Multiple recrystallization behavior of poly(1,1-dimethysilacyclobutane): A combined calorimetric and small angle X-ray scattering study

Abstract

The crystallization and melting behavior of a series of poly(1,1-dimethysilacyclobutane) (PDMSB) samples accessible via living anionic polymerization protocols with different molar masses were studied by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and X-ray diffraction (XRD). It has been found that in the cooling process only one crystallization exotherm was observed by using DSC, but in subsequent heating two clear endotherms appeared. DSC measurements additionally revealed that there will be two melting peaks when the isothermal crystallization temperature, Tc, is low enough, but only one melting peak could be obtained when Tc is high. The value of the lower melting peak Tm1 increased with Tc, showing the increase of lamella thickness due to increase of Tc, which was also evidenced by using SAXS measurements. However, the value of the higher melting peak Tm2 stayed almost constant with Tc. XRD analysis showed that there is only one crystal form independent of Tc. While SAXS results revealed only one long range order, indicating this multiple endotherms phenomenon was caused by the recrystallization process during heating, but not isothermal thickening and thinning process as observed for e.g. poly(ethylene oxide)s (PEOs) with low molar masses. Heating rate dependent DSC experiments showed that there are two recrystallization processes with different time scales. The recrystallization at low temperature always showed up independent of heating rate while the recrystallization at high temperature showed up only when the heating rate was very low. This interesting phenomenon was explained by the different energy barrier for the recrystallization process at low and high temperature as shown by in situ SAXS measurements during heating.