Thermal stress hysteresis and stress relaxation in an epoxy film

Thermal cycling of an epoxy coating on silicon through the glass transition temperature (Tg) revealed a large stress hysteresis on the first thermal cycle through Tg and a change in the stress–temperature slope at Tg resulting from the change in the epoxy elastic properties due to the glass transition. This stress hysteresis was not observed on subsequent thermal cycles through Tg. However, after the coating was annealed (aged) below Tg (for hours or longer)—during which the stress relaxed exponentially with time—the stress hysteresis returned. The magnitude of stress hysteresis, on cycling through Tg, was found to correlate to the magnitude of long-time relaxation that occurred during annealing at temperatures below Tg.