Preparation of a Natural Rubber Core/Polymer Shell in a Nanomatrix by Graft Copolymerization Wanvimon Arayapranee, 1 Garry L. Rempel 2 1 Department of Chemical and Material Engineering, Rangsit University, Phathum Thani 12000, Thailand 2 Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada Received 24 July 2007; accepted 28 May 2008 DOI 10.1002/app.28765 Published online 20 August 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The grafting of a vinyl monomer, methyl methacrylate (MMA) or styrene (ST), onto a natural rubber (NR) exhibited a core–shell structure, with NR as the core and poly(methyl methacrylate) or polystyrene as the shell. The grafting efficiency (GE) of the graft copolymer was determined by a solvent-extraction technique. The synthe- sized graft copolymer was purified and then characterized by Fourier transform infrared spectroscopy analysis. The effects of the graft parameters, including time, initiator content, and concentration, and the type of monomer, MMA or ST, were investigated. A longer time was favor- able for the graft copolymerization. GE first increased and then decreased with increasing concentration of initiator. GE decreased with increasing monomer content, and it was confirmed that the graft copolymerization was a sur- face-controlled process. The grafting ST monomer had a higher GE compared to MMA under the same conditions. The characterization of the particles by transmission elec- tron microscopy and scanning electron microscopy con- firmed the formation of a core–shell structure. From the micrographs, we inferred that at 71% GE, the NR seed particle had a complete closed shell several nanometers in thickness. Therefore, the NR particle was dispersed in a polymer nanomatrix. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2475–2482, 2008 Key words: core–shell polymers; graft copolymers; nanolayers; polystyrene; rubber INTRODUCTION Natural rubber latex (NRL) obtained from Hevea bra- siliensis is a natural biosynthesis polymer with an attractive range of modifying properties that can be devised to provide more desirable properties. NRL is supplied as a natural latex that has a broad parti- cle size distribution, ranging from 100 nm to 2 lm, stabilized with ammonia. 1 In general, the improved products from natural rubber (NR) have potentially wide applications as a result of physical or chemical modification. Chemical modification of NR by graft- ing with vinyl monomers with various initiator systems has gained considerable importance in the modification of the properties of NR. 2–6 The core– shell particles may be prepared by the graft copoly- merization of NR in latex form because the NR, an unsaturated elastomer with double bonds in its chains, can be readily grafted with a variety of monomers with the well-established technique of seeded emulsion polymerization. Latex particles with a soft core and a hard shell are modified as impact modifiers, whereas particles with a hard core and a soft shell are usually used in the coating and adhesive fields. 7 The graft copolymer of vinyl mono- mers, such as methyl methacrylate (MMA or C 5 H 8 O 2 ) or styrene (ST or C 8 H 8 ) onto NR, compris- ing an inner soft polymer sphere, the core, and an outer hard polymer, the shell, can be expected to have better impact-resistance properties. A poly- meric material composed of a thin matrix several nanometers in thickness (nanolayers) and dispersiod a micrometer in diameter is proposed to be a multi- functional soft material, namely, a nanomatrix- dispersed polymer. The graft copolymer of vinyl monomers onto NR comprising the rubbery core makes up for the defects without the loss of the out- standing properties because NR is grafted with a small amount of functional polymer when the poly- mer forms a nanomatrix. A number of reports 3,4,8–10 have appeared on the grafting of vinyl monomers onto NRL particles with an amine-activated hydro- peroxide. Hydroperoxides have been found to be particularly susceptible to activation by polyalkyle- nepolyamines. 10 A redox initiation system consisting of organic hydroperoxide and tetraethylene pentam- ine [TEPA or HN(CH 2 CH 2 NHCH 2 CH 2 NH 2 ) 2 ] was chosen for this study because it operates efficiently at the high pH values normally encountered in NRL. Journal of Applied Polymer Science, Vol. 110, 2475–2482 (2008) V V C 2008 Wiley Periodicals, Inc. Correspondence to: W. Arayapranee (wanvimon@rsu.ac. th). Contract grant sponsor: Rangsit Research Institute of Rangsit University.