Crystal structure and packing disorder of poly(p-phenylenebenzobisoxazole): Structural analysis by an organized combination of X-ray imaging plate system and computer simulation technique

To clarify the crystal structure of poly(p-phenylenebenzobisoxazole) (PBO) fibers, the analysis has been made for the X-ray diffraction data collected by the imaging plate system in combination with the computer simulation technique. The characteristic features of the X-ray diffraction pattern of PBO fiber are seen in the spotlike equatorial reflections and the diffuse reflections along the layer lines. The peak positions of the diffuse layer-line reflections were found to fit well to those of the molecular transform calculated for a uniaxially oriented isolated chain, suggesting an appreciably low correlation in the relative height between the neighboring chains. Among many types of the energetically minimized crystal structural models constructed by a computer simulation technique, a model giving a good fit between the observed and calculated X-ray profiles was found, in which the chains are assumed to form the layers extending along the 110 planes with the relative heights of the adjacent chains in the sheets almost confined to either +c/4 or -c/4 (c = fiber identity period) and these sheets are stacked together in the b-axis direction with random heights along the chain axis.