Molecular Dynamics Simulation Of Star And Comb Polymers In Start-up Shear

With the rapid development of synthesis technology,researchers can synthesize more and more kinds of polymers with different topologies,such as ring,H-shaped,comb,star,and dendritic polymers.Because of their special topological structure,these polymers have many excellent performances in physical and chemical properties and have a wide application prospect in the industry,such as star-shaped polymers and comb-shaped polymers.The study of polymer rheological behavior is very important for its processing and molding process,because most of the polymer molding process is in the melt state,so the study of polymer melt rheological behavior in shear field is favored by more and more researchers.In particular,because of their complex molecular structure,branched polymers have a variety of structural responses in the shear field.The emergence of these hard cores makes star polymers have an additional relaxation mode-local structure relaxation.Therefore,there are two levels of structural response of star polymers in the flow field,namely,chain level and local structure level.In addition,because comb polymers have two types of chain structures,namely main chain and branched chain,their rheological behavior is also different from that of traditional linear polymers.Therefore,clarifying the responsibilities of these microstructures to the flow field is the key to understanding the rheological behavior of these branched polymer systems.In this paper,we use molecular dynamics simulations to study the properties of star and comb polymers during the start-up shear process from macroscopic and microscopic perspectives,respectively.The specific research work is as follows:1.Molecular dynamics simulation of star polymer melt during start-up shear:In this work,we analyze the stress,structure and dynamic characteristics of star polymer melts with different arm numbers under shear flow.It is found that the stressstrain responses of multiarm star polymer melts and small arm star polymer melts are different quantitatively and qualitatively.The 60-arm star polymer has an overshoot peak under weak shear strength.This phenomenon does not exist in 3-arm and 20-arm star polymers.Therefore,this means that the local structure relaxation in the multiarm star polymer will lead to overshoot even when the shear strength is relatively weak.When the shear rate is equivalent to the reciprocal of the relaxation time of the star polymer,we found that the 60-arm star polymer showed two-step yield,which was not observed in the 3-arm and 20-arm star polymers.Finally,all star polymer samples show one-step overshoot behavior at sufficiently strong shear strength.This special stress overshoot behavior of multiarm star polymers does not exist in linear polymers and small arm star polymers,and is obviously related to the local structure relaxation in multiarm star polymers.Therefore,the local structure,chain conformation and dynamic characteristics of star polymer melt under start-up shear are analyzed in detail.2.Molecular dynamics simulation of comb polymer melt during start-up shear:In this part,the non-equilibrium molecular dynamics simulation method was used to study the response of non-entangled comb polymer melt with short branched chain to start-up shear flow,and compared with linear polymer.By selecting different shear strengths,the simulation found that the stress and structural properties of non-entangled comb polymer with short branched chain are basically similar to those of linear polymer under start-up shear,and there is no obvious difference.Under weak shear strength,the stress reaches the platform value after short temporary increase,there is no stress overshoot behavior.The main chain of comb polymer has only slight deformation and orientation and the branched chain is not basically stretched.Under strong shear strength,the stress first increases rapidly and then decreases to the plateau value.In this process,there is an overshoot peak.At this time,we also observed an overshoot peak for the ratio of the mean square end-to-end distance of main chain of comb polymer,while the ratio of the mean square end-to-end distance of branched chain of comb polymer increases only slightly.The simulation results in this paper will help to understand the macroscopic mechanical properties and microstructural changes of star and comb polymer systems,which are of great significance for designing new polymer materials with desired properties.