Effect of sonication on polymeric membranes

This article studies the effect of 47 kHz ultrasonic (US) waves on polymeri c membranes immersed in an aqueous bath. The membranes under study are made from three different polymers: polyethersulfone (PES), polyvinylidenefluor ide (PVDF) and polyacrylonitrile (PAN) and present various molecular weight cut-off (MWCO). The evolution of the polymeric structure exposed to US was followed by the measurement of the water permeability and the A(k)/Delta ( x) parameter which represents the ratio of surface porosity to thickness. R esults showed that important variations occurred on certain membranes after irradiation. In addition, microscopic imaging using field emission electro n scanning microscopy (FESEM) was performed on irradiated membranes in orde r to visualize the nature of the degradation. An image analysis method give s the evolution of the pore density, porosity and pore size distribution of a homogeneous area of this membrane before and after irradiation. It has been shown that, over the three materials tested, only the PES is af fected by the ultrasonic treatment over all its surface, whereas the others present no significant change in the measured parameters except the PAN (5 0 kDa) and PVDF (40 kDa) membranes whose edges are affected. In conclusion, in spite of their great efficiency in enhancing filtration processes. ultr asonic waves have to be used with care as the polymeric material itself is sensitive to the ultrasonic waves at the chosen frequency. (C) 2001 Elsevie r Science B.V. All rights reserved.