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ARTICLES Unperturbed Volume Transition of Thermosensitive Poly-(N-isopropylacrylamide) Microgel Particles Embedded in a Hydrogel Matrix Judith Musch,† Stefanie Schneider,† Peter Lindner,‡ and Walter Richtering*,† Institute of Physical Chemistry, RWTH Aachen UniVersity, Landoltweg 2, D-52074 Aachen, Germany, and Institute Laue-LangeVin, 6 rue Jules Horowitz, BP 156-38042, Grenoble Cedex 9, France ReceiVed: December 20, 2007; ReVised Manuscript ReceiVed: February 27, 2008
The thermoresponsive behavior of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in a covalently cross-linked polyacrylamide hydrogel matrix was investigated using ultraviolet-visible (UV-vis) spectroscopy, small-angle neutron scattering (SANS), and confocal laser scanning microscopy. The hydrogel synthesis was performed at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or fully collapsed PNiPAM microgel particles during the incorporation step. UV-vis spectroscopy experiments verify that the incorporation of thermosensitive microgels leads to temperaturesensitive optical properties of the composite materials. SANS measurements at different temperatures show that the thermosensitive swelling behavior of the PNiPAM microgels is fully retained in the composite material. Volume and structure criteria of the embedded microgel particles are compared to those of the free microgels in acrylamide solution. To visualize the temperature responsive behavior of larger PNiPAM particles, confocal fluorescence microscopy images of PNiPAM beads, of 40-µm size, were taken at two different temperatures. The micrographs also demonstrate the retained temperature sensitivity of the embedded microgels. Introduction Hydrogels are of high interest for technical and medical applications. Especially, stimuli responsive or so-called “smart” polymer gels have been studied extensively during the past decades. The gels react to external stimuli, e.g., change in pH or temperature, by a drastic change of their volume. By far, the most studied temperature-sensitive polymer gel is cross-linked poly-(N-isopropylacrylamide (PNiPAM), which has a volume phase transition temperature (VPTT) of 32 °C.1,2 Hydrogels by themselves show heterogeneities with respect to cross linking and segment density. These heterogeneities depend on the conditions of gel synthesis and have a strong influence on the properties of the gel.3 The incorporation of a second component as, e.g., nanoparticles or clay minerals, provides a further possibility to introduce heterogeneities, and such particles have a significant influence on hydrogel properties as well.4–6 By use of particles that are stimuli-sensitive, one can obtain hydrogels, the properties of which can be manipulated via the stimulus thus enabling the preparation of complex hydrogels with tailor-made properties. Analogous to macroscopic hydrogels, PNiPAM microgels reveal a large decrease of the volume with rising temperature as well.7 However, the kinetics of their temperature-dependent swelling are much faster as compared to macroscopic PNiPAM hydrogels due to the smaller dimension of the microgel.8,9 To obtain macroscopic gels, with a fast response to external stimuli, sensitive microgel particles can be embedded into a nonrespon* To whom correspondence should be addressed. Phone: +49 (0) 241 80 94760. Fax: +49 (0) 241 80 92327. E-mail:
[email protected]. † Institute of Physical Chemistry, RWTH Aachen University. ‡ Institute Laue-Langevin.
sive hydrogel matrix, which at the same time serves as a solvent reservoir. The hydrogel matrix furthermore prevents the flocculation of microgel particles at temperatures above the VPTT. Chu et al. synthesized PNiPAM hydrogels with different temperature-sensitive microgels to improve the thermosensitive properties of the macroscopic gel. They showed via scanning electron microscopy that PNiPAM hydrogels, which contain microgel, exhibit a tighter network than the pure PNiPAM hydrogels.10 Reese et al. developed a nanosecond photonic crystal material by incorporating PNiPAM microgels that selfassemble into crystalline colloidal arrays into a polyacrylamide matrix and investigated the optical properties of the system.11 The deswelling behavior of thin microgel composite films has been reported by Lyon et al.12 Our group investigated the long-range heterogeneities of polyacrylamide hydrogels with embedded PNiPAM microgels by means of static and dynamic light scattering. Mechanical properties of the composite materials as well as the gelation process were studied by rheological measurements.13,14 Concomitantly Lynch et al. started investigations on a similar system.15 They also used a composite gel with PNiPAM microgels in a non-temperature-sensitive hydrogel matrix and focused on the release of labeled molecules and the diffusion of small polymers through the network.16 The cross-linker content of their composite gel was similar to the cross-linker concentration that we used in this work, but in their case the microgel concentration was more than twice the concentration we used. Lynch et al. showed that the incorporated microgels have only a small influence on the diffusion of poly(ethylene glycol) (PEG) through the hydrogel. The structure of the
10.1021/jp711939v CCC: $40.75 2008 American Chemical Society Published on Web 04/30/2008
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Musch et al. this study. Thus the full thermosensitive properties of the PNiPAM microgels are retained in the composite materials. Experimental Section
Figure 1. Schematic representation of the synthesis of filled hydrogels in aqueous solution:13 component 1, acrylamide (monomer); component 2, N,N′-methylenbisacrylamide (BIS, cross linker); component 3, ω,ω′diethoxyacetophenone (DEAP, starter). The reaction was performed at constant temperature and under UV irradiation with a wavelength of 254 nm.
embedded microgels and their temperature-dependent behavior has not been investigated.17 The temperature-dependent swelling of the microgel particles used in this study has been investigated in detail by small-angle neutron scattering (SANS) measurements.18 The PNiPAM microgel particles in the fully swollen state at 20 °C have a hydrodynamic radius Rh of 128 nm. At higher temperatures (above the VPTT), the PNiPAM particle size decreases to a radius of 49 nm at a temperature of 40 °C. The refractive index difference between particles and the surrounding increases, and light is scattered more strongly.11 This effect leads to an increased turbidity of the microgel dispersion when crossing the VPTT. Core-shell particles with the same PNiPAM core and a shell of poly-N-isopropylmethacrylamide (PNiPMAM), which has a higher VPTT (44 °C) than PNiPAM, were prepared by Berndt et al. and investigated using SANS.19 It was found that the shell strongly influences the swelling of the core.18,20–23 Therefore one could expect that a macroscopic hydrogel network influences the swelling of embedded PNiPAM microgels in a similar fashion. In this work PNiPAM microgels were incorporated into a polyacrylamide (PAAm) hydrogel matrix at two different temperatures (see Figure 1 for the reaction scheme). At low preparation temperatures (25 °C) swollen microgel particles were embedded, whereas at a preparation temperature of 40 °C, i.e., above the VPTT of PNiPAM, the embedded particles were fully collapsed. Different cross-linking densities in the polyacrylamide matrix were investigated, and we address the question whether or not the temperature-dependent swelling of the microgel particles is retained in the composite material or if swelling/ shrinking is hindered by the surrounding hydrogel matrix. An influence of the preparation temperature on the swelling behavior of the microgels can be expected if the interaction between microgel particles and the hydrogel matrix is strong. For composite gels prepared at low temperatures, the shrinking of the microgels at T > VPTT might be hindered due to entanglements and/or chemical bonds between the two networks. Similarly, at high preparation temperatures, the microgels are fully collapsed during the embedding process and have to expand against the hydrogel matrix when the sample is cooled afterward. A strong hydrogel network might restrict this swelling, even if there are no entanglements and/or chemical bonds between microgels and hydrogel matrix. The temperature-dependent size of the microgel particles was determined by analyzing the form factor obtained by SANS. In addition, the size of large embedded PNiPAM beads was visualized at two different temperatures using confocal fluorescence microscopy. It will be shown below that the temperature-dependent swelling of the microgels was not significantly altered by the hydrogel for all preparation conditions used in
Materials. N,N-Methylenbisacrylamide (BIS), N,N,N′,N′tetramethylethylendiamin (TEMED), and potassium peroxodisulfate (KPS) were obtained from Merck. N-Isopropylacrylamide (NiPAM) was purchased from Acros. The surfactant Synperonic PE/L61 and acrylamide was commissioned from Fluka/Serva and the starter Diethoxyacetophenon (DEAP) from ABCR GmbH and Co KG. The dye PolyFluor TM 570 (Methacryloxyethyl thiocarbonyl rhodamine B) was obtained from Polyscience Inc. All chemicals were used without further purification. The prepared solutions and the reactions were all preformed in double distilled water or in D2O (SANS samples), which was obtained from Deutero. For the SANS samples, also deuterated acrylamide purchased from Camebridge Isotopes Laboratories Inc. was used. The PNiPAM microgel particles used in the SANS and UV-vis measurements were synthesized via dispersion polymerization and characterized by Berndt et al. The molar ratio of cross linker to monomer was 1:71. Details of the synthesis are described in the literature.19 PNiPAM Bead Synthesis. A 500-mL three-neck flask, equipped with KPG-stirrer, reflux condenser, and a nitrogen inlet, was filled with 200 mL of paraffin oil and flushed with nitrogen overnight at room temperature. Synperonic (1.8 g) was added, and after stirring 45 min, a solution of 0.6 g of NiPAM, 0.08 g of BIS, 0.08 g of KPS, and 0.01 g of methacryloxyethyl thiocarbonyl rhodamine B in 4 mL of water was added. After 60 min of stirring the reaction was started by adding 0.5 g of TEMED. The solution was stirred for about three hours at room temperature. Finally, the solution was centrifuged three times with a Hettich centrifuge universal 320 R for 60 min to remove the oil from the samples. Between each centrifugation step the supernatant was removed and replaced by doubly distilled water. Hydrogel Synthesis. The filled and unfilled hydrogels were prepared in a glovebox under argon atmosphere directly either in neutron-scattering cells, in UV-vis cells, or in temperaturecontrollable cells for the confocal fluorescence microscopy. The gels were prepared from a 3.75 wt % aqueous acrylamide solution (D2O for SANS experiments) with BIS used as crosslinker. For the preparation of composite gels 0.2 wt % microgel or microgel beads were added as well. To start the polymerization a drop of DEAP was added to the solution and the samples were exposed to UV light (λ ) 254 nm) for about 10 min.14 For the SANS measurements, we used two different cross linker-to-monomer ratios (molar ratio 1:15 and 1:60). Because of the fact that hydrogels with a molar cross linker-to-monomer ratio of 1:15 are turbid we used a cross linker-to-monomer ratio of 1:150 for the UV-vis-measurements. The confocal fluorescence microscopy measurements were all performed with samples with a cross linker-to-monomer ratio of 1:60. The exact compositions of UV-vis and SANS samples are shown in Table 1. UV-Vis Spectroscopy. The samples for the UV-visexperiments were prepared in plastic cuvettes with a thickness of 1 cm. The measurements were performed with a Jasco spectrometer V-530. As a reference we used a cuvette filled with doubly distilled water. All extinction measurements were performed at a wavelength of 550 nm. The temperature was varied between 20 and 40 °C with a step width of 1 K.
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TABLE 1: Summary of the Composition of the Composite Gels and of the Effective Microgel Volume Fraction and Temperature during Preparation of the Hydrogels
a
sample
[microgel]
ΦPrepa
[BIS]
[acrylamide]
BIS:AAm
TPrepb
UV-vis 60 20 UV-vis 60 40 UV-vis 150 20 UV-vis 150 40 SANS 60 20 SANS 60 40 SANS 15 20 SANS 15 40
0.25 wt % 0.25 wt % 0.25 wt % 0.25 wt % 0.25 wt % 0.25 wt % 0.25 wt % 0.25 wt %
0.0684 0.0068 0.0684 0.0068 0.0684 0.0068 0.0684 0.0068
0.18 wt % 0.18 wt % 0.072 wt % 0.072 wt % 0.18 wt % 0.18 wt % 0.72 wt % 0.72 wt %
3.75 wt % 3.75 wt % 3.75 wt % 3.75 wt % 3.75 wt % 3.75 wt % 3.75 wt % 3.75 wt %
1:60 1:60 1:150 1:150 1:60 1:60 1:15 1:15
20 °C 40 °C 20 °C 40 °C 20 °C 40 °C 20 °C 40 °C
Volume fraction while in preparation. b Temperature while in preparation.
SANS. SANS measurements were performed at the D11 beam line of the Institut Laue-Langevin (ILL) in Grenoble, France. A broad q range from 0.0015 to 0.2 Å was covered with three sample-detector distances of 2.5, 10.0, and 36.7 m and a neutron wavelength of 6 Å. Hellma quartz glass cells with a sample thickness of 1 mm were used. The data were collected on a two-dimensional multidetector (64 ) 64 elements of 1 ) 1 cm2) and corrected for background scattering (scattering of the microgel-free hydrogel matrix). The data were calibrated on absolute scale using the incoherent scattering of H2O according to the standard procedures at the ILL. All experiments were carried out using fully deuterated polyacrylamide for the hydrogel matrix and D20 as a solvent. However, the cross linker was not deuterated leading to a background scattering. The microgel concentration in all samples was 0.2 wt %. The microgel scattering data were analyzed using a form factor model of an inhomogeneous sphere as described elsewhere.18,24 For a schematic representation of the model see Figure 2. In this model R and σsurf are the two main adjustable parameters, where R is the radius of the particle at which the scattering length density profile decreased to 1/2 of the core density and σsurf is a parameter defining the width of the smeared particle surface. The inner region of the microgel, which shows a higher degree of cross linking, is described by a radial box profile extending to a radius of Rbox ) R - 2 σsurf. In dilute solution, the density profile approaches zero at RSANS ) R + 2σsurf. The overall size of the particle is therefore given by RSANS. Confocal fluorescence microscopy measurements were performed on a Micro Time 200 inverse time-resolved Fluorescence Microscope (MT200) as described elsewhere.25 A laser PicoTA530N with a wavelength of 532 nm was used. The laser was pulsed with a frequency of 40 MHz and a pulse width of 50 ps. The confocal setup includes a pinhole of 10 µm and a laser clear up filter (HQ580/70m). We used two single-photon avalanche diodes (SPAD, PDM series) for the detection of the fluorescence light with a detection diameter of 50 µm. The temperature of the sample is controlled by a homemade temperature-controlled cell. Results and Discussion UV-Vis Spectroscopy. We used UV-vis measurements to investigate the temperature-dependent optical properties of the composite materials. As mentioned before, a PNiPAM microgel solution becomes turbid at a temperature of 32 °C, i.e., the cloud point of composite gels can be observed with UV-vis spectroscopy. Light scattering is difficult to employ for these samples due to the high background scattering of the acrylamide matrix and the immobility of the microgel particles in the gel matrix, which leads to heterogeneities in the sample. In dynamic lightscattering measurements of free microgels in solution, a collapse
Figure 2. Structure of PNiPAM microgels. A highly cross-linked core is characterized by a radial box profile up to Rbox ) R - 2σsurf. The cross-linking density decreases with increasing distance from the core and is described by σsurf. At R the profile has decreased to half the core density. The overall size obtained by SANS is given by RSANS ) R + 2σsurf where the profile approaches zero. RSANS is slightly smaller than the hydrodynamic radius Rh.24
of the microgels at 32 °C from a hydrodynamic radius of 128 nm to a radius of 49 nm was observed.26 Figure 3 shows the temperature-dependent extinction of the composite materials. The extinction is given as E ) log(I0/I), where I0 is the incident and I is the transmitted intensity. All UV-vis measurements were performed at a wavelength of 550 nm. The samples were prepared at two different cross-linker concentrations and two different preparation temperatures (one well below (20 °C) and one well above (40 °C) the VPTT, see Table 1). The absolute values of the extinction depended both on the temperature and on the cross-linker concentration. With higher amount of cross linker the turbidity increased. Therefore UV-vis experiments with composite gels with a molar crosslinker to monomer ratio of 1:15 were difficult to perform, and samples with a monomer to cross-linker ratio of 1:150 were used.
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Figure 3. UV-vis extinction as a function of temperature for PNiPAM microgels in a cross-linked polyacrylamide matrix with cross linkerto-monomer concentrations of 1:60 (squares) and 1:150 (diamonds). The microgels were prepared either at 20 °C (solid symbols) or at 40 °C (open symbols).
All four investigated samples behaved similarly. At temperatures around 32 °C, the extinction increased drastically, and the temperature of the extinction increase was the same for all samples and corresponds to the VPTT of the microgels in solution. Thus the thermoresponsive properties of the microgels are retained when embedded in the hydrogel matrix. All composite gels show the same VPTT (32 °C) independent of cross-linker concentration and preparation temperature. SANS. Figures 4 and 5 show the scattering curves of the samples with microgel particles in deuterated acrylamide solution and embedded in the deuterated polyacrylamide matrix at two different cross-linker concentrations and at two different preparation temperatures, namely, 25 (Figure 4) and 40 °C (Figure 5). Figures 4a and 5a show results from SANS measurements at 25 °C, and Figures 4b and 5b display the results from measurements at 40 °C. All samples show similar scattering curves independent of the surrounding of the microgels. Obviously, the hydrogel matrix has only little effect on the temperature-dependent swelling of the microgels. The fits according to the form factor model described above show good agreement with the measured data. At temperatures below the VPTT the particles are swollen, and the particle radius, RSANS, was found to be approximately 110-115 nm for samples with a surface parameter σsurf of about 18-22 nm and R between 66 and 69 nm. The polymer volume fraction in the core of these particles was found to be about 11%. At high temperatures, the microgel particles radius decreased to RSANS ≈ R between 45 and 48 nm with polymer volume fractions of roughly 50%. The slope of I ≈ q-4 at high q gives evidence of a sharp particle surface at temperatures above the VPTT. The parameters obtained from the fits of the experimental intensity I(q) are in good agreement with the results obtained with the microgels in aqueous solution that were reported previously by Berndt et al.18 A closer inspection of the scattering data, however, reveals that no perfect agreement of the fits and the scattering data was achieved for the composite gel samples. In Figure 6 one can see that the fit slightly deviates from the data, especially at low q. To obtain the scattering curve of the microgels, we measured the background scattering of the polyacrylamide gel matrix without embedded microgel particles and subtracted it from the total scattering of the composite sample leading to the data displayed in Figure 6. The scattering intensity of the polyacrylamide matrix was much weaker but revealed an intensity increase at low q, indicating that the scattering contributions
Figure 4. SANS curves of samples prepared with fully swollen microgels at 20 °C and measured at (a) 25 °C and (b) 40 °C. The SANS intensity was plotted as a function of the momentum transfer q for PNiPAM microgels in a cross-linked deuterated polyacrylamide matrix with a cross linker-to-monomer ratio of 1:15 (red triangles) and 1:60 (blue squares) and in deuterated acrylamide solution (black circles). The lines show the numerical fits.
from the polyacrylamide gel are not fully matched. This is mainly due to the cross linker, which is not deuterated. One can assume that the cross-linker distribution in the microgelfree polyacrylamide gel was homogeneous, whereas the distribution of cross linker might have become inhomogeneous when microgel particles were present during the polymerization of the matrix. Therefore, the background scattering in the gel sample could be different from the scattering of the microgelfree polyacrylamide gel, leading to an incorrect background subtraction. Nevertheless, we expect mainly the forward scattering of the hydrogel matrix to change, and therefore the q position of the first minimum of the microgel form factor should remain unaffected. Thus from the q position of the form factor minimum and from the overall shape of the curve, we can safely conclude that the particles in the composite material can swell and deswell to the same size as the particles in free solution. Confocal Microscopy. To observe the shrinking behavior of microgels in composite hydrogels in real space, we investigated the influence of temperature on the size of microgel beads using confocal fluorescence microscopy employing PNiPAM beads with a size of ca. 40 µm. These beads were synthesized in suspension polymerization at temperatures below the VPTT and will a have different morphology as compared to the smaller microgels that were synthesized by dispersion polymerization above the VPTT. Nevertheless, the temperature sensitivity of the particle size, which is the relevant property for our study, is essentially the same for beads and microgels. Therefore the beads can safely be employed for comparing the general aspects of temperature-dependent volume change of PNIPAM particles
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Figure 7. Confocal fluorescence microscopy pictures of PNiPAM beads in aqueous solution (left) and in a hydrogel matrix with a cross linker-to-monomer ratio of 1:60 (right). All samples were prepared at room temperature and measured at 25 (top) and 40 °C (bottom). The scale bars represent a size of 20 µm.
Figure 5. SANS curves of samples prepared with collapsed microgels at 40 °C and measured at (a) 25 °C and (b) 40 °C. The SANS intensity was plotted as a function of the momentum transfer q for PNiPAM microgels in a cross-linked deuterated polyacrylamide matrix with a cross linker-to-monomer ratio of 1:15 (red triangles) and 1:60 (blue squares) and in deuterated acrylamide solution (black circles). The lines show the numerical fits.
VPTT (40 °C). The polymerization of the hydrogel matrix was carried out at room temperature, i.e., with beads in the swollen state. The pictures on the left-hand side show the shrinking behavior of PNiPAM beads in water. The central bead (the center of which was in the focal plane) shrank from a diameter of 40 µm to a size of 13 µm. The PNiPAM beads embedded in a polyacrylamide matrix, shown on the right-hand side, displayed similar shrinking behavior. Although only few individual particles are monitored in confocal microscopy, these results clearly indicate that the gel matrix does not significantly influence the temperature sensitivity of the embedded PNiPAM beads. Thus the results from microscopy obtained with 40 µm beads agree nicely with the SANS results obtained from much smaller microgels. Conclusions
Figure 6. SANS intensity as a function of the momentum transfer for PNiPAM microgels in a cross-linked deuterated polyacrylamide matrix with a cross linker-to-monomer ratio of 1:60 (blue, open squares) and of the corresponding microgel-free polyacrylamide matrix (red diamonds). The background scattering of the polyacrylamide matrix was subtracted from the total scattering (black, solid squares) to obtain the microgel scattering. The samples were prepared at 20 °C and measured at 40 °C.
observed on the one hand with beads in real space and on the other hand with microgels in reciprocal space. Figure 7 shows confocal fluorescence microscopy images of such microgel beads in aqueous solution and embedded in a hydrogel matrix at two different temperatures, below (25 °C) and above the
Composite materials of temperature sensitive PNiPAM microgels and a non-thermosensitive polyacrylamide matrix have been prepared at different temperatures (below and above the VPTT of PNiPAM) and with different cross-linker concentrations. UV-vis measurements revealed that the composite material became thermosensitive due to the incorporation of the thermosensitive microgels. Furthermore, the VPTT of the PNiPAM microgels was not affected by the presence of the polyacrylamide matrix, independent of the matrix cross-linking density or the preparation temperature. SANS experiments demonstrated that the embedded microgel particles were able to swell and deswell to the same size as the corresponding particles in free solution. Similarly, larger PNiPAM beads synthesized at 25 °C were able to deswell freely, as observed with confocal fluorescence microscopy. Therefore, we can conclude that we successfully synthesized macroscopic thermosensitive composite gels by embedding thermoresponsive microgel particles into a polyacrylamide gel matrix while retaining the full temperature sensitivity of the microgel particles. For sophisticated applications as, e.g., in controlled release or sensing, it can be beneficial to combine particles with different
6314 J. Phys. Chem. B, Vol. 112, No. 20, 2008 sensitivities. In such mixtures, however, interactions between different particles, e.g., caused by flocculation above the VPTT of one component, can affect materials properties. Embedding different particles in a hydrogel matrix offers the possibility to avoid such particle interactions while retaining the particles’ different environmental sensitivities. Work on multicomponent hybrid gels is in progress and will be reported in the future. Acknowledgment. This study was supported by the Deutsche Forschungsgemeinschaft within the SPP 1259. References and Notes (1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81 (12), 6379– 6380. (2) Nayak, S.; Lyon, L. A. Angew. Chem., Int. Ed. 2005, 117, 7886– 7709. (3) Shibayama, M. Macromol. Chem. Phys. 1998, 199 (1), 1–30. (4) Haraguchi, K.; Li, H.-J.; Song, L.; Murata, K. Macromolecules 2007, 40, 6973–6980. (5) Haraguchi, K.; Song, L. Macromolecules 2007, 40 (15), 5526– 5536. (6) Okay, O.; Oppermann, W. Macromolecules 2007, 40, 3378. (7) Pelton, R. AdV. Colloid Interface Sci. 2000, 85 (1), 1–33. (8) Wang, J. G., D.; Lyon, L. A.; El-Sayed, M. A. J. Am. Chem. Soc. 2001, 123, 11284. (9) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214–1218.
Musch et al. (10) Zhang, X.-Z.; Chu, C.-C. Polymer 2005, 46, 9664. (11) Reese, C. E.; Mikhonin, A. V.; Kamenjicki, M.; Tikhonov, A.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 1493–1496. (12) Nayak, S.; Debord, S. B.; Lyon, L. A. Langmuir 2003, 19, 7374– 7379. (13) Soddemann, M. Ph.D. thesis, Universita¨t Freiburg, 2002. (14) Soddemann, M.; Richtering, W. Prog. Colloid Polym. Sci. 2004, 129, 88–94. (15) Lynch, I.; Dawson, K. A. J. Phys. Chem. B 2003, 107, 9629–9637. (16) Lynch, I.; de Gregorio, P.; Dawson, K. A. J. Phys. Chem. B 2005, 109, 6257–6261. (17) Salvati, A.; So¨derman, O.; Lynch, I. J. Phys. Chem. B 2007, 111, 7367–7376. (18) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2006, 22, 459–468. (19) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780–8785. (20) Berndt, l.; Pedersen, J. S.; Richtering, W J. Am. Chem. Soc. 2005, 127, 9372. (21) Berndt, I.; Pedersen, J. S.; Richtering, W.; Angew. Chem. Int. Ed. 2006, 45 1737; Angew. Chem. 2006. 118, 1769. (22) Gan, D.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 8203. (23) Jones, C. D.; McGrath, J. G.; Lyon, L. A. J. Phys. Chem. B 2004, 108, 12652. (24) Stieger, M.; Richtering, W.; Pedersen, J. S.; Lindner, P. J. Chem. Phys. 2004, 120, 6197–6206. (25) Bo¨hmer, M.; Pampaloni, F.; Wahl, M.; Rahn, H. J.; Erdmann, R.; Enderlein, J. ReV. Sci. Instrum. 2001, 72 (11), 4145–4152. (26) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705–1711.
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