Molecular dynamics computer simulation of polymer fiber microstructure

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Abstract
Microscopic multichain models of amorphous and crystalline regions of an oriented polymer fiber have been examined using molecular dynamics computer simulation. A lamina model was used in which the chains were suspended between two fixed parallel planes representing the faces of adjacent crystallites. The ordered structure consisted of 16 parallel chains each containing 25 segments. The amorphous structure contained 24 chains and was characterized by a distribution of chain lengths, entanglements, end defects, and loops. The values of the geometric and interaction parameters for the polymer chains were taken from previous studies of low molecular mass liquid alkanes, so that the model approximates to polyethylene, with each CH2 group represented as a single interaction site. The structures were brought to equilibrium at 300 K and stress–strain curves obtained for elongation/compression in a direction perpendicular to the crystal faces. The influence of interchain forces on these curves and on the Young’s moduli (which differed by more than an order of magnitude for the two structures) have been analyzed and compared with theoretical predictions. The frequency dependent stress across the interfaces of the ordered structure was analyzed in terms of angle bending motions of the chains (so‐called accordion modes). The relation between the associated frequency spectrum and chain length was similar to that observed in real crystalline polyethylene. Young’s moduls estimated from the velocity of sound was in good agreement with that obtained from the static stress/strain curve.