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

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 r 0 of SiPs was 65 nm, and the surface density σ 0 of PMMA grafts at the SiP surface was about 0.7 chains/nm 2 (about 36 000 chains per particle). The hydrodynamic thickness of the swollen brush layer h (= D h /2 − r 0 ) was qualitatively interpretable by a modified Daoud−Cotton-type scaling model. Namely, for short graft chains, h obeyed the universal relation, h [1 + ( h /2 r 0 )] ∼ L c σ 0 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.