About the suitability of the seeded-dispersion polymerization technique for preparing micron-sized silica-polystyrene clusters† David Nguyen, ab Serge Ravaine, b Elodie Bourgeat-Lami c and Etienne Duguet * a Received 15th December 2009, Accepted 5th February 2010 DOI: 10.1039/b926438h Dispersion polymerization of styrene was performed in the presence of methacryloxymethyltrimethoxysilane-derivatized silica seeds in various experimental conditions in order to get micron-sized polystyrene/silica clusters of controlled morphology. It is demonstrated that asymmetric clusters (dumbbell-like or snowman-like) are easily prepared in a large range of sizes by taking advantage of the coalescence phenomenon which occurs between growing polymer nodules in pure alcoholic medium, at high monomer concentration and/or at high temperature. A model describing the evolution of the morphologies as a function of the seed size and concentration is proposed. Moreover, micron-sized multipod-like or raspberry-like particles were also obtained by adding water in the dispersing medium. The experimental parameters which allow control of the cluster morphology are compared to those of the seeded-emulsion polymerization, which is efficient for preparing submicron-sized clusters. Introduction Colloidal clusters, and in particular asymmetric ones, may offer new opportunities in a large variety of applications, e.g. photonic band-gap materials, nanopatterns by colloidal lithography, and catalyst supports. 1,2 The controlled aggregation of colloidal particles has appeared as a possible route to create such colloidal clusters whose shape, functionality and packing capability may be tuned. Several clever assembling routes were successively investigated through the use of surface coupling reactions, 3 planar substrates which have been topographically 4 or chemically 5 structured, or physical confinement in emulsion droplets. 6 Another strategy consists of using a preformed colloidal particle on the surface of which one or several other particles are subsequently created through a physical or chemical process: the dewetting phenomenon in core-shell particles, 7 phase separation in latex-seeded emulsion polymerization 8 and controlled surface nucleation and growth. 9 A few years ago, we demonstrated a seeded-growth emulsion polymerization process that is capable of letting a controlled number of polystyrene (PS) latex nodules grow onto the surface of silica seed particles. 10–15 The surface of these silica seeds needs to be previously treated by compatibilizers, e.g. meth- acryloxyalkyltrialkoxysilanes in low surface density, in order to create reactive (co)polymerizable loci promoting the surface capture of the growing macromolecules and therefore the nucleation of the PS latex particles. 16 A large range of colloidal clusters were produced from snowman-like particles 10 to tetra- pods, hexapods and octopods 11,12 by varying the concentration of the silica seeds and/or by varying the size of silica from a few tens to a few hundreds of nanometers. Unfortunately, for larger silica seeds, only raspberry-like morphologies were observed. That is why we investigated a new a route based on disper- sion polymerization. It is a process working in organic media where the initiator and monomer are soluble and the polymer insoluble. 17 As soon as the growing macromolecules reach a critical length, they precipitate from the medium and become primary nuclei. As in the case of the homogeneous nucleation mechanism in emulsion polymerisation, the primary nuclei coagulate to a stable one, which is stabilized by a suitable steric stabilizer present in the reaction medium and captured by a chain transfer reaction. Then, the growing particles absorb monomers and capture oligomeric radicals and primary nuclei formed in the medium, and the polymerization site shifts from the continuous phase to the growing particles. Styrene is easily polymerised through a dispersion route in particular in alco- holic polar media in the presence of poly(N-vinylpyrrolidone) (PVP) as steric stabilizer. 18 At high monomer-to-polymer Scheme 1 Examples of colloidal clusters based on spheres of two different chemical compositions and obtainable from seeded-emulsion polymerization (in that case, left or surrounding spheres are made of organic polymers and right or central spheres are silica seeds) a CNRS, Universit e de Bordeaux, ICMCB, 87 avenue du Dr Albert Schweitzer, F-33608 Pessac, France. E-mail: duguet@icmcb-bordeaux. cnrs.fr; Fax: +33 540 002 761; Tel: +33 540 002 651 b CNRS, Universit e de Bordeaux, Centre de Recherche Paul Pascal, avenue du Dr Albert Schweitzer, F-33600 Pessac, France c Chimie, Catalyse, Polym ere, Proc ed es, C2P2/LCPP-UMR 5265 CNRS/ CPE/UCBL, 43 boulevard du 11 Novembre 1918, F-69616 Villeurbanne, France † This paper is part of a Journal of Materials Chemistry themed issue on Advanced Hybrid Materials, inspired by the symposium on Advanced Hybrid Materials: Stakes and Concepts, E-MRS 2010 meeting in Strasbourg. Guest editors: Pierre Rabu and Andreas Taubert. 9392 | J. Mater. Chem., 2010, 20, 9392–9400 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/materials | Journal of Materials Chemistry