CRYSTALLINE-STRUCTURE OF POLY(METHYL-N-PROPYLSILANE)

The thermal behavior and physical structure of atactic poly(methyl-n-p ropylsilane) (PMPrS) have been investigated by complementary technique s. Temperature-dependent wide-angle X-ray scattering as well as therma l analysis clearly indicate that atactic PMPrS crystallizes below 40 d egrees C in a monoclinic lattice with PMPrS adopting an all-trans plan ar zigzag conformation. Above 40 degrees C, the polymer is in the isot ropic amorphous state. A restricted analysis of the structure factors of PMPrS has been performed, indicating that the zigzag planes most pr obably lie in ((1) over bar 10) planes. The chains pack with little in terpenetration, and the crystals may be considered as bundles of long, closely packed prisms. The restricted interlocking of neighboring cha ins results, in turn, in a poor register of the chains along the c-axi s. Moreover, transmission electron microscopy reveals that the crystal lized polymer adopts a lamellar microstructure, with parallel lamellae tending to form tight bundles. Both electron microscopy and small-ang le X-ray scattering indicate crystal thicknesses of about 60 Angstrom. Finally, PMPrS was found to crystallize with a nucleation-controlled type of kinetics. Avrami exponents were calculated as n approximate to 1, suggesting a fibrillar growth geometry compatible with the absence of spherulitic superstructure. A double-melting behavior is also obse rved for PMPrS. (C) 1997 John Wiley & Sons, Inc.