CONCENTRATION AND CHAIN-LENGTH DEPENDENCE OF THERMODYNAMIC INTERACTIONS IN POLYETHYLENE ISOTOPE BLENDS

Small-angle neutron scattering (SANS) measurements of interactions in polymer blends, chi(NS), generally depend on blend concentration phi, even though chi(NS) is evaluated with a model that assumes that the th ermodynamic interaction parameter chi(FH)=chi(NS) is independent of ph i. Londono et al. have reported chi(NS) to increase by similar to 4x w hen phi drops below 0.05 in polyethylene isotope blends. The relation between scattering and thermodynamics is addressed with incompressible Flory-Huggins theory wherein the thermodynamic interaction parameter chi may vary with concentration phi and degree of polymerization N; he re chi(NS)(phi) not equal chi(phi), For polyethylene isotope and simil ar polyolefin blends, the strong upward curvature of chi(NS) implies a modest (ca. 30%) increase of chi. Macroscopic phase behavior is unaff ected because the shape of the binodal remains essentially unchanged. The phi-dependence of chi(NS) in turn depends on N, leading to the fol lowing empirical expression for the thermodynamic interaction paramete r: chi(phi, N) = beta - (2 gamma'/N phi(1) phi(2))(phi(1) ln phi(1) phi(2) ln phi(2)). For polyethylene isotope blends at 155 degrees C, b eta = 2.85 x 10(-4) and gamma' = 0.15. Simple Flory-Huggins behavior w ith chi(FH) = beta is recovered when N approaches infinity. The source of the phi- and N-dependent second term is not known. (C) 1997 John W iley & Sons, Inc.