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
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:32021 |
| Date | 25 October 2018 |
| Creators | Chang, Baobao |
| Contributors | Heinrich, Gert, Androsch, Rene, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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