CO 2 -solubility of oligomers and polymers that contain the carbonyl group Z. Shen a, * , M.A. McHugh a , J. Xu b , J. Belardi b , S. Kilic b , A. Mesiano b , S. Bane b , C. Karnikas b , E. Beckman b , R. Enick b a Department of Chemical Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284, USA b Department of Chemical and Petroleum Engineering, 1249 Benedum Engineering Hall, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 3 October 2002; received in revised form 12 December 2002; accepted 12 December 2002 Abstract Poly(vinyl acetate) (PVAc) is miscible with CO 2 over a broad range of molecular weights at 298 K. The cloud-point pressures needed to dissolve , 5 wt% poly(methyl acrylate) (PMA) at 298 K are significantly greater than those needed to dissolve PVAc, even though a PMA repeat group has the same number of carbon, hydrogen, and oxygen atoms as in PVAc. This large difference in dissolution pressures is attributed to the lack of accessibility of the carbon dioxide to the carbonyl group in PMA. In addition, experimental data for poly(dimethyl siloxane) (PDMS) copolymers with readily accessible side groups suggest that an acetate group is slightly more CO 2 -philic than an acrylate group. PVAc is more CO 2 -soluble than other hydrocarbon homopolymers, including poly(propylene oxide) (PPO) and poly(lactide) (PLA). However, PVAc is significantly less miscible with CO 2 than PDMS and poly(fluoroalkyl acrylate) (PFA). q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Carbon dioxide; Phase behavior; Carbonyl 1. Introduction The identification of highly CO 2 -soluble polymers composed of only carbon, hydrogen, and oxygen could facilitate the design of safe, inexpensive, environmentally benign ‘CO 2 -philes’. These compounds could enhance the performance and economics of CO 2 -based technologies that require the dissolution of amphiphiles, such as surfactants, chelating agents, thickeners, and dispersants and homo- polymers or copolymers for foam, fiber, and film appli- cations. Heller, et al. [1] performed an extensive review of polymer solubility in dense carbon dioxide to identify a thickener that would reduce the mobility of supercritical CO 2 flowing through porous media. Although no viscosity- enhancing polymer was identified, hydrocarbon-based polymers that exhibited slight solubility in CO 2 (0.1 – 1.0 wt%) were water-insoluble, atactic, amorphous, and had solubility parameters less than 8 (cal/cm 3 ) 0.5 . For reference, the solubility parameter of liquid CO 2 is calculated to be in the range of 4–5 (cal/cm 3 ) 0.5 at 298 K and pressures above 10 MPa [2]. Subsequent published solubility studies were more successful in identifying polymers capable of dissolving at much higher concentrations in CO 2 . In one particular study, cloud-point data at a concentration of approximately 5 wt% polymer, which is expected to be the maximum of a pressure-concentration isotherm, were reported for a series of polyacrylates along with poly(vinyl acetate) (PVAc) [3]. PVAc (M w ¼ 125,000) was much more soluble in CO 2 at 298 K than poly(methyl acrylate) (PMA) (M w ¼ 31,000) even though PMA had a much lower molecular weight. The PVAc cloud-point pressures increased with temperature over the 295–423 K range while the PMA cloud-point pressures decreased over approximately the same tempera- ture range. Nevertheless, the PVAc two-phase locus remained lower than that of PMA by approximately 150– 100 MPa. It should be noted that the pressure required to dissolve PVAc increased from 60 MPa at 295 K to 100 MPa at 423 K, which is high relative to most proposed CO 2 -based technologies. The glass transition temperature of PVAc is 21 K higher than that of PMA, indicative of the stronger polar interactions between acetate groups relative to methyl acrylate groups. PVAc was therefore considered to be more polar than PMA, facilitating the formation of a weak complex between CO 2 and vinyl acetate especially at low to 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00020-X Polymer 44 (2003) 1491–1498 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 1-804-827-7000x460; fax: þ 1-804-828- 3846. E-mail address: zshen@vcu.edu (Z. Shen).