Received August 6, 2007

Revised Manuscript Received October 9, 2007

Abstract:

We previously reported that monodisperse silica particles (SiPs) afforded with a high-density brush of poly(methyl methacrylate) (PMMA) and suspended in a good solvent for PMMA formed a colloidal crystal in a certain concentration range (Macromolecules 2006, 39, 1245). Here we investigated similar hybrid particles with respect to the influence of graft chain length L_c on their hydrodynamic diameter D_h in dilute suspension and on colloidal crystallization in more concentrated suspension. The average radius r₀ of SiPs was 65 nm, and the surface density ₀ of PMMA grafts at the SiP surface was about 0.7 chains/nm² (about 36 000 chains per particle). The hydrodynamic thickness of the swollen brush layer h (=D_h/2 - r₀) was qualitatively interpretable by a modified Daoud-Cotton-type scaling model. Namely, for short graft chains, h obeyed the universal relation, h[1 + (h/2r₀)] ~ L_c0^1/2, applicable to concentrated polymer brushes on flat as well as spherical surfaces, and for chains longer than a critical length, h showed positive deviations from this linear relation, indicating the brush layer getting into the semidilute polymer brush regime. Suspensions of the hybrid particles showed a phase transition from a (disordered) fluid to a fully crystallized system with a narrow fluid/crystal coexisting regime. The critical concentration of crystallization (melting point) decreased with increasing graft chain length, and the nearest-neighbor interparticle distance D_dis in the crystal approached to a micrometer scale as the graft molecular weight reached 500 000. Good correlation was observed between D_h and D_dis such that D_dis = (1.15 ± 0.05)D_h. Confocal laser scanning microscopic observation suggested that the colloidal crystal generally include both hexagonal close-packed (hcp) and face-centered cubic (fcc) lattice arrangements with the fcc arrangement likely to increase with increasing chain length.

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