Macromolecules 2000, 33, 4213-4219
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pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes S. S. Shiratori† and M. F. Rubner*,‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Applied Physics and Physico-informatics, Keio University, Yokohama 223, Japan Received September 28, 1999
ABSTRACT: A detailed study of the role that solution pH plays in the layer-by-layer processing of the weak polyelectrolytes poly(acrylic acid) and poly(allylamine hydrochoride) was carried out. It was found that dramatically different polymer adsorption behavior is observed as one systematically increases (or decreases) the charge density of a weak polyelectrolyte including transitions from very thick adsorbed layers (ca. 80 Å) to very thin adsorbed layers (ca. 4 Å) over a very narrow pH range. By controlling pH, it is possible to vary the thickness of an adsorbed polycation or polyanion layer from 5 to 80 Å. In addition, control over the bulk and surface composition of the resultant multilayer thin films is readily achieved via simple pH adjustments. These studies have provided new insights into the polyelectrolyte sequential adsorption process and have already opened up some interesting technological applications.
Introduction Polyelectrolyte complexes, formed by the mixing of dilute solutions of a polycation and a polyanion, have been extensively studied in past years and are known to exhibit a unique combination of physical properties due to their ionically cross-linked nature.1 These interesting polymeric salts, however, are oftentimes insoluble and hence very difficult to manipulate into useable forms such as uniform thin film coatings. A novel layerby-layer deposition process initially introduced by Decher and co-workers2 has changed this situation. In this process, polyelectrolyte complexes are fabricated one molecular layer at a time via a simple sequential adsorption procedure. By repeatedly dipping a substrate into a dilute polycation solution followed by a dilute polyanion solution, it is possible to fabricate highly uniform polyelectrolyte multilayer thin films with precisely controlled thicknesses and molecular architectures. Currently, this approach has been utilized to assemble a wide variety of quite different materials into multilayer thin films.2 In general, most researchers have utilized strong polyacids and polybases to construct these multilayer thin films. With such materials, adding salt to the polyelectrolyte dipping solutions best controls the thickness of an adsorbed layer. Our approach has been to use weak polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) whereby control over the layer thickness and molecular organization of an adsorbed polymer chain can be achieved by simple adjustments of the pH of the dipping solutions.3 In this case, pH controls the linear charge density of an adsorbing polymer as well as the charge density of the previously adsorbed polymer layer. As will be further demonstrated in this paper, the net result is an unprecedented ability to control the blending of a polycation and polyanion at the molecular level. In addition, these studies have revealed that dramatic changes in the thickness of an adsorbed layer can be † ‡
Keio University. Massachusetts Institute of Technology.
induced by very small changes in the pH of the dipping solutions. An understanding of the molecular origin of these thickness transitions is expected to provide new insights into the basic polymer physics of the sequential adsorption process as well as to provide new possibilities for their technological application. For example, soon we will show that it is possible to fabricate uniform, microporous thin films from these weak polyelectrolyte multilayers. Experimental Section Poly(acrylic acid) (Mw ) 90 000) (PAA) was obtained from Polysciences. Poly(allylamine hydrochloride) (Mw ) 55 00065 000) (PAH) and the methylene blue dye were obtained from Aldrich. All polyelectrolytes and the dye were used as received without further purification. Polyelectrolyte dipping solutions of 10-2 M (based on the repeat unit molecular weight) were made from 18 MΩ Millipore water and pH adjusted with either HCl or NaOH. Details concerning substrate preparation, thickness measurements, contact angle measurements, methylene blue staining, and the automated layer-by-layer dipping process can be found in a previous paper.3 To avoid substrate effects, incremental thickness measurements were started on films containing a minimum of 10 bilayers. After the substrates were dipped into a polyelectrolyte solution, they were rinsed in three separate bins of pH neutral water (pH of 5.5-6.5). No drying step was used in the dipping procedure. All multilayer films were dried at 90 °C for 1 h prior to any measurements. Surface roughness measurements were done on 20 layer films deposited on silicon wafers with a Dimension 3000 scanning probe microscope (Digital Instrument). A silicon cantilever was used for all measurements. The spring constant of the cantilever was 20-100 N/m. Typically, the surface morphology of three 10 × 10 mm spots near the center of each sample was observed by the tapping mode of the scanning probe microscope.
Results and Discussion An automatic dipping method was used to fabricate a number of different PAH/PAA multilayer thin films on both silicon wafers and hydrophilic glass slides. The pH of the PAH and PAA dipping solutions was systematically varied from 2.5 to 9.0 in order to determine how
10.1021/ma991645q CCC: $19.00 © 2000 American Chemical Society Published on Web 04/22/2000
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Figure 2. Average incremental thickness contributed by a PAA and PAH adsorbed layer as function of solution pH. Both the PAH and PAA dipping solutions in this case were at the same pH. Solid line represents the PAA layer thickness, and the dashed line is the PAH layer thickness. Figure 1. Complete pH matrix showing the average incremental thickness contributed by a PAH/PAA bilayer as a function of dipping solution pH.
dipping solution pH influences layer thickness and organization. Figure 1 shows the full pH matrix generated in this study. This figure displays the average incremental thickness contributed to each multilayer thin film by the bilayer building block (polycation plus polyanion: measured on dried films). A quick survey reveals that the deposition of this layer pair is dramatically influenced by the pH of the individual dipping solutions. By simply controlling pH, it is possible to deposit unusually thick bilayers (>120 Å) or very thin bilayers (40 Å/layer) is observed in region IV of the pH matrix. In this pH region, the PAA chains are always adsorbing in their fully ionized state whereas the degree of ionization of the PAH chains starts to decrease with increasing pH. Again, we have the situation that a fully charged chain is alternately adsorbed onto a nearly fully charged chain, the net result being the creation of unusually thick PAA and PAH layers. Clearly this thickness transition from very thin adsorbed layers to very thick adsorbed layers does not depend on which polyelectrolyte is in the fully charged state but only on the fact that a fully ionized chain and a nearly fully ionized chain are being alternately deposited. An exploration beyond pH 9 to determine if this region completely mirrors region I and II is not possible due to solubility and film quality/precipitation problems. To our knowledge, a thickness transition of this type has not been previously reported in any single layer or multilayer adsorption studies on polyelectrolytes. To understand the driving force behind this behavior, Barrett et al. have recently carried out model studies involving the adsorption of fully charged PAA chains onto methoxysilane self-assembled monolayer surfaces (SAMs) with pH-controllable surface charge densities of cationic ammonium groups.6 These studies also reveal an abrupt transition from a molecularly thin adsorbed layer (ca. 5 Å) to a much thicker adsorbed layer (ca. 25 Å) over a very narrow pH range of about a half a pH unit. A thermodynamic model that captures all of the features observed experimentally was put forth to explain this behavior.6 In essence, the model indicates that, at a critical high surface charge density (modeled as neutral “stickers”), the entropic penalty for spreading a chain into a flat conformation on the surface (molecularly thin layer) is overcome by the enthalpic gain to the free energy of adsorption. As the surface charge density decreases below this point, however, the energy gain for spreading a chain over the surface is not sufficient to overcome the loss in configurational energy, and a sharp transition to a thicker layer with a high segmental population of loops occurs. As observed experimentally, this transition to a thick layer was predicted to occur when the surface charge density dropped somewhat below its fully charged state. In the case of the multilayer thin films of PAH/PAA, the interaction between the two sequentially adsorbed polyions must also be considered. We surmise that two fully charged chains of this type in the absence of added salt will strive to form a cooperatively stitched 1:1 polyelectrolyte complex with extended sections of polycation/polyanion double-strand like units. Such “zippedup” structures are known to occur in the waterinsoluble, stoichiometric polyelectrolyte complexes formed by the mixing of solutions containing two oppositely and fully charged polyions.1,7 In the layer-by-layer deposition process, these insoluble complexes are constructed one layer at a time. The net result in the case of fully charged chains is the deposition of very thin adsorbed layers that are highly interpenetrated and lying essentially flat within the multilayer. In support of this
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description, we have found that the thickness of an adsorbed polymer layer deposited when both macromolecules are fully charged (for example, when PAH and PAA are deposited at a pH of 6.5) is independent of the molecular weight of the adsorbing polymer over a range of at least 3000-106 g/mol. Hence, larger molecular weight chains simply spread out and occupy more surface area without increasing the thickness of the adsorbed layer. When a fully ionized chain and a nearly fully ionized chain are alternately deposited, this cooperative zipping process is frustrated somewhat and the chains cannot spread out flat over the surface but instead adopt a conformational arrangement of dense loops that extend away from the surface leading to the deposition of much thicker layers. The layers within the film are still highly interpenetrated but organized in a different fashion. As expected for a loopy conformational arrangement,8 the thickness of an adsorbed layer deposited in region II (for example with the dipping solutions at a pH 5.0) is dependent on the molecular weight of the adsorbing polymer chain. Preliminary results indicate that the layer thickness in this case scales approximately as T ∝ M0.3. The idea that the polycation/polyanion chains are still highly interpenetrated (as opposed to forming discrete layers) is supported by measurements of advancing water contact angles and methylene blue staining studies. We have previously shown3 that such measurements provide complementary information about the composition of the multilayer surface. Contact angle measurements made on films fabricated with the dipping solutions set at pH 5.5-8.5 (regions II-IV) all indicate that the surface is comprised of an approximately equal volume fraction of segments from both polymers regardless of which polymer is the outermost layer. In all cases, an advancing water contact angle of 25 ( 6° is observed. Even at a pH of 5.0, where the thickest adsorbed layers are deposited, contact angle measurements indicate that the PAH and PAA outermost layers are well interpenetrated by segments from the previously adsorbed layer (contact angles: PAH as the outermost layer, 38°; PAA as the outermost layer, 24°). According to the well-known Cassie’s equation,9 a contact angle of about 30° for this polymer combination would indicate a 50/50 mixture of both segments (the contact angle measured from a thin film of pure PAA prior to dissolution is less than 5° whereas that of pure PAH is 50-55°).3 If the surfaces of these multilayer thin films are well interpenetrated, it seems reasonable to conclude that the internal layers are also highly interpenetrated. In the case of methylene blue surface staining measurements, we find that very little methylene blue is adsorbed onto a PAA or PAH outermost layer film when multilayers are fabricated from pH 5.0-6.0 solutions (absorbance at λmax of about 0.04 when PAA is the outermost layer and less than 0.02 when PAH is the outermost layer). Essentially no methylene blue is adsorbed onto multilayers fabricated from solutions of pH 6.5 or higher (absorbance at λmax of 0.02 or less). We have previously suggested3 that, under the conditions employed, methylene blue will only adsorb onto a multilayer surface that contains free binding sites in the form of carboxylic acid or nonpolycation bound carboxylate groups. Thus, these measurements suggest that the interpenetrated surfaces of the multilayer films
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Figure 3. RMS surface roughness of 20 layer PAH/PAA films as a function of dipping solution pH. Both the PAH and PAA dipping solutions in this case were at the same pH.
fabricated from solutions of pH 5.0 and higher are comprised primarily of polycation-polyanion ion pairs. The weak methylene blue absorption observed with PAA outermost layer films in the pH range of 5-6 is consistent with the fact that the PAA chains do not become fully ionized (i.e., not fully charged) until a pH of about 6.5. The conformational state of a multilayer surface (i.e., whether it is dominated by loop and tail segments or train segments) can be explored indirectly via measurements of the surface roughness of dried films. A solvated surface comprised of a significant population of loops and tails, upon drying, will produce a molecularly rough surface whereas a surface dominated by flat, trainlike segments will produce a more molecularly smooth surface. Figure 3 shows the rms surface roughness of dried PAH/PAA multilayer films fabricated from dipping solutions covering the range of 2.5-9 as determined by AFM. For the most part, the shape of this curve maps onto the thickness data found in Figure 2; i.e., the large increase in thickness that occurs as one moves from region III to region II or IV is accompanied by a large increase in surface roughness. In the pH range of 6-7.5, it can be seen that the multilayer films exhibit very low surface roughness (
