Programmable localized wrinkling of thin films on shape memory polymers with application in nonuniform optical gratings

Abstract

Shape memory polymers (SMPs) can remember different shapes and can be recovered to their permanent shapes from temporary shapes with appropriate stimuli, such as heat, humidity, and electrical field. Using programmed thermal responsive SMPs as substrates, we demonstrate a self-assembly fabrication method for programmable surface wrinkling within a highly confined area that is accurately controllable. Different from global wrinkling reported in most of the literature, Joule heating through a heating wire embedded in the SMP substrate leads to temperature increase and thus recovery in a highly confined area of the SMP substrate, inducing localized wrinkling of the stiff thin film on SMPs. The patterns show good sinusoidal profiles, with the wrinkling wavelength and amplitude decreasing gradually with the distance from the heat source. The surface wrinkling area can be accurately tuned by controlling the heat input, such as power and duration. Based on this unique surface wrinkling phenomenon, we demonstrate a nonuniform reflective optical grating device, whose peak wrinkling wavelength and amplitude decrease gradually away from the heat source. This study offers a simple method to fabricate programmable localized wrinkling patterns, with potential applications in surface engineering, advanced manufacturing, optical gratings, and other demanding areas.