ULTRASONIC MEASUREMENTS OF AN EPOXY-RESIN NEAR ITS SOL-GEL TRANSITION

Measurements of the change in attenuation and transit time of high-fre quency (5 MHz) longitudinal sound waves through an epoxy resin as it p roceeds through the sol-gel transition are presented. The transition f rom viscoelastic liquid to elastic solid results from the formation of permanent cross-links and is well suited to an interpretation in term s of percolation concepts. At high frequency, the exponents k and t, w hich describe the critical behavior of the viscosity and elasticity, r espectively, are modified considerably as compared with corresponding static values, but in a manner consistent with extensions of percolati on theory to the high-frequency regime. Interestingly, it is observed that at high frequency, ''dangling chains'' appear to contribute to th e elasticity, resulting in the exponent t approaching beta, that of th e gel fraction.