Monodisperse silica particles (SiPs) of diameter between 100 and 1500 nm were surfacemodified in a mixture of ethanol/water/ammonia with a newly designed triethoxysilane having an atom transfer radical polymerization (ATRP) initiating site, (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane. The surface-initiated ATRP of methyl methacrylate (MMA) mediated by a copper complex was carried out with the initiator-fixed SiPs in the presence of a "sacrificial" (free) initiator. The polymerization proceeded in a living manner in all examined cases, producing SiPs coated with well-defined PMMA of a target molecular weight up to 480K with a graft density as high as 0.65 chains/nm 2 . These hybrid particles had an exceptionally good dispersibility in organic solvents. Transmission electron microscopic and atomic force microscopic observations of their monolayers prepared at the air-water interface revealed that they formed an ordered 2-dimensional lattice extending throughout the monolayer.
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/nm2 (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/2r
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.
A colloidal crystal was newly identified for a liquid suspension of the hybrid particles having a spherical silica core and a shell of well-defined poly(methyl methacrylate) "concentrated brush". With increasing particle concentration, the suspension progressed from a (disordered) fluid to a fully crystallized system, going through a narrow crystal/fluid coexisting regime. The crystal had a face-centered cubic structure with a surprisingly large nearest-neighbor interparticle distance, suggesting that the graft chains, highly extended due to the "concentrated brush effect", exerted a long-range steric interaction. This type of colloidal crystal is new with respect to the origin of long-range interparticle potential and the controllability of many of the system parameters.
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