The effect of different siloxane chain-extenders on the thermal degradation and stability of segmented polyurethanes

The thermal degradation and stability of siloxane-containing segmented polyurethane and poly(urethane-urea) polymers (OH-SiPU and NH 2-SiPU), both prepared from 4,4′-diphenylmethane diisocyanate (MDI), polytetramethylene ether glycol (PTMG) and silicon-containing chain-extenders, have been studied by thermogravimetric analysis (TGA) and Ozawa-Flynn kinetic analysis. The analyses identically revealed that OH-SiPU and NH 2-SiPU polymers showed two stages of thermal degradation. The thermal stability and degradation depended on the kinds of hard and soft segment in the backbone of the polymers. The OH-SiPU polymers whose hard segment consists of siloxane-urethane displayed more thermal stability than the NH 2-SiPU polymers, whose hard segment consist of siloxane-urea, in the initial degradation at the first stage of reaction (stage I). However, NH 2-SiPU polymers were more thermally stable than OH-SiPU polymers and typical polyurethane as the weight loss went beyond about 5 wt.%. The length of the soft segment (PTMG molecular weight) affected the thermal stability of these polymers at various stage of degradation: polymers with a long soft segment had a high thermal stability in stage I, and those with a short soft segment had a higher thermal stability in the second stage of degradation (stage II). The Ozawa method was used to evaluate the activation the energies (Ea) of the whole process of degradation and the average activation energy (Ea,av) in each stage of degradation. The Ea and Ea,av values determined for the OH-SiPU and NH 2-SiPU polymers according to the Ozawa method corresponds with the TG and DTG curve displaying the character of the degradation.