ELECTRON CRYSTALLOGRAPHIC ANALYSIS OF POLY (p-OXYBENZOATE):A COMBINED MODEL AND DIRECT PHASING APPROACH. D. L. Dorset, Electron Diffraction Department, Hauptman-Woodward Institute, Buffalo, NY 14203; J. Liu, B.-L. Yuan and P. H. Geil, Department of Materials Science and Engineering, and Materials Research Laboratory, University of Illinios, Urbana, IL 61801

Electron diffraction patterns from phase I of poly(p-oxybenzoate) were collected from ca. 100 Å thick single crystal lamellae (hk0 data) grown by confined thin film melt polymerization. Alternative orientations (e. g. 0kl data) were obtained from high molecular weight whiskers grown (polymerized) from dilute solution. The quality of the intensity data were ascertained by a R_merge criterion using various patterns for any given orientation. The orthorhombic unit cell has dimensions: a = 7.42, b = 5.70, c = 12.45 Å and the crystal structure was determined in two orthogonal projections, [001] and [100] by direct methods. In the former case, the centrosymmetric projection down the chains (plane group pgm) was easily determined as a molecular profile by symbolic addition. The latter, noncentrosymmetric projection (pg), was found via a multisolution approach using the Sayre equation. After location of all atomic positions (via Fourier refinement) in the latter projection, the final solution was within 0.2 Å deviation from the model found independently using the Cerius² program, where the chains were packed to match the observed diffraction patterns qualitatively. The chain packing is found to be polar, with two staggered monomer units in the asymmetric unit. This study, among other things, demonstrates the facility of innovative crystallization techniques, in addition to the epitaxial orientation procedures often used, for providing essential single crystal diffraction data from a projection onto the polymer chain axis.

Supported, in part by the NSF Polymer and Physical Chemistry Programs.