Molecular Simulations Of Strain Induced Polymer Crystallization

Since Staudinger put forward the concept of Macromolecule,polymer materials have been developed very rapidly,making them a more and more important role in both polymer industry and everyday life.Crystallization will supply polymer materials with necessary strength.Up to now,more than 2/3 of polymer materials in volume are semi crystalline.In order to enhance materials and develop new types of materials,we need to investigate the process of crystal nucleation and growth systematically on the molecule level.Specially,orientation,including strain or shearing field,has been used to enhance the strength of polymer materials.In the thesis,the main focus is strain-induced polymer crystallization.The thesis focuses mainly on the mechanism of polymer crystal nucleation and growth,morphology and the crystallization behavior of system with polydispersity in the process of strain-induced polymer crystallization.Based on dynamics Monte Carlo simulation,the thesis mainly deals with 3 topics:strain-induced random copolymer memory effects,effects of branches in strain-induced branched polymer crystallization and strain-induced enhanced SC formation in binary blends.Polymer crystallization has been a very important field of polymer physics.In this filed,some basic concepts like chain folding and lamella growth have been set up.There have also been basic research methods like chain conformation statistics and scaling analysis.Apart from experimental technique and theoretical research,dynamic Monte Carlo simulation can be used to analyze polymer crystallization with a lot of random processes and output effective and reliable results in statistics due to the accords in time scale.Prof.Hu Wenbing creatively introduced the concept of "Parallel Packing Energy Ep"and has well developed the method in the simulation studies on polymer crystallization.On the basis of the method,the thesis has focused on the investigation on revealing the mechanism of polymer crystal nucleation and growth in the process of strain-induced crystallization polymer systems,linear random copolymer,branched polymer and binary blends.In Chapter 1,we will make a brief demonstration on the physics background of strain-induced polymer crystallization.Firstly,the mechanisms of polymer crystal nucleation and growth are introduced,followed by theories of morphology evolution,different trends of chain folding in different stages,nucleation transition from intramolecular nucleation to intermolecular nucleation and melting points change due to strain or existence of comonomers and the nucleation in the binary blends.In Chapter 2,we will give a detailed introduction about dynamic Monte Carlo simulation.We start with lattice space used in the simulation and continue the introduction of simulation details like periodic boundary condition and micro-relaxation model(made of "single site jumping" and "sliding diffusion").The next part is about "Metropolis sampling" and energy parameters like parallel packing energy Ep,conformational energy Ec and mixing parameter B.In Chapter 3,we perform dynamic Monte Carlo simulations of stretching-retraction cycles of bulk homopolymer and random copolymers.We observe that random copolymers exhibit a significant memory of strain-induced crystallization but homopolymer not,which shifts down the onset strain of crystallization and raises the copolymer crystallinity.We attribute this strong memory effect to the sequence-length segregation,which is initiated by the first-round strain-induced crystallization and remained in the second-round crystallization after crystal melting upon the first-round retraction.Our observation evidences the basic mechanism proposed in previous experimental and simulation observations on the strong memory of copolymer crystallization during heating-cooling cycles.In Chapter 4,dynamic Monte Carlo simulations of a lattice polymer model are performed to investigate strain-induced crystallization behaviors of short-chain branched polymers with variable sequence distributions,branch numbers and lengths,and their blending compositions with linear polymers.The results revealed that the branch density and distribution bring the dominant effects.In the blends,there is no phase separation prior to crystallization,except that the strain-enhanced mixing is temporarily hindered by chain-folding upon crystallization.Our observations shed lights onto strain-induced crystallization of short-chain branched polymers.In Chapter 5,we perform dynamic Monte Carlo simulations to investigate the competition in strain-induced crystal nucleation between stereo-complex crystals and homo-crystals of half-half stereo-isomer symmetric polymer blends.The results demonstrated the strain-enhanced stereo-complex crystal nucleation,in agreement with the experiments of polylactides.We attribute the results to the preferred intermolecular crystal nucleation under polymer stretching.Our observation provides new evidence on the preferred intramolecular crystal nucleation of polymers under the quiescent conditions.In Chapter 6,we will make a summary in the thesis and put forward future direction.