Phase Behavior of Oxygen-Containing Polymers in CO₂

Abstract

The cloud point curves of a series of oxygen-containing polymers in CO 2 were measured to attempt to deduce the effect of oxygen functional groups within a polymer on the polymer/CO 2 phase behavior. The addition of an ether oxygen to a hydrocarbon polymer, either in the backbone or the side chain, enhances “CO 2 -philicity” by providing sites for specific interactions with CO 2 as well as by enhancing the entropy of mixing by creating more flexible chains with higher free volume. Ab initio calculations show that both ether and ester oxygens provide very attractive interaction sites for CO 2 molecules. The binding energy for an isolated ether oxygen with CO 2 is larger in magnitude than that for a carbonyl oxygen/CO 2 complex. However, acetate functionalized polymers are more CO 2 -soluble than polymers with only ether functionalities possibly because acetate functional groups contain a total of three binding modes for CO 2 interactions, compared with only one for the ether functional group. Experiments clearly indicate that adding a single methylene group as a spacer between a polymer backbone and either an ether or acetate group exhibits a strong deleterious effect on phase behavior. This effect cannot be explained from our ab initio calculations.