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

Abstract

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 ($T_{\mathrm{prog}}$) and isothermal-recovery temperatures ($T_{\mathrm{iso}}$) 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 $T_{\mathrm{iso}}$: 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 $T_{\mathrm{g}}$. The shape-recovery process is slower in materials with more densely crosslinked and hindered network structures. The shape-recovery time ($t_{\mathrm{sr}}$) is significantly reduced when the isothermal-recovery temperature $T_{\mathrm{iso}}$ increases from below to above $T_{\mathrm{g}}$ because the network relaxation dynamics accelerates. However, the temperature range from the beginning to the end of the recovery process is hardly affected by $T_{\mathrm{iso}}$; at higher $T_{\mathrm{iso}}$ 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 $T_{\mathrm{iso}} < T_{\mathrm{g}}$ to maximize the effect of the structure and/or by increasing $T_{\mathrm{iso}}$ to minimize the effect but increasing the shape-recovery rate.