Relaxations in thermosets. XVIII. Ultrasonic studies of curing kinetics of ethylene‐diamine‐cured epoxide

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Abstract
Velocity and attenuation of longitudinal acoustic waves in a frequency range of 5–16 MHz have been measured during the curing of diglycidyl ether of bisphenol‐A with ethylene diamine. The velocity monotonically increases and reaches a limiting value and the attenuation reaches a maximum and thereafter decreases as curing proceeds. Both kinds of data have been transformed into a complex‐longitudinal‐modulus formalism. The complex‐plane plots of the longitudinal modulus show an arc‐skewed shape at the long (curing) time end. It is fitted, for the initial phase of curing, to a stretched exponential decay function with an exponent γ = 0.22 to 0.28. These results then are considered in terms of a process with a negative feedback between molecular diffusion and chemical reactions to obtain the increase in the relaxation time with the curing time. This increase is sigmoidal. Calculations of the complex longitudinal modulus show the consistency of our formalism.