The hydrophobicity of poly (vinyl chloride) (PVC) is the main problem for its application as porous membrane for water purification. In the present work, using poly acrylonitrile (PAN) as the second polymer, poly (vinyl chloride) membrane was modified. The miscibility of PVC/PAN blend was investigated before membrane fabrication using a simple thermodynamic theory based on the compressible regular solution model. Results indicated that PVC/PAN blend system was immiscible in all compositions. Membranes were prepared using phase inversion method, whereas N, N-dimethylformamide (DMF) and water were used as solvent and nonsolvent, respectively. The PVC/PAN membranes were characterized by scanning electron microscopy (SEM), contact angle and porosity measurement. Distilled water and BSA solution were used in order to investigate the pure water flux and rejection of fabricated membranes under an operating pressure of 1 bar. The results showed an enhancement in pure water flux of PVC membrane by adding 20 wt% PAN, from 37.93±1.52 to 66.73±5.79 L/m2h. This increase of membrane flux was attributed to not only the increase of hydrophilicity but also the immiscibility of PAN/PVC blend which resulted in polymer-polymer phase separation and interfacial microvoid formation. The blend membrane showed lower rejection than pure PVC membrane, due to the change of surface morphology by adding PAN to PVC.

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