NMR based investigations of the effects of aging on the motional properties of cellular silicone foams
The aging of polymeric composite materials, such as filled polydimethylsiloxane foams, 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. We have used multinuclear nuclear magnetic resonance (NMR) spectroscopy to characterize aging signatures in a series of PDMS based composite materials. Unfortunately, {sup 13}C, {sup 29}Si, and {sup 1}H magic angle spinning NMR spectra remain unchanged with gamma radiation exposure up to 50Mrad. This suggests that the speciation related changes are small and occur at a frequency of less than approximately 1% of the monomer units. As a result, we have shifted focus and have employed relaxation studies to monitor changes in motional properties of the copolymer foams caused by irradiation. We have measured spin-lattice, spin-spin, and rotating frame spin-lattice relaxation times for PDMS model rubbers with variable cross link density and filler content, for M9760 foams irradiated from 0 to 50Mrad, and for dehydrated M9760 foams. Spin-lattice relaxation times, in general, are sensitive to fast molecular motions in the MHz frequency range. Spin-spin and rotating frame relaxation times, on the other hand, are sensitive to changes in slower motion processes in the kHz range. Comparison of changes in these relaxation parameters in the irradiated samples to mechanical properties, cross-link density, and filler content dependencies observed in the model compounds have lead to a picture of the changes in motional properties of the foam due to irradiation and water content. The characterization of motional changes in the foams could provide important data for predictive modeling efforts. In addition, we have also developed empirical relationships between relaxation times and interfacial and bulk polymer motional properties that might allow rapid NMR based screening methods to compliment solvent swelling experiments.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15005155
- Report Number(s):
- UCRL-JC-140914; TRN: US200414%%634
- Resource Relation:
- Conference: 23rd Aging, Compatibility and Stockpile Stewardship Conference, Livermore, CA (US), 11/14/2000--11/16/2000; Other Information: PBD: 4 Oct 2000
- Country of Publication:
- United States
- Language:
- English
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