Preparation of Composite Materials by Using Graphene Oxide as a Surfactant in Ab Initio Emulsion Polymerization Systems Stuart C. Thickett and Per B. Zetterlund* Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia * S Supporting Information ABSTRACT: In this letter, we report a simple and unexpected method of producing polymer-graphene oxide (GO) composite materials via ab initio emulsion polymerization in water. On the basis of the recent reports concerning the surfactant-like behavior of GO for stabilizing oil-in-water emulsions, we prepared exfoliated GO sheets with lateral dimension approximately 200 nm for use as surfactant in the emulsion polymerization of styrene. We observed an expected “classic” surfactant behavior to produce stable nanoparticles at extremely low GO loadings (<0.1% w/w); however, at higher concentrations a highly aggregated, fibrous morphology was formed. This morphology is predominantly due to the electrolyte concen- tration (ionic strength) of the aqueous phase resulting in heterocoagulation of growing oligomers with dispersed GO sheets, which offers a convenient route toward preparing hybrid materials. G raphene oxide (GO), the oxidized form of graphene prepared via the oxidation and exfoliation of bulk graphite, is often considered the most convenient material for the realization of graphene-based composite materials. 1 With respect to preparing functional polymeric materials, GO possesses numerous hydroxyl, epoxy, and carboxylate groups that permit polymeric functionalization and incorporation into a polymer matrix. 2-5 Of particular interest to our group is the recent report which describes the amphiphilic nature of GO, 6 whereby oil-in-water (o/w) emulsions are stabilized by adsorbed GO sheets at the oil-water interface (i.e., a Pickering emulsion). 7,8 This behavior, which is due to the ionizable carboxylate groups at the periphery and hydrophobic graphitic regions within the basal plane of GO, has given rise to its use as a colloidal stabilizer in heterogeneous polymerization, in particular miniemulsion polymerization. 9-12 The resultant materials, which in the case of recent work from our own group consist of polymer particles “armored” with a shell of GO, 11 represent a simple method to create hybrid materials. The aim of this work was to demonstrate if GO could be used as a colloidal stabilizer in Pickering emulsion polymer- ization systems, as opposed to miniemulsion polymerization. This was recently achieved using Laponite clay discs as surfactant in the emulsion polymerization of various mono- mers 13 under appropriate conditions; other Pickering stabilizers successfully employed in emulsion polymerization include modified silica sols 14,15 and nanoparticles. 16,17 Emulsion polymerization has several technical advantages over mini- emulsion polymerization, in particular the absence of a high shear emulsification step to create submicrometer-sized monomer droplets. To our knowledge, emulsion polymer- ization using GO as a surfactant has not been successfully achievedhowever it has been incorrectly reported; previous reports claiming the preparation of polymer-GO hybrid materials by emulsion polymerization have used an ultra- sonication step to create preformed submicrometer-sized monomer droplets 10,18 or droplet nucleation with an oil- soluble initiator, 19 which is inconsistent with the mechanism of emulsion polymerization 20,21 and more consistent with miniemulsion polymerization. In the case of a true emulsion polymerization, the locus of polymerization is either monomer- swollen micelles or newly nucleated particles formed via aqueous-phase initiation and propagation (as opposed to monomer droplet nucleation). In this work, nanodimensional GO sheets were prepared by the oxidation of graphite nanofibers using a modified Hummers’ method as reported previously. 11,22 These small sheets (Z-average hydrodynamic diameter 188 nm by dynamic light scattering (DLS), zeta potential = -48.4 mV in Milli-Q water) were dispersed in water via sonication and used as the aqueous phase for the ab initio emulsion polymerization of styrene for 24 h at 70 °C (target solids content 10% w/w, initiator (potassium persulfate) concentration ∼13 mM), which would be considered “typical” emulsion polymerization conditions. 23 At very low GO concentrations (<0.1% w/w relative to monomer), we observe that GO does behave as a surfactant; the average particle size decreases, and the particle Received: June 3, 2013 Accepted: July 2, 2013 Letter pubs.acs.org/macroletters © XXXX American Chemical Society 630 dx.doi.org/10.1021/mz400280t | ACS Macro Lett. 2013, 2, 630-634