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Responsive Thin Hydrogel Layers from Photo-Cross-Linkable Poly(N-isopropylacrylamide) Terpolymers† Patrick W. Beines, Iris Klosterkamp, Bernhard Menges, Ulrich Jonas, and Wolfgang Knoll* Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed NoVember 8, 2006. In Final Form: December 8, 2006 The structural features and swelling properties of responsive hydrogel films based on poly(N-isopropylacrylamide) copolymers with a photo-cross-linkable benzophenone unit were investigated by surface plasmon resonance, optical waveguide mode spectroscopy, and atomic force microscopy. The temperature-dependent swelling behavior was studied with respect to the chemical composition of the hydrogel polymers containing either sodium methacrylate or methacrylic acid moieties. In the sodium methacrylate system, a refractive index gradient was found that was not present in the free acid gel. This refractive index gradient, perpendicular to the swollen hydrogel film surface, could be analyzed in detail by application of the reversed Wentzel-Kramers-Brillouin (WKB) approximation to the optical data. This novel approach to analyzing thin-film gradients with the WKB method presents a powerful tool for the characterization of inhomogeneous hydrogels, which would otherwise be very difficult to capture experimentally. In AFM images of the hydrogel layers, a macroscopic pore structure was observed that depended on the polymer composition as well as on the swelling history. This pore structure apparently prevents the often-observed skin barrier effect and leads to a quickly responding hydrogel.
Introduction Responsive hydrogels, which are composed of water-swollen polymer networks, can change their swelling state and accordingly their water content in response to an external stimulus such as temperature, pressure, or pH change. These gels represent a particularly interesting class of “intelligent” materials for biological, medical, pharmaceutical, sensing, and actuator applications.1 One particular type of hydrogel polymer that has received much attention in the literature is poly(N-isopropylacrylamide) (PNIPAAm) in various modifications.2 The swelling response of such PNIPAAm hydrogels depends critically on the chemical composition, in particular, on ionic groups in the polymer backbone3,4 and on the cross-linking density.5,6 The volume change that is accompanied by the volume-phase transition may be employed for the actuation and control of microvalves7 and can be monitored in thin hydrogel films by surface plasmon resonance and optical waveguide mode spectroscopy.8 A known problem of NIPAAm (co-)polymers that do not contain any ionic species is the fact that they suffer from a socalled “skin barrier” effect when collapsing.2,9 This barrier is formed by a thin, dense outer layer (the skin) upon collapse. The skin prevents water from diffusing out of the gel and thus slowing down or completely hindering the transition from the swollen to the collapsed state. Two common ways to circumvent this † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * Corresponding author. E-mail:
[email protected]. Fax: +49 6131 379 100.
(1) (a) Qiu, Y.; Park, K. AdV. Drug DeliVery ReV. 2001, 53, 321. (b) Hoffman, A. S. AdV. Drug DeliVery ReV. 2002, 54, 3. (c) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. AdV. Mater. 2006, 18, 1345. (2) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (3) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (4) Yu, H.; Grainger, D. W. J. Appl. Polym. Sci. 1993, 49, 1553. (5) Kuckling, D.; Harmon, M. E.; Frank, C. W. Macromolecules 2002, 35, 6377. (6) Harmon, M. E.; Kuckling, D.; Frank, C. W. Macromolecules 2003, 36, 162. (7) Richter, A.; Kuckling, D.; Howitz, S.; Gehring, T.; Arndt, K. F. J. Microelectromech. Syst. 2003, 12, 748. (8) Knoll, W. Annu. ReV. Phys. Chem. 1998, 49, 569. (9) Xue, W.; Hamley, I. W.; Huglin, M. B. Polymer 2002, 43, 5181.
issue are to create hydrogels with macropores and to introduce ionic groups into the hydrogels.4,9 These polyelectrolyte moieties serve as hydrophilic, channel-like regions that allow the water to diffuse out of the collapsing hydrogel. To produce novel responsive hydrogels that can be spin-coated as photo-cross-linkable thin films5,6,10,11 and to investigate their swelling behavior4,12 by optical techniques, terpolymers of N-isopropylacrylamide (NIPAAm), sodium methacrylate (SMA) (or optional methacrylic acid (MAA)), and 4-methacryloyloxybenzophenone (MaBP) were synthesized. NIPAAm is responsible for the “smart” behavior of these gels, whereas SMA (or MAA) provides the polyelectrolyte character and MaBP serves as the cross-linking unit. The temperature-dependent swelling behavior and the morphology of these hydrogel layers with respect to the dependence of their chemical composition were studied by surface plasmon resonance/optical waveguide mode spectroscopy and atomic force microscopy. Experimental Section Materials. N-Isopropylacrylamide (NIPAAm, Aldrich) was purified by recrystallization from a mixture of toluene/hexane (1/4) and dried in vacuum. Methacrylic acid (MAA, Aldrich) and ethanethioic S-acid (Aldrich) were distilled prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. Dioxane and methanol used for the polymerization were distilled over calcium hydride. Amberlyst 15 cation exchanger (Fluka), sodium methacrylate (SMA, Aldrich), and all other reagents of analytical grade were used as received. 4-Methacryloyloxybenzophenone (MaBP) monomer, 4-allyloxybenzophenone, and 4-(3′-chlorodimethylsilyl)propyloxybenzophenone (1) were prepared according to the literature.11 Synthesis of the Polymers. The P(NIPAAm) terpolymers were obtained by free radical polymerization of NIPAAm, sodium methacrylate (SMA) or methacrylic acid (MAA), and MaBP, initiated by AIBN. The polymerization solvent for the SMA terpolymers was (10) Prucker, O.; Naumann, C. A.; Ru¨he, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766. (11) Toomey, R.; Freidank, D.; Ru¨he, J. Macromolecules 2004, 37, 882. (12) Mamada, A.; Tanaka, T.; Kungwatchakun, D.; Irie, M. Macromolecules 1990, 23, 1517.
10.1021/la063264t CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007
2232 Langmuir, Vol. 23, No. 4, 2007 methanol, and that for the MAA terpolymers, dioxane. The reactions were carried out at 60 °C under argon for 24 h. The polymers were precipitated directly from the reaction mixture in ice-cold diethyl ether, purified by reprecipitation from methanol into ice-cold diethyl ether and freeze-dried from tert-butanol in vacuum. The yield was between 64 and 83%. 1H NMR (methanol-d , δ): 0.67-1.20 (m, -CH ), 1.20-1.74 4 3 (m, -CH2-), 1.74-2.40 (m, NIPAAm backbone CH), 3.87 (s, CH3CH-CH3), 7.21-8.09(m, C-Harom.). In order to obtain free acid polymer P1A, a sample of SMA terpolymer P1S was dissolved in ethanol and stirred with Amberlyst 15 cation exchanger for 15 h. The solution was filtered, and the residue was rinsed with ethanol. The solvent of the polymer solution was removed in vacuum, and the polymer was freeze-dried from tert-butanol in vacuum. Yield: 86%, not further characterized (the overall polymer characteristics, such as size and size distribution, of polyacid P1A are identical to those of polysalt P1S). Polymer Characterization. To determine the polymer compositions, the 1H NMR spectra were recorded on a Bruker Spectrospin 250 (250 MHz) spectrometer. The solvent (methanol-d4) was used as an internal reference. The NIPAAm and MaBP fractions can be determined independently by integration of the CH peak of the isopropyl group and by integration of the aromatic CH peak. However, the methacrylic acid or sodium methacrylate fraction has to be calculated by subtracting the given amounts of NIPAAm and MaBP from the CH3 peak. This bears an inaccuracy (approximately 5-10%), so the value of the methacrylic acid/sodium methacrylate fraction should be treated with caution. The molecular weights (Mw) and the molecular weight distributions (polydispersity Mw/Mn) of the polymers were determined by gelpermeation chromatography with a Waters instrument equipped with UV and RI detectors and using PSS GRAM columns. The samples were measured at 60 °C in DMF with a flow rate of 1.0 mL/min. Synthesis of S-3-(4-Benzoylphenoxy)propyl Ethanthioate (2). The benzophenone thioate was synthesized by radical addition of ethanethioic S-acid to 4-allyloxybenzophenone.10 Typically, 2 g (19.9 mmol) of 4-allyloxybenzophenone was dissolved in 20 mL of chloroform to which 0.138 g (0.845 mmol) of AIBN and 1.8 mL (25.39 mmol) of ethanethioic S-acid were added. The mixture was heated to reflux for 4.5 h at 80 °C and then cooled to room temperature. The solution was extracted twice with 100 mL of aqueous NaHCO3. The aqueous phase was washed twice with 45 mL of petroleum ether, and the organic phase was extracted twice with 45 mL of brine. The combined organic phases were dried over Na2SO4, and the solvent was evaporated. The crude product was purified by column chromatography with a mixture of cyclohexane/ethyl acetate (10/1) as the solvent. The yield was 56%. 1H NMR (CDCl , δ): 2.13 (pent, 2H, -CH -CH -CH -), 2.37 3 2 2 2 (s, 3H, -CH3), 3.10(t, 2H, -S-CH2-), 4.12 (t, 2H, -CH2-O-), 6.9-7.9 (various m, 9H, -C-Harom.). Preparation of Samples for SPR Measurements. Immobilization of 1 on SiOx Surfaces. The SPR substrates were LaSFN9 glass slides coated with a 2 nm of chromium, 50 nm of gold film, and a 200 nm SiOx film on top, which were deposited by evaporation with a Balzers PLS 500 E evaporator. Silane 1 was chemisorbed on the SiOx surface at room temperature from toluene solution (20 mL of a 0.025 M solution) using Et3N (4 mL) as a catalyst and acid scavenger. The solution with the substrates was left to stand overnight, and then the samples were cleaned by successively rinsing with dichloromethane, methanol, toluene, and again dichloromethane. After each rinsing step, the sample was blown dry with nitrogen. Immobilization of 2 on Gold Surfaces. The SPR substrates were LaSFN9 glass slides coated with a 2 nm chromium film and a 50 nm gold film, which were deposited by evaporation with an Edwards Auto 306 evaporator. Thioate 2 was immobilized on the gold surface at room temperature from 5 mM toluene solution. The solution with the substrates was left to stand for 24 h, and then the samples were cleaned by successively rinsing with toluene and ethanol. After each rinsing step, the samples were blown dry with nitrogen.
Beines et al. Scheme 1. Schematic Setup with a Hydrogel Layer in the Kretschmann Configuration
Deposition of the Polymer Films and Cross-Linking. Thin polymer films were deposited on the substrates by spin-coating from polymer/ethanol solutions onto the adhesion promoter layers. The polymer content of the solutions was 8 wt % and yielded a film thickness of approximately 1 µm at 4000 rpm. The SPR samples were dried in vacuum at 50 °C and irradiated with a Stratagene UV Stratalinker 2400 operating at 75 W with a peak wavelength of λ ) 365 nm for 1 h.10,11 SPR Technique. Surface plasmon resonance spectroscopy was performed in the Kretschmann configuration with a setup described in the literature.8 Here, only a short overview of the experimental technique is given. The sample glass slide (LaSFN9 glass, Hellma Optik GmbH Jena, refractive index n ) 1.8449, corresponding to ) 1.3583) is optically matched to the base of a glass prism (refractive index n ) 1.8449, corresponding to ) 1.3583). Monochromatic light (He/Ne laser, Uniphase, λ ) 632.8 nm) with linear, transverse-magnetic polarization (Glan-Thompson polarizer, Owis) is directed through the prism as depicted in Scheme 1. Variation of the external angle of incidence θ (two-cycle goniometer with a resolution of 0.005°, Huber) and collecting the light with a photodiode (BPW 34 B silicon photodiode, Siemens) yield angle-dependent intensities I(θ). The angle-dependent reflectivity of the prism base is modeled by solving Fresnels equations by a transfer matrix algorithm13 for a planar multilayer system consisting of glass (glass), chromium (dchromium, chromium), gold (dgold, gold), an adhesion promoter (dadhesion promoter, adhesion promoter), a hydrogel (dhydrogel, hydrogel), and water (water) with and d being the dielectric constants and thickness of the layers, respectively. For every , the real part ′ and the imaginary part ′′ have to be provided. Reflection losses at the prism-air interfaces as well as refraction are included to produce a modeled I(θ). Layer parameters are extracted by minimizing the difference between experimental and modeled I(θ) upon variation of a subset of layer parameters that can be uniquely determined from the I(θ) values as follows. Before deposition of the adhesion promoter and polymer film, a reference scan is performed that allows us to determine the substrate parameters (dchromium, chromium, dgold, gold, and water). A second I(θ) scan after deposition of the adhesion promoter was used to determine dadhesion promoter and adhesion promoter, assuming that all other and d values are fixed. The third I(θ) scan after deposition and cross-linking of the polymer layer was used to determine dhydrogel and hydrogel in the dried state, assuming that all other and d values are fixed. If the polymer film is sufficiently thick (d J 500 nm), then optical waveguide spectroscopy (OWS) measurements can be conducted with such a setup. In this case, additional minima are observed as a result of coupling of the laser beam into waveguide modes. As with the surface plasmon resonance mode, these optical waveguide modes directly depend on the thickness and refractive index of the hydrogel layer, which allow for a detailed characterization of these layers. Characterization of Refractive Index Profiles. A common method used to analyze the SPR/OWS data of thin organic layers from reflection angle scans usually assumes an average refractive (13) Born, M.; Wolf, E. Principles of Optics, 7th ed.; Cambridge University Press: Cambridge, U.K., 1999.
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Figure 1. SPR/OWS measurements of the hydrogel layer at 26 °C with sodium methacrylate P1S (1a) and of ion-exchanged polymer P1A with methacrylic acid (2a), including magnifications (1b and 2b) of the waveguide mode regions. index for the whole layer. This so-called box model is accurate for homogeneous systems. Complex systems such as the present P1S hydrogel film, however, may possess a nonuniform density or refractive index profile perpendicular to the substrate surface. For these systems, such a simple method of analysis may yield very unreliable film parameters or does not even allow for fitting of the experimental data (which is shown for polymer P1S). An appropriate analysis method that can principally capture more complex film architectures represents the reversed Wentzel-Kramers-Brillouin (WKB) approximation, which was initially developed for inorganic waveguides with refractive index gradients normal to the waveguide surface. Assuming that the hydrogel film is laterally homogeneous (parallel to the substrate surface), we now deal with a variation of the refractive index perpendicular to the substrate surface. Such a gradient index profile can be solved with the help of the reversed WKB approximation. This method is a common approximation used for planar waveguide gradient index profile analysis.14 The reversed WKB approximation is based on the fundamental idea that at the position of the electromagnetic field distribution, for which the oscillating and evanescent solution of the wave equation are identical, the physical refractive index is equal to the measured effective refractive index Neffm for each mode. To determine the refractive index at the gold-hydrogel interface (x ) 0 µm), the differences between the slopes of the linear segments between neighboring data (14) Weisser, M.; Thoma, F.; Menges, B.; Langbein, U.; Mittler-Neher, S. Opt. Commun. 1998, 153, 27.
points (each described by xm and Neffm) from the measurement are squared, and all differences are summed. By varying the assumed refractive index (as an undefined parameter), the minimum of this sum is searched. The refractive index at this minimum is taken as the real refractive index at the substrate interface (x ) 0 µm) and is used to compute a smooth refractive index profile as a function of the vertical distance from the substrate interface (x coordinate in Figure 3). More details of this calculation are given in the literature.15,16 With the effective refractive indices determined by measuring the coupling angles with a prism coupler in the Kretschmann configuration, the refractive index profile in the hydrogel layer is calculated. The data from these calculations are shown in Figure 3, together with a fit of the refractive index profile based on the expression below from ref 17 n(x) ) ncover +
∆n [1 - (1 - γ)erfc(x/df)] γ
Here, ncover is the refractive index of the cover, ∆n is the maximum increase of the refractive index, χ is a material constant, and df is the effective layer thickness. Swelling Experiments. Investigations on the temperaturedependent behavior of the hydrogels were conducted by placing the sample in a flow cell that was connected to a peristaltic pump and (15) Karte, W.; Mu¨ller, R. Integrierte Optik; Akademische Verlagsgesellschaft Geest and Porttig KG: Leipzig, Germany, 1991. (16) Tien, P. K. ReV. Mod. Phys. 1977, 49, 361. (17) Linares, J.; Prieto, X.; Montero, C. Opt. Mater. 1994, 3, 229.
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Figure 2. (a) Coupling angles of TM1 waveguide modes from the OWS measurements of polysalt P1S and of free polyacid P1A. (b) Heating curve of the temperature-dependent swelling behavior with refractive index and thickness changes of free polyacid P1A.
Figure 3. WKB calculations for the refractive index profile of the hydrogel layers from polysalt P1S and polyacid P1A (for a temperature of 26 °C). situated on a hot stage. The temperature was adjusted by resistive heating of the hot stage and by cooling with a water mantle that was mounted surrounding the hot stage. After the measurement of the hydrogel in the dried state, the flow cell was filled with deionized water from a Milli-Q system with a resistivity of 18.2 MΩ·cm. The hydrogel was left to swell for 1 h at 15 °C. During this initial swelling of the hydrogel layer, some irreversible restructuring and loss of non-cross-linked polymer material into the water phase may occur. To achieve a stable system, a temperature-dependent swelling and collapse cycle is first performed before any SPR/OWS data are collected. After this initial treatment, the layer composition and structure are not changing over time, and the swelling and collapse process is fully reversible. Then an I(θ) angle scan measurement was performed at constant temperature. After the completion of the angle scan, the temperature was increased in steps of 1 to 5 °C. After each temperature increase, the hydrogel was left to swell for 15 min to reach thermal equilibrium, and then the angle scan measurement was repeated. Investigations of the swelling dependence on the salt concentration were performed with sodium chloride concentrations of 0.01, 0.1, 1, and 6.1 mol/L in Milli-Q deionized water. The measurements were conducted at 14 °C starting with the lowest salt concentration.
After the hydrogel was immersed in the salt solution for 20 min to reach the equilibrium state, the angle scan was performed. Then the salt solution was replaced by the next higher concentration. This process was performed four times.
Results and Discussion To investigate the effect of the ionizable units on the responsive properties of these photo-cross-linkable PNIPAAm hydrogels, three different polymer types (P1, P2, and P3) were synthesized by free radical polymerization. Addition of the affixed “A” to these abbreviations indicates the free acid (-COOH) as an ionizable function, whereas an “S” indicates the sodium salt (-COO-Na+). Sodium methacrylate was used as the polyelectrolyte in the first polymer (P1S), methacrylic acid was utilized for the second polymer (P2A), but no ionizable unit was present in the third polymer (P3). The general polymerization reaction is shown in Scheme 2, and the structures of the synthesized polymers are depicted in Scheme 3. The polymer composition as well as the GPC results detailing the molecular weights and weight distributions are
Thin Hydrogel Layers from PNIPAAm Terpolymers Scheme 2. General Polymerization Reaction for the Statistical (stat) PNIPAAm Terpolymer with Methacrylic Acid as the Ionic Species
summarized in Table 1. Compared to the monomer composition, increased incorporation of sodium methacrylate in the polymer backbone was observed for P1S (polymerized in methanol), whereas an increased incorporation of MaBP was found for P2A and P3 (both polymerized in dioxane). Surface attachment was achieved by utilizing either silane chemistry on silicon oxide surfaces (glass, silicon wafer) or the affinity of sulfur for gold surfaces. For this purpose, a benzophenone monochlorosilane10 and a novel benzophenone ethanthioate were synthesized. The adhesion layers from both compounds show good stability and surface attachment in pure water. The different synthesis steps are illustrated in Scheme 4 and described in detail in the Experimental Section. Upon contact with water, a dry hydrogel film (with refractive index nHG-dry ) 1.498) swells by the diffusion of water (with lower refractive index nH2O ) 1.33) into the gel. This results in an increase in the film thickness d and in a decrease in the refractive index of the hydrogel (nHG-swollen ) 1.356). Thus, two parameters (nHG and d) change simultaneously during swelling and accordingly also during collapse. To unambiguously determine these two parameters from the optical measurements (SPR and OWS), at least two optical modes are needed to make an appropriate fit and independently extract the thickness and the refractive index. The surface plasmon probes the hydrogel layer only 100 to 150 nm perpendicular to the substrate-hydrogel interface. Beyond this point, it is “blind”. The reason for this is the exponentially decaying evanescent field. Waveguide modes, however, are guided within the complete layer. Therefore, they can be utilized to determine the thickness and the refractive index of hydrogel layers. The lowest-order waveguide mode in the Kretschmann configuration, the TM1 mode, with absorption at the highest angle of incidence (below the surface plasmon resonance), is most sensitive to changes in the refractive index whereas the highest-order mode, in our case the TM4 mode, with absorption at the lowest angle of incidence, is most sensitive to changes in the thickness. This sensitivity difference is due to the electric field distribution of the modes within the film, which is highest in the center of the layer for the TM1 mode, whereas the TM4 mode has very high field intensities at both interfaces of the hydrogel film with the substrate and the water phase. For the optical measurements, two types of hydrogel layers were prepared in order to investigate the effect of salt or acid units on the hydrogel film properties. The first film was formed from the terpolymer with sodium methacrylate P1S by spincoating and subsequent irradiation to cross-link the layer. In the second case, polymer P1S was first subjected to ion exchange, prior to spin-coating, and photo-cross-linking in order to convert the salt to free acid form P1A. The scan data of the SPR/OWS measurements for both polymer types are presented in Figure 1.
Langmuir, Vol. 23, No. 4, 2007 2235 Scheme 3. Structures of Synthesized Statistical (stat) Polymers P1S, P1A, P2A, and P3
Scheme 4. Syntheses of the Adhesion Promoters for Silicon Oxide and Gold Surfaces
The parameters for the curve fitting of polymers P1S/P1A were as follows: the thicknesses of the hydrogel film layers were fixed as 3560 nm/2875 nm, ′ values as 1.869/1.917, and ′′ values both as 0. For terpolymer P1S (Figure 1: 1a, 1b), the spectra show clear angular deviations of the measured SPR/OWS minima at 48.1 and 49.2° compared to the corresponding calculated fit. It seems that the simple box model, which assumes a homogeneous refractive index throughout the whole hydrogel layer and on which the calculation of the fit is based, is not appropriate for this particular sample. In contrast, the measurement of free acid polymer P1A, treated with the cation-exchange resin, revealed that the calculated curve of the waveguide modes fits the measured data much better. The deviation of the measured surface plasmon resonance angle and the corresponding fit indicates a difference in the real refractive index close to the substrate-hydrogel interface and the assumed average value of the refractive index
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Table 1. Monomer Composition, Polymer Composition, Polymerization Solvent, Molecular Weight, and Polydispersity Summarized for the Synthesized Polymers monomer composition P1S P2A P3
NIPAAm
SMA
97 94 99
2
polymer composition
MAA
MABP
NIPAAm
SMA
1 1 1
85 86 93
13
5
for the whole hydrogel layer. This is due to the fact that the surface plasmon is most sensitive close to the gold surface, where the adhesion promoter is located, and forms a dense region of cross-links with the hydrogel. This massive cross-linking results in a higher refractive index close to the substrate surface compared to that for the rest of the hydrogel. The good fit of the waveguide modes indicates that the real structure of this polymer P1A film (Figure 1: 2a, 2b) resembles the assumed box model much more than the real structure of the polymer P1S film. It seems that the presence of ionic groups from sodium methacrylate introduces an inhomogeneity, probably a density gradient, perpendicular to the sample surface. It is possible that the polar ionic groups lead to phase segregation because of unfavorable interactions with the hydrophobic groups of the terpolymer backbone and the apolar adhesion promoter layer at the substrate surface. The temperature-dependent behavior of sodium methacrylate terpolymer P1S and the free acid derivative P1A was also measured by SPR/OWS. To prevent the use of the (for P1S inappropriate) box model, the raw data, being the angles of incidence as a function of temperature, are shown in Figure 2a for polymers P1S and P1A. The raw data does not require any model for representation. The expected volume-phase transitions can clearly be observed from the TM1 modes of the two samples. The transition temperatures of the volume-phase transitions are defined by the inflection points (second derivative equals zero) of the curves. These two, almost parallel shifted curves show a temperature difference of 5 °C. For polysalt P1S, the transition temperature is 37 °C, whereas free acid polymer P1A shows a drastic change in the transition temperature, which has dropped to 32 °C. These observations strongly suggest that the presence of polar ionic groups leads to a more hydrophilic hydrogel that exhibits a higher transition temperature compared to that of the free acid form of the otherwise identical polymer. Applying the box model to the data obtained by the SPR/ OWS measurement of P1A (Figure 2b) shows that the refractive index’s temperature dependence is the same as the temperature dependence of the TM1 mode of P1A (Figure 2a). There is no
Figure 4. Temperature-dependent swelling behavior of polyacid P2A, which was directly polymerized with the free methacrylic acid monomer.
MAA
MABP
solvent
Mn (g/mol)
Mw (g/mol)
PDI
5
2 9 7
MeOH dioxane dioxane
201 000 194 000 137 000
478 000 404 000 279 000
2.4 2.1 2.0
observable difference in the transition temperature. The inflection points of both curves are the same. By applying the reversed WKB approximation to the hydrogel films, it was found that the layer composed of polysalt P1S exhibited a significant refractive index gradient as shown in Figure 3. The refractive index continually decreases with increasing distance from the substrate-hydrogel interface toward the interface of the hydrogel with the water phase. The profile of the polymer layer with methacrylic acid units P1A was found to resemble much more a box with a homogeneous refractive index throughout the whole hydrogel film. The refractive index decreases only slightly from the substrate to a distance of 2.5 µm and then drops very rapidly to the value of pure water beyond this distance. It is worthwhile to note that the profile of polymer P1S exhibits not only a further extension into the water phase (x > 4 µm) but also a lower refractive index close to the gold surface (x ) 0 µm) than polymer P1A. This indicates the higher water content and stronger swelling of the more hydrophilic salt-containing polymer. The detailed description of the layer structure by the WKB fit and the match between the calculated refractive index at the substrate-hydrogel interface and the refractive index from the surface plasmon fit show that the reversed WKB approximation is a very powerful tool for the analysis of inhomogeneous layers with vertical refractive index gradients. Because polyacid P1A, which was obtained from polysalt P1S by treatment with an ion exchanger, formed well-defined hydrogel layers with a homogeneous refractive index throughout the film, this type of polymer (P2A) was directly synthesized from the free acid monomer by copolymerization of Nisopropylacrylamide, methacrylic acid, and 4-methacryloyloxybenzophenone. The temperature-dependent behavior of this polymer is depicted in Figure 4 and shows a volume-phase transition temperature of 38 °C (from the refractive index curve). These measurements were conducted in thermal equilibrium with an equilibration time of 15 min between two measurements. In polymer systems P2A and P3, which are described further below, no gradient structures were observed, and thus no WKB analysis was required. It is worthwhile to note that polyacid P1A from the ion-exchange reaction has a methacrylic acid content of 13% and shows upon heating a volume-phase transition temperature at 32 °C, whereas polyacid P2A formed by direct copolymerization with methacrylic acid has an acid content of only 5% but shows a transition temperature of 38 °C. However, the opposite behavior would be expected because P1A should be intrinsically more hydrophilic and thus should show the higher transition temperature. Furthermore, the cross-linker content in P1A is 2% whereas in P2A it is 9%, thus a higher cross-link density and consequently a lower swelling ratio would be expected. The swelling ratio for P1A is about 2.7 (at 26 °C) whereas for P2A with a higher cross-linker ratio it is 3.5 (at 26 °C), which is counterintuitive. Because the molecular weights are very similar for P1A and P2A and only the weight distributions vary, possible reasons for this odd behavior may be attributed to differences in the repetition unit sequence along the polymer backbone, which
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Figure 5. NaCl-dependent swelling of free acid hydrogel P2A. Table 2. Thicknesses, Refractive Indices, and Swelling Ratios of P2A and P3 in the Dry State and in the Wet State in Contact with the Water Phase (1) and after Drying (2) of the Previously Swollen Layers dry
thickness refr. index swollen thickness refr. index collapsed thickness refr. index swelling ratio (dry f swollen) swelling ratio (collapsed f swollen)
P2A (1)
P2A (2)
P3 (1)
P3 (2)
725 nm 1.499 6536 nm 1.357 892 nm 1469 9.0
715 nm 1.499 6536 nm 1.357
715 nm 1.498 3858 nm 1.359 629 nm 1.471 6.4
472 nm 1.498 3858 nm 1.359
7.3
9.1
8.2
6.1
may cause local inhomogeneities, or differences in the overall film morphology (refer to AFM investigations below). Because potential applications of responsive hydrogels in biological environments will involve buffer solutions with salts and because a strong osmotic and electrostatic screening effect of salt solutions on polyelectrolytes is known, the response of free acid hydrogel P2A to the addition of sodium chloride was investigated at 14 °C in the swollen state and is shown in Figure 5. It was observed that the layer thickness of d0 ) 5.7 µm at maximum swelling in pure water is already reduced to 5.3 µm at low salt concentration (0.01 mol/L). This becomes more prominent at a concentration of 1 mol/L, with a thickness of 4.9 µm, and reaches its maximum with the 6.1 mol/L saturated NaCl solution. In this medium, the swollen hydrogel has a thickness of 2.6 µm, which is still 50% of the maximum swelling in pure water. The exclusion of water from the hydrogel layer at higher salt concentration may be mainly due to osmotic effects whereas at lower salt concentrations screening of the charge around the carboxylic acid groups prevails. To investigate the particular effect of free methacrylic acid groups on the swelling behavior, two types of polymers were comparedsterpolymer P2A containing methacrylic acid and P3, a copolymer of N-isopropylacrylamide and 4-methacryloyloxybenzophenone without acid. In the SPR/OWS measurements, it was found that the swelling ratio is slightly increased by a factor of about 1.2 if methacrylic acid is present in the polymer backbone. The results are shown in Table 2. The response time was found to be below 5 min for the complete swelling and collapse process. This value is an upper limit imposed by the time required for
Figure 6. AFM images of the hydrogel films of the polyacid P2A as prepared (a) and after swelling, collapse, and drying (b) and of the polymer without ionizable groups P3 as prepared (c) and after swelling, collapse, and drying (d, note the 5× larger image size).
a complete reflection-angle scan but may be significantly smaller. Indeed, from experiments in small capillaries we know that the collapse time is within seconds. Sample P3 without methacrylic acid was expected to have a much longer response time because of the anticipated formation of a skin barrier. However, it was found that the response time is similar to that of hydrogel P2A with methacrylic acid. To understand the swelling behavior from a morphological point of view, AFM measurements were performed on hydrogel layers of P2A and P3. The films of acidcontaining hydrogel P2A revealed in the AFM a smooth surface after spin-coating (Figure 6a), whereas a pore-covered surface was found after swelling, collapse, and drying (Figure 6b). These pores have diameters of 40-70 nm and depths of 2-10 nm. In contrast, the surface of the acid-free hydrogel P3 film already reveals pores in the dry state after spin-coating. The pore diameters range from 40 to 100 nm, and their depths range from 3 to 12
2238 Langmuir, Vol. 23, No. 4, 2007
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The presence of polar groups such as methacrylic acid slightly increases the swelling ratio, but the intrinsic film morphology of the acid-free P3 copolymer with pores still allows efficient water diffusion in the hydrogel layer for a rapid swelling and collapse process.
Summary and Conclusions
Figure 7. AFM cross-sections of the pore structures.
nm (Figure 6c). After swelling, collapse, and drying, the sizes of many pores have increased tremendously to diameters of 90650 nm and depths of 5-320 nm (Figure 6d). The AFM cross-sections of the pore structures are shown in Figure 7 and illustrate the approximate depth profile (which is accessible by the used AFM tip with a nominal tip radius below 10 nm and an opening angle of less than 35°). Apparently, these pores prevent the formation of a tight skin barrier, and thus the water is released quickly through these pores from the collapsing hydrogel. Pore formation upon exposure to water in the polyacid P2A film may be caused by the partial dissolution of some non-cross-linked polymer chains, which would also explain the slightly reduced layer thickness after swelling, collapsing, and drying (values for P2A (2) in Table 2). Additionally, the polymer network might be partially restructured, and the more hydrophilic units may segregate to form the pores as water-release channels. In acid-free copolymer P3, pores are already present in the dry state, which may be caused by the segregation of the very apolar benzophenone units or partial dewetting of the polymer film during spin-coating and soft baking. Upon hydration, the pore size increases dramatically, apparently because of the dissolution of large amounts of non-cross-linked polymer upon swelling, as the substantially decreased layer thickness upon drying indicates (P3 (2) in Table 2). The film thickness of dry hydrogel P3 after spin-coating was determined to be 607 nm, whereas the thickness after one swelling and collapse cycle was found after drying to be 472 nm. Therefore, apparently about 25% of the hydrogel material was extracted from the film during the first swelling process.
Novel PNIPAAm terpolymers with benzophenone photo-crosslinking units and sodium methacrylate (P1S) or methacrylic acid units (P1A, P2A) were synthesized; they can form thermally responsive hydrogel networks after irradiation and are, as such, interesting as active sensor materials. Thin surface-attached films of these polymers were easily obtained by spin-coating and photocross-linking. Characterization of these films with surface plasmon resonance/optical waveguide spectroscopy (SPR/OWS) in the Kretschmann configuration showed inhomogeneities in the sodium methacrylate-containing hydrogels, whereas the free methacrylic acid-containing hydrogels (of otherwise identical structure) were found to be mainly homogeneous. First-time application of the Wentzel-Kramers-Brillouin (WKB) approximation to these hydrogel layer systems allowed for a detailed description of the layer structures and revealed a pronounced refractive index gradient profile for the film of P1S, which was not present in P1A. From these results, it can be concluded that the overall layer gradient structure can be directly controlled by the protonation state of the carboxylic acid functions in the linear precursor polymer before photo-cross-linking. To investigate the effect of free acid groups in the hydrogels further, two polymers with methacrylic acid groups (P2A) and without (P3) were synthesized and compared. In the more polar gel with methacrylic acid functions, the swelling ratio was increased by a factor of 1.2 compared to that of the less-polar polymer with only NIPAAm and benzophenone units. The response times of the two hydrogel types were of the same order of magnitude, which shows for our benzophenone-NIPAAm copolymer even in the absence of polar acid groups unexpectedly good swelling behavior. However, AFM measurements revealed that the film morphology strongly depends on the chemical composition and swelling history. Free acid-containing film P2A initially showed a smooth film surface after spin-coating. When this film was swollen in water and then dried again, a perforated surface with pore diameters around 60 nm was observed. Less polar polymer P3 possessed such a pore structure already in the as-spun film, and after swelling and drying, the pore dimensions substantially increased up to 650 nm. In conclusion, the reported study could show that film structure and morphology strongly depend on the chemical composition of the polymer whereas the swelling behavior is affected only to a small extent. Acknowledgment. We thank Ru¨diger Berger and Uwe Rietzler for assistance with the AFM measurements, Natalie Horn for assistance with the adjustments of the SPR/OWS setup, and Dirk Kuckling, Cathrin Corten, Ju¨rgen Ru¨he, and Oswald Prucker for many helpful discussions. This work was partially supported by the Deutsche Forschungsgemeinschaft (priority program “Intelligente Hydrogele” SPP 1259, KN 224/18-1), the Bundesministerium fu¨r Bildung und Forschung (project DPPPMPIP, FKZ: NMT/03X0014D), and the European Commission (project Nano3D, NMP4-CT-2005-014006). LA063264T
