A study of the water sorption kinetic of Poly(HEMA) hydrogels by SLAM and FT-IR measurements

The relative changes in the ultrasonic attenuation measured by scanning laser acoustic microscopy (SLAM) during water sorption in poly-hydroxy-ethil-methacrilate[Poly(HEMA)] are analyzed. Acoustical images performed at 10 and 30 MHz of Poly(HEMA) during water absorption show an ultrasonic attenuation increase followed by a decrease. This phenomenon is analyzed in terms of reflections, scattering, and absorption of the acoustic waves. However, the contribution of scattering and reflection may be neglected and the time dependence of acoustic attenuation is attributed to absorption of the acoustic waves occuring in accordance with the glass transition, although induced by a solvent and not by temperature. Fourier transform infrared spectrometry (FT-IR) measurements indicate that bound water is responsible for the glass-rubber transition. On the other hand, the longitudinal velocity changes are only affected by the variations of density and mechanical properties of the polymer during water sorption. The traditional weight gain data are compared with the attenuation changes observed by ultrasonic measurements.