Structural Analysis of “Semisoft” Colloidal Crystals by Confocal Laser Scanning Microscopy

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Semisoft colloidal crystals, which are formed by the work of steric or excluded-volume interactions among the concentrated polymer brushes (CPBs) afforded on spherical particles, were studied with respect to their crystalline structure as a function of graft chain length, while the graft density was approximately fixed at a high value. Rhodamine-labeled, low-polydispersity silica particles (average diameter 590 nm) were synthesized using the sol−gel chemistry based on the Stöber method and then grafted with a shell of well-defined poly(methyl methacrylate) high-density brush by surface-initiated atom transfer radical polymerization. The internal structure of colloidal crystals formed in a liquid suspension of those hybrid particles was directly (in situ) observed by confocal laser scanning microscopy in fluorescence mode. The crystalline structure was of close-packed type in all cases, and the nearest-neighbor interparticle distance in the crystal increased with increasing graft chain length. The crystals consisted of two-dimensional hexagonal close-packed planes which are stacked in a statistical sequence of hexagonal close-packed (hcp) and face-centered cubic (fcc) lattice arrangements. The probability α of finding a fcc stacking was about 0.59 in a region of relatively short graft chains and substantially equal to 1.0 (perfect fcc) in a region of relatively long graft chains, exhibiting a rather narrow intermediate transition region, where α steeply increased with increasing chain length. This transition of crystalline structure from a nearly random stacking to the fcc arrangement was ascribed to a qualitative change in graft chain conformation and hence interparticle potential curve accompanying the previously noted CPB-to-SDPB (semidilute polymer brush) crossover in the effective graft density of the polymer layer ( Ohno et al. Macromolecules 2007, 40, 9143). These results are the first to experimentally show a greater prevalence of fcc stacking as the interaction becomes of longer range. Semisoft colloidal crystals of CPB-afforded hybrid particles can thus cover, by simply changing the graft chain length, a wide range of crystallization concentrations and interparticle potentials between those of typical hard and soft colloidal crystals.