Characterization of a self-standing and stress-free two way shape memory effect in semicrystalline networks based on poly(ethylene glycol)/poly(ε–caprolactone)

The two-way shape memory effect (SME) is a peculiar feature of some polymeric materials, among which semicrystalline networks, that allows to obtain reversible shape changes upon cycling the material across its crystallization and melting regions under the application of an external load. Recently, the possibility of achieving this effect also under stress-free conditions was proved by subjecting semicrystalline networks to a suitable thermomechanical protocol. This early treatment leads to the formation of an internal stress at the macromolecular level, usually obtained by only partially melting the crystalline structure. Here, we synthesized networks based on poly(ethylene glycol)/poly(ε-caprolactone) by photocrosslinking, starting from methacrylated precursors of different molecular weights and in different ratios. The stress-free two-way SME was obtained and the role of some thermomechanical parameters, such as the applied pre-strain and the actuation temperature of the reversible cycle, was investigated. The composition of the material was fundamental for the achievement of a significant effect and it is related to the presence of a low-melting phase which elongates during crystallization and of a crystalline skeleton which provides the driving force.