5980
Langmuir 2000, 16, 5980-5986
pH-Controlled Adsorption of Polyelectrolyte Diblock Copolymers at the Solid/Liquid Interface D. A. Styrkas,†,‡ V. Bu¨tu¨n,§,| J. R. Lu,† J. L. Keddie,*,† and S. P. Armes§ School of Physics and Chemistry, University of Surrey, Guildford, GU2 7XH, UK, and School of Chemistry, Physics, and Environmental Science, University of Sussex, Falmer, Brighton, BN1 9QJ, UK Received December 8, 1999. In Final Form: April 13, 2000 We investigated the pH dependence of the adsorption of polyelectrolyte diblock copolymers from aqueous solution onto the native oxide surface of silicon using spectroscopic ellipsometry. The observed adsorption behavior is closely related to the chemical structure and the hydrophilic-hydrophobic balance of the copolymers. These amphiphilic copolymers contain hydrophobic residues comprising either 2-(diethylamino)ethyl methacrylate (DEA) or 2-(diisopropylamino)ethyl methacrylate (DPA). The copolymers also contain hydrophilic residues that are either (1) methyl-quaternized or benzyl-quaternized 2-(dimethylamino)ethyl methacrylate (designated Me-DMA or Bz-DMA, respectively) or (2) sulfopropyl betainized 2-(dimethylamino)ethyl methacrylate (Bet-DMA). The DEA and DPA residues can be tuned to become hydrophilic by adjusting the solution pH. Thus, these diblock copolymers can be molecularly dissolved in acidic media without using cosolvents as a result of the protonation of the tertiary amine groups. At low solution pH, adsorption of the copolymers is only about 0.5-1.5 mg/m2, which is expected for polyelectrolyte adsorption. Above a pH of 7, there is a pronounced increase in the adsorbed amount. This change in adsorption coincides with the formation of copolymer micelles in the bulk solution. Hence, it is likely that the interfacial layer consists of adsorbed micelles. In the Bz-DMA-b-DEA copolymer, only a small fraction of DEA blocks (22%) is needed to achieve a relatively large increase in the adsorption at higher pH. On the other hand, control experiments confirm that the corresponding homopolymers show no sharp change in the extent of adsorption with pH. Changing the hydrophobic residues from DEA to DPA does not significantly affect the extent of adsorption. However, substitution of the hydrophilic Bz-DMA residues with Bet-DMA significantly increases the extent of adsorption at higher pH. This is probably because the electrically neutral betainized block is less hydrophilic than the cationic Bz-DMA block. This work provides insight into the major influences on the block copolymer adsorption and thus creates a framework for tuning adsorption behavior.
Introduction The adsorption of polyelectrolytes at solid/liquid interfaces is of considerable interest because of their widespread use in many technological processes and formulations, for example, in cosmetics, paint, paper, and water purification.1 The possibility of varying the amphiphilic character of polyelectrolytes with block architectures opens up a range of adsorption behaviors that depend on the relative contributions of the hydrophobic and hydrophilic blocks. The presence of amphiphilic residues within the block copolymer structure, which can be tuned to become either hydrophilic or hydrophobic by adjusting the solution pH, is expected to result in new adsorption behavior. Recent advances in synthetic methodology now provide access to such block copolymers.2-5 Once the adsorption behavior of these copolymers is fully charac* To whom correspondence should be addressed. † University of Surrey. ‡ Current address: Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, England. § University of Sussex. | Permanent address: Osmangazi University, Faculty of Arts and Science, Department of Chemistry, TR 26020, Eskisehir, Turkey. (1) Dautzenberg, H.; Jaeger W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina, D. Polyelectrolytes; Hanser Publishers: Munich, 1994. (2) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Frank, F.; Arnold, M. Macromol. Chem. Phys. 1996, 197, 2553. (3) Creutz, S.; Teyssie, P.; Jerome, R. Macromolecules 1997, 30, 6. (4) Baines, F. L.; Armes, S. P.; Billingham, N. C. Macromolecules 1996, 29, 3416. (5) Bu¨tu¨n, V.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1997, 671.
terized and understood, control can be achieved. Ultimately, it is expected that the amount of adsorption will be “tuned” by the judicious combined adjustment of the solution pH and the copolymer architecture. It is also relevant to note here that the relative dearth of experimental investigations of the structure of polyelectrolyte brushes, in particular, has been pointed out recently6 as a major shortcoming in the literature. Herein, we report the adsorption behavior of a novel series of near-monodisperse tertiary amine methacrylatebased water-soluble diblock copolymers onto a planar silica surface. In each case, one of the blocks is permanently hydrophilic and the other block is amphiphilic, that is, it can be made hydrophilic or hydrophobic depending on the pH of the aqueous solution. Such copolymers can be dissolved molecularly in acidic media without cosolvents. The adsorption behavior of these diblock copolymers was studied as a function of the solution pH using the corresponding homopolymers as reference materials. We use ellipsometry to determine the amount of polymer adsorbed at the silica/water interface. Ellipsometry has been used recently by Walter et al.7 in a study of the adsorption of zwitterionic block copolymers at the silica/water interface as a function of solution pH. Such copolymers become water-insoluble at their isoelectric point (iep). Thus, adjusting the solution pH to the iep led to substantial adsorption (up to 10 mg m-2) due to macroscopic precipitation of the neutral zwitterionic (6) Jones, R. A. L.; Richards, R. W. Polymers at Surfaces and Interfaces; Cambridge University Press: Cambridge, 1999; p 261. (7) Walter, H.; Harrats, C.; Muller-Buschbaum, P.; Jerome, R., Stamm, M. Langmuir 1999, 15, 1260.
10.1021/la991605f CCC: $19.00 © 2000 American Chemical Society Published on Web 06/13/2000
Adsorption of Polyelectrolyte Diblock Copolymers
Langmuir, Vol. 16, No. 14, 2000 5981
Figure 1. Chemical structures of some of the homopolymers and diblock copolymers: (a) DEA homopolymer; (b) Me-DMA homopolymer; (c) Bz-DMA homopolymer; (d) Bet-DMA-b-DEA diblock copolymer, (e) Bz-DMA-b-DEA diblock copolymer; and (f) Me-DMA-b-DPA diblock copolymer. Table 1. Average Molecular Weights, Refractive Indices, and Compositions of the Homopolymers and Diblock Copolymers namea
DMA content (mol %)
Mn,b Da
estimated19,20 refractive index
Ave. micelle diamc at pH ) 12 (nm)
DEA (Figure 1a) DPA Me-DMA (Figure 1b) Bz-DMA (Figure 1c) Bet-DMA-b-DEA (Figure 1d) Bz-DMA-b-DEA (Figure 1e) Bz-DMA-b-DEA Bz-DMA-b-DPA Me-DMA-b-DPA (Figure 1f)
homopolymer homopolymer homopolymer homopolymer 50% 50% 78% 80% 80%
5.6 k 4.8 k 9.0 k 8.6 k 25.0 k 20.5 k 19.9 k 18.4 k 19.5 k
1.55 1.57 1.39 1.54 1.51 1.54 1.54 1.55 1.42
no micelles no micelles no micelles no micelles 29 15 11 15 14
a Abbreviations are as follows: DMA ) 2-(dimethylamino)ethyl methacrylate; DEA ) 2-(diethylamino)ethyl methacrylate; DPA ) di(isopropylamino)ethyl methacrylate; Me ) methyl quaternized; Bz ) benzyl quaternized; Bet ) sulfopropyl betainized. b Determined with GPC as described in text. c Determined with PCS as described in text.
chains. In contrast, the diblock copolymers described in the present study always possess a net positive charge. The adsorption of copolymers with permanent hydrophilic blocks and pH-“tuneable” amphiphilic blocks at the solid/ liquid interface has never been explored. Here, we systematically determine how their adsorption is influenced by the hydrophilic block length, the nature of the amphiphilic block, and the nature of the permanent hydrophilic block. Experimental Details Materials. The polymers are methacrylate-based amphiphilic homopolymers and diblock copolymers. The hydrophobic residues comprise either 2-(diethylamino)ethyl methacrylate (DEA) or 2-(diisopropylamino)ethyl methacrylate (DPA). The permanent hydrophilic residues are either (1) methyl-quaternized or benzylquaternized 2-(dimethylamino)ethyl methacrylate (Me-DMA or Bz-DMA, respectively) or (2) sulfopropyl betainized 2-(dimethylamino)ethyl methacrylate (Bet-DMA).
The homopolymers and diblock copolymers were synthesized via group transfer polymerization, using sequential monomer addition where appropriate. Quaternization of the DMA residues in the diblock copolymer is fully selective under mild conditions. The synthetic details for the precursor DMA-DEA diblock copolymer,5 the betainized copolymers,8 and the quaternized copolymers9 have been reported elsewhere. Figure 1 illustrates the chemical structures of some of the polymers. Table 1 lists the chemical structures, molecular weights (Mn), and estimated refractive indices (n) of all the homopolymers and block copolymers studied here. Mn and polydispersity indices (Mw/Mn) of the homopolymers and the precursor polymers were measured using gel permeation chromatography with an eluent of HPLC-grade THF stabilized with BHT at a flow rate of 1 mL min-1. Calibration was carried out with poly(methyl methacrylate) standards (Polymer Laboratories). Mn values of the copolymers were then calculated assuming 100% betainization or (8) Bu¨tu¨n, V.; Bennett, C. E.; Vamvakaki, M.; Lowe, A. B.; Billingham, N. C.; Armes, S. P. J. Mater. Chem 1997, 7, 1693. (9) Bu¨tu¨n, V. Ph.D. Thesis, University of Sussex, Falmer, UK, 1999.
5982
Langmuir, Vol. 16, No. 14, 2000
quaternization, as appropriate. Note that the molecular weights of all of the diblock copolymers are about 2 × 104 D, which allows easy comparison of their adsorption. All copolymers have narrow molecular weight distributions, with polydispersity indices (Mw/Mn) ranging from 1.10 to 1.16. Elsewhere,5 it was reported that the DMA-DEA diblock copolymer (78 mol % DMA; Mn ) 12,400 D) is present as unimers at pH ) 2, but at pH ) 9.5 it is present as micelles with an average size of ca. 20 nm. Subsequently, 1H NMR spectroscopy confirmed that the betainized DMA-b-DEA and DMA-b-DPA copolymers likewise form micelles, with the Bet-DMA block forming the micelle’s corona and the DEA or DPA block forming the hydrophobic core.8 Likewise for the benzyl-quaternized DMADEA diblock copolymer, 1H NMR spectroscopy indicates that micelles are present at a pH of 12, at which the Bz-DMA blocks constitute the corona.9 In all of the block copolymers studied here, the micelles at higher pH dissolve into unimers when the pH is lowered, that is, micellization is fully reversible. Photon correlation spectroscopy (PCS, using a Malvern PCS 4700 spectrometer) was performed on dilute (1 wt %) solutions of the homopolymers and copolymers at varying pH values. There is no evidence from PCS for micelle formation in any of the homopolymers, but there is evidence for micelle formation for each of the diblock homopolymers at higher pH values. Table 1 shows the average micelle diameter measured after titration to a pH of 12 at a temperature of 20 °C. In the case of small-molecule surfactants, plots of the surface tension of the air/water interface as a function of concentration are routinely used to determine the critical micelle concentration (cmc) by detecting the change in activity coefficient associated with micellization. A similar approach has been used with block copolymers that form micelles,10,11 but there are limitations to the method due to errors in surface tension measurement. Measurement of the surface tension of solutions of a DMA-bDEA precursor copolymer showed that a limiting value of the surface tension is obtained at a concentration of 0.02 wt %, which is expected to correlate with the cmc. After sulfopropyl betainisation, the cmc was estimated by the same method to be 0.04 wt %.9 The substrates for adsorption were single crystal silicon wafers with (111) orientation. The surface of the silicon contained a native oxide layer, typically 2- to 4-nm thick. Prior to the adsorption experiments, the wafers were cleaned with a surfactant solution (DECON-90), followed by rinsing in water and immersion in a H2SO4/H2O2 solution (7:1 volume ratio) for 10 min at 90 °C. Finally, the wafers were rinsed copiously with deionized water. Techniques. Ellipsometry is a powerful optical analytical technique that has been used in the analysis of copolymer thin films and interfaces for over 30 years.12 The principles of the technique have been discussed in detail elsewhere.13 Ellipsometry measurements determine the change of the polarization of incident light upon reflecting from an interface. These changes (expressed in terms of two ellipsometric angles, Ψ and ∆) are dependent on the thickness of any surface layer and its optical constants. Ellipsometry has the advantage of being a laboratorybased technique that is noninvasive and thatsbecause it is relatively fastsenables adsorption processes to be studied as a function of time. Ellipsometry measurements were performed in a liquid cell with permanent glass windows that facilitates measurements with the incident light beam at 75° with respect to the sample normal. We used a variable angle spectroscopic ellipsometer (J. A. Woollam Co. Inc., USA) with a rotating analyzer configuration. In principle, from the measurement of the ellipsometric angles, Ψ and ∆, in the 350 to 700 nm range for the incident light, both (10) Su, T. J.; Styrkas, D. A.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 6892. (11) An, S. W.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P.; Penfold, J. J. Phys. Chem. B 1998, 102, 5120. (12) Smith, T. J. Opt. Soc. Am. 1968, 58, 1069. (13) Styrkas, D. A.; Doran, S. J.; Gilchrist, V.; Keddie, J. L.; Lu, J. R.; Murphy, E.; Sackin, R.; Su, T.; Tzitzinou, A. Application of ellipsometry to polymers at interfaces and thin films. In Polymer Surfaces and Interfaces III; ed. Richards, R. W., Peace, S. K., Eds.; John Wiley & Sons: Chichester, 1999; p 1.
Styrkas et al.
Figure 2. Schematic diagram of the experimental cell used in this study. The polarized light enters the cell through thin windows at an angle of 75° from the sample normal. the thickness and the refractive index of an adsorbed layer can be determined with high accuracy. Figure 2 shows schematically the experimental cell in which copolymer solutions are placed in contact with the silicon substrate. Ellipsometry analysis requires fitting a model to the experimental data. Our data modeling assumes a uniform adsorbed copolymer layer. Optical parameters for the silicon and native oxide layer are known from other studies.14 Window effects were taken into account by measuring ellipsometry spectra for pure deionized water in the cell prior to adsorption experiments and by adjusting the ∆ offset in the fitting procedure. Spectroscopic measurements allow modeling of the data over a wide range of wavelengths, which increases the reliability of the chosen model. For thin adsorbed layers, there is a strong correlation between the modeled values of thickness (d) and index (n), and, therefore, no unique solution can be obtained15 unless a complementary technique is used.16 However, the optical thickness dn (the product of two parameters) of the adsorbed layer is invariant, and it is from this value that we can reliably determine the adsorbed amount (Γ) of the copolymer:17
Γ)
d(n - n0) dn/dc
(1)
where dn/dc indicates the change in the refractive index of the copolymer solution with increasing concentration and n0 is the index of the neat solvent (e.g., aqueous salt solution). Refractive indices of 1 wt % aqueous solutions of the Bz-DMA-DEA copolymer at different pH values were measured with a refractometer (Abbe 60/ED, Bellingham Ltd) at three wavelengths (405, 480, and 644 nm). The copolymer was first dissolved in HCl solution (pH ) 2) and then titrated with KOH stepwise to a pH of 12. The indices of the solution were found to be independent of pH within the resolution of the measurement (0.001). These measured values were used for modeling the ellipsometry data. The dn/dc was measured directly with the refractometer for BzDMA-b-DEA and was found to be 0.17 ( 0.02 cm3/g at three different pH values (1.5, 7, and 12). These measurements were performed by diluting concentrated solutions while keeping the pH constant by the addition of HCl or KOH solutions, as appropriate. We note that recently the dn/dc of poly(acrylic acid), which is a weak polyelectrolyte, has been shown to depend on the degree of disassociation and hence the pH.18 In our case, however, this pH dependence is not observed. For a reliable fitting procedure, it is desirable to have as few variables in the model as possible. One way to reduce the number of variable parameters is to use a fixed refractive index for the adsorbed layer, rather than determining its value via fitting to the data. The refractive index of the dense copolymer can be estimated using the molecular refractivities method, which uses the known refractive indices of the fragments of the molecule and applies the Lorenz-Lorentz equation.19,20 Table 1 lists (14) Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: London, 1991. (15) Barradas, N. P.; Keddie, J. L.; Sackin, R. Phys. Rev. E 1999, 59, 6138. (16) Styrkas, D. A.; Keddie, J. L.; Lu, J. R.; Su, T. J.; Zhdan, P. A. J. Appl. Phys. 1999, 85, 868. (17) Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (18) Currie, E. P. K.; Sieval, A. B.; Avena, M.; Zuilhof, H.; Sudholter, E. J. R.; Cohen Stuart, M. A. Langmuir 1999, 15, 7116. (19) Beevers, R. B. Lab. Pract. 1973, 4, 272.
Adsorption of Polyelectrolyte Diblock Copolymers
Langmuir, Vol. 16, No. 14, 2000 5983
Figure 4. Adsorption as a function of pH for various homopolymers: DEA (2); DPA (×); Me-DMA (O); and Bz-DMA (9). Vertical lines indicate the pKa values of the DEA (dashed line) and DPA (solid line) homopolymers as determined by titration. Typical uncertainty on Γ is (0.5 mg/m2 for this and subsequent figures.
Figure 3. Experimental ellipsometry data for the Bz-DMAb-DEA diblock copolymer adsorbed from solution onto an oxidized silicon substrate: b pH 2.7 and 9 pH 9.2. The solid line shows the best fit corresponding to adsorbed layers of thicknesses 1.2 and 3.9 nm, respectively, and n ) 1.54. The standard deviation of the Ψ and ∆ values is smaller than the corresponding size of the data points. estimated values of the refractive index for each polymer and copolymer. The weak dependence of n on wavelength was assumed to be negligible in our data analysis. With the index values fixed, the adsorbed layer thickness was thus the only fitting parameter in our data analysis. Adsorption Measurements. We used 0.10% w/w copolymer aqueous solutions in all experiments, so that the only variables in the experiments were pH and polymer composition. At a concentration of 0.10 w/w, all copolymers show surface activity at higher pH values. We separately measured the adsorbed amount of the DMA-DEA copolymer at concentrations of 0.05 w/w and 0.25 w/w and found it to be the same as at 0.1 w/w. We infer from these experiments and the cmc measurements that the concentrations used in all experiments is such that a plateau value of surface excess is obtained in an adsorption isotherm. The pH of the solutions was adjusted by titration of an initial solution with a pH of approximately 2.3 (prepared by adding a few drops of 2 M HCl to the aqueous copolymer solution) with concentrated KOH. Ellipsometry data were acquired approximately 5 min after each adjustment to the pH. The adsorbed amounts did not change over time. At least three separate trials were performed for each copolymer over a range of pH. The reproducibility of the trials is about ( 0.5 mg/m2. We report Γ values averaged over three trials.
Results and Discussion Figure 3 shows typical ellipsometry spectra for a silicon substrate immersed in a diblock copolymer solution at both low and high pH. The best fit to the uniform singlelayer model is also shown. There is good agreement between the experimental data and the model. There are clear differences between the spectra at these two solution pH values. The uncertainty in Ψ is typically (0.01°, and in ∆ it is (0.2°. The observed differences between the two spectra are much larger than this uncertainty. (20) Born, M. Principles of Optics: Electromagnetic Theory of Propagation, Interference, 3rd rev. ed.; Pergamon Press: London, 1965.
We have checked the time dependence of adsorption by repeating the spectroscopic scans every five minutes for up to 2 h. The first scan was performed approximately three minutes after the solution was poured into the sample cell. After the initial adsorption, there was no change in the ellipsometry spectra over time, which indicates that adsorption was complete within the first three minutes. This stability of the adsorbed layer over time was confirmed at both high and low pH. We begin by considering the adsorption of homopolymers in order to understand how the solution pH affects the degree of protonation and thus the charge density of the polymer. Figure 4 shows the adsorbed amount, Γ, as a function of pH for various homopolymers. The typical experimental error on these and subsequent measurements is (0.5 mg/m2. Vertical lines in Figure 4 indicate the pKa values for the two homopolymers. At low pH, the homopolymers are fully protonated and therefore soluble in aqueous solution as weak cationic polyelectrolytes. Adsorption at low pH is therefore small and corresponds to classical polyelectrolyte adsorption,21 in which cationic chains adsorb onto an anionic surface as a fairly flat molecular monolayer. Γ increases progressively with increasing pH, as the positive charge (i.e., the degree of protonation) on the polymer chains decreases, and they thus become more hydrophobic. Above their pKa values, the DEA and DPA homopolymers become water-insoluble, because they have insufficient charge density to remain hydrophilic. Polymer chains precipitate onto the surface but also precipitate in bulk solution, making it opaque and preventing ellipsometry investigation. The low but measurable extent of adsorption (
