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

Pulsed N.M.R. measurements of the proton spin-lattice, T₁, and spin-spin, T₂, relaxation times for γ-irradiated linear polydimethylsiloxane ($\text{M}$_w = 0.6 × 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 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.