Investigation on the stress-free two-way shape memory effect of semicrystalline networks towards reversible shape-shifting hinges and metamaterial structures

Soft materials capable of autonomously changing their shape with an on-demand and reversible response are considered very interesting in the fields of soft robotics and bioengineering, for the development of artificial muscles, smart grippers and active hinges. To this aim, the use of two-way shape memory polymers results promising. These materials are capable of switching between two shapes when exposed to cooling/heating cycles across their crystallization and melting regions both under stress-driven and stress-free conditions. Here, we characterized semicrystalline networks based on poly(ε-caprolactone) crosslinked by a sol-gel approach or by photocrosslinking. All the materials displayed an excellent two-way shape memory effect under the application of an external load. Interestingly, after the set-up of a proper thermo-mechanical protocol as a training step, it was possible to achieve a stress-free reversible deformation under tensile and bending conditions. The effect of the macromolecular architecture and the employed thermo-mechanical parameters (actuation temperature and applied pre-strain) was investigated to identify optimal conditions for the development of responsive hinges and of an auxetic unit cell capable of repeatedly self-expanding and shrinking.