Abstract
In this study, we combine molecular dynamics (MD) simulation and molecular theory such as Rouse and reptation theory to develop a method for predicting the viscoealstic properties. Reptation theory can be used to descrie phenomenon of entangled polymer and analyze it quantitatively. However, some material parameters in these models are difficult to obtain. MD simulation is based on atomic level which can simulate material properties with less assumption. But for large systems it is time consuming and inefficiency. Hence, we take advantage of the strong points of the two methods. i.e. we used MD simulation to calculate the material parameters in the reptation model then applied it to the reptation model to predict the viscoelastic properties of material. In this work, double reptation theory was also appled to describe the effect of molecular weight distribution. The prediction of high molecular weight polyethylene are agree with the experimental results by applying double reputation theory MD simulation was also used to investigate the viscoelastic properties of short chain PE under oscillatory shear flow. Rheometric simulations of an ultra-thin molecular film are studied and compared with the results of a bulk simulation. Strain amplitude sweep tests at a fixed frequency show that strain thinning (the dynamic modulus monotonically decreases with increasing strain amplitude) exists at extreme strain for both bulk and thin film systems. Fourier analysis is performed to characterize the nonlinear behavior of the viscoelasticity. No even harmonic was found in our study even though wall slip occurs. Furthermore, we show that a Fourier series with odd harmonics can be used to perfectly describe the simulation results by plotting Lissajous loops. Shear wave propagation appears when the frequency is larger than a certain value. Moreover, the molecular orientation and molecular potential energies, including those for bonding potential, intra- and intermolecular van der Waals interactions are plotted against the strain amplitude to examine the changes in the microscopic structures with respect to the macroscopic thermodynamic states.In addition, we found that the linear region is decreaseing with the increase of frequency. By stress decomposition the individual contribution of elastic or viscous effect is determined. Therefore, the origin of decrase of linear region could be investigated.