Pulsed N.M.R. studies of radiation-induced crosslinking and gel formation in linear polydimethylsiloxane

Pulsed N.M.R. measurements of the proton spin-lattice, T1, and spin-spin, T2, relaxation times for γ-irradiated linear polydimethylsiloxane (molecular weight = 0.6 x 104) as a function of dose up to 200 Mrad are reported. The dependences of T1 and T2 on dose are discussed in terms of the effects of the introduction of intermolecular crosslinks on the molecular motion. Above the gel point the spin-spin relaxation exhibits a double exponential decay characteristic of a two-phase system. This behaviour is attributed to the heterogeneous phase separation of the material into gel and sol fractions. The value of T2 for each phase provides information regarding the differing molecular mobilities of the gel and sol phases and their dependence on radiation dose. The sol fractions determined from the N.M.R. T2 measurements are compared with those obtained using the conventional sol extraction technique and are shown to be reasonable quantitative agreement. The applicability of N.M.R. as a quantitative method for the determination of gel points and gel fractions is demonstrated and discussed. (author)