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XX3.03 - Diffusive Photoplymer Structuring of RAFT Materials for Pre-Programmed Origami 
April 22, 2014   2:30pm - 3:00pm

Traditional shape memory materials are initialized with a several-state “program” via mechanical constraints applied at various temperatures. The program is then initiated by the uniform stimulus of temperature change. Photo-origami, demonstrated recently by one of us (Qi), instead uses a complex stimulus - one or more patterns of structured illumination - to locally relax an applied stress. The shape change is accomplished by polymer network relaxation via photo-initiation of a reversible addition-fragmentation chain transfer (RAFT) process. This method has the advantage of potentially complex shape response but the limitation that the stimulus, in the form of the applied optical pattern, must be similarly complex. Here we explore photo-origami using the stress-relaxing RAFT mechanism with the goal of complex programmed shape in response to simple stimulus, e.g. uniform optical exposure.This goal may be accomplished by locally modifying the optical or mechanical properties of the host polymer prior to initiation of the RAFT mechanism. We demonstrate that a low molecular-weight monomer freely diffusing through the solid polymer network can be locally attached to this network via radical photo-polymerization. Subsequent diffusion of the small monomer results in compositional gradients approximately proportional to the local optical dose. Finally, illumination can then initiate the RAFT mechanism of this inhomogeneous polymer volume which then exhibits bending based on the pre-programed internal structure. We report on a predictive model for the material structure in response to the first illumination and show that the the RAFT mechanism is maintained through this programming step. This internal mechanical (e.g. modulus) and/or optical (e.g. refractive index) structure can be arbitrarily patterned within the limits imposed by diffraction of the writing optical field and the reaction/diffusion kinetics of the photo-polymerization process. We show that this large design space maps well to the numerical design method known as Topology Optimization, which efficiently finds optimal distributions of 3D material properties to meet a design goal such as final shape.

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