Strain- and temperature-induced polymorphism of poly(dimethylsiloxane)

Phase behavior of silica-filled poly(dimethylsiloxane) (PDMS) network was investigated by wide-angle X-ray diffraction (WAXD) under various strain ratio between room temperature and -100 A degrees C, and anomalous polymorphic behavior was discovered. At room temperature, when sufficient strain was applied, PDMS network was found to transform into the mesomorphic phase from which only a pair of sharp equatorial reflections and faint meridional scattering were obtained in the WAXD pattern. At low temperature, PDMS network crystallized into one of three different crystal forms according to strain ratio. These crystal forms were denoted as alpha, transient, and beta forms in the descending order of corresponding strain ratio. The mesomorphic phase at room temperature transformed into the crystalline alpha form by reducing temperature. There was an anomalous feature about the transition of the crystalline forms that the position of reflections in the WAXD pattern changed continuously and reversibly with strain between the alpha and the beta forms through the transient form, while keeping the diffraction angles almost unchanged.