In-situ monitoring of polysulfone membrane formation via immersion precipitation using an ultrasonic through-transmission technique

Abstract

An ultrasonic through-transmission technique as a non-invasive technique was developed to monitor the formation process of asymmetric polysulfone (PSF) membrane via immersion precipitation. The effect of polymer content in the casting solution in water/1-methyl-2-pyrrolidinone (NMP)/PSF and ethanol (EtOH)/NMP/PSF systems on the membrane morphology were investigated. Results showed that the systematic movement of ultrasonic signals in the arrival time observed during the membrane formation is associated with the changes of sound velocity in the media. The shift of ultrasonic signals in the arrival time decreased with an increase in the PSF concentration in both water and EtOH coagulations. The arrival time of ultrasonic signals kept stable at the initial time of membrane formation and then gradually shifted with the membrane formation when using water as a coagulation agent. It is relevant to the formation of the dense skin layer and finger-like macrovoids as well as the out-diffusion of NMP from the bottom surface of the casting solution. The systematic shift of ultrasonic signals forward along transmission direction of the sound wave in time-domain with the membrane formation time using EtOH as a coagulation agent is correlated with the formation of the porous surface and sponge-like structure. It is the reason because the double diffusion between EtOH and NMP leaded to liquid–liquid phase separation and the shrinkage in the lateral layer, mainly dominated by the penetration of EtOH from the coagulation to the casting solution. The SEM and AFM observation of membrane morphology corroborated ultrasonic measurements.