Self-folding of Engineering Materials Actuated by Heat Shrinkable Polymer

Shape shifting is an important process for both natural and artificial structures. Animals and plants need to change their shape as a response to environmental stimuli. Some man-made tools also change their shape during the usage. The dissertation focuses on the transformation from 2D sheets into 3D structures. The idea is from origami, an ancient art of paper folding. Various 3D structures can be obtained by folding 2D sheets along pre-designed crease patterns. Besides fabricating 3D structures, origami can be used to design deployable structures and tailor the mechanical behavior of metamaterial structures. In the dissertation, we developed multiple ways to actuate the folding, which are divided into three approaches.;In the first approach, folding is caused by local bending, which is induced by strain gradient across the thickness. We demonstrated the self-folding of a heat shrinkable polymer sheet using a local Joule heater. Strain gradient is induced by unsymmetrical heating. The folding is influenced by thermal force and mechanical force. By modifying the interface between heater and polymer sheet, the folding direction can be changed. We also demonstrated the self-folding of aluminum foil actuated by a local bimorph (heat shrinkable polymer sheet & aluminum foil). For aluminum foil with isotropic stiffness, folding direction is determined by the orientation of the bimorph strip. We used topographical features to enable anisotropic bending stiffness in the aluminum foil, which can be used to tune the folding direction.;In the second approach, folding occurs as a result of compressive buckling, which is localized at the creases due to their lower stiffness. The heat shrinkable polymer sheet was attached to the pre-designed 2D precursor. Upon heating, the polymer sheet shrinks equibiaxially. This compresses the 2D precursor and causes the buckling. The buckling deformation is guided by the crease patterns. Various 3D origami structures were folded. Their application in crash energy absorber and thermal insulator were demonstrated.;In the third approach, folding occurs as a result of changing Gaussian curvature. By embedding origami structures into the flat heat shrinkable polymer sheets, the shrinkage of the polymer is constrained and becomes non-uniform. The Gaussian curvature of the composite sheet is changed due to the non-uniform shrinkage, resulting in shape morphing of the composite sheet. The shape morphing is guided by the origami mechanism.;Those geometric rules developed in this dissertation are independent of length scale. The actuating component can be replaced by other materials under other stimuli. The shape transformation model developed in the dissertation can be used to design smart structures which can respond to environmental change. It can also be used to design actuators that can achieve different modes of movements.