Thermo-mechanical transformation of shape memory polymers from initially flat discs to bowls and saddles

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This paper describes the transformation of shape memory polymer (SMP) discs, from flat sheets to complex three-dimensional (3D) shapes, in response to heat generated by localized absorption of external infrared (IR) light. The gray-scale ink darkness printed on the surface of the SMP sheet determines the amount of absorbed light and the amount of heat generated on the surface of the sheet. Consequently, the lateral pattern of the ink governs the out-of-plane deformation of the SMP sheets due to variations in localized heating and shrinking. While recent studies have focused primarily on out-of-plane deformations of planar, rectangular substrates printed with linear patterns of ink and featuring either discrete or gradient ink variation, only limited studies have been performed on circular substrates with axisymmetric ink patterns. When heated by IR light, the axial symmetry of these ink designs produces unique out-of-plane deformations of the sheets, such as saddle shapes or bowl-like structures. We investigate these designs by utilizing a finite element analysis of material shrinkage and deformation, and we validate the model with experimental measurements and observations. This investigation provides insights into the mechanisms that cause axisymmetric geometries and ink patterns to form non-axisymmetric 3D structures, which can lead to the ability to program planar geometries that form complex 3D shapes when exposed to external stimuli.