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Temperature-Sensitive Hybrid Microgels with Magnetic Properties Andrij Pich,*,† Sanchita Bhattacharya,† Yan Lu,† Volodymyr Boyko,‡ and Hans-Juergen P. Adler† Institute of Macromolecular Chemistry and Textile Chemistry, Dresden University of Technology, D-01062 Dresden, Germany, and Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany Received June 9, 2004. In Final Form: August 9, 2004 In the present paper, we report the preparation of hybrid temperature-sensitive microgels which include magnetite nanoparticles in their structure. Polymeric microgels have been prepared by surfactant-free emulsion copolymerization of acetoacetoxyethyl methacrylate (AAEM) and N-vinylcaprolactam (VCL) in water with a water-soluble azo-initiator. The obtained microgels possess a low critical solution temperature (LCST) in water solutions, with a rapid decrease of the particle size being observed at elevated temperatures. Magnetite was deposited directly into microgels, leading to the formation of composite particles which combine both temperature-sensitive and magnetic properties. The influence of magnetite load on microgel size, morphology, swelling-deswelling behavior, and stability is discussed.
Introduction Polymeric microgels form an important subdivision of polymer colloids which utilize properties of water-soluble macromolecules, bulk hydrogels, and hydrophobic latex particles. In recent years, increasing attention has been focused on the preparation and characterization of microgels containing thermosensitive polymers.1-3 These spherical particles display a strong thermoresponsivity; for example, below a characteristic lower critical solution temperature (LCST), they are highly swollen in water, but after heating, they shrink rapidly to become a collapsed polymer globule. The swelling ability of a microgel depends on the type of microgel, its affinity to the solvent, which in turn is dependent on the monomer (and/or comonomer) composition/concentration, as well as the degree of crosslinking. The nature of the monomer used in the preparation of the microgels will determine the overall properties of the final product. The addition of monomers with different functionalities to the microgel can create particles with a wide range of properties and widen the range of applications. Microgels have been used as containers for different drugs,4 metal nanoparticles,5-7 or magnetite.8 There has been much interest in utilizing magnetic nanoparticles in biological applications such as magnetic resonance imaging contrast enhancement9 and drug * Corresponding author. E-mail: andrij.pich@ chemie.tu-dresden.de. † Institute of Macromolecular Chemistry and Textile Chemistry. ‡ Physical Chemistry and Electrochemistry. (1) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (2) Duracher, D.; Elaı¨ssari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 905. (3) Hu, Z.; Lu, X.; Gao, J.; Wang, C. Adv. Mater. 2000, 12, 1173. (4) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2003, 19, 8675. (5) Biffis, A.; Sperotto, E. Langmuir 2003, 19, 9548. (6) Antonietti, M.; Gro¨hn, F.; Hartmann, J.; Bronstein, L. Angew. Chem., Int. Ed. 1997, 36, 2080. (7) Whilton, N. T.; Berton, B.; Bronstein, L. Adv. Mater. 1999, 11, 12. (8) Kawai, T.; Gobe, M.; Satou, Y.; Kon-No, K.; Kitahara, A. Yukagaku 1994, 43, 1011. (9) Babes, L.; Denizot, B.; Tanguy, G.; Le Jeune, J.; Jallet, P. J. Colloid Interface Sci. 1999, 212, 474.
delivery.10,11 Incorporation of magnetite into smart microgel particles can solve the problem of stabilization in physiological environments. In a previous work,12 we studied the colloidal and temperature behavior of a novel microgel system based on N-vinylcaprolactam (VCL) and the hydrophobic comonomer acetoacetoxyethyl methacrylate (AAEM), prepared by precipitation polymerization with a cationic initiator in surfactant-free conditions. VCL/AAEM microgels have been successfully used for the incorporation of conducting polymer (polypyrrole), and obtained composite particles combine both temperature sensitivity and electrical conductivity.13,14 Here, we first demonstrate that magnetic nanoparticles can be incorporated into microgel particles with no loss of temperature sensitivity or colloidal stability. Materials and Methods Chemicals were generally reagent grade from commercial sources. Acetoacetoxyethyl methacrylate (AAEM) was obtained from Aldrich, purified by conventional methods, and then stored under nitrogen. N-vinylcaprolactam (VCL) was obtained from Aldrich and purified by distillation. The initiator, 2,2′-azobis(2-methylpropyonamidine) dihydrochloride (AMPA), the crosslinker N,N′-methylenebisacrylamide (BIS), iron (III) chloride (FeCl3), and iron (II) chloride (FeCl2) were obtained from Aldrich and used as received. Distilled water was employed as the polymerization medium. Ammonium hydroxide (NH4OH) in the form of 25% water solution was obtained from Fluka. Microgel Synthesis. Appropriate amounts of AAEM (0.16 g), VCL (1.98 g), and the cross-linker (3 mol %) were added to 145 mL of deionized water. A double-wall glass reactor equipped with a stirrer and a reflux condenser was purged with nitrogen. The solution of the monomers was placed into the reactor and stirred for 1 h at 70 °C with purging with nitrogen. After that, the 5 mL water solution of initiator (5 g/L) was added under continuous stirring. The reaction was carried out for 8 h. (10) Scientific and Clinical Applications of Magnetic Carriers; Hafeli, U., Schutt, W., Teller, J., Zborowski, M., Eds; Plenum Press: New York, 1997. (11) Safarik, I.; Safarikova, M. Monatsh. Chem. 2002, 133, 737. (12) Boyko, V.; Pich, A.; Lu, Y.; Richter, S.; Arndt, K. F.; Adler, H. J. Polymer 2003, 44/25, 7821. (13) Pich, A.; Lu, Y.; Boyko, V.; Arndt, K. F.; Adler, H. J. Polymer 2003, 44/25, 7651. (14) Pich, A.; Lu, Y.; Boyko, V.; Arndt, K. F.; Adler, H. J. Polymer 2004, 45/4, 1079.
10.1021/la040084f CCC: $27.50 © 2004 American Chemical Society Published on Web 10/20/2004
Hybrid Microgels with Magnetic Properties
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Figure 1. Chemical structure of the VCL/AAEM copolymer (a) and SEM image of cross-linked microgel particles (b). Magnetite Deposition. Deposition of magnetite was performed according to the modified method described in ref 15. Microgel dispersions were placed into a stirred reactor, and the mixture was stirred for 15 min under nitrogen flow at 25 °C. Solutions of FeCl2 and FeCl3 were prepared in separate flasks and added to the stirred dispersion under a nitrogen blanket (the molar ratio FeCl3/FeCl2 was kept constant at 2:1). A water solution of NH4OH was added dropwise to start the magnetite formation process. Immediately after base addition, the dispersion became dark brown, indicating that magnetite has been formed in the system. After 60 min, the formed composite particle dispersion was removed from the reaction vessel and cleaned by dialysis to remove all byproducts. Purification Procedure. The VCL/AAEM microgel dispersion as well as the composite particles with magnetite were purified by dialysis with deionized water for 4 days. The dispersions were dialyzed by using a Millipore device operating with Biomax 100 membrane (MWCO 100.000). Dynamic Light Scattering. A commercial laser light scattering (LLS) spectrometer (ALV/DLS/SLS-5000) equipped with amultiple digital time correlator (ALV-5000/EPP) and a laser goniometer system (ALV/CGS-8F S/N 025) was used with a helium-neon laser (Uniphase 1145P, output power of 22 mW and wavelength of 632.8 nm) as the light source. In dynamic LS, the Laplace inversion (the CONTIN procedure) of each measured intensity-time correlation function, G(2)(q,t), in the self-beating mode can be related to a line-width distribution, G(Γ). For a pure diffusive relaxation, Γ is related to the translational diffusion coefficient, D, by Γ/q2 ) D at q f 0 and c f 0 or the hydrodynamic radius, Rh, by Rh ) kBT/(6πηD), with kB, T, and η being the Boltzmann constant, absolute temperature, and solvent viscosity, respectively. Dynamic light scattering (DLS) measurements were performed at angles from 30 to 130°. The particle concentration was varied from 0.001 to 0.1 g/L. The accuracy of the measurements for the hydrodynamic radius was (3%. Stability Measurements. Stability measurements were performed with the separation analyzer LUMiFuge 114 (L.U.M. GmbH, Germany) in glass cuvettes at an acceleration velocity of 3000 rpm. The slopes of the sedimentation curves were used to compare the stabilities of the samples. Scanning Electron Microscopy (SEM). SEM images were taken at a voltage of 4 kV with a Gemini microscope (Zeiss, Germany). Dispersions were diluted with distilled water, dropped onto an aluminum support, and freeze-dried. Atomic Force Microscopy (AFM). Measurements were made with a Dimension 3100 microscope (Digital Instruments Inc.) using the tapping mode regime (set point ratio, 90%; integral gain, 0.2; proportional gain, 2.0; amplitude setpoint, 0.7 V; scan rate, 0.901 Hz). Samples were prepared by the spin coating technique (2000 rpm, 10 min) on previously cleaned glass supports. Thermogravimetrical Analysis (TGA). TGA measurements were made with a TGA 7 Perkin-Elmer instrument, and Pyris-Software, version 3.51, was used. Samples were dried in a vacuum for ∼48 h and analyzed in closed aluminum cups at a heating rate of 5 °C/min under a nitrogen atmosphere. ξ-Potential Measurements. ξ-potential measurements were performed with a Zetasizer 2000 instrument (Malwern Instruments). pH was adjusted by the addition of 0.01 M NaOH or (15) Buske, N.; Sonntag, H.; Go¨tze, T. Colloids Surf. 1984, 12, 195.
Table 1. Reagents Used for the Synthesis of Composite Microgelsa run
microgel (g)
FeCl2 (g)
FeCl3 (g)
1 2 3 4 5 6 7 8 9
0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
0.0059 0.0119 0.0170 0.198 0.0230 0.0251 0.0261 0.0293
0.0161 0.0321 0.0459 0.534 0.0650 0.0676 0.0709 0.0803
a
NH4OH water Fe3O4T Fe3O4P (g) (g) (wt %) (wt %) 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
-58.8 -58.8 -58.8 -58.8 -58.8 -58.8 -58.8 -58.8 -58.8
0 3.6 7.3 10.5 12.2 14.8 15.4 16.2 18.3
0 2 4 6.8 7.1 8.22 9.4 11.57 16.2
T and P stand for theoretical and practical load, respectively.
0.01 M HCl. The average value of at least 10 measurements was adopted as the ξ-potential at a given pH value. Magnetization Measurements. The vibrating-sample magnetometer (VSM, Oxford) was used to study the magnetic properties of composite particles with a maximum magnetic field of 1 T at 290 K.
Results Preparation of Composite Microgels. Thermosensitive microgels based on VCL and AAEM have been prepared under surfactant-free conditions.12 It was found that overall the VCL/AAEM ratio determines the size of microgels below the LCST due to regulation of crosslinking (swelling), but only slightly influences the size of collapsed particles. The microgels have a core-shell structure due to fast consumption of the more reactive methacrylic monomer. Due to the presence of AAEM in the copolymer structure, the transition temperature of the PVCL-rich shell is shifted to ∼28 °C as compared to the reported 32 °C value for pure PVCL. The AAEM-rich particle core is more hydrophobic and is less temperature sensitive compared with the VCL-rich shell. The chemical structure of the VCL/AAEM copolymer is presented in Figure 1a. Figure 1b shows the SEM image of VCL/AAEM microgel particles. Since drying at room temperature leads to the formation of dense film, the microgel sample was freezedried. Nevertheless, there is still a high tendency for film formation and fusion of particles, leading to nonspherical and polydisperse structures. In contrast, VCL/AAEM microgels are quite stable in the swollen state in water solution and can be used as smart containers for magnetite deposition. Table 1 shows the amounts of ingredients used in the magnetite preparation process in the presence of microgel particles. The amounts of iron salts were changed to increase stepwise the final magnetite load inside the microgels. Our aim was to determine the maximal Fe3O4 amount which can be deposited into porous particles without altering the colloidal stability and thermosensitive properties.
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Figure 2. Hydrodynamic radii of hybrid microgels as a function of magnetite load (a) and distribution curves for selected microgel samples (1, pure microgel; 2 and 3, 8.22 and 9.4% magnetite, respectively) (b).
Properties of Composite Microgels. Particle Size as a Function of Magnetite Load. Incorporation of inorganic material into the polymer network should influence the particle size of the microgels. Therefore, microgel particles with different magnetite loads were investigated by DLS. It has been found that the initial hydrodynamic radius of microgels decreases when the magnetite content is increased. When ∼7% magnetite was deposited into the microgels, the hydrodynamic radius started to increase gradually (see Figure 2a). When the magnetite content was higher than 16%, the system became unstable. Figure 2b indicates that there was no strong change in particle size distribution for microgel particles with different magnetite loads (the numbers in Figure 2a indicate the samples measured by DLS presented in Figure 2b). This indicates that the initial decrease of the hydrodynamic radii cannot be caused by the presence of small magnetite particles beside large composites. The previous decrease of the particle size can be explained by the specific interaction of magnetite nanoparticles and the polymer network. It can be assumed that β-diketone groups of AAEM can form complexes with iron, because such functionalities are well-known as metalbinding species. Powel et al.16 reported that copper was successfully extracted from pulverized crystals utilizing β-diketone functions of AAEM which was copolymerized with fluorinated acrylate. Amphiphilic block copolymers with bidentate β-dicarbonyl ligands were prepared via the chemical modification of suitable precursor polymers that are the acetoacetylation of hydroxylated polymers.17 Initial studies indicate that the copolymers of this type form complexes with metal salts to yield sterically stabilized colloidal hybrid materials. The complexation effect mentioned above provides probably efficient fixation of magnetite nanoparticles in the microgel core region which possesses a higher concentration of β-diketone sites. When the magnetite amount in microgel particles is low, filler particles are located within a certain distance of each other and the VCL/AAEM polymer chains can interact effectively with the particle surface of magnetite inclusions. Thus, stepwise increase of the magnetite particle content leads to shrinkage of the microgel template. This situation is possible up to a critical content of magnetite particles in the microgel structure. If the Fe3O4 content reaches a certain value, the distance between the inclusions will be smaller. Therefore, the repulsion forces between charged magnetite filler particles (16) Powell, K. R.; McCleskey, T.; Tumas, W.; DeSimone, J. M. Ind. Eng. Chem. Res. 2001, 40, 1301. (17) Schlaad, H.; Krasia, T. Macromolecules 2001, 34, 7585.
Figure 3. Morphology of composite microgels at different magnetite loads.
will dominate and the size of the composite microgel increases with further increase of the magnetite content. This situation is schematically presented in Figure 3. If the magnetite content reaches the critical value, some magnetite particles will be weakly connected to the composite microgel or located beside the template, leading to flocculation of composite microgels. Similar changes of the hydrodynamic radius were detected by the incorporation of nanoinclusions of polypyrrole into VCL/AAEM microgels by Pich et al.13,14 It seems that such effects are not really dependent on the nature of the incorporated material but require a strong affinity and compatibility of the polymer network and filler particles. These interactions will determine the location of nanoinclusions inside the swollen microgel and therefore the morphology of the formed composite. Particle Morphology. The method of magnetite preparation used in the present study provides the formation of superparamagnetic nanoparticles with average particle sizes
