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

A microscopically realistic theory for modeling the structure, thermod ynamics, and phase behavior of polyolefin blends is developed on the b asis of the polymer reference interaction site model (PRISM theory). T he thermodynamics of mixing is treated using perturbation theory with the corresponding athermal mixture as the reference system. As an illu stration of the approach we modeled the polyethylene/isotactic polypro pylene blend (PEA-PP). The polypropylene monomers were constructed fro m three independent interaction sites representing the CH2, CH, and CH 3 united atom groups constituting the monomer. Likewise polyethylene m onomers consisted of two identical interaction sites, each representin g a CH2 moiety. The intramolecular structure functions, required as in put to PRISM theory, were determined from single-chain Monte Carlo sim ulations using the rotational isomeric state approximation. The Suter- Flory rotational isomeric state parameters were used for isotactic pol ypropylene. PRISM theory was used to compute the ten independent inter molecular pair correlation functions needed to characterize the interm olecular 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 compu ted 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 incom patible with UCST behavior. Local or short range correlations were fou nd to significantly increase the heat of mixing, resulting in critical temperatures approximately 10-15 times larger than predicted on the b asis of Flory-Huggins theory under the assumption of random mixing.