title: Thermal degradation in a trimodal poly(dimethylsiloxane) network studied by (1)H 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 (1)H multiple quantum (MQ) NMR methods. Multiple domains of polymer chains were detected by MQ NMR exhibiting residual dipolar coupling (<Omega(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 <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 (1)H 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.
authors.affiliation: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551, USA.
authors.forename: Jason R
authors.initials: JR
authors.lastname: Giuliani
authors.forename: Erica L
authors.initials: EL
authors.lastname: Gjersing
authors.forename: Sarah C
authors.initials: SC
authors.lastname: Chinn
authors.forename: Ticora V
authors.initials: TV
authors.lastname: Jones
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Wilson
authors.forename: Cynthia T
authors.initials: CT
authors.lastname: Alviso
authors.forename: Julie L
authors.initials: JL
authors.lastname: Herberg
authors.forename: Mark A
authors.initials: MA
authors.lastname: Pearson
authors.forename: Robert S
authors.initials: RS
authors.lastname: Maxwell