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 CO2 and 1–4wt% polydimethyl siloxane (PDMS, Mw 237, 410, 1250, 2000, 3780, 5970, 9430) terminated with CO2-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 23MPa for PDMS 9430. If PDMS is end-functionalized with CO2-phobic groups, however, the resultant molecules will exhibit higher cloud point pressures. Further, there is an optimal PDMS molecular weight for enhancing CO2 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 1–4wt% of the compound in CO2 are attained for a PDMS molecular weight of 2357. If the PDMS has a molecular weight of 1207 or less, the compound becomes less CO2-soluble because the unfavorable enthalpic interactions of CO2 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 CO2, coupled with the unfavorable enthalpic effects between the end groups and CO2, result in the compound being CO2-insoluble. Finally, these design principles were used to generate 10 other end-functionalized PDMS-based compounds capable of dissolving at 1wt% in CO2 at 23°C and a pressure below 5000psi based on PDMS segments containing 10–40 repeat units.