MOLECULAR MODELING OF POLY(ARYL ETHER KETONES) .2. CHAIN PACKING IN CRYSTALLINE PEK AND PEEK

A molecular mechanics study of the crystal structures of aryl ether ketone materials has been undertaken using a model based on ab initio calculations and crystal data of small, aromatic molecules. The model can reproduce the global minima of the poly(pheylene oxide), PE, and PEK crystals and also correctly estimate the ring conformation in these materials. The calculations show the different unit cell dimensions in PEK, relative to PE (suggested to be due to the additional bulk of the carbonyl group) to favour a lower ring twist angle and therefore complement the respective single chain conformations which, due to the increased torsional barrier around the ketone bridge, also predict a lower twist angle for PEK. To reproduce the PEEK ring conformation satisfactorily it is necessary to considerably reduce the torsional terms employed. This change can be rationalized by n.m.r. evidence which suggests the conjugation of the PEEK chain to be somewhat reduced relative to PEK. The nature of the chain packing in PEK and PEEK has also been examined with respect to the alignment of the ether and ketone bridges and, for both materials, it is suggested that increased packing order involves horizontal (bc plane) E..K alignment and vertical E..E. (and K..K) alignment. The relative energy difference between ordered and disordered packing is much greater for PEK and it is suggested that this provides an explanation for the differing behaviour of the two unit cells with increased crystallization temperature.