Asymmetric membranes based on poly(vinyl chloride): effect of molecular weight of additive and solvent power on the morphology and performance

Poly(vinyl chloride) (PVC)-based asymmetric membrane was prepared by nonsolvent-induced phase separation method using N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP) as solvents. Poly(ethylene glycol) (PEG) of three different molecular weights (PEG-400, PEG-4000, PEG-20000) was used as the pore former and hydrophilic additive. Solvent effect on the phase inversion mechanism and morphology was interpreted using the Hansen solubility parameter. Effect of molecular weight of PEG additive on the morphology and performance of the membrane was systematically investigated. It was observed that the pure water flux has been increased initially (up to 236.8 ± 3.3 Lm⁻² h⁻¹) and then decreased (134.3 ± 0.7 Lm⁻² h⁻¹) with the increase in the molecular weight of PEG in the PVC/PEG/DMAc system. However, pure water flux value of PVC/PEG/NMP system progressed (61.1 ± 1.2 to 184.1 ± 3.8 Lm⁻² h⁻¹) with the rise in molecular weight of PEG. As the molecular weight of PEG increased, phase separation was enhanced, and membrane with higher porosity was formed. It was found that the shape and size of finger-like structures in the sub-layer increase with the increase in the molecular weight of PEG. Residual PEG content in the membrane also showed a positive gradation with the molecular weight of PEG, which extends the hydrophilicity of the membrane. Between the two solvents used, NMP showed better interaction with PVC than DMAc. The membranes exhibited sufficient thermal stability, mechanical strength and antifouling property suitable for ultrafiltration operation.