Shape memory polymers for self‐folding via compression of thermoplastic sheets

Abstract

ABSTRACTWe report a simple method to strain, and thereby program, shape memory polymers by compressing planar thermoplastic sheets. This work is motivated by the limited number of commercially available prestrained polymer sheets; current examples include: Shrinky Dinks, Eastman's Embrace, and polyurethane shrink films. However, these commercial specimens limit the sample thickness, polymer composition, and amount of stored strain. We show here that melt pressing can strain thermoplastic sheets over a range of thicknesses and polymer chemical compositions. After pressing (and thus, straining), the polymer sheets can self‐fold out‐of‐plane into complex geometries using two different actuation mechanisms, both of which locally release strain stored in the polymer. Three‐dimensional geometries are attained experimentally with both thick (~12 mm) and thin (~1 mm) strained polymer samples with a range of polymer compositions. Digital image correlation maps the strain profile within the melt pressed samples while a Mooney–Rivlin and geometric model predicts the average strain and folding response of the samples, respectively. The model predictions agree well with experimental results. These findings enable self‐folding with a broader design space such as polymer chemical composition, sample thickness, strain within the sample, and external stimulus. Techniques presented here should translate to other thermoplastic polymers, thus making this technique a viable tool to increase the available pool of materials available for self‐folding devices. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46889.