Polypropylene membranes prepared via non-solvent/thermally induced phase separation: effect of non-solvent nature

The goal of this work was to investigate the effect of non-solvent nature on the formation of porous membranes via non-solvent/thermally induced phase separation (N-TIPS). The hot solution of polypropylene (PP) in a mixture of dioctyl phthalate (DOP) and dibutyl phthalate (DBP) at 210°C was placed as a thin film on a polyethylene terephthalate (PET) substrate, and then precipitated by immersion in the non-solvent (water, iso-propanol, 1-hexanol, or 1-decanol) at room temperature. It was found that the non-solvent nature greatly affected the morphology of the thin skin layer of the membrane facing the precipitation bath, which can be attributed to non-solvent induced phase separation (NIPS). The affinity between the polymeric solution and corresponding non-solvent was evaluated by using Hansen solubility parameters of components taken at room temperature and extrapolated to the temperature of 210°C. An increase in the affinity between the non-solvent and the polymer transformed the surface layer structure from almost monolithic to cellular (with different pore sizes and porosity) and to spherulitic types. The non-solvent nature played a less pronounced role in the formation of the porous structure of the membrane bulk and the back side of the membrane (facing the PET substrate). Since the morphology of the rest of the membrane was correlated with thermophysical properties of non-solvents, it was concluded that the membrane formation took place due to temperature induced phase separation (TIPS). In the case of water, which has the highest cooling rate, the polypropylene crystallized by forming a "smectic" structure, while the standard α-lamellar structure was observed for other non-solvents. To gain insight into the TIPS process, a model of unsteady one-dimensional heat transfer was applied to simulate the change in the temperature profile of the hot, thin film of polymeric solution placed in the corresponding non-solvent. The resulting membranes were mainly in the microfiltration range with a mean through pore size of 0.05-0.61 μm, and iso-propanol permeance of 2.1-8.4 m3∙m–2∙h–1∙bar–1. The rejection of 500 nm polystyrene microspheres was in the range of 45-98%. The tensile strength was in the range of 2.9-3.2 MPa, and elongation at break was 30-190% with respect to the non-solvent used.