Confirmation of the crystal structure of poly(p-phenylene benzobisoxazole)by the X-ray structure analysis of model compounds and the energy calculation

To check the previously proposed crystal structure of poly(p-phenylene benz obisoxazole) [PBO], we performed an X-ray structure analysis for single cry stals of low molecular weight model compounds with the following chemical f ormulas: [GRAPHICS] Both of these two model compounds show essentially tile same molecular and subcell structures as those of PBO: the molecular chains take an almost per fect planar conformation and are packed together with a relative height bet ween the adjacent chains of about 3 A along the chain axis, although fur th e polymer the chains are shifted by the same value but in a disordered mode with respect to the direction of the shift (upward or downward), different fi om the regular packing in model compounds. These structural features ar e reproduced well with energy calculations. Structural ordering in PBO fibe rs caused by heat treatment at high temperatures, as clarified by X-ray dif fraction measurement, are interpreted on the basis of the energy calculatio ns. (C) 2001 John Wiley & Sons, Inc.