Switching periodic membranes via pattern transformation and shape memory effect

We exploited mechanical instability in shape memory polymer (SMP) membranes consisting of a hexagonal array of micron-sized circular holes and demonstrated dramatic color switching as a result of pattern transformation. When hot-pressed, the circular holes were deformed to an array of elliptical slits (with width of tens of nanometers), and further to a featureless surface with increasing applied strain, therefore, switching the membrane with diffraction color to a transparent film. The deformed pattern and the resulting color change can be fixed at room temperature, both of which could be recovered upon reheating. Using continuum mechanical analyses, we modeled the pattern transformation and recovery processes, including the deformation, the cooling step, and the complete recovery of the microstructure, which corroborated well with experimental observations. We find that the elastic energy is roughly two-orders of magnitude larger than the surface energy in our system, leading to autonomous recovery of the structural color upon reheating. Furthermore, we demonstrated two potential applications of the color switching in the SMP periodic membranes by (1) temporarily erasing a pre-fabricated Penn logo in the film via hot-pressing and (2) temporarily displaying a Penn logo by hot-pressing the film against a stamp. In both scenarios, the original color displays can be recovered.