Radiation induced structural and motional changes occurring in silica filled silicone polymer foams as probed by multinuclear NMR

The aging of polymeric composite materials through factors such as thermal and mechanical stresses, environment, radiation, and chemical attack can affect the length of time for which a given material can maintain its engineering performance. Iterative interactions and cumulative reactions may result in the material or device reaching a critical age where its properties fail unexpectedly and catastrophically. The mechanical property changes associated with multi-mechanism aging may be subtle, and may not necessarily change linearly as a function of time in service. Since such linear relationships are often used in lifetime predictions, there is a fundamental need to develop and employ spectroscopic methods to investigate the structural and motional changes that occur in these organic-inorganic materials as a result of aging in chemically, thermally, or radioactively harsh environments. Silica filled polydimethylsiloxane (PDMS) composite systems are of technological interest due to their chemical and environmental resilience. Silica is usually chosen as the filler phase due to the significant reinforcement of the composite material through hydrogen bonding between the polymer chains and the surface groups on the filler. Unfilled PDMS is known to crosslink when exposed to high-energy radiation. The presence of a silica filler phase, which has a higher electron density than the polymer matrix, has been proposed to result in an increased incidence of crosslinking or scission due to backscatter of the incident radiation. Cohen-Addad has used 1H relaxation times to characterized cross-link density in unirradiated filled PDMS and Charlesby has reported 1H relaxation studies of irradiation induced changes in unfilled PDMS systems of average molecular weights up to 1 MDalton. However, no specific studies have been reported on aging of silica-filled PDMS based polymers systems. To this end the authors have applied Nuclear Magnetic Resonance (NMR) methods to gain insight into the processes that are contributing to mechanical failure of silica filled polydimethylsiloxane (PDMS) based cushions. The studies so far have concentrated on (A) 1H, 13C, and 29Si Magic Angle Spinning (MAS) measurements of chemical speciation from chemical shifts, and (B) 1H relaxation measurements