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 co ntains water, can exist in two forms: a solid grassy phase when it is dry, a rubber phase in equilibrium with water. Scanning laser acoustic microscop e (SLAM) technique has been used to monitor the change in the ultrasonic at tenuation, during water sorption in crosslinked poly(hydroxyethylmethacryla te) and poly-vinyl alcohol (PVA) hydrogels of different thickness at the fr equencies of 10 and 30 MHz. The pulse-echo technique has been applied to th e measurement of the longitudinal velocity and ultrasonic attenuation and t o monitor the advancement of the swollen-unswollen fronts. During the hydro gel water sorption a peak in the ultrasonic attenuation and a decrease in t he longitudinal velocity have been detected. The increment in the ultrasoni c attenuation has been analysed in terms of reflections from the swollen-un swollen boundary, microvoids scattering and absorption of the ultrasonic wa ves due to the glass to rubber transformation. (C) 2000 Elsevier Science S. A. All rights reserved.