Molecular Dynamic Simulations Of Flow Induced Conformational Transitions Of Polymers

Polymeric material is everywhere to be seen in our daily life,and the developing of polymers effects every bit of our lives.Polymer processing is commonly involving strong flow,during which flow-induced conformation transition plays a central role on the structural hierarchy of semi-crystalline polymeric products and consequently the final properties.Polymers are versatile and useful because they can be processed easily due to they can be stretched,pulled,and squeezed into any shape we desire.But what happens to the stringy polymers in such processing?Understanding the shapes that the polymers are forced into when their host materials are processed is key to controlling the processing.About these problems,shorter time resolution and smaller size detection in-situ techniques like synchrotron radiation small-and wide-angle X-ray scattering(SR-SAXS/WAXS)have been developed.The aim of these in-situ techniques is to detect the evolution of the structural and conformational information and the relationship between these parameters of the polymers.However,SR-SAXS/WAXS is not capable to acquire the molecular details like intra-chain conformation,which is remains unknown.Thus,the mechanism of flow induced polymer crystallization and the rheology properties of the polymer under flow have been studied by the non-equilibrium molecular dynamics(NEMD)simulations in this thesis.The main researches and conclusions have been shown in follows:(1)Non-equilibrium molecular dynamics(NEMD)simulations have been performed to study the molecular mechanism of flow-induced crystallization(FIC)of polyethylene(PE).The bond orientational order parameter Qa is first taken to depict inter-chain hexagonal and orthogonal orders.Our simulation results show that upon imposing extension intra-chain conformational ordering or gauche-trans transition occurs first,which further couples with inter-chain ordering and results in hexagonal order.The growth of the all-trans segments and hexagonal order finally leads to the formation of orthorhombic order.This process clearly demonstrates that FIC is a multi-stage ordering process involving intra-chain and inter-chain orderings.Based the simulation results,we propose that coupling between intra-chain conformational and inter-chain orientation and density orderings is the essential physics of FIC,in which intra-chain conformational ordering is directly related to stress.In this intra-chain and inter-chain coupling mechanism,not only entropic reduction but also inter energy,either due to gauche-trans transition or from inter-chain packing,contribute simultaneously in FIC.(2)Using combination of NEMD simulations and theoretical analytical calculations,the molecular origin of stress and orientation overshoots of entangled polymers under different shear rate have been studied.To explain why the shear orientation overshoot occurs at a constant strain(?S=2),the concept of non-coaxial strain ellipsoid method is first imposed to explain this phenomenon.The theoretical calculation shows that simple shear is a continuous attenuation flow field.The results proved that the orientation overshoot is caused by the rotation of the system,whilst y is equal to 2 is a constant for the orientation overshoot.The theoretical result is consistent with our simulation results,which indicates that the orientation overshoot of the segments is only determined by the rotation of the chain segment orientation.Clearly physical pictures of stress-strain response of entangled polymers under the simple shear have been displayed in this work.(3)The molecular mechanism of the chain stretching and orientation for the stress overshoot response of comb polymers under shear are analyzed using the non-equilibrium molecular dynamics simulation.To clarify the stretching and orientation of the backbone and branches,we analyzed the overall and segmental stretch ratios and order parameters of linear and comb polymers.The peak strain ?max vs.the Rouse-Weissenberg number WiR displays two scaling law regions for linear system.However,the ?max vs.WiR displays three scaling law regions for comb systems.From the detail segmental orientation and stretching dynamics,the following conclusions can be drawn:(i)The occurrence of orientation is in advance of stretching for different length segment.(ii)The stretching of chains mainly comes from the ordered alignment of segments when WiR<10.(iii)The stretching of chains mainly comes from the coil-stretch transition is not ignored when WiR>10.And the molecular pictures are given for the three scaling law regions.(4)Structure and rheological properties of ring and linear polymers under shear are analyzed using the non-equilibrium molecular dynamics(NEMD)simulation.The simulation results have shown that the linear have more prominent stress overshoot than the ring chains.Since the overshoot reflects the maximum flow-induced deformation of the polymer,this qualitative observation already implies that the rings experience less deformation than its linear precursors in simple shear flows.This is consistent with the recent experimental results.In order to further study the molecular mechanism of this phenomenon,the segmental structure and orientation angle distribution as function of strain under the different Weissenberg number have been given in this study.The weak overshoot of the stretching of the ring polymers proved that the weak shear thinning and peak strain is due to the weak deformation of the segment chain of the ring in the shear flow.According to the gyration tensorand orientation angle(tan 2?),the power-law relationship is obtained for the given Weissenberg number(WiR)rang in this work.The main innovations are summarized as follows:(1)Non-equilibrium molecular dynamics(NEMD)simulations have been performed to stu.dy the molecular mechanism of flow-induced crystallization(FIC)of polyethylene(PE).The bond orientational order parameter Q4 is first taken to depict inter-chain hexagonal and orthogonal orders.(2)To explain why the shear orientation overshoot occurs at a constant strain(?s=2),the concept of non-coaxial strain ellipsoid method is first imposed to explain this phenomenon.Thus,it provides a new experimental method for the study of rheology properties of polymer under flow.(3)The molecular mechanism of the chain stretching and orientation for the stress overshoot response of comb polymers under shear are analyzed using the non-equilibrium molecular dynamics simulation.(4)The structure and rheological properties of ring and linear polymers under shear are analyzed using the non-equilibrium molecular dynamics(NEMD)simulation.