pubs.acs.org/Langmuir © 2009 American Chemical Society
Amphoteric Core-Shell Microgels: Contraphilic Two-Compartment Colloidal Particles Kostas E. Christodoulakis and Maria Vamvakaki* Institute of Electronic Structure and Laser Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece and Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 710 03 Heraklion, Crete, Greece Received June 21, 2009. Revised Manuscript Received August 25, 2009 pH-responsive amphoteric core-shell microgel particles were synthesized by emulsion copolymerization of the appropriate functional monomers with ethylene glycol dimethacrylate as the cross-linker. 2-(Diethylamino)ethyl methacrylate (DEA) was used as the ionizable basic monomer, and tert-butyl methacrylate served as the hydrophobic monomer precursor, which gave the methacrylic acid (MAA) moieties following acid hydrolysis of the ester groups. The core of the polyampholyte microgels comprised a cross-linked poly(2-(diethylamino)ethyl methacrylate) (PDEA) or poly(methacrylic acid) (PMAA) network surrounded by a cross-linked PMAA or PDEA shell, respectively. A polyampholyte random copolymer microgel with the DEA and MAA units randomly distributed within the gel phase was also prepared. Scanning electron microscopy studies showed spherical particles of a narrow size distribution, and transmission electron microscopy verified the core-shell topology of the particles. Potentiometric titration curves revealed two plateau regions for the polyampholyte core-shell microgels attributed to the independent ionization process of the core and the shell of the particles, in contrast to the random copolymer microgel particles that exhibited a single plateau region as a result of the simultaneous protonation/deprotonation process of the basic and acidic moieties of the microgels. The core and the shell of the particles were found to swell independently upon ionization of the DEA or MAA moieties at low or high pH, respectively, whereas collapsed latex particles were obtained in the intermediate pH range when both the core and the shell of the particles were neutral, in agreement with the potentiometric titration data. These core-shell microgels comprise novel two-compartment nanostructures that exhibit contraphilic properties in the core and the shell of the particles in response to a single external stimulus.
Introduction Responsive microgels typically comprise lightly cross-linked latex particles of submicrometer dimensions that can become highly swollen in response to certain external stimuli. These materials have attracted particular attention because of their potential applications in various fields such as drug delivery,1-6 *Author to whom correspondence should be addressed. E-mail: vamvakak @iesl.forth.gr. (1) Vinogradov, S. V. Curr. Pharm. Des. 2006, 12, 4703–4712. (2) Lopez, V. C.; Hadgraft, J.; Snowden, M. J. Int. J. Pharm. 2005, 292, 137–147. (3) Das, M.; Mardyani, S.; Chan, W. C.; W.; Kumacheva, E. Adv. Mater. 2006, 18, 80–83. (4) Nayak, S.; Lee, H.; Chmielewski, J.; Lyon, L. A. J. Am. Chem. Soc. 2004, 126, 10258–10259. (5) Qiu, X. P.; Leporatti, S.; Donath, E.; Mohwald, H. Langmuir 2001, 17, 5375–5380. (6) Eichenbaum, G. M.; Kiser, P. F.; Dobrynin, A. V.; Simon, S. A.; Needham, D. Macromolecules 1999, 32, 4867–4878. (7) Elaissari, A. A.; Ganachaud, F.; Pichot, C. In Colloid Chemistry II; Antonietti, M., Ed.; Topics in Current Chemistry; Springer: Berlin, 2003; Vol. 227, pp 169-193. (8) Murthy, N.; Xu, M. C.; Schuck, S.; Kunisawa, J.; Shastri, N.; Frechet, J. M. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4995–5000. (9) Ogawa, K.; Wang, B.; Kokufuta, E. Langmuir 2001, 17, 4704–4707. (10) Ali, M. M.; Su, S.; Filipe, C. D. M.; Pelton, R.; Li, Y. Chem. Commun. 2007, 4459–4461. (11) Lynn, D. M.; Amiji, M. M.; Langer, R. Angew. Chem., Int. Ed. 2001, 113, 1757–1760. (12) Grabstain, V.; Bianco-Peled, H. Biotechnol. Prog. 2003, 19, 1728–1733. (13) Serpe, M. J.; Yarmey, K. A.; Nolan, C. M.; Lyon, L. A. Biomacromolecules 2005, 6, 408–413. (14) Cao, R.; Gu, Z. Y.; Patterson, G. D.; Armitage, B. A. J. Am. Chem. Soc. 2004, 126, 726–727. (15) Saunders, B. R.; Laajam, N.; Daly, E.; Teow, S.; Hu, X.; Stepto, R. Adv. Colloid Interface Sci. 2009, 147, 251–262. (16) Hendrickson, G. R.; Lyon, L. A. Soft Matter 2009, 5, 29–35. (17) Ulbricht, M. J. Chromatogr., B 2004, 804, 113–125.
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biotechnology,7-15 sensor technology,16 membranes,17 catalysis,18,19 and magnetic20 and semiconducting materials.21,22 Photo-, thermo-, and pH-responsive microgels have been reported in the literature.23-32 The latter comprise acidic or basic monomer repeat units and exhibit reversible swelling properties in response to changes in the solution pH.33-40 pH-responsive (18) Zhang, M. C.; Zhang, W. Q. J. Phys. Chem. C 2008, 112, 6245–6252. (19) Li, D. J.; Dunlap, J. R.; Zhao, B. Langmuir 2008, 24, 5911–5918. (20) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H. A. P. Langmuir 2004, 20, 10706–10711. (21) Zhang, J. G.; Xu, S. Q.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908–7914. (22) Gao, J.; Hu, Z. B. Langmuir 2002, 18, 1360–1367. (23) Bradley, M.; Vincent, B.; Warren, N.; Eastoe, J.; Vesperinas, A. Langmuir 2006, 22, 101–105. (24) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546–5552. (25) Gan, D. J.; Lyon, L. A. Macromolecules 2002, 35, 9634–9639. (26) Gan, D. J.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511–7517. (27) Peng, S. F.; Wu, C. J. Phys. Chem. B 2001, 105, 2331–2335. (28) Peng, S. F.; Wu, C. Macromolecules 2001, 34, 568–571. (29) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482–487. (30) Dalmont, H.; Pinprayoon, O.; Saunders, B. R. Langmuir 2008, 24, 2834–2840. (31) Nayak, S.; Lyon, L. A. Chem. Mater. 2004, 16, 2623–2627. (32) Garcia, A.; Marquez, M.; Cai, T.; Rosario, R.; Hu, Z.; Gust, D.; Hayes, M.; Vail, S. A.; Park, C.-D. Langmiur 2007, 23, 224–229. (33) Amalvy, J. I.; Wanless, E. J.; Li, Y.; Michailidou, V.; Armes, S. P.; Duccini, Y. Langmuir 2004, 20, 8992–8999. (34) Palioura, D.; Armes, S. P.; Anastasiadis, S. H.; Vamvakaki, M. Langmuir 2007, 23, 5761–5768. (35) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381–3387. (36) Dupin, D.; Rosselgong, J.; Armes, S. P.; Routh, A. F. Langmuir 2007, 23, 4035–4041. (37) Dupin, D.; Armes, S. P.; Connan, C.; Reeve, P.; Baxter, S. M. Langmuir 2007, 23, 6903–6910. (38) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544–2550.
Published on Web 09/15/2009
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microgels become highly swollen when ionized because of the electrostatic repulsive forces between the charged monomer repeat units and the counterion partitioning in the gel phase and collapse when they become neutral. Responsive polyampholyte microgel particles consist of both negatively and positively charged monomer units.41-48 These microgels swell when ionized at low and high pH, whereas in the intermediate pH range they possess a mobility close to zero, a minimum in their size, and a negative second virial coefficient because of the overall charge neutralization near their isoelectric point.43 Polyampholyte random copolymer microgel particles also possess unique “antipolyelectrolyte” behavior and swell with increasing salt concentration, in contrast to their polyelectrolyte analogues.45,46 Another type of microgel particle that has attracted attention lately is that with a core-shell topology.26,49-57 These microgels are prepared by a two-step polymerization process involving first the synthesis of the cores of the particles that serve as the seeds for the formation of the shells in the second reaction step. Ballauff and co-workers investigated the volume transition in colloidal core-shell particles comprising a polystyrene core and a thermosensitive cross-linked poly(N-isopropylacrylamide) (PNIPAM) shell by small-angle X-ray scattering.58 The core-shell microgels exhibited distinctively different solution behavior compared to the PNIPAM homopolymer thermosensitive gels because of the presence of a solid boundary between the core and the shell of the particles, which decreased the maximum degree of swelling of the shell at low temperatures but also prevented the full collapse of the network above the volume transition temperature of PNIPAM. Hollow thermoresponsive PNIPAM microgels were also synthesized from core-shell nanoparticles upon selective degradation of the particle core.59 The hollow particles exhibited higher equilibrium swelling volumes below the lower critical solution temperature (LCST) and greater deswelling ratios at temperatures above the LCST compared to those of the nondegraded core-shell particles as a result of the loss of material from the core. Multiresponsive core-shell microgels consist of a core that is responsive to a particular stimulus, whereas the shell is sensitive to a different external stimulus.55,56 Li et al. prepared narrowly (39) Debord, J. D.; Lyon, L. A. Langmuir 2003, 19, 7662–7664. (40) Suzuki, D.; Tsuji, S; Kawaguchi, H. J. Am. Chem. Soc. 2007, 129, 8088– 8089. (41) Harding, I. H.; Healy, T. W. J. Colloid Interface Sci. 1982, 89, 185–201. (42) Bradley, M.; Vincent, B.; Burnett, G. Aust. J. Chem. 2007, 60, 646–650. (43) Tan, B. H.; Ravi, P.; Tam, K. C. Macromol. Rapid Commun. 2006, 27, 522– 528. Tan, B. H.; Ravi, P.; Tan, L. N.; Tam, K. C. J. Colloid Interface Sci. 2007, 309, 453–463. Ho, B. S.; Tan, B. H.; Tan, J. P. K.; Tam, K. C. Langmuir 2008, 24, 7698– 7703. (44) Ni, H.; Kawaguchi, H.; Endo, T. Macromolecules 2007, 40, 6370–6376. (45) Hampton, K. W.; Ford, W. T. Macromolecules 2000, 33, 7292–7299. (46) Neyret, S.; Vincent, B. Polymer 1997, 38, 6129–6134. (47) Das, M.; Kumacheva, E. Colloid Polym. Sci. 2006, 284, 1073–1084. (48) Ogawa, K.; Nakayama, A.; Kokufuta, E. Langmuir 2003, 19, 3178–3184. (49) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301–8306. (50) (a) Berndt, I.; Pedersen, J. S.; Richtering, W. Angew. Chem., Int. Ed. 2006, 45, 1737–1741. (b) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780–8785. (c) Berndt, I.; Pedersen, J. S.; Richtering, W. J. Am. Chem. Soc. 2005, 127, 9372–9373. (d) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2006, 22, 459–468. (51) Nayak, S.; Lyon, L. A. Angew. Chem., Int. Ed. 2004, 43, 6706–6709. (52) Lu, Y.; Wittemann, A.; Ballauff, M.; Drechsler, M. Macromol. Rapid Commun. 2006, 27, 1137–1141. (53) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988–1993. (54) Blackburn, W. H.; Lyon, A. Colloidal Polymer Science 2008, 286, 563–569. (55) Lapeyre, V.; Ancla, C.; Catargi, B.; Ravaine, V. J. Colloid Interface Sci. 2008, 327, 316–323. (56) Li, X.; Zuo, J.; Guo, Y. L.; Yuan, X. H. Macromolecules 2004, 37, 10042– 10046. (57) Bradley, M.; Vincent, B. Langmuir 2008, 24, 2421–2425. (58) Dingenouts, N.; Norhausen, Ch.; Ballauff, M. Macromolecules 1998, 31, 8912–8917. (59) Nayak, S.; Gan, D.; Serpe, M. J.; Lyon, L. A. Small 2005, 1, 416–421.
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distributed spherical core-shell nanogels comprising a temperature-responsive cross-linked PNIPAM core and a pH-sensitive poly(4-vinylpyridine) (P4VP) shell.56 They showed that the temperature-induced volume phase transition of the PNIPAM core was not significantly affected by the P4VP shell whereas the deswelling of the latter, upon deprotonation of the 4VP moieties, was influenced by the core-shell topology. However, P(NIPAMco-acrylic acid)-based core-shell microgel particles displayed complex pH-dependent swelling behavior and a multistep volume phase transition at high pH when the acrylic acid units were highly charged.49 Recently, Bradley and co-workers employed pHresponsive core and temperature-sensitive shell microgel particles for the controlled uptake and release of active species.57 The uptake and release of an anionic surfactant from the microgels were investigated as a function of solution pH and temperature and indicated that the electrostatic attraction between the anionic surfactant and the cationic core of the microgel particles dominated the absorption of the surfactant molecules. In the present study, we report the synthesis of novel polyampholyte core-shell microgel particles comprising 2-(diethylamino)ethyl methacrylate (DEA) and methacrylic acid (MAA) moieties. Scanning electron microscopy (SEM) was used to determine the shape and the size distribution of the microgels whereas transmission electron microscopy (TEM) was employed to verify their core-shell topology. The complex ionization and swelling behavior of the microgel particles were investigated by potentiometric titrations and dynamic light scattering (DLS), respectively. To the best of our knowledge, this is the first example of contraphilic core-shell microgel particles in which the charge, hydrophilicity, size, and softness of the core and the shell of the particles can be tuned independently in a reverse manner by a single external stimulus and are opposite in the two microgel compartments at high and low pH values. This renders these materials very attractive for use in numerous applications such as in controlled drug delivery and catalysis.
Experimental Section Materials. All chemicals used were commercially available and were purchased from Sigma-Aldrich (Germany). DEA, tert-butyl methacrylate (t-BuMA), and ethylene glycol dimethacrylate (EGDMA) were passed through a basic alumina column to remove the inhibitor before use. Potassium persulfate (K2S2O8), methoxy poly(ethylene glycol) methacrylate (PEGMA) (Mn = 2000 g/mol), sodium dodecyl sulfate (SDS), dioctyl sulfosuccinate salt (AOT), trifluoroacetic acid (TFA), tetrahydrofuran (THF), and dichloromethane (DCM) were used as received. Milli-Q water (18.2 MΩ) and 0.1 and 0.5 M HCl and NaOH standard solutions were used for all preparations. Microgel Synthesis. Microgel particles with core-shell topology comprising a poly(2-(diethylamino)ethyl methacrylate) (PDEA) or a poly(tert-butyl methacrylate) (P(t-BuMA)) core and a P(t-BuMA) or PDEA shell, respectively, were prepared. The chemical formulas of PDEA and P(t-BuMA) are shown in Figure S1 (Supporting Information). The synthesis procedure followed for the preparation of the core-shell particles involved a two-step polymerization process.49 First, the core of the microgels was formed by emulsion copolymerization of the appropriate functional monomer with the cross-linker in the presence of an anionic stabilizer, SDS or AOT. In the next step, the second monomer was copolymerized with the cross-linker in the presence of the core particles by seed polymerization to form the cross-linked shells. A polymerizable stabilizer, PEGMA, was used in the second step of the synthesis, which conferred steric stabilization to the core-shell microgel particles. The use of a polymerizable steric stabilizer in the shell ensures that the amphoteric microgels obtained after deprotection Langmuir 2010, 26(2), 639–647
Christodoulakis and Vamvakaki of the t-BuMA units will remain stable over the whole pH range, whereas an ionic stabilizer could cause particle flocculation near the IEP point as as result of overall charge neutralization.43 A typical synthesis procedure followed for the preparation of the PDEA core-P(t-BuMA) shell microgel particles is summarized below. Water (37.6 g) and SDS (0.24 g, 20 wt % based on the monomer) were added to the reaction flask. A mixture of DEA (1.2 g) and EGDMA (0.012 g, 1 wt % based on the monomer) was added next under stirring. The solution was degassed and stirred for 30 min under nitrogen at 65 °C. The polymerization commenced on addition of a previously degassed aqueous solution of the K2S2O8 initiator (0.012 g, 1 wt % based on the monomer). The reaction was allowed to proceed for 5 h. A sample was extracted for analysis before proceeding to the second step of the polymerization. The electrostatically stabilized PDEA latexes prepared in the first step served as the core particles for the synthesis of the cross-linked shells by seed polymerization. The dispersion of the core particles was heated again to 65 °C, and a degassed mixture of water (10 g), t-BuMA monomer (1.2 g), EGDMA (0.012 g, 1 wt % based on the monomer), K2S2O8 (0.012 g, 1 wt % based on the monomer), and PEGMA stabilizer (0.24 g, 20 wt % based on the monomer) was added under a nitrogen atmosphere via a double-tipped needle over a period of 30 min. The reaction was allowed to proceed for 6 h before cooling with water. After the synthesis, the excess stabilizers and traces of unreacted monomers and initiator were eliminated by ultrafiltration in order to purify the core-shell latex particles. P(t-BuMA) core-PDEA shell microgel particles were also prepared following a similar procedure in which the order of monomer addition was reversed and AOT was used instead of SDS as the anionic stabilizer for the synthesis of the P(t-BuMA) core particles. Random copolymer microgel particles comprising DEA and t-BuMA monomer repeat units randomly distributed within the particle gel phase were also prepared in a single polymerization step by the simultaneous addition of the two monomers to an aqueous solution of the polymerizable PEGMA stabilizer. Deprotection of the Microgel Particles. The t-BuMA monomer repeat units of the random copolymer and the coreshell microgel particles were deprotected by acid hydrolysis to obtain the MAA moieties. Hydrolysis was carried out by TFA in DCM.60 In a typical reaction, 5 mL of the as-synthesized microgel dispersion was first dried in a vacuum oven to remove all traces of water before being redispersed in 5 mL of DCM. TFA (10-fold excess with respect to the number of moles of t-BuMA) was next added to the DCM solution. The solution was allowed to stir for 3 days before the addition of 10 mL of a 50 v/v% THF/water mixture, followed by a 10 M NaOH solution (the number of moles of NaOH was equal to the number of moles of added TFA) to neutralize the added acid. The organic solvents were removed under vacuum to obtain the deprotected microgel particles in water. Finally, the excess salt formed by the addition of TFA and NaOH was eliminated by ultrafiltration, and the pure microgel particles were isolated. Potentiometric Titrations. Milli-Q (18.2 MΩ) water was used for the preparation of all samples. A 20 mL aqueous microgel dispersion at c = 0.2 wt % was prepared by serial dilution of a 5 wt % stock solution. The solution pH was then adjusted to pH ∼2 using 0.5 M HCl, and the sample was stirred for 24 h. Titration curves for the microgels dispersions were obtained by monitoring the increase in the solution pH upon addition of 0.1 M NaOH (increments of 20 to 100 μL). The pH was measured with a Crison (GLP 21) pH meter and titration curves in the pH range from 2 to 12 were obtained. Dynamic Light Scattering. Samples for dynamic light scattering were prepared following a procedure similar to that described above. A 5 wt % aqueous microgel dispersion was first diluted with milli-Q water prefiltered through a 0.2 μm filter to (60) Li, Z.; Day, M.; Ding, J.; Faid, K. Macromolecules 2005, 38, 2620–2625.
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Article a concentration of c = 0.005 wt %. Next, the net charge of the microgel sample was adjusted to the required value by the addition of 0.1 M NaOH or 0.1 M HCl, and the solution was stirred for 24 h before the measurement. We define as the effective net charge of the microgel the ratio CHCl/Cm or CNaOH/Cm where CHCl is the concentration of added acid, CNaOH is the concentration of added base, and Cm is the polymer concentration in the sample in terms of ionizable monomeric units (DEA or MAA) assuming a complete yield of the polymerization reaction. When defined in this way, the net charge approximates the degree of ionization of the ionizable monomer repeat units and takes values between -1 (100% ionized MAA) and 1 (100% ionized DEA) if one assumes that all of the added acid or base ionizes the polymer. The samples were filtered carefully through a 5 μm pore size filter and were left to equilibrate for about 1 h before being measured. A ALV spectrophotometer with an Nd:YAG laser at λ = 532 nm was used, and all measurements were performed at 20 °C. (See Supporting Information for a detailed experimental description.) Transmission Electron Microscopy. Samples for TEM were prepared following a procedure similar to that described above for the DLS experiments. All samples were previously diluted with water to c = 0.05 wt % and adjusted to the appropriate microgel net charge by the addition of HCl or NaOH. Potassium hexachloroplatinate and cadmium nitrate at a monomer repeat unit/ salt 2/1 molar ratio were used to selectively stain the DEA and MAA units, respectively, and provide the necessary contrast for TEM imaging. A drop of the diluted sample was then placed on a carbon-coated copper grid and was left to dry in air overnight. A JEOL JEM-100C instrument at an electron accelerating voltage of 80 kV was employed for the measurements. Scanning Electron Microscopy. The microgel samples for SEM were prepared by serial dilution of a 5 wt % stock solution with Milli-Q water to c = 0.05 wt %. The samples were spincoated onto glass slides at 1000 rpm for 45 s. A JEOL JSM6390LV instrument at an electron acceleration voltage of 10 kV was used for the measurements.
Results and Discussion Synthesis of the Microgel Particles. The synthesis of the microgel particles was followed by DLS. The composition of the two monomers in the feed was kept constant at 50/50 wt/wt % (56/44 mol/mol %) t-BuMA/DEA for all microgel syntheses. A sample of the microgel core was extracted from the reaction flask after the completion of the first step of the polymerization and was analyzed by DLS. The hydrodynamic size of the core particles was compared to that of the core-shell particles obtained after the second step of the synthesis. Figure 1 shows the autocorrelation functions for 0.005 wt % aqueous dispersions of the PDEA core and the PDEA core-P(t-BuMA) shell microgel particles at a 60° scattering angle. The inset shows the distribution of relaxation times from the inverse Laplace transformations of the correlation functions. A single process with very strong intensity and diffusive dynamics dominates the autocorrelation functions of the core particles. From the distribution of relaxation times, the diffusion coefficient D ¼ limq f 0 ðΓ=q2 Þ for this process is 2.43 10-8 cm2/s and the corresponding hydrodynamic radius, Rh, is 90 ( 2 nm, attributed to the hydrophobic PDEA latex particles. After the formation of the shell by seed polymerization, a single process is again obtained with a diffusion coefficient of 1.82 10-8 cm2/s, which corresponds to a hydrodynamic radius of 120 ( 2 nm. The increase in hydrodynamic size from 90 nm for the core particles to 120 nm for the core-shell microgels suggests the successful synthesis of the core-shell structure. Using the monomer volume ratio in the feed and assuming full conversion, the geometrically calculated increase in size for a 90 nm sphere upon addition of the shell was found to be 114 nm, which is in good agreement with the size found by DLS DOI: 10.1021/la902231b
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for the as-prepared dense core-shell latex particles. Although the hydrodynamic size is not directly comparable to the calculated geometrical size, the above results verify the incorporation of a single core particle within the cross-linked shell and thus the formation of true core-shell structures. Similar results were obtained for the P(t-BuMA) core-PDEA shell microgel particles (not shown). The diffusion coefficient of the single process obtained for the core sample was 3.28 10-8 cm2/s corresponding to a hydrodynamic radius of 67 ( 2 nm, which was attributed to the hydrophobic P(t-BuMA) latex particles, whereas the diffusivity for the core-shell sample was 2.38 10-8 cm2/s corresponding to a hydrodynamic radius of 92 ( 2 nm. As discussed above, the increase in the hydrodynamic size after the formation of the shell suggests the successful synthesis of the core-shell structure. The theoretically calculated increase in size for a 67 nm sphere upon the addition of the shell, based on the monomer feed ratio, was 85 nm for the core-shell particles, which is in good agreement with the size found for the collapsed latex particles by DLS and verifies the incorporation of a single core in each core-shell particle. Finally, random copolymer microgel particles comprising DEA and t-BuMA monomer repeat units randomly distributed within the gel phase were also synthesized. A single process was again identified in the autocorrelation function of the random
Figure 1. Intensity autocorrelation functions of a PDEA core microgel (0) and a PDEA core-P(t-BuMA) shell microgel (O) dispersion at c = 0.005 wt % and a 60° scattering angle. Insets: distributions of relaxation times multiplied by the total scattering intensity (normalized to that of toluene) for the respective microgel samples.
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copolymer microgel particles with very strong intensity and a diffusion coefficient of 1.29 10-8 cm2/s corresponding to a hydrodynamic radius of 170 ( 2 nm that was attributed to the hydrophobic PDEA-P(t-BuMA) random copolymer latex particles. Following acid hydrolysis of the t-BuMA ester groups, polyampholyte random copolymer and core-shell microgel particles were obtained. The large size of the particles prohibited their characterization by NMR spectroscopy, and the presence of PDEA and PEGMA hindered their analysis by FTIR spectroscopy in order to quantify the conversion of t-BuMA to MAA. However, we have carried out the deprotection of linear P(t-BuMA) to poly(methacrylic acid) (PMAA) using similar conditions and found quantitative hydrolysis by NMR. (See Figure S2 and relevant text in Supporting Information.) Moreover, the potentiometric titration data discussed below suggest nearly complete conversion to MAA. SEM and TEM Characterization of the Microgel Particles. Figure 2 shows the SEM images for the hydrophobic PDEA-P(t-BuMA) random copolymer microgel particles before deprotection and the polyampholyte PDEA-PMAA random copolymer microgel particles after hydrolysis of the ester groups at a net charge equal to zero. From the SEM images, particles with a spherical shape and a very narrow size distribution were observed both before and after deprotection with diameters of 340 ( 13 and 350 ( 27 nm, respectively, which are in good agreement with the DLS results discussed below (Dh = 360 and 370 nm, respectively). It should be noted that the random copolymer microgel particles were very uniform in size and retained their spherical shape and narrow size distribution after deprotection, suggesting that the particle morphology was not affected by the mild conditions used for the hydrolysis of the ester groups. The results for the core-shell microgel particles were similar. Before deprotection, P(t-BuMA) core-PDEA shell spherical particles of a narrow size distribution were observed (Figure S3a, Supporting Information) with a diameter of 170 ( 10 nm (Dh = 184 nm). The core-shell microgels retained their spherical shape and narrow size distribution after hydrolysis (Figure S3b, Supporting Information), whereas their diameter increased to 177 ( 20 nm in agreement with the DLS results discussed below (Dh = 204). The minimal particle coalescence observed for the microgels both before and after deprotection was attributed to the soft nature and the film-forming properties of the PDEA shell. The random copolymer and core-shell particles were also characterized by TEM upon selective staining of the weak base
Figure 2. SEM images of the PDEA-P(t-BuMA) random copolymer latex particles (a) and the PDEA-PMAA random copolymer microgel particles after deprotection at degree of ionization equal to zero (b). 642 DOI: 10.1021/la902231b
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Figure 3. TEM images of the P(t-BuMA) core-PDEA shell microgel particles stained with potassium hexachloroplatinate (a) and the PMAA core-PDEA shell microgel particles stained with cadmium nitrate (b) and potassium hexachloroplatinate (c).
or weak acid residues. TEM images of the PDEA-PMAA random copolymer microgel particles stained with K2PtCl6 and cadmium nitrate (Cd(NO3)2) are shown in Figure S4a,b (Supporting Information), respectively. The metal species were incorporated within the ionized microgels at a monomer/salt 2/1 molar ratio at low pH for PtCl62-, which interacted selectively with the positively charged DEA units, and at high pH for Cd2þ, which bound to the negatively charged MAA groups. The successful staining of the microgels by both the positively and negatively charged metal-containing species verified the presence of the DEA and MAA moieties within the particles. Figure 3a shows the P(t-BuMA) core-PDEA shell microgels before deprotection, stained with K2PtCl6, which allowed the selective imaging of the PDEA shell. A lighter-colored core attributed to the hydrophobic P(t-BuMA) domain that was not stained is clearly observed, surrounded by a darker-colored ring due to the stained PDEA shell, which verifies the core-shell topology of the microgel particles. The sharp interface observed between the core and the shell of the particles suggests the absence of significant partitioning and interpenetration of the monomer from the shell synthesis into the seed particles. The TEM images of the polyampholyte PMAA core-PDEA shell microgel particles after deprotection, stained with cadmium nitrate (Cd(NO3)2) (Figure 3b) and potassium hexachloroplatinate (K2PtCl6) (Figure 3c), are shown. In Figure 3b, the PMAA core of the particles was selectively stained by the Cd2þ cations, which resulted in the observed darker-colored core surrounded by the lighter-colored unstained PDEA shell and verified the core-shell topology of the polyampholyte microgels after deprotection. However, when the same particles were stained with K2PtCl6 (Figure 3c) they showed a rather uniform distribution of the metal species, which was attributed to the partial diffusion of the metal Langmuir 2010, 26(2), 639–647
ions within the marginally hydrophilic PMAA core of the particles and hindered the selective staining of the microgel shell. Ionization and Swelling Behavior of the Amphoteric Microgel Particles. The ionization behavior of the amphoteric microgel particles was investigated as a function of solution pH by potentiometric titration whereas their swelling properties were studied by DLS. The hydrodynamic radii of the particles before deprotection and the pKa values and minimum and maximum hydrodynamic sizes after hydrolysis of the ester groups are summarized in Table 1. Figure 4 shows the potentiometric titration curve for the PDEA-PMAA random copolymer microgel particles. A plateau region is observed in the titration curve at around pH 7 signifying the presence of the ionizable tertiary amine and methacrylic acid groups, which behave as a weak base and a weak acid, respectively, and participate in an acid-base equilibrium in solution (Figure S1, Supporting Information). It should be noted that the single plateau region in the titration curve of these random copolymer microgel particles signifies the simultaneous deprotonation (from left to right) of the ionizable DEA and MAA groups and is attributed to the similar effective pKa values reported for the two respective homopolymer microgels, 5.9 for the PDEA and 7.2 for the PMAA particles, and the random distribution of the two monomer repeat units within the microgel particle.61,62 In the inset, the plateau region is plotted as the solution pH versus the net charge of the microgel assuming that the added base is used for the deprotonation of the DEA units and the neutralization of the MAA groups. A net charge of 1 (deflection point at low pH) (61) Christodoulakis, K. E.; Vamvakaki M. Macromol. Symp., 2009, accepted for publication. (62) Homola, A.; James, R. O. J. Colloid Interface Sci. 1977, 59, 123–134.
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Christodoulakis and Vamvakaki Table 1. pKa Values and Hydrodynamic Radii of the Microgel Particles before deprotection Rh core (nm)
Rh core-shell (nm)
random copolymer PDEA -PMAA 170 PDEA core-PMAA shell 90 120 PMAA core-PDEA shell 67 92 a Isoelectric point of the polyampholyte microgel particles
Figure 4. Potentiometric titration curve for a 0.2 wt % solution of the PDEA-PMAA random copolymer microgel. (Inset) Potentiometric titration data plotted as pH versus the microgel net charge in the range of -1 < net charge < 1.
Figure 5. Hydrodynamic radius (Rh) of the PDEA-PMAA random copolymer microgel particles as a function of the microgel net charge in the range of -1 < net charge < 1. The solid line serves as a guide to the eye.
signifies that all of the DEA monomer repeat units in the microgel are positively charged and the MAA residues are neutral. The decrease in the particle net charge from 1 to 0 as NaOH is added to the solution signifies the gradual deprotonation of the DEA units or the neutralization of the MAA moieties. At net charge equal to 0, the number of protonated DEA units is equal to the number of neutralized MAA groups and the pH value of 6.8 corresponds to the isoelectric point of the microgel. When the pH increases above the isoelectric point, the number of negatively charged MAA units is higher than the positively charged DEA groups and the microgel attains a negative overall net charge. Finally, at net charge equal to -1 the MAA units are all neutralized and negatively charged and the DEA groups are deprotonated and neutral. 644 DOI: 10.1021/la902231b
after deprotection pKa core
pKa shell
Rh max low pH (nm)
Rh min zero net charge (nm)
Rh max high pH (nm)
8.8 4.8
234 180 152
185 135 102
330 217 119
6.8a 5.3 9.0
The swelling behavior of the polyampholyte random copolymer microgels was followed by DLS as a function of the microgel net charge. First, the amphoteric random copolymer microgels obtained after hydrolysis of the t-BuMA ester groups were studied at their isoelectric point (pH 6.8). A single process with very strong intensity and diffusive dynamics dominated the autocorrelation functions. The diffusion coefficient for this process was 1.18 10-8 cm2/s corresponding to a hydrodynamic radius of 185 ( 2 nm. The increase in size from 170 nm for the hydrophobic PDEA-P(t-BuMA) random copolymer latex particles before deprotection to 185 nm for the polyampholyte PDEA-PMAA random copolymer particles after hydrolysis was attributed to the greater hydrophilic nature of PMAA compared to that of P(t-BuMA), which causes the swelling of the microgels. Figure 5 shows the hydrodynamic radii for the PDEA-PMAA random copolymer particles as a function of the microgel net charge. The left part corresponds to a larger number of ionized MAA units at high pH and thus to a negative net charge, and the right part of the plot represents excess protonated DEA units at low pH when the particles attain a positive overall net charge. The net charge is zero at the isoelectric point. As seen in Figure 5, the microgels swell at both high and low pH when they possess an excess of either positive or negative charge. This swelling is due to the electrostatic repulsions and the osmotic pressure created within the microgels by the counterions to the charged monomer repeat units. The minimum size of microgels observed at the isoelectric point is attributed to charge neutralization, which results in the collapse of the particles in the aqueous medium.42-44 It is also interesting that the degree of swelling of the microgels is higher for a negative net charge compared to that at a positive net charge. This was partially attributed to the slightly higher mole fraction of MAA (56 mol %) compared to that of DEA (44 mol %) in the particles and, more importantly, to the greater hydrophilic nature of the ionized MAA monomer repeat units compared to the protonated DEA moieties.61 The maximum volumetric swelling factors (VSFs) calculated to be the cube of the ratio of the maximum hydrodynamic radii of the swollen particles over the hydrodynamic radius of the particles at zero net charge were found to be 5.6 and 2.1 at a negative and a positive net charge, respectively. These values are more than 2 times lower than those reported for the two homopolymer microgels (13.4 for PMAA and 5.8 for PDEA) in an earlier study61 and are attributed to the copolymerization of the two monomers in the microgel. At high pH (negative net charge), only the excess ionized MAA units cause the swelling of the particles, whereas the neutral, hydrophobic DEA units and the neutralized MAA and DEA units lead to the collapse of the microgel. Similarly, at low pH (positive net charge), the excess ionized DEA units result in the swelling of the particles and the neutral MAA units and the neutralized DEA and MAA units hinder the extensive swelling of the microgel. The equilibrium swelling of the random copolymer microgel particles is thus dictated by the interplay between the excess ionized monomer repeat units that Langmuir 2010, 26(2), 639–647
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Figure 6. Potentiometric titration curve for a 0.2 wt % solution of the PDEA core-PMAA shell microgel (a) and the potentiometric titration data plotted as pH vs the degree of ionization (R) of the PMAA shell and the PDEA core in the range of 0 < R < 1 (b).
cause their swelling and the hydrophobic and electrostatic interactions within the particles that contribute to their shrinkage. Next, the solution behavior of the core-shell particles was investigated in water. Figure 6a shows the potentiometric titration curve for the PDEA core-PMAA shell microgel particles. It is noted that two plateau regions are observed in the titration curve of the core-shell microgels. This is in contrast to the single plateau region found above for the random copolymer PDEAPMAA microgel particles, which suggested the simultaneous ionization of the basic and acidic moieties within the microgels. In the core-shell particles, the two types of ionizable groups were spatially separated and became ionized independently in a different pH range. (See Supporting Information for further discussion.) This is a very interesting characteristic that verifies the core-shell topology of the particles and allows the independent tuning of the properties of the core and the shell of the particles by a single external stimulus, leading to true contraphilic core-shell particles. Between the two plateau regions at around pH 7.6, the particles are not charged and comprise a hydrophobic PDEA core and a neutral PMAA shell. Below this pH value, the DEA groups in the core of the microgels become protonated and positively charged and the MAA units in the shell of the particles are neutral. Above pH 7.6, the MAA groups in the shell of the microgels become neutralized and negatively charged whereas the DEA monomer repeat units in the core of the particles are deprotonated and hydrophobic. From the two plateau regions, the effective pKa values for the PMAA shell and the PDEA core were calculated as the pH at 50% ionization (Figure 6b). A pKa of 5.3 was obtained for the PDEA core (Figure 6b, bottom), which is similar to the value reported earlier for the PDEA homopolymer microgel (pKa 5.9).61 However, for the PMAA shell an effective pKa of 8.8 (Figure 6b, top) was calculated, which is more than 1 unit greater than the effective pKa found for the PMAA homopolymer microgel (pKa 7.2).59 This was attributed to the increased hydrophobicity of the core-shell particles as a result of the presence of the hydrophobic PDEA core that led to a lower dielectric constant and thus hindered the ionization of the MAA groups in the microgel shell.63 Finally, from the width of the two plateau regions the composition of the microgels was calculated as the mole fraction of the two components, DEA and MAA. The composition determined experimentally was 49 mol % DEA, which is in good agreement with the theoretical composition (44 mol % DEA) calculated from the monomer feed ratio used (63) Kali, G.; Georgiou, T. K.; Ivan, B.; Patrickios, C. S. J. Polym. Sci., Part A: Polym. Chem 2009, 47, 4289–4301.
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Figure 7. Hydrodynamic radius (Rh) and the solution pH (second y axis) of the PDEA core-PMAA shell microgel particles as a function of the microgel net charge in the range of -1 < net charge < 1. The solid line serves as a guide to the eye.
during synthesis and verifies the quantitative polymerization and the complete hydrolysis of t-BuMA to MAA. The swelling behavior of the core-shell particles as a function of the degree of ionization of the weak base and weak acid moieties was studied by DLS. Figure 7 shows the hydrodynamic radii of the core PDEA-shell PMAA particles as a function of the net microgel charge. On the second y axis, the measured solution pH of the samples is also shown. The left part of the plot (negative values on the x axis) corresponds to the ionization of the MAA repeat units at high pH, and the right part (positive values on the x axis) represents the protonation of the DEA units at low pH. The point at pH 7.85 that is very close to the pH value at maximum deflection in the titration curve corresponds to a microgel net charge equal to zero where neither the DEA nor the MAA moieties are charged. As seen in Figure 7, these contraphilic microgels swell at both high and low pH when either the shell or the core of the particles is ionized, whereas they exhibit a minimum size at zero net charge when both the core and the shell of the microgels are neutral and collapsed. It should be noted that the driving force for the collapse of the particles at zero net charge is different for the core-shell and the random copolymer microgel particles discussed above. The random copolymer particles collapsed because of charge neutralization at their isoelectric point when the number of positive and negative charges within the particle was equal, and hence their shrinkage is expected to depend on the ionic strength DOI: 10.1021/la902231b
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of the solution. However, the collapse of the core-shell particles occurred when both the core and the shell of the microgels became neutral, and thus it would not depend on the addition of salt. The reduction in particle size at extreme pH is attributed to the high ionic strength of the solution, which results in charge screening in the microgel particles. Assuming that the PMAA shell does not swell at low pH, a VSF of 5.5 was found for the PDEA core, which is in good agreement with the value obtained for the PDEA homopolymer microgel (VSF = 5.8)61 and suggests that the swelling of the PDEA core is not affected by the core-shell topology. This is attributed to the moderate hydrophilicity of the neutral PMAA shell, which allows water to diffuse into the core, and its sufficient elasticity, which can accommodate the increase in the size of the core. However, a VSF of 5.4 was found for the PMAA shell, which is significantly smaller than that obtained for the PMAA homopolymer microgel (VSF = 13.4)61 and suggests that the swelling of the shell is hindered by the presence of the collapsed core. This finding is in good agreement with an earlier study by Dingenouts et al.,58 which showed that the spatial constraint by the solid hydrophobic core boundary decreased the maximum degree of swelling of the cross-linked chains in the particle shell. For the particles discussed herein, the hydrophobic PDEA core forms a solid boundary that prohibits the full expansion of the ionized PMAA shell at high pH. The PMAA core-PDEA shell microgels exhibited similar ionization and swelling behavior. The potentiometric titration curve for the PMAA core-PDEA shell microgel particles is shown in Figure S5 (Supporting Information). Two plateau regions are again observed in the titration curve of the core-shell particles, verifying the contraphilic nature of the microgels; the first, at pH ∼5, again corresponds to the ionization of the tertiary amine moieties in the shell of the microgel, which become positively charged, and the second plateau, at pH ∼9, is due to the neutralization of the MAA groups in the microgel core, which acquires a negative charge. In the intermediate region at pH ∼7.6, the MAA units are protonated and the DEA groups are deprotonated and hence the particles are neutral. An effective pKa of 9.0 was found for the PMAA core (Figure S5b, top), and 4.8 was found for the PDEA shell (Figure S5b, bottom), which are similar to the values found for the PDEA core-PMAA shell microgel particles. The high effective pKa value found for PMAA was again attributed to the presence of the hydrophobic PDEA shell, which led to a lower dielectric constant and hindered the ionization of the MAA groups in the microgel. Finally, from the width of the two plateau regions the composition of the microgel was found to be 40-60 mol % DEA-MAA, which is in good agreement with the theoretical composition (44-56 mol % DEA-MAA) calculated from the monomer feed ratio used during synthesis and verifies the complete conversion of t-BuMA to MAA. The size of these core-shell microgels was studied as a function of the degree of ionization of the DEA and MAA units by DLS. Figure 8 shows the hydrodynamic radii of the core-shell microgels and the respective solution pH values as a function of the amphoteric microgel net charge. Again, the left part of the plot corresponds to the ionization of the MAA units at high pH and hence to a negative net microgel charge that caused the swelling of the particles whereas the right part represents the protonation of the DEA units at low pH, when the contraphilic microgels acquired a positive net microgel charge and swelled. At pH 7.42, neither the DEA nor the MAA moieties were ionized, and hence the particle net charge was equal to zero and the microgels collapsed. The reduction in the size of the microgels at extreme pH 646 DOI: 10.1021/la902231b
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Figure 8. Hydrodynamic radius (Rh) and solution pH (second y axis) of the PMAA core-PDEA shell microgel particles as a function of the microgel net charge in the range of -1 < net charge < 1. The solid line serves as a guide to the eye.
is again attributed to the high ionic strength of the solution, which results in charge screening in the microgel particles. A VSF of 5.3 was calculated for the PDEA shell at low pH, which is in good agreement with the VSFs found for the PDEA homopolymer and the PDEA core in the PDEA core-PMAA shell microgels. This suggests that the swelling behavior of PDEA in the shell is not affected by the presence of the flexible PMAA core. However, a VSF of 2.4 was found for the PMAA core, which is considerably lower than the value obtained for the PMAA homopolymer microgel (VSF = 13.4) and the PMAA shell of the PDEA core-PMAA shell particles (VSF = 5.4) and suggests that the collapsed hydrophobic PDEA shell surrounding the microgel particles further restricts the expansion of the PMAA core and significantly affects its maximum swelling. A similar influence on the swelling behavior of the microgel core in the presence of the shell was shown by Richtering and co-workers for doubly temperature-sensitive core-shell microgel particles.50 They found that at intermediate temperatures the collapsed hydrophobic shell restricted the core to a more compact structure and prohibited its swelling. This was attributed to the weak thermodynamic hydration forces in the core, which are not sufficient to expand the network in the shell of the microgel in order to accommodate the maximum swelling of the core. From Figures 7 and 8 and the discussion above, we also note that the core-shell amphoteric microgels with the balanced 50-50 wt % composition reported herein swell more when their shell is ionized than when their core is fully charged because the shell is spatially less constrained to swelling, being attached only on the inside to the collapsed core but free to swell outward, compared to the core that is surrounded by the collapsed shell.
Conclusions Amphoteric random copolymer and core-shell microgel particles have been synthesized by emulsion copolymerization. Particles of a spherical shape and a narrow size distribution were obtained, and their core-shell topology was verified by the selective staining of the shell of the microgels. Potentiometric titrations of the core-shell particles have shown the independent protonation/deprotonation of the two microgel compartments in contrast to the polyampholyte random copolymer microgels that exhibited a simultaneous ionization process of the basic and acidic moieties of the particles. This results in the independent swelling of the core and the shell of Langmuir 2010, 26(2), 639–647
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the particles at low or high pH upon ionization of the DEA or MAA moieties, respectively, and verifies the contraphilic character of the microgels whereas collapsed latex particles are obtained in the intermediate pH range when the core and the shell of the particles are neutral. It was also shown that whereas the marginally hydrophilic PMAA gel does not affect the swelling behavior of the PDEA network when located in either the core or the shell of the particles the presence of the hydrophobic PDEA network reduces the swelling of the crosslinked PMAA in the particle shell and strongly suppresses its expansion when the latter is restricted in the core of the particles. These microgels comprise novel contraphilic nanostructures with independently tunable core and shell properties that can be advantageous for many applications. Acknowledgment. The European Union (NMP3-CT-2005506621) and the Greek General Secretariat for Research and
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Technology are acknowledged for funding. We thank Ms. S. Papadogiorgaki and Ms. Aleka Manousaki for their help with the TEM and SEM images, respectively. Supporting Information Available: Chemical formulas of P(t-BuMA), PDEA, and PMAA and the reversible ionization process of PDEA and PMAA as a function of solution pH. Experimental details for the DLS measurements. Evidence for the quantitative conversion of P(t-BuMA) to PMAA by NMR. SEM images of the P(t-BuMA) core-PDEA shell microgels before and after deprotection and TEM images of the PDEA-PMAA random copolymer microgel particles. Ionization behavior of the polyampholyte core-shell microgel particles in relation to the local distribution of the ionizable groups and their mutual interactions. Titration data for the PMAA core-PDEA shell microgels. This material is available free of charge via the Internet at http://pubs.acs.org.
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