Ultrasonic characterization of the kinetics of water sorption in hydrogels

A complex mechanism characterizes the water uptake kinetics in hydrogels, a s a consequence of the strong structural changes occurring in the material during the sorption process. Water acts as a plasticizer, reducing the glas s 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-alco hol (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 att enuation. The structural changes in PVA hydrogels, monitored by Wide Angle X-ray Diffractometry (WAXD), performed during the sorption process, are cor related with the ultrasonic data.