| Abstract: | A series of polymeric networks were prepared by the γ-radiation crosslinking of two ethylene-propylene copolymers containing 62 and 69 mol % ethylene units. The elastomeric materials thus obtained were studied in elongation, both swollen and unswollen, over the temperature range ?30 to 95°C. The experiments involved measurements of both the elastic force and birefringence, using primarily changes in temperature at constant length (rather than changes in length at constant temperature). There is evidence for crystallization in these networks, as manifested by marked decreases in force and increases in birefringence at relatively low elongations and at temperatures as high as 70°C. As expected, the birefringence and related quantities were found to be more sensitive to crystallization than the force, with the stress-optical coefficient showing the greatest sensitivity. The results obtained in the elongation-temperature regions presumably free from effects of network crystallization were used to calculate values of the temperature coefficient of the unperturbed dimensions of the chains, and values of the optical-configuration parameter. Both are widely used to characterize (spatial) configurations of chain molecules. Values of the former quantity are in very good agreement with those predicted some years ago using a rotational isomeric state model for ethylene-propylene chains of various chemical and stereochemical compositions. The extension of these theoretical calculations to include the stress-optical coefficient should provide further information on crystallization in ethylene-propylene elastomers and the configurational characteristics of these copolymeric chains. |
