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DC745 is a commercially available silicone elastomer consisting of dimethyl, methyl−phenyl, and vinyl−methyl siloxane monomers cross-linked with a peroxide vinyl specific curing agent. It is generally considered to age gracefully and to be resistant to chemical and thermally harsh environments. However, little data exist on the radiation resistance of this commonly used silicone elastomer. We report static ¹H NMR studies of residual dipolar couplings in DC745 solid elastomers subject to exposure to ionizing γ radiation. ¹H spin-echo NMR data shows that with increasing dose the segmental dynamics decrease is consistent with radiatively induced cross-linking. ¹H multiple quantum NMR was used to assess changes in the network structure and observed the presence of a bimodal distribution of residual dipolar couplings, 〈Ωd〉, that were dose dependent. The domain with the lower 〈Ωd〉 has been assigned to the polymer network while the domain with the higher 〈Ωd〉 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 cross-linking. In addition, DSC thermograms obtained following a 3 h isothermal soak at −40 °C showed the presence of a second melt feature at Tm ~ −70 °C consistent with a network domain with significantly reduced segmental motion.

Structural speciation of glasses in the systems PbO−B2O3−SiO2, PbO−B2O3−Al2O3−SiO2, and PbO−Al2O3−SiO2 were studied using solid state ²⁹Si, ²⁷Al, ¹¹B, and ²⁰⁷Pb nuclear magnetic resonance (NMR) and Raman spectroscopy. Application of these methods provided insight into the role of Al2O3 incorporation in the lead borosilicate glass networks. The general composition range studied was (PbO)x[(B2O3)1-z(Al2O3)z]y(SiO2)y where x = 0.35, 0.5, and 0.65, y = (1 − x)/2 and z = 0.0, 0.5, and 1.0. Additional insight was obtained via ²⁷Al 2D-3QMAS experiments. The ²⁰⁷Pb spin−echo mapping spectra showed a transition from ionic (Pb²⁺) to covalently bound lead species with increased PbO contents in the borosilicate glasses. The addition of aluminum to the glass network further enhanced the lead species transition, resulting in a higher relative amount of covalent lead bonding in the high PbO content alumino-borosilicate glass. The number of BO4 units present in the ¹¹B MAS NMR decreased with increasing PbO contents for both the borosilicate and the alumino-borosilicate glass systems, with the addition of aluminum further promoting the BO3 species. A deshielding of the ¹¹B chemical shifts and the ²⁷Al chemical shifts with increased lead contents (independent of changes in the quadrupolar coupling) was also observed for both glass systems and was attributed to an increasingly homogeneous glass structure. The ²⁹Si spectra of the borosilicate and alumino-borosilicate glasses showed a downfield shift with increased PbO concentrations representing incorporation of Pb into the silicate and aluminosilicate networks. The Raman spectra were characterized by increased intensities of Si−O−Pb peaks and decreased intensities of Q³ peaks with increased PbO contents and showed no evidence of BO3 or BO4 ring species. Both the NMR and the Raman data point toward systems where lead is increasingly incorporated into the B2O3−SiO2 and the B2O3−SiO2−Al2O3 networks at high PbO concentrations, with the addition of Al2O3 enhancing the trend.