Phase Behaviour and Thermal Stability of Imide-Containing Siloxane-Urethane Copolymers

Imide-containing siloxane-urethane copolymers (I-PU copolymers) were synthesized from 4,4'-diphenylmethane diisocyanate (MDI), OH-terminated polydimethylsiloxane (PDMS) and pyromellitic dianhydride (PMDA). The hard segment consisted of urethane and imide segments and the soft segment was PDMS. The DSC of I-PU copolymers suggests that they had a high degree of phase separation in their low hard segment contents and phase mixing in high hard segment contents. DMA indicates similar phase behaviour and shows that incorporating imide segments into the copolymers can maintain the high modulus at high temperature. The progress of degradation of I-PU copolymers was determined by TGA and standard kinetics methods and there were three stages at around 250~425 °C, 425~560 °C and 560~700 °C, respectively. The first stage was dominated by the hard-segment structure and content. Incorporating imide segments into PU raises the characteristic temperatures of degradation in the first stage. The second stage resulted from the degradation of the PDMS soft segment to form oligomers by the interchange reaction. The degradation of the imide segment leads to the third stages because the maximum rate of degradation in DTG curves increases with the imide content.