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 2philicity" 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 functionalitiesspossibly 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.

Influence of tert-amine groups on the solubility of polymers in CO2

“…The interaction energies are computed to be À19.1 and À21.2 kJ/mol for 4-methyl pyridine/CO 2 and 2-methyl pyridine/CO 2 , respectively. These energies are comparable to the interaction energy of trimethylamine/CO 2 complex and stronger than those of IPA/CO 2 system [14].…”

“…PPMAEI and PEOX have CO 2 -philic acrylate and carbonyl side groups, respectively. We had postulated that these groups would induce CO 2 -solubility of the polymers [14]. Unfortunately, even the addition of these groups failed to achieve any measurable solubility.…”

“…Spectroscopic [7][8][9][10] and theoretical [11][12][13] investigations have provided insight related to the favorable thermodynamic interactions between dense CO 2 and the fluorinated and siloxane-based polymers. There is a general consensus that the ability of these polymers to dissolve in CO 2 is related to their low glass transition temperature, low cohesive energy density [14,15], and Lewis acid-Lewis base interactions between the polymers and CO 2 [3,16].…”

“…Hydrocarbon-based polymers would likely have the advantages of being more environmentally benign and less expensive. Although no polymers composed solely of carbon and hydrogen were found to be remarkably CO 2 -soluble, theoretical and experimental studies led to a handful of CO 2 -soluble oxygenated hydrocarbon polymers being identified [14]. Enhanced solubility of these polymers in CO 2 was attributed to a specific interaction between the carbon atom of CO 2 and the lone pairs on the oxygen of a carbonyl group, in which CO 2 acts as a Lewis acid and the carbonyl group as a Lewis base [17].…”

“…Subsequently, Raveendran and Wallen's ab initio calculations indicated that a second interaction, a cooperative interaction between oxygen atoms of CO 2 and hydrogen atoms attached directly to the carbonyl carbon or the a-carbon atom [27], was responsible for the superior CO 2 -philicity of carbonyl-based groups. Our group employed ab initio calculations to reveal that the carbon of CO 2 can interact to a comparable degree with the etherlike ester oxygen in the acetate group [6,14], confirming earlier reports that interactions of CO 2 with this oxygen could occur [28,29]. The occurrence of multiple binding sites in an amorphous polymer with a low glass transition temperature is thought to be responsible for poly(vinyl acetate) displaying a greater degree of solubility in CO 2 than any other high molecular weight oxygenated hydrocarbon [26].…”