Comparative study of PCL shape-memory networks with Diels-Alder or Alder-ene adducts

Shape-memory polymers (SMPs) are remarkable stimuli-responsive materials able to switch from one stable macroscopic shape to another one, which can find application as smart medical devices. For this purpose, poly(ε-caprolactone) (PCL) networks are widely studied because PCL is biocompatible, degradable and has good mechanical properties. The integration of reversible bonds in these networks allows (re)processing and recycling of the SMP material. In this work, different reversible reactions, i.e. Diel-Alder (DA) addition of TAD-anthracene or maleimide-Anthracene and Alder-ene (AE) addition of TAD-indole, were used for the synthesis of reprocessable SMPs. Bis-TAD and star-shaped PCL end-capped by maleimide, anthracene or indole were synthesized as reported elsewhere. They were melt blended at 120°C in a mini-extruder in equimolar ratio followed by thermal curing at 65°C for blend 1. The cross-linking density of the resulting networks was determined by swelling experiments, their crystallinity by DSC and SMP properties by DMA. The 3 blends reached a similar swelling rate (around 1000%) typical of a highly cross-linked network. Remarkably, it is reached directly after extrusion for blends 2 and 3 thanks to the fast DA and AE additions while a post-curing of 24h at 65°C is required for blend 1. Expectedly, all the three networks exhibited a similar crystallinity degree above 35%, accounting for the good fixity of these SMPs. If recovery ratios higher than 99% were measured for the three samples, a creep effect was observed for the blend 3 upon reaching the temporary shape originating from the stress-sensitive AE adducts able to break upon deformation at 90°C. Finally, the reprocessing of these networks at 120°C was achieved only for blends 2 and 3 thanks to their fast retro-reaction. Various DA or AE reactions were investigated to introduce reversible bonds in PCL networks leading all to SMPs with high fixity and recovery. Among them, the anthracene-TAD equilibrium combines (i) fast addition to build the network, (ii) high stress stability at 90°C avoiding creep phenomena during processing of the temporary shape, (iii) high reversibility above 120°C offering efficient modification of the permanent shape and material (re)processing and recycling.