Liquid-crystalline behaviour of a series of poly(di-n-alkylsilanes)

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Abstract
A homologous series of poly(dialkylsilanes), with alkyl side chains from butyl to tetradecyl, has been studied using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. It was shown that these polymers undergo a first order transition from a low temperature crystalline to a high temperature columnar mesomorphic phase; the transition temperatures range from 9 to 82°C. As the length of the side chains is increased, the transition temperature initially drops drastically and then rises again. These two regimes are connected to the relative part played in the phase transition by the aliphatic side chains and the polymer backbone. The enthalpies also revealed two regimes. Starting from the butyl polymer the enthalpy first grows more and more rapidly and, beyond a chain length of about ten methylene groups, it then grows linearly. From the Y intercept of the corresponding curve it was inferred that the transition into the mesomorphic phase is followed by a stabilization of the backbone conformation. The structure of the columnar mesophase was described as a two dimensional hexagonal packing of columns consisting of straight polymer backbones surrounded by alkyl chains in a disordered conformation. From the slope of the square of the intercolumnar distance as a function of the number of methylene groups per silicon atom, and using the known value of the volume of one methylene group in the liquid state, the stacking period of the silicon atoms along the columnar axis was found to be 1·68 Å. This relatively small value indicates a coiled structure involving a significant number of gauche conformers. The Y intercept of the corresponding straight line, related to the volume of one silylene repeat unit deprived of all its methylene groups, suggests a great compacity of the atomic packing near the silicon backbone.