Ultrasonic characterization of the kinetics of water sorption in hydrogels

Abstract

A complex mechanism characterizes the water uptake kinetics in hydrogels, as a consequence of the strong structural changes occurring in the material during the sorption process. Water acts as a plasticizer, reducing the glass transition temperature of the polymer below the sorption temperature and determining a glass transition in the polymer. In this study the changes in the ultrasonic attenuation and velocity in semicrystalline Poly‐vinyl‐alcohol (PVA) hydrogel films during water sorption are measured by a pulse‐echo system. The ultrasonic wave propagation is applied to monitor the position of the swollen/unswollen fronts and to the measurement of velocity and attenuation. The structural changes in PVA hydrogels, monitored by Wide Angle X‐ray Diffractometry (WAXD), performed during the sorption process, are correlated with the ultrasonic data.