Article pubs.acs.org/Macromolecules
Size-Tunable Nanoparticle Synthesis by RAFT Polymerization in CO2Induced Miniemulsions Siqing Cheng, S. R. Simon Ting, Frank P. Lucien, 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: A novel environmentally friendly low-energy emulsification method that relies on pressurization with CO2 to low pressure has been applied to reversible addition−fragmentation chain transfer (RAFT) polymerization of styrene-in-water miniemulsions with the anionic surfactant Dowfax 8390. This method circumvents traditional high-energy homogenization, and over a certain CO2 pressure range, a transparent miniemulsion is formed. RAFT polymerization of styrene using benzyldodecyl trithiocarbonate and the aqueous phase initiator VA-044 was carried out successfully in CO2-induced miniemulsions at 50 °C with good control/livingness. Interestingly, the particle size could be conveniently tuned via the CO2 pressure without altering the recipe, with 6.00, 6.50, and 7.50 MPa generating numberaverage particle diameters of 98, 89, and 48 nm, respectively, at ∼70% conversion. The smallest particle size corresponded to the pressure range within which the emulsion was transparent.
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for nonpolymerization purposes.1b The approaches available mainly make use of accessible catastrophic phase transitions occurring during the emulsification process due to the change in the spontaneous curvature of the surfactant as a result of the variation in the physicochemical properties of the system, such as changing temperature (phase inversion temperature (PIT) method6), adjusting the composition of the system (emulsion inversion point (EIP) method7), or using additives to change the pH8 or ionic strength.9 In addition to the obvious benefit of low energy, such methods may also be advantageous with regards to reproducibility and generation of uniform droplet size distributions. High-energy homogenization methods normally generate fairly ill-defined droplet size distributions and are sensitive to process variables such as the power, the size and shape of the container, and the location of the sonifier tip. Recently, miniemulsions based on low-energy emulsification have been attracting attention for synthesis of polymeric nanoparticles. Spernath et al.10 reported miniemulsion polymerization of lauryl acrylate based on the sequential use of high-energy homogenization and the PIT method. Sadtler et al.11 and Galindo-Alvarez et al.12 synthesized poly(ethylene oxide)-covered nanoparticles using miniemulsion polymerization of styrene in connection with the EIP and PIT methods, respectively, for miniemulsion generation. In our recent work,13 miniemulsion polymerization of styrene based on the EIP low-energy emulsification method was implemented successfully. An alternative means of effecting low-energy miniemulsion formation is based on in situ formation of
INTRODUCTION Miniemulsions (also referred to as nanoemulsions) are kinetically stable but thermodynamically unstable submicrometer-sized (typically in the diameter range 50−500 nm) dispersions of oil (e.g., vinyl monomer) in water.1 The miniemulsion concept was originally introduced by Ugelstad et al.,2 who proposed the notion of polymerization within small monomer droplets (monomer droplet nucleation) during emulsion polymerization. Miniemulsion polymerization is distinct from the traditional emulsion polymerization process in that polymer particles are generated from monomer droplets, as opposed to nucleation in the continuous phase via micellar and/or homogeneous nucleation. This mechanistic aspect is the reason for the attractiveness of miniemulsion polymerization for nanoparticle synthesis. Since miniemulsion polymerization possesses the inherent advantage that ideally each monomer droplet is transformed into a polymer particle and diffusion across the aqueous phase is not required,3 the process conveniently lends itself to synthesis of hybrid polymer particles,4 hollow polymer particles,4a and implementation of controlled/living radical polymerization (CLRP).5 Since miniemulsions are thermodynamically unstable, energy input is required for their formation, traditionally in the form of highenergy homogenization via ultrasonication or high-pressure homogenization. This requirement remains an impediment to industrial implementation of miniemulsion polymerization, and it is therefore of interest to develop alternative low-energy methods to generate miniemulsions for polymerization as well as to device convenient methods to tune the droplet/particle size. Considerable efforts have been geared toward development of methods for low-energy generation of miniemulsions, mainly © 2012 American Chemical Society
Received: December 20, 2011 Revised: February 2, 2012 Published: February 8, 2012 1803
dx.doi.org/10.1021/ma202744f | Macromolecules 2012, 45, 1803−1810
Macromolecules
Article
surfactant at the oil−water interface;14 this method has been implemented successfully for both conventional radical polymerization and nitroxide-mediated radical polymerization (NMP). Some of the above approaches are of limited use for polymerizations; the required composition changes (EIP method) are not always easily achieved in practice, and a variation in temperature (PIT method) is not practical in connection with radical polymerization initiated by thermal initiators. The development of effective, controllable, environmentally benign, and low-energy emulsification methods for preparation of miniemulsions for polymerizations is challenging and of great importance. As an attractive green solvent, supercritical or compressed CO2 has been widely used in many chemical and industrial processes because it is readily available, inexpensive, and nontoxic and its physical properties can be tuned continuously by pressure and/or temperature.15 The dissolution of compressed CO2 in aqueous solution can change the properties of the aqueous solution considerably, and thus the properties of aqueous solutions can be tuned by controlling the CO2 pressure. The ability to control the properties of an aqueous surfactant solution by CO2 is interesting16the properties of the surfactant aggregates in aqueous solution can be tuned by the CO2 pressure because of the tunable nature of compressed CO2. This principle has been widely applied successfully in reversibly controlling the properties of solutions containing surfactant by pressurization and depressurization, e.g., reverse micelles,17 creating CO2 continuous microemulsion or CO2/ water emulsions,18 and triggering phase transitions between different surfactant aggregates.19 Most interestingly, a recent investigation20 demonstrated that CO2 can induce the formation of miniemulsions (in the absence of high-energy mixing) in a wide range of water-to-oil volume ratios in the presence of a low concentration of surfactant. Miniemulsions obtained in this way have been applied to the synthesis of nanomaterials, enhanced oil recovery, and polymerizations in preliminary investigations.20,21 This novel means of preparing miniemulsions is of great interest not only because it circumvents the traditional high-energy mixing approaches but also because the formation and breakage of the miniemulsion can be controlled reversibly by pressurization and depressurization without contaminants (e.g., salt to break the emulsion). Although the exact miniemulsion formation mechanism remains to be clarified,20 this technique clearly has great potential for miniemulsion polymerization. CLRP 22 makes it possible to prepare polymer of predetermined microstructure and various complex architectures by free radical means, and the past decade has seen significant progress in the area of CLRP in dispersed systems.5e,f Reversible addition−fragmentation chain transfer (RAFT) polymerization is one of the most frequently employed techniques.23 RAFT miniemulsion polymerization can be challenging mainly due to issues related to colloidal stability (superswelling) but can be implemented with success under appropriate conditions.5f,24 To date, the only examples of miniemulsion CLRP based on low-energy emulsification are our works on NMP14c and RAFT25 polymerization in miniemulsion using in situ surfactant formation without use of high-energy homogenization. In the current work, we investigatedfor the first time RAFT polymerization of styrene in aqueous miniemulsion induced by compressed CO2, i.e., in the absence of traditional high-energy mixing techniques. It is demonstrated that
polymeric nanoparticles of diameters
