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

Abstract

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 (〈Ω_d〉) 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 〈Ω_d〉 values and changes in 〈Ω_d〉 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 〈Ω_d〉, a broadening in the distribution was observed with aging time. For the domain with high 〈Ω_d〉, increases in both the mean 〈Ω_d〉 and the width in 〈Ω_d〉 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.