Entropy‐Driven Crystallization of Polymeric Colloids Swollen by Photocurable Resin for Ultranarrow Stopband

Abstract Colloidal crystals hold promise as structural colorants, but developing photonic platforms with narrow bandwidth, high reflectance, flexibility, and scalable production is challenging. To address this, all‐polymer photonic crystals are crafted by harnessing the spontaneous crystallization of polymeric colloids dispersed in photocurable resin. These colloid dispersions undergo a fluid‐to‐crystal transition within a narrow volume fraction range, akin to hard spheres. As the crosslinked polymeric colloids swell in the resin, their effective volume fraction increases, reducing the refractive index contrast. This reduced contrast diminishes van der Waals attraction, causing the colloids to behave like hard spheres. The colloids crystallize when their effective volume fraction exceeds 0.494 and fully crystallize at 0.545. Elevated temperatures induce phase transitions at lower volume fractions due to increased swelling. The reduced index contrast leads to ultra‐narrow bandwidth spectra while maintaining high reflectance due to high crystallinity and uniform orientation. Photopolymerization of the dispersions results in flexible all‐polymer photonic films with further reduced bandwidth. This narrow bandwidth enables the creation of multiple peaks within a short wavelength range through film stacking, beneficial for programmable optical codes. Additionally, entropy‐driven crystallization applies to various monomers that swell the crosslinked polymeric particles, opening new opportunities for diverse applications.