The solubility of low molecular weight Poly(Dimethyl siloxane) in dense CO2 and its use as a CO2-philic segment

Abstract Bubble point and cloud point pressures for mixtures of CO 2 and 1–4 wt% polydimethyl siloxane (PDMS, Mw 237, 410, 1250, 2000, 3780, 5970, 9430) terminated with CO 2 -philic trimethylsilyl groups are presented. The two-phase pressure loci at 23 °C and 40 °C are relatively flat and the cloud point pressures increase with PDMS molecular weight to a value of approximately 23 MPa for PDMS 9430. If PDMS is end-functionalized with CO 2 -phobic groups, however, the resultant molecules will exhibit higher cloud point pressures. Further, there is an optimal PDMS molecular weight for enhancing CO 2 solubility of the end-functionalized compound. For example, when PDMS is terminated at each end with ethyl-naphthalene, the lowest cloud point pressures at 40 °C for mixtures of 1–4 wt% of the compound in CO 2 are attained for a PDMS molecular weight of 2357. If the PDMS has a molecular weight of 1207 or less, the compound becomes less CO 2 -soluble because the unfavorable enthalpic interactions of CO 2 and the ethyl-naphthalene groups become dominant. For PDMS of molecular weight 5664 or more, the entropic effects associated with higher molecular weight PDMS mixing with CO 2 , coupled with the unfavorable enthalpic effects between the end groups and CO 2 , result in the compound being CO 2 -insoluble. Finally, these design principles were used to generate 10 other end-functionalized PDMS-based compounds capable of dissolving at 1 wt% in CO 2 at 23 °C and a pressure below 5000 psi based on PDMS segments containing 10–40 repeat units.