SIMULATION OF GLASSY POLYMETHYLENE STARTING FROM THE EQUILIBRATED LIQUID

A simulation of glassy polymethylene, as represented by a periodic assembly of 32 chains of 24 methylene units each, was carried out by energy minimization. The starting structures were equilibrium liquid samples taken from a constant-pressure and -temperature Monte Carlo simulation. Volume was allowed to participate in the minimization so that densification and the property changes accompanying it could be studied. The C24 methylene system both experimentally and by simulation has a noticeably higher specific volume than long-chain polyethylene, but the fractional degree of densification on quenching to 0 K is in agreement with that for PE. The calculated cohesive energy of both the starting liquid and the glasses were in good agreement with experiment for a C24 alkane. The intermolecular radial distribution functions in the glass show much sharper features than those for the liquids and indicate well-developed coordination of neighbors about the methylene units. The distribution of free volume and cavity sizes is discussed. The mechanical elastic properties of the glasses were also derived from the calculated compliance constant matrix. The derived shear modulus for the C24 glass, at the density from simulation, is somewhat lower than that found experimentally from extrapolations for semicrystalline PE. However, when calculated at the density of high molecular weight PE, the agreement is much better.