Disorder in stimuli-responsive microgel photonic crystals: A confocal microscopic study

Abstract

Crystalline arrays of sub-micron sized particles have lattice constants in the visible range and serve as photonic crystals. Highly diffracting photonic crystals (PCs) can be prepared through colloidal route by self-assembly of monodisperse particles. Size polydispersity (SPD) is known to induce second type disorder in these crystals.SPD is inherent to hard-sphere as well as charged colloidal suspensions and cannot be tailored through external conditions. We employ a confocal laser scanning microscope (CLSM) to study the disorder in PCs of poly(N-isopropylacrylamide-co-Acrylic acid) (PNIPAM-co-AAc) stimuli-responsive microgel suspensions and show here for the first time, that osmotic pressure reduces the SPD; hence the second type disorder in these crystals. Different samples with varying particle concentrations have been prepared by subjecting the dilute suspensions to different amounts of osmotic pressure. The crystallized samples were investigated for their structure and disorder using CLSM. (111) planes of single crystalline domains have been imaged and real-space coordinates of the particles and the SPD have been determined by image analysis. Different types of disorder present in these crystals are investigated by analyzing the diffraction peaks computed from the particle coordinates. The analysis revealed presence of second-type disorder in these crystals. Our results indicate that second type disorder in these crystals arises due to SPD which is found to decrease with increase in osmotic pressure. © 2011 American Institute of Physics.