NMR measurements of residual dipolar couplings for lifetime assessments in γ-irradiated silica-PDMS composite materials

Abstract

We have measured changes in 1H NMR residual dipolar couplings, 〈Ω d 2〉 , and the mean squared fluctuations in the residual dipolar couplings, 〈 δΩ d 2〉 , associated with cross-link density changes in a complex silica-filled PDPS/PDMS block copolymer composite material. The cross-links were induced by exposure of the composite material to γ-radiation from a Co-60 source. We have detected 1H NMR responses from polymer chains directly interacting with the silica and network polymer chains not directly interacting with the silica. The residual dipolar couplings change in a straight-forward 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.