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RECEIVED DATERunning title: "MQ-NMR of Radiation damage in filled siloxanes" CORRESPONDING AUTHOR FOOTNOTE: Mail code: L-231, (925) 423-4991, (925) 422-3750, maxwell7@llnl.gov ABSTRACT: DC745 is a commercially available silicone elastomer consisting of dimethyl, methylphenyl, and vinyl-methyl siloxane monomers crosslinked with a peroxide vinyl specific curing agent. It is generally considered to age gracefully and to be resistant to chemical and thermally harsh Maxwell, et al. MQ-NMR of radiation damage in filled silicones p.2 environments. However, little data exists on the radiation resistance of this commonly used silicone elastomer. We report static 1 H NMR studies of residual dipolar couplings in DC745 solid elastomers subject to exposure to ionizing gamma radiation. 1 H spin-echo NMR data shows that with increasing dose, the segmental dynamics decrease is consistent with radiatively induced crosslinking. 1 H multiple quantum NMR was used to assess changes in the network structure and observed the presence of a bimodal distribution of residual dipolar couplings, <Ω d >, that were dose dependent. The domain with the lower <Ω d > has been assigned to the polymer network while the domain with the higher <Ω d > has been assigned to polymer chains interacting with the inorganic filler surfaces. In samples exposed to radiation, the residual dipolar couplings in both reservoirs were observed to increase and the populations were observed to be dose dependent. The NMR results are compared to Differential Scanning Calorimetry (DSC) and a two-step solvent swelling technique. The solvent swelling data lend support to the interpretation of the NMR results and the DSC data show both a decrease in the melt temperature and the heat of fusion with cumulative dose, consistent with radiative crosslinking. In addition, DSC thermograms obtained following a 3 hr isothermal soak at -40 ºC showed the presence of a second melt feature at T m ~ -70 ºC consistent with a network domain with significantly reduced segmental motion.

Radiation-induced degradation of polymeric materials occurs through numerous, simultaneous, competing chemical reactions. Although degradation is typically found to be linear in adsorbed dose, some silicone materials exhibit nonlinear dose dependence due to dose-dependent dominant degradation pathways. We have characterized the effects of radiative and thermal degradation on a model filled-PDMS system, Sylgard 184 (commonly used in electronic encapsulation and in biomedical applications), using traditional mechanical testing, NMR spectroscopy, and sample headspace analysis using solid-phase microextraction (SPME) followed by gas chromatography/mass spectrometry (GC/MS). The mechanical data and 1H spin−echo NMR spectra indicated that radiation exposure leads to predominantly cross-linking over the cumulative dose range studied (0−250 kGy) with a rate roughly linear with dose. 1H multiple-quantum NMR spectroscopy detected a bimodal distribution in the network structure, as expected from the proposed structure of Sylgard 184. The MQ NMR spectra further indicated that the radiation-induced structural changes were not linear in adsorbed dose and that competing chain scission mechanisms made a greater contribution to the overall degradation process in the range of 50−100 kGy (although cross-linking still dominated). The SPME−GC/MS data were analyzed using principal component analysis (PCA), which identified subtle changes in the distributions of degradation products (the cyclic siloxanes and other components of the material) as a function of age that provide insight into the dominant degradation pathways at low and high adsorbed dose.