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Optical Detection of Nonequilibrium Swelling Behavior of a Polymer Gel upon Solvent Substitution Akiko Toyotama,† Tsutomu Sawada,*,† Junpei Yamanaka,‡ and Kenji Kitamura† National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Graduate School of Pharmaceutical Sciences, Nagoya City UniVersity, 3-1 Tanabe-dori, Mizuho, Nagoya, Aichi 467-8603, Japan ReceiVed September 2, 2005 We report a novel transient swelling and shrinking behavior of a thin poly(acrylamide)-based gel film upon solvent substitution between water and ethylene glycol. These size changes could be optically detected through a change in the Bragg diffraction wavelength for the colloidal crystal of charged polystyrene latex particles that was fixed in the gel. The transient size change that was observed in this study could not be explained on the basis of the equilibrium characteristics, but it was attributable to the transient variation of osmotic pressure in the gel.
A polymer gel is a 3D polymer network that swells in a solvent. Its volume changes depending on the affinity between the network and the solvent. On the basis of this characteristic, a number of studies have been conducted on the application of polymer gels to chemical and physical sensors.1-3 Recently, polymer gels containing periodic arrays of colloidal particles (colloidal crystals) have attracted considerable attention.3,4 Because the lattice spacing of a colloidal crystal is typically on the order of 100 nm, it diffracts visible or near-infrared light. Therefore, a volume change in the gel is spectroscopically detected as a shift in the diffraction wavelength. This implies that a gel that contains colloidal crystals can be used as an optical sensor. Almost all of the gelled colloidal crystal sensors reported thus far detected environmental changes by determining the difference in the degrees of equilibrium swelling. In the present letter, we report a novel nonequilibrium swelling-shrinking behavior of a gelled colloidal crystal that enabled dynamic optical sensing. The transient swelling-shrinking behaviors of thin gel films have been reported by Osada et al.1,5 and Asher et al.6 In particular, Asher et al. succeeded in the optical detection of the transient size change by using the gelled colloidal crystal. In both cases, the gels had special functional groups that played essential roles in the dynamic size changes. However, in the present system, a poly(acrylamide)-derived gel, which does not possess special stimulus-responsive functionality, undergoes a rapid and large transient volume change upon solvent substitution between water and ethylene glycol (EG) despite the fact that the equilibrium swelling volume of the gel for these solvents is approximately * Corresponding author. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Nagoya City University. (1) Osada, Y.; Gong, J. P. AdV. Mater. 1998, 10, 827. (2) Vlatakis, G.; Andersson, L. I.; Muller, R.; Mosbach, K. Nature 1993, 361, 645. (3) (a) Kamenetzky, E. A.; Magliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (b) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (c) Asher, S.; Holtz, J.; Weissman, J. MRS Bull. 1998, 23, 44. (d) Hu, Z. B.; Lu, X. H.; Gao, J. AdV. Mater. 2001, 13, 1708. (e) Takeoka, Y.; Watanabe, M. AdV. Mater. 2003, 15, 199. (f) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (4) (a) Fudouzi, H.; Xia, Y. AdV. Mater. 2003, 15, 892. (b) Yamanaka, J.; Murai, M.; Iwayama, Y.; Yonese, M.; Ito, K.; Sawada, T. J. Am. Chem. Soc. 2004, 126, 7156. (5) (a) Gong, J. P.; Matsumoto, S.; Uchida, M.; Isogai, N.; Osada, Y. J. Phys. Chem. 1996, 100, 11092. (b) Osada, Y.; Gong, J. P.; Uchida, M.; Isogai, N. Jpn. J. Appl. Phys. 1995, 4B, L511. (6) Asher, S. A.; Peteu, S. F.; Reese, C. E.; Lin, M. X.; Finegold, D. Anal. Bioanal. Chem. 2002, 373, 632
Figure 1. Equilibrium behavior of the gel. (a) Diameter of the gel in EG/water mixtures for various mixing ratios. The dashed line acts as a guide for the eye. (b) Bragg wavelength of the sample in EG/ water mixtures for various mixing ratios. The open circles represent the values calculated from the data in Figure 1a, and the closed circles represent the measured values.
the same. We demonstrate that this transient behavior can be optically detected as a significant shift in the diffraction peak of the colloidal crystals in the gel. The polymer gel used in this study is a poly(N-methylol acrylamide) gel. Polystyrene latex particles (Duke no. 5020; particle diameter, 198 nm) were mixed in a reaction solution containing N-methylol acrylamide(0.79 M) as a monomer, N,N′methylene-bis-acrylamide (10 mM) as a cross linker, and camphorquinone (0.4 mM) as a photoinduced polymerization initiator. Pure water was used as the solvent. The reaction solution exhibiting iridescence, which is indicative of colloidal crystallization, was held between two glass plates and then polymerized by photoirradiation. We thus obtained a gel film (0.1 mm in thickness) containing colloidal crystals. Disk-shaped samples (8 mm in diameter) were cut from the film for the following measurements. Reflection spectrometry revealed that the colloidal crystal immobilized in the gel had a face-centered cubic (fcc) structure with the (111) lattice plane parallel to the gel surface. Because the lattice of the colloidal crystal is fixed to the polymer network of the gel, we can optically detect a change in the gel thickness through a shift in the Bragg peak. The lattice constant for the equilibrium swelling state in water was approximately 570 nm, which corresponds to 7.9% of the particle volume fraction. The reflection spectra were obtained by using a multichannel spectrophotometer with a Y-branch fiber probe (Ocean Optics Inc., Dunedin, FL; types NIR-512 and USB2000) in the visible/near-IR region. All experiments were performed at 25 °C. First, we examined equilibrium behaviors of the gel. Figure 1a shows the diameter of the gel in EG/water for various compositions in which the gel was equilibrated. On the basis of the Bragg’s equation for the normal incidence λ ) 2nd111, the
10.1021/la052395b CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Figure 2. Photographs showing the transient response of the gel sample upon solvent substitution from water to EG (a) before the immersion, (b) at time t ) 6.3 min, and (c) equilibrated in EG and from EG to water (d) before the immersion, (e) at time t ) 0.46 min, and (f) equilibrated in water. Dashed circles represent the gel sizes before the substitution processes. The scale bar applies to a-f.
change in length can be converted to the Bragg shift for the (111) reflection using λ/λw ) (n/nw)(d/dw) by assuming isotropic swelling (i.e., the proportionality between the lateral and normal size change upon swelling, d111/d111w ) d/dw). In this case, λ denotes the Bragg wavelength; n, the refractive index of the gel;7 d, the diameter of the gel; d111, the interplanar spacing of the (111) planes; and subscript w, the quantity of the gel that is swollen with pure water. Figure 1b shows the Bragg shift thus calculated from the change in gel diameter along with values observed as reflectance peaks in spectral measurements. Good agreement between the observed and calculated Bragg shifts supports the isotropic swelling assumption. Figure 1a and b indicates that the swelling size and Bragg wavelength of the present gel are unaffected by the EG content under equilibrium conditions. However, a significant transient change was observed in the size of the gel after the substitution of the two solvents. Figure 2 presents overviews of the gel sample upon solvent substitutions from water to EG ((a) before the immersion, (b) at time t ) 6.3 min, (c) equilibrated in EG) and from EG to water ((d) before the immersion, (e) at time t ) 0.46 min, (f) equilibrated in water). The dashed circles in the photographs indicate the original gel sizes before the solvent substitutions. Figure 3a shows the time course of the reflection spectra when the gel swollen with water was immersed in 100% EG. The peak position immediately shifted to the shorter-wavelength side and then gradually relaxed toward the equilibrium position over time. In contrast, when the gel swollen in EG was immersed in pure water, the Bragg peak primarily shifted to the longer-wavelength side immediately after the immersion and relaxed over time (Figure 3b). In this case, the gel expanded temporarily. Figure 3c quantitatively expresses the transient peak shift, as a function of time, in terms of the ratio of the wavelength of the peak to the value λ0 when the gel was immersed in water. This Figure indicates that the transient peak shift reaches up to approximately 20% of the initial value in both processes, which significantly differs from the equilibrium behavior shown in Figure 1b. The transient behavior was further examined for solvent substitution processes between water and aqueous solutions with (7) The refractive index of the gel is approximately given by n ) npφp + nwφw + nEGφEG, where n and φ are the refractive index and volume fraction of the material indicated by the suffix, respectively, and suffixes p, w, and EG denote particle, water, and EG, respectively. In this case, np ) 1.59, nw ) 1.33, and nEG ) 1.45. The contribution of the gel polymer is disregarded because of its negligible impact on the conclusions.
Figure 3. Dynamic shifts in the reflection spectra of the gel during solvent substitution when (a) the gel equilibrated in water was immersed in EG and (b) the gel equilibrated in EG was immersed in water. (c) Changes in the diffraction peak wavelength reduced by that at time t ) 0, λ0. Open circles, substitution process of EG to water (λ0 ) 960 nm); closed circles, substitution process of water to EG (λ0 ) 914 nm). The inset shows an enlargement for the first 5 min of EG to water.
Figure 4. Influence of EG concentration on the normalized peak wavelength of the gel at the maximum swelling or minimum shrinking (open circles, substitution process of EG to water; closed circles, substitution process of water to EG).
various EG concentrations (20, 40, 60, and 80 vol %) instead of pure EG. The transient shrinking and swelling were observed in a similar manner, and the Bragg wavelengths for the minimal and maximal shifts in their transient process monotonically varied with EG concentration, as shown in Figure 4. The volume changes are more drastic under high EG conditions. In equilibrium systems, a reentrant phase transition of the gel has been observed with respect to binary solvents in which the gel size is maximized or minimized in an intermediate mixing ratio of the solvents.8 However, as discussed above, the transient behavior in the present system cannot be explained by the equilibrium characteristics. To understand the present behavior, it is necessary to consider a nonequilibrium mechanism. The transient shrinking/swelling behaviors of gels have been reported by Osada et al.1,5 and Asher et al.6 In both cases, the gel had a special functionality. Osada et al. reported the dynamic behavior of a gel, which was swollen in an organic solvent, (8) (a) Hirotsu, S.; Hirokawa, H.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (b) Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. J. Phys. Chem. 1993, 97, 737. (c) Hirotsu, S. J. Chem. Phys. 1998, 88, 427.
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immersed in water. They demonstrated that the cross-linked hydrophobic-hydrophilic copolymer gels swollen in organic solvents exhibited spontaneous motion when placed in water. This was attributed to the simultaneous production of high osmotic pressure between the gel/water interface and the formation of an organized structure of the hydrophobic segments inside the gel. However, Asher et al.6 reported the optical detection of the transient response of the gelled colloidal crystal for a poly(acrylamide)-based copolymer gel having 18-crown ether moieties in its side chains. This intelligent gel had the ability to sense Pb2+ chemically through a change in the Bragg diffraction wavelength upon a volume change of the gel due to the chelation of Pb2+. When the gel containing a sufficient amount of Pb2+ was exposed to pure water, the osmotic pressure difference between the gel and the solvent was sufficiently large to cause a transient swelling of the gel. With time, the Pb2+ inside the gel gradually diffused to the outer solution, following which the gel shrunk. This is attributed to the difference in ion concentration, which causes the difference in the osmotic pressure. In both cases, the transient behavior was explained by the osmotic pressure. Although the present gel does not have any special functionality, the observed dynamic size changes are commonly explained on the basis of the transient change in the osmotic pressure. It should be noted that EG is often used in dehydration applications to remove water from various substances. In the present case, the water in the outer solution is extracted from the gel. At the molecular level, the transient response of the gel is explained as follows: Because the mobility of the EG molecules is significantly less than that of water molecules, on a short time scale the surface of the gel can function as a semipermeable membrane that transmits water molecules. When the gel swollen in water is immersed in EG, the osmotic pressure outside the gel is higher than that inside the gel. Because of this pressure difference, the inner solvent, water, flows out, thereby causing the gel to shrink. In contrast, when the gel swollen in EG is immersed in water, the inner osmotic pressure is higher, and the outer solvent, water, flows into the gel, thus causing expansion. On a longer time scale, the EG molecules also diffuse, and the entire system relaxes to an equilibrium. The difference in the relaxation times is reasonable because the diffusion of EG in the shrunken gel should be slower than that in the expanded gel because of the dense polymer network. Because the osmotic pressure difference should
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be smaller if the pure EG is replaced by an EG aqueous solution, the above-mentioned mechanism also qualitatively explains the concentration dependence of the EG shown in Figure 4. Assuming that the gel can be modeled by an osmometer cell equipped with semipermeable membranes, we measured the time dependence of the meniscus height in the cell,9 which is proportional to the excess hydrostatic pressure inside the cell, accompanying the solvent substitution from EG/water with various concentrations to water. The experimental details and a result are shown in the Supporting Information. We confirmed that the meniscus height first increased with time. After passing through a maximum, which was larger for a higher EG concentration, it decreased. This confirmed the occurrence of the transient change in the osmotic pressure accompanied by solvent substitution. If the sample cell volume is not fixed, then it should exhibit a transient volume change. We observed this tendency with regard to the gel. It should be noted that the present gel does not have special functionality. Despite this, it exhibits a transient size change, presumably due to the osmotic pressure difference. This implies that the transient size change in the gel is universal. In conclusion, we demonstrated the transient and rapid swelling and shrinking processes of the polymer gel upon solvent substitution, which can be detected by spectroscopic measurements. This behavior cannot be explained on the basis of the equilibrium characteristics; it appears to be due to a transient difference in the internal and external osmotic pressures of the gel. This phenomenon may present an application of the colloidal crystal containing gel as a sensing material for the dynamic properties. Acknowledgment. We acknowledge Dr. Yoshitsugu Hirokawa for helpful discussions on reentrant shrinking behavior. We thank Ikuko Maki for providing assistance in preparing the manuscript. Supporting Information Available: Membrane osmometer used and time dependence of the osmotic pressure upon solvent substitution. This material is available free of charge via the Internet at http://pubs.acs.org. LA052395B (9) Katchalsky, A.; Curran, P F. Nonequilibrium Thermodynamics in Biophysics; Harvard University Press: Cambridge, MA, 1965; Chapters 5 and 10.
