Effect of the molecular structure of semicrystalline polyethylene on mechanical properties studied by molecular dynamics

摘要

We present results from molecular dynamics simulations for an all-atoms model of a semicrystalline polyethylene under uniaxial tensile test. Our model has a realistic semicrystalline organization of two high molecular chains which are involved in two crystalline and two amorphous phases. The tie-molecules link these phases. A high content of tie-molecules is compatible with the stability of the semicrystalline organizations in the initial state. The main objective of the study is to check the effect of some molecular parameters, poorly known from experiments, on the mechanical properties under uniaxial tensile test. It is a way to have an insight on the value of the probable molecular parameters. We show that the number of tie-molecules mainly acts on the elastic modulus and the lamellar thickness acts on the yield stress. A nearly constant stress, a plateau, results from the narrow length-distribution of tie-molecules. This plateau is not observed during mechanical experiments. This plateau is the direct result of isomolecular tie-molecules. The only way to predict the progressive increase of the stress up to the yields stress observed experimentally is to consider a large length-distribution of tie-molecules. This progressive increase of stress implies a progressive deformation of the crystalline phase. The basic mechanism of plastic deformation is the pull-out of crystalline chain segments by the tie-molecules in extended conformation.