THE ROLE OF DIRECT METHODS IN POLYMER CRYSTALLOGRAPHY. D. L. Dorset, Electron Diffraction Department, Hauptman-Woodward Institute, 73 High Street, Buffalo, NY 14203-1196 USA

Because of the sparse sampling of the reciprocal lattice in electron or polymer diffraction studies of linear polymers, most structural analyses have relied on model-based trial and error searches for simultaneous minima of the crystallographic residual and the lattice packing energy. Recently, however, true ab initio structure determinations, using direct phasing methods, have proven to be very effective. For single crystal electron diffraction data, these determinations are facilitated if care is taken to collect intensites from a complete reciprocal lattice, requiring observation of data from two orthogonal crystal orientations in the tilt experiments. If only one crystal orientation is available for tilting (e. g. a chain-folded lamella), then the missing information in the unsampled `dead zone' may degrade details along the polymer chain axis. In this case, prediction of the missing information by the Sayre equation may be useful. An alternative approach is to combine data from fibers with the lamellar single crystal data, when epitaxial orientation is not possible for achieving the orthogonal orientation of the chain packing in a single crystal form. Tests with fiber x-ray data from polyethylene and poly ([[epsilon]]-caprolactone) demonstrate that the structure can be determined ab initio, and also that a reasonably good approximation deconvolution of overlapped intensities can be made without assumption of a molecular model a priori. Such procedures have been repeated with electron diffraction intensities (lamella and fiber) from poly (ethylene sulfide) with good success. Numerous direct methods approaches have been evaluated but the most powerful techniques found so far seem to be the Sayre equation in a multisolution approach or the SnB technique based on the minimal principle.

Supported by grants from the NSF (CHE-94-17835) and NIGMS (GM-46733)