Molecular Simulations Of Stretch-and Flow-induced Polymer Crystallization

About two-third of the synthetic polymer materials are crystallizable. Crystallization remarkably affects physical properties of polymer materials. Thus, Comprehension of crystallization behavior of polymers is the basis of the preparation of high-performance materials. The strain of polymers will be changed during processing. This variable strain will enhance the mechanical properties of semi-crystalline polymers. Thus, the strain-induced crystallization is one of important issues in the field of polymer physics.Compard with theory and experiment, computer simulation holds its own advantages, which can be complemental to the other two methods. The main simualtion methods contain molecular dynamics, Brownian dynamics and Monte Carlo simulation. Monte Carlo simulation has been widely used in the field of polymer physics, such as chain conformation statistics, polymer chain dynamics and stretching of rubber network. Professor Hu Wenbing et al. studied polymer crystallization in-depth in the lattice space by using Monte Carlo simulation and many microscopic phenomena of crystallization were successfully explained. This thesis is focused on the crystallization under an oriented external field by Monte Carlo simulation.In Chapter1, we introduced the basic theory of polymer crystallization, such as two basic models of polymer crystal nucleation:folded chain nucleation and fringed-micelle nucleation. In order to explain the special phenomena of polymer crystallization, different models for crystal growth kinetics were proposed. Then, we outlined the important results in the stretch-induced crystallization and shear-induced crystallization, and further summarized the unsolved problems, such as the nature of precursors, the roles of long and short chains during shish formation. This paper is an attempt to answer these challenge questions.In Chapter2, we introduced Monte Carlo simulation, including the principle, molecular model and sampling methods. On-lattice Monte Carlo simulation was used. The object of the study is a chain-like molecule. These chain molecules can move in the lattice space by a single point jumping and slipping along the local chain. We introduce the anisotropic parallel interaction (corresponding to the enthalpy of crystallization) for the driving force of polymer crystallization.In Chapter3, we focusd on the strain-induced polymer crystallization. We established an uniform method of stretching polymers, and successfully achieved the affine deformation. We found that there exists a competition between intramolecular nucleation mode and intermolecular nucleation mode during stretching. Stretching can induce the nucleation mode transition from intramolecular nucleation to intermolecular nucleation. During stretching, a hierarchical evolution has been observed. First, the oriented segments took part in nucleation, followed by the chain-folded crystal growth. At high strains, further stretching extended the folded-chain blocks in crystal to form the micro-fiber structure.In Chapter4, we studied the microstructure changes of copolymer during drawing. Stretching enhanced the sequcence segregation. The oriented nuclei appeared firstly in the aggregation domains of oriented monomer sequences. For low content of comonomer, folded-chain crystals formed due to the enough sequcence segregation prior to crystallization. For high content of comonomer, only small crystal blocks can be formed due to weak sequcence segregation.In Chapter5, we constructed a shear flow with symmetrical velocity distribution. We then studied the crystallization in the mixed system. In addition to deformation of long chains, phase separation between long and short chains also occurs. The results showed that the formation of nuclei happened within the aggregation domains of deformed long chain segments, and thus the aggregation domains can be considered as precursor for shish formation. Meanwhile, both long and short chains were involved in the formation of shish. The orientation of the long chains caused the reduction in entropy, while short chains with high diffusion ability resulted in the decrease in diffusion activation energy. By changing the shear temperature and strength, we found three different precursors. In the end of this chapter, we studied the melting of shish-kebab crystals and memory effect. It was found that when the crystal was completely melted, the melt still contained some local orientation and phase separation regions.In chapt6, we summarized conclusions, and suggested directions for future studies.