Factors affecting the nodule size of asymmetric PMMA membranes

Several factors that may affect the surface nodule size of a polymeric memb rane were under investigation. The increase of polymer concentration and mo lecular weight were found to increase the surface nodule size. The increase of casting temperature also resulted in an increase in nodule size. These results supported that the radius of gyration and the collision frequency b etween polymer chains were the key factors affecting the nodule size. Howev er, when the radius of gyration was reduced by the use of a poor solvent or by pre-adding nonsolvent in the casting solution, the surface nodule size increased. It suggested that there existed other factors affecting the nodu le size on membrane surface besides the gyration radius and the collision f requency of polymer chains. In this study, we found in most cases that the surface nodule size decreased along with the surface tension difference bet ween the casting solution and the coagulant. To demonstrate the effect of s urface tension, we examined the nodule size inside the membranes where the nodule formation was not significantly affected by the inter-facial tension . Opposite to what was observed on the surface, the nodule size increased w ith the solvation power of the solvent. This result suggested that it was t he interfacial tension that overpowered the gyration radius in affecting th e surface nodule size. (C) 2001 Elsevier Science B.V. All rights reserved.