Gas and Vapor Solubility in Cross-Linked Poly(ethylene Glycol Diacrylate)

Abstract

Polymers containing poly(ethylene glycol) (PEG) have been investigated for CO2 removal from mixtures with light gases such as CH4, N2, and H2 because of the good affinity of PEG for CO2. In this study, the solubilities of CO2 and several hydrocarbons (CH4, C2H4, C2H6, C3H6, and C3H8) were determined as a function of gas fugacity (i.e., pressure) at temperatures ranging from −20 °C to 35 °C in cross-linked amorphous poly(ethylene glycol) diacrylate (XLPEGDA), which was prepared by UV photopolymerization of poly(ethylene glycol) diacrylate and contains about 82 wt % PEG. Sorption isotherms were described using the Flory−Huggins model. Unlike sorption in nonpolar rubbery polymers, solubility of gases such as CO2 and olefins in polar XLPEGDA do not vary systematically based on parameters used conventionally to characterize gas condensability such as critical temperature (Tc) or (Tc/T)2, so a new model is proposed to correlate the data. CO2 exhibits the lowest Flory−Huggins interaction parameter (i.e., χ) values among all of the penetrants considered, indicating favorable interactions with XLPEGDA. Although gas solubility at a given pressure generally increases as temperature decreases, χ values increase with decreasing temperature for all penetrants except CO2. These results reflect the interplay between increases in penetrant condensability as temperature decreases, which tend to increase solubility, and increases in the unfavorable interactions between the gas and polymer segments (reflected in the increased χ values) as temperature decreases, which tend to reduce solubility. χ values for CO2 in XLPEGDA decrease with decreasing temperature, which is another indication of the affinity between CO2 and XLPEGDA. The following example illustrates the effect of temperature on intermolecular interactions: CO2/C2H6 solubility selectivity at infinite dilution increases from 2.7 to 6.6 as temperature decreases from 35 °C to −20 °C, even though these two penetrants have almost the same condensability as characterized by Tc. Furthermore, gas solubility values in XLPEGDA are very similar to those in amorphous PEG at infinite dilution and 35 °C, suggesting that the PEG linkages in XLPEGDA dominate its sorption properties.