@article{hubbard_davis_dickey_genzer_2018, title={Shape memory polymers for self-folding via compression of thermoplastic sheets}, volume={135}, ISSN={["1097-4628"]}, url={https://doi.org/10.1002/app.46889}, DOI={10.1002/app.46889}, abstractNote={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.}, number={47}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, publisher={Wiley}, author={Hubbard, Amber M. and Davis, Duncan S. and Dickey, Michael D. and Genzer, Jan}, year={2018}, month={Dec} }
 @article{davis_chen_dickey_genzer_2016, title={Self-Folding of Thick Polymer Sheets Using Gradients of Heat}, volume={8}, ISSN={["1942-4310"]}, DOI={10.1115/1.4032209}, abstractNote={Self-folding converts two-dimensional (2D) sheets into three-dimensional (3D) objects in a hands-free manner. This paper demonstrates a simple approach to self-fold commercially available, millimeter-thick thermoplastic polymer sheets. The process begins by first stretching poly(methyl methacrylate) (PMMA), polystyrene (PS), or polycarbonate (PC) sheets using an extensometer at elevated temperatures close to the glass transition temperature (Tg) of each sheet. Localizing the strain to a small strip creates a “hinge,” which folds in response to asymmetric heating of the sheet. Although there are a number of ways to supply heat, here a heat gun delivers heat to one side of the hinge to create the necessary temperature gradient through the polymer sheet. When the local temperature exceeds the Tg of the polymer, the strain in the hinged region relaxes. Because strain relaxation occurs gradually across the sheet thickness, the polymer sheet folds in the direction toward the heating source. A simple geometric model predicts the dihedral angle of the sheet based on the thickness of the sheet and width of the hinge. This paper reports for the first time that this approach to folding works for a variety of thermoplastics using sheets that are significantly thicker (∼10 times) than those reported previously.}, number={3}, journal={JOURNAL OF MECHANISMS AND ROBOTICS-TRANSACTIONS OF THE ASME}, publisher={ASME International}, author={Davis, Duncan and Chen, Bin and Dickey, Michael D. and Genzer, Jan}, year={2016}, month={Jun} }