A criterion of diluent selection for the polymeric membrane formation via thermally induced phase separation process based on Hansen solubility parameter theory

The Hansen solubility parameter (HSP) theory, which includes the Hansen dispersion (D), polar (P), and hydrogen (H) components and a derivative ‘solubility parameter distance, Ra’ parameter, was adopted to evaluate the interaction between different polymers and their various diluents, so as to obtain a valuable and feasible criterion for the diluent selection of the thermally induced phase separation (TIPS) process for the preparation of polymeric membranes. Firstly, a full-scale database of the four HSP parameters of typical polymer-diluent systems was obtained based on a complete literature review about the phase separation process of all the polymer/diluents systems that have been applied to prepare polymeric membranes via TIPS and our additional exploratory experiments for membrane formation mechanism, in which two different phase separation processes including solid to liquid and liquid to liquid were distinguished. Relationships between the Ra parameter and the phase separation behavior were figured out to get a criterion for selecting the single diluent for the typical polymers. Moreover, the diluent selection was extended by adding a second diluent, and a schematic three-dimensional phase diagram was drawn to provide a feasible understanding of the TIPS process of the polymer-binary diluent system. Taking polypropylene (PP) as the representative example, plenty of exploratory experiments for the membrane formation mechanism based on a literature review were conducted to propose a guide based on the Hansen polar and hydrogen solubility component parameters to help select a proper binary diluent system.