Thermodynamic Interactions in Model Polyolefin Blends Obtained by Small-Angle Neutron Scattering

The dependence of Flory-Huggins interaction parameter χ 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 H₂ and D₂, the latter to provide contrast for small-angle neutron scattering (SANS) experiments. Values of χ were extracted from SANS data in the single-phase region for two series of blends, H97/D88 and H88/D78, using the randomphase 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 ϕ= 0.25, 0.50, and 0.75. Their temperature dependence (27-170 °C) obeys the form χ(T) =A/T + B with coefficients that connote upper critical solution behavior, yielding T_c ∼ 40 °C for one blend series (H97A/D88) and T_c ∼ 60 °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 χ(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.