Investigations of silica alcogel aging using coherent light

Light scattering methods have previously been used to monitor the formation of gels. In this report we present new light scattering techniques to stud y the properties of silica alcogels during the aging process. Monitoring on e particular polarization transformation of scattered light with time revea ls a clear increase in internal strain in standing alcogels with time. The stress birefringence coefficient of an acid-catalyzed SiO2 gel was found to he 1.3 Brewsters. Additionally, the evolution of the stiffness of alcogels was investigated using laser speckle methods. Specifically, image analysis of specklegrams obtained during multi-frequency acoustic excitation of agi ng gels was used to non-destructively measure the hardening of alcogels. Fo r an acid-catalyzed gel with a theoretical density of similar to0.05 g/cm(3 ) SiO2, the rate of hardening is found to be greatest between gelation and 2x the gel time, and drops considerably thereafter. The Young's modulus of the gel can be monitored over time with this method and was found to range from 6.2 x 10(3) N/m(2) after 6 h to 2.2 x 10(5) N/m(2) after 24 h for acid -catalyzed silica gels. (C) 2001 Elsevier Science B.V. All rights reserved.