Shape Actuation of Dual-Cured Networks

Conventional two-way shape-memory polymers switch between two shapes in response to temperature in the presence of an external load. In this work, we demonstrate a single phase, two-way shape actuator which is thermal-responsive without an external load. Double networks were formed by Thiol-acrylate Michael addition, followed by additional chemical cross-linking using UV irradiation under pre-strained conditions. Prepared samples exhibit contractile forces along the pre-strained direction upon melting of crystalline domains due to entropic elasticity of the first network.  Samples reversibly elongate, upon cooling, due to internal stress-induced crystallization along the same direction. The degree of orientation and reversible crystallization was further investigated using a combination of X-ray diffraction and calorimetry.  Shape actuation was studied at various pre-strained conditions. Under optimized conditions, reversible actuation of around 15% strain can be reproducibly achieved and cycled many times with nearly uniform actuation.