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Swelling of a Polyelectrolyte Brush in Humid Air M. Biesalski and J. Ru¨he* Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received July 1, 1999. In Final Form: September 13, 1999 We present an experimental study on the swelling of polyelectrolyte brushes covalently attached to planar solid surfaces in contact with humid air. Monolayers of poly-N-methyl-[4-vinylpyridinium]iodide (MePVP) with film thicknesses of several hundred nanometers were used in this study. The MePVP brushes were attached to the surfaces of silicon wafers as well as to evaporated silicon oxide films on solid substrates by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The film thicknesses of the surface-bound monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The MePVP brushes show strong increases in thickness as well as a strong decrease of the refractive index of the surface-attached layer due to water incorporation caused by the exposure to the humid environment.
Introduction The modification of solid surfaces with polymeric materials has received increasing interest in a wide field of interdisciplinary research topics in chemistry and physics and also in recent years in biology.1 Systems in which molecularly thin polymeric coatings are covalently attached to surfaces are very interesting for basic scientific studies because the properties of polymers in such a confined geometry greatly differ from those of bulk materials. Furthermore, several interesting new phenomena arise. If, for example, the polymer chains are tethered with one end to a surface and if the distance of the anchored polymer molecules is smaller than 2 times the radius of gyration, the conformation of the polymer molecules at the surface is completely different from that of “unperturbed” polymer molecules in solution, as first shown by Alexander and de Gennes2,3 and more recently by a large number of theoretical groups.4-6 From a technological point of view polymer-coated surfaces are of great interest in a variety of applications, particularly in the area of sensor development, especially for “chemical sensors”, which can detect, identify, and quantify chemical substances in the environment.7 In recent years a rapidly growing area of research has been directed toward the development of devices that make use of a combination of the interactions between biological systems and synthetic (or modified natural) materials to produce a so-called “biosensor”.8,9 All systems, especially polymer films, that are used in a “biosensing” application in contact with biological material have to fulfill some * To whom correspondence should be addressed. (1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (2) Alexander, S. J. Phys. 1977, 38, 977. (3) de Gennes, P. G. J. Phys. 1976, 37, 1443. (4) Milner, S. T. Science 1991, 251, 905. (5) Szleifer, I.; Carignano, M. A. In Advances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; John Wiley Inc.: New York, 1996; Vol. 44, p 165. (6) Halperin, A.; Tirell, M.; Lodge, T. P. In Advances in Polymer Science; Springer-Verlag: Berlin, Heidelberg, 1992; Vol. 100, p 31. (7) Go¨pel, W.; Jones, T. A.; Kleitz, M.; Lundstro¨m, J.; Seiyama, T. In Sensors, A Comprehensive Survey; Go¨pel, W., Hesse, J., Zemel, J. N., Eds.; VCH: Weinheim, 1992; Vol. 2. (8) Go¨pel, W.; Jones, T. A.; Kleitz, M.; Lundstro¨m, J.; Seiyama, T. In Sensors, A Comprehensive Survey; Go¨pel, W., Hesse, J., Zemel, J. N., Eds.; VCH: Weinheim, 1992; Vol. 3. (9) Ratner, B. D.; Hoffman, A. S.; Schoen, F. J.; Lemons, J. E. Biomaterials Science, An Introduction to Materials in Medicine; Academic Press: San Diego, 1996.
important requirements. First, almost all biological molecules of interest are only stable in aqueous buffer solutions. Accordingly the polymer coating has to be swellable in such an environment to allow good contact between functional groups in the polymer and the molecules in the analyte. Second, completely reversible changes in interface properties are required to bind molecules with high sensitivity, selectivity, and reproducibility in sensors and biosensors.7 One of the reasons why polymer films are frequently used for the buildup of a chemical sensor is the favorable signal-to-noise ratio possible in such systems. In most cases the signal of a specific reaction originates from an interaction of the analyte molecule with a specific functional group at the surface of the sensor. For polymeric materials, the functional groups are in most cases located in the repeat units of the polymer and the polymer consists of high molecular weight chains, and the signal is not caused only by a few, but by a very large number of such groups. One kind of parameter used for the detection of the molecules could be thickness changes of the employed surface coating, due to the interaction of the surfaceattached molecules with a low molecular weight gaseous and/or liquid compound. Whereas self-assembled monolayers, which are frequently used for surface modification, respond to external compounds only with thickness changes on the order of a few angstroms, thick polymer films, as we describe here, can show a much stronger response to the presence of low molecular weight compounds in the environment. If a polymer film at a surface is used for the buildup of a sensor, an important issue is the stability of such films in different environments. If the polymer film is established at the solid surface of the device by physisorption of polymer molecules to the substrate material, problems concerning the long-term stability arise. If such a layer is exposed to a liquid or a gas, in which the polymer material is swellable, the film can be displaced from the surface.1 In addition, strong mechanical stresses can occur because of the lateral expansion of the film caused by the swelling. Such stress can ultimately cause the whole film or network to leave the surface of the device (delamination). This problem can be overcome if the polymer layer is covalently linked to the surface of the substrate.11 (10) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592.
10.1021/la990863+ CCC: $19.00 © 2000 American Chemical Society Published on Web 11/27/1999
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Furthermore, as discussed above, the layer used for the detection of the analyte should have a high film thickness to give a good signal-to-noise ratio. Polymer layers, which are covalently linked to the surface of a substrate and which have a high film thickness, must consist of polymer chains with high graft density and high molecular weight. Such films can only be obtained by “growing” the polymer in situ at the surface of the substrate.10-14 Film formation through chemisorption of preformed polymer molecules, on the other hand, is intrinsically limited for kinetic and thermodynamic reasons and film thicknesses of typically only 1-10 nm can be obtained by this method.1 We recently showed that it is possible to generate polyelectrolyte (PEL) monolayers in situ at a surface of a planar solid substrate, using a “grafting from” approach. Both positively13 as well as negatively14 charged “brushes” can be obtained through this method. “Polymer brushes” is a term frequently used for polymer molecules that are at one end irreversibly bound to a solid (or liquid) surface and where the distance between the anchoring points, which connect the polymer to the surface, is smaller than the radius of gyration of the polymer chain. The chains are stretched away from the surface due to intra- and intermolecular correlations which occur because of the high segment density at the surface. As the first in a series of papers in which we describe the behavior of PEL monolayers in contact with aqueous environment, we report here on the behavior of poly-Nmethyl-[4-vinylpyridinium]iodide (MePVP) brushes in contact with humid air. For this purpose we synthesized MePVP brushes at the surface of silicon wafers as well as on evaporated silicon oxide. The monolayers were prepared as schematically shown in Figure 1. Starting from a selfassembled monolayer of initiator molecules, which had been immobilized on the substrate, a monolayer of poly4-vinylpyridine (PVP) is “grown” through a radical chain polymerization process in situ. This PVP brush is transformed into the resulting positively charged MePVP brush by a polymer-analogous quarternization reaction, using methyl iodide in nitromethane solution. Experimental Section Materials and Synthesis of the PEL Monolayer. The synthesis of the end-functionalized azo initiator was described previously.10,15 Glass slides (LaSFN9; Helma; n ) 1.844) were used as substrates for the deposition of the polymer layer. Onto the slide an approximately 50 nm thick gold and a 30 nm thick silicon oxide layer were evaporated prior to polymer deposition. As alternative substrates silicon wafers (Aurel, Germany) with a natural silicon oxide layer were used. The immobilization of the initiator was carried out in dry toluene at room temperature under argon. The reaction time was about 15 h. The concentration of the initiator was approximately 0.5 mmol/L16 and triethylamine was used as catalyst for the surface attachment reaction. After completion of the immobilization reaction nonattached initiator and other byproducts of the reaction were removed by careful extraction with toluene and methanol. The polymerization of 4-vinylpyridine with the surfaceattached initiator was carried out in bulk solution. After the immobilization of the initiator, the substrates were transferred into Schlenk tubes, which were subsequently filled with the (11) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 1, 602. (12) Ru¨he, J. Nachr. Chem. Techn. Lab. 1994, 42(12), 1237. (13) Biesalski, M.; Ru¨he, J. Macromolecules 1999, 32(7), 2309. (14) Biesalski, M.; Ru¨he, J. Submitted for publication in Macromolecules. (15) Prucker, O. Ph.D. Thesis, University of Bayreuth, Germany, 1995. (16) The initiator is applied in large excess (the number of moles is several thousand times higher than the number of hydroxyl groups on an SiOx surface), to ensure that to all hydroxyl groups that can possibly react, an initiator molecule is attached.
Figure 1. Synthesis route for the preparation of the covalently to a planar solid substrate attached poly-N-methyl[4-vinylpyridinium]iodide brush. monomer. After removal of all oxygen traces from the solution under vacuum during repeated freeze-thaw cycles, the Schlenk tubes were placed into a thermostat at 60.0 °C. After 14 h of polymerization time the substrates were removed from the polymerization solution, rinsed carefully, and extracted for at least 15 h in a Soxhlet extractor with methanol, which is a good solvent for poly(vinylpyridine) PVP. This procedure was found to be necessary to remove all physisorbed polymer from the covalently bound polymer monolayer.15 The quarternization of the PVP was carried out using 1.0 M methyliodide in nitromethane at 45 °C following the procedure described by Fuoss and Strauss for the quarternization of free PVP in solution.17 The reaction time was 6 h. The samples were rinsed afterward extensively with nitromethane, to remove all of the nonreacted quarternization agents, and dried in a vacuum. Instrumentation. For a qualitative characterization of the generated monolayer, Fourier transform infrared (FTIR) transmission measurement was carried out using a Nicolet Omnic 850 spectrometer. For these measurements approximately 1 mm thick silicon wafers, which were polished on both faces, were used as substrates. Typically 750 scans were accumulated for each spectrum with a resolution of 4 cm-1. For the determination of the thicknesses of the surfaceattached polymer layers, waveguide spectra were measured. The measurements were carried out in an attenuated total reflection (17) Fuoss, R. M.; Strauss, P. J. Polym. Sci. 1948, 3(2), 246.
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Figure 2. Setup for waveguide spectroscopy scan and kinetic measurements. Table 1. Relative Humidities Over Saturated Salt in a Closed Environment salt LiBr CaCl2 K2CO3 KI NH4Cl ZnSO4 KNO3 a
temperature relative temperature relative (°C) humidity (%) (°C) humidity (%)a 25 25 20 25 20 (25b) 20 20 (25b)
6 29 44 69 79 (79b) 90 98 (93b)
22 22 22 22 22 22 22
12 35 43 70 78 87 93
Reference 19a. b Reference 19b.
(ATR) setup. A glass prism was used to couple the polarized laser light (HeNe laser, 632.8 nm) into the gold film, which had been evaporated onto the glass substrates as described above. For index matching of the prism and the glass slide, a refractive index oil (Cargille Laboratories Inc., NJ; n ) 1.800) was used. During angular scans the reflected intensity was measured as a function of the angle of incidence (Θ). By comparison of a calculated reflection curve (fresnel calculation, using a simple box model) and the experimental data, the thicknesses of the deposited polymer layer as well as the refractive index of the layer can be determined.18 The waveguide spectroscopy experiments were carried out using both p- and s-polarized light. To determine the swelling kinetics and the layer thickness in moist air as a function of the humidity, essentially the same setup was used. As shown in Figure 2, the sample was brought into contact with a cell, which can be held at constant temperature and at constant relative humidity. The humidity inside the cell was controlled by using small vessels filled with saturated aqueous salt solutions in contact with excess salt. The high salt content of the solution reduces the vapor pressure of the water to a distinct value. The chosen salt solutions as well as the corresponding relative humidity inside the environmental chamber (measured with a humidity sensor 601 from Testo Corperation, Germany) are shown in Table 1.19 To measure the kinetics of the swelling, the sample was dried in moisture free air. This was achieved by placing solid potassium hydroxide instead of the salt solution inside the cell. During moisture exposure the change in thickness was monitored by measuring the shift of the minimum of a waveguide mode as a function of the time. The time resolution during the experiment was better than 1.5 s. When the equilibrium of the swelling was reached, the swollen thickness as well as the refractive index of the polymer film was determined by carrying out a waveguide scan measurement. (18) Knoll, W. MRS Bull. 1991, 7, 29. (19) (a) Wexler, A. In CRC Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, New York, London, Tokyo, 1995-1996. (b) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: Oxford, 1992.
Results and Discussion To investigate the behavior of the surface-attached PEL brushes in humid environments, we first prepared neutral PVP brushes on the surfaces of silicon wafers and on the surfaces of LaSFN9/Au/SiOx substrates. Therefore, polymerization reactions of 4-vinylpyridine were carried out in bulk monomer for 14 h at 60 °C. After termination of the polymerization reaction, the samples were carefully extracted from physisorbed material using methanol, which is a good solvent for the polymer.20,21 The quarternization to the surface-attached PEL was carried out under mild conditions similar to the process for the quarternization of free PVP in solution.17 Methyl iodide was used as the quarternizing agent. The reactions were carried out at slightly elevated temperature (45 °C) in nitromethane for 6 h. We recently showed for a comparable system that the covalent attachment of the PVP chains has no influence on the reaction process and that the transformation in the positively charged monolayer can be carried out to quantitative conversion.13 To prove the chemical identity of the attached polymer layer and to determine the degree of conversion of the quarternization reaction, we performed FTIR measurements on the PVP layers attached to a silicon wafer before and after quarternization. The transmission IR spectra are shown in Figure 3. The spectrum of the prepared PVP shows typical vibrational bands of the PVP such as the C-H stretching vibrations around 3000 cm-1 and the vibrational bands from the CdC double bonds of the pyridine ring at 1600 and 1556 cm-1. The area below 1500 cm-1 shows the typical absorption bands of the bulk spectrum of PVP.22 When PVP is transformed into the cationic species a new infrared adsorption band can be observed at 1642 cm-1 (for CdC double bond stretching vibrations of the MePVP). The intensity of the signals, which are attributed to the unquarternized PVP (e.g., at 1600 and 1556 cm-1 ), decreases with increasing reaction time until at high conversion it cannot be detected any longer (Figure 3A). To show the changes in the infrared spectra more clearly, the spectral region between 1670 and 1530 cm-1 of such a sample before and after completion of the quarternization reaction is depicted in Figure 3B. To determine the layer thickness we performed optical waveguide spectroscopy (OWS) experiments on the prepared PVP monolayers as well as after the quarternization on the MePVP monolayers on the LaSFN9/Au/SiOx substrates. In Figure 4 spectra before (a) and after (b) the
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Figure 4. Waveguide spectra (p-polarization) of (a) a 490 nm thick PVP brush on a LaSFN9/Au/SiOx substrate. The brush was prepared as described in Figure 4; and (b) the same sample at a constant relative humidity of 70% after quarternization with methyliodide. Thickness: 870 nm.
Figure 3. (A) FTIR spectra of (a) a 430 nm thick PVP monolayer attached to both sides of a silicon wafer. The monolayer was prepared on the surface of the substrate by radical chain polymerization of 4-vinylpyridine in bulk for 14 h at 60 °C. After the polymerization the substrate was extracted with methanol for 15 h; and (b) the same sample after quarternization in 1 M methyliodide/nitromethane for 6 h at 45 °C. (B) Detailed FTIR spectra obtained from Figure 3(A).
transformation into the PEL brush are shown. The angular positions of the waveguide modes allow us to determine the thickness and the refractive index of the polymer layer using Fresnel equations. After the quarternization reaction the angular positions of the resonance signals shift to higher values and an additional mode can be observed, indicating a large increase in thickness and/or change in the refractive index. From the fresnel fit curves the thickness of the neutral PVP monolayer was calculated to be 490 nm (refractive index: n ) 1.581) and after
quarternization the thickness of the MePVP was 870 nm (n ) 1.54). This increase in film thickness is caused by an increase of the molecular weight of the surface-attached polymer chains during the quarternization reaction. The molecular weight of the repeat units of the polymer increases from 105 g/mol (PVP) to 246 g/mol (MePVP). However, note that the degree of conversion of the polymer-analogous quarternization cannot be calculated in a straightforward manner from the increase in film thickness because both films were measured at ambient humidity (70% relative humidity at 22 °C). Although the film thickness of the unquarternized PVP layer is not very sensitive to the humidity of the environment, the MePVP layer is, in that particular case, in a rather highly swollen state. Therefore, the film thickness had to be remeasured in a moisturefree atmosphere (dried air). Under these conditions the film thickness was determined to be 741 nm. This large difference of the film thickness in dry and humid air is already a first indication that the polymer monolayer shows a strong response to the water content of the environment. In addition, the density of the polymer film increases during quarternization from 1.01 ( 0.02 (PVP) to 1.40 ( 0.07 (MePVP), mainly due to the incorporation of the iodide counterions. According to eq 1, the conversion of the quarternization reaction of the sample shown in Figure 4 was calculated to be f ) 92% ( 8%.
f)
LMePVP MPVP δMePVP LPVP MMePVP δPVP
(1)
where f is the degree of conversion of the reaction, LPVP and LMePVP are the dry thicknesses of the respective layers, MPVP and MMePVP are the molecular weights of the repeat units of the polymers, and δPVP and δMePVP are the densities of the polymers at 0% relative humidity. The results of the calculations show that almost quantitative conversion can be achieved and that during the quarternization reaction no significant amount of polymer is lost, and accordingly, no significant changes of the graft density (number of polymer molecules per surface area) occur. This is in good agreement with earlier results on the polymer-analogous reaction on a similar system.13
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Figure 5. Waveguide spectra (p-polarization) of a 740 nm thick (dry thickness) MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate. The spectra were measured at different constant relative humidities as shown in the figure. The solid lines are the calculated reflection curves obtained from fresnel equations.
To investigate the swelling behavior of thus-obtained PEL monolayers in contact with humid air, we performed additional OWS measurements in controlled humidity environments. To this purpose the OWS substrates were attached to a closed chamber in which a small vessel containing either solid KOH pellets (for determination of dry thicknesses) or saturated aqueous salt solutions was placed. The salt contents of the solutions reduces the vapor pressure of the water to a well-defined and highly reproducible value. For the determination of the thickness of the MePVP layer at a given relative humidity, the reflectivity of the sample was recorded as a function of the angle of incidence at constant temperature. The reflectivity curves are shown in Figure 5. Clearly, in all samples waveguide modes can be excited and with increasing humidity of the environment, the waveguide modes of higher order, which are located at lower angles of incidence, shift to higher angles. At high relative humidity even an additional waveguide mode can be observed, indicating a large increase of thickness. While the resonance signal of these modes shift to higher angles of incidence, the lower waveguide mode (around 75° incidence angle at “zero” humidity) shifts to lower angles of incidence. This indicates a large decrease in the refractive index due to incorporation of water into the layer. Reflectivity curves can be calculated using the Fresnel equations. A simple box model was used and the thicknesses of the swollen MePVP layers were calculated, as shown in Figure 5 (solid lines). The layer thicknesses as a function of the humidity of the environment are shown in Figure 6A. It is evident that with increasing relative humidity the thickness of the polymer layer increases greatly up to values of more than 150% of the dry thickness.
Figure 6. (A) (a) Thickness of a 740 nm thick MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate as a function of the relative humidity of the environment. (b) Refractive index of the same sample as a function of the relative humidity of the environment. (B) Relative increase of the thickness of the brush in Figure 6(A) as a function of the relative humidity of the environment. The lines are simulations according to a Flory-Huggins-type of sorption behavior. The different interaction parameters used for the calculations are given in the figure.
The occurrence of several waveguide modes in the reflectivity curves allows us to determine thickness and refractive index of the layer independently from each other.
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Because of the incorporation of water molecules (n ) 1.33) into the polymer layer (ndry ) 1.67) the refractive index of the layer decreases, as shown in Figure 6A. At 100% relative humidity a refractive index of n ) 1.54 is measured, which is in good agreement with approximately 50% water incorporation into the film. The results of the refractive index determinations were verified by measuring waveguide spectra using s-polarized light (not shown here). The results of the two sets of measurements were in good agreement. Note that with increasing water uptake of the film the width of the resonance signal of the waveguide mode becomes narrower and the minimum intensity of the modes decreases. This is especially clear if the modes located at 55 and 70° (reflectivity curves from the dry film) are compared at different humidity values. This signal narrowing and deepening of the mode indicates that with increasing water uptake the layer roughness decreases significantly. Preliminary atomic force microscopy (AFM) investigations support this interpretation of the reflectivity curves. The changes in film morphology are most likely directly related to changes in surface tension of the surface-attached monolayer during the drying process and changes of the chain mobility caused by the swelling/deswelling of the layer. Note that the spectra shown were recorded in random order and the sample had always been dried back to the starting thickness value in a “zero” humidity environment. Therefore, it can be concluded that the observed changes in layer roughness are not an artifact of the preparation of the individual sample, but are intrinsic to the swelling process. The observed swelling behavior of the MePVP brushes in humid air (Figure 6A) is a very typical example for a “Flory-Huggins”-type of sorption behavior in which a mobile permeant (water vapor molecules in this case) diffuses into a polymer matrix. Such swelling behavior is observed when the interactions between permeant molecules are strong compared to the interactions between permeant and polymer.23-25 If this is the case the concentration of absorbed molecules increases exponentially with increasing pressure of the permeating molecules and the sorption behavior can be described by the FloryHuggins relationship23,25
ln
p ) ln φ1 + (1 - φ1) + χ(1 - φ1)2 p0
(2)
Here φ1 is the volume fraction of small molecules in the polymer matrix, p0 is the saturation vapor pressure, p is the vapor pressure and χ is the Flory-Huggins interaction parameter. In principle, a further term should be added to eq 2, which accounts for the elastic pressure of the polymer due to the stretching of the chains in the brush. However, because the thickness of the monolayer increases “only” by a factor of 1.5 at the highest humidity studied, the change of the free energy of the system due to chain stretching is small compared to other contributions. If we take ∆Eel ∝ 3kT L/L0, where ∆Eel is the change of the elastic energy and L/L0 ≈ 1.5 (the thickness increase of (20) Luskin, L. S. In Functional Monomers; Yocum, R., Nyquist, E., Eds.; Marcel Dekker: New York, 1974. (21) Boyes, A. G.; Strauss, U. P. J. Polym. Sci. 1956, 22, 463. (22) Hummel, D. O. Atlas of Polymer and Plastics Analysis, 3rd ed.; VCH: Weinheim, New York, Basel, 1991. (23) Naylor, T. Permeation Properties. In Comprehensive Polymer Science; Pergamon Press: Elmsford, NY, 1989; Vol. 2, p 643. (24) Neogi, P. Transport Phenomena in Polymer Membranes. In Diffusion in Polymers; Neogi, P., Ed.; Marcel Dekker, New York, 1996. (25) Barrie, J. A.; Machin, D. Trans. Faraday Soc. 1971, 67, 244.
the layer), the energy contribution due to chain stretching is about ∆Eel ≈ 5kT per surface-attached polymer chain. This contribution to the free energy of the whole system is small compared to that caused by the changes in the chemical potential of the solvent molecules described by eq 2, because the number of solvent molecules in the swollen film exceeds the number of surface-attached polymer chains by several orders of magnitude. Hence we can neglect the effect of changes of the polymer conformation in the discussions here. However, in systems where the degree of swelling is significantly higher, such entropy changes will be of utmost importance. In Figure 6B the relative increase in thickness (L/L0 1) is shown as a function of the relative humidity (p/p0). The relative increase in thickness is directly proportional to the change in volume and, therefore, the relative increase in brush thickness can be directly taken as a measure of the volume fraction φ1 in eq 2. By fitting the experimental data using eq 2 it is possible to calculate the Flory-Huggins interaction parameter χ, which is directly correlated to the strength of interactions of the absorbed water vapor molecules in the PEL brush. The lines in Figure 6B represents calculated curves from p/p0 ) (L/L0 - 1) * exp[(L/L0) + χ(L/L0)2] using different interaction parameters as indicated in the figure. A comparison of the simulated curves with the measured data indicate that the interaction parameter for the swelling process of a MePVP brush in humid air is χ ) 0.85 ( 0.10. The high value of the interaction parameter in this case indicates that the interaction of the mobile phases (water vapor molecules) with each other is stronger compared to interactions with the polymer segments in the brush. At first consideration this behavior is unexpected because PELs are hydrophilic polymers. However, one has to consider that even at saturation (100% relative humidity) the water vapor pressure at 22 °C is p0 ) 26.4 mbar, and therefore, the total amount of water molecules in the air is less than 2.5%.26 Air is a bad solvent for the polymer studied here even if it contains a few percent of water molecules. The polymer will be in a good solvent environment only if the layer would be very hygroscopic and water would condense inside the film to a continuous water phase. This, however, is not the case in the system described here. In addition, the water content in the vapor swollen film is not sufficient to allow for dissociation of the macroions and the low molecular weight counterions. Note that the relatively high value obtained for the interaction parameter is consistent with the assumed model of a Flory-Huggins-type sorption process where the interaction between the mobile small permeant molecules is strong relative to the polymer/permeant interactions. An important aspect of any swelling experiment is whether the process eventually reaches an equilibrium state upon long-time exposure and whether the solvent uptake is reversible. The kinetics of the swelling process was measured by monitoring the shift of one minimum of one of the waveguide modes as a function of time. In the measurements shown in Figure 7A, the third mode of the waveguide spectrum, located at about 30° in the spectrum of the dry film, was chosen because it shows the strongest angular shift upon changes of the layer thickness. Note that for all experiments shown the sample was dried in a “moisture-free” atmosphere before the next swelling experiment was started with the same layer. It is observed that most of the water uptake of the film (70-80%) occurs (26) Weast, R. C. Ed.-in-Chief. Handbook of Chemistry and Physics, 64th ed.; CRC Press: Boca Raton, 1984.
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Figure 7. (A) Kinetic measurements of the swelling of a 740 nm thick (dry thickness) MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate. The swelling experiments were carried out using different relative humidities at constant temperature as noted in the figure. The baseline indicates the position of the third waveguide mode (Figure 5) at a relative humidity of 0% (thickness: 740 nm). (B) Relative shift of a waveguide mode as a function of the time of swelling of a 740 nm thick MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate. The data were obtained from Figure 7(A).
during the first minute. The initial rate of the increase of the film thickness is apparently only governed by the rate of the diffusion of water into the layer. Generally, the transport of small molecules into a polymer film can be described by simple diffusion equations:27
Lt ) ktm LM
(3)
Here Lt is the thickness of the film at time t, LM is the equilibrium thickness, and k is a constant. For fickian diffusion the exponent is m ) 0.5. The film thicknesses are related to the positions of the resonance minima of the waveguide modes in the spectra. In Figure 7B the shift of the minimum of the waveguide caused by the uptake of water molecules is shown as a function of the (27) Alfrey, T.; Gurnee, E. F.; Lloyd, W. G. J. Polym. Sci. C 1966, 12, 249.
exposure time in a double logarithmic plot. The slope of all curves (except the curve at 93% relative humidity) during the first few seconds is m ≈ 1.0, which indicates that the mobile permeant molecules diffuse much faster through the polymer layer, as would be expected for normal fickian diffusion. This can be explained because the incoming water vapor molecules plasticize the polymer chains at the diffusion front, thus allowing more molecules to enter the brush rapidly.28 A detailed phenomenological description of this type of diffusion behavior was given in a theoretical paper by Rossi, Pincus, and de Gennes.29 This rapid increase is followed by a much slower increase of the thickness until what appears to be the equilibrium state is reached after several hours. The measurement of the layer thickness was continued for several days, but no further increase could be detected. The second, much slower, process is most likely due to relaxation phenomena of the surface-attached chains, in which conformational changes of the polymer molecules allow the layer to absorb additional water molecules. However, if the equilibrium degree of swelling of the surface-attached monolayer is influenced by the conformation of the brush-forming polymer molecules, it is expected that the ratio between the thicknesses of the completely swollen and the dry layers is not a constant value, but a function of the graft density of the surfacebound chains. Brushes with a high graft density of the tethered chains prepared by the described method are already to some extent stretched in the dry state, where all polymer chains are completely collapsed. For example, the sample shown in Figure 7 has a film thickness of 741 nm and the distance between anchoring points of the chains is in this case less than 3 nm. Solvent uptake will lead to further stretching of the chains and accordingly higher loss of entropy. It is evident that the more the chains are stretched before the start of the swelling experiment, the lower the tendency will be to take up additional solvent and stretch even further. Indeed, in agreement with these considerations it was observed that PEL brushes with low graft density swell more strongly in a humid environment than the same brushes with a high graft density if both are exposed to the same humidity environments. To prove the reversibility of the swelling process, the MePVP brush was swollen and “dried” alternately in air having 90% and 43% relative humidity, as shown in Figure 8. Clearly, the swelling process is completely reversible and upon repetition of the swelling experiment the same thickness values are obtained. To explore the resolution limits of such a system that uses PEL brush layers for the measurement of the moisture contents of the surrounding medium, the MePVP layer was exposed to an environment having 78.0% relative humidity and subsequently “dried” in air having 77.1% relative humidity. The angular position of the resonance minimum was recorded as a function of the time of vapor exposure. To change the vessel containing the salt solution, the cell had to be opened and the film was briefly exposed to ambient humidity (approximately 55%). Accordingly, the resonance angle shifted strongly to lower values, but when the vessel with the second salt solution was added and the cell was sealed, the signal quickly recovered, as shown in Figure 9. From the observed shift of the minimum it can be concluded that a change in the relative humidity of even less than 1% can be easily detected. In this example (28) Thomas, N. L.; Windle, A. H. Polymer 1982, 23, 529. (29) Rossi, G.; Pincus, P. A.; deGennes, P. G. Europhys. Lett. 1995, 32(5), 391.
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Langmuir, Vol. 16, No. 4, 2000
Figure 8. Position of the resonance minimum of a waveguide mode of a 740 nm thick (dry thickness) MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate as a function of time at two different humidities.
the change in relative humidity caused a decrease of thickness of about 7 nm, which is clearly above the resolution of the OWS method.18 Conclusions As an example of a positively charged polymer monolayer, MePVP was prepared on the surfaces of silicon wafers and evaporated silicon oxide surfaces by “growing” the polymers from the substrate using immobilized azo initiators and polymer-analogous transformation. Monolayers with film thicknesses of several hundred nanometers were generated. The MePVP monolayers interact strongly with water vapor. From a large increase of the thickness as well as a large decrease of the refractive index of the layer upon water vapor exposure, it can be concluded that the polymer monolayer is very hydrophilic and that such a thick polyelectrolyte brush can absorb a very large amount of water vapor molecules. The changes in film thickness due to the swelling are so very large that they are already visible by eye through changes of the interference colors of the film. It is clearly evident from these measurements that it is indispensable to determine the precise humidity of the environment in which the film thickness determination of a hydrophilic polymer film is carried out. If swelling by water vapor is not taken into account, the mass of the surface-attached monolayer can be highly overestimated. The results of the studies present here show that the difference between the dry thickness and the thickness in
Biesalski and Ru¨ he
Figure 9. Thickness of a 740 nm thick (dry thickness) MePVP brush covalently attached to the surface of a LaSFN9/Au/SiOx substrate as a function of the relative humidity. The swollen layer thickness was 890 nm at 78% relative humidity and 883 nm at 77.1%. The solid lines are guides to the eye.
ambient humidity can easily lead to errors of about 40% of the measured thickness. The exact difference between dry and humidity-swollen film thicknesses will vary from case to case and depend greatly upon the chemical composition and the graft density of the polymer film. The strong response of the film to the presence of water as a typical low molecular weight compound shows the potential for such brushes to be used as elements of a (chemical) sensor. The high sensitivity, which is 2-3 orders of magnitude higher than that which would be expected for a self-assembled monolayer, is a direct consequence of the high molecular weight, the high graft density of the attached chains, and therefore the large number of surfaceattached polymer segments. By using such thick PEL brushes, changes in the relative humidity of even less than 1% can be detected without any difficulties. In further communications we will report on the swelling behavior of PEL brushes in aqueous solution. The degree of swelling as a function of the graft density and the molecular weight of the attached chains will be reported. Furthermore, we will describe the behavior of such monolayers as a function of the ionic strength (salt concentration) of the surrounding medium. Acknowledgment. The German Research Council (DFG, Schwerpunkt “Polyelektrolyte mit definierter Moleku¨larchitektur”) is thanked for financial support. D. Johannsmann is thanked for helpful discussions. LA990863+
