Phase structure and transitions in a poly(methyloctadecylsilane) oligomer

A poly(methyloctadecylsilane) oligomer was synthesized by a typical Wurtz coupling reaction. Upon cooling, three transitions were observed at temperatures of 39.9, 37.5 and 33.9°C at a rate of 2.5°C/min in differential scanning calorimetry (DSC). The first transition, with enthalpy change of 0.47 kJ/mol and supercooling of 0.2°C, was characteristic of the conformational change in the Si–Si backbone into an all-trans conformation, which was detected by temperature-dependent Fourier transform infrared (FT-IR) spectroscopy. The second and the third transitions with large supercooling were identified as the formation of two-dimensional hexagonal crystal packing and three-dimensional two-chain orthorhombic crystal packing, respectively. The crystal structure was determined by the combination of WAXD and transmission electron microscopy (TEM) experiments.