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Lateral Structures of Thin Films of Ampholytic Diblock Copolymers Adsorbed from Dilute Aqueous Solution at the Solid/Liquid Interface H. Walter,† P. Mu¨ller-Buschbaum,†,‡ J. S. Gutmann,†,§ C. Lorenz-Haas,† C. Harrats,†,| R. Je´roˆme,⊥ and M. Stamm*,† Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany, and Centre for Education and Research on Macromolecules, University of Lie` ge, Sart-Tilman B6, B-4000 Lie` ge, Belgium Received February 24, 1999. In Final Form: June 9, 1999 The lateral structures of dried thin films of the ampholytic diblock copolymer poly((methacrylic acid)block-((dimethylamino)ethyl methacrylate)) adsorbed from dilute aqueous solution onto silicon substrates were investigated by scanning force microscopy (SFM) and diffuse X-ray scattering. The adsorbed amount of polymer, as a function of pH, reveals a maximum near the isoelectric point (IEP) of the polyampholyte. Different lateral structures are determined for samples adsorbed at pH values above the maximum of the adsorbed amount, as compared to the ones adsorbed at pH values below this maximum. At a pH around the IEP, the polyampholyte precipitates and the layers are formed by adsorption of flocks with sizes in the micrometer range. Lateral structures measured by SFM are similarly detected by diffuse X-ray scattering. The later technique delivers a statistical description of the lateral surface structures averaged over a larger area. The structures investigated are related to the polyampholyte structure in solution and can be explained by the interplay of the electrostatic interactions of the two oppositely charged blocks of the polyampholyte and the charged surface, as well as by the dependencies of charge densities on pH.
Introduction Polyelectrolytes are used in many technological applications such as superabsorbers, washing powders, paper production, and many others.1 Adsorption of polyelectrolytes at the solid/liquid interface is often one of the most important steps in these applications. Polyampholytes and adsorption of polyampholytes also play an essential part in many biological processes.2 Polyelectrolytes and polyampholytes in solution have been investigated to some extent,3,4 but there are still a lot of open questions concerning the adsorption at the solid-liquid interface. Only one investigation on the adsorption of ampholytic block copolymers5 and just a few publications concerning SFM investigations of synthetic polyelectrolyte layers6-11 * To whom correspondence should be addressed. † Max-Planck-Institut fu ¨ r Polymerforschung. ‡ Present address: TU Mu ¨ nchen, Lehrstuhl E13, James-FrankStr. 1, D-85747 Garching, Germany. § Also connected to Universita ¨ t Hamburg, Germany, and GKSS Geesthacht, Germany. | Present address: Katholieke Universiteit Leuven, Afdeling Polymeerchemie, Celestijnenlaan 200F, B-3001 Heverlee, Belgium. ⊥ University of Lie ` ge. (1) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina, D. Polyelectrolytes; Hanser Publishers: Munich, 1994. (2) Drake, B.; Prater, C. B.; Weisenhorn, A. L.; Gould, A. C.; Albrecht, T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989, 243, 1586. (3) Fo¨rster, S.; Schmidt, M. Adv. Polym. Sci. 1995, 120, 53. (4) Kudaibergenov S. E. Adv. Polym. Sci. 1999, 144, 115. (5) Walter, H.; Harrats, C.; Mu¨ller-Buschbaum, P.; Je´roˆme, R.; Stamm, M. Langmuir 1999, 15, 1260. (6) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y.-H.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (7) Saremi, F.; Maassen, E.; Tieke, B.; Jordan, G.; Rammensee, W. Langmuir 1995, 11, 1068. (8) Stipp, S. L. S. Langmuir 1996, 12, 1884. (9) Akari, S.; Schrepp, W.; Horn, D. Langmuir 1996, 12, 857. (10) Akari, S.; Schrepp, W.; Horn, D. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1014. (11) Uchida, E.; Ikada, Y. Macromolecules 1997, 30, 5464.
have been reported up to now. For industrial utilization the understanding of adsorption and desorption of these polymers under different solution conditions is fundamental. Any successful application of such thin adsorbed polyampholyte layers depends on many factors such as their stability and lateral structure. The adsorption from dilute aqueous solutions of an ampholytic diblock copolymer at the solid/liquid interface is driven by the interplay of the long-range electrostatic attraction and repulsion between the positively and the negatively charged blocks on the one side and the charged surface of the substrate on the other side.12 Hydrophobic short range and sterical interactions can provide an additional contribution. Since the two blocks of the investigated polyampholyte are weak polyelectrolytes, the electrostatic interactions are hardly influenced by solution conditions such as pH or ionic strength. The degree of dissociation of the ionic groups, and therefore the charge density, depends on the pH for the two blocks of the polyampholyte13,14 and for the surface of the substrate.15 The two isoelectric points (IEP)sof the polyampholyte and of the substratesare important for the adsorption and desorption, as well as for the conformation of the chains in solution and at the surface.16,17 Consequently formation of quite different topographies of the adsorbed layers can be expected by varying solution conditions. We have studied the topographies of dried layers of the ampholytic diblock copolymer poly((methacrylic acid)block-((dimethylamino)ethyl methacrylate)), P(MAA-bDMAEMA), adsorbed onto silicon surfaces from dilute (12) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. Macromolecules 1997, 30, 4332. (13) Leyte, J. C.; Mandel, M. J. Polym. Sci. 1964, A2, 1879. (14) Hoogeveen N. G.; Cohen Stuart M. A.; Fleer G. J. J. Colloid Interface Sci. 1996, 182, 133. (15) Joppien, G. R. J. Phys. Chem. 1978, 82, 2210. (16) Kamiyama., Y.; Israelachvili, J. Macromolecules 1992, 25, 5081. (17) Eirich, F. R. J. Colloid Interface Sci. 1977, 58, 423.
10.1021/la990216i CCC: $15.00 © 1999 American Chemical Society Published on Web 08/19/1999
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aqueous solutions at different pH by scanning force microscopy (SFM) and diffuse X-ray scattering. To detect the influence of conformation of the chains in solution on the structures of the adsorbed layers, the transmission of red laser light in solution and the hydrodynamic radius of the ampholytic diblock copolymer were measured by light scattering as a function of pH. The structure at the surface will be compared to the solution properties and the adsorption behavior, which has been investigated recently.5 Experimental Section Materials. The precursor diblock copolymer poly((tert-butyl methacrylate)-block-((dimethylamino)ethyl methacrylate), P(tBMA-b-DMAEMA), was synthesized by anionic polymerization in THF at -78 °C as described in the literature.18,19 To obtain the ampholytic diblock copolymer poly((methacrylic acid)-block((dimethylamino)ethyl methacrylate)), P(MAA-b-DMAEMA), the tBMA monomers were hydrolyzed by dissolving the precursor polymer in dioxane (5-10% solution) in the presence of hydrochloric acid.20 More details about the polymerization and the hydrolysis can be found in the literature.5 The characteristics of the studied polyampholyte A-B/62 are as follows: molecular weight Mn ) 62 000 g/mol, Mw/Mn ) 1.12, and block composition 67% PMAA and 33% PDMAEMA. The molecular weight of the first PtBMA block was determined by size exclusion chromatography (SEC) based on PMMA standards. The composition of the precursor diblock copolymer was analyzed by nuclear magnetic resonance spectroscopy (1H NMR), which allowed the molecular weight of the second PDMAEMA block (and thus the weight of the whole diblock) to be calculated.21 Substrates for adsorption experiments were silicon 100 wafers with a native oxide layer of typically 2 nm. Prior to use, they were cleaned in a special manner. The first cleaning step was an ultrasonic bath with dichloromethane at about 50 °C. The samples were kept in this bath for 15 min. Afterward, they were washed with water. For all cleaning solutions and experiments, fresh Milli-Pore water was used. Next, the samples were put in an oxidation bath of a mixture of H2O2, NH3, and water (at a ratio of 1:1:15) at a temperature of 75 °C for about 20-30 min, depending on pollution level. The surfaces of the samples were cleaned from organic pollution when the size of the bubbles, evolving at the samples surface, changed significantly. After being rinsed several times with water, the samples were dried with clean nitrogen and stored in an oven at 50 °C. Null Ellipsometry. With a computer-controlled null ellipsometer in a vertical polarizer-compensator-sample-analyzer (PCSA) arrangement,22 the amount of adsorbed polymer was determined as a function of pH as reported elsewhere.5 The adsorbed amount obtained from measurements in solution and in air was equal within error for all investigated samples. For the samples investigated with SFM and diffuse X-ray scattering, adsorption was done at polyampholyte concentrations of 3.24 µmol/L (=0.2 g/L), which is well in the plateau of the adsorption isotherm. The salt concentration was fixed at 0.01 mol/L NaCl. Therefore, the influence of changing the pH by adding acid (HCl) or base (NaOH) on the ionic strength in solution is negligible in the investigated pH range. The native silicon oxide layer of the substrate is no longer stable above pH ) 10.23 Therefore only experiments at lower pH were performed. All adsorption experiments were done at room temperature. Light Scattering. The intensity of the laser light of the null ellipsometer passing through the solution in the Teflon cell was measured for the determination of the transmission. Adding acid (18) Creutz, S.; Teyssie´, P.; Je´roˆme, R. Macromolecules 1997, 30, 6. (19) Antoun, S.; Teyssie´, P.; Je´roˆme, R. Macromolecules 1997, 30, 1556. (20) Creutz, S.; van Stam, J.; Antoun, S.; De Schryver, F. C.; Je´roˆme, R. Macromolecules 1997, 30, 4078. (21) Orth, J.; Meyer, W. H.; Bellmann, C.; Wegner, G. Acta Polym. 1997, 48, 490. (22) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (23) Axelos, M. A. V.; Tchoubar, D.; Bottero, J. Y. Langmuir 1989, 5, 1186.
Langmuir, Vol. 15, No. 20, 1999 6985 or base to the aqueous solution led to a variation of pH and, in a small pH range near the IEP of the polyampholyte, to the formation of complexes (clusters). As a consequence, the intensity of the laser light was reduced. Therefore, the pH range of insolubility and the IEP of the polyampholyte could be measured. Normalization of the measured intensity to its maximum value provides the transmission in percent. Micelle formation was investigated by dynamic light scattering (DLS) measurements performed on a commercial ALV 3000 digital correlator with a 400 mW krypton ion laser (λ ) 647 nm) as a light source. Autocorrelation functions, gq(t), were measured at the already mentioned polyampholyte and salt concentration for several pH values. The scattering angle was 90° and temperature was set to 22 °C. The autocorrelation function of the scattered intensity of monodispersed spheres can be well represented by the singleexponential function gq(t) ) exp(-Dq2t),24 where D 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, frequently two diffusion processes (slow and fast mode) are detected. In the experimental correlation function, first the slow diffusion process (Ds) is fitted by a sum of two exponential functions. The remaining fast diffusion process (Df) is fitted by just one exponential function, as described elsewhere.25 The free particle diffusion coefficient and the hydrodynamic radius Rh of particles are connected for both diffusion processes by the Stokes-Einstein relation
Rh ) kT/6πηD
(1)
where k is the Boltzmann constant, T the temperature, and η the viscosity of the solvent. Scanning Force Microscopy. The topography of the dried adsorbed films was studied with a commercially available SFM (Autoprobe CP/Park Scientific Instruments). Microfabricated gold-coated Si cantilevers were used. To minimize tip-induced damages of the soft polyampholyte layers, all scans were performed in noncontact mode at a frequency in the range of 315-410 kHz, depending on the individual tip resonance. The samples were scanned in air using different scanning areas from 0.8 × 0.8 µm2 to 20 × 20 µm2. The topographies were measured at different positions of the surface of the samples, and several samples, prepared under the same conditions, were studied to ensure reproducibility of the results. Line scans in the SFM pictures contain information about the radius r in the plane of the surface and the height h of adsorbed structures. From these data it is possible to calculate the volume of the structures V ) πh(3r2 + h2)/6 by modeling them as spherical caps. With this volume and the mass density (F ) 1.25 g/cm3), just as the molecular weight of the polyampholyte, a rough estimation of the number of molecules per adsorbed structure
p ) FVNA/Mn
(2)
can be given. NA is Avogadro’s number. The uncertainty of these numbers is about 30%.26 Diffuse X-ray Scattering. While SFM gives only local information about lateral structures (at an area of typically some µm2), diffuse X-ray scattering delivers this information at typical length scales on the order of the illuminated sample area (some mm2, depending on the angle of incidence). Therefore these two techniques complement each other. Dried adsorbed polyampholyte layers were measured at the BW4 USAX beamline of the DORIS III storage ring at HASYLAB/DESY in Hamburg.27,28 The wavelength was set to 1.13 Å. At a distance of 11.34 m from the samples, the off-specular intensity was recorded with a twodimensional detector which enables a resolution of the parallel in-plane momentum transfer qy better than 2 × 10-3 nm-1. In (24) Pecora, R.; Berne, B. J. Dynamic Light Scattering; John Wiley & Sons: New York, 1976. (25) Fo¨rster, S.; Schmidt, M.; Antonietti, M. Polymer 1990, 31, 781. (26) Meiners, J. C.; Quintel-Ritzi, A.; Mlynek, J.; Elbs, H.; Krausch, G. Macromolecules 1997, 30, 4945. (27) Gehrke, R. Rev. Sci. Instrum. 1992, 63, 455. (28) Mu¨ller-Buschbaum, P.; Vanhoorne, P.; Scheumann, V.; Stamm, M. Europhys. Lett. 1997, 40, 655.
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Figure 1. Adsorbed amount A of sample A-B/62 (circles) 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 (lower part) indicate where the silicon surface (S) and the polyampholyte (P) are carrying a positive (S+, P+) or negative (S-, P-) net charge. this notation, the xy-plane denotes the sample surface. The samples were measured in a reflection geometry, to realize diffuse X-ray scattering under ultra-small-angle scattering conditions.29 A setup of high-quality entrance slits was used, and the pathway was evacuated. At a fixed angle of incidence, Ri, the twodimensional intensity distribution consist of several vertical and horizontal slices. Vertical slices correspond to so-called detector scans,30 which are mainly qz-dependent. They deliver, for example, information about the correlation between interfaces. The in-plane information of the most prominent structure size, ξ, of the adsorbed layers is yielded from the qy-dependence. It is obtained from peak positions qy* in horizontal slices corresponding to “out-of-plane” scans by
ξ ) 2π/qy*
(3)
Examining horizontal slices at an angle R ) Ri + Rc, where Rc is the critical angle of total reflection for P(MAA-b-DMAEMA), enhances the resolution for the lateral structures of the adsorbed polyampholyte layers. All out-of-plane scans are horizontal slices at this angle. Without the restriction of common diffuse X-ray scattering imposed by the sample surface, the used geometry permits a high parallel momentum transfer qy. Therefore, lateral length scales in the range 15 nm < ξ < 3000 nm can be investigated.31,32 The limits of the detectable in-plane length scale, ξ, are given by the geometric setup (like the sample-detector distance and the detector size), as well as by the resolution, given by slits and monochromator settings.
Results The adsorbed amount of the studied polyampholyte P(MAA-b-DMAEMA) shows as a function of pH a distinct maximum near the IEP of the polyampholyte,5 as can be seen in Figure 1. This adsorption behavior is similarly observed for polyampholytes with a statistical distribution of charges.17 With the maximum of adsorbed amount a sharp minimum in the transmission of the solution (T < 20%) is correlated between pH ) 5.4 and pH ) 6.3. In this pH range, macroscopic aggregates are visible in the solution, because of the precipitation of the polyampholyte.4 Between pH values of 4.2 and 5.4, as well as for (29) Salditt, T.; Metzger, T. H.; Peisl, J. Phys. Rev. Lett. 1994, 73, 2228. (30) Salditt, T.; Metzger, T. H.; Peisl, J.; Goerigk, G. J. J. Phys. D: Appl. Phys. 1995, 28, A236. (31) Salditt, T.; Metzger, T. H.; Brandt, C.; Klemradt, U.; Peisl, J. Phys. Rev. B 1995, 51, 5617. (32) Salditt, T.; Metzger, T. H.; Peisl, J.; Reinker, B.; Koske, M.; Samwer, K. Europhys. Lett. 1995, 32, 331.
Figure 2. Comparison of structures at the surface and in solution: Diameter of lateral structures on top of dried adsorbed layers as obtained from SFM (solid cycles) and diffuse X-ray scattering investigations (open cycles) and diameter of characteristic micelles in solution (stars) and transmission of solution T (dashed line) from light scattering as a function of pH. The error bars of the lateral structure sizes at pH ) 7.3 and pH ) 7.6 are of the size of the open cycles. The solid lines are guides for the eye. The bars and arrows (lower part) indicate where the silicon surface (S) and the polyampholyte (P) are carrying a positive (S+, P+) or negative (S-, P-) net charge.
pH values between 6.3 and 6.8, the transmission is close to, but still less than, 100%. There are nevertheless a few smaller clusters in solution caused by the increasing intraand interchain interactions for pH values approaching the IEP of the polyampholyte,16 but no precipitation of the polyampholyte occurs. For all other pH values the transmission is essentially 100%. Dried samples adsorbed in the three pH ranges, above, close to, and below the IEP, reveal lateral structures of quite different size (see Figure 2), as will be discussed in the following sections. 1. Lateral Structures at pH above the IEP. The investigated layers of sample A-B/62, adsorbed at pH above the IEP, were all built up by blobs with a lateral size of about 40 nm. The surfaces of these samples were completely covered with such blobs. At pH ) 7.3-7.6 the structures were very dense packed and quite unique in size (39 ( 5 nm) and height (5 ( 1 nm), as can be seen in the SFM topography image of Figure 3a. Larger scanning areas (up to 20 × 20 µm2) and surfaces of other samples, prepared under the same conditions, exhibit the same structures. Out-of-plane scans of diffuse X-ray scattering measurements of these samples (see upper curve in Figure 3b) displayed a well-pronounced peak at a momentum transfer qy* ) 0.167 nm-1 (marked with an arrow), which is characteristic for the length scale ξ ) 38 ( 5 nm. Therefore, the structures, which are observed in the SFM pictures, exist all over the samples and can be regarded as representative for this pH-regime. As the SFM measurements reveal the same structures before and after the X-ray investigation, the radiation did not significantly damage the layers. Above pH ) 7.6 adsorbed spherical structures of about 40 nm in size are still found, but they are less dense packed and no longer as unique in size as shown in Figure 3a. At pH values between 6.4 and 6.9, additional larger adsorbed blobs (typically 136 ( 15 nm in size and 20 ( 4 nm in height) were found by SFM. They are placed on top of the layer, built up by the 40 nm structures, but these smaller structures are no longer as unique in size and as numerous as those at pH ) 7.6. Simultaneously, a small peak (marked with an arrow) is visible in the diffusely scattered X-ray intensity at qy* ) 0.048 nm-1 (see Figure 3b lower curve). It corresponds to a charac-
Thin Films of Diblock Copolymers
a
b
Figure 3. (a) Topography picture from SFM investigations of the dried layer of sample A-B/62 adsorbed at pH ) 7.6 scanned in air. Scanning area was 2 × 2 µm2 and measurements are performed in noncontact mode. In the lower part of the figure a line scan through the upper figure at the location of the white line is presented. (b) Out-of-plane scans of diffuse X-ray scattering investigations of dried layer of sample A-B/62 adsorbed at pH ) 7.6 (upper curve) and pH ) 6.8 (lower curve). Angle of incidence was 0.65°. The diffusely reflected intensity as a function of the parallel momentum transfer qy was integrated over ∆qz ) (5 × 10-3 nm-1. The lower curve is shifted for clarity. The positions qy* of the characteristic lateral structures are marked with arrows. The dashed lines are guides for the eye, and the resolution limit of the instrument is also indicated.
teristic length scale of ξ ) 131 ( 20 nm. Thus, both techniques yield the same lateral structure size again. As
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the peak in the diffuse scattered intensity is distinctly larger for the sample adsorbed at pH ) 7.6 compared to the one adsorbed at pH ) 6.8, there should be more of the smaller structures present at the surface of the former sample, as of the larger ones at the surface of the latter sample. This observation is confirmed by the SFM topography pictures. Dynamic light scattering measurements of solutions of sample A-B/62 deliver, for all investigated samples, a hydrodynamic radius of single chains of 4.5 nm, independent of pH. On the other hand larger clusters or micelles were observed, too. Above the IEP of the polyampholyte two different sizes of such clusters or micelles were found in solution. Between pH ) 6.4 and pH ) 8.1 the smaller ones of these clusters or micelles reveal a diameter of nearly the same size as the investigated lateral structures on the surface of the adsorbed layers (see comparison in Figure 2). At pH ) 9.0 the size of the micelles is about three times larger than the size of the adsorbed structures. 2. Lateral Structures at pH close to the IEP. As the polyampholyte precipitates at its IEP, the transmission through the solution is quite low for 5.4 e pH e 6.3, while the adsorbed amount is at its maximum (Figure 1). In this pH range, macroscopic aggregation with a flock size up to a few micrometers occurs. The adsorption of these flocks is visible by eye and has some resemblance to snowing. As it is not possible to measure structures of some micrometers in height with the used SFM, just samples adsorbed at pH ) 5.4, at the edge of the pH range of precipitation (where slightly smaller aggregates adsorb) were studied. This adsorbed layer is formed by large structures of 370 ( 80 nm in diameter and, between them, by smaller ones of 120 ( 30 nm (see Figure 2). The average height of these structures is 100 ( 20 nm and 20 ( 10 nm, respectively. SFM pictures show that the large structures are built up by the small ones. Investigations with an optical microscope prove that the large structures are adsorbed all over the sample. Diffuse X-ray measurements of samples adsorbed near the IEP of the polyampholyte were not performed, as they are not very useful due to the large surface roughness (σRMS ) 20 nm). Also, the lateral structures are not homogeneous in size, and therefore, no distinct peak can be expected. 3. Lateral Structures at pH below the IEP. Below the IEP of the polyampholyte the adsorbed layers show two regimes of distinct different types of lateral structures. Above the isoelectric point of the silicon substrate (pHIEP ) 3.9) the layers are mainly formed by blobs with increasing lateral diameter with increasing pH (see Figure 2). All layers cover the surface of the substrate completely. For example, at pH ) 4.0 the adsorbed layer is built up by blobs with a diameter of 55 ( 13 nm and a height of 8 ( 3 nm (Figure 4a). Larger scanning areas show the same topography. The size distribution of these blobs is larger, and they are not arranged as regular on the surface as the structures of the sample adsorbed at pH ) 7.6. In the diffusely scattered intensity, the blobs are observed as a broad shoulder instead of a distinct peak, as in the case of the more regular structures. The position qy* ) 0.116 nm-1 (marked with an arrow in Figure 4b) corresponds to a characteristic length scale of ξ ) 54 ( 15 nm. Thus, both techniques deliver, within error, the same structural size. These structures can be regarded as representative for the whole sample. Above pH ) 4.0 and below pH ) 4.8 the adsorbed blobs increasingly tend to form clusters at the surface, but still all these blobs are placed in one plane. No formation of multilayers was observed by SFM. In contrast there are, at pH ) 4.8 like
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Walter et al.
c
b
Figure 4. (a) Topography picture from SFM investigations of the dried layer of sample A-B/62 adsorbed at pH ) 4.0 scanned in air. Scanning area was 2 × 2 µm2 and measurements are performed in noncontact mode. In the lower part of the figure a line scan through the upper figure at the location of the white line is shown. (b) Out-of-plane scans of diffuse X-ray scattering investigations of a dried layer of sample A-B/62 adsorbed at pH ) 4.0 (upper curve) and pH ) 2.6 (lower curve). Angle of incidence was 0.65°. The diffusely reflected intensity as a function of the parallel momentum transfer qy was integrated over ∆qz ) (5 × 10-3 nm-1. The lower curve is shifted for clarity. The positions qy* of the characteristic lateral structures are marked with arrows. The dashed lines are guides for the eye, and the resolution limit of the instrument is also indicated. (c) Topography picture from SFM investigations of the dried layer of sample A-B/62 adsorbed at pH ) 2.6 scanned in air. Scanning area was 2 × 2 µm2 and measurements are performed in noncontact mode. In the lower part of the figure a line scan through the upper figure at the location of the white line is given.
at pH values close to the IEP, larger aggregates (size 190 ( 50 nm; height 40 ( 16 nm) on top of the dried sample, besides the individual adsorbed blobs, but just a few. Below pH ) 3.9 adsorption of blobs still occurs, but the surface of the substrate is no longer completely covered with polymer (see Figure 4c). The coated area at pH ) 2.6 is only about 10%. The average size and height of the
structures of these sample are 125 ( 19 and 8 ( 2 nm, respectively. As a result of the largely uncoated area between the adsorbed blobs, the out-of-plane scan exhibits no visible characteristics for the structures observed with the SFM. The scattered intensity just decreases monotonically already close to the resolution limit (see Figure 4b lower curve).
Thin Films of Diblock Copolymers
Figure 5. Schematic model for the adsorption of micelles. The diameters of the micelles in solution (2Rh) and of the adsorbed lateral structures (2Rads) are marked with arrows.
Unfortunately, no dynamic light scattering data are available for 4.0 < pH < IEP, because of the turbidity of the solution. For pH e 4.0, one typical size of clusters or micelles in solution was found. Below pH ) 4.0, these micelles reveal the same size as the adsorbed lateral structures (see Figure 2). At pH ) 4.0, the size of the micelles in solution is distinctly smaller than the blob size at the surface. Discussion 1. Lateral Structures at pH above the IEP. As already mentioned, at 6.4 e pH e 6.9 the transmission is a little bit lower than 100%. Thus the speculation is obvious that aggregates, clusters, or micelles can play an important role in the adsorption process. This assumption is supported by ellipsometric investigations of the kinetics of the adsorption process for this system.5 The apparent diffusion coefficient during adsorption at the surface shows qualitatively the same dependencies on pH, polyampholyte, and salt concentration as the slow diffusion coefficient of polyelectrolytes in solution (slow mode). This slow mode is correlated to the diffusion of clusters in solution.25 The adsorption of micelles or clusters is driven by the long-range electrostatic attraction between the surface and the oppositely charged block of the polyampholyte.12 As the diameter of the structures in solution, measured by light scattering, is for 6.4 e pH e 8.1 approximately the same as the lateral diameter of the adsorbed structures (see Figure 2), the micelles or clusters adsorb at the silicon surface without significant changes in size parallel to the surface, even during drying. This may be due to the fact that immediately after the first contact of these structures to the surface relatively tight binding sites develop, which fix the lateral size at the surface. The adsorption of micelles is schematically drawn in Figure 5. The volume of the large adsorbed structures with a size of about 135 nm is five times smaller than the volume of the clusters or micelles in solution, and they are built up by around 1800 chains. This collapse factor is much smaller, and the number of chains distinctly larger, than those reported in the literature, even for adsorbed micelles of neutral chains in nonpolar solvent, where only shortrange interactions play a role.26,33-36 In contrast to our investigations, these studies deal mostly with a much higher polymer concentration and a distinctly larger adsorbed amount and thickness of the adsorbed layers. (33) Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Khougaz, K.; Eisenberg, A.; Lennox, R. B.; Krausch, G. J. Am. Chem. Soc. 1996, 118, 10892. (34) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (35) Fossum, E.; Matyjaszewski, K.; Sheiko, S. S.; Mo¨ller, M. Macromolecules 1997, 30, 1765. (36) Koutsos, V.; van der Vegte, E. W.; Pelletier, E.; Stamouli, A.; Hadziioannou, G. Macromolecules 1997, 30, 4719.
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The authors interpret their data, obtained from SFM and light scattering, by the adsorption of micelles on top of a brushlike monolayer of single chains. The highest reported number of chains per adsorbed micelle is p ) 358 for an asymmetric diblock copolymer of poly(styrene-block2-vinylpyridine) with a molecular weight of 140 000 g/mol.26 Thus, the large adsorbed structures are clusters, built in solution due to the electrostatic attraction near the IEP between the oppositely charged blocks of the chains. These structures are no micelles, which consist of an insoluble block in the core and a swollen soluble block in the corona.37 The adsorbed structures, with a size of about 40 nm, are collapsed in their volume by a factor of approximately 20. These structures are micelles, since they consist of around 35 chains, which is of the same order as results obtained for micelles of the triblock polyampholyte poly(((dimethylamino)ethyl methacrylate)-block-(methyl methacrylate)-block-(methacrylic acid)), with a molecular weight of 4000 g/mol and a composition of 1:1:1.38 At pH ) 5 this system forms micelles, consisting of 23 polyampholyte chains. The higher collapse factor of the 40 nm structures is to be expected, since micelles are more swollen in solution as the clusters are near the IEP, which is due to the higher charge density of the PMAA block. The high regularity in the arrangement of the adsorbed micelles (see Figure 3a) is caused by the electrostatic repulsion between the negative charged micelles combined with the slow adsorption. Besides the layers adsorbed at pH around 7.6 are very similar to monolayer films of latex particles with a size of 80 ( 12 nm prepared by evaporation of a latex particle containing solution on a hydrophilic substrate at 20 °C.39 After adsorption at pH ) 9.0, only a few structures with a size of about 40 nm are found by SFM on top of silicon substrates. These structures are distinctly flatter than the one observed at lower pH and they reveal a broad size distribution. Their volume is a factor of 450 smaller than one of the micelles in solution. Thus, it is not easy to propose adsorption of micelles at this pH. Since the charge density of the negatively charged silicon surface rises quickly with increasing pH for pH > 5,15 as well as the charge density of the negatively charged PMAA block for pH > 6,13 the electrostatic repulsion between the surface and the micelles rises with increasing pH too. Simultaneously, the charge density of the positively charged PDMAEMA block and, consequently, the attractive part of the electrostatic interaction to the surface tend to zero. This leads to an increasing adsorption time and a decline in the adsorbed amount.5 Due to the quickly increasing electrostatic repulsion, the micelles might be dissolved at pH g 9.0 before they can adsorb, as is already found for micelles of the triblock polyampholyte poly(((dimethylamino)ethyl methacrylate)-block-(methyl methacrylate)block-(methacrylic acid)) in solution at extremely high or low pH.38 Thus, at pH ) 9.0 just single chains would adsorb at the surface and form the observed structures during adsorption or during drying. This model is supported by SFM investigations of layers of diblock copolymers of poly(styrene-block-butadiene), P(S-b-B), adsorbed from toluene, where the PB block was functionalized for adsorption.40 The nonadsorbing PS block is expanded, since toluene is a good solvent for it, while the PB with the sticky urazole groups is not. Siqueira et al. report a quite (37) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (38) Patrickios C. S.; Hertler, W. R.; Abbott, N. L.; Hatton, T. A. Macromolecules 1994, 27, 930. (39) Rakers, S.; Chi, L. F.; Fuchs, H. Langmuir 1997, 13, 7121. (40) Siqueira, D. F.; Ko¨hler, K.; Stamm, M. Langmuir 1995, 11, 3092.
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similar topography on top of dried layers, which were built by single-chain adsorption. However charges do not play any role there. Hydrophilic-hydrophobic block copolymersspoly((tert-butylstyrene)-block-(styrenesulfonate)), P(tBS-b-SS)swith a highly charged, not adsorbing polyelectrolyte block exhibit, after adsorption on silicon substrates from aqueous solution, a topography similar to the one of the sample adsorbed at pH ) 9.0, too.6 Amiel et al. discuss likewise just single chain adsorption. 2. Lateral Structures at pH near the IEP. The smaller adsorbed structures of about 120 nm in size consist of about p ) 1400 chains, which is of the same order as the number of chains of the adsorbed clusters investigated above the IEP of the polyampholyte. These clusters built up the larger flocks, which consist on average of 70 000 chains. Thus, the electrostatic attraction is just near the IEP, where precipitation occurs, strong enough to enhance sticking of the smaller cluster and the formation of the larger flocks, while outside this pH range of precipitation the formation of the clusters indicate a different process. 3. Lateral Structures at pH below the IEP. Due to the increasing electrostatic repulsion between different chains, as a result of the increasing positive net charge of the polyampholyte, the size of the adsorbing clusters declines with decreasing pH between the two isoelectric points. For example, at pH ) 4.0 the adsorbed clusters consist only of p ) 75 chains. For the same reason, the trend to cluster formation in the plane of the surface of the substrate diminishes. The reason for the smaller regularity in the arrangement of the adsorbed structures in this pH range, compared to the structures adsorbed at pH above the IEP of the polyampholyte (see Figure 3a), might be the much faster adsorption at pH with oppositely charged polyampholyte and surface (see arrows below Figure 1 or 2), as compared to the case where both carry charges of the same sign.5 Due to the attractive interaction to the surface and the fast adsorption process, the clusters adsorb at the surface in a quite statistical fashion, which is less influenced by already adsorbed clusters. The structures in solution at pH ) 4.0, which seem to consist of just around six chains, are much smaller than the adsorbed ones. The volume of the adsorbed structures is two times larger than one of the structures in solution. Therefore, both should not be correlated. The high amount of these small structures in solution probably masks the contribution of the 55 nm clusters to the scattered intensity in the DLS measurement, and so it was not possible to detect the adsorbing clusters in solution. Below the IEP of the substrate the adsorption behavior can be explained by a model analogous to the one for pH above the IEP of the polyampholyte. The net charge of the polyampholyte as well as the charge of the surface is positive and adsorption is slow. Again micelles, which consist of a charged PDMAEMA block in the corona and a more or less neutral PMAA block in the core, adsorb at the substrate without changing their lateral dimension (see Figures 2 and 5). As the charge density of the PMAA block decreases and the charge densities of the PDMAEMA block and of the surface increase with decreasing pH, the size of the micelles in solution as well as at the surface
Walter et al.
increases. Simultaneously the number of adsorbed micelles decreases and the adsorbed amount diminishes. Since the positive charge density of the silicon surface at pH ) 2.6 is distinctly lower than the negative charge density at pH ) 9.0,15 even the large micelles at pH ) 2.6, which consist of around p ) 500 chains, are not dissolved before they can adsorb. Below a certain pH the electrostatic repulsion between the micelles and the surface becomes too strong and no adsorption occurs anymore.5 Conclusion Thin layers of the ampholytic diblock copolymer P(MAAb-DMAEMA) with a block composition of 67% and 33%, respectively, show distinctly different lateral structures adsorbed on silicon substrates as a function of pH. These structures are explained in the investigated pH range by the electrostatic interaction between clusters and micelles in solution, formed by the chains of the polyampholyte, and the charged surface. For the first time, adsorption of micelles of diblock copolymers from dilute aqueous solutions has been investigated. Above the IEP of the polyampholyte the adsorbed layer is mainly created by densely packed micelles, which are unique in size. Near the IEP larger clusters of polyampholyte chains are formed in solution, which reveal a broad size distribution due to electrostatic inter- and intrachain attraction. These aggregates adsorb on the surface of the substrate with some resemblance to snowing. Below the IEP of the polyampholyte and above the IEP of the substrate, smaller clusters adsorb on top of the substrate in a nonregular manner, due to the attractive electrostatic interaction and the fast adsorption. Below the IEP of the substrate, again micelles adsorb, but the surface is no longer completely covered with polymer. Thus the surface is in all investigated cases laterally structured. The subject of ongoing work is to investigate on one hand the influence of the concentration of the polyampholyte on the structuring of the adsorbed layers, since it is known that micelle formation occurs in solution only above a critical polymer concentration.26 On the other hand the effect of the temperature during adsorption is an interesting parameter, since it is known that a lowering of temperature can lead to a higher order in monolayer films of particles.39 Acknowledgment. We owe many thanks to S. Cunis and G. von Krosigk for their technical help at the BW4 beamline and R. Gehrke for his general support of the experiment at HASYLAB/DESY. Further we thank B. Mu¨ller, C. Rosenauer, and W. Ko¨hler for their assistance with light scattering measurements and H. Buchhammer from the Institute of Polymer Research IPR Dresden for zeta potential 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). J.S.G. acknowledges financial support by GKSS project VG.1.01.G.01-HS3, and C.L.-H. by Graduiertenkolleg “Physik und Chemie Supramolekularer Systeme” at Mainz University. LA990216I
