Suspensions of Silica Particles Grafted with Concentrated Polymer Brush: Effects of Graft Chain Length on Brush Layer Thickness and Colloidal Crystallization

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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 Lc on their hydrodynamic diameter Dh in dilute suspension and on colloidal crystallization in more concentrated suspension. The average radius r0 of SiPs was 65 nm, and the surface density σ0 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 (=Dh/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)] ∼ Lcσ01/2, 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 Ddis in the crystal approached to a micrometer scale as the graft molecular weight reached 500 000. Good correlation was observed between Dh and Ddis such that Ddis = (1.15 ± 0.05)Dh. 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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