Molecular Dynamic Simulation Of Linear And Star Polymers In The Grid Model

The relation between the complex viscoelastic properties of entangled polymer liquids and their microscopic structure and dynamics is a key issue in materials science and biophysics.Essentially,entanglements in polymer system are transient topological constraints arising from the restriction that the backbones of fluctuating polymer molecules cannot cut through each other.During the past several decades,the proposal and development of tube theory based on the principle of mean-filed approximation make the study of entangled polymer system possible.Tube theory provides systematic methods to explain and describe the rheological properties of a polymer system.In this work,we have used computer simulation methods based on the tube theory,coarse-grained approximation,molecular dynamic simulation method,to confront some of the problems in entangled polymer rheology.The first chapter we introduces the background and some basic concepts of polymers,then introduces the properties related to polymer chains,and briefly summarizes the development of computer simulation research and related issues.Meanwhile,we introduce the theoretical basis of polymer and the model used in this research.The second chapter we introduce a new method which can derive an important parameter in entangled polymer system,tube diameter based on the measurement of entropy force,called as“entropy force analysis method(EFA method)”.We use the grid model to simulate the entanglement environment based on the EFA method to measure the value of the tube diameter corresponding to various chain lengths and grid sizes(entanglement degree).Compared with the values measured by some other methods,the data provided by the EFA method shows better accuracy.In the third chapter we record the diffusion coefficient D,the relaxation time for the arms of star polymer?_armand some other parameters corresponding to different branch length.By analyzing the data,results that are inconsistent with the theoretical qualitative prediction begin to emerge.In chapter four we introduce a special device based on the structure of gird model,in which the topological rejection effect in the study of biomacromolecule traslocation competes with the entropic tensile force in tube theory.The expected results for the translocation of the polymer chain given by these two different theories do not coincide with each other,thus we call the problem proposed by this device the“translocation paradox”.We study the dynamic of the polymer chain in such device by computer simulation method and propose several possible simulation results.Surprisingly,the result obtained by the simulation does not completely accord with all the expected results.To explain this phenomenon,We provide a plausible explanation which matches our simulation results.Meanwhile,this explanation provides a possible solution for the design of translocation device in biophysics field.