Unconventional Shape Memory Mechanisms of Nanoporous Polymer Photonic Crystals: Implications for Nano-Optical Coatings and Devices

Shape memory photonic crystals may hold the key to the continuing development of smart optical coatings and next-generation all optical integrated circuits. The reconfigurability of these materials in response to various external stimuli is not only aesthetically appealing but also fundamentally important in guiding the design of emerging reconfigurable nanophotonic devices. Here we report a new type of polymer shape memory photonic crystal (PSMPC) that shows autonomous Laplace pressure-driven, elastic modulus-dependent microstructural programming and solvent-swelling-triggered shape memory recovery, all occurring at room temperature. By varying the compositions of their constituent polymers, the elastic moduli of the PSMPCs can be systematically modulated, leading to different photonic bandgaps (i.e., diffractive colors) in response to different solvents, such as water, ethanol, and acetonitrile. The different diffractive colors represent varied strains stored in the semideformed nanoporous PSMPCs. A new physical competing relationship, denoted by a dimensionless parameter, mu between the elastic modulus of polymer and the surface tension of solvent, was established to characterize the range within which the nanoporous photonic crystal structure can stay in the semideformed stable state upon the application of solvents with different surface tensions. Good overlapping in the range of mu was observed from PSMPCs deformed by water, ethanol, and acetonitrile, which verified the applicability of this competing relationship in predicting the "cold" programming behaviors of the nanoporous PSMPCs. This fundamental study will pave the way for the rational design of nanoporous PSMPCs with optimized mechanochromic properties that can be used in a broad spectrum of tunable nano-optical applications.