Water-triggered shape memory of multiblock thermoplastic polyurethanes (TPUs)

Abstract

In this article we describe the preparation and characterization of a water-triggered shape memory polymer (SMP) family, PCL–PEG based thermoplastic polyurethanes (TPUs). Upon immersion in water, water molecules selectively swelled the hydrophilic PEG domains, resulting in durable hydrogels with strain-to-failure values greater than 700%. Dry samples fixed in a temporary shape underwent water-triggered shape recovery wherein only the oriented PEG domains recovered, causing incomplete shape recovery toward the equilibrium shape upon contact with liquid water. Addressing the limited recovery observed for dry-fixing samples that led to some PCL domain deformation, we developed a novel, “wet-fixing” SM cycle, where the temporary shape is achieved by deforming the material in the hydrogel state (wet drawing) and is later fixed via PEG recrystallization upon drying. The fixing and recovery ratios were substantially improved using this new shape memory programming method, the mechanism of which was proven by X-ray diffraction analysis. The recovery speed of this material system was studied by varying the thickness of bulk films and demonstrated that water-recovery is diffusion-limited. By processing the TPUs as a web of microfibers, rapid shape recovery was achieved in water at room temperature within 1.3 s. The controllable actuation speed, the high recoverable strain, and the simple fixing and recovery process make these materials potential candidates for applications as water responsive sensors, actuators, and medical devices.