Brownian Dynamics Numerical Simulation For Stress Relaxation Process Of Polymer

The rheological properties of the polymer have great influence on the appearance shape and quality of products, and it is the central link to determine the final structure and properties of polymer products. With consumers'demanding of high quality products increasingly, the depth study for the behavior and microstructure of polymer in the fluid flowing of the polymer processing is needed. Using molecular dynamics and numerical simulation methods to describe the molecular structure, and get the macro-rheological properties of polymers through the ensemble average. Brownian Dynamics simulation is based on the molecular model, using computer to simulate the structure and behavior of molecular, which reveals the various physical and chemical properties of molecular systems.According to the theory of Doi-Edwards molecular model, there is such a point in time during the flow of thick system of polymer:before this time, the molecular chain movement is not constrained by other chains, manifested as Rouse chain of free space, and motion driving force expressed by Rouse model; after this time, the movement of molecular chains will be constrained by other chains, and chains aggregation and entanglement with each other, the driving force of motion expressed by Doi-Edwards model. According to this, we will use Brownian Dynamics simulation method for numerical simulation of polymer fluids:(1) Based on Rouse model, using Brownian Dynamics to simulate polymer fluids. we established the control equation of polymer fluids, using finite difference method derivation discrete iteration formulas, and got the micro evolution configurational field of molecular chain ensemble at any time, then using ensemble average to calculate the macro-stress of polymer.(2) Based on Doi-Edwards model, we analyzed the linear viscoelastic and nonlinear viscoelastic of polymer theoretically; discussed the stress relaxation after strain which is described by Doi-Edwards model; researched the numerical simulation of parameter selection criteria and simplified simulation conditions; established the control equations for solving the steady shear flow, and obtained the ensemble average shear stress.(3) Through theoretical analysis and numerical simulation, we obtained the curve of shear stress relaxation as function of dimensionless time, the first normal stress difference curves under different strain, the relationship curve between shear stress and strain of step strain, and the tensile stress relaxation curve of elongational step strain, and these curves were analyzed.