Atomistic dynamics of macromolecular crystals

Abstract

In this article we review recent computational results on the dynamics of macromolecular crystals. From these studies it has been demonstrated that conformational defects can be created at temperatures as much as 100 K below the melting point of crystalline polyethylene and the concentration of the defects continues to increase (exponentially) with temperature, ultimately leading to a disordered crystal along the polymer chains (CONDIS crystal). Although the rate of formation of conformational defects is relatively high, approximately 1×10¹⁰ s⁻¹ at 350 K, these defects do not by themselves lead to any macroscopic motion that could give rise, for example, to lamellar thickening. The mechanism appears to involve coupling of the large-amplitude torsional motions with the transverse and longitudinal vibrations of the crystal lattice, which can subsequently lead to the formation of short-range twists in the chains (twist defects). Defects like the twist can, under the correct conditions, move coherently toward the end of the crystal, thereby causing a chain diffusion process that leads to lamellar thickening or deformation processes. For smaller systems such as paraffins, disorder occurs by a collective twisting (so-called rotator phase) of the chains, which is not strongly influenced by conformational defects. The hexagonal or pseudo-hexagonal structure of the asymmetric motifs is caused by a dynamic multidomain arrangement of the twisting chains. Overall, a more accurate description of the thermodynamic, spectroscopic, and kinetic behavior is possible and gives a new understanding of the deformation, relaxation, annealing, and motion in macromolecular crystals.