Molecular dynamics study of polyethylene chain folding: the effects of chain length and the torsional barrier

Abstract

We report on molecular dynamics simulations for model polyethylene chains, varying in length from 30 to 1000 CH₂ units. We found that a certain minimum chain length is required for the formation of stable folded-chain lamellae. The present calculations show that this occurs when the chains have more than 150 CH₂ units, which is in agreement with the experimental observations of Keller et al. and Lee and Wegner. For stable folds to be formed, the decrease in the intramolecular van der Waals energy, due to the attractive interaction between the segments upon folding, should be large enough to overcome the increase in energy due to torsion-angle and bond-angle deformations. Use of a ‘soft’ energy profile with a barrier of 2 kcal mol^–1 for torsions around the skeletal bonds, together with the united-atom approximation, leads to lamellar dimensions which are significantly smaller than experimental values. Increasing the torsional barrier to 6 kcal mol^–1, so as to reduce the population of bonds which exercise transitions from the trans to gauche and non-staggered conformations, leads to lamellar dimensions which are in the range obtained from experiments. We speculate that this ‘hard’ torsional barrier essentially mimics solvent and interchain interactions in these simulations with single chains.