of water cooled and annealed iPP 44 5.3 Microstructure of iPP with different thermal history 45 5.4 Melting behavior of iPP with different thermal history 50 5.5 Mechanical relaxation behavior of iPP with different thermal history 52 5.6 Conclusion 57 6 Critical strains for lamellae deformation and cavitation during uniaxial stretching of annealed iPP 59 6.1 Introduction 60 6.2 The true stress-strain curves of iPP uniaxial stretched at 75 oC 61 6.3 In Situ SAXS and WAXS Results 63 6.3.1 Synchronize mechanical test and in-situ SAXS/WAXS measurement 66 6.4 Lamellae deformation 67 6.4.1 The evolution of the long period 67 6.4.2 The evolution of the crystal size 69 6.4.3 The orientation of the c-axis of the crystal 71 6.4.4 The evolution of the crystallinity 72 6.5 Cavitation behavior 74 6.5.1 The onset strain of the voids formation and the voids direction transition 74 6.5.2 The evolution of the voids size 75 6.5.3 The scattering invariant (Q) of the voids 76 6.5.4 The morphology of voids 77 6.6 Final discussion 79 6.7 Conclusion 82 7 Accelerating shear-induced crystallization and enhancing crystal orientation of iPP by controlling the morphology of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide 83 7.1 Introduction 84 7.2 The self-assembly process of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide 85 7.3 Rheological behavior 88 7.3.1 Frequency sweep test 88 7.3.2 Strain sweep test 88 7.3.3 Steady-state shear test 89 7.4 Shear-induced crystallization 91 7.4.1 Crystallization kinetics studied by rheological method 91 7.4.2 In-situ SAXS measurement 93 7.4.3 Microstructure of iPP after shear-induced crystallization 96 7.4.4 The morphology of the sample 98 7.4.5 The crystallization mechanism 99 7.5 Conclusion 100 8 Influence of nucleating agent self-assembly on structural evolution of iPP during uniaxial stretching 101 8.1 Introduction 102 8.2 The morphology of the NJS in the compression molded iPP 103 8.3 Microstructure of iPP with different NJS morphologies 104 8.4 In-situ SAXS results 105 8.4.1 Cavitation behavior 107 8.4.2 Evolution of the long period 110 8.5 In-situ WAXS results 111 8.5.1 The β-α phase transition behavior 112 8.5.2 The orientation of the crystal 115 8.6 Conclusion 117 9 Microstructural evolution of iPP during creep: an in-situ study by synchrotron SAXS 119 9.1 Introduction 120 9.2 The creep curve 121 9.3 In-situ SAXS results 123 9.3.1 Evolution of long period and domain thickness 125 9.3.2 Lamellae tilting and rotation 128 9.3.3 Lamellae orientation and fibrillary structure formation 129 9.4 Conclusions 132 10 Microstructural evolution of iPP during stress relaxation 133 10.1 Introduction 134 10.1.1 The structural evolution during stress relaxation at 60 oC 135 10.1.2 The structural evolution during stress relaxation at 90 oC 140 10.2 Conclusion 145 11 Conclusion and outlook 146 12 References 148 13 Appendix 158 13.1 List of symbols and abbreviations 158 13.2 List of figures and tables 163 13.3 List of publications 171 14 Acknowledgements 173 15 Eidesstattliche Erklärung 175