Casting Solution Additives

Abstract

A casting solution is normally prepared to be filmed as a flat sheet, or extruded as a hollow fiber, in polymeric membrane preparation via phase inversion. The two main components of a casting solution are the selected polymer (P) and a suitable solvent (S) (or a diluent, for membrane preparation via TIPS). Different types of additives can be introduced in the casting solution composition to improve/enhance some selected membrane properties, depending on the application. Additives can be divided in two categories: soluble and insoluble additives. Soluble additives can be, in general, salts, as LiCl and LiClO 4 ; small molecules, as glycerol or ethylene glycol (EG); or polymers, as polyethylene glycol (PEG) or polyvinyl pyrrolidone (PVP), with different molecular weights (Mw). Most soluble additives are introduced to enhance the formation of membrane pores and are called "pore former" or "pore forming." They are normally leached out from the membrane matrix in the coagulation bath. However, high Mw additives, as PVP, can be retained in the membrane structure. This effect can be exploited to tailor membrane wettability, since PVP is hydrophilic. Similarly, a class of poly(ethylene oxide)-b-poly(propylene oxide)-bpoly(ethylene oxide) triblock copolymers, referred to as Pluronic ® , can be used as additives to improve membrane hydrophilicity, thanks to their stable incorporation in the polymeric matrix. The effect of soluble additives on the membrane morphology depends on a trade-off between thermodynamic and kinetic factors. Soluble additives can increase the thermodynamic instability of the casting solution. This can be detected from a shift of the binodal curve toward the P/S axis, in ternary phase diagrams, which indicates less non-solvent (NS) tolerance of the P/S/Additive system, with respect to the P/S system having the same polymer concentration. This normally promotes phase inversion, resulting in more porous structures, and may enhance macrovoids formation as well. However, additives can also increase the dope viscosity. This will hinder the S/NS exchange and, hence, delay phase inversion. As a result, membrane structure can shift to more spongy type, while macrovoids formation may be reduced. For instance, PVP affects both the thermodynamic and kinetic of the phase-inversion process due to its hydrophi-licity and its effect on the dope viscosity (Simone et al. 2010). Hence, the effect of PVP on the final membrane morphology and properties depends both on its concentration and molecular weight. On the one hand, PVP concentration is directly connected with the thermodynamic of the P/S/ Additive/NS system. On the other hand, the casting solution viscosity increases with polymer concentration and molecular weight. Finally, the