| Abstract: | The molecular mechanisms involved in the orthorhombic-to-monoclinic phase transformation in polyethylene were investigated by the computer simulation of a structure–energy map based on empirically justified intermolecular potential functions. Stable packing structures for the orthorhombic and monoclinic form were isolated as relative minima, cohesive energies were determined from the energy minima, specific chain motions involved in the transformation were identified by the minimum energy path connecting the packing minima, and the activation energy for the transformation was determined from the energy barrier along the minimum energy transformation path. The packing structure parameters predicted from the energy map were in excellent agreement with unit cell dimensions observed near 0°K. The activation energy predicted for the transformation is relatively low (～0.5 kcal/mole of ethylene at 0°K and 0.25 kcal/mole of ethylene near the melting point, 411°K). Monoclinic packing was predicted to be slightly more stable than orthorhombic. Since this result is inconsistent with a large body of observations, we propose that the intramolecular energy of chain folds plays a dominant role in establishing chain-packing geometry. The inclusion of fold-transition energetics could give rise to transformation mechanisms which differ in details from those proposed in this work. |
