Network structure dependence on unconstrained isothermal-recovery processes for shape-memory thiol-epoxy "click" systems

The shape-memory response (SMR) of "click" thiol-epoxy polymers produced using latent catalysts, with different network structure and thermo-mechanical properties, was tested on unconstrained shape-recovery processes under isothermal conditions. Experiments at several programming temperatures (T-prog) and isothermal-recovery temperatures (T-iso) were carried out, and the shape-memory stability was analyzed through various consecutive shape-memory cycles. The temperature profile during the isothermal-recovery experiments was monitored, and it showed that the shape-recovery process takes place while the sample is becoming thermally stable and before stable isothermal temperature conditions are eventually reached. The shape-recovery process takes place in two different stages regardless of T-iso: a slow initial stage until the process is triggered at a temperature strongly related with the beginning of network relaxation, followed by the typical exponential decay of the relaxation processes until completion at a temperature below or very close to T-g. The shape-recovery process is slower in materials with more densely crosslinked and hindered network structures. The shape-recovery time (t(sr)) is significantly reduced when the isothermal-recovery temperature T-iso increases from below to above T-g because the network relaxation dynamics accelerates. However, the temperature range from the beginning to the end of the recovery process is hardly affected by T-iso; at higher T-iso it is only slightly shifted to higher temperatures. These results suggest that the shape-recovery process can be controlled by changing the network structure and working at T-iso < T-g to maximize the effect of the structure and/or by increasing T-iso to minimize the effect but increasing the shaperecovery rate.