Atomistic calculation of chain conformations and crystal structures of polyoxymethylene

Abstract

A set of potential functions for hydrocarbons has been extended to oxygen interactions by fitting the parameters to a large number of experimental data of small ether molecules. This set is applied to the conformational and packing analysis of polyoxymethylene (POM).

The energy minimization yields several helix conformations of the unperturbed chain. The experimentally well-known 2 * 9/5 helix (or 2 * 29/16) helix is not obtained in single chain calculations; the problem, however, is solved by means of crystal calculations. It is shown that there is an influence of crystalline packing on chain conformation: the 2 * 2/1 helix is changed to a 2 * 9/5 helix in a hexagonal packing. This is a case where Natta’s second postulate is not strictly satisfied.

In contrast to this result the orthorhombic modification of POM is built up of chains (2 * 2/1) in the minimum of intramolecular energy.

The lattice calculations give some further hexagonal and triclinic modifications of POM, which have not yet been observed experimentally.