Effect of Grafted Lewis Base Groups on the Phase Behavior of Model Poly(dimethyl siloxanes) in CO₂

Abstract

The impact of various Lewis bases on the miscibility of siloxane polymers in CO 2 was investigated using both ab initio calculations and experimental phase behavior studies. A series of sidechain functional silicones were synthesized containing various Lewis bases in the side chain, and their phase behavior was compared in CO 2 at 295 K. Calculations showed that interactions between CO 2 and ethers (either a dialkyl ether or the ether oxygen in an ester group) should be as favorable as interactions between CO 2 and a carbonyl oxygen. Indeed, phase behavior results seemed to support this, as ether-functional silicones exhibited miscibility pressures as low or lower than acetate-functional analogues. Further, a keto-functional material was not nearly as CO 2 -philic as the acetate functional analogue. In general, the location of the phase boundary in CO 2 is governed by a balance between forces working to increase miscibility pressures, such as increased cohesive energy density of the polymer or factors suppressing the entropy of mixing, and those working to lower miscibility pressures, such as enhanced specific interactions with CO 2 and increased free volume or chain flexibility.

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

“…We have found that the MP2 level of theory gives results in fairly good agreement with coupled-cluster with single, double, and perturbative triple excitations calculations on similar systems [6]. A larger basis set, aug-cc-pVDZ, was used for single point energy calculations on the 6-31 þ G(d) optimized structures.…”

“…Propyl acetate (PA) and propyl dimethylamine (PDMA) functional siloxane copolymers were prepared according to the procedure described earlier [6].…”

“…Phase behavior measurements of the polymers were performed in the same way as described earlier [6]. Typical variability in the cloud point measurement is less than AE0.7 MPa.…”

“…Perfluoropolyethers are also quite miscible with CO 2 , although the cloud point pressures associated with these polymers increase dramatically as molecular weight increases above 10,000 [2]. Poly-(dimethylsiloxane) (PDMS) has the highest solubility of any known non-fluorinated polymers [3][4][5][6]. 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.…”

“…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].…”