Control of thermal cross-linking reactions and the degree of crystallinity of syndiotactic 1,2-polybutadiene

Thermal stability, crystallization, morphological development, subsequently melting, and crystallinity control of a syndiotactic 1,2-polybutadiene sample were carefully carried out by thermogravimetry (TGA), polarized optical microscopy (POM), differential scanning calorimetry (DSC), temperature-modulated differential scanning calorimetry (TMDSC), and wide-angle X-ray diffraction (WAXD), respectively. The experiments indicate that thermal cross-linking reaction rates under nitrogen protection and in air are different for this polymer at temperature above 155 degrees C. Under nitrogen protection, the thermal cross-linking reaction rate is delayed and the mechanism of melt crystallization obtained from the DSC results is in good accordance with that from POM observation. TMDSC results indicate that melting-recrystallization-melting model is more proper to explain the double melting events of this sample. At the same time, the evolution of the degree of crystallinity as the function of the time was investigated by WAY-D profiles for the samples firstly crystallized at 145 degrees C for 1 h and then kept at 163 degrees C mediated between the temperatures of the double peaks. It shows that as prolonging the annealing time at 163 degrees C thermal cross-linking reactions possibly occur, leading to gradual reduction of the apparent crystallite sizes, evaluated by Scherrer equation and the degree of crystallinity. The changing sequence of the relative intensity of the stronger four diffraction peaks with time due to thermal cross-linking reactions is (111)/(201) > (210) > (010) > (200)/(110). (c) 2005 Wiley Periodicals, Inc.