THERMODYNAMIC INTERACTIONS IN MODEL POLYOLEFIN BLENDS OBTAINED BY SMALL-ANGLE NEUTRON-SCATTERING

The dependence of Flory-Huggins interaction parameter chi on temperature, composition, and chain length was investigated for binary blends of amorphous model polyolefins, materials which are structurally analogous to copolymers of ethylene and butene-1. The components were prepared by saturating the double bonds of nearly monodisperse polybutadienes (78 %, 88 %, and 97 % vinyl content) with H-2 and D2, the latter to provide contrast for small-angle neutron scattering (SANS) experiments. Values of chi were extracted from SANS data in the single-phase region for two series of blends, H97/D88 and H88/D78, using the random-phase approximation and the Flory-Huggins expression for free energy of mixing. These values were found to be insensitive to chain length (one test only) and to the component volume fractions for phi = 0.25, 0.50, and 0.75. Their temperature dependence (27-170-degrees-C) obeys the form chi(T) = A/T + B with coefficients that connote upper critical solution behavior, yielding T(c) approximately 40-degrees-C for one blend series (H97A/D88) and T(c) approximately 60-degrees-C for the other (H88/D78). These estimates are consistent with SANS pattern changes and supplemental light scattering results that indicate two-phase morphologies at lower temperatures. The chi(T) coefficients for the two series are also consistent with the random copolymer equation, although the interaction parameter obtained for branch C4-linear C4 chain units is much larger than that found by Crist and co-workers for saturated polybutadienes with lower ethyl branch contents.