Thermodynamic Interactions in Polydiene/Polyolefin Blends Containing Diverse Polydiene and Polyolefin Units

Thermodynamic interactions were investigated in blends of melts containing 1,4-polyisoprene (1,4-PI) and copolymers of 1,2polybutadiene (1,2-PBD), poly(ethyl ethylene) (PEE), and/or polyethylene (PE) units. Specifically, two classes of blends were examined: one contained the polydiene 1,4-PI and a diene-olefin copolymer of 1,2-PBD and PEE units (1,2-PBD-co-PEE, with the PEE volume fraction varying from 0 to 1), and the other contained 1,4-PI and a fully olefinic copolymer of PE and PEE units (PE-co-PEE, with the PEE volume fraction varying from 0.05 to 1). The Flory-Huggins interaction parameter, chi, was measured using small angle neutron scattering (SANS) for homogeneous blends at or close to their critical compositions. SANS data were analyzed through the Zimm method and fitting of the random phase approximation model to extract chi(T). Random copolymer theory (RCT) was applied to understand the resulting chi(T) behavior. In 1,2-PBD-co-PEE/1,4-PI blends, predictions using the RCT model showed excellent agreement with the measured chi(T), indicating the presence of random mixing and dominance of dispersive forces in these blends. By contrast, RCT failed to describe the chi(T) behavior in PE-co-PEE/1,4-PI blends. The PE-co-PEE/1,4-PI blends were further examined using lattice cluster theory and solubility parameter formalism, yet neither approach could explain the chi(T) behavior. The locally correlated lattice model, which correlates the equation of state properties with the polymer blend miscibility, was found to reasonably describe the phase behavior of PE-co-PEE/1,4-PI blends.