In this paper, we report on how interaction strength varies with pressure and temperature
for several polyolefin mixtures. We find that the interaction energies that govern phase behavior in
polymer blends are only a function of density for UCST polyolefin blends far from a critical point. As a
result, the effects of pressure on miscibility can be predicted for such blends from knowledge of the effects
of temperature on the interactions combined with PVT data. This remarkable simplification appears to
be related to the van der Waals nature of the interactions between saturated hydrocarbons. Density
dependence predicts the trends correctly for LCST polyolefin blends, but for these mixtures the interactions
depend in a more complex way on T and P.
The cloud-point pressures of copolymers of ethylene with propylene, butene, hexene, and octene in propane were measured with a variable-volume optical batch cell to investigate the effects of the number and length of branches on the phase behavior in the temperature range from 25 to 200 °C and at pressures up to 700 bar. As the degree of branching increased in the ethylene-propylene copolymers, ethylene-butene copolymers, and ethylene-hexene copolymers, the cloud-point pressures decreased. At the same degree of branching, the cloud-point pressures decreased slightly upon increasing branch length. The copolymer SAFT (statistical association fluid theory) equation of state was found to correlate the experimental cloud-point data by adjusting the branch segment energy.
Cloud-point and coexistence pressures, sub-and supercritical, are determined experimentally for a series of ethylene and propylene solutions of nearly monodisperse and polydisperse poly-(ethylene-co-propylene), referred to as PEP. The cloud-point and coexistence pressures are found to be close for solutions of nearly monodisperse PEP polymers but not for solutions of polydisperse PEP. The SAFT parameters derived from the experimental cloud points for nearly monodisperse solutes alone are found to predict the cloud-point pressures and solubilities for polydisperse PEP as well.
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