Further NMR studies of polydimethylsiloxanes: Effects of radiation‐induced crosslinking

Abstract

Previous nuclear magnetic resonance measurements on a polymethylsiloxane gum (MW = 75,000) had been interpreted to indicate the presence of considerable segmental motion in both the amorphous and crystalline components at temperatures above the glass temperature. Further investigation again showed the NMR line to be completely narrowed just above the glass point indicating such motions must extend into the crystalline component. This work was extended by the measurement of NMR absorption line widths for temperatures of 80–300°K. on samples of the same gum subjected to gradually‐increased crosslinking by electron doses of 3–600 × 10⁶ rep. Here, no change in the NMR properties of low temperature, rigid states were observed, but the room temperature line width progressively increased from 0.019 gauss to over 0.100 gauss peak‐to‐peak due to the increase in bulk viscosity of the liquid. The “first‐order” activation energy associated with the temperature‐dependent line narrowing decreased from 10 to 2 kcal./mole. The NMR data indicate that the primary effects of radiation‐induced crosslinking are to reduce the chain rotational energy by the creation of defects and to increase the short‐range viscosity by the lowering of translational freedom.