Sustainable Opal-Like Polymer Particles Unveiled

Abstract

Structurally colored colloids, or photonic pigments, offer a sustainable alternative to conventional dyes, yet existing systems are constrained by limited morphologies and complex synthesis. In particular, achieving angle-independent color typically relies on disordered inverse architectures formed from synthetically demanding bottlebrush block copolymers (BCPs), hindering scalability and functional diversity. Here, we report a conceptually distinct strategy to assemble three-dimensional inverse photonic glass microparticles using amphiphilic linear BCPs (poly(styrene-block-4-vinylpyridine), PS-b-P4VP) via an emulsion-templated process. By employing trans-1,2-dichloroethylene to promote interfacial water infiltration, nanoscale aqueous domains form within the organic phase and direct short-range-ordered pore structures. Evaporative solidification arrests these structures into porous photonic beads with angle-independent color. Systematic control of surfactant alkyl chain length and BCP molecular weight enables precise tuning of pore size, shell thickness, and visible-range optical output. Furthermore, post-chemical modification via quaternization of P4VP provides an orthogonal chemical handle to modulate interfacial instability and photonic behavior. This work expands the self-assembly capabilities of linear BCPs and establishes a modular, scalable platform for producing structurally and chemically programmable photonic pigments.

A research team, affiliated with UNIST have developed a novel method to synthesize polymer-based particles that mimic the stunning iridescence of opal gemstones. This innovative approach employs nanostructured, porous microparticles composed of linear block copolymers, offering a sustainable and scalable alternative to conventional dyes and pigments.

Opal gemstones are renowned for their mesmerizing, color-shifting appearance, which arises from their unique internal nanostructure of silica spheres arranged in a specific pattern. Inspired by this natural architecture, Professor Kang Hee Ku and her research team in the School of Energy and Chemical Engineering at UNIST utilized amphiphilic linear block copolymers-specifically poly(styrene-block-4-vinylpyridine) (PS-b-P4VP)-to create inverse photonic glass microparticles with angle-independent, vivid colors. These particles feature nanoscale pores arranged within a polymer matrix, enabling the production of structurally colored pigments without relying on chemical dyes that fade over time.

The key innovation lies in a scalable emulsion-templating process that induces water infiltration at the interface, forming nanoscale aqueous domains within the organic phase. As the solvent evaporates, these domains solidify into porous, nanostructured particles resembling the inverse of natural opal's silica sphere arrangement. The resulting microparticles are approximately tens of micrometers in size, with internal pore structures that are hundreds of times smaller, effectively controlling their optical properties.

This process exploits principles of interfacial science, where water penetrates the polymer particles through surface instability phenomena. The outer shell of the particles is composed of polystyrene, which is hydrophobic and prevents water infiltration, while the internal structure is driven by the self-assembly characteristics of the block copolymer. The distinct chemical composition of the blocks enables precise tuning of pore size, shell thickness, and consequently, the visible color output across the entire spectrum.

Remarkably, the pigments produced exhibit consistent color regardless of viewing angle-a significant advantage over natural opal, which displays color variations depending on the angle of observation. The researchers demonstrated versatile color control by adjusting surfactant types, molecular weights, and chemical modifications of the copolymers. They also successfully dispersed these particles into high-moisture-content hydrogels to fabricate optical inks, capable of producing intricate printed patterns via standard printing techniques.

Professor Ku commented, "By employing relatively simple linear block copolymer structures, we have developed a versatile platform for generating vibrant, angle-independent structural colors. This technology holds promise for applications in displays, security features, and functional coatings."

This breakthrough was published in Angewandte Chemie International Edition on September 6, 2025. It has been supported by the National Research Foundation of Korea (NRF), the Ministry of Science and ICT (MSIT), and the Korea Toray Science Foundation.

Journal Reference

Juyoung Lee, Hyeong Seok Oh, Soohyun Ban, et al., "," (2025).