EFFECT OF DEUTERIUM SUBSTITUTION ON THERMODYNAMIC INTERACTIONS IN POLYMER BLENDS

We have investigated the effect of deuterium labeling on the thermodynamic interactions in blends of labeled and unlabeled saturated hydrocarbon polymers. Small-angle neutron scattering (SANS) was used to evaluate the Flory-Huggins interaction parameter chi at several temperatures and compositions. Light scattering was also used in several cases to confirm the location of phase boundaries. We find that deuterium labeling changes chi relative to the value for hydrogenous components and that the direction of the change depends on which of the two components is labeled. For blends of hydrogenated polybutadienes, chi always increases when the more branched component is labeled, a pattern first noted by Crist and Rhee and also consistent with the expectation that deuterium substitution reduces the cohesive energy density (solubility parameter) of hydrocarbon substances. A solubility parameter formalism is developed by which chi for hydrogenous components can be estimated with reasonable accuracy from SANS data obtained for the two combinations of singly-labeled components. It also provides a method for assigning relative values of the solubility parameter for a wide class of saturated hydrocarbon polymers.