Langmuir 2002, 18, 10095-10099
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Thermosensitive Two-Dimensional Arrays of Hydrogel Particles Kazumichi Nakahama and Keiji Fujimoto* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received June 11, 2002. In Final Form: October 9, 2002 We prepared two-dimensional (2D) arrays composed of nanoscale hydrogel particles as a model of suprastructured assemblages. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel particles and the coreshell particles, which possess a rigid polystyrene core encased in a PNIPAM hydrogel shell, were selected as components for arrays. Below and above 32 °C, these two kinds of particles swelled and shrank, respectively, due to temperature-dependent solubility of a PNIPAM chain. The particle dispersion was spread and then compressed to produce particle monolayers closely packed at the air-water interface. To study thermal behavior of 2D arrays, surface pressure and surface area for obtained 2D arrays were measured at 40 and 25 °C with leaving the monolayer on the water surface. Both of them varied in response to temperature, indicating that each particle of 2D arrays swelled and shrank at the water surface. Although the pressure and the area for the 2D array of PNIPAM hydrogel particles exhibited great responses at the beginning, they gradually decreased and eventually disappeared with repeating the temperature cycle, due to particle coalescence. On the other hand, the 2D array of core-shell particles maintained the thermosensitive response without relaxation though its variation between 40 and 25 °C was still small. By assembling the mixture of PNIPAM and core-shell particles into the monolayer, the 2D array with large and steady responses could be obtained.
Introduction Biomaterials found in cells, tissues, and organs have highly hierarchical structures built up from biological entities. They are widely diverse in structure and properties, and their collective features and functionalities are different from those obtained by just adding individual components. A lot of important ideas about how to make the novel materials (e.g., bottom-up, self-assembly, and templating methods) will come up from biological systems. In the technological and industrial fields, highly ordered structures with unique material properties have attracted much attention, and numerous methods have been developed to produce a variety of materials assembled into two-dimensional and three-dimensional materials. For instance, particles and proteins could be twodimensionally assembled into a hexagonal form by solvent evaporation and capillary force.1 Alternatively, the particles were injected into the cell that was constructed from two glass substrates and accumulated at the bottom to form a close-packed structure.2 Three-dimensionally assembled materials have been produced by the use of emulsion droplets as templates.3-5 Velev et al. reported that microstructured particles were synthesized by growing colloidal crystals in aqueous droplets suspended on fluorinated oil.6 Whitesides and co-workers have employed molecular self-assembly to fabricate two-dimensional arrays and three-dimensional objects ranging from nanoto millimeter scale.7-9 Such particle assemblies have been * To whom correspondence should be addressed. Telephone and fax: +81-45-566-1580. E-mail:
[email protected]. (1) Nagayama, K. Phase Transitions 1993, 45, 185. (2) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266-273. (3) Velev, O. D.; Furusawa, K.; Nagayama, K. Langumuir 1996, 12, 2374. (4) Velev, O. D.; Furusawa, K.; Nagayama, K. Langumuir 1996, 12, 2385. (5) Velev, O. D.; Nagayama, K. Langumuir 1997, 13, 1856. (6) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240. (7) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233.
utilized to produce a variety of functional materials for catalysis and separations, and synthetic opals, microlense arrays,10 and porous materials for tissue regeneration. Recently, particle assemblages have been utilized as a template for the photonic crystal.11 The spin-coating method also provides the formation of particle monolayers arranged in a hexagonal form on a polystyrene dish.12,13 Particle monolayers formed by spreading the particles on a water surface from organic dispersions have been extensively investigated.14-18 We have previously utilized the Langmuir-Blodgett (L-B) technique to produce unsymmetrical microspheres.19,20 The technique can allow us to prepare two-dimensional arrays by assembling particles that spread on a water surface. The thin membrane of entangled gel-like structures of β-lactoglobulin has been previously prepared on the air/water boundary. In this study, we intended to produce thin materials (particle monolayers) as elastic and robust as biomaterials by assembling hydrogel particles. We used hydrogel particles including poly-N(8) Terfort, A.; Bowden, N.; Whitesides, G. M. Nature 1997, 386. (9) Choi, I. S.; Weck, M.; Xu, B.; Jeon, N. L.; Whitesides, G. M. Langmuir 2000, 16, 2997. (10) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538-547. (11) Subramanian, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261-1265. (12) Fujimoto, K.; Takahashi, T.; Miyaki, M.; Kawaguchi, H. J. Biomater. Sci., Polym. Ed. 1997, 8, 879-891. (13) Miyaki, M.; Fujimoto, K.; Kawaguchi, H. Colloids Surf., A: Physiochem. Eng. Aspects 1999, 153, 603-608. (14) Clint, J. H.; Taylor, S. E. Colloids Surf. 1992, 65, 61. (15) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Colloids Surf., A: Physicochem. Eng. Asp. 1993, 71, 327. (16) Schuller, H.; Kolloid, Z. Z. Polym. 1967, 380, 216. (17) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Langmuir 1996, 12, 997. (18) Mate, M.; Fendler, J. H.; Ramsden, J. J.; Szalma, J.; Horvolgyi, Z. Langmuir 1998, 14, 6501. (19) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630. (20) Nakahama, K.; Kawaguchi, H.; Fujimoto, K. Langmuir 2000, 16, 7882-7886.
10.1021/la020541x CCC: $22.00 © 2002 American Chemical Society Published on Web 12/17/2002
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isopropylacrylamide (PNIPAM), which could change their volume in response to temperature,21,22 as “building blocks”. The surface pressure response to deformation of the interface was measured using a Langmuir trough while the surface area was subjected to changes.23,24 Furthermore, surface area was also measured in the trough to evaluate a dynamic response of obtained monolayers to temperature change. Materials and Methods Materials. N-Isopropylacrylamide (NIPAM), donated by Kohjin Co., was purified by recrystallization from a mixture of hexane and toluene (1:1). Styrene, purchased from Wako Pure Chemicals Co., was distilled under reduced pressure. N-Octadecyl acrylamide (ODA, Polyscience, Inc.) was used without further purification. Potassium persulfate (KPS) (Wako Pure Chemicals Co.) was recrystallized from water. Other chemicals were purchased from Wako Pure Chemicals Co. and used without further purification. Preparation of Thermosensitive Particles. PNIPAM hydrogel particles were prepared by precipitation polymerization.21 A mixture of 2.664 g of NIPAM, 0.216 g of methylenebisacrylamide (MBAAm), and 124 g of distilled water was put into a 300-mL three-necked round-bottom flask equipped with a stirrer, a nitrogen gas inlet, and a condenser. Nitrogen gas was bubbled into the mixture to purge oxygen. Four grams of distilled water containing 0.04 g of KPS was added into the flask to initiate the polymerization, and the polymerization was continued at 70 °C for 4 h. The resulting PNIPAM particles were dialyzed and purified by repeating centrifugation, decantation, and redispersion into water. To obtain parent particles for seeded polymerization, styrene and NIPAM were copolymerized in a soap-free aqueous medium using KPS according to a method described previously.21 Styrene (9.09 g), NIPAM (0.909 g), and distilled water (90 g) were mixed, and then 10 g or distilled water containing 0.1 g of KPS was added to initiate the emulsion polymerization. The polymerization was continued at 70 °C for 20 h. The obtained particles (SN particles) were collected by centrifugation and then washed four times with water. Then, core-shell type particles with a shell of a PNIPAM hydrogel (SNN particles) were prepared by the following procedures. A mixture of 1.11 g of NIPAM and 0.09 g of MBAAm was added to 2 g of SN particles. Then, 2 g of distilled water containing 0.02 g of KPS was added for the initiation of the seed polymerization, and the polymerization was continued at 70 °C for 3.5 h. The size of dried particles was measured by transmission electron microscopy (TEM). The hydrodynamic diameter of particles was measured in 0.001 N KCl aqueous solution by dynamic light scattering using a laser particle analyzer system, PAR-IIIs (Otsuka Electric Co). Copolymerization of NIPAM and ODA, which were charged at the weight ratios 4.6 and 0.4, respectively, was carried out in toluene at 60 °C using AIBN (2,2′-azobis(isobutyronitrile)) as an initiator. The resulting copolymer was precipitated by pouring the polymer solution into a large excess of diethyl ether, and it was subsequently dried under vacuum. The weight-average molecular weight and the number-average molecular weight were 2.25 × 105 and 4.76 × 104, respectively. Assembling of Particles into Monolayers. The dispersion of the ethanol solution of particles (4-8 mg/mL, 150-550 µL) was gently added dropwise on the surface of the aqueous solution containing 0.001 M KCl in a Teflon trough (75 mm × 320 mm) using a micropipet. The spread particles were allowed to stand on the water surface for 30 min, and then surface compression was carried out at a speed of 10 mm/min by a L-B film deposition apparatus (NL-LB 240-MWC, Nippon Laser and Electronics Lab.). During the compressing, the surface pressure was measured using a Wilhelmy plate of filter paper. The Wilhelmy plate was positioned at 13 mm from the moving barrier. The surface area of the surface pressure-area isotherms was (21) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53. (22) Fujimoto, K.; Mizuhara, Y.; Tamura, N.; Kawaguchi, H. J. Intell. Mater. Syst. Struct. 1993, 4, 184-188. (23) Petkov, J. T.; Gurkov, T. D. Langmuir 2000, 16, 3703-3711. (24) Petkov, J. T.; Gurkov, T. D. Langmuir 2001, 17, 4556-4563.
Figure 1. Hydrodynamic diameters of the particles in 0.001 N KCl aqueous solution as a function of temperature. expressed as the effective radius of particles (R). R was calculated by assuming that particles were assembled on the water surface in a hexagonal manner. The particle radius at the collapse pressure (Rc) was estimated from the isotherm by drawing the intersection point of the two tangents, as seen in Figure 2. To study the stability of particle monolayers on the water surface, they were compressed until the specified pressure (see Figure 3), and their areas were kept constant to detect the change in the surface pressure for 30 min. The dynamic surface pressure responses for compressed monolayers were measured as follows. The monolayers were compressed at 40 °C. After a 30-min incubation, the temperature was lowered to 25 °C and raised to 40 °C at the speed 0.5 °C/min. This procedure was repeated three times with keeping the trough area constant. We also investigated the responses in the dynamic surface area for compressed monolayers. The monolayers of SNN particles or PNIPAM particles were compressed up to 40 mN/m at 40 °C and kept compressed for 30 min. Then, the temperature was lowered to 25 °C and raised to 40 °C at the speed 1.0 °C/min, while the surface pressure was kept constant by changing the trough area automatically. This procedure was carried out three times. The pressure-area behavior of copolymer chains on a water surface was also studied in the same manner.
Results and Discussion The diameters of all dried particles were monodisperse in size (approximately 350 nm), whereas their hydrodynamic sizes changed in response to temperature, as shown in Figure 1. This is because PNIPAM chains of the hydrogel particle undergo the volume transition based on their lower critical solution temperature (LCST) in aqueous solution. Thus, SNN and PNIPAM particles shrank above the LCST, whereas the size of SN particles that were mainly composed of polystyrene changed little in response to temperature. Formation of two-dimensional particle monolayers was assessed by the measurement of the surface pressure and the surface area with an L-B apparatus. A Wilhelmy film balance of an L-B apparatus has been utilized to determine the surface pressure versus surface area isotherms for particle monolayers.14,15 Schuller et al. reported that the surface pressure for PS particles suddenly increased at a certain area as well as those for amphiphilic molecules when the region of spreading particles was gradually reduced.16 Large repulsion between particles is known to lead to the reduction of surface tension and the increase in surface pressure. Figure 2a shows the surface pressure-particle radius isotherms for SN particle monolayers at different temperatures. The pressure suddenly rose at a certain area,
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Figure 2. Pressure-radius isotherms of (a) SN particles, (b) SNN particles, (c) PNIPAM particles, and (d) NIPAM/ODA copolymer chains at different temperatures. Samples were spread on the water surface and compressed at 10.0 mm/min.
Figure 3. Changes in surface pressure of particle monolayers and the copolymer monolayer as a function of time.
indicating that particles were closely packed as well as the solid L-B membrane. In general, a surface pressurearea isotherm is much influenced by temperature because the pressure relaxation depends on temperature. However, the shape of each isotherm was similar over the temperature ranging from 25 to 40 °C at the compression speed 10 mm/min. This is because particles are so large compared with small amphiphilic molecules that their thermal
fluctuation cannot influence the isotherm behavior. Figure 2b and c shows that surface pressures increased in a twostep manner. This strongly suggests that the type of interaction changes along the interparticle distance from soft to hard sphere repulsion. The pressure for the monolayers of SNN and PNIPAM particles gradually increased with compressing them at the first step. This indicates that the water surface was gradually covered with PNIPAM chains tethered from the particle surface with succeeding compression (soft repulsive force). At the second step, the surface pressure sharply increased when the monolayer was further compressed. This is probably attributed to the stronger steric repulsion arising from the chain overlapping (hard repulsive force). At this region, monolayers collapsed and some creases parallel to the moving barrier were formed in the particle monolayers. As can be seen in Figure 2a, the surface pressure for SN particles abruptly rose because they have nothing but the thin layer of PNIPAM chain. It is also observed in Figure 2b and c that the isotherms were shifted to smaller areas with increasing temperature. This indicates that monolayers assembled from SNN and PNIPAM particles became contracted in response to temperature. This is attributed to the shrinking of individual particles above the LCST of PNIPAM.
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Table 1. Collapse Radius per Particle at Different Temperatures code SN
SNN
PNIPAM
T (°C)
Rc (nm)
Rh (nm)
deformation ratio
25 30 35 40 25 30 35 40 25 30 35 40
200.23 198.92 198.06 193.65 301.82 289.09 290.91 282.36 171.59 153.41 132.95 128.41
214.70 216.53 217.54 215.23 371.25 338.58 292.35 282.90 349.98 317.55 230.90 219.83
1.07 1.09 1.10 1.11 1.23 1.17 1.00 1.00 2.34 2.07 1.74 1.71
The collapse radii (Rc) and hydrodynamic radii (Rh) for SN, SNN, and PNIPAM particles are tabulated in Table 1. Rc represents the particle radius at the pressure where the particles were fully compressed, whereas the radius of the uncompressed particle is referred to as the hydrodynamic radius, Rh. Clint et al. reported that Rc of the rigid particle was in good agreement with Rh.14 However, Rc of rigid SN particles was smaller than Rh, as shown in Table 1. Scanning electron microscopic observation demonstrated that some of the SN particles in the monolayer transferred onto the glass plate aggregated. This is probably because particles were not dispersed well in ethanol before they were spread over the water. This indicates that the Rc of SN particles shown in Table 1 is a low estimate due to particle aggregation. It is concluded that the compressibility of SN particles apparently became larger compared with that of SNN ones as well. Rc of SNN particles was nearly equal to their Rh above the LCST; Rc of SNN particles was smaller than Rh below the LCST. We could observe few aggregated particles in the monolayers below the LCST with SEM. Hammond et al. reported that linear-dendritic diblock copolymeric amphiphiles became distorted at high compression.25 Probably, the shell layer of SNN particles was distorted at high compression as well, leading to the relatively small Rc. On the other hand, Rc of PNIPAM particles was much smaller than their Rh irrespective of temperature. Moreover, their deformation ratio, which is the collapse radius per hydrodynamic radius (Rh/Rc), was higher than that of SNN. This is because PNIPAM particles are a hydrogel without a rigid core and were easily deformed by the compressive stress. The low Rh/Rc of SNN particles might be attributed to a rigid core of polystyrene. Therefore, their size and shape were not so difficult to return to the original ones once they were deformed. Particle monolayers were compressed until the collapse pressure at the prescribed temperature. Then, their surface pressures were measured for 30 min with keeping the temperature and the surface area constant to estimate the stability of particle monolayers. The results are shown in Figure 3. Surface pressures became constant within 30 min. It is probable that the relaxation of surface pressures was caused by redistribution and reorientation of particles into the stable packing. The large depression indicates the state of preformed monolayers is metastable. It was found that monolayers of SNN and PNIPAM particles were more stable than those of SN particles, and the former two monolayers prepared at 25 °C were more stable than those prepared at 40 °C. These results indicate that the PNIPAM hydrogel shell is important to make up the stable monolayers from the particles. The dynamic surface pressure responses of water, a temperature-sensitive copolymer, and particle monolayers (25) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299.
Figure 4. Response of dynamic surface pressure to periodic temperature changes. The trough area was kept constant and the subphase temperature changed between 40 and 25 °C within 30 min.
Figure 5. Response of the dynamic surface area to periodic temperature changes. The pressure was kept constant and the subphase temperature changed between 40 and 25 °C within 15 min.
were measured between 25 and 40 °C at the constant surface area. These results are shown in Figure 4. The pressure of water responded to temperature. This is because the surface tension of water depends on the temperature. The monolayer of SN particles also exhibited the temperature-dependent pressure response as well. The copolymer was spread on the air/water interface, and its surface pressure was high and responded to temperature as well as water and SN particles. When the temperature was lowered to 25 °C, copolymer chains were solubilized and extended into not the water surface but the subphase to form loops. In contrast, the pressure of the monolayers of SNN and PNIPAM particles was found to inversely respond to temperature. We found that these two particles swelled and shrank when the temperature was lowered and raised, respectively.21 This volume change would lead to deformation of the particle monolayers. Accordingly, it can be concluded that dilatation of the monolayers upon cooling led to increasing the surface pressure. It can also be seen in Figure 4 that the surface pressure of the PNIPAM particles
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Figure 6. SEM of SNN particle monolayer and PNIPAM particle monolayer.
monolayer gradually decreased with repeating the temperature cycle. We suppose that compression led to coalescence among the particles through entanglement of PNIPAM chains at the outermost surface, resulting in the relaxation of stress. In contrast, the pressure of the monolayer of SNN particles changed little. This indicates that the restoring force generated by the rigid core of SNN particles caused a strong repulsion between PNIPAM hydrogel shells. The dynamic responses of the area of the monolayers of SNN and PNIAPM particles are shown in Figure 5. The A0 and A represent the initial area of the monolayer and the area of the monolayer after temperature change. The A/A0 of the PNIPAM particle monolayer was high but rapidly dropped with temperature cycles. This is quite different from its surface pressure response. The value of 0.54 observed for the monolayer of PNIPAM particles after two cycles means that the monolayer contracted until the limited position of the trough. On the other hand, the response of the monolayer of SNN particles was found to be small but relatively reversible as well as the pressure response. As shown in Figure 6, SEM observation demonstrated that PNIPAM particles were prone to coalescence whereas SNN particles were not. This difference may be attributed to the existence of the latter’s inner core. The core must be resistant to coalescence of the gel particles. Therefore, the monolayer of core-shell particles, which possess the rigid core encased in the PNIPAM hydrogel shell, maintained the thermosensitive response without relaxation. The monolayer was prepared by
mixing SNN and PNIPAM particles. As shown in Figure 5, the dynamic responses of particle monolayers could be changed with the mixed ratio of two types of particles. This indicates that the monolayer possesses both the reversibility derived from SNN particles and the large change derived from PNIPAM particles. The SEM photograph revealed that each particle was scattered and randomly mixed together (data not shown). The L-B technique seems to provide a novel means to measure the volume change associated with stimulisensitive hydrogel particles. New insights regarding the properties of hydrogel particles will be obtained by this method. Furthermore, our developed technique makes it possible to assemble multicomponent monolayers from a variety of particles and to imprint unique functionalities. The volume change caused by thermal stimulation allows us to apply this assemblage of thermosensitive particles to actuators, switching devices and rewritable media. Monolayers assembled from particles sensitive to different stimuli such as pH and specific substances will be applicable to catalysts, biosensors, multiple electrodes, and optical materials. A study is currently being undertaken to bind particles of monolayers for producing a flexible membrane possessing stimuli-sensitive nanopores. Acknowledgment. We acknowledge the financial support from Sekisui Chemical Co Ltd. LA020541X
