Controlling Cellulose Membrane Performance via Solvent Choice during Precursor Membrane Formation

Abstract

Fabrication of cellulose membranes by alkaline hydrolysis of cellulose acetate (CA) membranes is a simple alternative method to preparing cellulose membranes via phase inversion from their ionic liquid solutions. In this study, three different solvent systems were used to fabricate cellulose acetate membranes by phase inversion, which are dimethyl sulfoxide and its mixtures with acetone and acetic acid. Acetone as cosolvent led to an asymmetric morphology with the densest selective layer, whereas acetic acid led to a loose, almost isotropic structure with high porosity and pore connectivity, also reflected in the pure water permeance and molecular weight cutoff (MWCO) of the CA membranes. Comparing the effects of these cosolvents on the phase inversion process, acetic acid decreased the solvent quality, increased nonsolvent-solvent interactions, and in total decreased the polymer solutions’ stability against phase separation. The phase inversion rate when acetic acid was used as cosolvent was also markedly lower, which was attributed primarily to the solvent’s much higher viscosity. After alkaline hydrolysis of the CA membranes, which converted them into cellulose membranes, the permeance and MWCO of large-pore, small-pore, and dense membranes changed in different directions. This was attributed to a changing membrane structure due to partial degradation of cellulose chains in an alkaline medium, which in turn can affect transport through both the pores and the membrane matrix.