MICROSCOPIC PARAMETERS INFLUENCING THE PHASE-SEPARATION IN COMPRESSIBLE BINARY BLENDS OF LINEAR SEMIFLEXIBLE POLYMERS

The lattice cluster theory (LCT) is used to determine the essential mi croscopic parameters that influence the phase separation in binary ble nds of linear semiflexible lattice chains with equal polymerization in dices. The LCT and the polymer reference interaction site model are sh own to predict nearly identical and universal constant volume phase be haviors (after simple numerical rescaling of the polymerization indice s) for ''athermal'' blends with vanishing van der Waals attractive ene rgies. Phase separation in these systems is driven solely by stiffness disparities. LCT computations are extended to ''thermal'' systems in which the van der Waals interactions are large enough to produce liqui d densities at standard temperature and pressure. Both the stiffness d isparity between the blend components and the relative magnitudes of t he van der Waals interaction energies influence the phase behavior of the model blends. We find a family of universal constant volume spinod als, parameterized by the exchange energy. Compressibility is shown to produce significant enthalpic contributions to phase separation, even when all van der Waals energies are identical. We also study the pres sure dependence of these model blends, as well as the variety of quali tatively different phase behaviors exhibited. A future work will deter mine the combined influence of monomer structure, semiflexibility, van der Waals interactions, and the energetic implications of compressibi lity on the phase behavior of polyolefin blends. (C) 1997 American Ins titute of Physics.