THE INFLUENCE OF HIGH-PRESSURE CARBON DIOXIDE ON THE PHASE BEHAVIOR OF PDMS/PEMS BLENDS: AN EXPERIMENTAL AND THEORETICAL INVESTIGATION

摘要

In this work the effect of high-pressure CO_2 on the miscibility of low-polydispersity poly(dimethylsiloxane)/poly(ethylmethylsiloxane) (PDMS/PEMS) blends in which M_(w, PDMS) = 44.5 kg/mol and M_(w, PEMS) = 14.6 kg/mol is investigated by complementary experimental and theoretical methods. Experimental measurements of phase equilibria in ternary PDMS/PEMS/ CO_2 systems are obtained as functions of blend composition and CO_2 pressure by high-pressure spectrophotometry. Results indicate that the cloud point of the blend (T_(cp)) increases with increasing CO_2 pressure. This observation reveals that CO_2 serves to reduce blend miscibility by translating the entire phase envelope of our PDMS/PEMS blend to higher temperatures, in marked contrast to previous studies of PDMS/PEMS blends subjected to hydrostatic pressure (which enhances blend miscibility by depressing T_(cp)), We have modeled the phase behavior of these PDMS/PEMS/CO_2 systems using the SAFT equation of state, which is a molecular-based formalism that is designed to account for the effects of molecular association and chain flexibility, as well as repulsive and dispersion interactions. The SAFT parameters for PDMS are obtained from binary cloud curves previously reported in the literature for PDMS/CO_2 mixtures, whereas PEMS parameters are obtained from an extended group contribution approach established for PDMS systems. The ability of the model to predict the phase behavior of the ternary PDMS/PEMS/CO_2 system is corroborated by comparing theoretical predictions with the experimental data reported herein.