THERMODYNAMICS OF MIXING FOR BLENDS OF MODEL ETHYLENE BUTENE COPOLYMERS

This paper describes a study of thermodynamic interactions in binary b lends of AyB1-y copolymers. The blend components are hydrogenated poly butadienes, consisting of chains with branched C4 and linear C4 units and corresponding to model ethylene-butene copolymers. Overall composi tions range from nearly polyethylene (y = O) to nearly poly(butene-1) (y = 1) through a series of nearly random branched C4-linear C4 copoly mers. Values of the Flory-Huggins interaction parameter chi were deter mined by small-angle neutron scattering (SANS) from binary blends (one component deuterated) and then corrected for the effect of isotopic s ubstitution. The reduced interaction parameter chi/y2 - y1)2 was found to be a strong function of the average blend composition, contrary to classical copolymer theory. Higher order theories, based on diad or t riad interactions, could be made to fit, but the physical significance of the fits was unclear. The same results were also expressed in term s of a relative solubility parameter, delta - delta(ref), for each cop olymer. These values were found to agree well with calculations based on PVT properties of the pure components. Thus, it seems clear that, f or this system at least, departures from the copolymer equation are fo retold in the pure component properties and have nothing to do with an omalies associated with mixing. More generally, inferences about the m ixing process that are based on the temperature dependence of chi alon e must be viewed with great caution.