1260
Langmuir 1999, 15, 1260-1267
Adsorption of Ampholytic Diblock Copolymers from Dilute Aqueous Solution at the Solid/Liquid Interface H. Walter,† C. Harrats,†,‡ P. Mu¨ller-Buschbaum,† R. Je´roˆme,§ and M. Stamm*,† Max Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany, Centre for Education and Research on Macromolecules, University of Lie` ge, Sart-Tilman B6, B-4000 Lie` ge, Belgium Received September 8, 1998. In Final Form: November 16, 1998 The adsorption of the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) (PMAA-b-PDMAEMA) and the corresponding homopolyelectrolytes of the two blocks, PMAA and PDMAEMA, was investigated from dilute aqueous solution on silicon substrates. The adsorbed amount of polymer as a function of pH, polyampholyte concentration, and salt concentration in solution has been determined by ellipsometry at room temperature. As a function of pH the adsorbed amount reaches its maximum at the isoelectric point of the polyampholyte. Some adsorption takes place even in pH ranges where the surface charge has the same sign as the net charge of the polyampholyte. By variation of the polyampholyte concentration, typical adsorption isotherms were determined at several pH values. With increasing salt concentration the adsorbed amount increases, and above a critical concentration it diverges to very large values. The measured dependencies can be explained by the adsorption of one or the other of the two blocks depending on acidity and ionic strength and are in good agreement with theoretical predictions. Adsorption kinetics has also been studied in detail, and the diffusion coefficient of the polyampholyte toward the surface has been obtained in the early state of adsorption as a function of pH, polyampholyte concentration, and salt concentration.
Introduction There is a considerable interest in the adsorption of polyelectrolytes at the solid/liquid interface. Many technological processes such as wastewater treatment, separation by flotation, paper production, etc., are closely connected with adsorption phenomena. Adsorption of ampholytic polyelectrolytes such as proteins plays a key role in many biological processes, e.g., the coagulation of blood on artificial implants. While polyelectrolytes in solution are well investigated,1,2 there are only a few examinations of the adsorption at the solid/liquid interface.3-12 Investigations on adsorption of ampholytic block copolymers are not reported up to now. The understanding of adsorption and desorption of these polymers under different solution conditions is fundamental for technological utilization and improvements. Successful application of such thin adsorbed polyampholyte films depends * To whom correspondence should be addressed. † Max Planck-Institut fu ¨ r Polymerforschung. ‡ Present address: Katholieke Universiteit Leuven, Afdeling Polymeerchemie, Celestijnenlaan 200F, B-3001 Heverlee, Belgium. § University of Lie ` ge. (1) Fo¨rster, S.; Schmidt, M. Adv. Polym. Sci. 1995, 120, 53. (2) Kassapidou, K.; Jesse, W.; Kuil, M. E.; Lapp, A.; Egelhaaf, S.; van der Maarel, J. R. C. Macromolecules 1997, 30, 2671. (3) Lyklema, J. Fundamentals of Interface and Colloid Science Vol. II; Academic Press: London, 1995. (4) Robb, I. D. Comprehensive Polymer Science Vol. II; Pergamon Press: Oxford, 1989. (5) Van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 8, 6661. (6) Evers, O. A.; Fleer, G. J.; Scheutjens, J. M. H. M.; Lyklema, J. J. Colloid Interface Sci. 1986, 111, 446. (7) Blaakmeer, J.; Bo¨hmer, M. R.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1990, 23, 2301. (8) Kamiyama., Y.; Israelachvili, J. Macromolecules 1992, 25, 5081. (9) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. Faraday Discuss. Chem. Soc. 1994, 98, 161. (10) Shubin, V.; Linse, P. J. Phys. Chem. 1995, 99, 1285. (11) O ¨ dberg, L.; Sandberg, S. Langmuir 1995, 11, 2621. (12) Killmann, E.; Reiner, M. Tenside, Surfactants, Deterg. 1996, 33, 220.
on many factors such as their stability and structure under different conditions. Adsorption from dilute aqueous solutions of an ampholytic diblock copolymer like the investigated poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) (PMAA-b-PDMAEMA) depends on the balance of long-range electrostatic attraction and repulsion between the charged surface of the substrate with the positive and the negative charged block.13 Possible hydrophobic short-range interactions can play an additional role. The electrostatic interactions are strongly influenced by solution conditions such as pH or ionic strength, since for both, the blocks of the polyampholyte and the surface of the substrate, the degree of dissociation of their ionic groups depends on pH.14,15 The two isoelectric points (IEP)sof the polyampholyte and of the substratesare important for the adsorption16 and desorption, as well as for the conformation of the chains in solution and at the surface. We studied the adsorption of the ampholytic diblock copolymer PMAA-b-PDMAEMA on silicon surfaces from dilute aqueous solution as a function of pH, polyampholyte, and salt concentration in solution using ellipsometry. Static and kinetic aspects of this process are discussed. An asymmetric block distribution of the polyampholyte was chosen with a smaller basic block and a bigger acidic block, where the basic block can act as an anchor to the surface and the acidic block dangles in solution. This behavior is expected for pH above the isoelectric point of the silicon surface where the surface is negatively charged. Further we studied the adsorption of homopolyelectrolytes of the two blocks as a function of pH to get an idea how each block interacts alone with the substrate. The (13) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. Macromolecules 1997, 30, 4332. (14) Leyte, J. C.; Mandel, M. J. Polym. Sci. 1964, A2, 1879. (15) Joppien, G. R. J. Phys. Chem. 1978, 82, 2210. (16) Eirich, F. R. J. Colloid Interface Sci. 1977, 58, 423.
10.1021/la981178q CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
Adsorption of Diblock Copolymers
Langmuir, Vol. 15, No. 4, 1999 1261
Table 1. Molecular Characteristics of Studied Polymersa sample
Mn (g/mol)
Mw/Mn
NPMAA
A/31 A-B/62 B/15
30 700 61 800 15 000
1.04 1.12 1.12
354 393
NPDMAEMA
fPMAA
F (g/cm3)
194 104
1.00 0.67 0
1.218 1.251 1.318
a As described in the text molecular weights M are calculated n based on GPC measurements. NPMAA and NPDMAEMA are the degrees of polymerization of the poly(methacrylic acid) and of the poly((dimethylamino)ethyl methacrylate) block, and fPMAA is the composition of the diblock copolymer as obtained from NMR. F is the mass density measured using the “principle of Archimedes” (floating technique).
transmission of the solution as a function of pH and the hydrodynamic radius of the ampholytic diblock copolymer under different solution conditions were measured by light scattering to check the influence of the conformation on adsorption of the investigated polyampholyte. Experimental Section Materials. The diblock copolymer poly(tert-butyl methacrylate)-block-poly((dimethylamino)ethyl methacrylate) (PtBMAb-PDMAEMA) and the homopolymers PtBMA and PDMAEMA were synthesized by anionic polymerization in THF at -78 °C as described elsewhere.17,18 The initiator used was (sec-BuLi/ diphenylethylene). To get monodisperse polymers lithium chloride (LiCl) coordinating agent has been used. To obtain the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) (PMAA-b-PDMAEMA) and the homopolyelectrolyte PMAA, the hydrolysis of the tBMA monomers was necessary.19 The polymers were dissolved in dioxane (5-10% solution) in the presence of hydrochloric acid. The polymer solution was left under reflux during at least 12 h. After removal of the solvent by vacuum-drying, the polymers were dispersed in water. Following neutralization with NaOH at a pH of 9, the polymers became water soluble. To remove the excess of salt and base, the solutions were dialyzed during at least 3 days. The polymers were then stored in aqueous solution. Molecular weights of the two homopolymers, the precursor diblock copolymer and its PtBMA block, were determined by gel permeation chromatography (GPC) based on a PMMA standard. The composition fPMAA ) NPMAA/(NPMAA + NPDMAEMA) of the diblock copolymer was measured by nuclear magnetic resonance spectroscopy (1H-NMR). The degree of polymerization N of the polymers was calculated out of these data (Table 1). The molecular weights M after hydrolysis of the homopolyelectrolyte PMAA and the polyampholyte PMAA-b-PDMAEMA were then obtained from the GPC measurement taking into account the reduced weight of each tBMA monomer as a result of hydrolysis.20 For this calculation a degree of hydrolysis of 100% was assumed. Silicon wafers (Wacker Chemitronics) with a native oxide layer of typically 2 nm were used as substrate for adsorption experiments. Prior to use they were cleaned 15 min in dichloromethane in an ultrasonic bath at about 50 °C. Afterward the wafers were washed with water. The next cleaning step was an oxidation bath with a mixture of H2O2, NH3, and water at a temperature of 75 °C for about 20-30 min, depending on pollution level. When the organic contamination of the surface is completely removed, the size of the bubbles evolving at the samples changes significantly. After the wafers were rinsed several times with water, they were dried with clean nitrogen and stored in an oven at 50 °C. For all cleaning solutions and all experiments, fresh Milli-Pore water was used. Null Ellipsometry. With null ellipsometry the amount of adsorbed polymer and the kinetics of adsorption were determined by variation of solution parameters. All measurements were (17) Creutz, S.; Teyssie´, P.; Je´roˆme, R. Macromolecules 1997, 30, 6. (18) Antoun, S.; Teyssie´, P.; Je´roˆme, R. Macromolecules 1997, 30, 1556. (19) Creutz, S.; van Stam, J.; Antoun, S.; De Schryver, F. C.; Je´roˆme, R. Macromolecules 1997, 30, 4078. (20) Orth, J.; Meyer, W. H.; Bellmann, C.; Wegner, G. Acta Polym. 1997, 48, 490.
performed with a computer-controlled null ellipsometer in a vertical polarizer-compensator-sample-analyzer (PCSA) arrangement.21 Angle of incidence was set to 70.0° to obtain the best sensitivity for our system. A He-Ne laser (λ ) 632.8 nm) was used as light source. In principle the thickness d1 and the index of refraction n1 of a polymer film can be obtained from the ellipsometric angles Ψ and ∆, measured at null intensity of reflected light, assuming a multilayer model formed by an adsorbed homogeneous isotropic film on a silicon wafer in contact with a dilute polymer solution with index of refraction n0.22 Since changes in the two angles are small due to formation of thin adsorbed layers, the errors, in d1 and n1 are very large for measurements in solution, and thus for thin films no unique values can be found. Instead a reasonable value for the refractive index can be guessed and the corresponding thickness calculated.23 With these values the optical thickness d1n1 or the adsorbed amount of polymer A can be obtained.24
A ) d1(n1 - n0)/(dn/dc)
(1)
dn/dc is the increment of refractive index of the polymer. For measurements of dried samples in air the adsorbed amount is
A ) F1d1
(2)
using an analogous multilayer model. F1 is the mass density of the adsorbed layer. The obtained adsorbed amount of the layer was for all samples equal within error for the two different kinds of measurements. The adsorption experiments were performed in a specially designed Teflon cell at room temperature and fixed humidity. First the cell was filled with an aqueous solution with a fixed salt concentration (NaCl) at a certain pH, and ellipsometric angles were measured to test the stability of the oxide layer. Every 20 s a pair of the angles Ψ and ∆ was recorded. A stirring magnet placed beneath the sample stirred the solution softly. Subsequently a small volume of concentrated polymer solution was added. The changes in Ψ and ∆ as a result of the adsorption were followed as a function of time. After a characteristic time, depending on solution conditions (see Chapter 5), adsorption achieved its equilibrium. When no more changes in the ellipsometric angles were detected, the sample was taken out of the cell, washed with Millipore water, and afterward dried with clean nitrogen. All shown data of adsorbed amount as a function of solution conditions were taken in equilibrium of adsorption. Above pH ) 10 the oxide layer is no longer stable,25 and therefore all measurements were done at pH below this value. Light Scattering. For the determination of the transmission the intensity of the laser of the null ellipsometer passing through the solution in the Teflon cell was measured. Variation of pH by adding acid or base led to the formation of complexes (clusters) of the polyampholyte near its IEP and therefore to a reduced intensity of the laser light. Thus the IEP and the pH range of insolubility of the polyampholyte can be measured. Normalization of the measured intensity to its maximum value provides the transmission in percent. Micelle formation in solution was investigated by dynamic light scattering (DLS) measurements performed on a commercial ALV 3000 digital correlator. Light source was a 400 mW krypton ion laser with a wavelength of λ ) 647 nm. Autocorrelation functions, gq(t), were measured for solutions of sample A-B/62 at several pH values at fixed polyampholyte and salt concentrations. Scattering angle was 90° and temperature was set to 22 °C. (21) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (22) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publication: Amsterdam, 1987. (23) Zhang, Y.; Tirrell, M.; Mays, J. W. Macromolecules 1996, 29, 7299. (24) Siqueira, D. F.; Breiner, U.; Stadler, R.; Stamm, M. Langmuir 1995, 11, 1680. (25) Axelos, M. A. V.; Tchoubar, D.; Bottero, J. Y. Langmuir 1989, 5, 1186. (26) Pecora, R.; Berne, B. J. Dynamic Light Scattering; John Wiley & Sons: New York, 1976.
1262 Langmuir, Vol. 15, No. 4, 1999
Walter et al.
For monodispersed spheres the autocorrelation function of the scattered intensity can be well represented by the following single-exponential function26
gq(t) ) exp(-Dsolq2t)
(3)
where Dsol is the translational diffusion coefficient of free chains or micelles in solution, q is the magnitude of the scattering vector, and t is the time delay. For polyelectrolytes often two diffusion processes are detected (slow and fast mode).27 In the experimental correlation function first the slow diffusion process (Dssol) is fitted to a sum of two exponential and the remaining fast diffusion process (Dfsol) is fitted by just one exponential function as described elsewhere.27 The hydrodynamic radius Rh of particles and the free particle diffusion coefficient are interrelated by the Stokes-Einstein relation
Rh ) kT/6πηDsol
(4)
where k is the Boltzmann constant, T the temperature, and η the viscosity of the solvent. Rh can be calculated for both diffusion processes.
Results and Discussion 1. Adsorption of the Homopolyelectrolytes as a Function of pH. To understand the adsorption behavior of the ampholytic diblock copolymer A-B/62, first the adsorption of the acidic PMAA and the basic PDMAEMA block was determined with the two homopolyelectrolytes A/31 and B/15 (see Table 1). Both are weak polyelectrolytes with a degree of dissociation R of their ionic groups depending on pH.14 Adsorption on silicon substrates from dilute aqueous solution was investigated by ellipsometry. (a) PMAA Adsorption. Silicon surfaces are negative charged above their IEP. For the wafers used in this work this is the case above a pH of about 3.9 in an aqueous solution with a NaCl concentration of 0.01 mol/L, below this IEP they are slightly positive charged.15,28 The sign of the charges on the surface is indicated as a function of pH in the corresponding figures by the arrows below the x-axis (S+ or S-). PMAA is negatively charged at high pH and neutral at low pH.14 Therefore no adsorption was detected for the acidic homopolyelectrolyte PMAA (A/31) at pH g 4.4 up to pH ) 10.0 (Figure 1) as the surface and the polyelectrolyte are carrying charges of the same sign. This behavior is theoretically predicted for weak polyacids.3,6 At pH < 4.4 a rapid increase of the adsorbed amount of PMAA was detected as the polymer and the surface are charged with opposite sign below the IEP of the silicon wafer. With decreasing pH the number of positive charges of the silicon substrate rises slowly15 while the charge density of PMAA decreases.14 Therefore the adsorbed amount reaches a maximum at pH of about 3.1. This was already theoretically predicted6 and experimentally found for poly(acrylic acid) (PAA).7 At lower pH the adsorbed amount decreases as fast as it increased previously since the charge density of PMAA and therefore the electrostatic interaction tends to zero. The observed adsorption of a polyelectrolyte on an identically charged surface in a small pH range close to the IEP (Figure 1) was already found for PAA15 and for DNA.29 Such an adsorption dominated by nonionic interactions can occur if the adsorption energy is stronger (27) Fo¨rster S.; Schmidt M.; Antonietti M. Polymer 1990, 31, 781. (28) Suhara, T.; Fukui, H.; Yamaguchi, M. Colloids Surf., A 1995, 101, 29. (29) Chattoraj, D. K.; Chowrashi, P.; Chakravarti, K. Biopolymers 1967, 5, 173.
Figure 1. Adsorbed amount A of the homopolyelectrolytes A/31 (solid diamonds) and B/15 (open diamonds) as a function of pH. Salt concentration in solution was 0.01 mol/L and polyelectrolyte concentration 6.51 µmol/L (A/31) and 5.83 µmol/L (B/15), respectively. The solid lines are guides for the eye. The bars and arrows (below) indicate where the silicon surface carries a positive (S+) or a negative (S-) net charge. The inlay illustrates two different possible conformations of adsorbed chains.
than the electrostatic repulsion. In our case this repulsion becomes too strong to promote adsorption at pH g 4.4. The adsorbed amount below pH ) 4.1 is too high for the assumption of flat monolayers. PMAA is quite weakly charged under these conditions14 and the chains adopt a coil-like conformation in solution7 similar to strong polyelectrolytes at high ionic strength.1 As the surface is only weakly charged, too, the chains can be expected to adsorb in a coil-like conformation which leads to a higher number density of chains at the surface and thus to the high adsorbed amount (see inlay of Figure 1). In summary with increasing distance between charges on the polymer chains the adsorbed amount on uncharged or weakly charged surfaces increases, too.4 (b) PDMAEMA Adsorption. The basic PDMAEMA homopolyelectrolyte is neutral at high pH and positively charged at low pH. Thus it should adsorb on silicon wafers at pH above the IEP of this substrate, while PMAA adsorbs below this point. As can be seen in Figure 1 sample B/15 adsorbs at pH > 5.2. The adsorbed amount below pH ) 6.8 is typical for flat monolayer formation3 (see inlay of Figure 1). With increasing pH the charge density of PDMAEMA gets lower and the adsorbed chains form more and more loops and tails. Therefore the chains adsorb in a denser conformation which leads to the increasing adsorbed amount with increasing pH. Below pH 5.2 no adsorption was detected. Adsorption can only take place if the net interaction energy is stronger than the thermal energy kT.13 As the charge density of the silicon substrate becomes really low between the IEP and pH 5.215 the attraction is weak and adsorption is prevented by thermal fluctuations although the polymer and the surface carry charges of opposite sign. As a result of the adsorption experiments with the two homopolyelectrolytes, one can conclude that the PMAA block of the ampholytic copolymer should act as an anchor to the silicon surface at pH < 4.4 and the PDMAEMA block at pH > 5.2. 2. Adsorption of the Polyampholyte as a Function of pH. At room temperature and fixed salt concentration of 0.01 mol/L NaCl the adsorbed amount of the ampholytic diblock copolymer A-B/62 (Figure 2a) exhibits as a function of pH three typical regimes (below pH ) 5.4, 5.4 e pH < 6.4, and pH g 6.4), which are discussed in the following. (a) 5.4 e pH < 6.4. As a function of pH we observe a maximum in the adsorbed amount between pH ) 5.4 and
Adsorption of Diblock Copolymers
Langmuir, Vol. 15, No. 4, 1999 1263
negative charges. This leads to a strong attractive interaction between different chains8 and for ampholytic blockcopolymers additional between the two blocks. Thus macroscopic flocks or clusters are formed in solution. This precipitation has been theoretically predicted for polyampholytes with a statistical distribution of charges by Dobrynin et al.13 As the polyampholyte chains and also the flocks only exhibit a net charge close to zero in this pH range, still ionic bonds between the positively charged groups of the polyampholyte and the negatively charged surface can be formed which results in the adsorption in a dense conformation with a high adsorbed amount. Additional hydrophobic interactions may play a role in the adsorption since water is under these conditions a poor solvent for the net uncharged polyampholyte.3 With increasing pH, starting from the IEP of the polyampholyte, the number of charged monomers of the PMAA block increases while the one of the PDMAEMA block decreases. With decreasing pH, it is vice versa. This lowers the attractive interaction between the two blocks and between different chains. The chains become soluble in water, and flocculation does not occur anymore as can be seen in the transmission in Figure 2a. At pH where the transmission is 100%, formation of monolayers was found, while in the pH range near the IEP of the ampholytic diblock copolymer the layers are formed by adsorption of flocks (with some resemblance to snowing). The net charge on the polyampholyte as a function of pH is indicated by the arrows below the x-axis (P+ or P-) (Figure 2a). (b) pH g 6.4. At pH > 6 the diblock copolymer carries a negative net charge: more monomers of the acidic PMAA block are charged than of the basic PDMAEMA block. Under these conditions the short basic block acts as the anchor to the surface of the silicon substrate that is negatively charged, too. For this case adsorption was experimentally found8 and theoretically predicted for polyampholytes with a statistical distribution of charges if the net interaction energy between polyampholyte and surface, W, overcomes the thermal energy (W + kT < 0).13 This energy consists of three different parts.
W ) Wa + W r + W i
Figure 2. (a) Adsorbed amount A of sample A-B/62 at two polyampholyte concentrations (open circles 0.81 µmol/L and solid circles 3.24 µmol/L) and transmission T of the solution (dashed line) as a function of pH. Salt concentration in solution was 0.01 mol/L. The solid lines are guides for the eye. The bars and arrows (below) indicate where the silicon surface and the polyampholyte are carrying a positive (S+, P+) or a negative (S-, P-) net charge. (b) Adsorbed amount A of sample A-B/62 as a function of polyampholyte concentration cp at pH ) 3.7 (open squares) and pH ) 6.5 (solid squares). Salt concentration in solution was 0.01 mol/L. The dotted lines are fits based on the Langmuir model as explained in the text. (c) Adsorbed amount A of sample A-B/62 as a function of salt concentration cs at pH ) 6.7. Polyampholyte concentration in solution was 3.24 µmol/L. The lines are guides for the eye. The data point in parentheses indicate that adsorption at this concentration did not reach an equilibrium adsorbed amount after 1 week.
pH ) 6.4. Correlated with this maximum is a sharp minimum in transmission of the solution at pH ) 5.9. A similar behavior was found by Eirich16 and Kamiyama8 investigating the statistical polyampholyte gelatin. They discovered that the adsorbed amount increases to a maximum at a pH close to the IEP of gelatin. At this point a polyampholyte carries an equal number of positive and
(5)
The polarization of the chain by the electric field of the charged surface leads to the attractive part Wa, while Wr is the repulsive energy between the net charge on the chain and this surface field. If the adsorbing surface has a dielectric constant 1 smaller than the one of the solution 0, both media are polarized by the presence of the polyampholyte chain and an additional repulsive interaction Wi has to be included. The investigated adsorbed amount of the polyampholyte decreases above its IEP with increasing pH (Figure 2a) as the surface of the silicon substrate and the big acidic PMAA block become more and more negatively charged. Therefore the polyampholyte chains should adsorb in an expanded conformation.8 These increasing charge densities lead to a growing repulsion between the PMAA block of the polyampholyte and the surface; Wr and Wi are rising. Additionally the basic PDAEMA block becomes increasingly uncharged and therefore the attractive interaction to the opposite charged surface of the silicon substrate and its energy Wa tend to zero. At high pH the thermal energy overcomes the net interaction energy W and no adsorption occurs anymore. (c) pH < 5.4. Below pH 6 and above the IEP of the wafer the surface is oppositely charged to the polyampholyte chain, which is carrying now a positive net charge below
1264 Langmuir, Vol. 15, No. 4, 1999
Walter et al.
its IEP (arrows in Figure 2a). Although the investigated PDMAEMA homopolyelectrolyte does not adsorb in this pH range on the weakly negatively charged silicon substrate and the PMAA homopolyelectrolyte only adsorbes below pH ) 4.4 (Figure 1) there is a big adsorbed amount of the polyampholyte which decreases rapidly with decreasing pH. Thus at 4.4 e pH < 5.4 the reason for the observed adsorption is not obvious. Our model for the adsorption of the polyampholyte in this regime is that the weakly charged PMAA blocksconnected with the positive charged PDMAEMA blocksand the cluster formation between the two blocks or between different chains yields an increased immobility of the polyampholyte as compared to the PDMAEMA homopolyelectrolyte. Thus the thermal energy cannot overcome the attractive adsorption energy of the polyampholyte as for the homopolyelectrolyte and the basic block anchors to the surface. This different adsorption behavior is comparable with snowing again: At a temperature well below 0 °C snow flocks are usually light and cannot reach ground at windy weather. The wind keeps them in the air as the gravity is not strong enough. Around 0 °C snow flocks become wet and heavier as they cluster together. Thus they fall straight to the ground, only slightly affected by the wind. In our case the thermal energy acts as the wind, the moment of immobility of the chains and clusters as the weight of the snow flocks and the electrostatic interaction as the gravity. As the charge density of the silicon surface declines with decreasing pH in the discussed pH range 4.4 e pH < 5.4, the electrostatic attraction is lowered and the adsorbed amount decreases too. Since the transmission of solution is not 100% under these conditions (Figure 2a) there are still some flocks or clusters in solution. These flocks or clusters can adsorb at the surface, and we indeed observe them by topologic investigations with atomic force microscopy (AFM) of the dried adsorbed layers. We will report about the lateral structure investigations of such adsorbed polyampholyte layers elsewhere.30 As already mentioned below pH ) 4.4 the big PMAA block of the polyampholyte can act as an anchor to the surface. The adsorbed amount as a function of pH can be treated in this pH range in a similar way as at pH g 6.4. The density of positive charges on the silicon surface increases with decreasing pH. Similarly the positive net charge of the polyampholyte increases with decreasing pH. Thus the repulsive parts of the adsorption energy Wr and Wi are growing and the attractive energy Wa declines. Subsequently the adsorbed amount tends to zero. At low pH the thermal energy kT overcomes the net interaction energy W and no adsorption can take place. 3. Adsorption as a Function of Polyampholyte Concentration. As a function of polyampholyte concentration typical adsorption isotherms were measured at pH values above and below the maximum of adsorbed amount (Figure 2b). They are similarly already known for polyelectrolytes (i.e., PAA on a cationic latex7). In both cases the plateau is reached at low concentrations. The fits to the data in Figure 2b are based on a simple adsorption model by Langmuir as described elsewhere.31 The covered fraction θ in dynamic equilibrium and therefore the adsorbed amount as a function of polymer concentration cp can be calculated
Figure 3. Adsorbed amount A of sample A-B/62 as a function of square root of time t1/2 at pH ) 2.6 (up triangles), pH ) 4.0 (down triangles), and pH ) 8.1 (crosses). Salt concentration and polyampholyte concentration in solution were 0.01 mol/L and 3.24 µmol/L, respectively. The dotted lines are fits based on the diffusion model as explained in the text.
(6)
polymer chain. All binding sites will be occupied in the plateau regime of adsorbed amount. In both fits of the measured adsorption isotherms K is about 8 × 1010 cm3/mol and therefore the adsorption process turns out to be irreversible. Actually even extended rinsing with pure water did not change the adsorbed amount. Only a drastic change of pH to values below 2.2 leads to desorption of a layer adsorbed at pH above the IEP of the polyampholyte. All previously discussed measurements by variation of pH (Figure 2a) were done at a polyampholyte concentration in the plateau region of adsorbed amount. 4. Adsorption as a Function of Salt Concentration. Below a critical salt concentration and at a fixed pH the adsorbed amount of the polyampholyte increases slightly with increasing salt concentration (Figure 2c). This is already known for simple polyelectrolytes such as for instance PSS, if the interaction driving adsorption is not purely electrostatic.3 As the added salt screens the charges of the chains, the electrostatic repulsion between the polyelectrolyte chains is reduced. The chains can adsorb in a dense manner, which leads to the higher adsorbed amount, but still adsorption of monolayers is found. Above a critical salt concentration, however, adsorption did not stop even after 1 week when the experiments was broken off, and not even the speed of the adsorption process decreased. Thus formation of multilayers occurs. The polyampholyte chain can adsorb on top of an already adsorbed polyampholyte layer. This adsorption process can be described as a kind of phase separation of the polyampholyte at the silicon surface, as water at this salt concentration is a bad solvent for the chains. This phase separation is theoretically predicted for adsorption of polyelectrolytes that contain a hydrophobic contribution to the adsorption interaction.3 Another possible reason for the formation of multilayers at high salt concentration is the screened electrostatic repulsion between already adsorbed polyampholyte chains and the chains in solution. 5. Adsorption Kinetics and Diffusion Coefficient of the Polyampholyte. The amount of adsorbed polyampholyte increases monotonic with time in all our measurements (Figure 3) and reaches a plateau value after a characteristic time depending on pH, polyampholyte, and salt concentration.
where A∞ is the adsorbed amount at infinity high polymer concentration and K ) kads/kdes is the ratio of the rate coefficients for adsorption kads and desorption kdes of the
(30) Walter, H.; Mu¨ller-Buschbaum, P.; Harrats, C.; Jerome, R.; Stamm, M. To be published. (31) Atkins, P. W. Physical Chemistry; University Press: Oxford, 1982.
A(cp) ) A∞θ ) A∞(Kcp/(1 + Kcp))
Adsorption of Diblock Copolymers
Langmuir, Vol. 15, No. 4, 1999 1265
(a) Influence of pH. At pH > 6 the adsorption process takes 2 to 3 h, while below pH 6 and above the IEP of the substrate it is finished after several minutes. Below pHIEP of the silicon substrate the adsorption time rises up to several hours again. Typical adsorption kinetics for the three interesting pH ranges are compared in Figure 3. In the two pH ranges where the surface and the polyampholyte chains possess a net charge with the same sign (see arrows in Figure 2a) the adsorption is very slow. The repulsive parts of the interaction energy play an important role, and adsorption is controlled by an interplay of repulsion and attraction. In the pH range between the IEP of the polyampholyte and the IEP of the silicon substrate, the adsorption is very fast. The kinetics of adsorption under these conditions is as fast as for the studied homopolyelectrolytes on the opposite charged substrate. In the initial state the time dependence of adsorption is controlled by a diffusion process of chains in solution. This is observed for instance for the adsorption of PSPEO block copolymers from toluene onto silicon surfaces.21 The diffusion coefficient of the polymer toward the surface Dsur can be obtained out of kinetic measurements from the slope of the adsorbed amount as a function of square root of time t
A(t) ) 2cp(Dsurt/π)1/2
(7)
in the very initial state of adsorption (see dashed lines in Figure 3) where cp is the polymer concentration in solution. In expression 7 it is assumed that each polymer chain reaching the surface is immediately adsorbed.21 If one has to consider an activation potential for adsorption as discussed previously, the diffusion coefficient in eq 7 is only an apparent diffusion coefficient Dapp and the adsorption process may be divided into two steps, the diffusion of chains to the surface and the actual adsorption process at the surface.32,33 As a function of pH at a fixed polyampholyte and salt concentration the diffusion coefficient Dsur approaches a maximum at the IEP of the polyampholyte (Figure 4a) similar to the adsorbed amount. DLS measurements, only possible at pH where no flocculation of the polyampholyte occurs, deliver the fast and the slow diffusion coefficient, Dfsol and Dssol, of the polyampholyte in solution. The fast diffusion process on one hand is attributed to a cooperative diffusion of the polyions and counterions. The slow diffusion process on the other hand is referred to the diffusion of interchain domains.27 For polyelectrolytes the two diffusion coefficients in solution and the one toward the surface are different, as the long-range electrostatic attraction or repulsion between the surface and polyelectrolytes provides a big influence on the diffusion near the surface. From Dfsol and Dssol one can obtain the hydrodynamic radii Rh of the single chains and the clusters or micelles in solution by using eq 4. The radius of the single chains was for the studied system between around 4.5 nm independent of pH. As can be seen in Figure 4a the radius of the micelles in solution decreases toward the IEP of the polyampholyte. The micelles in solution above the IEP of the polyampholyte are bigger as the one below the IEP since at high pH the big acidic PMAA block is charged and stretched while the small PDMAEMA block is neutral and collapsed. At low pH it is vice versa (see model structures in Figure 4a). Thus the dependence of the (32) Ravera, F.; Liggieri, L.; Steinchen, A. J. Colloid Interface Sci. 1993, 156, 109. (33) Filippova, N. Langmuir 1998, 14, 2864.
Figure 4. (a) Diffusion coefficient Dsur for adsorption of sample A-B/62 (circles) and radius of micelles in solution Rh (crosses) as a function of pH. Salt concentration and polyampholyte concentration in solution were 0.01 mol/L and 3.24 µmol/L, respectively. The lines are guides for the eye (solid line belongs to Dsur and dotted line to Rh). The inlays illustrate the micelle formation at different pH. (b) Diffusion coefficient Dsur for adsorption of sample A-B/62 as a function of polyampholyte concentration cp at pH ) 6.7. Salt concentration in solution was 0.01 mol/L. The dotted line is a linear fit with the slope -0.99 ( 0.08. (c) Diffusion coefficient Dsur for adsorption of sample A-B/62 as a function of salt concentration cs at pH ) 6.7. Polyampholyte concentration in solution was 3.24 µmol/L. The dotted line is a linear fit with the slope 0.73 ( 0.12.
diffusion coefficient Dsur as a function of pH can be partially explained by the slower diffusion of the micelles with a radius depending vice versa on pH as bigger aggregates got a slower diffusion. Nevertheless the diffusion coefficient Dsur reaches its biggest value at the IEP of the polyampholyte although there are very large aggregates in solution that should lead to a slower diffusion. This behavior can be explained taking the electrostatic interaction between the polyampholyte and the surface into account. The polyampholyte behaves like a neutral polymer in the pH range near the IEP of the polyampholyte
1266 Langmuir, Vol. 15, No. 4, 1999
while above and below the IEP the diffusion of the charged polyampholyte chains and clusters is hindered by electrostatic interactions which should lead to the lower diffusion coefficient. A similar behavior is known in the literature for the diffusion of poly(methacrylic acid) in aqueous solution at very low ionic strength.34 As a function of degree of neutralization R the slow diffusion coefficient Dssol (slow mode) increases as the charge density of the polyelectrolyte decreases. The diffusion coefficient Dfsol of the fast diffusion process on the other hand rises with increasing charge density. Thus the diffusion toward the surface during adsorption may be driven by the slow diffusion process, although the mass fraction associated with the slow mode is very small (of the order of 10-3). The total adsorbed amount is also only of the order of 2 × 10-4 of the mass of polyampholyte dissolved in solution. The values of the diffusion coefficient vary as a function of pH over 3 magnitudes and at the used polyampholyte concentration they are significantly lower as the ones known for comparable polymers in organic solvents. For instance a diblock copolymer PS-b-PEO with a molecular weight of Mw ) 80 000 g/mol has a diffusion coefficient of Dsur ) 4.91 × 10-7cm2/s.21 The obtained values for the fast adsorption kinetics in some cases had large error bars as there are only few measurement points in the very early stages of adsorption. The error is caused by the uncertainty in the time needed to add the polyampholyte. (b) Influence of Polyampholyte Concentration. For both measured adsorption isotherms (pH ) 3.7 and pH ) 6.5) the time for adsorption increases with decreasing polyampholyte concentration. This is similarly known in the literature for adsorption of nonionic as well as for cationic surfactants on silica.35,36 At pH ) 3.7 the time to reach equilibrium increases from a few minutes (cp ) 3.24 µmol/ L) to about 3 h (cp ) 0.02 µmol/L) while at pH ) 6.5 it increases from 3 h (cp ) 3.24 µmol/L) to at least 50 h (cp ) 0.02 µmol/L). At low concentration it takes a longer time until a certain number of chains hit the surface and adsorb than at higher concentration.37 Contrary to that, the diffusion coefficient Dsur increases drastically with decreasing polyampholyte concentration (Figure 4b). The exponent from a linear fit in the double logarithmic plot is -0.99 ( 0.08. A qualitatively similar behavior was found by Kona´k et al.34 for diffusion of fully dissociated poly(methacrylic acid) in solution. They report a slow diffusion process for two different molecular weights, with an exponent for the dependence of Dssol on the polyelectrolyte concentration of -0.3 (Mw ) 30 000 g/mol) and -0.7 (Mw ) 400 000 g/mol), changing above a critical concentration to -1.4, respectively. The investigated concentration range was cp ) 0.33 g/L to cp ) 45 g/L. The fast diffusion process, only reported for the low molecular weight, increases with increasing polyelectrolyte concentration. Sedla´k and Amis38 investigated the strong polyelectrolyte sodium poly(styrenesulfonate) (NaPSS) at three different molecular weights (Mw ) 5000, 100 000, and 1 200 000 g/mol) in the concentration range cp ) 0.01 g/L to cp ) 45.6 g/L. They found for all three samples a decrease of the slow diffusion coefficient with increasing polyelectrolyte concentration. The exponent of the polyelectrolyte concentration dependence for the (34) Sedla´k, M.; Kona´k, C.; Sˇ tepa´nek, P.; Jakesˇ, J. Polymer 1987, 28, 873. (35) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (36) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333. (37) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y.-H.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (38) Sedla´k, M.; Amis, E. J. J. Chem. Phys. 1992, 1, 826.
Walter et al.
three molecular weights was -0.096, -0.34, and -0.35, changing to -0.83 above a critical concentration. Schmidt et al.27 report about diffusion of cationic quaternized poly(2-vinylpyridine) (P2VP) of different molecular weights (from 109 000 to 2 260 000 g/mol) and degree of quaternization (40-100 mol %) in solution as a function of polyelectrolyte concentration, too. They likewise find a slow diffusion process with a coefficient Dssol decreasing with increasing polyelectrolyte concentration while the fast diffusion coefficient Df increases. Depending on molecular weight and concentration, the exponent of the concentration dependence of the slow diffusion process observed in ref 27 was between -0.1 and -1. Thus it seems to be possible that the diffusion of the polyampholyte toward the surface during adsorption may be determined by the slow diffusion process. (c) Influence of Salt Concentration. At different salt concentration adsorption of the polyampholyte at pH ) 6.7 reaches equilibrium after about 2-3 h, except for the measurement at cs ) 0.1 mol/L where adsorption did not come to an end after 1 week, as already mentioned. The diffusion coefficient, determined in the early state of adsorption, rises with increasing salt concentration since the screening by the added salt reduces the size of the individual particles in solution (Figure 4c). A linear fit in the double logarithmic plot yields an exponent of -0.73 ( 0.12. Again the slow diffusion process of polyelectrolytes in solution shows qualitatively the same dependence, for P2VP in the salt concentration range 10-5 to 10-3 mol/L, but no exponent of the dependence is reported.27 For the strong acidic polyelectrolyte sodium poly(styrenesulfonate) (NaPSS) a slight dependence of Dssol on cs is found at high molecular weight between cs ) 5 × 10-3 mol/L and cs ) 0.1 mol/L.39 Therefore again a correlation between the slow diffusion process of polyelectrolytes in solution and the diffusion of the polyampholyte toward the surface in the early state of adsorption may be concluded. Conclusion The adsorption behavior of the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) depending on the pH, polyampholyte, and salt concentration can be explained with reference to the adsorption of homopolyelectrolytes of the acidic and the basic block at the same conditions. By a variation of the polyampholyte and the salt concentration, an adsorption behavior similar to simple polyelectrolytes is found while by variation of pH a maximum of adsorbed amount at the isoelectric point of the polyampholyte is observed. Additional adsorption took place even at pH where the charge of the surface and the net charge of the polyampholyte have the same sign. This is in good agreement with theoretically predictions for polyampholytes with a statistical distribution of charges. The diffusion coefficient of the polyampholyte toward the surface could be measured in the early state of adsorption and shows as a function of pH, polyampholyte, and salt concentration qualitatively quite similar dependencies as the slow diffusion process (slow mode) of polyelectrolytes in solution. The complex adsorption behavior of polyampholytes consisting of diblock copolymer chains offers many possibilities for the manipulation of the adsorbed films. We therefore have also investigated the properties of the adsorbed polyampholyte films by atomic force microscopy (39) Sedla´k, M. J. Chem. Phys. 1996, 105, 10123.
Adsorption of Diblock Copolymers
and diffuse X-ray scattering, which is the topic of a subsequent publication. Acknowledgment. We thank H. Buchhammer from the Institute of Polymer Research IPR Dresden for zetapotential measurements and B. Mu¨ller, C. Rosenauer, and
Langmuir, Vol. 15, No. 4, 1999 1267
W. Ko¨hler for their help with light scattering measurements. This work was supported by the DFG Schwerpunkt “Polyelektrolyte” IIC10-322 1009. R.J. is grateful to the “Services Fe´de´raux des Affaires Scientifiques, Techniques et Culturelles” for general support to CERM (PAI 4/11). LA981178Q
