Radiation-Induced Cross-Linking in a Silica-Filled Silicone Elastomer As Investigated by Multiple Quantum ¹H NMR

Abstract

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.

MQ NMR and SPME Analysis of Nonlinearity in the Degradation of a Filled Silicone Elastomer

“…H vs. Y crosslinking via varying free radical mechanisms), bonding to the filler matrix, etc. [1][2][3][4][6][7][8][9][10][11][12][13][14][15] For both thermally and radiation induced degradation multiple, competing reaction pathways can exist and occur simultaneously to varying extent dependent on variables such as total input energy, monomer types (i.e. stabilizing phenyl groups), additives (fillers, stabilizers, etc.…”

Mechanical properties and cross-linking structure of cross-linked natural rubber

“…Previous work [1][2][3] on the analysis of the cross-linking junctions primary used high-resolution solid-state NMR spectroscopy, because the cross-linked natural rubber was insoluble in organic solvents as a result of the threedimensional network formed after vulcanization and the structural heterogeneity of the cross-linking junctions. However, in the previous work, pulse sequences applicable to analyze the cross-linking junctions of the cross-linked natural rubber were limited to onedimensional solid-state 13 C-NMR spectroscopy because of highpower decoupling.…”

Lifetime Assessments of Weapon Organics and Polymers FY05 Annual Report

“…Detailed descriptions of the experimental methods as well as thermal and radiative aging methods can be found elsewhere [4][5][6][7][8].…”

“…We have used combined use of traditional polymer assessment methods with advanced NMR relaxometry methods to assess changes in the network structure in a series of radiatively aged PDMS based elastomers based on the Dow Corning DC745 gum stock [5]. The NMR data suggests that with increasing exposure, DC745 undergoes radiative crosslinking reactions that, up to 25 MR do not increase significantly the sol fraction of the network.…”