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Langmuir Isotherms of Quenched and Annealed Polyelectrolyte Brushes C. Prinz, P. Muller, and M. Maaloum* Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg Cedex, France Received December 21, 1999. In Final Form: April 7, 2000 We report here Langmuir isotherms of diblock copolymers containing a short neutral hydrophobic sequence (polystyrene) and a long water-soluble polyelectrolyte sequence (polyvinyl-2-pyridine). This study focuses on the polyelectrolyte sequence, which is a weak polyelectrolyte when unmodified and strong polyelectrolyte when quaternized. In pure water, the surface pressure of the quenched brush is proportional to the osmotic pressure of the counterions. In water at pH 2, the annealed brush isotherms are those of a polyelectrolyte brush in the strong screening limit. In water at pH 3, a plateau appears in the annealed layer isotherms, revealing a transition from a state where the polyelectrolyte sequence is adsorbed at the interface to a strong polyelectrolyte brush state. It is not clear whether this transition is a first-order transition or a continuous compression-induced solubility of the PVP chains.
Introduction The physical properties of neutral polymer brushes have been the subject of many experimental and theoretical investigations recently,1-10 and the behaviors of these systems is now well understood. For instance, at the surface of colloidal particles in an organic solvent, they can prevent the suspension from flocculating. The use of polyelectrolyte allows us to extend the brush application domain to the case of polar solvents such as water. There exist two types of polyelectrolytes: strong polyelectrolytes (often called quenched polyelectrolytes in theoretical papers) and weak polyelectrolytes (annealed polyelectrolytes). The quenched polyelectrolytes have their charges fixed along the chain, independently of the external conditions. This is the case of quaternized poly(vinylpyridine) and polystyrene sulfonate, for example. For the annealed polyelectrolytes, the distribution and overall fraction of charges vary according to the pH of the solution and the polymer concentration. For quenched polyelectrolyte brushes, it has been shown11-13 that these systems are insensitive to salt up to a certain concentration. This could be of great interest for the stability of colloidal suspensions in aqueous media where the salt concentra* Author for correspondence. Tel: (33).(0).3.88.41.40.02. Fax: (33).(0).3.88.41.40.99. E-mail:
[email protected]. (1) de Gennes, P. G. Macromolecules 1980, 13, 1069. (2) Milner, S. T.; Witten, T. A.; Cates, M. E. Europhys. Lett. 1988, 5, 413. (3) Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610. (4) Skvortsov, A. M.; Pavlushkov, I. V.; Gorbunov, A. A.; Zhulina, E. B.; Borisov, O. V.; Priamitsyn, V. A. Polym. Sci. U.S.S.R. 1988, 30, 1706. (5) Wijmans, C. M.; Scheutjens, J. M. H. M.; Zhulina, E. B. Macromolecules 1992, 25, 2657. (6) Hadziioannou, G.; Patel, S.; Granick, S.; Tirrell, M. J. Am. Chem. Soc. 1986, 108, 2869. (7) Auroy, P.; Auvray, L.; Le´ger, L. Phys. Rev. Lett. 1991, 66, 719. (8) Cosgrove, T.; Heath, T. G.; Phipps, J. S.; Richardson, R. B. Macromolecules 1991, 24, 94. (9) Field, J. B.; Toprakcioglu, C.; Ball, R. C.; Stanley, H. B.; Dai, L.; Barford, W.; Penfold, J.; Smith, G.; Hamilton, W. Macromolecules 1992, 25, 434. (10) Mir, Y. Thesis, Universite´ Paris VII, 1995. (11) Pincus, P. Macromolecules 1991, 24, 2912. (12) Guenoun, P.; Schlachli, A.; Sentenac, D.; Benattar, J. J. Phys. Rev. Lett. 1995, 74, 3628. (13) Guenoun, P.; Muller, F.; Delsanti, M.; Auvray, L.; Chen, Y. J.; Mays, J. W.; Tirrell, M. Phys. Rev. Lett. 1998, 81, 3872.
tion is uncontrolled. Moreover, the annealed polyelectrolytes may be neutral at a given pH and charged at a different pH value. Considering the fact that these polymers usually have an organic backbone, they are a bad solvent in water when neutral. Their ambivalent properties can stabilize or flocculate the suspension depending on the pH of the solution. For instance, let us consider a weak polybase. When grafted at the surface of colloids, the suspension is stable in acidic solutions, where the polyelectrolyte is charged. If a base is added to the solution, the polymer becomes neutral and the suspension flocculates. This could be of great interest for water treatment, where the particles have to be small enough for the transport in water in the canalization and have to aggregate before filtration of the water. For annealed polyelectrolyte brushes in the osmotic regime (i.e., when the osmotic pressure of the counterions is dominant), Zhulina et al. predicted that the charge fraction decreases when the grafting density is increased.14 We report here Langmuir isotherms of diblock copolymers containing one short sequence of polystyrene (PS) and one long sequence of polyvinyl-2-pyridine (PVP). In the present study, we have examined the influence of PVP on the Langmuir isotherms. The PVP behaves as a basic polyelectrolyte in acid solutions and is neutral and insoluble at pH higher than pH 4. The isotherms are studied as a function of the pH, the ionic strength, and the molecular weight. The resulting Langmuir isotherms are compared with those where the PVP is quaternized. Materials and Method Samples. The brushes were made from a diblock copolymer composed of a short sequence of polystyrene (PS) and a long sequence of polyvinyl-2-pyridine (PVP). The copolymers were provided by l’Ecole de Chimie des Polyme`res et Mate´riaux (ECPM, Strasbourg). They were synthesized by anionic polymerization. The nature and masses of the copolymers studied are reported in Table 1. The chemical formulas of the copolymers in the neutral and charged states are shown in Figure 1. The PVP is only soluble in acidic aqueous solutions, where it acts as a weak base and acquires a net linear charge density owing to protonation of the nitrogen in the pyridine rings. It is well-known that for annealed polyelectrolytes, as a result of the buildup of an electrostatic (14) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491.
10.1021/la9916631 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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Table 1. Characteristics of the Polymer Chains Used name
Mw (PS) (g/mol)
M (PS)
Mw (PVP) (g/mol)
N (PVP)
Mw (PS + PVP) (g/mol)
PS177-PVP1581 PS177-PVP863 PS179-PVP412 quaternized PS177-PVP1581
18 400 18 400 18 600 18 400
177 177 179 177
166 000 90 600 43 300 246 090
1581 863 412 1581
184 400 109 000 61 900 264 490
Figure 1. Chemical formula of (a) PS-PVP diblock copolymers; (b) charged PVP monomers; (c) quaternized PVP monomers. potential, the pKa is not constant but varies with both the degree of polymer protonation R and the electrolyte concentration.15 A. Elaissari et al. have estimated the degree of PVP protonation between pH 1 and 4.16 The solubility of a single PVP chain in water is limited to pH below 4. At pH 2 it is almost fully protonated (R ) 90%). At pH 3, the PVP charge fraction is 50%. Meanwhile, if we take into account the Manning condensation,17 only the ions, which are separated at a distance above the Bjerrum length, are dissociated. This gives an effective degree of ionization RM for our polymer equal to RM ) a/lB ) 0.36, where a is the monomer size (2.5 Å) and lB is the Bjerrum length (7 Å in water). We cannot determine which value should be considered as the real charge fraction for a single chain. Moreover, for weak polyelectrolytes, R varies with the polymer concentration. Thus, the charge fraction in the brushes probably never attains the above given values. The quaternized PVP is charged independently of the external conditions. Its charge degree R has been estimated by elementary analysis of iode and nitrogen and was found to be equal to 0.35. Langmuir Isotherms. The polyelectrolyte brushes are obtained by spreading the copolymer in a Langmuir trough (commercialized by Riegler & Kirstein), using water (MilliQ from Millipore) as the subphase. The spreading solvent is chloroform (Merck) for the annealed polyelectrolyte and is composed of 83% and 17% by volume of chloroform and methanol (Merck), respectively, for the quenched polyelectrolyte. The methanol is used to dissolve the charged monomers of quaternized PVP that are not soluble in chloroform. It is assumed that the methanol is not solved in the subphase but evaporate as chloroform does. Anyway, the quantity of methanol added in the trough is negligible compared to the subphase volume (10 × 10-6 L of methanol for 2 × 10-1 L of water). Before the water is poured into the Langmuir trough, the pH is measured with a pH meter (Tacussel electronique). If necessary, the pH is adjusted with hydrochloric acid (prolabo). The salt concentration is changed by adding a known mass of sodium chloride (prolabo) to a given volume of water. Once the monolayer is deposed at the water surface, we wait 5 min and 45 min for the annealed and quenched brush, respectively, before compressing the layer. This step is necessary in order to let the spreading solvent evaporate. The compression speed is of 15 Å2 s-1. The surface pressure is measured with a Wilhelmy plate.18 The surface pressure measurement precision is of 0.1 mN/m. All isotherms are reversible except when mentioned. The main experimental errors associated with these measurements arise from the adjustement of the pH in the subphase. Indeed, the PS-PVP surface pressure is very sensitive to small pH variations, which cannot be measured by the pH meter. Thus, when the pH changes a little from one experiment to another, both performed on the same copolymer, the isotherms are not perfectly superposed. This point will be discussed later in the Results section. (15) Ouahes, R.; De´vallez, B. Chimie Ge´ ne´ rale; Editions PubliSud, 1982. (16) Elaissari, A.; Pefferkorn, E. J. Colloid Interface Sci. 1990, 141, 522. (17) Manning, G. J. Chem. Phys. 1969, 51, 924. (18) Gaines, G. L. Insoluble monolayers at liquid-gas interfaces; Interscience Publishers: New York, 1966.
Figure 2. Langmuir isotherm of quaternized PS177-PVP1581 in pure water. The solid line represents the best fit π ∝ σ-1.
Results For the brush isotherms, because of the small proportion of PS monomer, we have checked that the PS block has no contribution to the surface pressure.19 The theoretical background concerning the surface pressure of polyelectrolyte brushes has been summarized elsewhere.19 Langmuir Isotherms of the Quaternized PS177PVP1581 in Pure Water. Figure 2 shows the Langmuir isotherm of the quaternized PS177-PVP1581 in pure water (pH 6). The x-axis is the area per chain σ. The solid line represents the best surface pressure fit with the function π ) Aσ-β. The best fit gives β ) 1 ( 0.1. For this exponent value, the prefactor is equal to A ) (9 ( 0.5) × 10-19 N m. This agrees well with Bringuier’s predictions for quenched polyelectrolyte brushes.20 Indeed, for quenched systems in a salt-free solution, he showed that the surface pressure is proportional to the surface concentration of the counterions (kTRN)/σ, where k is the Boltzmann constant, T is the temperature, R is the charge fraction, N is the chains degree of polymerization, and σ is the area per chain. Langmuir Isotherms of PS-PVP in pH 2. We have studied the Langmuir isotherms of PS-PVP copolymers at pH 2. As mentioned above, at this pH value, the charge dissociation fraction of a single PVP chain is equal to 0.9. Figure 3 represents the Langmuir isotherm of the PS177PVP1581 at pH 2. The surface pressure scales as σ-2(0.1, as shown by the solid line. The same measurement was repeated for the copolymers PS177-PVP863 and PS179PVP412. Figure 4 shows the surface pressure of the three copolymers as a function of N/σ2. The lines are superposed for the longer copolymers, showing that the surface pressure is proportional to N/σ2. However, this is not the case for the PS179-PVP412. This may be due to the change in PS/PVP proportions. In this situation, the copolymer could form surface micelles.24 Further investigations would be required to confirm it. (19) Prinz, C.; Muller, P.; Maaloum, M. To be published in Macromolecules. (20) Bringuier, E. J. Phys. 1984, 45, L-107. (21) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491. (22) Witten, T.; Pincus, P. Europhys. Lett. 1987, 3, 315. (23) Williams, C. Private communication. (24) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5585.
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Figure 3. Langmuir isotherm of PS177-PVP1581 in water at pH 2. The surface pressure scales as σ-2 (solid line). The dashed line represents the best fit with the function π ) Aσ-5/3.
Figure 4. Surface pressure of PS177-PVP1581, PS177-PVP863, and PS179-PVP412 copolymers in water at pH 2 as a function of N/σ2.
For annealed polyelectrolyte brushes in the osmotic regime, Zhulina et al. calculated the charge fraction R and the monomer concentration cp as a function of the area per chain σ. They found21 cp ) a-4/3σ-4/3[{(1 - RB)/ RB}{1/(CH+ + CS)}]1/3 and R ) [{RB/(1 - RB)}{(CS + CH+)/ cp}]1/2, where RB is the charge fraction of a single chain in solution, Cs is the salt concentration, and CH+ is the H+ ions concentration (CH+ ) 10-pH). Assuming that the surface pressure is proportional to the osmotic pressure of the counterions, we find π ∝ σ-1/3. This is not the case in our experiments. On the other hand, the surface pressure of a quenched polyelectrolyte brush in the strong screening limit is given by20,22 π ) (kTR4/3N)/(4CS2/3a2/3σ5/3). The -5/3 exponent is close to the -2 exponent found experimentally. The strong screening regime is the relevant one when the salt concentration is large compared to the counterions concentration, i.e., (CS + CH+) . Rcp. Now, considering the fact that the H+ ions concentration arising from the chlorhydric acid added to the solution is equal to 10-pH ) 10-2 M, it could be large enough for the brush to be in the “salted” regime. The dashed line in Figure 3 represents the isotherm best fit using the function π ) Aσ-5/3. The best fit gives A ) (7.8 ( 0.05) × 10-30 N m7/3. Since we have A ) (kTR4/3N)/(4CH+2/3a2/3), we can deduce the value of the charge fraction R from the prefactor A. Taking CH+ ) 6.04 × 1024 m-3, a ) 2.5 × 10-10 m, kT ) 4 × 10-21 N m, and N ) 1581 gives R ) 0.12. This value is below the threshold given by Manning condensation (RM ) 0.36). Similar phenomena have been observed for quenched polyelectrolytes in dilute solution.23 An estimation of the polymer concentration has been provided by AFM force measurements on the brush, indicating that it is of the same order of magnitude as the salt concentration in the subphase.19
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Figure 5. Langmuir isotherm of PS177-PVP1581 in water at pH 2, for different values of salt concentrations.
Figure 6. Langmuir isotherm of PS177-PVP1581 in water at pH 3, in a double-logarithmic representation.
We measured the Langmuir isotherms PS177-PVP1581 at pH 2 with sodium chloride (NaCl) added to the subphase. Figure 5 shows the corresponding isotherms in a doublelogarithmic representation for different values of NaCl concentration. The surface pressure is lowered by the salt addition. This is due to the decrease of the difference of osmotic pressure inside and outside the brush. We adjusted the isotherms with a power law π ∝ σ-β. The β exponent passes continuously from -2 without salt to -2.4 at 10-1 M of NaCl. This change in β can be attributed to an increase in the PVP surface activity due to different ionization state or to the screening of the electrostatic interactions in the subphase. Langmuir Isotherms of PS-PVP in pH 3. Figure 6 represents the Langmuir isotherm of the PS177-PVP1581 at pH 3 in a double-logarithmic representation. Because of the small compression coefficient of the trough, we had to make three distinct isotherm measurements in order to cover the range of areas shown in Figure 6. This explains the two kinks on the isotherm, arising from littles changes in the pH from one experiment to another, as explained in the experimental section. A plateau appears in the isotherm. This plateau could correspond to a compressioninduced solubilization of PVP chains previously forming surface micelles with a PS core,24 as shown in previous studies. On the other hand, although the plateau is not perfectly flat, it could be the signature of a first-order phase transition. Indeed, for a first-order phase transition, it has been shown that unless extremely severe experimental precautions are taken, the plateau is not flat in the Langmuir isotherms.25,26 The surface pressure scales as σ-1.8(0.1 and as σ-1(0.1, respectively, below and above the plateau. If we assume (25) Middleton, S. R.; Iwahashi, M.; Pallas, N. R.; Pethica, B. A. Proc. R. London, Ser. A 1984, 386, 143. (26) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509.
Langmuir Isotherms of Polyelectrolyte Brushes
Figure 7. Langmuir isotherms of PS177-PVP1581, PS177PVP863, and PS179-PVP412 copolymers in water at pH 3. The x-axis is the area per PVP monomer.
Figure 8. Langmuir isotherms of PS177-PVP1581 and PVP1352 at pH 3, below the plateau.
that the layer is in a “brush” state, above and below the plateau, then, from a theoretical point of view, these exponents indicate a (salted polyelectrolyte brush)(strong polyelectrolyte brush) phase transition. Indeed, for a salted polyelectrolyte brush, the surface pressure is22 π ) (kTR4/3N)/(4CS2/3a2/3σ5/3) while for a strong polyelectrolyte brush, it is equal to20 π ) (kTRN)/σ. In order to check whether these conclusions are valid, we studied the influence of the degree of polymerization N on the surface pressure. Figure 7 shows the Langmuir isotherms of PS177-PVP1581, PS177-PVP863, and PS179-PVP412 at pH 3. The x-axis is the area per PVP monomer σ/N. The three curves can be considered as superposed within the experimental errors that are due to small differences in the pH of the subphase. Indeed, at pH 3 when the suphase hydrochloric concentration is changed to 2 × 10-5 M, the measured pH remains unchanged, but the PS-PVP surface pressure at the beginning of the transition is lowered by 0.5 mN/m. The fact that the curves are superposed means that the surface pressure scales as N/σ above the plateau, which is consistent with the strong polyelectrolyte brush behavior assumption. In the same way, this also means that the surface scales as (N/σ)1.8 below the plateau, which is not the case for polyelectrolyte brushes in the strong screening limit. Thus the layer is not in a brush state below the plateau. On the other hand, the fact that the phase transition is independent of the chain length may indicate a state where the PVP chains would be adsorbed at the interface. Figure 8 shows the Langmuir isotherms under the plateau of PS177-PVP1581 and a PVP homopolymer containing 1352 monomers, both obtained in the water at pH 3. The isotherms are similar, showing that the PVP is adsorbed at the interface. Figure 9 shows the isotherms of the same polymers as above at smaller areas per monomer. While the surface pressure increases for the copolymer, it remains constant for the
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Figure 9. Langmuir isotherms of PS177-PVP1581 and PVP1352 at pH 3, at and above the plateau.
Figure 10. Langmuir isotherms of PS177-PVP1581 in water at pH 3, below the plateau, in a double-logarithmic representation and for different values of salt concentration.
Figure 11. Langmuir isotherms of PS177-PVP1581 in water at pH 3 and pH 6.
homopolymer. Moreover, when the PVP layer is expanded, the surface pressure decrease rapidly to zero. This hysteresis of the homopolymer isotherm shows that the chains are expulsed into the bulk at the plateau. For a more thorough investigation of the system, we considered each state separately and studied the effect of added salt. “Adsorbed” State (below the Plateau). Figure 10 shows the PS177-PVP1581 isotherm below the plateau in a double-logarithmic scale, for different salt (NaCl) concentrations. The surface pressure is independent of the salt concentration. The first explanation for this is that the chains are neutral. Figure 11 shows the PS177PVP1581 isotherms in water at pH 3 and pH 6, where the PVP is neutral.27 The surface pressure is higher at pH 3 than at pH 6, indicating that the PVP chains are charged at pH 3. Thus, a reasonable description of the adsorbed layer consists of a neutral part at the interface and a (27) Note that the isotherm remains unchanged from pH 4 to pH 12, which shows that the layer is neutral at these pH values.
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Figure 12. Langmuir isotherms of PS177-PVP1581 in water at pH 3 above the plateau, in a double-logarithmic representation and for different values of salt concentration.
strongly charged part in contact with the solution in a way that the screening length associated with the counterions is smaller than the screening length due to the salt. This “charge gradient” in the layer could explain the fact that the PVP is adsorbed at the interface and insensitive to any salt addition. Note that this state requires an heterogeneous charge distribution along the chain and could not exist for quenched polyelectrolytes. There are no theoretical results for these systems in the adsorbed state. Strong Polyelectrolyte Brush State (above the Plateau). Figure 12 represents the PS177-PVP1581 isotherms at pH 3, above the plateau for different salt concentrations. Both axes are in the logarithmic scale. The plateau disappears in the isotherm at a salt concentration of 10-1 M. As mentioned previously, the surface pressure in a salt-free solution scales as σ-1. At 5 × 10-3 M added salt, it scales as σ-1.4 and from 10-2 to 10-1 M NaCl it scales as σ-2. In order to see if the PS-PVP brush still behaves as a strong polyelectrolyte brush, we compared the isotherms with those of the quaternized PS177-PVP1581 (i.e., with the quenched polyelectrolyte brush isotherms). Figure 13 represents three isotherms of the quaternized PS177-PVP1581: in water at pH 6 (i.e., pure water), in water at pH 6 with 10-2 M NaCl, and at pH 3. As seen previously, in pure water, the quenched polyelectrolyte brush surface pressure scales as σ-1. When HCl is added in order to adjust the pH of the solution to pH 3, the isotherm remains unchanged and is the same as for the annealed brush. Finally, with 10-2 M of salt in the solution, we have π ∝ σ-2 for the quenched brush, as for the annealed brush. Thus, we have studied the effect of salt, polymerization degree, and grafting density for
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Figure 13. Langmuir isotherms of quaternized PS177-PVP1581 in water at pH 3, pH 6, and pH 6 with 10-2 M of NaCl.
the annealed brush at pH 3 above the plateau, and we can conclude that it behaves consistently as a quenched polyelectrolyte brush (i.e., with a fixed charge fraction). Conclusion We have studied the Langmuir isotherms of PS-PVP and PS-(quaternized PVP) copolymers at the air-water interface. In pure water the surface pressure of the quaternized copolymer is proportional to the counterions surface concentration and behaves as predicted by the theory. At pH 2, the annealed PVP chains form a polyelectrolyte brush firmly anchored by the PS block. Its surface pressure corresponds to the theoretical results for a polyelectrolyte brush in the strong screening limit. At pH 3 the PS-PVP layer undergoes a transition from a state where the PVP is adsorbed at the interface at large areas per chain to a state of quenched polyelectrolyte brush at small areas per chain. This transition has been studied using scaling arguments. The adsorbed state was found to be charged but insensitive to salt addition in the solution. A charge gradient in the layer may explain this latter state. A direct investigation of the surface on the plateau using atomic force microscopy is needed to determine the nature of the transition. Thus the visualization of phase coexistence domains at the transition would allow one to ascribe this phenomenon to the firstorder transition. For a better understanding of the system, new theoretical developments are necessary. Moreover, neutron reflectivity measurements could be of great interest. Indeed, the concentration profile of annealed layers and brushes could be measured. These profiles are expected to differ from those for quenched systems because of the charges mobility along the chains. LA9916631
