Color-shifting Crystalline Colloidal Arrays from Polymers With Upper Critical Solution Temperature

Crystalline colloidal arrays (CCAs) composed of core-shell microspheres with thermoresponsive structural iridescence governed by Bragg's law have garnered significant attention for diverse applications. While core-shell microspheres with lower critical solution temperature (LCST) properties are extensively studied, upper critical solution temperature (UCST) counterparts remain unexplored, offering the potential to expand the application scope of thermoresponsive CCAs. In this study, poly(N-acryloyl glycinamide) (PNAGA), a UCST homopolymer, is employed for the first time to synthesize core-shell microspheres. By copolymerizing NAGA with the hydrophilic co-monomer acrylamide (AM) to form the shell, microspheres with soft shells capable of assembling into CCAs with bright iridescence are obtained. Owing to Bragg's law and the UCST properties of the shell, the diffraction wavelength of these CCAs depends on concentration, observation angle, and temperature. The CCAs exhibit thermoresponsive behavior, with a size transition temperature around 14°C. Upon heating, the shells swell, and the microspheres transition from a rigid to a soft state, leading to an increase in interparticle distance and enhanced stabilization of the ordered microsphere packing. This process results in a red shift and a significant increase in the intensity of the diffraction peak. The thermoresponsive properties of these CCAs highlight their potential as intelligent temperature-sensing materials.