Interfacial Engineering Approach to Pattern Resilient Polymer Photonic Crystals with Temperature‐Responsive Optical Properties

Abstract

structures, which are stacks of alternating layers of high and low refractive index domains. At the interfaces of these domains, a portion of incident light can be reflected. The amount of reflecting light at each interface is dependent upon the contrast between the refractive index of the distinct layers, while the wavelengths that are in-phase upon reflection are determined by the product of refractive index and thickness of each layer.In general, assembly of 1D-PCs can be accomplished through bottom-up and top-down approaches, where the latter involves subsequent depositions of alternating layers, providing the ability to precisely control the reflecting wavelengths without sophisticated syntheses that are typically required in bottomup methods. [11][12][13] Many model systems have been demonstrated for constructing 1D-PCs through layer-by-layer casting, with an essential requirement that selected polymers need to have orthogonal solubilities or be crosslinkable for preventing possible film loss from dissolution. [14][15][16][17][18] However, further implementing these systems into practical applications would demand additional functionalities, preferably incorporated either through intrinsic material properties and/or simple processing methods. For example, producing patterns on 1D PC films is needed for display-relevant applications. [19,20] Previous approaches for achieving this goal often involve a combination of different synthesis and processing steps including functionalization with reactive units, [21] photolithography, [22] and crosslinking processes. [23] In these cases, portions of 1D-PCs can be removed, leaving patterns on the substrate where structural color can be present. A recent study introduced a method of generating patterns in polymer photonic crystal films through light induced, in-film polymerization of a block copolymer (BCP) to selectively swell domain sizes in desired regions of the film. [20,24] While this technology can be broadly applied to different BCP systems, it involves a batch process (closed chamber system) and requires inert atmospheres for radical polymerization, potentially hindering their scalability. Therefore, development of simple and effective methods for patterning 1D-PCs is highly desired for broadening the applications of 1D polymer PCs at large scales. Planar, 1D photonic crystals (1D-PCs) are stacks of alternating layers with different refractive indices, which can reflect specific wavelengths of light through the formation of a photonic bandgap. Typical systems for 1D-PC fabrication possess relatively limited thermal and/or chemical stabilities and may require several steps for producing patterned features. Additionally, enabling stimuli-responsive behaviors in 1D-PCs are often achieved through relatively complex chemistries that might be difficult to scale-up due to associated cost and energy consumption, presenting challenges toward their implementation in practical systems. Herein, through sequential depositions of phenolic resin (resol) and poly(vinylidene f...