Rubber-modified epoxy resins: 2. Dielectric and ultrasonic relaxation studies

Abstract

Dielectric and ultrasonic relaxation measurements are reported on a series of rubber-modified epoxy resins over a temperature range from 190 to 423K. The resins were prepared by the reaction of the diglycidyl ether of bisphenol A with a chain-extender formed from carboxyl-terminated acrylonitrile-butadiene copolymer and cured with either triethylene tetramine or diethylene glycol bis-propyl amine. At low temperature, a dielectric relaxation was detected which was also active ultrasonically and can be associated with molecular motion of the acrylonitrile-butadiene chain extender. Changes in the concentration of the rubbery phase and type of curing agent used have marked effects on both the activation energy for molecular relaxation and in the breadth of the relaxation curves. Extensive studies of the dielectric properties at higher temperatures failed to reveal a resolvable relaxation associated with either of the higher temperature transitions observed using equilibrium measurements. Electrical conductivity, permittivity and acoustic velocity measurements performed as a function of temperature, however, indicate the existence of a change in slope at approximately the value of the glass transition of the epoxy phase. A further change in slope at approximately the temperature of the highest temperature transition was detected. The observed relaxation properties are compatible with the resins being phase-separated at low rubber concentrations and changing to a semicontinuous interpenetrating network structure at higher rubber concentrations.