doi:10.1016/j.jcis.2007.01.052
Copyright © 2007 Elsevier Inc. All rights reserved.
Responsive colloidal systems: Reversible aggregation and fabrication of superhydrophobic surfaces
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Mikhail Motornova, Roman Sheparovycha, Robert Lupitskyya, Emily MacWilliamsa and Sergiy Minko
, a, 
aDepartment of Chemistry & Biomolecular Science, Clarkson University, 8 Clarkson Ave., Potsdam, NY 13699-5810, USA
Received 7 December 2006;
accepted 23 January 2007.
Available online 1 March 2007.
Abstract
We report on a method of fabricating stimuli-responsive core–shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the “grafting to” approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.
Graphical abstract
Reversible aggregation of hybrid nanoparticles consisting of a silica core with a grafted responsive mixed-block copolymer brush shell.
Keywords: Coatings; Responsive colloids; Mixed polymer brushes; Ultrahydrophobic surface
Scheme 1. Schematics of the grafting route to fabricate mixed-block copolymer brushes on the silica particle surface: Step 1, chemisorption of BTMS; Step 2, grafting the diblock (a) and triblock (b) copolymers.
Fig. 1. The synthesized particles' effective diameter (a) and their zeta-potential (b) as a function of pH in aqueous suspensions.
Fig. 2. AFM topographical images with corresponding cross sections of the synthesized particles deposited onto Si wafers: silica spheres with the grafted triblock copolymer brush (sample S) in water at pH 2 (a) and in toluene (b); primary aggregates of fumed silica with the grafted diblock copolymer brush (sample F2) in water at pH 2 (c) and in toluene (d).
Scheme 2. Schematic representation of different morphologies of particle shells prepared from the PS-2VP-EO brush: collapsed P2VP, and stretched PS chains in toluene, a good solvent for PS, and a poor solvent for P2VP (a); collapsed PS, and stretched P2VP, PEO in water at pH < 4, a good solvent for P2VP and PEO, and a poor solvent for PS (b); collapsed P2VP, PS, and stretched PEO chains in water at pH > 4, a good solvent for PEO, a poor solvent for P2VP and PS (c).
Fig. 3. Reversible aggregation for silica spheres with grafted PS-4VP/7:1 (sample S2): schematics (a, b) and experiment (c) for particles stabilized by stretched P4VP (red chains) in water at pH < 4. PS (blue chains) is collapsed (a); particle aggregates in water at pH > 5 (b).
Fig. 4. 3-D AFM images obtained from the coatings on silicon wafers prepared from aqueous suspensions of F2 at pH 2 (a) and pH 6 (b), and of S at pH 2 (c) and pH 7 (d) with the corresponding water contact angles (CA).
Fig. 5. Root-mean-square roughness (a) and contact angles (b) of coatings obtained from aqueous suspensions of samples F2 and S on silicon wafers as a function of pH.
Table 1.
Characteristics of the synthesized nanoparticle samples
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