In this work, a positively charged nanofiltration membrane was developed by chemical modification of a P84 copolyimide ultrafiltration membrane using branched polyethylenimine. The effect of reaction temperature on membrane morphology, surface charge, and separation performance was investigated. A reaction temperature of 70 degrees C was optimal as the membrane showed best performance in terms of salt rejections and water fluxes when Na2SO4, NaCl and CaCl2 aqueous solutions were used as feeds. Such membrane had a molecular weight,cut off (MWCO) of 226 Da indicating its effectiveness for separation of organics. Rejections to three dyes in methanol solutions were in the order of methyl orange (negative, MW: 327 Da) < disperse red (neutral, MW: 314 Da) < safranine O (positive, MW: 351 Da). The results suggest that the positive charge on membrane surface induce higher rejection to positive organic molecules due to the charge repulsion effect. Additionally, the membrane had very high stability in organic solvents; no significant changes in membrane performance and membrane structure were observed after being, immersed in dimethyl formamide (DMF) for 1 month. (C) 2016 Elsevier B.V. All rights reserved.