Ultrasonic measurements of an epoxy resin near its sol-gel transition

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Abstract
Measurements of the change in attenuation and transit time of high-frequency (5 MHz) longitudinal sound waves through an epoxy resin as it proceeds through the sol-gel transition are presented. The transition from viscoelastic liquid to elastic solid results from the formation of permanent cross-links and is well suited to an interpretation in terms of percolation concepts. At high frequency, the exponents k and t, which describe the critical behavior of the viscosity and elasticity, respectively, are modified considerably as compared with corresponding static values, but in a manner consistent with extensions of percolation theory to the high-frequency regime. Interestingly, it is observed that at high frequency, ‘‘dangling chains’’ appear to contribute to the elasticity, resulting in the exponent t approaching β, that of the gel fraction.