Entanglement-based shape memory polyurethanes: Synthesis and characterization

In this paper, we report on the shape memory behavior of a family of hardblock-free multiblock thermoplastic polyurethanes (TPUs) consisting of poly(ε-caprolactone) (PCL) and poly(ethylene glycol) (PEG). Due to high molecular weight (M_n > 200 kDa), high degrees of entanglement were achieved. Instead of conventional "hard" blocks, entanglements served as the physical crosslinks in this system, slowing stress relaxation (suspending flow) above the melting temperatures of the soft blocks long enough to allow facile elastic deformation for shape fixing. Moreover, highly deformed samples (>800%) imparted by plastic deformation at room temperature completely recovered their as-cast shape within 1 min when heating above the transition temperature. Upon tensile deformation, the constituent chains and domains of both PCL and PEG phases became oriented, while heating-induced recovery reversed this orientation, as evidenced by wide-angle and small-angle X-ray diffraction studies. The observed large recoverable deformation and fast actuation make these materials strong candidates for applications in such medical devices as self-tightening sutures.