Dynamic Changes in Structural Color of a Lamellar Block Copolymer Photonic Gel during Solvent Evaporation

A high molecular weight, lamellar block copolymer (BCP) can exhibit a photonic stop band in the visible range of wavelengths due to the one-dimensional periodic dielectric layer structure. After dissolving a high molecular weight BCP in a neutral solvent, solvent evaporation causes microphase separation, forming a so-called photonic gel at a quite low polymer concentration (similar to 10 wt %). Moreover, by allowing the solvent to further evaporate, the reflected color can be tuned over the entire range of the visible spectrum. Here, a polystyrene-polyisoprene diblock (PS-b-PI) is dissolved in cumene, and the solution was placed between two glass substrates. The radial concentration gradient during slow evaporation of the cumene generates a rainbow-like set of reflected colors from the photonic gel. Based on local measurement of the reflectivity along the radial direction of the photonic gel, the radial solvent composition was calculated and the experimentally observed reflectivity was compared to results simulated using the transfer matrix method. Balsara's scaling law for the lamellar period versus polymer concentration was adapted for the simulation of the concentration profile, which showed a reasonable agreement with a previous theoretical calculation. Compared to the simulated spectra, experimentally obtained reflectance spectra exhibited weaker peak reflectivity and additional peak broadening due to lamellar misorientation and decrease of the average grain size during solvent evaporation.