A Coarse-Grained Molecular Simulation Study On Thermal Conduction Of Polyethylene

Polymer materials are widely used due to their corrosion resistance,low density,easy processing and low cost.However,conventional polymer materials generally have poor thermal conductivity,which limits their application.The method of ordering polymers not only can greatly improve the thermal conductivity,but also maintain its original advantages.Generally ordering method is mechanically stretching polymers,whose parameters have impressive impact on ordering results.At present,experiments and all-atom molecular dynamics simulations have been conducted to study the thermal conductivity of ordered polymers.However,experimental method cannot accurately measure the internally molecular structure of the polymer and the all-atom molecular dynamics simulation cannot achieve the time and space scale required for studying the processing parameters of ordering polymers.Therefore,developing molecular-simulation force field for large-scale calculation of thermal transport in ordered polymers and studying influence of the order processing technology and operating parameters on the thermal conductivity can provide theoretical basis and guidance for realizing large-scale production of ordered polymers,which has important theoretical and applied value.Firstly,this paper develops a coarse-grained（CG）force field by reproducing the structural properties of bulk polyethylene and the thermal conductivity of single-chain polyethylene based on all-atom molecular dynamics simulations and validates its accuracy.The results show that the CG force field replace five methylene（-CH2-）with a coarse-grained bead,and describes bond stretching potential and nonbonded potential by the Morse function,angle bending potential by the cosine function.Meanwhile,the CG force filed predicts structural and thermal dynamics properties well in bulk polyethylene at 300 K and 500 K,and the thermal conductivities of fully extended single-chain polyethylene and disordered bulk polyethylene at 300 K accurately.It can also accurately reproduce phase changes in bulk polyethylene.These results indicate the accuracy of CG force field.Besides,the theoretically computational efficiency of CG force field is 9000 times that of AIREBO force field.Secondly,the effects of strain,stretching temperature and strain rate on the orientation order parameter parallel to the stretching direction and crystallinity of the thermally-drawn polyethylene were investigated.The results show that as the strain increases,the molecular orientation parallel to the stretching direction increases gradually while that firstly increases and then decreases under excessively high strain rate.Moderate stretching temperatures at the fixed strain rate can achieve higher molecular orientation.Medium strain rates at the fixed stretching temperature are found to achieve the maximum orientation order.Strain level,stretching temperature and strain rate have similar impact on degree of crystallinity and orientation order parameter.The optimum strain rates for maximum crystallinity and that for maximum orientation order parameter both increase from 5×10⁶ s^-1 to 5×10⁸ s^-1 as the stretching temperature increases from 400 K to 600 K.In addition,the influences of stretching temperature and strain rate on orientational order parameter and degree of crystallinity are dominated by relaxation of molecular chain and vancancy before annealed in bulk polyethylene.Finally,this paper studies the effect of strain,stretching temperature and strain rate on thermal conductivity in thermally-drawn bulk polyethylene,and develops the relation between morphology and thermal conductivity,and measures the mechanism of phonon transport in thermally-drawn bulk polyethylene.The results show that as strain increases,the thermal conductivity increases,and the contribution from bond stretching interactions to the total thermal conductivity reaches the maximum.The maximum thermal conductivity can be achieved at moderate stretching temperatures with strain rate fixed and moderate strain rate with stretching temperature fixed.Under a stretching temperature of 400 K and a strain rate of5×106 s^-1,the thermal conductivity reaches a maximum value of 1.54 W/（m?K）at a strain of10,13.6 times larger than that of unstretched polyethylene.The thermal conductivity of the thermally-drawn polyethylene is mainly affected by the orientation order and the crystallinity,and is positively correlated with them.The average relative deviation between the model-predicted thermal conductivities and the simulated thermal conductivities over all processing conditions is 14.5%,which indicates that the model has certain accuracy.The improvement of the thermal conductivity in the thermally-drawn polyethylene is mainly due to the increasing intrinsic phonon mean free path and group velocity in longitudinal mode.