Improving fluxes of polyimide membranes containing gold nanoparticles by photothermal heating

Polyimide (PI) is a well known polymer to obtain membranes for solvent resistant nanofiltration. However, when compounds with a very low molecular weight need to be separated, very dense PI membranes are required that generally have very low fluxes. It was previously shown for cellulose acetate membranes containing gold nanoparticles that the membrane permeance increased by photothermal heating of the membrane during filtrations. In this work, PI membranes were prepared containing different amounts of gold nanoparticles and the membranes were locally heated during filtrations by irradiation with continuous green laser light. The gold nanoparticles and the membrane structure were thoroughly characterized by UV–visible spectroscopy and diffuse reflection spectroscopy, scanning electron microscopy and transmission electron microscopy. The photothermal heating effect on the temperature and the permeance behavior of the membrane depended on the concentration of gold nanoparticles in the membrane, their aggregation and on the laser intensity. It was shown that the effect on the permeance was higher for membranes with an intrinsically lower flux, due to a lower energy loss by convection in the membrane pores and/or free volume. Most remarkably, the rejection of the membranes was not affected by the laser irradiation. Overall, it was thus shown that localized heating of PI membranes can significantly improve membrane performances in solvent resistant nanofiltration.