THEORY FOR THE PHASE-BEHAVIOR OF POLYOLEFIN BLENDS - APPLICATION TO THE POLYETHYLENE/ISOTACTIC POLYPROPYLENE BLEND

A microscopically realistic theory for modeling the structure, thermodynamics, and phase behavior of polyolefin blends is developed on the basis of the polymer reference interaction site model (PRISM theory). The thermodynamics of mixing is treated using perturbation theory with the corresponding athermal mixture as the reference system. As an illustration of the approach we modeled the polyethylene/isotactic polypropylene blend (PEA-PP). The polypropylene monomers were constructed from three independent interaction sites representing the CH2, CH, and CH3 united atom groups constituting the monomer. Likewise polyethylene monomers consisted of two identical interaction sites, each representing a CH2 moiety. The intramolecular structure functions, required as input to PRISM theory, were determined from single-chain Monte Carlo simulations using the rotational isomeric state approximation. The Suter-Flory rotational isomeric state parameters were used for isotactic polypropylene. PRISM theory was used to compute the ten independent intermolecular pair correlation functions needed to characterize the intermolecular packing of the athermal blend. The enthalpic contribution to the free energy was then computed from first-order perturbation theory. The entropy of mixing, heat of mixing, and spinodal curve were computed as a function of composition for the blend consisting of PE and i-PP chains of 200 monomer units. The blend was found to be highly incompatible with UCST behavior. Local or short range correlations were found to significantly increase the heat of mixing, resulting in critical temperatures approximately 10-15 times larger than predicted on the basis of Flory-Huggins theory under the assumption of random mixing.