Glass-rubber phase transformation detected in polymers by means of ultrasonic waves

Ultrasonic attenuation and velocity measurements were carried out in order to study the kinetics of water sorption process in hydrogels characterized by strong structural changes occurring in the material. Hydrogel, or gel contains water, can exist in two forms: a solid glassy phase when it is dry, a rubber phase in equilibrium with water. Scanning laser acoustic microscope (SLAM) technique has been used to monitor the change in the ultrasonic attenuation, during water sorption in crosslinked poly(hydroxyethylmethacrylate) and poly-vinyl alcohol (PVA) hydrogels of different thickness at the frequencies of 10 and 30 MHz. The pulse-echo technique has been applied to the measurement of the longitudinal velocity and ultrasonic attenuation and to monitor the advancement of the swollen-unswollen fronts. During the hydrogel water sorption a peak in the ultrasonic attenuation and a decrease in the longitudinal velocity have been detected. The increment in the ultrasonic attenuation has been analysed in terms of reflections from the swollen-unswollen boundary, microvoids scattering and absorption of the ultrasonic waves due to the glass to rubber transformation.