Thermoreversible Hydroferrogels with Tunable Mechanical Properties

Nov 24, 2010 - ... Jeyaraj Dhaveethu Raja , Jamespandi Annaraj , Kathiresan Sakthipandi ... Rudolf Weeber , Sofia Kantorovich , Christian Holm ... Sun...
1 downloads 0 Views 1MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Thermoreversible Hydroferrogels with Tunable Mechanical Properties Utilizing Block Copolymer Mesophases As Template Marina Krekhova,† Tobias Lang,‡ Reinhard Richter,‡ and Holger Schmalz*,† †

Makromolekulare Chemie II and ‡Experimentalphysik V, Universit€ at Bayreuth, D-95440 Bayreuth, Germany Received October 11, 2010. Revised Manuscript Received November 9, 2010

Thermoreversible hydroferrogels (FGs) have been prepared via gelation of aqueous maghemite ferrofluids (FFs) using the triblock copolymer Pluronic P123 as gelator. In the investigated concentration range of 28-42 wt % P123, long-term stable homogeneous FGs can be prepared from FFs with a maximum maghemite content of 14 wt %. For higher FF concentrations up to 29 wt %, however, homogeneous FGs were formed only for gelator contents up to ca. 33 wt %. A combination of rheology and μ-DSC was applied as an alternative method to construct the P123 phase diagram, without the need for visual methods or scattering techniques. Using this procedure, we could show that maghemite nanoparticles can be effectively templated by the cubic and hexagonal P123 mesophases in a concentration range of 33-38 wt % P123 and FF concentrations up to 14 wt %, respectively. Most importantly, the phase behavior and the corresponding phase-transition temperatures of P123 were not significantly altered. As a result, the FGs show a reversible temperature-triggered transition from a cubic hard gel to a hexagonal gel, which is linked with a softening of the gel. Furthermore, this concept can be applied to template cobalt ferrite nanoparticle effectively, too. Magnetization experiments revealed that the superparamagnetic behavior of the maghemite nanoparticles, which show a Neel type relaxation, is not altered in the corresponding FGs. In contrast, FGs based on blocked cobalt ferrite nanoparticles show a hysteretic behavior, which indicates a strong mechanical coupling between the P123 mesophase and the magnetic nanoparticles.

Introduction In the past two decades, owing to the significant technological and biomedical applications, considerable attention has been paid to the development of “smart” hydrogels.1-7 In general, “smart” hydrogels are physical or chemical networks of water-soluble polymers, which form/disintegrate or swell/contract upon the application of an external stimulus. Suitable stimuli are, for example, solution pH, temperature, ionic strength, light, or redox processes. Biodegradable hydrogels have been applied for tissue engineering and drug release purposes because no additional procedures are necessary to remove the remaining polymer after administration.8,9 The recent development of nanocomposite hydrogels, or hybrid hydrogels, via incorporation of inorganic nanoparticles into the gel matrix, has opened the access to hydrogels with tailored mechanical properties and allows us to manipulate the properties of the hydrogels by applying external magnetic or electric fields.10-12 These unique properties can be *To whom correspondence should be addressed. E-mail: holger.schmalz@ uni-bayreuth.de. (1) Responsive Gels: Volume Transitions I/II; Dusek, K., Ed.; Advances in Polymer Science Series 109/110; Springer: Berlin, 1993. (2) Ahn, S.-k.; Kasi, R. M.; Kim, S.-C.; Sharma, N.; Zhou, Y. Soft Matter 2008, 4, 1151–1157. (3) Dong, L.; Jiang, H. Soft Matter 2007, 3, 1223–1230. (4) Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173–1222. (5) Tsitsilianis, C. Soft Matter 2010, 6, 2372–2388. (6) Vogt, A. P.; Sumerlin, B. S. Soft Matter 2009, 5, 2347–2351. (7) van der Linden, H. J.; Herber, S.; Olthuis, W.; Bergveld, P. Analyst 2003, 128, 325–331. (8) Hawkins, A.; Satarkar, N. S.; Hilt, J. Z. Pharm. Res. 2009, 26, 667–673. (9) Kamath, K. R.; Park, K. Adv. Drug Delivery Rev. 1993, 11, 59–84. (10) Messing, R.; Schmidt, A. M. Polym. Chem., advance article. doi: 10.1039/ c0py00129e. (11) Satarkar, N. S.; Biswal, D.; Hilt, J. Z. Soft Matter 2010, 6, 2364–2371. (12) Schexnailder, P.; Schmidt, G. Colloid Polym. Sci. 2009, 287, 1–11. (13) Zrı´ nyi, M. Magnetic Polymeric Gels as Intelligent Artificial Muscles. In Intelligent Materials; Shahinpoor, M., Schneider, H.-J., Eds.; RSC: Cambridge, U.K., 2008; pp 282-300. (14) Nguyen, V. Q.; Ramanujan, R. V. Macromol. Chem. Phys. 2010, 211, 618–626.

Langmuir 2010, 26(24), 19181–19190

utilized for various applications, for example, actuators,13-16 damping elements,17,18 microfluidic devices,19,20 or biomedical applications like drug delivery and hyperthermia.11,21-23 Recently, magnetic fields have achieved growing interest as a highly selective stimulus. Magneto-responsive hydrogels, also called hydroferrogels (FGs), usually consist of a chemically crosslinked polymer network, which is swollen with an aqueous suspension of magnetic nanoparticles (ferrofluid (FF)).13,18 As a result, the shape and mechanical properties are fixed once the hydrogel is formed. Frequently used water-soluble polymers are poly(vinyl alcohol) (PVA),24-28 poly(acryl amide) and derivatives,29-31 (15) Fuhrer, R.; Athanassiou, E. K.; Luechinger, N. A.; Stark, W. J. Small 2009, 5, 383–388. (16) Monz, S.; Tsch€ope, A.; Birringer, R. Phys. Rev. E 2008, 78, 021404/1– 021404/7. (17) Abramchuk, S.; Kramarenko, E.; Grishin, D.; Stepanov, G.; Nikitin, L. V.; Filipcsei, G.; Khokhlov, A. R.; Zrı´ nyi, M. Polym. Adv. Technol. 2007, 18, 513–518. (18) Filipcsei, G.; Csetneki, I.; Szilagi, A.; Zrı´ nyi, M. Adv. Polym. Sci. 2007, 206, 137–189. (19) Ghosh, S.; Yang, C.; Cai, T.; Hu, Z.; Neogi, A. J. Phys. D: Appl. Phys. 2009, 42, 135501/1–135501/8. (20) Satarkar, N. S.; Zhang, W.; Eitel, R. E.; Hilt, J. Z. Lab Chip 2009, 9, 1773– 1779. (21) Meenach, S. A.; Anderson, K. W.; Hilt, J. Z. Hydrogel Nanocomposites: Biomedical Applications, Biocompatibility, and Toxicity Analysis. In Safety of Nanoparticles; Springer: New York, 2009; pp 1-27. (22) Ramanujan, R. V.; Ang, K. L.; Venkatraman, S. J. Mater. Sci. 2009, 44, 1381–1387. (23) Qin, J.; Asempah, I.; Laurent, S.; Fornara, A.; Muller, R. N.; Muhammed, M. Adv. Mater. 2009, 21, 1354–1357. (24) Collin, D.; Auernhammer, G. K.; Gavat, O.; Martinoty, P.; Brand, H. R. Macromol. Rapid Commun. 2003, 24, 737–741. (25) Franc-ois, N. J.; Allo, S.; Jacobo, S. E.; Daraio, M. E. J. Appl. Polym. Sci. 2007, 105, 647–655. (26) Szabo, D.; Szeghy, G.; Zrı´ nyi, M. Macromolecules 1998, 31, 6541–6548. (27) Zrı´ nyi, M.; Barsi, L.; B€uki, A. Polym. Gels Networks 1997, 5, 415–427. (28) Ramanujan, R. V.; Lao, L. L. Smart Mater. Struct. 2006, 15, 952–956. (29) Mayer, C. R.; Cabuil, V.; Lalot, T.; Thouvenot, R. Adv. Mater. 2000, 12, 417–420. (30) Xulu, P. M.; Filipcsei, G.; Zrı´ nyi, M. Macromolecules 2000, 33, 1716–1719. (31) Galicia, J. A.; Cousin, F.; Dubois, E.; Sandre, O.; Cabuil, V.; Perzynski, R. Soft Matter 2009, 5, 2614–2624.

Published on Web 11/24/2010

DOI: 10.1021/la1040823

19181

Article

Krekhova et al.

hydroxypropyl cellulose,32 and gelatin.33 In addition, magnetoresponsive microgels were reported; however, this is out of focus of the presented work.34,35 In these materials the unusual physical properties of FFs36 are coupled to the elastic behavior of the gel matrix. As a result, the application of a nonuniform magnetic field causes an attraction of the nanoparticles to regions of higher field strengths, and thus can lead to a contraction or deformation of the gel. This can be utilized in actuating systems, for example, for robots, or other devices like switches, seals, or damping controls.13,14,18,26,28,37 In contrast, uniform magnetic fields result in an alignment of the nanoparticles parallel to the field direction. This can be used to influence the strength/rigidity of the gel matrix to tune the release properties of hydrogels for drug delivery purposes.23,25,33,38,39 It is noted that micrometer-sized magnetic particles are frequently used in the construction of ferrogels as well as solid magneto-rheological rubber materials (MR elastomers) to induce large changes in the storage modulus by applying external magnetic fields.40-43 In addition, hydrogels with anisotropic swelling or mechanical properties can be produced by cross-linking the polymeric matrix while it is exposed to a uniform magnetic field, causing the alignment of the incorporated magnetic nanoparticles.17,24,44-46 Physical hydrogels have the advantage that the gelation process is reversible and thus can be repeated several times in contrast to chemical hydrogels. In the case of thermoreversible hydrogels, the gelation can be easily triggered by temperature, and the sol-gel transition temperature can be adjusted by the nature and the concentration of the gelator. However, only a few examples of physically cross-linked FGs are reported. Thermoreversible PVA hydrogels were prepared via consecutive freezethaw cycles, with physical cross-links based on hydrogen bonding or PVA crystallites.39,47 Magneto-responsive hydrogels based on polysaccharides,48 showing a temperature-induced coil-helix transition, and block copolymers,23,49 forming gels via a regular packing of micelles, were reported. Another approach is based on supramolecular hydrogelators, which build up a hybrid nanofiber network.50 The property of superparamagnetic particles to dissipate local heat upon the application of AC magnetic fields can be used as an additional trigger to tune the properties of FGs. The combination of magnetic heatability with a polymeric matrix, which is sensitive

to temperature changes, leads to systems that can be used for drug delivery,35,51 hyperthermia,52 and shape transition applications.53,54 Furthermore, core-shell-corona hybrid micelles with a thermoresponsive corona and superparamagnetic nanoparticles, being permanently fixed in the core of the micelles, can be used to trigger gelation remotely by applying AC magnetic fields.49 Pluronic or poloxamers are ABA-type triblock copolymers, consisting of a thermoresponsive poly(propylene oxide) (PPO) middle block and water-soluble poly(ethylene oxide) (PEO) end blocks (PEOm-b-PPOn-b-PEOm; subscripts denote the numberaverage degree of polymerization of the corresponding block). PPO exhibits a lower critical solution temperature (LCST ca. 5 °C), above which the PPO segments become hydrophobic and thus water insoluble.55 As a result, micelles with a PPO core and a PEO corona are formed above a certain critical micellization temperature (CMT). At high concentrations, these block copolymers form thermoreversible hydrogels based on close-packed micellar mesophases.55-59 Pluronic P123 (PEO20-b-PPO70-b-PEO20) is of particular interest because it exhibits a temperature-triggered transition from a cubic gel phase, formed predominantly by a face centered cubic (fcc) packing of spherical micelles, to a hexagonal gel phase, built from hexagonally packed cylindrical micelles. This transition is present in a narrow concentration range of 32-45 wt % and is linked to a significant softening of the hydrogel.55,58,60,61 Walker and coworkers have shown that P123 gels can be used as templates to create linear arrays of hydrophilic nanoparticles (silica, proteins) utilizing the temperature-induced transition from spherical to cylindrical micelles.62 As the nanoparticles are confined within the interstitial cavities of the crystalline mesophases, the transition to the hexagonal gel phase forces the nanoparticles to align in a linear fashion. In addition, Pluronic F127, forming exclusively cubic mesophases, can be used as a template to create ordered arrays of nanoparticles, too.63-66 The addition of disk-like clay particles to a hexagonal P123 mesophase was shown to induce a transition from hexagonally packed cylindrical micelles to a lamellar phase upon increasing temperature.67 The concept of using cylindrical micelles as templates has been applied to arrange magnetic nanoparticles in a linear fashion by confining the particles in the hydrophobic core of a swollen lyotropic hexagonal phase.68,69 It is noted that Pluronic F127 has been already used to construct magneto-responsive hydrogels;

(32) Chatterjee, J.; Haik, Y.; Chen, C. J. Colloid Polym. Sci. 2003, 281, 892–896. (33) Huang, L.-Y.; Yang, M.-C. J. Magn. Magn. Mater. 2007, 310, 2874–2876. (34) Bhattacharya, S.; Eckert, F.; Boyko, V.; Pich, A. Small 2007, 3, 650–657. (35) Dong, H.; Mantha, V.; Matyjaszewski, K. Chem. Mater. 2009, 21, 3965– 3972. (36) Odenbach, S. Ferrofluids: Magnetically Controllable Fluids and Their Applications; Springer: Berlin, 2003. (37) Zrı´ nyi, M.; Szabo, D.; Filipcsei, G.; Feher, J. Electrical and Magnetic FieldSensitive Smart Polymer Gels. In Polymer Gels and Networks; Osada, Y., Khokhlov, A. R., Eds.; Marcel Dekker: New York, 2002; pp 309-355. (38) Liu, T. Y.; Hu, S. H.; Liu, K. H.; Liu, D. M.; Chen, S. Y. J. Controlled Release 2008, 126, 228–236. (39) Liu, T.-Y.; Hu, S.-H.; Liu, T.-Y.; Liu, D.-M.; Chen, S.-Y. Langmuir 2006, 22, 5974–5978. (40) Bellan, C.; Bossis, G. Int. J. Mod. Phys. B 2002, 16, 2447–2453. (41) Lokander, M.; Stenberg, B. Polym. Test. 2003, 22, 677–680. (42) Mitsumata, T.; Abe, N. Chem. Lett. 2009, 38, 922–923. (43) Zhou, G. Y. Smart Mater. Struct. 2003, 12, 139–146. (44) An, H.; Picken, S. J.; Mendes, E. Soft Matter 2010, 6, 4497–4503. (45) Varga, Z.; Feher, J.; Filipcsei, G.; Zrı´ nyi, M. Macromol. Symp. 2003, 200, 93–100. (46) Varga, Z.; Filipcsei, G.; Zrı´ nyi, M. Polymer 2006, 47, 227–233. (47) Hernandez, R.; Sarafian, A.; Lopez, D.; Mijangos, C. Polymer 2004, 46, 5543–5549. (48) Mitsumata, T.; Sakai, K.; Takimoto, J.-I. J. Phys. Chem. B 2006, 110, 20217–20223. (49) Reinicke, S.; D€ohler, S.; Tea, S.; Krekhova, M.; Messing, R.; Schmidt, A. M.; Schmalz, H. Soft Matter 2010, 6, 2760–2773. (50) Yang, Z.; Gu, H.; Du, J.; Gao, J.; Zhang, B.; Zhang, X.; Xu, B. Tetrahedron 2007, 63, 7349–7357.

(51) Schmidt, A. M. J. Magn. Magn. Mater. 2005, 289, 5–8. (52) Lao, L. L.; Ramanujan, R. V. J. Mater. Sci.: Mater. Med. 2004, 15, 1061– 1064. (53) Schmidt, A. M. Macromol. Rapid Commun. 2006, 27, 1168–1172. (54) Kaiser, A.; Winkler, M.; Krause, S.; Finkelmann, H.; Schmidt, A. M. J. Mater. Chem. 2009, 19, 538–543. (55) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145– 4159. (56) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1–46. (57) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103–A124. (58) Newby, G. E.; Hamley, I. W.; King, S. M.; Martin, C. M.; Terrill, N. J. J. Colloid Interface Sci. 2009, 329, 54–61. (59) Prud’homme, R. K.; Wu, G.; Schneider, D. K. Langmuir 1996, 12, 4651– 4659. (60) Chaibundit, C.; Ricardo, Nagila M. P. S.; Ricardo, Nadja M. P. S.; Costa, F. de M. L. L.; Wong, M. G. P.; Hermida-Merino, D.; Rodriguez-Perez, J.; Hamley, I. W.; Yeates, S. G.; Booth, C. Langmuir 2008, 24, 12260–12266. (61) Chaibundit, C.; Ricardo, Nagila M. P. S.; Costa, F. de M. L. L.; Yeates, S. G.; Booth, C. Langmuir 2007, 23, 9229–9236. (62) Pozzo, D. C.; Walker, L. M. Eur. Phys. J. E 2008, 26, 183–189. (63) Pozzo, D. C.; Hollabaugh, K. R.; Walker, L. M. J. Rheol. 2005, 49, 759–782. (64) Pozzo, D. C.; Walker, L. M. Macromol. Symp. 2005, 227, 203–210. (65) Pozzo, D. C.; Walker, L. M. Colloids Surf., A 2007, 294, 117–129. (66) Pozzo, D. C.; Walker, L. M. Macromolecules 2007, 40, 5801–5811. (67) Castelletto, V.; Ansari, I. A.; Hamley, I. W. Macromolecules 2003, 36, 1694– 1700. (68) Ramos, L.; Fabre, P.; Fruchter, L. Eur. Phys. J. B 1999, 8, 67–72. (69) Ramos, L.; Fabre, P.; Ober, R. Eur. Phys. J. B 1998, 1, 319–326.

19182 DOI: 10.1021/la1040823

Langmuir 2010, 26(24), 19181–19190

Krekhova et al.

Article

however, the amount of incorporated magnetite nanoparticles was rather low (99%), cobalt(II) chloride hexahydrate (Sigma-Aldrich, g99.5%), ammonium hydroxide solution (Fluka, 25%), tetramethylammonium hydroxide (Fluka, p.a.), nitric acid (Merck, 65%), iron(III) nitrate (Sigma-Aldrich, p.a.), :: sodium citrate (Riedel-de Haen, >99.5%), and citric acid (Fluka, g99.5%) were used as received. Millipore (Milli-Q, deionized water) water was freshly taken from the Milliporeþ apparatus equipped with filtration packs QPAK2E (0.5 μm prefilter, macroreticular activated carbon, high-purity mixed-bed ion-exchange resin, Organex polisher). Ferrofluid Preparation. The maghemite (γ-Fe2O3)-based aqueous FFs were prepared by coprecipitation of FeCl2 3 4H2O and FeCl3 3 6H2O under basic conditions, followed by oxidation of the initially formed magnetite (Fe3O4) nanoparticles to maghemite (70) Bee, A.; Massart, R.; Neveu, S. J. Magn. Magn. Mater. 1995, 149, 6–9. (71) Massart, R. C. R. Acad. Sci. Paris, Ser. C 1980, 291, 1–3.

Langmuir 2010, 26(24), 19181–19190

using nitric acid.70,71 Subsequently, the γ-Fe2O3 nanoparticles were stabilized with trisodium citrate. A detailed description of the maghemite FF synthesis can be found elsewhere.49 The numberaverage diameter of the γ-Fe2O3 nanoparticles was 10 ( 2 nm (average over 400 particles, Figure S1A, Supporting Information), as determined by transmission electron microscopy (TEM; Zeiss CEM 902 electron microscope, operated at 80 kV). Cobalt ferrite (CoFe2O4) FFs were prepared by coprecipitation of FeCl3 3 6H2O and CoCl2 3 6H2O under basic conditions at a molar ratio of 2:1.72,73 The precipitated primary nanoparticles were washed several times with Millipore water and successively treated with nitric acid and iron(III) nitrate solution. Subsequently, the suspension was flocculated with citric acid, and the nanoparticles were redispersed in water by the addition of tetramethylammonium hydroxide solution. Consequently, the obtained cobalt ferrite nanoparticles are electrostatically stabilized by a tetramethylammonium citrate shell. The number-average diameter of the CoFe2O4 nanoparticles was 12 ( 3 nm (average over 110 particles, Figure S1B, Supporting Information), as determined by TEM. The concentration of the FFs was adjusted by slow evaporation of water using a rotary evaporator. The final solids content in the aqueous FFs was determined by gravimetric analysis using a Mettler Toledo TGA/SDTA851e by applying a temperature ramp from 30 to 1000 °C at a heating rate of 10 K min-1 in the presence of N2 (flow rate 60 mL min-1). Consequently, using the obtained weight of magnetic nanoparticles (ωNP) and the initial sample weight, the nanoparticle concentration (cFF) was determined according to cFF = [ωNP/(ωNP þ ωH2O)] 3 100 wt %. The FF samples were stored in the refrigerator at 5 °C and used as quickly as possible. The homogeneity of all FFs was proven by optical microscopy prior to use. Hydro(ferro)gel Preparation. Appropriate amounts of Millipore water for neat gels (NGs), or aqueous FF for FGs, and gelator P123 were dissolved at low temperatures (T = 0-5 °C) under gentle shaking for 2 to 3 days using a cooling-heatingthermomixer (MKR13, HLC BioTech). Before studying the temperature-dependent phase behavior by rheology and μ-DSC, we verified the homogeneity of the FGs by optical microscopy. All gel samples were stored in the refrigerator at 5 °C until further use. The nanoparticle content of the FGs was defined using the nanoparticle concentration in the corresponding FF, that is, cFF. Likewise, the P123 concentration in the FGs (cgel) was referred to the amount of water in the FF used for the preparation of the FGs, according to cgel = [ωgel/(ωgel þ ωH2O)] 3 100 wt %. This in turn allows us to compare directly FGs and NGs with identical gelator concentration regarding their phase behavior. Consequently, we used as a short notation FGxFFy, with x denoting the gelator concentration (cgel) and y denoting the concentration of nanoparticles in the ferrofluid (cFF), respectively. If necessary, the concentrations with respect to all FG components can be converted, according to cFF(FG) = [ωNP/(ωNP þ ωH2O þ ωgel)] 3 100 wt %, and cgel(FG) = [ωgel/(ωgel þ ωH2O þ ωNP)] 3 100 wt %.74 Rheology. Rheology measurements were performed with a Physica MCR 301 rheometer (Anton Paar) using a cone-andplate shear cell geometry (d = 50 mm, cone angle = 1°). The temperature was controlled by a Peltier element, and evaporation of water was minimized using a cover cap with built-in solvent trap (Anton Paar). For loading a gel sample, the plate was cooled to T = 0-5 °C. Subsequently, the gel was put onto the cooled plate, where it became liquid, and the cone was lowered to the measurement position. Prior to the measurements, we determined the linear viscoelastic regime by performing a strain sweep test at a frequency of 1 Hz for each sample. For the temperaturedependent measurements a frequency of 1 Hz, a strain of 0.3%, and a heating rate of 1 K min-1 were applied. (72) Bonini, M.; Wiedenmann, A.; Baglioni, P. Physica A 2004, 339, 86–91. (73) Wagner, J.; Fischer, B.; Autenrieth, T.; Hempelmann, R. J. Phys.: Condens. Matter 2006, 18, S2697–S2711. (74) Lattermann, G.; Krekhova, M. Macromol. Rapid Commun. 2006, 27, 1373– 1379.

DOI: 10.1021/la1040823

19183

Article

Micro-Differential Scanning Calorimetry (μ-DSC). The calorimetric measurements were performed with a Setaram μ-DSC III using closed “batch” cells at a scanning rate of 0.1 K min-1. Millipore water was used as reference substance. The μ-DSC allows measurements with an extremely high sensitivity using sample masses up to 1 g and hence the detection of phase transitions with very small enthalpy values. Optical Microscopy. The homogeneity of the aqueous FFs and the corresponding FGs, that is, the absence of micrometersized nanoparticle aggregates, was examined using a Nikon Diaphot 300 inverted microscope. In the case of FGs, we prepared thin samples by placing an FG sample on a cooled microscope slide (T = 0-5 °C), which was covered with a thin coverslip after the sample became liquid. After heating to room temperature to induce gelation, the samples were studied in the microscope. We performed examination of the gel samples for birefringence by placing the sample between crossed polarizers, and we adjusted the temperature by using a Mettler FP82HT hot-stage by applying a heating rate of 0.1 K min-1. Vibrating Sample Magnetometry. We measured the magnetization curves M(H) for the FFs and the corresponding ferrogels by means of a commercial vibrating sample magnetometer (Lakeshore 7404). It is equipped with a single-stage variable temperature option (Lakeshore 74035), which allows a control of the temperature in the range of 100 to 950 K. The liquid FFs were filled brimful in the commercial liquid sample holder (Lakeshore 730935). Its volume was measured to be 70 μL. The FG samples were first liquefied in a water-ice bath at 273 K and thereafter filled in the liquid sample holders as well. A bubble-free filling of the sample holder was checked by means of X-ray pictures.75 The cavity of the sample holder has the combined shape of a hollow cylinder and a cone. To estimate the demagnetization factor, D, of the sample in the holder, we approximated the filled orifice by means of a prolate spheroid with the inner radius and volume of the cavity.76

Results and Discussion Neat P123 Hydrogels (NGs). The phase diagram of concentrated P123 solutions is well-established in literature.55,58,60,61 Commonly, a combination of different methods is used to construct the phase diagram, that is, test tube inversion, rheology, visual inspection, polarized light microscopy, and scattering techniques. Sol-gel transitions can be easily determined via test tube inversion and rheology. However, the temperature-induced transition from a cubic phase to a hexagonal phase in the concentration range of 32-45 wt % is usually determined by visual inspection, as the clear cubic gel transforms into a hazy (turbid) hexagonal gel. In addition, the typical birefringence of the hexagonal phase can be used for identification. The problem encountered with FGs is that visual methods cannot be used for detecting the transition from clear cubic gels to turbid hexagonal gels simply because of the deep-brown to black color of the FGs for high nanoparticle contents; that is, the samples are not transparent anymore. This, in turn, limits the applicability of polarized light microscopy, too, because the onset of birefringence cannot be detected unambiguously. The same accounts for the use of scattering techniques such as small-angle X-ray scattering. Here the scattering contrast of the magnetic nanoparticles is much larger compared with that of the polymer; therefore, no information about the structure of the block copolymer mesophase can be extracted (results not shown). Consequently, we established a combination of rheology and μ-DSC to construct the phase diagram without the need of visual methods or scattering techniques, and we will compare our (75) Gollwitzer, C.; Matthies, G.; Richter, R.; Rehberg, I.; Tobiska, L. J. Fluid Mech. 2007, 571, 455–474. (76) Raikher, Y. L.; Stolbov, O. V. J. Appl. Mech. Tech. Phys. 2005, 46, 434–443.

19184 DOI: 10.1021/la1040823

Krekhova et al.

Figure 1. Temperature-dependent storage (G0 ) and loss (G00 ) modulus in (A) semilogarithmic and (B) linear representation for a 33 wt % P123 neat gel (γ = 0.3%, f = 1 Hz, 1 K min-1). Tsg = sol-gel transition temperature; phase transitions: CG-TG (cubic hard gel-turbid hexagonal gel), TG-SG (turbid hexagonal gel-soft gel).

results with the already published phase diagram of P123 in the following section.58,60,61 For the preparation of FGs, we are mainly interested in the cubic-to-hexagonal mesophase transition, which will be utilized to template the magnetic nanoparticles and to obtain FGs with tunable mechanical properties. Therefore, we have restricted our investigations to the concentration range of 28-42 wt % P123 and a maximum temperature of ∼60 °C. Rheology. The phase behavior of the NGs was investigated by rheology using a cone-and-plate shear cell geometry. We applied an oscillatory stress to the sample and monitored the storage (G0 ) and loss (G00 ) modulus in dependence on temperature. Regimes with G0 > G00 are referred to as gel state, whereas the sol state is characterized by a loss modulus being significantly higher compared with the storage modulus.77 The sol-gel transition temperature upon heating was determined from the crossover point of the G0 and G00 traces. To compare our results with literature data,61 the transition from a soft to a hard gel was taken as the temperature at which G0 (1 Hz) passes through 1 kPa. In addition, we will use an identical notation for the different phases observed in dependence on concentration and temperature, that is, S = sol state, SG = soft gel with G0 < 1 kPa, CG = cubic hard gel, TG = turbid hard gel (hexagonal), TF = turbid fluid, and CLG = cloudy lowmodulus gel (see later discussion on the P123 phase diagram, Figure 3).60 As an example, the temperature-dependent storage and loss modulus for a NG with a gelator concentration (cgel) of 33 wt % is shown in Figure 1A. At low temperatures, G00 exceeds G0 , which is consistent with the sol state. At T = 13.5 °C, the G0 and G00 graphs cross, and now G0 exceeds G00 ; that is, the free-flowing solution (77) Winter, H. H.; Mours, M. Adv. Polym. Sci. 1997, 134, 165–234.

Langmuir 2010, 26(24), 19181–19190

Krekhova et al.

transforms into a gel. The corresponding temperature is taken as the sol-gel transition temperature, Tsg. It is noted that the sol-gel transition is very sharp, as both storage and loss modulus show a steep increase within a narrow temperature window. A second characteristic transition can be extracted from the inflection point of G0 or the corresponding shoulder at the left side of the G00 peak at ∼44 °C. The inflection point in the G0 trace can be detected more clearly when the moduli are plotted in a linear fashion versus temperature (Figure 1B). This temperature corresponds to the transition of the cubic hard gel (CG) to the turbid hexagonal gel (TG), denoted as CG-TG in Figure 1. This assignment is confirmed by μ-DSC measurements, as will be discussed later on. Furthermore, an identical rheological behavior was observed for all NGs in the concentration range of 33-42 wt %, which is consistent with literature data.55,60,61 It can be clearly seen that this transition is linked to a significant softening of the gel; however, G0 still exceeds the limit of 1 kPa for hard gels. Upon a further increase in temperature, the storage modulus continues to decrease. Finally, at ∼50 °C, G0 drops below 1 kPa, which is taken as the transition to a soft gel, that is, TG-SG, as denoted in Figure 1A. The applicability of the used method to determine the CG-TG transition temperature is further confirmed by the corresponding cooling trace shown in Figure S2 (Supporting Information). Upon cooling, the SG-TG and TG-CG transitions are wellseparated, and both transitions are linked with an inflection point in G0 and a corresponding maximum in G00 . However, upon heating, the respective CG-TG and TG-SG transitions are comparatively close; consequently, the CG-TG transition shows up as a shoulder in G00 (Figure 1). μ-DSC Investigations. Because of its high sensitivity, μ-DSC is a powerful tool not only for the study of micellization processes, especially for Pluronic type block copolymers, but also to probe transitions from disordered micellar solutions to ordered micellar mesophases, which are characterized by very weak endothermic transitions.55 As an example, the results of calorimetric measurements for NGs with varying P123 concentration are shown in Figure 2. The large endothermic peak at about 0-20 °C, which is visible in all samples, is attributed to the temperature-induced micelle formation (Figure 2A). This peak is linked to the desolvation of the PPO block upon heating, and the peak maximum is therefore assigned to the CMT.55 This transition is reproducible and shifted to lower temperatures with increasing gelator content. Upon further heating, a small endothermic peak is visible at the right shoulder of the micellization peak for NGs with concentrations above 25 wt %, that is, the concentration range where cubic hard gels are formed.55,60,61 This endothermic peak can be attributed to the formation of a close cubic packing of micelles and is denoted as Tcub.55 A comparison with Figure 1A shows that for a 33 wt % P123 solution, the sol-gel transition temperature determined by rheology (Tsg = 13.5 °C) perfectly fits to Tcub = 13.5 °C obtained from μ-DSC. The phase transition from a cubic hard gel to a turbid hexagonal gel (CG-TG) is linked to a transition of spherical micelles to cylindrical micelles. It was shown that the transition to cylindrical micelles can be detected by μ-DSC as a very weak endothermic peak for low-molecular-weight surfactants as well as for Pluronic-type (78) Armstrong, J. K.; Chowdhry, B. Z.; Snowden, M. J.; Leharne, S. A. Langmuir 1998, 14, 2004–2010. (79) Grell, E.; Lewitzki, E.; Schneider, R.; Ilgenfritz, G.; Grillo, I.; von Raumer, M. J. Therm. Anal. Calorim. 2002, 68, 469–478. (80) Heerklotz, H.; Tsamaloukas, A.; Kita-Tokarczyk, K.; Strunz, P.; Gutberlet, T. J. Am. Chem. Soc. 2004, 126, 16544–16552.

Langmuir 2010, 26(24), 19181–19190

Article

Figure 2. (A) μ-DSC heating traces for NGs with different gelator concentrations and (B) corresponding magnifications in the temperature range 20-80 °C (0.1 K min-1). CMT = critical micellization temperature, Tcub = transition temperature for the formation of a cubic hard gel (CG); phase transitions: CG-SG (cubic hard gel-soft gel), CG-TG (cubic hard gel-turbid hexagonal gel), SG-TF (soft gel-turbid fluid).

block copolymers.78-82 A magnification of the μ-DSC traces in the relevant temperature range reveals that this weak endothermic transition attributed to the formation of cylindrical micelles can be detected in P123 gels, too (Figure 2B). The end-set of this peak corresponds well to the inflection point of G0 observed by rheology (Figure 1) for concentrations between 33 and 42 wt % and thus can be clearly ascribed to the CG-TG transition. In addition, this assignment is supported by the respective cooling traces obtained by rheology and μ-DSC (Figure S2, Supporting Information). Upon cooling, the transition from the hexagonal (TG) to the cubic (CG) gel phase is shifted to lower temperatures (ca. 25 °C). Accordingly, this transition is evident from the appearance of an inflection point in G0 (maximum in G00 ) in the temperature-dependent moduli, as well as from a corresponding exothermic transition in μ-DSC. A comparison of the CG-TG transition temperatures extracted from rheology and μ-DSC (Table S1, Supporting Information), respectively, reveals that in dependence on the gelator concentration, the transition temperatures determined by μ-DSC are on average 2 °C lower compared with that from rheology. This is reasonable, as μ-DSC probes the transition from spherical to cylindrical micelles, that is, changes in the hydration state of PEO, whereas rheology is sensitive to changes in the mechanical response of the hydrogel, which are most prominent when the transition to cylindrical micelles and the subsequent formation of the hexagonal phase is finished. In addition, different heating rates were used for rheology (1 K min-1) and μ-DSC (0.1 K min-1), which might contribute to the observed (81) L€of, D.; Niemiec, A.; Schillen, K.; Loh, W.; Olofsson, G. J. Phys. Chem. B 2007, 111, 5911–5920. (82) Michels, B.; Waton, G.; Zana, R. Colloids Surf., A 2001, 183-185, 55–65.

DOI: 10.1021/la1040823

19185

Article

Krekhova et al.

Figure 3. Phase diagram for NGs determined by rheology (], 0)

and μ-DSC (y). Lines represent the P123 phase diagram taken from literature data.60,61 Phase assignments: S = sol state, SG = soft gel with G0 < 1 kPa, CG=cubic hard gel, TG=turbid hexagonal gel, TF=turbid fluid, and CLG=cloudy low-modulus gel.

effect, too. Consequently, the CG-TG transition can be determined with an accuracy of about (2 °C, using a combination of both methods. At 30 wt % P123, the respective endothermic peak at ∼45 °C (Figure 2B) arises from the hard cubic gel to soft gel transition (CG-SG), as hexagonal phases are only observed for concentrations above ca. 32 wt %. This is supported by the rheology data, revealing a transition to a soft gel at this temperature, that is, G0 < 1 kPa (results not shown). The on-set of the last broad endothermic transition at high temperatures (g50 °C) can be attributed to phase separation or clouding, that is, the formation of a turbid fluid (TF) or a cloudy low-modulus gel (CLG), respectively, depending on concentration.55,78,81,82 Phase Diagram. Our results from rheology and μ-DSC show that all phase transitions occurring in concentrated P123 solutions can be clearly identified by a combination of both methods without using visual methods or scattering techniques. This, in turn, is the prerequisite to study the phase behavior of the FGs, as discussed above. In Figure 3, the phase transitions for NGs, determined by rheology and μ-DSC, are summarized and compared with the P123 phase diagram described in literature.60,61 All results are in good agreement with each other and with the literature data. This holds, in particular, for the transition from the cubic (CG) to the hexagonal (TG) gel phase upon heating at concentrations of 33-42 wt % P123, which will be utilized to template magnetic nanoparticles in the corresponding FGs. There are some deviations for 42 wt % P123, as our results indicate a sol-gel transition at significantly lower temperatures. However, some minor disparities between different studies might be expected because of slightly different properties of P123 depending on the used supplier.83,84 It is noted that the transition temperature from the hexagonal (TG) to the cloudy low-modulus (CLG) gel phase for 42 wt % P123 was extracted from rheology (Figure S3, Supporting Information). Here the temperaturedependent storage modulus G0 shows a characteristic decrease, and the corresponding on-set is taken as the transition temperature (TG-CLG). Maghemite (γ-Fe2O3)-Based Hydroferrogels. Before studying the phase behavior of FGs based on maghemite FFs, we studied the time-dependent stability and homogeneity of the FGs at room temperature by visual inspection and by optical (83) Mortensen, K.; Batsberg, W.; Hvidt, S. Macromolecules 2008, 41, 1720– 1727. (84) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, T. A. Eur. Polym. J. 1997, 33, 673–677.

19186 DOI: 10.1021/la1040823

Figure 4. Consistency of prepared FGs in dependence on gelator (cgel) and FF (cFF) concentration. The pictures show optical microscopy images for representative homogeneous and heterogeneous FG samples, respectively.

microscopy. The concentration range of P123 (cgel) and maghemite (cFF) with respect to water in which long-term stable homogeneous FGs are formed is shown in Figure 4. As discussed above, we have limited our investigations to a gelator content of 28-42 wt % P123 per water. Heterogeneous FGs can be clearly distinguished from homogeneous samples using optical microscopy by the appearance of small dark domains in the micrometer range, formed because of agglomeration of maghemite nanoparticles. The prepared FGs are homogeneous for FF concentrations up to 14 wt %. However, homogeneous FGs with a higher maghemite content (up to 29 wt %) are accessible only for lower gelator concentrations (28-33 wt %). In general, the higher the P123 concentration, the lower the amount of maghemite nanoparticles that can be incorporated in the FGs without phase separation. This is reasonable because the accessible free volume in a cubic packing of micelles decreases with increasing volume fraction of the micelles. It is noted that the prepared homogeneous FGs are stable for several months without any sign of phase separation. Rheology and μ-DSC. Figure 5 shows the results from rheology and μ-DSC for an FG with 35 wt % P123 and an FF concentration of cFF = 11 wt %, that is, FG35FF11 using our short notation. All phase transitions can be determined by applying identical procedures as described for the NGs. In particular, the transition from the cubic to the hexagonal gel phase (CG-TG) can be clearly assigned from rheology by the inflection point in G0 and the maximum in G00 (Figure 5A) as well as from the small endothermic peak in the corresponding μ-DSC trace (Figure 5B). In accordance with the behavior of the NGs, the storage modulus in the hexagonal phase is significantly lower compared with that of the cubic phase. Moreover, FG35FF11 shows the same phase transitions as the corresponding NG with 35 wt % P123 (Figure 3); that is, the magnetic nanoparticles do not alter the phase behavior. Investigations by polarized light microscopy (PLM) revealed that both the NG with 35 wt % P123 and FG35F11 exhibit the typical birefringence of the hexagonal gel phase, as shown exemplary by the PLM images at 53 °C (Figure S4, Supporting Information). Consequently, we determined the phase behavior of all prepared homogeneous FGs by using a combination of rheology and μ-DSC. The concentration dependence of the CMT for P123 solutions in water and in maghemite-based FFs with different maghemite contents is shown in Figure 6A. The CMT decreases with increasing gelator concentration, as expected,55 and the maghemite nanoparticles obviously do not influence the CMT significantly. Langmuir 2010, 26(24), 19181–19190

Krekhova et al.

Figure 5. Structural transitions for FG35FF11 determined by:

(A) rheology (γ = 0.3%, f = 1 Hz, 1 K min-1) and (B) μ-DSC (0.1 K min-1). CMT = critical micellization temperature, Tcub = transition temperature for the formation of a cubic hard gel (CG); phase transitions: CG-TG (cubic hard gel-turbid hexagonal gel), TG-SG (turbid hexagonal gel-soft gel), SG-TF (soft gel-turbid fluid).

This indicates that the interaction between P123 and citratestabilized maghemite nanoparticles is negligible. In the case that P123 acts as a cosurfactant for the magnetic nanoparticles, the concentration of “free” P123 in solution would be reduced, and thus an increase in the CMT might be expected with respect to that observed for the neat P123 solution at identical gelator content. An efficient templating of the magnetic nanoparticles is possible only when the ordered cubic and hexagonal mesophases are still present in the FGs. A very sensitive method to prove the presence of the ordered cubic mesophase (CG) in the FGs is μ-DSC. Only for the formation of a close-packed cubic phase can the corresponding small endothermic peak (Tcub) at the right shoulder of the micellization peak be observed, whereas this peak is absent when irregularly packed micellar gels (SG) are formed (Figure 2A).55 In Figure 6B, the transition temperatures for the formation of close-packed cubic gel phases (Tcub) in the FGs are compared with the corresponding NGs. First of all, the transition temperatures observed for the FGs are very similar to those of the NGs, except for the FG with the highest gelator concentration. For this sample, a significant shift to higher temperatures can be observed, which might be attributed to the decreasing free interstitial volume available for the incorporation of the magnetic nanoparticles at high P123 concentrations; that is, the presence of the nanoparticles disturbs the formation of a close packing of gelator micelles. A closer look reveals that the maximum maghemite concentration in the FGs for the formation of a ordered cubic gel phase is 14 wt % for gelator concentrations of 33-38 wt %. At 30 and 42 wt % P123, only 11 wt % maghemite can be incorporated, and at 28 wt % gelator, FGs do not form a cubic gel Langmuir 2010, 26(24), 19181–19190

Article

Figure 6. Dependence of (A) the critical micellization temperature CMT and (B) Tcub on the gelator concentration (cgel) for NGs (O) and FGs (9, cFF = 11 wt %; 4, cFF = 14 wt %; /, cFF = 29 wt %).

Figure 7. Sol-gel transition temperatures (Tsg) for NGs (O) and

FGs (9, cFF = 11 wt %; 4, cFF = 14 wt %) in dependence on the gelator concentration (cgel), determined by rheology (γ = 0.3%, f = 1 Hz, 1 K min-1).

phase. The absence of a cubic gel phase for FGs with maghemite contents above 14 wt % is clearly indicated by rheology as well as μ-DSC, as shown for FG35FF29 (Figure S5, Supporting Information). The sol-gel transition determined by rheology is significantly broadened (Figure S5A, Supporting Information), and the small endotherm attributable to the formation of a cubic phase (Tcub) is absent in the corresponding μ-DSC trace (Figure S5B, Supporting Information). Consequently, an effective templating of the maghemite nanoparticles by the cubic P123 mesophase can be achieved for gelator concentrations of 30-42 wt % and maghemite contents up to 14 wt %, respectively. As a result, we will focus on FGs based on FFs with a maximum maghemite concentration of cFF = 14 wt % in the following discussion. Figure 7 compares the sol-gel transition temperatures (Tsg) for NGs and FGs, extracted from rheological measurements by the DOI: 10.1021/la1040823

19187

Article

Krekhova et al.

Figure 8. G0 max values versus gelator concentration (cgel) for NGs (O) and FGs (9, cFF = 11 wt %; 4, cFF = 14 wt %) determined by rheology according to Figure 1B.

crossover of G0 and G00 . The transition temperatures for the FGs are comparable to those of the NGs for gelator concentrations up to 35 wt %. For higher concentrations, the sol-gel transition temperatures are shifted to higher values, in agreement with the μ-DSC results (Figure 6B). With increasing gelator content, that is, increasing volume fraction of P123 micelles, the formation of a close cubic packing is slightly hindered by the incorporation of magnetic nanoparticles, resulting in the observed shift of the sol-gel transition to higher temperatures. An important issue connected to the templating of magnetic nanoparticles by P123 mesophases is the question of whether the incorporation of the magnetic nanoparticles has an effect on the mechanical properties of the FGs. As a measure for the gel strength in the cubic gel phase (CG), we have used the characteristic maximum (G0 max) in the temperature dependence of the storage modulus G0 (Figure 1B). In Figure 8, the corresponding G0 max values for FGs (cFF = 11 and 14 wt %) are compared with the respective values of the NGs in dependence on the gelator concentration. In general, G0 max increases with the gelator concentration because of an increasing volume fraction of the P123 micelles. Up to a gelator concentration of 35 wt %, the gel strengths of the NGs and FGs are comparable, and thus the magnetic nanoparticles obviously do not exert a significant influence on the mechanical properties of the cubic gel phase. However, for higher gelator concentrations, the FGs exhibit significantly reduced G0 max values. This is most likely attributed to the increased volume fraction of P123 micelles, that is, decreasing interstitial volume available for the magnetic nanoparticles. As a result, the cubic packing of the P123 micelles might be slightly disturbed, resulting in the observed decreased gel strength. This is consistent with the observed effect on the sol-gel transition temperature (Figure 7) and on Tcub (Figure 6B), determined by rheology and μ-DSC, respectively. Phase Diagram. The phase diagrams for FGs with maghemite contents of 11 and 14 wt % per water were constructed, using the obtained results from rheology (Figure 9A) and μ-DSC (Figure 9B), respectively, and are compared with the phase diagram of neat P123 hydrogels as well as with literature data.60,61 Here an identical procedure as that described for the neat P123 gels was used, and as discussed above, we have focused on gelator concentrations of 28-42 wt %, that is, the range where hexagonal (TG) and cubic (CG) mesophases are expected. The data for the sol-gel transitions (Figures 6B and 7) have been omitted in the phase diagram for means of clarity. It can be clearly seen that the FGs show an identical phase behavior with respect to that of the NGs. Moreover, the phasetransition temperatures are very close, revealing the successful templating of the maghemite nanoparticles by the P123 mesophases. 19188 DOI: 10.1021/la1040823

Figure 9. Comparison of phase diagrams for NGs (O) and FGs (9, cFF = 11 wt %; 4, cFF = 14 wt %) determined by: (A) rheology (γ = 0.3%, f = 1 Hz, 1 K min-1) and (B) μ-DSC (0.1 K min-1). Lines represent the P123 phase diagram taken from literature data.60,61 Phase assignments: S = sol state, SG = soft gel with G0 < 1 kPa, CG = cubic hard gel, TG = turbid hexagonal gel, TF = turbid fluid, and CLG = cloudy low-modulus gel.

In particular, the transition from the cubic (CG) to the hexagonal (TG) gel phase can be unambiguously determined for the FGs using a combination of rheology and μ-DSC, without the need of applying visual methods or scattering techniques. Consequently, the magnetic nanoparticles act as inert fillers up to contents of 14 wt % without exerting a significant influence on the gel morphology and the phase-transition temperatures. Vibrating Sample Magnetometry. The identical phase behavior of the FGs with respect to that of the NGs already points to the absence of any nanoparticle agglomeration because otherwise, an effective templating by the P123 mesophases cannot be achieved. This has been further proven by quasi-static magnetization experiments. Figure 10 shows the magnetization graphs of a maghemite FF (cFF = 14 wt %) and the corresponding FG38FF14 FG at two temperatures, that is, 25 and 45 °C. At 25 °C, FG38FF14 forms a cubic gel phase, whereas 45 °C corresponds to the hexagonal gel phase (Figure 9). The three graphs show an identical shape, indicating that neither the incorporation into the P123 mesophase nor the phase transition from a cubic to a hexagonal gel significantly influences the magnetization behavior. The lack of hysteresis (low coercitivity and remanence) confirms the superparamagnetic behavior that is expected in this particle size range85 and the internal remagnetization mode (Neel relaxation) for virtually all particles. On the contrary, if particle rotation (Brown relaxation) played a significant role in the field (85) Berkowitz, A. E.; Schuele, W. J.; Flanders, P. J. J. Appl. Phys. 1968, 39, 1261–1263.

Langmuir 2010, 26(24), 19181–19190

Krekhova et al.

Figure 10. Magnetization curves of a maghemite FF (cFF = 14 wt %) and the corresponding hydroferrogel FG38FF14 (cgel = 38 wt %, cFF = 14 wt %) at 25 and 45 °C, respectively.

Figure 11. Characteristic transition temperatures of NGs (open symbols) and cobalt-ferrite-based FGs (cFF = 4 wt %, closed symbols) in dependence on the gelator concentration (cgel), as determined by rheology and μ-DSC. CMT = critical micellization temperature, Tcub = transition temperature for the formation of a cubic hard gel (CG), CG-TG = phase-transition temperature cubic hard gel-turbid hexagonal gel; phase assignments: S = sol state, CG = cubic hard gel, TG = turbid hexagonal gel.

orientation of the particles, a different behavior would be expected for the FGs, as will be discussed later on for cobalt-ferrite-based FGs. The saturation magnetization (Ms) of the FG is lower compared with that of the corresponding FF at both temperatures, which is attributed to the lower overall maghemite content in the FG (cFF(FG) = 10.1 wt %). The slightly decreased saturation magnetization for the FG at 45 °C compared with that at 25 °C is most likely attributed to the general trend of a decreasing Ms with increasing temperature, observed in FFs, too. Cobalt Ferrite (CoFe2O4)-Based Hydroferrogels. We performed initial experiments with cobalt ferrite FFs (cFF e 4 wt %) to prove that the templating effect of P123 mesophases is not only restricted to the use of maghemite nanoparticles. The preparation and characterization methods are identical to those described for the maghemite-based FGs. Figure 11 summarizes the obtained results from rheology and μ-DSC for FGs (cFF = 4 wt %) with varying gelator content in comparison with the corresponding NGs. In agreement with the results obtained for the maghemite-based FGs, the micellization and phase behavior of P123 is not altered by the presence of cobalt ferrite nanoparticles, and the respective phase-transition temperatures are very close to those observed for the NGs. Consequently, cobalt ferrite nanoparticles can be effectively incorporated in P123 mesophases, too. As an example, results from rheology and μ-DSC for a CoFe2O4-based FG with 35 wt % gelator and cFF = 4 wt % (FG35FF4) are shown in Figure S6 (Supporting Information). We are currently working on alternative methods to stabilize Langmuir 2010, 26(24), 19181–19190

Article

Figure 12. Magnetization curves of a cobalt ferrite FF (cFF = 1.0 wt %) and the corresponding hydroferrogel FG38FF01 (cgel = 38 wt %, cFF = 1.0 wt %) at 20 °C.

concentrated cobalt ferrite FFs to explore the maximum amount of CoFe2O4 nanoparticles that can be incorporated in P123 mesophases without altering the phase behavior. Eventually, we studied the impact of the gelation on the magnetization curve. As shown in Figure 12, the cobalt ferrite FF exhibits the familiar superparamagnetic curve. In contrast, the corresponding FG follows a hysteretic magnetization curve. This effect is known from chemically cross-linked FGs.16 However, this is so far unique for thermoreversible FGs. The observed hysteresis indicates a strong coupling between the gel matrix and the magnetic moment of the individual nanoparticles. This coupling is based on a two-fold gear transmission: (i) a mechanical coupling between the particles and the gel network, as the nanoparticles are fixed within the close packed cubic P123 mesophase at 20 °C, and (ii) a coupling between the cobalt ferrite crystal and its magnetic moment. The latter is favored by the anisotropy energy K V, where K denotes the anisotropy constant of the crystal and V is its volume. This anisotropy energy may be overcome by thermal activation. The Neelian relaxation time for this process is given by   KV τN ¼ τ0 exp kB T where τ0 ≈ 10-9 s is the Korringa relaxation time.86 For CoFe2O4, the anisotropy constant K is about 15 times larger compared with that for maghemite. Consequently, τN becomes very large, and a rotation of the magnetic dipole moment within the CoFe2O4 crystal is effectively blocked for the size range of the used CoFe2O4 nanoparticles.36 Consequently, in both types of FGs, a mechanical coupling between the gel matrix and the magnetic nanoparticles may exist. However, in the magnetization curve, it becomes prominent for only CoFe2O4-based FGs, where the magnetic moment is also blocked.

Conclusions We have shown that long-term stable homogeneous FGs can be prepared using Pluronic P123 as gelator for aqueous maghemitebased FFs. In the investigated concentration range of 28-42 wt % gelator, the prepared FGs are homogeneous for FF concentrations up to 14 wt %. FGs with a higher maghemite content (up to 29 wt %) are accessible only for lower gelator concentrations (28-33 wt %). Concentrated P123 solutions show an interesting phase transition from a cubic hard gel, formed by a close packing of spherical (86) Neel, L. Adv. Phys. 1955, 4, 191–243.

DOI: 10.1021/la1040823

19189

Article

micelles, to a turbid hexagonal gel, built up from cylindrical micelles. This transition is linked to a significant softening of the hydrogel. We have utilized this phase transition to template effectively maghemite nanoparticles within the cubic and hexagonal gel phase, respectively, which allows us to construct FGs with tunable mechanical properties. For this purpose, we first developed an alternative method based on rheology and μ-DSC to study the phase behavior of P123 neat gels (NGs) as well as FGs without using visual methods or scattering techniques that cannot be applied for FGs. The constructed P123 phase diagram corresponds well to the literature data, confirming the applicability of our method. On the basis of this method, a phase diagram for FGs with maghemite contents of 11 and 14 wt % per water was constructed, showing that the phase behavior resembles that of neat P123 hydrogels. Furthermore, the corresponding phase-transition temperatures are close to that of the corresponding NGs, and the mechanical properties of the cubic gel phases are not significantly altered by the presence of the maghemite nanoparticles. Consequently, the magnetic nanoparticles can be regarded as inert fillers in the investigated concentration range. Combined results from rheology, μ-DSC, and polarized light microscopy showed that the maghemite nanoparticles can be effectively templated by the cubic and hexagonal P123 mesophases for gelator concentrations of 33-38 wt % and maghemite contents up to 14 wt % per water, respectively. These FGs exhibit a temperature-triggered transition from a cubic hard gel to a hexagonal gel, which is linked to a significant softening of the gel analogous to the behavior of the NGs. As a result, the mechanical properties of the FGs can be

19190 DOI: 10.1021/la1040823

Krekhova et al.

easily tuned by temperature. Initial experiments using cobalt ferrite FFs indicated that this approach can be applied to template effectively CoFe2O4 nanoparticles, too. Magnetization experiments revealed that the incorporation of the maghemite nanoparticles in the cubic and hexagonal P123 mesophases has no significant influence on the superparamagnetic behavior and the magnetic moment distribution of the nanoparticles, which show a Neel-type remagnetization mode. This in turn supports the effective templating of the nanoparticles in the FGs because any agglomeration of nanoparticles is expected to alter the magnetization behavior significantly. In contrast, CoFe2O4-based FGs show a hysteretic magnetization curve. This reveals a strong mechanical coupling between the P123 mesophase and the cobalt ferrite nanoparticles because in this case the magnetic nanoparticles are blocked; that is, remagnetization is dominated by Brownian-type relaxation. Acknowledgment. Financial support by the German Science Foundation (Research Group FOR 608, project 8) is gratefully acknowledged. We thank Martin Hufnagel for his help in the preparation of the CoFe2O4-based FGs. Supporting Information Available: Particle size distributions of the used FFs, additional results from rheology and μ-DSC for NGs, as well as maghemite-based FGs, polarized light microscopy images for NG35 and FG35FF11, and rheology and μ-DSC data for FG35FF4 based on a cobalt ferrite FF. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(24), 19181–19190