Hydrogen-Bonding Interactions in Polymer-Organic Solvent Mixtures

Polymer solubility in organic solvents generally is predicted by the Hildebrand solubility parameter (delta) approach based on the like dissolves like principle. A conspicuous exception is that there exist many soluble pairs that the solubility parameter approach predicts to be insoluble due to the large delta difference between the pair. These cases were attributed to specific attractive interactions (e.g., hydrogen bonding) between the solute and solvent that lead to a negative enthalpy of mixing to promote the solubility. Recently, we find that the polymer solubility in ionic liquids is dominated by hydrogen-bonding interactions (Phys. Chem. Chem. Phys.2021,23, 21893-21900). More importantly, three principles based on the Kamlet-Abraham-Taft multiple-polarity scale for dealing with hydrogen-bonding interactions were proposed. Two parameters (alpha is the hydrogen-bond acidity parameter, and beta is the basicity parameter) are required to describe hydrogen-bonding complementarity. The product of Delta alpha Delta beta, where Delta is the difference in parameters between the polymer and solvent, is an indicator of the competition between cross-association and self-association hydrogen bonding. Therefore, an excess interaction parameter chi(HB), which is proportional to Delta alpha Delta beta, can be introduced to account for the solubility promotion. In this study, these principles were used to test the solubility of polymer/organic solvent mixtures, for which the solubility parameter approach fails. These polymer/organic solvent pairs that consist of 12 polymers and 27 organic small molecules have discrepant solubility parameters (>5 MPa1/2 at least). A good correlation between the solubility data and the solubility criterion (Delta alpha Delta beta < 0) reveals that hydrogen-bonding interactions are responsible for the solubility. Therefore, the excess interaction parameter, chi(HB) (similar to Delta alpha Delta beta), provides a significant correction in the Flory-Huggins theory. This finding can also serve as a prescreening tool for the solubility of polar polymers besides the like dissolves like principle.

Automated summary

The solubility of polymers in ionic liquids is mainly determined by hydrogen bonding interactions. The excess interaction parameter chi(HB), which accounts for the difference in hydrogen-bonding parameters between the polymer and solvent, provides a significant correction in the Flory-Huggins theory. This finding offers a new screening tool for the solubility of polar polymers, in addition to the like dissolves like principle.