(327e) Patterning Shape Memory Polymer Photonic Crystal Membranes through Supplemental UV Exposure

Shape memory polymers (SMPs) can transition between a temporary and permanent shape in response to external stimuli. Typically, SMPs are utilized for a transition in macroscale shape after exposure to high temperatures. Here, we show the less common ability of SMPs to perform a change in shape on the nanoscale and without the use of high temperatures. A post curing exposure to ultraviolet (UV) light can be used to deactivate the stimuli responsive ability of specific regions of the SMPs. SEM images show that the exposed regions no longer retain the ability to recover their permanent nanoscale structure and nanoindentation shows the elastic modulus of these regions nearly doubles. As a result, we show that this photolithography-like exposure of UV light through a photomask can be used to selectively pattern nanostructures. We fabricated proof of concept photonic crystal SMP films with iridescent structural color by filling the interstitial regions of a sacrificial colloidal crystal template with a polymer blend of ethoxylated trimethylolpropane triacrylate (ETPTA) and polyethylene glycol diacrylate (PEGDA). The 3D ordered structure of these ETPTA-co-PEGDA photonic crystal films can be temporarily disordered by submersion in water due to high capillary pressure during evaporation, resulting in a loss of structural color. However, exposure to a swelling solvent (e.g., ethanol or acetone) will trigger the recovery of the ordered structure and therefore the iridescent structural color. These SMP films can be modified with an exposure of UV light to display locally switchable structural color in the patterned regions only. This control over switchable structural color can enable the concealing of hidden patterns, words, and symbols. Moreover, this process of UV-induced disabling of the shape memory effect can be used as a low-cost alternative to pattern various functional nanostructures controlling disparate properties.