Experimental and Modeling Studies of Water-Silica-PDMS Interactions in M97-Based Stress Cushions
In filled PDMS based composites, such as M97XX stress cushions, significant mechanical reinforcement of the polymer component is obtained from hydrogen bonding between the silica filler surface hydroxyls and the siloxane polymer backbone. It is expected that these interactions are influenced by the amount and structure of interfacial water. We have chosen to investigate in detail the effect of chemisorbed and physisorbed water on the interfacial structure and dynamics in silica-filled PDMS-based composites. Toward this end, we have combined classical molecular dynamics simulations and experimental studies employing nanoindentation, temperature programmed desorption (TPD), Dynamic Mechanical Analysis (DMA), and Nuclear Magnetic Resonance (NMR) analyses. Our TPD results suggest that moisture desorption and adsorption in M9787 can be approximated by the interaction of its silica constituents (Cab-0-Sil-M-7D and Hi-Sil-233) with moisture. Our experimental data also reveal that, in general, as heat-treated silica particles are exposed to moisture, chemisorbed states, then physisorbed states are gradually filled up in that order. Molecular modeling results suggest that the polymer-silica contact distance and the mobility of interfacial polymer chains significantly decreased as the hydration level at the interface was reduced. The reduced mobility of the PDMS chains in the interfacial domain reduced the bulk motional properties of the polymer, thus causing an effective ''stiffening'' of the polymer matrix. This finding is consistent with both NMR and modal analysis experimental data on desiccated M97XX samples. We show that both the motional dynamics of the polymer and the structure in the interfacial region are indeed controlled by the concentration of water in this region and that by considering the thermodynamic and kinetic parameters derived from TPD experiments, we can begin to approach a predictive capability for the time dependence of water speciation and thus the mechanical properties of the composite material as a function of service environment and age.
- 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:
- 15005360
- Report Number(s):
- UCRL-JC-148029; TRN: US200322%%385
- Resource Relation:
- Conference: 24th Aging Compatibility and Stockpile Stewardship Conference, Amarillo, TX (US), 04/29/2002--05/03/2002; Other Information: PBD: 18 Apr 2002
- Country of Publication:
- United States
- Language:
- English
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