Multiscale Simulation Research Of Polymer Viscoelastic Rheological Behavior

Polvmers play a very important role in many advanced materials because of the high flexibility.The high flexibility of polymers is closely related to its molecular structure,so it is important to predict the properties of the polymer from the molecular level and to control its properties rationally.It’s conducive to the development of new materials and processes.In the polymer molding process,the theoretical analysis has a certain degree of difficulty,and experimental study has limitations,the general will melt polymer in the actual production department to study the rheological behavior using the numerical method in the die,which is of great significance for the optimization of polymer processing technology process,improving the forming performance.The mechanical properties of viscoelastic materials deformed when viscous and elastic behavior exhibit.These properties are the most important mechanical properties of polymer materials.It is very significant to establish a strict relationship between molecular structure and viscoelastic prope-ties in the optimization,design and application of polymer materials.Since the efficiency of the all atol molecular dynamics silulation is very low.a coupling method with the all-atom method is generated,which operates on a larger length and time scale,the so called cross-scale simulation.In this paper,the static structure about the all-atom model of polyethylene and polypropylene is analyzed using Boltzmann converse method,then the coarse-grained model is constructed.The coarse-grained model is a bridge of nano-scale to meso-scale.The diffusion coefficients at different shear rates are obtained through coarse-grained molecular dyna,nics simulation.The primitive path analyzed the entanglement network of polymer chains in coarse-grained model.The key parameters,such as pipe diameter and original chain length,have been extracted as well.The primitivepath analysis is a pure Geometric method.The original path parameters at different temperatures are analyzed.As the temperature increases,the melt flow of the polymer becomes better and the original path entanglement is getting lower and lower,the molecular chain increases and the mean square end distance increases.Based on the affine-deformation assumption,using the key parameters extracted from the primitive path analysis,the linear viscoelast’ic and nonlinear viscoelastic constitutive equations have been derived.The equations realized the transformation between micro-scale and macro-scale.In this way,different scales are crossed by using different bridging method which using the bottom-up method directly predict the viscoelasticity of polymer materials.Namely using small scale sinulation results to obtain the parameters are needed in large scale simulation.The cross-scale simulation is based on atomic simulation to calculate the phenolenological parameters,such as elastie coefficient tensor,diffusion coefficient viscosity coefficient,etc.,for continuous modeling.The multi-scale model 1s used to predict the dynanic modulus,zero shear viscosity and steady shear viscosity of polvethvlene and polypropylene.The rheolosiical properties of polyethylene and polypropylene are tested as well.It is found that the simulation results are greatly coinciding with the experimental results,which is verified the correctness of the multi-scale model.The multi-scale analysis method proposed in this paper can directly predict the relationship between molecular structure and rheological properties of polymer.