DIFFRACTION STUDIES OF POLYSILANES AND POLYSILAETHYLENES. B. L. Farmer, D. B. Holt and E. K. Karikari^*, Dept. of Materials Science and Engineering, University of Virginia, and L. V. Interrante, Dept. of Chemistry, Rensselaer Polytechnic Institute; *Present address: High Performance Polymers and Ceramics Center, Clark Atlanta University

Poly(di-n-alkyl silanes), having the formula -(SiR₂)-, display both an ordered structure (the exact nature of which depends on the specific alkyl group) and a disordered structure, described as hexagonal packing of rod-like molecules. Polysilaethylenes, having the general formula -(SiR₂-CH₂)-, also undergo a transition to a disordered phase prior to melting. Diffraction studies are being used to characterize the ordered structures and the nature of the disordered phases for these series of materials.

Poly(di-n-pentyl silane) [PdnPS] has a 7/3 helical conformation in the ordered phase at room temperature. A planar zig-zag conformation has also been observed in a sample of PdnPS cooled to -15C for several hours. The diffraction pattern is consistent with a unit cell having dimensions a=13.7Å, b=21.4Å, c=4.0Å, and angles of 90 deg. A transformation to the 7/3 helical structure occurs at 35C, and the structure disorders at about 70C. In the disordered phase, neither lateral registry between chains nor a specific helical structure remains.

There are considerable similarities in the powder patterns for the unsubstituted (R=H) and for the symmetrically substituted polysilaethylenes having ethyl, propyl, butyl, or pentyl side chains. Unlike the highly ordered poly(di-n-hexyl silane), only amorphous scattering has been obtained thus far from the poly(di-n-hexyl-silaethylene). The fiber pattern of -(SiH₂CH₂)- indicates that the polymer adopts a planar zig-zag conformation and a unit cell having dimensions a=5.70Å, b=8.75Å, c=3.25Å and =97.6 deg. In parallel with efforts to obtain fiber patterns, analysis of the powder patterns of the other polysilaethylenes is currently underway, assisted by various modeling techniques.