pubs.acs.org/Langmuir © 2010 American Chemical Society
Emulsion Polymerization Routes to Chemically Anisotropic Particles Eric B. Mock and Charles F. Zukoski* Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, Illinois 61801 Received May 18, 2010. Revised Manuscript Received July 26, 2010 Methods are presented to synthesize suspensions of chemically and shape anisotropic colloids on submicrometer length scales. Particles are synthesized through seeded emulsion polymerization where a weakly cross-linked seed is swollen with monomer that phase separates at the reaction temperature resulting in a protrusion. The final particles can be considered to be composed of interpenetrating spheres. pH-sensitive anisotropy is created through the use of different surface coatings on each of the interpenetrating spheres. Dark-field imaging, dynamic light scattering, and scanning electron microscopy are used to characterize the particles.
Introduction Anisotropic pair potentials are expected to result in suspensions with different structures and phase behavior than are observed when particles interact with isotropic potentials.1-13 Anisotropy can be created through the differential spatial distribution of different chemical compositions such as has been introduced with Janus particles with heterogeneous hydrophobicity and/or charge on different hemispheres of the spherical particles.14-20 A second method would be to introduce both shape and chemical anisotropy as we describe here. In developing an understanding of shape and chemical anisotropy on the clustering and phase behavior in suspension, key issues concern the particle size and shape uniformity, the degree of anisotropy, and the ability to make sufficiently large quantities of the particles for them to be characterized and put to use. A number of *Corresponding author. E-mail:
[email protected]. (1) Bianchi, E.; Largo, J.; Tartaglia, P.; Zaccarelli, E.; Sciortino, F. Phys. Rev. Lett. 2006, 97, 68301. (2) Blaak, R.; Miller, M. A.; Hansen, J.-P. Europhys. Lett. 2007, 78, 26002. (3) Chen, T.; Zhang, Z.; Glotzer, S. C. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 717–722. (4) Gay, S. C.; Beale, P. D.; Rainwater, J. C. J. Chem. Phys. 1998, 109, 6820– 6827. (5) Goyal, A.; Hall, C. K.; Velev, O. D. Phys. Rev. E 2008, 77, 031401. (6) McGrother, S. C.; Jackson, G. Phys. Rev. Lett. 1996, 76, 4183–4186. (7) Shelley, J. C.; Patey, G. N. J. Chem. Phys. 1995, 103, 8299–8301. (8) Van Workum, K.; Douglas, J. F. Phys. Rev. E 2005, 71, 031502. (9) Van Workum, K.; Douglas, J. F. Phys. Rev. E 2006, 73, 031502. (10) Whitelam, S.; Bon, S. A. F. J. Chem. Phys. 2010, 132, 074901. (11) Giacometti, A.; Lado, F.; Largo, J.; Pastore, G.; Sciortino, F. J. Chem. Phys. 2010, 132, 174110. (12) Solomon, M. J.; Zeitoun, R.; Ortiz, D.; Sung, K. E.; Deng, D.; Shah, A.; Burns, M. A.; Glotzer, S. C.; Millunchick, J. M. Macromol. Rapid Commun. 2010, 31, 196–201. (13) Fejer, S. N.; Chakrabarti, D.; Wales, D. J. ACS Nano 2010, 4, 219–228. (14) Chen, C. H.; Shah, R. K.; Abate, A. R.; Weitz, D. A. Langmuir 2009, 25, 4320–4323. (15) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495–9499. (16) Hong, L.; Cacciuto, A.; Luijten, E.; Granick, S. Nano Lett. 2006, 6, 2510– 2514. (17) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408–9412. (18) Roh, K. H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759–763. (19) Shah, R. K.; Kim, J. W.; Weitz, D. A. Adv. Mater. 2009, 21, 1949–1953. (20) Shepherd, R. F.; Conrad, J. C.; Rhodes, S. K.; Link, D. R.; Marquez, M.; Weitz, D. A.; Lewis, J. A. Langmuir 2006, 22, 8618–8622. (21) Dendukuri, D.; Hatton, T. A.; Doyle, P. S. Langmuir 2007, 23, 4669–4674. (22) Chastek, T. T.; Hudson, S. D.; Hackley, V. A. Langmuir 2008, 24, 13897– 13903. (23) Snyder, C. E.; Yake, A. M.; Feick, J. D.; Velegol, D. Langmuir 2005, 21, 4813–4815. (24) Ge, J.; Hu, Y.; Zhang, T.; Yin, Y. J. Am. Chem. Soc. 2007, 129, 8974–8975.
Langmuir 2010, 26(17), 13747–13750
techniques to prepare chemically anisotropic particles have been reported.14,15,17-24 However, because of difficulties in synthesizing large yields of uniform particles in the colloidal size domain, few studies characterizing the behavior of chemically anisotropic colloids have been reported.16,25,26 Here, seeded emulsion polymerization is studied to synthesize polymer colloids with shape and chemical anisotropy. Large batches of uniformly shaped anisotropic polymer particles with length scales of 0.2-10 μm are routinely prepared using seeded emulsion polymerization.27-29 These dicolloids may be thought of as interpenetrating spheres, where particles with equally sized spheres are called homonuclear dicolloids, and for spheres of different sizes, they are called heteronuclear dicolloids. Dicolloid synthesis via seeded emulsion polymerization involves swelling crosslinked seed spheres with monomer near room temperature and then raising the temperature to initiate free radical polymerization. Shape anisotropy results from the immiscibility of monomer in the cross-linked polymer network.30-34 Reports have been made using monomers in the swelling stage that are different from those used in synthesizing the seed particles to create chemical anisotropy, but the differences in surface properties have, to date, been sufficiently subtle that reports of unique microstructures resulting from this chemical anisotropy have not been reported.31,33,35,36 Here we extend this technique to synthesize two types of chemically anisotropic particles. The strategy pursued is that surface groups covalently bound to the cross-linked seed cannot migrate to cover monomer that phase separated from it during the second polymerization step. Using different surface-active (25) Hong, L.; Cacciuto, A.; Luijten, E.; Granick, S. Langmuir 2008, 24, 621– 625. (26) Gangwal, S.; Cayre, O. J.; Velev, O. D. Langmuir 2008, 24, 13312–13320. (27) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. 1990, 28, 653–667. (28) Mock, E. B.; De Bruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037–4043. (29) Kim, J. W.; Larsen, R. J.; Wetiz, D. A. Adv. Mater. 2007, 19, 2005–2009. (30) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629–651. (31) Kim, J. W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374– 14377. (32) Kegel, W. K.; Breed, D.; Elsesser, M.; Pine, D. J. Langmuir 2006, 22, 7135– 7136. (33) Chen, Y. C.; Dimonie, V.; El-Aasser, M. S. Macromolecules 1991, 24, 3779– 3787. (34) Okubo, M.; Wang, Z.; Yamashita, T.; Ise, E.; Minami, H. J. Polym. Sci., Polym. Chem. 2001, 39, 3106–3111. (35) Shi, S.; Kuroda, S.; Kubota, H. Colloid Polym. Sci. 2003, 281, 331–336. (36) Du, Y.-Z.; Tomohiro, T.; Kodaka, M. Macromolecules 2004, 37, 803–812.
Published on Web 08/02/2010
DOI: 10.1021/la101982c
13747
Letter
Mock and Zukoski A 500 mL round-bottomed flask fitted with a glass impeller and a PTFE paddle attached to a Glas-Col 099D GT31 motor running at ∼250 rpm was immersed in a constant-temperature bath at room temperature. Next, 150 mL of seed latex was discharged into the flask, followed by styrene in a monomer-to-seed mass ratio of 1.2:1. The swelling was allowed to continue for 21 h, after which the constant-temperature bath was heated to 80 °C. After waiting 3 h for the vessel contents to reach 80 °C, we added cetyltrimethylammonium bromide (CTAB, 1.20 g, Sigma, for molecular biology, approximately 99%) dissolved in 90.0 mL of DIW, followed by a 10.0 mL rinse with DIW and hydroquinone (4.00 g, Sigma-Aldrich, ReagentPlus 99%) dissolved in 80.0 mL of DIW. After allowing 1 h for the reactants to reach the bath temperature, we charged the flask with 2,20 -azobisisobutyronitrile (AIBN) (0.240 g, Aldrich, 98%) dissolved in 4.00 mL of styrene and allowed the reaction to proceed for 24 h.
Results and Discussion Figure 1. Schematic of the synthesis strategy, where a cross-linked seed particle is synthesized using an anionic initiator and an ionic comonomer that functionalizes the particle surface with amine groups, A, that are positively charged at low pH and neutral at high pH. The seeds are then (a) swollen with monomer and (b) undergo a second polymerization at low pH in the presence of a cationic surfactant to yield dicolloids with a protrusion that has a constant positive charge and a seed that can be changed from positively charged to negatively charged via increasing pH.
monomers in each step leads to particles that are expected to have different surface chemistries at each end. Of course, the surface chemistries have to be chosen such that during synthesis the heterogeneous surface chemistries do not lead to aggregation. Thus this strategy involves creating particles where the anisotropy can be triggered after dicolloid synthesis. Particles described here are synthesized such that the surfaces contain weak base groups. By altering the concentration of these groups in the seed and in the second polymerization step, at intermediate pH values one end of the particle is positive and the other is negative (Figure 1). Under these conditions, particles cluster into strings and circles.
Experiment Amphoteric polystyrene spheres with a diameter of ∼330 nm, cross-linked with divinylbenzene (DVB), were prepared as follows on the basis of the recipe of Homola and James.37 Deionized water (DIW) (135 mL) was added to a 1 L round-bottomed flask followed by 2-(diethylamino)ethyl methacrylate (DEAM) (3.32 g, Sigma-Aldrich, 99%). Hydrochloric acid (Fisher Scientific, ACS PLUS) was added to bring this solution to pH 1.2. This low pH is chosen to ensure that the ionic comonomer has a positive charge. Afterwards, the vessel was charged with styrene (18.8 mL, Sigma-Aldrich, 99%) and DVB (0.128 g, Aldrich, 55% mixture of isomers, tech grade). Potassium persulfate (0.338 g, Fisher Scientific, 99.5%) dissolved in 15.0 mL of DIW and brought to pH 2.0 with hydrochloric acid was finally added to the flask, which was fitted with a glass impeller and a poly(tetrafluorethylene) (PTFE) paddle attached to a Glas-Col 099D GT31 motor running at 333 rpm and immersed in a constanttemperature bath at 70 °C for 24 h. The resulting suspension was dialyzed in SpectraPor 4 dialysis tubing (molecular weight cutoff 12 000-14 000 Da) for 24 h against DIW that was replaced four times during the course of dialysis. Suspension volume fractions were determined by weight loss upon drying an ∼0.5 mL suspension at 110 °C using a polymer density of 1.05 g cm-3. (37) Homola, A.; James, R. O. J. Colloid Interface Sci. 1977, 59, 123–134.
13748 DOI: 10.1021/la101982c
The seed particles are synthesized under conditions where they have a net positive charge at the end of the reaction. When the pH is increased, the amine groups deprotonate and the strong acid groups remaining from the initiator give the particles a negative charge. Because the seeds are weakly cross-linked, we expect these charge groups to be largely retained on the seed particle during subsequent swelling and polymerization steps. In the swelling and polymerization steps, a nonionic initiator is used. Under these conditions, we found that the reaction mixture aggregated if a surfactant was not added. We attribute this to the hydrophobic nature of the protrusion leading to aggregation. To mitigate this effect, CTAB was added during the second polymerization step to adsorb to the protrusion and provide stability. Thus by carrying out the second polymerization reaction at low pH where both portions of the dicolloid will take on a positive charge, stable particles were produced. The resulting particles (Figure 2) have a seed containing negatively charged sulfate groups from the potassium persulfate initiator and tertiary amine groups from DEAM, and the protrusion is stabilized with positively charged CTAB. After synthesis at low pH, where tertiary amine groups are protonated and positively charged, both ends of the dicolloid surfaces are positively charged. With increasing pH, the tertiary amine groups become deprotonated such that the seed part has a negative charge from the sulfate groups while the protrusion retains its positive charge from the CTAB, resulting in dicolloids with negatively charged seeds and positively charged protrusions. Evidence for the amphoteric character of the seed and dicolloids can be found in the pH dependence of electrophoretic mobilities (Figure 3) where both the seeds and the dicolloids have isoelectric points near pH 7.5. Here the pH was adjusted using 10-3 M solutions of HCl and KOH. As originally discussed by Homola and James,37 the seed particles aggregate in a region where the particles carry a small charge. The region where the particles aggregate is shown in Figure 3 by cross hatching and extends from a pH of ∼7.5 to a pH of ∼8.5. For pH values outside this range, the particles form stable suspensions. The dicolloids aggregate at a pH near 7.9. At this pH, the particles retain a net negative charge. As indicated for the seeds, at this pH there has been a sufficient reduction in the charge of the DEAM ionic comonomer such that the seed particle is negatively charged. However, the CTAB groups will retain their positive charge, thus producing dipolar particles. Of particular interest is that as the pH is further increased and the electrophoretic mobility continues to grow more negative, the dicolloids do not restabilize. Despite a substantial net negative charge carried by the dicolloids at a pH of 8-10, these particle are not stable and Langmuir 2010, 26(17), 13747–13750
Mock and Zukoski
Letter
Figure 2. (a) Charge anisotropic dicolloids and (b) heterodicolloids where the protrusion is composed of poly-NiPAM.
Figure 4. (a) Dark-field image of the charge anisotropic dicolloids at pH >10, where single spheres are individual particles, and (b) SEM image of the same particles dried at pH 8.2, where particles align predominantly in a head-to-tail configuration.
Figure 3. Electrophoretic mobility as a function of suspension pH for the amphoteric (a) seed particles and (b) charge anisotropic dicolloids, where shaded areas indicate regions of instability at which the particles form clusters as shown in the inset of part b. The ionic strength of the solutions is 10-3 M.
aggregate. This observation is taken as evidence that the particles are dipolar-strong acid groups on the seed and quaternary ammonium cations on the protrusion resulting in strong opposite charge attractions and aggregation. Further evidence for this head-to-tail aggregation can be found in dark-field microscopy images of these dicolloid suspensions taken at pH 10.5 showing strings and circular structures (Figure 4) (38) Jiang, S.; Schultz, M. J.; Chen, Q.; Moore, J. S.; Granick, S. Langmuir 2008, 24, 10073–10077. (39) Butter, K.; Bomans, P. H. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P. Nat. Mater. 2003, 2, 88–91.
Langmuir 2010, 26(17), 13747–13750
as predicted and experimentally observed for spheres with dipolar interactions9,11,38,39 and nanorods with anisotropic interactions.40,41 Such structures have been observed for suspensions of colloids with isotropic interactions in the case of lock-and-key colloids in the presence of depletion forces,42 nanorods deposited on nanotubes serving as templates,43 colloids configured by microfluidics,44 and nanoparticles with long-range isotropic electrostatic interactions in the presence of dipole interactions.45 However, under the conditions of this study, in the absence of depletion forces, templating methods, microfluidic confinement, and any dipolar interactions outside of those from particle charge anisotropy, isotropic particles are not observed or predicted to self-assemble into strings and circular structures. These unique microstructures are thus attributed to the anisotropic interactions (40) Liu, K.; Nie, Z.; Zhao, N.; Li, W.; Rubinstein, M.; Kumacheva, E. Science 2010, 329, 197–200. (41) Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Nat. Mater. 2007, 6, 609–614. (42) Sacanna, S.; Irvine, W. T. M.; Chaikin, P. M.; Pine, D. J. Nature 2010, 464, 575–578. (43) Correa-Duarte, M. A.; Perez-Juste, J.; Sanchez-Iglesias, A.; Giersig, M.; Liz-Marzan, L. M. Angew. Chem., Int. Ed. 2005, 44, 4375–4378. (44) Sung, K. E.; Vanapalli, S. A.; Mukhija, D.; McKay, H. A.; Mirecki, J. M.; Burns, M. A.; Solomon, M. J. J. Am. Chem. Soc. 2008, 130, 1335–1340. (45) Zhang, H.; Wang, D. Angew. Chem., Int. Ed. 2008, 47, 3984–3987.
DOI: 10.1021/la101982c
13749
Letter
of the dicolloids in this work. In scanning electron microscope images prepared from dilute suspensions at pH 8.2, particles were seen to be dried into head-to-tail ordering (Figure 4), again suggesting strong dipolar attractions. Isotropic colloids have previously been dried into similar stringlike structures using capillary condensation,46 although taken together with the particle stability as a function of electrophoretic mobility and dark-field microscopy images of particles in suspension, the data indicates that the novel microstructures observed in this work are the result of chemically anisotropic particle interactions. A similar strategy can be used to produce thermally active particles. For these particles, the seeds are swollen with toluene and N-isopropylacrylamide (NiPAM). The resulting particles show a protrusion whose size depends on the NiPAM-to-seed particle mass ratio. Suspensions of these particles are stable at room temperature, but in keeping with the lower critical solution temperature (LCST) of poly-NiPAM at 35 °C,47 the particles form clusters at 50 °C. With vigorous shaking, the clusters are broken up and the suspensions are again stable upon cooling to room temperature. Examples of these particles are shown in Figure 2. Below the LCST, surfactant may desorb from the hydrophilic poly-NiPAM protrusion, although the stability of these dicolloids at room temperature would suggest that if this does happen it is either a slow process or the seed is sufficiently charged to keep the particles electrostatically stabilized. Surfactant desorption would be an interesting idea to explore in future studies to generate another type of chemically anisotropic dicolloid, where the seeds are charged and the protrusions are uncharged. (46) Li, F.; Stein, A. J. Am. Chem. Soc. 2009, 131, 9920–9921. (47) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (48) Kraft, D. J.; Vlug, W. S.; van Kats, C. M.; van Blaaderen, A.; Imhof, A.; Kegel, W. K. J. Am. Chem. Soc. 2009, 131, 1182–1186. (49) Kraft, D. J.; Groenwold, J.; Kegel, W. K. Soft Matter 2009, 5, 3823–3826. (50) Chen, T.; Zhang, Z.; Glotzer, S. C. Langmuir 2007, 23, 6598–6605.
13750 DOI: 10.1021/la101982c
Mock and Zukoski
Conclusions Conditions have been designed to synthesize dicolloids with chemical anisotropy using seeded emulsion polymerization. Particles with charge anisotropy can be created by coating dicolloid seeds and protrusions with different chemical species. We expect that similar particles can be formed by swelling polymer seeds with monomer that has different chemical properties from that of the monomer, and preliminary results show that swelling the seed with toluene and NiPAM results in a poly-NiPAM protrusion. The advantage of the described technique for synthesizing shape and interaction energy anisotropic particles is the versatility of free radical polymerization, the large volumes where particles can be synthesized, the uniformity of the resulting particles, and the extensive variety of particle shapes that can be achieved.48,49 Of particular interest for studies of complex clustering and phase behavior,1-13,50 there will be a need to characterize the strengths of the anisotropic interactions and to ensure that the strengths are sufficiently small that aggregates are reversible such that thermal fluctuations will result in spontaneous assembly. Acknowledgment. This material is based upon work supported by the U.S. Department of Energy, Division of Materials Science under award no. DE-FG02-07ER46471 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. Scanning electron microscopy was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which are partially supported by the U.S. Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471. We thank Dr. Duohai Pan at the Imaging Technology Group at the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign for help with dark-field imaging.
Langmuir 2010, 26(17), 13747–13750
