Computation of low-energy crystalline arrangements of cellulose triacetate

Abstract

Low-energy crystal packings of cellulose triacetate (CTA) have been computed, based on a two-dimers unit cell as suggested for CTA I. In addition to the different packing arrangements, parallel versus antiparallel, the effect of changes in the main-chain conformation on the crystal packing was investigated. Minor alterations of the main-chain torsional angles were sometimes found to induce significant changes of unit-cell parameters, although yielding structures with comparable energies. Since small changes of the main-chain conformation do not involve large energy barriers, it is therefore assumed that, in crystalline CTA, local transitions between unit cells with slightly different parameters are possible. Even highly ordered CTA samples might thus consist of a distribution of marginally differing crystalline domains. The resulting imperfections at the interfaces of slightly different unit cells, conceivable on the basis of the computed structures, might explain the problems encountered in the experimental elucidation of the crystal structures in both CTA morphologies, CTA I and CTA 11.