Journal of Chemical Physics, Vol.116, No.23, 10492-10502, 2002
Residual dipolar coupling for the assessment of cross-link density changes in gamma-irradiated silica-PDMS composite materials
We have measured changes in transverse relaxation times (T-2e), residual dipolar couplings (), and the mean-squared fluctuations in the residual dipolar couplings (), 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 H-1 NMR responses from polymer chains directly interacting with the silica filler (>2x10(6) rad(2) sec(-2)), network polymer chains not directly interacting with the silica filler (similar to2x10(6) rad(2) sec(-2) and similar to2x10(6) rad(2) sec(-2)) and non-network, low molecular weight chains and chain ends (similar to1x10(5) rad(2) 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.