Controlled 3D Shape Transformation Activated by Room Temperature Stretching and Release of a Flat Polymer Sheet

Shape transformation of polymeric materials, including hydrogels, liquid crystalline, and semicrystalline polymers, can be realized by exposing the shape-changing materials to the effect of a variety of stimuli such as temperature, light, pH, and magnetic and electric fields. Herein, we demonstrate a novel and different approach that allows a flat sheet or strip of a polymer to transform into a predesigned 3D shape or structure by simply stretching the polymer at room temperature and then releasing it from the external stress, that is, a 2D-to-3D shape change is activated by mechanical deformation under ambient conditions. This particular type of stimuli-controlled shape-changing polymers is based on suppressing plastic deformation in selected regions of the flat polymer sheet prior to stretching and release. We validated the design principle by using a polymer blend composed of poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA), and tannic acid (TA) whose plastic deformation can be locally inhibited by surface treatment using an aqueous solution of copper sulfate pentahydrate (Cu2+ ink) that cross-links PAA chains through a Cu2+-carboxylate coordination and, consequently, increases the material's Young's modulus and yield strength. After room temperature stretching and release, elastic deformation in the Cu2+ ink-treated regions leads to 3D shape transformation that is controlled by the patterned surface treatment. This facile and effective "stretch-and-release" approach widens the scope of preparation and application for shape-changing polymers.