Effect of supercritical carbon dioxide on the thermodynamics of miscible polymer blends

The design of environmentally-benign polymer processing techniques is an area of growing interest, motivated by the desire to reduce the emission of volatile organic compounds. Recently, supercritical carbon dioxide (scCO$_{2})$ has gained traction as a viable candidate for various processes as either a polymer solvent or diluent. To elucidate the impact of scCO$_{2}$ on polymer miscibility, the phase behavior and thermodynamic interactions of multicomponent mixtures comprising scCO$_{2}$, styrene-acrylonitrile copolymer (SAN), and poly(methyl methacrylate) (PMMA) were studied by small angle neutron scattering. Application of the Random Phase Approximation and Flory-Huggins Theory allowed quantitative analysis of scattering profiles to obtain the dependence of pairwise interaction parameters on scCO$_{2}$ activity. The location of the spinodal boundary was found to have a non-trivial dependence on scCO$_{2}$ processing conditions which can be interpreted in the context of balancing interaction strengths. The presence of scCO$_{2}$ was shown to disrupt the miscibility of SAN-PMMA induced by intramolecular repulsion, and decrease the accessible demixing temperature by over 130 $^{\circ}$C.