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Simulation Studies on Heterogeneous Transport and Mechanical Properties of Semi-crystalline Amorphous Soft Materials

  • 박형식 (Hyungshick Park, 서강대학교 일반대학원)

초록 (요약문)

Soft materials ranged from polymers, colloids, liquid crystals to cell membranes, and they have been extensively utilized for both industrial and academic purposes due to their mechanical properties —large bulk modulus and small shear modulus. Most soft materi- als consist of macromolecules such as polymers and show collective motions, resulting in non-linear responses even with small perturbations. Among these soft materials, lots of them can be categorized as two-phased materials with ordered (crystalline) and disordered (amorphous) phases. Most representative materials would be semi-crystalline polymers, in which several parts of chains form crystalline domains while other parts of chains are amorphous random coiled. These ordered and disordered phases introduce the intrinsic dynamic heterogeneity to the material. Ordered phases show slower dynamics and increase the mechanical properties due to their confined structure on crystalline lattices while dis- ordered phases show faster dynamics and cause flexibility in the system. In this thesis, representative examples of these systems will be discussed, and the ways in which these two phases contribute to heterogeneous dynamics and affect the mechanical responses will be suggested. The first three chapters of the thesis treat the soft material called Organic Ionic Plastic Crystals (OIPCs) and Ionic liquids. OIPCs belong to the family of ionic liquids and they are crystalline materials composed of organic ionic molecules. Surprisingly, molecules in OIPCs undergo rotational motions even in the crystalline lattice and these rotational mo- tions cause OIPCs to have soft mechanical properties. Numerous experimental and simu- lation studies have suggested that OIPCs could serve as a solid electrolytes due to the high ionic conductivity with alkali ion doping. Recent experimental studies suggested that de- fect such as point vacancy or grain boundary would be the major ion transport mechanism. Therefore, in the chapter 1 and 2, we provide how these defects break the local structures and introduce amorphous phase into OIPCs, and cause spatial dynamic heterogeneity in OIPCs as solid electrolytes. In chapter 3, we investigate the long-ranged structures hidden in the ionic liquids. In these concentrated electrolytes, long-ranged charge-charge interac- tions cause the spatial correlation even further than 1nm scales. We investigate how dopants disrupt these long-ranged correlation between ions and the relation between these hidden structures and the correlative diffusion of ions. We suggest that the decay length of this long-ranged correlation (screening length) is a key to understand correlative ion transport mechanisms in ionic liquids. The remaining two chapters are focus on the study about thermoplastic elastomers (TPEs). TPEs have been widely used due to their good mechanical properties and process- ability since the 1990s. TPEs are block copolymers composed of hard (crystallizable or high Tg) and soft (amorphous or low Tg) blocks. TPEs form micro-separated structures dur- ing cooling. Then, hard blocks form crystalline domains under their Tc. These crystalline domains act as cross-linkers while soft amorphous segments act as network strands. These complex network structures resemble the traditional elastomers and TPEs also behave sim- ilar to elastomers at room temperatures. However, when we increase the temperatures after cooling, TPEs return to melt states and their shape can be easily molded. Although these excellent characteristics of TPEs cause them to be generally utilized, their complex net- work structures and the effect on mechanical responses of TPEs remain elusive. In chapter 4 and 5, we investigate how crystallinity, conformational entropy, and network morphology affect the non-linear mechanical properties of TPEs. We find that not only the crystallinity but also the conformation entropy must be considered to understand the modulus above yield and energy dissipation in TPEs, and the complex network morphology can be tuned by altering the chain architectures.

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초록 (요약문)

연성 물질 (Soft Matters)은 고분자, 콜로이드, 액정과 같은 산업 소재에서부터 인지질 이중층, DNA와 같은 생체 분자에 이르는 다양한 물질들을 나타내며, 부드러운 기계적 특성으로 인해 다양한 연구 및 산업에서 활용된다. 연성 물질은 주로 고분자와 같은 거대 분자 또는 여러 분자들의 집단으로 구성되며 총체적인 움직임을 통해 작은 섭동에도 크고 비선형적인 반응을 나타내게 된다. 본 학위논문에서는 이러한 연성 물질 내부에서 일어나는 불균질한 확산 현상 및 기계적인 물성들을 분자동역학 (Molecular Dynamics; 이하 MD) 전산 모사 방법론을 통해 분자 수준에서 살펴본다. 유기 이온성 플라스틱 결정 (Organic Ionic Plastic Crystal; 이하 OIPC)은 유기 이온성 분자들로 이루어진 결정성 물질이며, 결정 상 속에서도 이온 분자들이 회전 자유도를 가지는 독특한 연성 물질이다. 기존 실험 연구를 통해, 결정 구조가 깨어져 나타나는 결함이 OIPC에서의 주요 이온 전도 메커니즘으로 제안되었다. 이번 학위 논문에서는, MD 방법론을 통해 빈자리 결함 또는 결정 경계에서 나타나는 비결정성 (Amorphous) 구조가 이온의 불균질한 확산을 어떻게 유발하고 이온 전도도와 연관되는지를 분자 수준에서 탐색하였다. 열가소성 탄성중합체 (Thermoplastic Elastomer; 이하 TPE)는 두 가지 다른 조성으로 구성되어 있는 블록 공중합체이다. 하드 블록의 경우, 상온에서 결정성 고분자 영역을 형성하며 탄성중합체에 가교 및 기계적 강성을 제공한다. 소프트 블록의 경우, 상온에서 유리 전이 온도 위에 있어 다양한 고분자 형태 (Conformation)를 나타내어 비결정성의 네트워크 구조를 형성하게 된다. 비록 TPE는 1950년대 이후에서부터 지금까지 활발하게 산업에서 사용되어 왔지만, 복잡한 모폴로지로 인하여 TPE의 비선형적인 기계적 성질은 분자 수준에서 이해되지 못하고 있었다. 본 학위 논문에서는 MD 방법론을 통해, 고분자의 결정성 및 배향, 비결정성 고분자 사슬의 형태 엔트로피, 고분자의 아키텍처 등이 다양한 비선형 기계적 물성에 대하여 미치는 영향을 분자 수준에서 연구하였다. OIPC, 이온성 액체, TPE 등 다양한 연성 물질 내부에는 결정성 (또는 장거리 구조 특성) 및 비결정성을 가지는 다양한 영역들이 혼재되어 있으며, 이러한 구조 및 계면들은 연성 물질 내의 동역학적 불균질성 (Dynamic Heterogeneity)을 야기하고, 나아가 본체 성질 (Bulk property)까지 결정하게 된다. 본 학위논문은 이러한 연성 물질에서의 특성을 분자 수준에서 조명하고 본체 성질과 동역학적 불균질성의 관계와 관련된 결과들을 정리하였다.

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목차

1 The Effects of Vacancies and their Mobility on the Dynamic Heterogeneity in 1,3-Dimethylimidazolium Hexafluorophosphate Organic Ionic Plastic Crystals 2
1.1 Introduction 2
1.2 Model and Methods 5
1.3 Data and Results 11
1.3.1 The hopping and diffusion of vacancies 11
1.3.2 Translational dynamic heterogeneity of [MMIM][PF6] crystals with vacancies 14
1.3.3 Rotational dynamic heterogeneity of [MMIM][PF6] crystals with vacancies 18
1.4 Summary and Conclusions 23
1.5 References 25
2 The Effects of Defects on the Transport Mechanisms of Lithium Ions in Organic Ionic Plastic Crystals 30
2.1 Introduction 30
2.2 Model and Methods 33
2.3 Data and Results 38
2.3.1 The structure of OIPCs with defects 38
2.3.2 The rotation of [MMIM]+ cations in OIPCs with defects 41
2.3.3 The translational diffusion of ions in [MMIM][PF6] OIPCs with Schottky vacancies 42
2.3.4 The translational diffusion of ions within the grain boundaries of [MMIM][PF6] OIPCs 45
2.3.5 Heterogeneous dynamics of matrix and Li+ions 47
2.3.6 Transference numbers of Li+ions 49
2.4 Summary and Conclusions 51
2.5 References 52
3 Screening and Correlated Ion Diffusion in Lithium-Doped Ionic Liquids 60
3.1 Introduction 60
3.2 Simulation Model and Methods 65
3.2.1 Simulation model: Li-doped [P1224][TFSI] 65
3.2.2 Simulation method 69
3.2.3 Computation of observables 69
3.2.3.1 Static properties: two-body correlation functions 69
3.2.3.2 Blob analysis: ionic fluctuations in a finite volume 72
3.2.3.3 Dynamic properties: Ion diffusion and conductivity 73
3.3 Results and Discussion 76
3.3.1 Lithium-ion solvation environment 78
3.3.2 Long-range correlations: Electrostatic interactions screening 78
3.3.3 Ion diffusion and correlated motions 80
3.3.4 Charge density fluctuations: Blob analysis 85
3.4 Conclusions 87
3.5 References 89
4 The Entropic Contribution to the Non-linear Mechanical Properties of Thermoplastic Elastomers 97
4.1 Introduction 97
4.2 Model and methods 100
4.3 Data and results 106
4.3.1 The structural changes of TPEs during the uniaxial deformation 106
4.3.2 The mechanical response in the linear regime 109
4.3.3 The mechanical response in the non-linear regime 110
4.3.3.1 The mechanical response during the loading-unloading cycle 112
4.3.3.2 The entropic contribution to the mechanical response in the non-linear regime 114
4.4 Summary and Conclusion 120
4.5 References 121
5 The Effect of Chain Architectures on the Mechanical Properties of Semi-crystalline Block Copolymers 127
5.1 Introduction 127
5.2 Model and methods 129
5.3 Data and results 134
5.3.1 Molecular level changes during cooling 134
5.3.2 The mechanical response during deformation 137
5.4 Summary and Conclusion 141
5.5 References 142

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