Langmuir 2006, 22, 863-866
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Growth of Columnar Hydrogel Colloidal Crystals in Water-Organic Solvent Mixture Jun Zhou,† Tong Cai,† Shijun Tang,† Manuel Marquez,‡,§ and Zhibing Hu*,† Departments of Physics and Chemistry, UniVersity of North Texas, P.O. Box 311427, Denton, Texas 76203, Los Alamos National Laboratory, Chemistry DiVision, Los Alamos, NM 87545, and INEST Group Research Center, PMUSA, Richmond, VA 23298 ReceiVed June 13, 2005. In Final Form: December 7, 2005 A novel emulsion method has been demonstrated to grow columnar hydrogel colloidal crystals by mixing an aqueous suspension of poly-N-isopropylacrylamide-co-allylamine microgels with organic solvent, driven by the coalescence of micelles consisting of organic oil droplets coated by many microgels. This method leads to microgel colloidal crystals of several centimeters growing from the top to the bottom along the gravity direction. Both temperature and polymer concentration play critical roles for the formation of columnar crystals. A phase diagram has been determined, and it can be used as a guide to selectively grow different crystals, including columnar crystals and randomly oriented crystals, and enable the coexistence of columnar crystals and randomly oriented crystals.
Introduction The development of methods obtaining colloidal crystals including sedimentation,1-3 diffusion of base,4 evaporation,5 electrostatic repulsion,6 templated growth,7 gradient temperature fields,8 and physical confinements9 is of paramount importance. This is because such crystals allow us to obtain useful functionalities not only from colloidal particles but also from the long-range ordering of these particles.10-12 Here we report a novel emulsion method of growing large columnar crystals by mixing an aqueous suspension of hydrogel colloids (or microgels) with organic solvent. The hydrogel colloidal crystals of several centimeters have grown from the top to the bottom along the gravity direction in a test tube with 10 mm diameter and 75 mm length. The growth was driven by the coalescence of micelles consisting of organic oil droplets coated by many microgels. This is in contrast to a conventional method to form a thin sheet of single crystals13 or randomly oriented hydrogel colloidal crystals, with the largest domain size on the order of several millimeters in pure water.14-16 Columnar crystals of hard spheres have been studied using a sedimentation2,3 or diffusion-of-base method.4 The silica spheres in these experiments were dispersed in an aqueous solution at * Corresponding author. E-mail:
[email protected]. † University of North Texas. ‡ Los Alamos National Laboratory. § INEST Group Research Center, PMUSA. (1) Pusey, R. N.; van Megen, M. Nature 1986, 320, 340. (2) Davis, K. E.; Russel, W. B.; Glantschnig, W. J. Science 1989, 245, 507. (3) Ackerson, B. J.; Paulin, S. E.; Johnson, B.; van Megen, W.; Underwood, S.; Phys. ReV. E 1999, 59, 6903. (4) Yamanaka, J.; Murai, M.; Iwayama, Y.; Yonese, M.; Ito, K.; Sawada, T. J. Am. Chem. Soc. 2004, 126, 7156. (5) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (6) Weissman, M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (7) Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (8) Cheng, Z.; Russel, W. B.; Chaikin, P. M. Nature 1999, 401, 893. (9) Park, S. H.; Qin, D.; Xia, Y. AdV. Mater. 1998, 10, 1028. (10) Yin, Y.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 2048. (11) Smay, J. E.; Cesarano, J. J.; Lewis, A. Langmuir 2002, 18, 5429. (12) Lellig, C.; Hartl, W.; Wagner, J.; Hempelmann, R. Angew. Chem., Int. Ed. 2002, 41, 102. (13) Debord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. AdV. Mater. 2002, 14, 658. (14) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705. (15) (a) Hu, Z. B.; Lu, X. H.; Gao, J. AdV. Mater. 2001, 13, 1708. (b) Gao, J.; Hu, Z. B. Langmuir 2002, 18, 1360. (16) Tang, S. J.; Hu, Z. B.; Cheng, Z. D.; Wu, J. Z. Langmuir 2004, 20, 8858.
volume fractions less than the freezing value3 or in a pH gradient solution4 to settle down on a flat surface to form columnar crystals. These methods and other previous ones cannot be directly used for hydrogel colloids. This is because, in contrast to silica or polystyrene hard spheres, the hydrogel colloids or microgels investigated in this work contain about 97 wt % water at room temperature.15b Consequently, the density and the hydrogel colloid refractive index of the microgels closely match up to those of the surrounding water, yielding a condition of mini-gravity (∼10-2 g) at room temperature.16 It is difficult to grow columnar crystals by natural sedimentation of microgels in water. Currently, the major method for preparing hydrogel colloidal crystals has relied on self-assembling hydrogel particles in water.13-16 Experimental Section Sample Preparation. Poly-N-isopropylacrylamide (PNIPAM)co-allylamine microgels were prepared by precipitation polymerization.17 PNIPAM monomer (3.8 g, 33.6 mmol), allylamine (0.2 g, 3.4 mmol, 10 mol % of NIPAM monomer), sodium dodecyl sulfate (0.08 g, 0.28 mmol), and N,N′-methylene-bis-acrylamide (0.067 g, 0.44 mmol, 1.3 mol % of NIPAM monomer) in water (240 mL) at room temperature were purged with nitrogen and stirred for 30 min, and then heated to 60 °C. Potassium persulfate (0.166 g) in 10 mL water was added to the reactor to initialize polymerization. The reaction was maintained at 59-61 °C under nitrogen for 5 h. After cooling to room temperature, the resultant microgels were dialyzed for 2 weeks to remove surfactant and unreacted molecules. The dialysis water was changed three times every day. The cutoff molecular weight of the dialysis membrane was 13 000. After dialysis, PNIPAM-co-allylamine microgels were concentrated by ultracentrifugation at 40 000 rpm for 2 h and redispersed with deionized (DI) water to a certain concentration. Allylamine in microgels was determined to be about 4.2 mol % of NIPAM monomer using a titration method. The solid concentration of the suspension was obtained by being completely dried at 70 °C in air and weighed. PNIPAM-co-allylamine Microgel Columnar Crystals. The centrifuged particle suspension was adjusted to concentrations ranging from 1.8 to 4.5 wt %. The defined amounts of dichloromethane (CH2Cl2), 0.27 g with 1 g particle suspension, were mixed by shaking for two minutes. The mixture was put into an incubator. The crystal formation was observed at each temperature for several days. Dynamic Light Scattering Measurements. A laser light scattering spectrometer (ALV, Co., Langen, Germany) was used with a helium-neon laser (Uniphase 1145P, output power of 22 mW and (17) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247.
10.1021/la0515773 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/06/2006
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Figure 1. Time-dependent growth of columnar crystals in a mixture of an aqueous suspension of PNIPAM-co-allylamine microgels and dichloromethane in a test tube with 10 mm diameter and 75 mm length. The time started after homogenization: (a) 0, (b) 4, (c) 33, (d) 43, (e) 55, (f) 72, and (g) 82 h.
Figure 2. The optical microscopic picture of a mixture of an aqueous suspension of PNIPAM-co-allylamine microgels and dichloromethane. The sizes of the oil droplets coated with microgels range from 10 to 40 µm. There is not enough resolution to see the microgels in this microscopic picture. wavelength of 632.8 nm) as the light source. The hydrodynamic radius distribution of the PNIPAM-co-allylamine microgels in water was measured at a scattering angle of 60°. UV-Visible Spectroscopy Measurements. The turbidity (R) of the gels was measured as a function of the wavelength using a diode array UV-visible spectrometer (Agilent 8453) by calculating the ratio of the transmitted light intensity (It) to the incident intensity (Io) R ) -(1/d) ln(It/I0), where d is the thickness (1 mm) of the sampling cuvette.
Results and Discussion PNIPAM-co-allylamine microgels showed phase behavior similar to that of a pure PNIPAM gel18 with a slightly higher volume phase transition temperature around 35 °C. The allylamine contributes amine groups that could be used as cross-linker sites if needed in water at a neutral pH.19 Although the propagation rate constants of allylamine and PNIPAM are different and may result in a nonrandom copolymer, the allylamine was indeed incorporated into PNIPAM microgels, as evidenced by the reaction of these particles to glutaric dialdehyde in water to form a microgel network. The average hydrodynamic radius of the particle was 135 nm at 22 °C, with a polydispersity index (PDI) 20 of about 1.08, and shrank to 65 nm at 37 °C with a PDI of about 1.01. The aqueous suspension of PNIPAM-co-allylamine microgels with polymer concentration 3.5 wt % was mixed with dichlo(18) Hirotsu, Y.; Hirokawa, T.; Tanaka,T. J. Chem. Phys. 1987, 87, 1392. (19) Hu, Z. B.; Huang, G. Angew. Chem., Int. Ed. 2003, 42, 4799. (20) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991.
Figure 3. UV-visible spectra of the PNIPAM-co-allylamine microgel crystals in a quartz cuvette (45 mm height × 1 mm width) at three locations: (a) for columnar crystals prepared in a waterdichloromethane mixture and (b) for randomly oriented crystals prepared in water. Both crystals were grown at 20 °C and had a polymer concentration of about 2.7 wt %.
romethane by shaking at 22 °C. All samples (Figures 1-4) contain microgels with an average hydrodynamic radius of about 135 nm and have the same suspension-to-oil weight ratio of 1:0.27. After homogenization, the mixture was left to stand. This initial mixture (Figure 1a) appears cloudy. The outside diameter of the test tubes is 1.0 cm. Within about 4 h (Figure 1b), we observed small columnar crystals growing from the top to the bottom, in contrast to the hard-sphere system that grew from the bottom to the top.2 The crystals grew longer with time along the direction of gravity and reached about 1.5 cm after 82 h (Figure 1g). The speed of crystal growth by this method is comparable with that by the natural sedimentation of hard spheres, 2 but slower than
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Figure 4. (a) Mixtures of an aqueous suspension of PNIPAM-co-allylamine microgels and dichloromethane at various PNIPAM concentrations at 22 °C: (1) 1.8, (2) 2.0, (3) 2.2, (4) 2.5, (5) 2.7, (6) 3.0, (7) 3.2, (8) 3.5, (9) 4.0, and (10) 4.5 wt %. (b) UV-visible spectra of columnar colloidal crystals at 2.5, 3.0, and 3.5 wt % at 22 °C, and (c) the phase diagram.
that by the dip-coating method.21,22 In contrast to these previous investigations, our method using emulsion is effective for hydrogel colloid systems. The mixture can be generally divided into three portions: the top one is the crystal phase, the bottom one (cloudy) is stable water-oil emulsion, and the middle portion is unstable emulsion (cloudy and white). The top crystalline layer has been successfully separated by first stabilizing the entire structure using chemical bonding and then physically removing the other phases. This result will be reported separately. Considering that PNIAPM particles have been used as emulsifiers,23 we suggested that our mixture initially formed an unstable oil-in-water emulsion with “micelles” consisting of organic oil droplets coated by many microgels. Using an optical microscope, we found that the sizes of the micelles ranged from 10 to 40 µm (Figure 2). The microgels are below the diffraction limit of the microscope and cannot be imaged under these conditions. However, previous SEM measurements supported the fact that PNIPAM microgels can cover the surfaces of oil droplets.23 These micelles, which are heavier than water due to the higher mass density of organic solvent (1.33 g/ml), gradually sink to the bottom of the cuvette. The mismatch of surface tension between the particle-oil and the oil-water results in coarsening. When such coarsening occurs, the microgels at the surface of the micelles are released. These released particles self-assemble into columnar crystals that originate in the interface between the mixture and air. The colors observed from columnar crystals are due to diffraction from the ordered colloidal arrays with a lattice spacing on the order of the wavelength of visible light according to Bragg’s law: 2nd sin θ ) mλ, where n is the mean refractive index of the suspension, θ is the diffraction angle, d is the lattice spacing, m is the diffraction order, and λ is the wavelength of the diffracted (21) Jiang, P.; Ostojic, G. N.; Narat, R.; Mittleman, D. M.; Colvin, V. L. AdV. Mater. 2001, 13, 389. (22) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (23) Ngai, T.; Behrens, S. H.; Auweter, H. Chem. Commun. 2005, 3, 331.
light.6 We have prepared columnar crystals in a waterdichloromethane mixture in a quartz cuvette (45 mm height × 1 mm width). The UV-visible spectra were measured along the long axis of the columnar crystals. Depending on the growth conditions of the crystals, the peak position of the UV-visible spectra can change or remain the same along the long axis. For high-quality crystals that grow directly from the top to the bottom of the cuvette, without bending, combining, or twisting columns, the peak position remains at about a constant value, as shown in Figure 3a. This shows that the interparticle spacing of columnar crystals is uniform across the crystals with a columnar length of about 13 mm. As a comparison, we performed the same measurement on randomly oriented crystals (Figure 3b) prepared in pure water; the peak position does not change significantly with the location in the cuvette. This indicates that the small crystal domains grown at the different parts in the cuvette have similar interparticle spacing. The different morphologies of colloidal crystals can be obtained by changing polymer concentration. Figure 4a shows mixtures of the aqueous suspension of PNIPAM-co-allylamine microgels with dichloromethane at various PNIPAM polymer concentrations, ranging from 1.8 to 4.5 wt % at 22 °C. For samples below 2.0 wt % (Figure 4a-1,2), no crystallization was observed. Near 2.2 wt % (Figure 4a-3), conventional, randomly oriented crystalline domains appeared. For samples near 2.5 wt % (Figure 4a-4), there was a coexistent region of columnar crystals and conventional crystal domains. For samples with polymer concentrations between 2.7 and 3.5 wt % (Figure 4a-5-8), columnar crystals were observed. In this concentration range, the color of the columnar crystals changed from red to blue as polymer concentration increased. UV-visible spectra on these crystals also demonstrated that the peak position shifts to a shorter wavelength with the increase in polymer concentration (Figure 4b) because of the decrease in interparticle spacing. Near 4.0 wt % (Figure 4a-9), a coexistent region of columnar crystals and randomly oriented crystalline domains was observed. At 4.5 wt % (Figure 4a-10), only randomly oriented crystalline domains
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were observed. It is interesting to note that the current method could help grow randomly oriented crystalline domains at high polymer concentrations at room temperature, while the previous method relies on heating-cooling cycles.13,19 Both temperature and polymer concentration play critical roles for the formation of columnar crystals. Figure 4c shows a phase diagram of the mixtures of the aqueous suspension of PNIPAMco-allylamine microgels with dichloromethane. The phase behavior has been divided into four areas: liquid, (randomly oriented) crystal, columnar crystal, and glass. As the temperature increases, the size of PNIPAM microgels reduces from 135 nm at 22 °C to 125 nm at 29 °C. The columnar crystals and randomly oriented crystals coexisting phases are indicated with thick blue lines. In the liquid-phase region, the top portion of the mixture flows easily, while, in the glass-phase region, it cannot flow. Growth kinetics of columnar crystals depends strongly on temperature. At 22 °C, it took about 2 or 3 days for crystals to grow to 1 cm long. However, above 26 °C, no crystals were observed after 7 days.
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Conclusion A novel emulsion method has been demonstrated to grow columnar microgel colloidal crystals by mixing an aqueous suspension of microgels with organic solvent. The microgels were made of PNIPAM-co-allylamine and contained about 97 wt % water. It is difficult to grow columnar crystals by the natural sedimentation of these microgels in water. This new method leads to microgel colloidal crystals of several centimeters, growing from the top to the bottom along the gravity direction. A phase diagram has been determined, and it can be used as a guide to selectively grow different crystals, including columnar crystals and randomly oriented crystals, and enable the coexistence of columnar crystals and randomly oriented crystals. Acknowledgment. We gratefully acknowledge the support from the U.S. Army Research Office (STIR W911NF-05-1-0546). LA0515773
