Article pubs.acs.org/Macromolecules
Redox-Responsive Polymer Brushes Grafted from Polystyrene Nanoparticles by Means of Surface Initiated Atom Transfer Radical Polymerization Markus Mazurowski,† Markus Gallei,†,* Junyu Li,‡ Haiko Didzoleit,‡ Bernd Stühn,‡ and Matthias Rehahn† †
Ernst-Berl Institute for Chemical Engineering and Macromolecular Science, Darmstadt University of Technology, Petersenstraße 22, D-64287 Darmstadt, Germany ‡ Institute of Condensed Matter Physics, Darmstadt University of Technology, Hochschulstraße 8, D-64289 Darmstadt, Germany S Supporting Information *
ABSTRACT: Well-defined poly(2-(methacryloyloxy)ethyl ferrocenecarboxylate) (PFcMA) brushes were synthesized by surface-initiated atom transfer radical polymerization (SIATRP) on cross-linked polystyrene particles. The ATRP of FcMA monomer was reinvestigated leading to molar masses up to 130 kg mol−1 with a good reaction control and high monomer conversion (91%). The SI-ATRP was done with different amounts of initiator in the PS particle shell leading to PFcMA surface conformations from “mushroom-like” to dense “brush-like” polymers, which could be confirmed by dynamic light scattering (DLS) experiments. Redox-responsive behavior of the PFcMA shell was investigated by DLS and cyclic voltammetry (CV) measurements indicating a tremendous increase in the hydrodynamic volume of the ferrocene-containing shell. The transformation of PFcMA-grafted PS particles to magnetic iron oxides after thermal treatment could be investigated by SQUID magnetization measurements showing the typical hysteresis for ferromagnetic material.
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separation techniques are only a few examples.20 For that reason various particles have been functionalized by the “grafting-from” methodology leading to considerably higher grafting densities compared to the ‘grafting-onto’ process due to the sterical hindrance of the polymers in the latter case.21 Controlled radical polymerization strategies, especially atom transfer radical polymerization (ATRP), seem to be the most promising route for decorating particles with polymer brushes as these methods lead to polymers precisely defined in their constitution and molar masses. Moreover the reactions can be carried out in the presence of water and have a large tolerance toward many functional groups. By suitable functionalization of the organic shell, it is already possible to generate high density polymer brushes in a wide range.18,20,22−28 The vast majority of reports about stimuli-responsive polymers deal with temperature-, light- and/or pH-dependent changes of polymer chain conformation16,29−35 but much less examples are known and well investigated for redox-responsive systems. The interconversion of thiols and disulfides in micellar core or shell structures is one example therefore.36 Another interesting redox-active couple in polymeric systems is the ferrocene/ ferrocenium motif. Ferrocene-containing polymers attracted a lot of attention in the past decade due to their promising
INTRODUCTION In recent years, nanoparticles have attracted enormous attention due to their ability to improve physical and chemical properties of common materials. Many researchers with interests in electronics, biomedical, optics, and catalysis endeavor to create core/shell particles consisting of inorganic cores and organic shells and vice versa.1−3 Studies in that direction have been extensively reviewed by Paria et al.4 Especially polystyrene (PS) (nano)particles have been intensively under investigation because of their simple synthesis leading to almost monodisperse particles. These particles can be easily modified with functional groups or shells by seeded emulsion polymerization techniques to gain access to surfacegrafting polymerization methodologies for functional monomers.5−10 Pioneering work in the field of surface-grafting of silica nanoparticles by free radical polymerization was achieved by Prucker and Rühe, who modified the particle surface with polystyrene.11,12 By controlling the particle surface and taking advantage of the stimuli-responsiveness of surface-anchored polymers so-called smart surfaces can be obtained.5,13−15 The resulting high functional materials can be stimulated by the change of solvent, temperature, ionic strength, light, or mechanical stress.16 Also a combination of different stimuli is therefore possible and favorable.17 Furthermore, a high grafting density allows access to stimuli-responsive polymer brushes for many important applications,18,19 for which mechanical actuation and sensing as well as drug release, cell growth, and © 2012 American Chemical Society
Received: September 25, 2012 Revised: October 31, 2012 Published: November 5, 2012 8970
dx.doi.org/10.1021/ma3020195 | Macromolecules 2012, 45, 8970−8981
Macromolecules
Article
Scheme 1. Synthesis of Redox-Responsive PFcMA Brushes Grafted from the Surface of Polystyrene Particles by Means of SIATRP
compare the ATRP kinetics for different ligand systems. Our main focus in this report is the investigation of SI-ATRP of FcMA monomer by a ‘grafting from’ methodology of functionalized PS particles (Scheme 1). The grafting densities of the surface-anchored initiators on polystyrene particles are varied leading to different densities of the surface-anchored ferrocene-containing polymers. The PFcMA-grafted particles are characterized by TEM, TGA, DLS, UV/vis spectroscopy, and cyclic voltammetry (CV). For additional investigations of the redox-responsiveness of the ferrocene-containing shell we oxidized the ferrocene moieties chemically and electrochemically. By DLS measurements, we observe that oxidation results in an enormous increase up to 20% of the average particle diameter due to the resulting polyelectrolyte effect of the poly(2-(methacryloyloxy)ethyl ferroceniumcarboxylate) (PFcMA+) shell. With L-ascorbic acid the hydrophilic shell can be reduced again. To the best of our knowledge redox-stimulus ferrocenecontaining polymer brushes grafted on polystyrene particles are reported for the first time and they will obviously have the potential for manifold applications, for which release and separation of organics or catalysis are only some examples. The new insights into the ATRP of ferrocenyl methacrylates will surely help to expand the synthesis to multistimuli block copolymer brushes. As another proof of the reliability of high density PFcMA shells we treat the functionalized particles thermally yielding magnetic iron oxide particles. The calcinated particles were characterized by TEM and superconducting quantum interference device (SQUID) magnetization measurements showing both the pristine particle structure and the ferromagnetic behavior in dependency of the degree of calcination. For the herein presented PFcMA-grafted particles we additionally anticipate applications as delivery systems or patterning assemblies after thermal treatment for the ceramics besides their novel applications as redox-responsive colloids.
combination of redox, mechanical, semiconductive, photophysical, optoelectronic, and magnetic properties.37−44 When the ferrocene moieties in cross-linked polymers or gels are oxidized, the molecular environment increases dramatically in hydrophilicity and a swelling of the resulting polyelectrolyte can be observed. The oxidation/reduction cycles of such redoxactive polymers can already be utilized in host−guest interactions for self-healing materials,45 controlled release of organics from gels,46 glucose sensor applications,47,48 enzymemediated biosensors,49,50 in composite colloidal crystal films,51 for changing the redox-responsive permeability in polyelectrolyte multilayer capsules,52 and for a redox-responsive release of a dye from patchy nanocapsules.53 Out of all laterally bonded ferrocene-containing monomers, that can be polymerized by anionic or reversible-deactivation radical polymerizations, ferrocenyl (meth)acrylates seem to be the most promising candidates.54,55 Huang et al. reported the ATRP of 2-acryloyloxyethyl ferrocenecarboxylate leading to narrowly distributed amphiphilic graft polymers with molar masses up to 8 kg mol−1 for the ferrocene-containing homopolymer.56,57 A detailed study is given by Tang et al., who further investigated the influence of the alkyl spacer between the ferrocene moiety and the propagating chain end in ATRP.58 Laschewsky and Barner-Kowollik reported the successful reversible addition−fragmentation chain transfer (RAFT) polymerization of two ferrocenyl acrylate derivatives and 4-ferrocenylbutyl methacrylate. In the latter case polydispersities below 1.3 with a highest molar mass of 15 kg mol−1 could be achieved.59 First insight into the SI-ATRP of a ferrocenyl methacrylate grafted from an indium tin oxide (ITO) substrate and the electrochemical characterization is given by Pyun et al.60 In this paper we describe the significantly improved synthesis of poly(2-(methacryloyloxy)ethyl ferrocenecarboxylate) (PFcMA) compared to the synthesis known from literature60 and the full characterization of the resulting homopolymers. The free PFcMA homopolymers are characterized by 1H NMR, 13 C NMR, DSC, TGA and cyclic voltammetry. By multiangle laser light scatter SEC (SEC−MALLS) measurements we are able to receive the absolute molar masses of the ferrocenecontaining polymers, which lead to crucial information for kinetics of the ATRP and additionally the characterization of the PFcMA-grafted particles. Compared to polystyrene standards a dramatic deviation in SEC measurements is observed similar to other previously reported laterally bonded ferrocenecontaining polymers such as poly(vinylferrocene).61 The ATRP of FcMA has been improved tremendously in regard to conversion (∼91%), reaction control (polydispersities
