Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers

Asymmetric cellulose acetate (CA) hollow fiber membranes were prepared by dry/wet spinning process from a dope composed of cellulose acetate (CA), N,N-dimethylformamide (DMF), and polyethylene glycol (PEG). Herein, PEG was the additive; DMF was the solvent; whereas water was the nonsolvent. The spinning parameters in this study were the contents and molecular weights of PEG and the external coagulation temperature. The surface and cross-section morphology of the resulting hollow fibers were examined using scanning electron microscopy (SEM). The pure water permeability (PWP) and retention of dextran were also measured. The results showed that the addition of PEG would suppress the formation of macrovoids. The effect on the suppression was more obvious for PEG with higher molecular weight and higher content. When PEG was added, the outer surfaces changed from smooth to microporous, whereas the inner surface remained smooth and dense. The PWP increased with the additive content but slightly decreased with the increase of molecular weight. Oppositely, the retention of dextrans decreased with the increase of additive contents but increased with the molecular weight. When adding PEG and coagulating at higher temperature simultaneously, the outer surface and cross-section of CA/PEG blended membrane exhibited macrovoids on outer surface and finger-like voids near the inner and outer edges. The resulting membranes showed higher PWP (4–7 times) with slight decrease in the retention. Hence, adding PEG and elevating coagulant temperature will promote the permeation performance of CA hollow fibers.