Kinetics of Shape-fixing in Semicrystalline Shape-memory Networks

Crystallization of elastic polymer networks can enable shape-fixing of elastically deformed shapes which can subsequently be melted to regenerate stress or revert to the original shape. The initial step of a shape-memory cycle is shape-programming and involves the formation of connected crystal domains that can prevent the release of stored elastic strain energy. Crystallization kinetics play a critical role in determining the conditions required for sufficient shape-fixing. Here, we assess how imposed strain and undercooling affect crystallization kinetics and concomitant stress loss of crosslinked poly(caprolactone)s. Isothermal crystallization experiments were performed that involve in situ x-ray scattering combined with stress decay measurements. The main finding is that that only a small amount of crystallization is needed to significantly reduce tensile stress. This result is valid for low levels of strain and undercooling, and it has broad implications for engineering new shape memory materials.