ISOTOPE AND MICROSTRUCTURE INTERACTIONS IN BLENDS OF RANDOM COPOLYMERS

Combined isotope and microstructure effects on thermodynamic interactions are examined by small-angle neutron scattering from binary blends of random copolymers having one partially deuterated component. These copolymers are based on linear and ethyl branched C4H8 units and are models for poly(ethylene-co-butene-1). The Flory-Huggins interaction parameter chi is found to be approximately 100% larger for blends in which the more branched copolymer is labeled with deuterium. This ''switching'' effect, where the chemical microstructure of the labeled component affects chi, is confirmed by critical temperatures in blends having all possible isotope combinations. Parallel studies by Graessley et al. show the same effects with copolymers having higher branch concentrations. Analysis in terms of copolymer/copolymer theory indicates that the repulsive interactions between monomers differing in both chemical microstructure and isotope depend on which repeat unit, linear or branched, is labeled with deuterium. A simple model based on molar volumes of linear and branched, hydrogenous and deuterous repeat units accounts for the observed effects in a semiquantitative manner. The contribution to chi from chemical microstructure does not obey simple copolymer theory; the strength of repulsive interactions depends on the average composition of the chains in addition to the composition difference between components.