Thermal Degradation in a Trimodal Poly(dimethylsiloxane) Network Studied by 1H Multiple Quantum NMR

Thermal degradation of a filled, cross-linked siloxane material synthesized from poly(dimethylsiloxane) chains of three different average molecular weights and with two different cross-linking species has been studied by 1H multiple quantum (MQ) NMR methods. Multiple domains of polymer chains were detected by MQ NMR exhibiting residual dipolar coupling (〈Ωd〉) values of 200 and 600 Hz, corresponding to chains with high average molecular weight between cross-links and chains with low average molecular weight between cross-links or near the multifunctional cross-linking sites. Characterization of the 〈Ωd〉 values and changes in 〈Ωd〉 distributions present in the material were studied as a function of time at 250 °C and indicate significant time-dependent degradation. For the domains with low 〈Ωd〉, a broadening in the distribution was observed with aging time. For the domain with high 〈Ωd〉, increases in both the mean 〈Ωd〉 and the width in 〈Ωd〉 were observed with increasing aging time. Isothermal thermal gravimetric analysis reveals a 3% decrease in weight over 20 h of aging at 250 °C. Degraded samples also were analyzed by traditional solid-state 1H NMR techniques, and off-gassing products were identified by solid-phase microextraction followed by gas chromatography−mass spectrometry. The results, which will be discussed here, suggest that thermal degradation proceeds by complex competition between oxidative chain scissioning and postcuring cross-linking that both contribute to embrittlement.