Shape-Memory and Self-Stretching Thermoset Elastomers

Over the last decade, macromolecular science and engineering have witnessed enormous progress in the field of shape-memory polymers (SMPs). SMPs can be deformed and fixed into a temporary shape upon crystallization or glass formation. When triggered, typically by heating, stored deformation energy is released, and the material reverts to its original shape. A myriad of applications, especially in the biomedical space, are poised to benefit from recent advances in SMPs. Materials can now robustly memorize multiple temporary shapes and can be triggered by various stimuli including light, moisture, and applied magnetic fields. However, most envisioned applications require SMPs to perform mechanical work against external loads, and researchers rarely measure the amount of elastic energy actually stored or released.

In this paper, networks from poly(caprolactones) (PCL) prepolymers are engineered to store large amounts of strain energy upon crystallization. The highly efficient thiol-acrylate coupling reaction ensures that the molecular weight between crosslinks is uniform, resulting in tougher, elastic materials with a high degree of crystallinity and outstanding shape-memory properties with high levels of elastic energy storage. Subtle characteristics of the thermomechanical shape-memory cycle will be discussed. The trigger temperature can also be tuned to be near the human body temperature. A two-stage curing process is also described that enables novel shape actuators that undergo fully reversible, elastic elongation in programmed direction, upon cooling. Unlike the two-way shape memory effect, actuation occurs without applied stress, and it is significant—exceeding 15% strain—placing this material in a class of only a few other known materials. Actuation is triggered as configurationally biased poly(caprolactone) chains undergo strain-induced crystallization.