Investigation of the phase behavior, and the crystallization and gelation processes at high pressures in binary fluid mixtures of supercritical carbon dioxide and an organic solvent such as n-pentane (for P4MP1) and toluene (for sPS) that we have conducted show that the nature of the solvent and its composition, the pressure, and the path followed (that is whether or not the solid-fluid phase boundary is crossed directly by lowering the temperature at constant pressure, or the solid-fluid phase boundary is crossed after first a liquid-liquid phase separation is induced by lowering the pressure at constant temperature which is then followed by a temperature reduction to induce crystallization) influence the final crystalline structures (polymorphic state) and morphology.
In this presentation we will discuss our most recent findings. Even though P4MP1 + n-pentane solutions undergo crystallization, they form gels when high levels of carbon dioxide are added to the solution. This is in contrast to sPS + toluene solutions which undergo gelation instead, and it is the crystallization that is now promoted at high levels of carbon dioxide additions. We will provide a mechanistic explanation of this opposite behavior in these polymer-solvent systems. Gelation in P4MP1 + n-pentane system at high CO2 additions is interpreted as a consequence of a liquid-liquid phase separation process along with crystallization in the polymer-rich phase which provides the anchoring domains between polymer chains in the gel structure. In the sPS + toluene system gel formation is a consequence of the formation of a “polymer-solvent compound” in which helix stabilization is achieved by toluene molecules that are intercalated between the phenyl groups. At high CO2 additions, carbon dioxide molecules compete and enter the cavities between the phenyl groups in the helical structure, which destabilizes the sPS-toluene compound and destroys the gel structure.
The presentation will also discuss the morphology of the polymers after gelation and/or crystallization processes as assessed by SEM. Gels of sPS formed from toluene or toluene + CO2 (low levels) solutions by directly crossing the S-F phase boundary are found to display fibrillar nano-porous morphology. If the L-L phase boundary is crossed first, and followed by cooling, then the polymer-rich phase displays a lamellar morphology. The polymer-lean phase displays characteristics that can be attributed to both a fibrillar and a lamellar morphology. Crossing the L-L phase boundary in a system containing high level of CO2 (which is similar to imposing a deeper pressure quench in a system at lower carbon dioxide content) leads to a polymer-rich phase that displays a stacked-lamellar morphology.