Surface Coated Iron Particles via Atom Transfer Radical Polymerization for Thermal–Oxidatively Stable High Viscosity Magnetorheological Fluid Joko Sutrisno, 1 Alan Fuchs, 1 Huseyin Sahin, 2 Faramarz Gordaninejad 2 1 Chemical Engineering Department, University of Nevada, Reno 2 Mechanical Engineering Department, University of Nevada, Reno Correspondence to: A. Fuchs (E-mail: afuchs@unr.edu) ABSTRACT: A surface grafting technique for poly(2-fluorostyrene) onto iron particles via atom transfer radical polymerization (ATRP) is described. Grafted poly(2-fluorostyrene)–iron particles were synthesized by immobilizing 2-4(-chlorosulfonylphenyl)-ethyl- trichlorosilane to the iron particles through the covalent bond of a silanol group, followed by the polymerization of 2-fluorostyrene monomer. The grafted polymer–iron particles display a higher thermal transition temperature compared to bulk polymer because the covalent bond between the polymer backbone and the surface of the iron particles restricts the molecular mobility. The molecular weight of the synthesized poly(2-fluorostyrene) has been measured and it has a narrow molecular weight distribution (M w /M n < 1.1). From thermogravimetric analysis, the thermal stability of poly(2-fluorostyrene) is superior to polystyrene. Also, the high viscos- ity magnetorheological fluid (HVMRF) prepared from surface coated iron particles has excellent thermo–oxidative stability, having nearly constant viscosity. These materials exhibit a large increase in shear yield stress for the off- and on-state as compared to a benchmark high viscosity magnetorheological fluid (HVMRF) and -coated iron particle HVMRF. In addition, this type of fluid eliminates iron particle settling which is a common problem found in traditional magnetorheological fluids (MRFs). V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: surface grafting; poly(2-fluorostyrene); ATRP; magnetorheological fluid (MRF) Received 28 April 2011; accepted 12 June 2012; published online DOI: 10.1002/app.38199 INTRODUCTION Atom transfer radical polymerization (ATRP) was introduced by Matyjaszewski’s group 1 and was widely used to synthesize poly- mers that have controlled morphologies, functionality, and com- positions. This polymerization technique involves an organic halide initiator, metal halides as a catalyst and a ligand to improve the solubility of the metal halides in the organic reac- tion system. 1,2 ATRP provides more flexibility in terms of the variety of monomers available. A wide range of monomers can be polymerized using ATRP either at mild conditions or ele- vated temperature. The polymers that have been synthesized using ATRP have a narrow polydispersity index due to the high ratio of dormant species to active species. 1–5 The grafting technique of thermo-responsive poly(N-isopropyla- crylamide) (poly(NIPAAm)) onto silica nanoparticles using ATRP has been investigated. 4,5 ATRP has been used for the po- lymerization of a series of substituted styrenes. 6 It has been shown that these polymers have polydispersities that are rela- tively low (M w /M n < 1.5). Surface polymerization of a block co-polymer of poly(styrene-b-methyl methacrylate) (PS-b- PMMA) and polystyrene (PS) with polydispersity index (PDI) of 1.29 on silica particles using ATRP has been investigated. 7 The properties of composite materials, such as magnetorheolog- ical fluids (MRFs) which consist of an inorganic substrate can be improved by introducing a polymer coating on the inorganic substrate. ATRP can be used as a tool to covalently graft various polymers onto the inorganic surface. MRFs are intelligent, composite materials which have controlla- ble rheological properties. MRFs consists of magnetic particles and a carrier fluid which is the dispersion medium for the mag- netic particles. MRFs have excellent mechanical and rheological properties which can be controlled using an external magnetic field. MRFs also have properties similar to Newtonian fluids in the off-state condition (without magnetic field). Magnetic par- ticles create chain-like structures within the MRF when external magnetic fields are applied and fluid flow is then restricted. This fluid-like material then transforms into a pseudo-solid material. V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38199 1