STRUCTURE AND PROPERTIES OF POLY(N-PENTYL-N-ALKYLSILANES) .1.

We have investigated the solid-state electronic properties and the structures of three asymmetric poly(n-pentyl-n-alkylsilanes) using UV absorption, calorimetry, and X-ray diffraction. Poly(n-butyl-n-pentylsilane) (C4-C5) has a 7/3 helical backbone conformation at 25-degrees-C, as has been previously observed for poly(di-n-butylsilane) and poly(di-n-pentylsilane). The X-ray diffraction pattern of a uniaxially oriented film of C4-C5 may be indexed with an orthorhombic unit cell containing hexagonally packed chains with a = 13.23 angstrom, b = 22.92 angstrom, and c = 13.88 angstrom. At 45-degrees-C a sharp first-order phase transition to a hexagonal, columnar structure with lattice parameter a = b = 13.4 angstrom is observed. Poly(n-propyl-n-pentylsilane) (C3-C5) shows a thermochromic shift of its UV absorption maximum from 324 to 339 nm upon cooling. Above the first-order phase transition observed at -2-degrees-C, C3-C5 has a hexagonal, columnar structure with lattice parameter a = b = 12.2 angstrom. Poly(ethyl-n-pentylsilane) is a poorly ordered polymer with a second-order phase transition at -35-degrees-C. The degree of order and the mechanical strength of the materials decrease markedly as the side chain mismatch increases in this series of polymers.