Solubility of Polymers in Supercritical Carbon Dioxide

A great deal of information is known about the solvent character of CO2 with a wide range of polymers and copolymers based on well-characterized and systematic solubility studies that are available in the literature (Kirby and McHugh, 1999). Nevertheless, the prediction of polymer solubility in CO2, or any solvent for that matter, presents a formidable challenge since contemporary equations of state are still not facile enough to describe the unique characteristics of a long-chain polymer in solution. The difficulty resides in accounting for the intra- and intersegmental interactions of the many segments of the polymer connected to a single backbone relative to the small number of segments in a solvent molecule. An additional challenge exists to describe the density dependence of the intermolecular potential functions used in the calculations since SCF–polymer solutions (SCF, supercritical fluid) can be highly compressible mixtures. In this brief review, the solvent character of CO2 is described using the principles of molecular thermodynamics and also using a select number of phase behavior studies to reveal the impact of polymer architecture on solubility. To form a stable polymer–SCF solvent solution at a given temperature and pressure, the Gibbs energy, shown in eq. 7.1, must be negative and at a minimum. . . . ΔGmix = ΔHmix − T ΔSmix (7:1) . . . where ΔHmix and ΔSmix are the change of enthalpy and entropy, respectively, on mixing (Prausnitz et al., 1986). Enthalpic interactions depend predominantly on solution density and on polymer segment–segment, solvent–solvent, and polymer segment–solvent interaction energies. The value of ΔSmix depends on both the combinatorial entropy of mixing and the noncombinatorial contribution associated with the volume change on mixing, a so-called equation-of-state effect (Patterson, 1982). The combinatorial entropy always promotes the mixing of a polymer with a solvent. However, the noncombinatorial contribution can have a negative impact on mixing as a result of monomer–monomer interactions that arise due to the connectivity of the segments in the backbone of the polymer chain.