Chain conformation and molecular packing in poly(p-oxybenzoate) single crystals at ambient temperature

The chain conformation and molecular packing of poly(p-oxybenzoate) (PpOBA) at room temperature have been determined using computer modelling and direct phasing based on electron diffraction data from lamellar and whisker single crystals. In the triclinic (metrically orthorhombic) phase I and orthorhombic phase II the unit cell parameters are a = 7.45, b = 5.64, c = 12.47 Angstrom, alpha = beta = gamma = 90 degrees for I and a = 11.15, b = 3.80 and c = 12.56 Angstrom for II. The best agreement between the computer modelling and the experimental diffraction patterns suggests the carbonyl groups in Phase I are nearly parallel to the planes of the phenyl rings in the [001] projection, the two phenyl rings being at an angle of 124 degrees with respect to each other along the c-axis. The two chains in the unit cell of these 'perfect' crystals have a parallel orientation and are staggered by +/-0.14 Angstrom along the c-axis from the positions they would have in the Pbc2(1) space group. In Phase II, space group Pca2(1) the conformation of the chain is nearly the same as in phase I (phenyl ring angle = ca128 degrees) and there is insufficient evidence to determine if chain stagger occurs. The results of the direct phasing are in good agreement, including the stagger in Phase I. Simulated X-ray diffraction scans are compared with measured scans for samples of various solution polymerization history and resulting morphology and phase composition. (C) 1997 Elsevier Science Ltd.