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 H-1 multiple quantum (MQ) NMR methods. Multiple domains of polymer chains were detected by MQ NMR exhibiting residual dipolar coupling (<Omega >) 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 <Omega(d)> values and changes in <Omega(d)> distributions present in the material were studied as a function of time at 250 degrees C and indicate significant time-dependent degradation. For the domains with low <Omega(d)>, a broadening in the distribution was observed with aging time. For the domain with high <Omega(d)>, increases in both the mean <Omega(d)> and the width in <Omega(d)> were observed with increasing aging time. Isothermal thermal gravimetric analysis reveals a 3% decrease in weight over 20 h of aging at 250 degrees C. Degraded samples also were analyzed by traditional solid-state H-1 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.