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Weak Polyacid Brushes: Preparation by LB Deposition and Optically Detected Titrations E. P. K. Currie,*,† A. B. Sieval,‡ M. Avena,† H. Zuilhof,‡ E. J. R. Sudho¨lter,‡ and M. A. Cohen Stuart† Department of Physical and Colloid Chemistry, University of Wageningen, 6703 HB Wageningen, The Netherlands, and Department of Organic Chemistry, University of Wageningen, 6703 HB Wageningen, The Netherlands Received June 2, 1999. In Final Form: August 12, 1999 We present a new technique for preparing and characterizing brushes consisting of weak acidic monomers. The brushes consist of polystyrene-polyacrylic acid block copolymers. The block copolymers are deposited from the air/water interface on a hydrophobically modified Si wafer via the Langmuir-Blodgett technique. The titration of such brushes at low ionic strength is measured with reflectometry. It is shown that the titration curve of the grafted PAA is shifted toward higher pH with increasing grafting density, as predicted by theoretical models for weak acidic brushes.
Introduction Polymers end-grafted at high densities onto surfaces or interfaces, so-called brushes, are interesting as an academic model system and are applied in various industrial applications as colloidal stabilizators or adsorption inhibitors.1 A special class of brushes are so-called annealed brushes. The intrinsic properties of the monomers in an annealed brush are not constant but depend on local conditions. A good example of annealed brushes is densely grafted weak acidic polymers. Several papers have examinedthepropertiesofsuchpolyacidbrushestheoretically.2-4 The fraction of charged monomers (R) in such brushes is predicted to depend not only on the pH but also on the salt concentration and grafting density. Experimental investigation, however, of annealed brushes is scant. Most experimental studies of charged brushes have focused on the properties of brushes with a constant R, so-called quenched brushes.5-8 A key issue in the theory of annealed brushes is the behavior of R as a function of the grafting density σ (defined as number of chains per unit area). At low ionic strength the local concentration of protons in the brush is significantly higher than that in the bulk phase and increases with increasing σ. The (average) value of R is thus lower than that in the bulk (which is solely determined by the pH) and is predicted to decrease proportional to σ-2/3 at a given pH.3,4 This adjustment of R to local conditions is predicted to result in various interesting properties of * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 00-31-317-482194. † Department of Physical and Colloid Chemistry. ‡ Department of Organic Chemistry. (1) Reviews on polymer brushes are as follows: (a) Milner, S. T. Science 1991, 251, 905. (b) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (c) Szleifer, I.; Carignano, M. A. Adv. Chem. Phys. 1996, 94, 165. (2) Israe¨ls, R.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087. (3) Lyatskaya, Y. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 3562. (4) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491. (5) Mir, Y.; Auroy, P.; Auvray, L. Phys. Rev. Lett. 1995, 75, 2863. (6) Guenoun, P.; Schlachli, A.; Sentenac, D.; Mays, J. W.; Benattar, J. J. Phys. Rev. Lett. 1995, 74, 3628. (7) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (8) Ahrens, H.; Fo¨rster, S.; Helm, C. A. Macromolecules 1997, 30, 8447.
annealed brushes at low ionic strength, such as a nonmonotonuous dependence of the brush height as a function of the ionic strength or σ.2-4 In this Letter we report our first results for annealed brushes consisting of polyacrylic acid chains. The brushes are prepared with the Langmuir-Blodgett (LB) technique. It is shown that the titration of such brushes can be followed with reflectometry. This entails that the behavior of the average value of R in the brush can be determined as a function of σ. Thus, the combination of the LB and optical techniques makes experimental investigation of the properties of annealed brushes possible. Materials and Method The refractive index of PAA solutions in the range from 0.006 to 0.125 g/mL was measured with an ABBE refractometer. The protonation-deprotonation of PAA was examined by potentiometric titrations using a fully automated titration system.9 The Si wafers coated with PS were prepared as described previously. In short, a HF-etched Si wafer is heated at 150 °C under reflux conditions in a 5% styrene solution in toluene for 2 h. This results in a chemically and thermally stable, covalently bonded styrene monolayer on the Si wafer, which ellipsometry showed to be approximately 2 nm thick.11 Subsequently, a PS layer of 80 nm was spin-coated on the wafer from a 15 g/L solution of PS (43K) in chloroform at 3000 min-1 and dried with N2. The result is a Si wafer hydrophobized with PS, the reflected intensity of which is insensitive to changes in the pH. The PS-PAA block copolymers were purchased from Polymer Source Inc., Quebec. The PS block consists of 34 monomers; the PAA block contains 368. The reported polydispersities of the blocks are 1.06 and 1.05, respectively. The block copolymers were dissolved in dioxane at 60 °C for 2 days, after which toluene was added to obtain a solution of 1 g/L, 60% dioxane and 40% toluene. After the solution was deposited on an air/water interface via a Hamilton micropipet and 30 min was allowed to pass, surface pressure isotherms were measured.12 These were found to be reversible up to a surface pressure of 45 mN m-1; i.e., no polymer (9) Kinniburgh, D. G.; Milne, C. J.; Venema, P. Soil Sci. Soc. Am. J. 1995, 59, 417. (10) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir, in press. (11) We remark that during the radical polymerization reaction a few oligimers may form in the styrene monolayer. As a PS layer of 80 nm is spin-coated on top of the monolayer, these oligomers are of no importance. (12) Currie, E. P. K.; Cohen Stuart, M. A., Fleer, G. J. In preparation.
10.1021/la990689p CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
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Figure 1. A schematic illustration of reflection at the wafer/ solvent interface. was lost during compression. Brewster angle microscopy showed the PS-PAA monolayer to be homogeneous. The PS-PAA block copolymers were transferred to the coated PS layers with the Schaeffer varaint of the familiar LB technique.13 The transfer ratio was close to 1. Following this the wafers were first dried in the air to evaporate the water layer on the LB layer, and subsequently heated at 150 °C for 10 min on a heating plate. During heating above the glass temperature of PS, the PS heads of the block copolymers diffuse into the PS layer.14 After cooling, the PS layer becomes glassy again, and the block copolymers are irreversibly fixed at the grafting density at which the LB film was prepared. Without the thermal annealing the PAA layer was unstable in solutions of high pH. Three wafers were prepared in this manner, with grafting densitites 18, 12.3, and 8 nm2 per molecule. The wafers with grafted PAA were brought in contact with several solutions of varying pH but constant ionic strength (10-3 M) in a reflectometer. These solutions were made by mixing a 10-3 M NaCl solution with 10-3 M HCl or NaOH; the ionic strength of such solutions is constant within 1%. A constant ionic strength is important, as the degree of dissociation in the brush is a function of the ionic strength, as well as the pH and grafting density.2-4 The technique of reflectometry has been discussed extensively.15 A polarized laser is reflected on a silicon surface with an adsorbed or grafted polymer layer, and both the intensity of the normal and parallel polarization directions with respect to the reflecting plane are measured, as shown in Figure 1. The phase difference resulting from the polymer layer is
Figure 2. The change in dn/dc of PAA in 10-3 M KNO3 with increasing R. where the subscript Γ denotes silicon with an adsorbed layer and 0 that of the bare silicon surface, and f is a sensitivity constant. From optical models (i.e., Abeles’ method16) Γ can be calculated for a given ratio Rp/Rs. It turns out that for most cases Rp/Rs is linear with Γ and insensitive to the exact distribution of material in the adsorbed layer.15 If we combine this with eqs 1 and 2, it is clear that upon adsorption Rp/Rs is proportional to φ for a given dn/dc. However, if Γ is constant and the dn/dc of the layer changes, it follows that a similar proportionality of Rp/Rs is retrieved. This result is used in the titration of annealed brushes. Under the assumption that the dn/dc is proportional with R,17 which is demonstrated in the following section, it follows that
R)
(
)
∆SR - ∆SR)0 ∆SR)1 - ∆SR)0
(4)
provided that Γ is constant during the measurement.
Results
(3)
We first demonstrate that the refractive index of a solution of weak acidic polymers is indeed proportional to the overall degree of dissociation R. In Figure 2 the dn/dc of a PAA solution is shown as a function of R for 10-3 M KNO3. The data points approximately fall onto a straight line. Several other polyacids were found to behave similary. This is as expected, since the refractive index is determined by the highly localized electronic properties of the constituting particles. Each carboxylate contributes a fixed amount to the overall refractive index, hence the linearity with R. Thus, the reflectometry signal ∆S of grafted PAA brushes is proportional to the (average) degree of dissociation of the brush. In Figure 3 the measured reflectivity signal ∆S is shown as a PAA-grafted wafer is brought into contact with several solutions with varying pH and constant ionic strength. It is clear that the change in signal is reversible: at the end of the cycle the pH changes from 7.2 to 3.0 and back, and no effective change in ∆S is found. This reversibility of ∆S is essential, as this indicates that the total grafted amount Γ remains constant during the titration. The titration curves of PAA, obtained by measurement of ∆S, are presented in Figure 4 for the three grafting densities. The curves drawn through the points are a guide
(13) Roberts, G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (14) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989. (15) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. J. Colloid Interface Sci. 1990, 51, 141.
(16) Born, M.; Wolf, E. Principles of Optics, 5th ed.; Pergamon Press: Oxford, 1975. (17) We remark that R is not a constant but varies as a function of the distance to the grafting plane.2,3 However, reflectometry is insensitive to this dissociation gradient, and an average value over the brush is measured. Therefore, with R the value averaged over the brush layer is implied.
φ)
4πdpnp cos θf λ0
(1)
where dp and np are the thickness and refractive index of the polymeric layer, respectively, and λ0 is the wavelength of the beam in a vacuum. If, for the sake of simplicity, we assume the layer to be homogeneous, then np can be written as
np ) ns +
Γ (dn dc ) d
(2)
p
where Γ is the total amount of polymer adsorbed or grafted on the silicon and dn/dc is the change in refractive index as a function of the polymer concentration. During reflectometry the output signal S measures the ratio of the parallel and perpendicular polarized reflectivities, Rp and Rs, respectively. During a (conventional) measurement of the adsorption of polymer or proteins, the change in signal is
∆S ) f
(( ) ( ) ) Rp Rs
-
Γ
Rp Rs
0
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librium the average value of R is given as3
Ka R ) 1 - R FH+ exp(-Ψe/kBT)
Figure 3. Reflection signal ∆S from PAA, 18.3 nm2 per molecule, for various pH values (indicated in the figure) and 10-3 M ionic strength.
Figure 4. Titration curves of PAA brushes at three grafting densities and 10-3 M ionic strength. Curves are drawn as a guide to the eye.
to the eye. It is seen that the titration curves effectively shift to higher pH values with increasing σ, as the theory of annealed brushes predicts.2,3 The shift in effective pKa value is approximately 1.5 units as σ increases from 18 to 8 nm-2. We remark that the reflectivities at pH values lower than 3 could not be measured, due to the necessity of constant ionic strength of 10-3 M. This is the reason that R increases stronger than expected at low pH, as the assumption is made that R ) 0 for pH values below 3. We can use the titration isotherms in Figure 4 to estimate the difference in electrostatic potential in the brush at two grafting densities. Using a Donnan equi-
(5)
where FH+ is the proton concentration in the bulk, Ka the dissociation constant of acrylic acid in dilute solutions, and Ψ the average electrostatic potential in the layer.18 If the pH values for R ) 0.5 are inserted, it follows that as σ increases from 18 to 8 nm-2, Ψ increases by approximately 90 mV. This is a significant increase, which indicates that the close grafting of polyelectrolyte chains greatly enhances the electrostatic potential in the layer. From the form of the titration isotherms of 8 and 18 nm2, it follows that this difference in Ψ increases with increasing R. In Figure 3 it can be seen that the equilibrium ∆S is obtained quite rapidly at low grafting densities. At 8 nm-2, however, a significant longer period was necessary for the equilibrium ∆S to be found (not shown). This may be due to the high electrostatic potential that is found at high pH values and high grafting densities. In following work we will examine this electrostatic potential in more detail. Conclusions We have prepared annealed brushes consisting of PAA polymers end-grafted to hydrophobized Si wafers and have measured the titration isotherms of these PAA brushes at low ionic strength with reflectometry. It is found that the grafting has a pronounced effect on the titration isotherms of the brushes. At the highest grafting density the apparent pKa of the PAA chains shifts approximately 1.5 units with respect to that of the lowest grafting density. This shift in apparent pKa indicates that the (average) electrostatic potential is approximately 90 mV higher in densely grafted layers than in sparsely grafted layers. This shift in apparent pKa agrees qualitatively with theoretical models for annealed brushes, which predict the (overall) degree of dissociation R to decrease strongly with increasing grafting density σ at constant pH and ionic strength. We note that scaling models for annealed brushes predict R to decrease as σ-2/3, whereas our measurements show a linear relationship, i.e., R ∼ σ-1. The number of densities measured is limited to three, however, and more measurements are necessary to ascertain the power exponent accurately. LA990689P (18) Actually a potential gradient Ψ(z) is present in the brush, which is coupled to the dissociation gradient R(z). As is the case for R, an average value for Ψ is estimated.
