We have measured changes in transverse relaxation times (T2e), residual dipolar couplings (〈Ωd2〉), and the mean-squared fluctuations in the residual dipolar couplings (〈δΩd2〉), associated with cross-link density changes in a complex silica-filled polydiphenylpolysiloxane/polydimethylsiloxane (PDPS/PDMS) block copolymer composite material. The crosslinks were induced by both chemical modification of the base polymer and by radiolytic aging. We have detected H1 NMR responses from polymer chains directly interacting with the silica filler (〈δΩd2〉≫2×106 rad2 sec−2), network polymer chains not directly interacting with the silica filler (〈Ωd2〉∼2×106 rad2 sec−2 and 〈δΩd2〉∼2×106 rad2 sec−2) and non-network, low molecular weight chains and chain ends (〈δΩd2〉∼1×105 rad2 sec−2). The network domain and the non-network domain were observed to exchange spin magnetization with a correlation time of 1 sec. No evidence of spin-exchange effects on the stimulated echo were observed between the PDMS and PDPS blocks, although the blocks were observed to be in spatial proximity by double quantum NMR methods. The residual dipolar couplings change in a straightforward manner with radiation and chemically induced cross-linking of the polymer network. The strength of the filler-polymer interaction was seen to affect only the residual dipolar couplings and the transverse relaxation times and not directly the mean-squared fluctuations of the residual dipolar couplings. Dipolar correlation effect NMR shows direct evidence for surface adsorbed species, however, and has measured changes in the amount of surface adsorption due to irradiation. The results suggest that siloxane polymer cross-linking was preceded by an initial disruption of the hydrogen bond interaction between the polymer backbone and the silica silanol groups at the polymer/silica interface and that noticeable radiation induced cross-linking then occurs at dosages above 100 kGray. The work reported here shows that detailed characterization of the relaxation processes of the various nuclei in the siloxane polymers under static conditions has the potential to provide detailed insight into changes in the mechanisms and energetics of motional processes brought about by polymer aging processes.
The effect of chemisorbed and physisorbed water on the interfacial structure and dynamics in silica-filled polydimethylsiloxane (PDMS) based composites have been investigated. Toward this end, we have combined molecular dynamics simulations and experimental studies employing dynamic mechanical analysis and nuclear magnetic resonance analysis. Our 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 overall, bulk, motional properties of the polymer, thus causing an effective 'stiffening' of the polymer matrix. The role of the long-ranged Coulombic interactions on the structural features and chain dynamics of the polymer were also examined. Both are found to be strongly influenced by the electrostatic interactions as identified by the bond orientation time correlation function, local density distribution and radial distribution functions. These results have important implications for the design and life performance behavior of nanocomposite silica -siloxane materials. q
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