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Articles PNIPAM-co-polystyrene Core-Shell Microgels: Structure, Swelling Behavior, and Crystallization Thomas Hellweg,*,† Charles D. Dewhurst,‡ Wolfgang Eimer,§ and Karl Kratz| Stranski-Laboratorium f. Physikalische und Theoretische Chemie, Technische Universita¨ t Berlin, Strasse des 17.Juni 112, D-10623 Berlin, Germany, Institute Laue-Langevin, Avenue des Martyrs, BP156-38042 Grenoble Cedex 9, France, Fakulta¨ t fu¨ r Chemie, Universita¨ t Bielefeld, Physikalische Chemie 1, 33615 Bielefeld, Germany, and mnemoScience GmbH, Carlstr. 50, D-52531 U ¨ bach-Palenberg, Germany Received August 12, 2003. In Final Form: February 11, 2004 The present contribution presents the single-step preparation and characterization of poly(N-isopropyl acrylamide)-co-polystyrene core-shell microgels with varying polystyrene content. The swelling behavior of the particles is investigated using dynamic light scattering and differs significantly from the swelling behavior of poly(N-isopropyl acrylamide) homopolymer particles. The lower critical solution temperature is found to be shifted to lower temperatures upon increasing the polystyrene content of the particles. The core-shell structure of the particles is revealed by means of small angle neutron scattering (SANS) using the method of contrast variation. Additionally, the formation of mesoscopic crystals of these particles is investigated by means of scanning electron microscopy and also by SANS. The particles seem to have preferable properties with respect to crystallization compared to homopolymer microgels.
1. Introduction During the last 2 decades thermosensitive hydrogels have received increasing attention due to their potential with respect to applications in drug delivery or as sensors.1-5 Most of the investigated systems are based on poly(N-isopropyl acrylamide) (PNIPAM), which has a lower critical solution temperature (LCST) of approximately 32 °C. However, macroscopic gels have rather long equilibration times with respect to swelling and deswelling. In some cases it might take days until the equilibrium state is reached.6,7 Hence, for applications as sensors or drug delivery systems so-called microgels8,9 have preferable properties compared to their macroscopic homologues. They are also well-suited to investigate the volume phase transition.10 Moreover, in microgel particles the properties arising from the gel network are combined with properties of classical colloids, e.g. crystallization11-14 or aggrega* To whom correspondence should be addressed. E-mail:
[email protected]. † Technische Universita ¨ t Berlin. ‡ Institute Laue-Langevin. § Universita ¨ t Bielefeld. | mnemoScience GmbH. (1) Dusek, K. Responsive Gels: Volume Transitions I. Advances in Polymer Science, 1st ed.; Springer-Verlag: Berlin, 1993; Volume 109. (2) Dusek, K. Responsive Gels: Volume Transitions II. Advances in Polymer Science, 1st ed.; Springer-Verlag: Berlin, 1993; Volume 110. (3) Shibayama, M. Macromol. Chem. Phys. 1998, 199, 1-30. (4) Annaka, M.; Tokita, M.; Tanaka, T.; Tanaka, S.; Nakahira, T. J. Chem. Phys. 2000, 112, 471-477. (5) Takeda, M.; Norisuye, T.; Shibayama, M. Macromolecules 2000, 33, 2909-2915. (6) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214-1218. (7) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636-1639. (8) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (9) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1-33. (10) Wu, C.; Zhou, S. Macromolecules 1997, 30, 574-576.
tion.15,16 This might be useful to generate colloidal crystals with fewer defects compared to classical colloids using the possibility to “temper” these materials by keeping them close to the LCST, where the particles shrink slightly and gain mobility again. This process is not possible with “classical” colloidal crystals (e.g. PMMA-based ones), which do not “melt” with increasing temperature. Another rather new aspect in the investigation of microgels is preparation of microgel hybrid materials with nanoparticles.17 Using copolymerization it is possible to alter the LCST of the microgel particles,18,19 and also core-shell particles can be obtained, for example, by preparing first a copolymer core particle and covering this in a second reaction step with a PNIPAM shell20 or by covering a PNIPAM core with, for instance, poly(N-isopropylmethacrylamide) (PNIPMAM). The second mentioned type of particle exhibits two shrinking steps when the temperature is changed.21 Also, acrylic acid containing coreshell microgels were already prepared.22 (11) Garcia-Salinas, M. J.; Romero-Cano, M. S.; de las Nieves, F. J. J. Colloid Interface Sci. 2002, 248, 54-61. (12) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705-1711. (13) Horn, F. M.; Richtering, W.; Bergenholtz, J.; Willenbacher, N.; Wagner, N. J. J. Colloid Interface Sci. 2000, 225, 166-178. (14) Hellweg, T.; Dewhurst, C. D.; Bru¨ckner, E.; Kratz, K.; Eimer, W. Colloid Polymer Sci. 2000, 278, 972-978. (15) Senff, H.; Richtering, W. Colloid Polym. Sci. 2000, 278, 830840. (16) Fernandez-Nieves, A.; Fernandez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Langmuir 2001, 17, 1841-1846. (17) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292-5293. (18) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf. A 2000, 170, 137-149. (19) Zha, L.; Hu, J.; Wang, C.; Fu, S.; Elaissari, A.; Zhang, Y. Colloid Polym. Sci. 2002, 280, 1-6. (20) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301-8306. (21) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780-8785. (22) Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 4544-4547.
10.1021/la0354786 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004
PNIPAM-co-polystyrene Core-Shell Microgels
It is also possible to use polystyrene core particles.23-25 Here, we are going to present a single-step synthesis of poly((N-isopropyl acrylamide)-co-polystyrene (PNIPAMco-PS) core-shell particles with extended PNIPAM shells. The used procedure is similar to the preparation described by Duracher and co-workers,26,27 but also particles with low styrene content are synthesized. Changing the shell thickness should allow for a variation of the interparticle interaction. Hence, several different PNIPAM-co-PS microgels were prepared containing different molar ratios of styrene to N-isopropyl acrylamide. The swelling curves for these microgels as obtained using dynamic light scattering (DLS) are presented and discussed. Additionally, using the method of contrast variation by means of deuteration, the core-shell structure of the prepared particles is revealed by small angle neutron scattering (SANS) experiments on dilute and concentrated solutions. SANS was already successfully used to reveal the inhomogeneous structure of PNIPAM homopolymer particles with high cross-linker content28 and the mesh size of the internal polymer network.33,32 In concentrated colloidal suspension, the PNIPAM-coPS particles are found to form crystallites, and slowly drying such a solution leads to a rather well ordered mesoscopic crystal. The crystals formed by the PNIPAMco-PS particles are imaged by scanning electron microscopy (SEM). 2. Materials and Methods 2.1. Synthesis of the PNIPAM-co-PS Microgels. N-isopropyl acrylamide (NIPAM), N,N-methylene bis-acrylamide (BIS), styrene, and potassium persulfate were obtained from Sigma-Aldrich. For the contrast variation experiments in SANS, deuterated styrene-d8 (isotopic purity 98%, Polymer Source, Inc., Lajoie, Canada) was used. All chemicals were reagent grade and used without further purification. The synthesis of the PNIPAMco-polystyrene particles was performed in a single step preparation employing a conventional stirring technique (300 rpm) as described elsewhere.8,29 After dissolving NIPAM, 0.50 mmol of BIS, and the desired amount of styrene (or styrene-d8) in 100 mL of triple-distilled, degassed water, the mixture was heated to 70 °C under a nitrogen atmosphere to exclude oxygen. Then, 2.4 mg of potassium persulfate dissolved in 1 mL of triple-distilled water was added to start the polymerization. The reaction proceeded for 4 h at constant temperature. Thereafter, the microgel suspension was cooled slowly to room temperature (4 h) under continued stirring. The final step of the preparation comprised of extensive dialysis for 14 days against double-distilled water. For all particles the molar ratio of NIPAM to BIS was kept constant at 3.8%. The NIPAM and styrene concentrations were adjusted to yield particles with a constant cross-linker molar ratio of 3.1% BIS. For the neutron scattering experiments, the (23) Dingenouts, N.; Nordhausen, C.; Ballauff, M. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1594-1596. (24) Dingenouts, N.; Nordhausen, C.; Ballauff, M. Macromolecules 1998, 31, 8912-8917. (25) Dingenouts, N.; Seelenmeyer, S.; Deike, I.; Rosenfeldt, S.; Ballauff, M.; Lindner, P.; Narayanan, T. Phys. Chem. Chem. Phys. 2001, 3, 1169-1174. (26) Duracher, D.; Sauzedde, F.; Elaissari, A.; Perrin, A.; Pichot, C. Colloid Polym. Sci. 1998, 276, 219-231. (27) Duracher, D.; Sauzedde, F.; Elaissari, A.; Perrin, A.; Pichot, C. Colloid Polym. Sci. 1998, 276, 920-929. (28) Fernandez-Barbero, A.; Fernandez-Nieves, A.; Grillo, I.; LopezCabarcos, E. Phys. Rev. E 2002, 66, 051803/1-10. (29) Kratz, K. Intelligente Poly-N-Isopropylacrylamid-Mikrogele unterschiedlicher chemischer Zusammensetzung. Einfluss von Konnektivita¨t, Ladungsdichte und Ionensta¨rke auf das Quellverhalten von PNIPA-Kolloiden. PhD-Thesis, University of Bielefeld, 1999. (30) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814-4820. (31) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213-217. (32) Kratz, K.; Hellweg, T.; Eimer, W. POLYMER 2001, 42, 65316539. (33) Crowther, H. M.; Saunders: B. R.; Mears, S. J.; Cosgrove, T.; Vincent, B.; King, S. M.; Yu, G.-E. Colloids Surf. A 1999, 152, 327-333.
Langmuir, Vol. 20, No. 11, 2004 4331 copolymer microgels were freeze-dried to remove water and afterward dissolved in D2O (isotopic purity g99%, Eurisotop, Groupe CEA, Saclay, France) or D2O/H2O mixtures to generate the desired scattering contrast conditions. 2.2. Dynamic Light Scattering (DLS). The dynamic light scattering experiments were performed using a classical goniometer setup (ALV Langen). The light source was an argon ion laser (Coherent Innova 90), which was operated in power mode at a wavelength of 488 nm. Moreover, by placing a thermostated Etalon into the laser resonator, single-mode output was obtained. The samples were positioned in an index-matching bath (toluene), which was thermostated with a temperature stability of (0.1 K. The scattering geometry was defined to be VV using a Glan polarizer for the incident beam and a Glan-Thompson polarizer in front of the photomultiplier (PM) employed for the detection of the scattered light. The collimation system for the scattered light consisted of two subsequent pinholes, which were chosen to optimize the scattering contrast. The fluctuations of the scattered intensity were analyzed with an ALV-5000 digital correlator. The measured intensity time correlation functions were analyzed using the method of cumulants30 and the Laplace inversion program CONTIN31 with 100 transformation grid points. From the relaxation rates, Γ, calculated on the basis of the experimental correlation functions, the translational diffusion coefficient, D, of the particles can be obtained using
Γ ) Dq2
(1)
Here, q ) 4πn/λ sin(θ/2) is the magnitude of the scattering vector. D is related to the hydrodynamic radius of the particles by the Stokes-Einstein equation. 2.3. Small-Angle Neutron Scattering (SANS). For the small-angle neutron scattering experiments presented here, the D11 machine installed at the high flux reactor of the Institute Laue Langevin (ILL) in Grenoble was used. This small-angle scattering machine allows us to cover a large q-interval ranging down to q-values usually probed in light scattering experiments. Therefore, the machine is well-suited to characterize colloidal particles with rather large diameters and crystals formed by these particles.13 The incident neutron wavelength was 10 Å and the sample to detector distances were chosen to be 2, 8, and 36.7 m, covering a q-range from 1 × 10-3 to 0.12 Å-1. The multidetector data were radially averaged about the beam-center, and the background counting rate was subtracted from the data using the program GRASP. After this procedure, the data from different sample to detector distances overlapped within the experimental error. However, in the present study no analysis of the absolute scattering intensity is performed and therefore no details of the procedure are given. The data analysis is performed using the indirect Fourier transformation method (IFT) developed by Glatter.36,37 The method does not require model functions for the form factor and is based on the numerical Fourier transformation of the experimental data smoothed by splines. The result is the pair distance distribution function p(r) of the scattering particles. For more details see refs 35-37. 2.4. Electron Microscopy. To obtain the dimensions of the synthesized particles, drops of concentrated microgel solutions were placed on microscope slides and dried afterward. The slides were then positioned on an aluminum plate and afterward covered with a gold layer using a Hummer VII sputtering system. For the electron micrographs we employed a Hitachi S450 microscope. The sample holder was tilted by 30° to increase the contrast of the samples. The radii, RSEM, obtained this way are given in (34) Williams, C. E. Contrast Variation in X-ray and Neutron Scattering. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier Science Publishers B. V.: Amsterdam, 1991; pp 101117. (35) Iampietro, D. J.; Brasher, L. L.; Kaler, E. W.; Stradner, A.; Glatter, O. J. Phys. Chem. B 1998, 102, 3105-3113. (36) Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Schubert, K.-V.; Kaler, E. W.; Glatter, O. J. Chem. Phys. 1999, 110, 10623-10632. (37) Glatter, O. Acta Phys. Austriaca 1977, 47, 83-102. (38) Burchard, W.; Richtering, W. Prog. Colloid Polym. Sci. 1989, 80, 151-163.
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Table 1. List of Particles Synthesized with Hydrogenated Styrenea styrene content (mol %)
Rh (nm)
RSEM (nm)
0 10 25 33
295 ( 15 265 ( 14 196 ( 10 92 ( 5
257 ( 15 204 ( 15
styrene content (mol %)
Rh (nm)
RSEM (nm)
50 66 75 100
79 ( 4 104 ( 5 119 ( 6 91 ( 5
82 ( 15 104 ( 15 118 ( 15 91 ( 15
a The given hydrodynamic radii were obtained at 293.2 K and at a scattering angle of 90°.
Figure 2. Swelling curves of microgels with different styreneto-NIPAM ratio. Increasing the styrene content leads to a decrease in swelling capacity and to a lower transition temperature.
Figure 1. Typical correlation function as obtained for the PNIPAM-co-polystyrene microgels at a scattering angle of 90°. The inset shows the relaxation rate distribution as obtained from a CONTIN analysis of the data. Additionally, the data were fitted with a simple single-exponential function. Table 1.The images of the crystals (see Figure 4) were obtained using a concentrated solution of the microgel particles. After slowly (1 day) evaporating the solvent at 45 °C, an opal-like shivering solid remained. This solid was broken and afterward treated as described above.
3. Results and Discussion 3.1. Swelling Behavior. DLS experiments were used to characterize the copolymer microgel particles with respect to their polydispersity and their swelling behavior. All investigated particles were prepared with an approximately constant amount of cross-linker (BIS). In Figure 1 a typical example for the obtained correlation functions is shown (PNIPAM-co-PS particles with 50 mol % styrene, scattering angle 90°, and T ) 293.1 K). All experimental correlation functions could be fitted by simple singleexponential functions. Already, this fact reveals the low polydispersity of the particles. The inset in Figure 1 shows the respective CONTIN result. The obtained relaxation rate distribution is rather narrow. For all particles that were prepared the polydispersity was found to be e10%. In addition the microgels were also characterized using SEM (images not shown), and the results from DLS and SEM are summarized in Table 1. From the hydrodynamic radius, Rh, calculated on the basis of the DLS experiments, the swelling ratio of the particles can be obtained using the equation
R)
( )
Vcollapsed Rh313.2K ) Vswollen R0288.2K
3
(2)
In Figure 2 swelling curves of PNIPAM-co-PS particles are shown. With increasing styrene content, the transition
Figure 3. SEM image as obtained for a microgel sample that was obtained by drying a drop of a concentrated suspension on the glass support. (The particles contained 50 mol % styrene.)
temperature is shifted toward lower temperatures and the swelling capacity decreases. Additionally, the transition is significantly broadened. This is in agreement with the results of Dingenouts and co-workers,25 who found a decrease of the swelling capacity of the NIPAM network in PNIPAM-co-PS core shell microgels. Using, for example, acrylic acid as comonomer, however, led to the opposite effect (a shift of the LCST toward higher temperature).18 At polystyrene contents of g75 mol, no significant shrinkage of the particles with increasing temperature is observable. Hence, with increasing styrene content the microgel particles behave more and more like polystyrene lattices and the copolymerization of NIPAM and styrene allows us to change the hardness of the particles. The results from the investigation of the swelling behavior are given in Table 2. Besides the changes in swelling behavior, it is also interesting that the radius of the obtained PNIPAM-co-PS microgels in the swollen state is decreasing with increasing styrene content. At high styrene content, the prepared particles are much smaller than homopolymer particles or PNIPAM-co-poly(acrylic acid) prepared under similar conditions and with similar amounts of cross-linker. The similarity of the radii
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Figure 4. Crystals made of PNIPAM-co-polystyrene microgels as seen by SEM. The visualized particles were prepared using 75 mol % styrene.
obtained from DLS experiments and from the electron micrographs indicates that the water seems to be more confined in the PNIPAM-co-PS microgels. For PNIPAM homopolymer particles, the radii obtained from electron microscopy are usually smaller due to the preparation conditions of the SEM samples, which leads to a loss of water and hence to a shrinkage of the microgels (see the first line of Table 1 and ref 32). 3.2. SANS Results. Small-angle neutron scattering is an excellent tool to characterize microgels in solution32,33 and in crystallized samples.13 In the present study the most important advantage of SANS compared to other experimental methods is the possibility to vary the scattering length density of the monomers by deuteration
Table 2. Table of the Results Concerning the Swelling Behavior of the PNIPAM-co-PS Microgelsa styrene content (mol %) 0 25 33 50 66 75 100
Tc (K) 305.2 303.2 302.7 300.7 299.2
RT)293.1K h (nm)
RT)318.1K h (nm)
295 ( 15 196 ( 10 92 ( 5 79 ( 4 104 ( 5 119 ( 6 91 ( 5
151 ( 8 98 ( 5 48 ( 3 66 ( 4 93 ( 5 120 ( 6 87 ( 5
R 0.134 0.125 0.142 0.583 0.894
a All data were obtained for particles cross-linked with 3.1 mol % BIS.
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Figure 5. Scattering curves for dilute samples in the swollen state (T ) 293 K) containing 50 wt % polystyrene-d8 using different contrast conditions. In a mixture of 10% D2O and 90% H2O, the PNIPAM is matched. The p(r) functions show that the obtained maximum dimension of the spheres is 120 nm in heavy water (entire particle) and 90 nm in the solvent mixture (polystyrene core). The solid lines in the left part of the figure represent the scattering curves calculated by Fourier transformation of the p(r) functions and indicate that the computed p(r) functions are in good aggreement with the experimentally observed scattering curves.
and to adjust the scattering length density of the solvent with respect to either styrene or NIPAM. This allows us to investigate the distribution of the two types of monomers in the obtained particles. 3.2.1. Dilute Solutions. The scattering length density of poly(styrene) and poly(styrene-d8) are 1.41 × 1010 and 6.47 × 1010 cm-2, respectively.34 The scattering length density of PNIPAM is 1.43 × 1010 cm-2.25 Therefore, with pure D2O as the solvent, both constituents of the microgel particles contribute to the scattering curves. A mixture of 10 vol % D2O and 90 vol % H2O has the same scattering length as PNIPAM. Hence, dissolving the microgels in this solvent mixture makes the NIPAM part of the particles “invisible” for the neutrons. In Figure 5 scattering curves of a dilute solution of the same particles (50 mol % styrene) are shown for the contrast conditions outlined above. The temperature was chosen to be 295 K and the particles are therefore in the swollen state. It is obvious that the minima of the form factor are shifted toward larger q-values when the scattering length density of the NIPAM part of the particles is matched by the solvent, clearly revealing the core-shell structure of the prepared particles. This structure is certainly formed due to the high hydrophobicity of styrene. Styrene will therefore be located in the cores of the emulsion droplets used to prepare the particles or, as proposed by Duracher et al., will react in the interior of previously formed PNIPAM precursor particles and expel the PNIPAM to the shell. In addition, Figure 5 also shows the p(r) functions for both contrast conditions. These functions were calculated using the program GIFT by Glatter.35-37 From these functions the radius of gyration of the PS core can be estimated to be ca. 45 nm and the radius of gyration of the entire particle is 60 nm. This leads to a core volume of 3.8 × 105 nm3 and to a shell volume of 5.2 × 105 nm3 for the swollen shell, respectively. Using the hydrodynamic radius obtained for the hydrogenated particles with 50 mol % styrene, the ratio F ) Rg/Rh is estimated to be 0.76.38 This is very close to the theoretical value for uniform spheres. However, this is only an estimate based on the assumption that the deuterated monomers behave in the
Hellweg et al.
Figure 6. Scattering curves for samples containing 50 wt % polystyrene-d8 in D2O (circles) and in a mixture of 10% D2O and 90% H2O (squares). The structure factor peak positions are not shifted, but the form factor is influenced by scattering contrast. In the solvent mixture, PNIPAM is matched; therefore, the shift of the form factor minima toward higher q-values clearly shows the core-shell structure of the particles.
Figure 7. Scattering curves for samples containing 30 wt % polystyrene-d8 in D2O (circles) and in a mixture of 10% D2O and 90% H2O (triangles). The observations are the same as in Figure 6.
same way as the hydrogenated styrene. These results are in good agreement with the findings of Duracher et al.26 using transmission electron microscopy. 3.2.2. Concentrated Samples. In concentrated samples the formation of crystallites is observable (iridescence). This is similar to previous observations of concentrated samples of microgels only made of NIPAM.11,13,14 In Figures 6 and 7 scattering curves for two different types of microgel particles are shown (50 and 30 mol % styrene-d8, respectively). Both samples contain crystallized regions, which leads to several pronounced peaks of the structure factor S(q) at low q-values. However, in both cases the first peak of the structure factor is still dominated by the liquid-like zones in the sample.13 Using the second and third peak in Figure 6 to calculate the unit cell length for the particles containing 50 mol % styrene leads to a value of 287 nm, assuming an fcc structure for the formed crystallites. This corresponds to a particle radius of ca. 100 ( 10 nm. The agreement with the radius from the DLS and the SEM experiments (see Table 1) is satisfying, taking into account that these measurements were performed using hydrogenated particles and the SANS experiments were performed on particles prepared with PS-d8. For the sample prepared using 30 mol % styrene-d8, the peak positions
PNIPAM-co-polystyrene Core-Shell Microgels
are not sufficiently well defined to allow for the estimation of the unit cell length. In the intermediate q-range the form factor minima are visible. Both figures display data for two different contrast conditions. For both scattering contrasts only the form factor minima are shifted. The structure factor peaks remain unchanged, again clearly showing the core-shell structure of the particles. 3.3. Crystallization As Seen by Electron Microscopy. The formation of mesoscopic crystals from the PNIPAM-co-PS microgels was also investigated by means of SEM. Already in the samples, which were prepared according to the procedure described in section 2.4, to characterize the particles with respect to their radius, crystal formation was observed for particles containing g50 mol % styrene. In Figure 3 an example for the found structures is shown. Therefore, to obtain larger crystals, higher volumes of concentrated suspensions of particles with high styrene content were crystallized by slowly evaporating the solvent (within 1 day at 45 °C). This leads to an opal-like, brittle solid, and in Figure 4 parts from this solid are imaged with increasing spatial resolution. The images reveal the high order of the obtained structures. A better control of
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the evaporation process might allow for the preparation of interesting new materials, and this will be investigated in the future. 4. Conclusions By copolymerization with styrene it is possible to obtain nearly monodisperse microgels. Copolymerization with styrene leads to a decrease in swelling capacity and shifts the LCST to lower temperatures. The core-shell structure of the particles is revealed by the presented SANS data using the method of contrast variation by deuteration. The thickness of the PNIPAM shell can be changed systematically by changing the monomer ratio during the synthesis of the particles. This allows for the variation of the hardness of the particles, becoming more latex-like with increasing polystyrene core radius. Moreover, it is shown that the particles crystallize in structures exhibiting a very good long-range order, making them interesting as templates for photonic band gap materials, for example. Hence, in the future the process of preparation of opals based on these materials using solutions containing colloidal crystals will be optimized. LA0354786
