STRUCTURAL ORIGIN OF THERMODYNAMIC INTERACTIONS IN BLENDS OF SATURATED-HYDROCARBON POLYMERS

The thermodynamic interactions in blends of saturated hydrocarbon poly mers originate from induced-dipole forces, and they differ in subtle b ut important ways, depending on the component structures. In this pape r we use six model polyolefins, four statistical copolymers and two al ternating copolymers, to examine some of the ways that have been sugge sted for organizing and interpretating these structural effects-statis tical segment length mismatch, random copolymer theory, and the solubi lity parameter formalism. The polymers and their partially deuterated counterparts are components in an interconnected matrix of miscible bi nary blends for which the interactions were determined by small-angle neutron scattering. The interactions, quantified as Flory-Huggins inte raction parameters, were broadly consistent with the length-mismatch i dea, but some notable exceptions were found. Interpretations based on random copolymer theory provided homopolymer interaction parameters wh ich conflicted with diblock copolymer results and other blend data. Te sts of consistency with solubility parameter ideas were applied to the matrix of blends. The results for one component pair indicated strong ly anomalous mixing, but those for all other pairs were consistent wit h the uniqueness criterion of the theory. Solubility parameters estima ted from PVT measurements on the pure components agreed fairly well wi th assignments based on the measured interactions. Some inconsistencie s remain, however, between diblock copolymer results and the solubilit y parameter formulation.