In-situ monitoring of asymmetric poly(ethylene-co-vinyl alcohol) membrane formation via a phase inversion process by an ultrasonic through-transmission technique

Abstract

An ultrasonic through-transmission technique as a novel protocol was developed to monitor asymmetric poly(ethylene-co-vinyl alcohol) (EVAL) membrane formation via the phase inversion process in real-time so as to reveal the relationship between the phase inversion rate and the membrane morphology. The EVAL membrane was prepared with dimethyl sulfoxide (DMSO) as a solvent and water as a nonsolvent at 25 °C. The concentration of EVAL used in the casting solution was 10–25 wt.%. A highly sensitive ultrasonic measurement system with two 10 MHz transducers was employed to pulse and receive the acoustic waves through the casting system. According to this study, the movement of the ultrasonic signals in the time-domain generated by the interface between the bath solution and casting solution/nascent membrane increased with an increase in membrane formation time. The shift rate of the ultrasonic signals in the arrival time-domain decreased with an increase in the EVAL concentration. The shift of the acoustic signals in the time-domain and changes in sound velocity through the media were associated with the double diffusion rate between DMSO and water in the coagulation bath and the precipitation phase. A fast diffusion of DMSO into the water bath resulted in the quick formation of very thin dense top layer and a sublayer containing macrovoids at a low EVAL concentration (10 wt.%). An increase in EVAL concentration led to the formation of finger-like structures owing to a low mass transfer rate at a high EVAL concentration. It implies that the mass transfer rate of solvent and nonsolvent strongly affected the membrane morphology. Ultrasonic measurements corroborated the observation of membrane morphology and properties.