Thermomechanical characterization of shape memory polymers using high temperature nanoindentation

This paper investigates the thermomechanical behavior of a thermosetting shape memory polymer (SMP) by using a high temperature nanoindentation technique. The nanoindenter is equipped with a microheater and a sophisticated temperature control and monitoring system. This allows the SMP to be activated at elevated temperatures enabling proper implementation of the thermomechanical cycle typically used to quantify the shape memory behavior. The load-depth curves of the SMP were obtained at various temperatures, from which the instantaneous moduli were calculated with a revised indenter-sample contact depth formula. The moduli from nanoindentation are consistent with those obtained from dynamic mechanical analysis on bulk samples. When activated at elevated temperatures, the SMP exhibits surface profiles different from those obtained when activated at room temperature. A large amount of "sink-in" is observed at the SMP surface when activated at temperatures above its glass transition temperature (T_g). It is seen that the large-strain elastic deformation is almost fully recoverable when recovery takes place at a recovery temperature, T_r > T _g.