Langmuir 2005, 21, 8785-8792
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Assembly of Multilayer Films from Polyelectrolytes Containing Weak and Strong Acid Moieties Elvira Tjipto, John F. Quinn, and Frank Caruso* Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Received May 4, 2005. In Final Form: June 23, 2005 Multilayer films were assembled from a copolymer containing both weakly and strongly charged pendant groups, poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA), deposited in alternation with poly(allylamine hydrochloride) (PAH). The strongly charged groups (styrene sulfonate, SS) are expected to form electrostatic linkages (to enhance film stability), while the weakly charged groups (maleic acid, MA) can alter multilayer film properties because they are responsive to external pH changes. In this study, we varied several assembly conditions such as pH, SS/MA ratio in PSSMA, and the ionic strength of the polyelectrolyte solutions. The multilayer films were also treated by immersion into pH 2 and 11 solutions after assembly. Quartz crystal microgravimetry and UV-visible spectrophotometry showed that the thickness of PSSMA/ PAH multilayers decreases with increasing assembly pH regardless of whether salt was present in the polyelectrolyte solutions. When no salt was added, the multilayers are thinner, smoother, and grow less regularly. Atomic force microscopy images indicate that the presence of salt in polyelectrolyte solutions results in rougher surface morphologies, and this effect is especially significant in multilayers assembled at pH 2 and pH 11. When both polyelectrolytes are adsorbed at conditions where they are highly charged, salt was necessary to promote regular multilayer growth. Fourier transform infrared spectroscopy studies show that the carboxylic acids in the multilayers are essentially ionized when assembled from different pHs in 0.5 M sodium chloride solutions, whereas some carboxylic acids remain protonated in the multilayers assembled from solutions with no added salt. This resulted in different pH stability regimes when the multilayers were exposed to different pH solutions, post assembly.
Introduction Layer-by-layer (LbL) adsorption of polymers is a versatile method that allows the construction of polymer thin films with fine control over thickness, composition, morphology, and chemical functionality.1-3 The method, as originally introduced, is based on the alternating adsorption of oppositely charged polyelectrolytes: with each adsorption step, the surface charge is reversed and this allows adsorption of the next polyelectrolyte. A key advantage of the technique is that it is readily transferred from planar surfaces to three-dimensional substrates such as colloidal particles. Using the LbL method, functional films and coated colloids have been fabricated to incorporate various materials such as dyes, proteins, and inorganic nanoparticles. Subsequent removal of the colloidal templates yields hollow capsules, with potential for controlled delivery applications where the multilayer is responsive to various external stimuli such as temperature,4-6 pH,7-9 salt,10 light, 11,12 or reducing potential.13 * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: + 61 3 8344 4153. (1) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (3) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (4) Quinn, J. F.; Caruso, F. Langmuir 2004, 20, 20. (5) Quinn, J. F.; Caruso, F. Macromolecules 2005, 38, 3414. (6) Jaber, J. A.; Schlenoff, J. B. Macromolecules 2005, 38, 1300. (7) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235. (8) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078. (9) Cho, J.; Caruso, F. Macromolecules 2003, 36, 2845. (10) Antipov, A. A.; Sukhorukov, G. B.; Mo¨hwald, H. Langmuir 2003, 19, 2444. (11) Radt, B.; Smith, T. A.; Caruso, F. Adv. Mater. 2004, 16, 2184. (12) Angelatos, A. S.; Radt, B.; Caruso, F. J. Phys. Chem. B 2005, 109, 3071.
Early studies of multilayer systems focused on the use of strong polyelectrolytes, where the charge density is effectively constant over a broad pH range. In these systems, electrostatic interactions dominate, and it has been demonstrated that the film assembly and film properties are influenced by the charge density and molecular weight of the adsorbing species, and the ionic strength of the adsorption solutions. More recently, multilayer systems have been assembled from weak polyelectrolytes, in which the ionization of the polyelectrolyte is dependent on the pH.14-25 One system that has been widely studied is that of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). This system is promising, as dramatic changes in film properties can be achieved by slight variations in pH. This enables greater control over not only layer thickness,15 but also film morphology16 and surface and internal composition of the polymer film14 as changes in pH alter the charge density and conformation of weak polyelectrolytes. (13) Haynie, D. T.; Palath, N.; Liu, Y.; Li, B. Y.; Pargaonkar, N. Langmuir 2005, 21, 1136. (14) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (15) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (16) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (17) Burke, S. E.; Barrett, C. J. Langmuir 2003, 19, 3297. (18) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780. (19) Mermut, O.; Barrett, C. J. J. Phys. Chem. B 2003, 107, 2525. (20) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (21) Xie, A. F.; Granick, S. Macromolecules 2002, 35, 1805. (22) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (23) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (24) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458. (25) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531.
10.1021/la051197h CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005
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Figure 1. Chemical structures of the polyelectrolytes used in this study.
An alternative method for preparing stimuli-responsive thin films incorporating weak polyelectrolyte moieties is to use copolymers. The advantage of this approach is that the film chemistry can be tailored for different applications. For instance, monomer units that are weakly charged, strongly charged, or hydrogen bonders can be readily incorporated into the chain, and the proportion of each unit varied by changing the copolymer composition. Such materials add a level of sophistication beyond those prepared simply from homopolymeric materials. Previously, we investigated a lowly charged copolymer of acrylamide and [3-(2-methylpropionamido)propyl]trimethylammonium chloride, (AM-MAPTAC-10) (10 mol % of cationic monomers), assembled in alternation with PAA.26 In this system, the MAPTAC groups are permanently charged, while the ionization of PAA is pH dependent. Below pH 4.3, hydrogen bonding (between the protonated carboxylic acid and primary amide group) occurs and is likely to be the driving force behind the (exponential) growth. However, above pH 4.3, growth is irregular and proceeds with adsorption-desorption steps. In another system, random copolymers of diallyldimethylammonium chloride (DADMAC) and acrylamide (AM) with varying contents (8-100 mol %) of the cationic DADMAC component were assembled into multilayers with poly(styrenesulfonate) (PSS).27 Instead of a continuous increase of adsorbed amount with decreasing charge density, two thickness maxima at intermediate charge densities were observed. Kharlampieva and Sukhishvili also studied the same copolymer (50% DADMAC component) assembled with poly(methacrylic acid) (PMAA) at different pH conditions.28 These authors found that growth was irregular above pH 6, and when films assembled at pH 2 were exposed to higher pH, PMAA was released from the film. Findings from these studies suggest that the reduction in hydrogen bonding at high pH between acrylamide and carboxylic acid groups destabilizes the multilayers to a large extent and that electrostatic forces between the fully charged cationic groups and the ionized acid are insufficient to hold the film together. While a range of different copolymers has been studied, to the best of our knowledge, a system in which both weak and strong moieties are present in one polyelectrolyte has not been investigated. In the current study, a copolymer of SS and MA (poly(4-styrenesulfonic acid-co-maleic acid), PSSMA) is assembled into multilayer thin films with PAH. (Structures of the polyelectrolytes used are shown in Figure 1.) Building a multilayer film with a copolymer consisting of both weak and strong polyelectrolyte pendant groups may obviate the need for chemical cross-linking to improve the stability of weak polyelectrolyte multilayers. In such (26) Schoeler, B.; Poptoschev, E.; Caruso, F. Macromolecules 2003, 36, 5258. (27) Schoeler, B.; Sharpe, S.; Hatton, T. A.; Caruso, F. Langmuir 2004, 20, 2730. (28) Kharlampieva, E.; Sukhishvili, S. A. Macromolecules 2003, 36, 9950.
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a case, the strongly charged groups can form electrostatic linkages (thereby enhancing film stability), while the weakly charged groups can be used to alter multilayer properties because they are responsive to external pH changes. It is also demonstrated that the assembly and morphology of the PSSMA/PAH multilayers can be varied by altering the adsorption conditions. Further, by exposing the preassembled multilayers to pH extremes, it is shown that the multilayer stability is highly dependent on the assembly conditions. These films may be useful where both film integrity and responsiveness are necessary, such as for biomaterials and controlled-release applications. Experimental Section Materials. Poly(4-styrenesulfonic acid-co-maleic acid, 1:1 SS:MA) sodium salt (PSSMA 1:1), and poly(4-styrenesulfonic acid-co-maleic acid, 3:1 SS:MA) sodium salt (PSSMA 3:1), both with molecular weights of 20 000 g mol-1, were obtained from Aldrich. Poly(allylamine hydrochloride), with a molecular weight of 15 000 g mol-1, and poly(ethyleneimine) (PEI) (Mw ) 20 000 g mol-1 water free) were also obtained from Aldrich. All polyelectrolytes were used as supplied. Quartz slides for UVvis measurements were obtained from Hellma Optik GmbH (Jena, Germany) and Herbert A. Groisse and Co. (Melbourne, Australia). Oxidized monocrystalline silicon wafers were purchased from MMRC Pty Ltd (Melbourne, Australia) and QCM electrodes were from Kyushu Dentsu (Nagasaki, Japan) and Uko Denshi (Sayama, Japan). An inline Millipore RiOs/Origin system was used to produce high-purity water with a resistivity greater than 18 MΩ cm. Substrate Preparation. Silicon wafers and quartz and glass slides were cleaned with Piranha solution (70:30 v/v% sulfuric acid/hydrogen peroxide). Caution! Piranha solution is highly corrosive. Extreme care should be taken when handling Piranha solution and only small quantities should be prepared. The RCA protocol was then applied to further clean and hydrophilize the substrates. This involved sonication in a 1:1 mixture of water and 2-propanol for 15 min, followed by heating at 60 °C for 15 min in a 5:1:1 mixture of water, hydrogen peroxide (30%), and ammonia solution (29%). Preparation of Multilayer Films. Aqueous polyelectrolyte deposition solutions were prepared to a concentration of 1 mg mL-1 in 0.5 M NaCl unless otherwise indicated. NaOH (1 M) was used to adjust the solution pH of PSSMA to pH 11 and HCl (1 M) was used to adjust the PSSMA solution pH to either 5.8 or 2. No acid or base was added to PAH deposition solutions (pH of PAH in 0.5 M NaCl is 5.25). The substrates were dipped for 15 min in PEI solutions to establish a highly charged precursor layer on the surface. The PEI-modified substrates were then dipped three times in Millipore water for 1 min each time and dried with a gentle stream of nitrogen. They were then immersed in the anionic PSSMA solution for 15 min, followed by further washing and drying. This procedure was repeated (alternating between PSSMA and PAH) until 20 layers were deposited. Post-treatment of Multilayer Films. After assembly, multilayer films were exposed to aqueous solutions of different pH to the assembly pH for 24 h. These films were then rinsed with water for 20 s and air dried before characterization. QCM Measurements. An in-house-built QCM device with a frequency counter from Agilent was used to determine the film mass after each adsorption step. The piezoelectric quartz crystal changes its fundamental oscillation frequency, F0, as mass is deposited onto (or depleted from) the surface. According to the Sauerbrey equation, the resonant frequency shift, ∆F, of a QCM is proportional to the mass change, ∆m:29
∆F ) -
2Fo2
∆m A(µqFq)1/2
(1)
where µq is the shear modulus of the quartz (2.947 × 1013 g m-1 s-2), Fq is the density of the quartz (2.648 × 106 g m-3), Fo is the (29) Sauerbrey, G. Z. Phys. 1959, 155, 206.
Multilayer Films from Polyelectrolytes operating frequency of the crystals (9 × 106 Hz), and A is the piezoelectric area of the electrode. Substituting these values into eq 1 gives:
∆F ) -(1.83 × 104)∆mA where ∆mA is the mass change per piezoelectric area in g m-2. The QCM electrodes from Kyushu Dentsu were gold coated on both sides and have a diameter of 4.5 mm, whereas electrodes from Uko Denshi were gold coated on only one side and have a diameter of 4 mm. By taking into account that the effective surface area is approximately 20% larger than that calculated directly from the diameter of the electrode (due to surface roughness),30 the mass adsorbed can be converted into thickness (assuming the density of the polyelectrolyte film is 1.2 × 106 ( 0.1 g m-3):31
Langmuir, Vol. 21, No. 19, 2005 8787 depends on factors such as the salt concentration in the deposition solutions and the strength of the electrostatic bonds within the multilayers. As such, the pKa values obtained from the titration experiments are used only as a qualitative indication for the degree of ionization.
Results and Discussion
where d is the thickness of the polyelectrolyte film. UV-Vis Measurements. UV-visible spectra were collected using an Agilent 8453 single-beam UV-vis spectrophotometer. An air blank was taken before each measurement. While PAH does not absorb in the region of interest (200-400 nm), PSSMA has an absorption peak at 226 nm due to the sulfonated phenyl ring in the SS moiety. FTIR Measurements. FTIR spectra were recorded using a Perkin-Elmer Spectrum 2000 FTIR spectrometer with a specular reflectance accessory set at an angle of 15°. Forty scans were taken for each spectrum at 4 cm-1 resolution and 0.5 cm-1 interval. Multilayers were assembled on glass slides precoated with a thin chromium adhesion layer and a thick gold layer. Atomic Force Microscopy (AFM) Imaging. AFM images were taken on air-dried films with a Nanoscope IIIa microscope (Digital Instruments Inc., Santa Barbara, CA) in tapping mode using silicon cantilevers with a resonance frequency of ca. 290 kHz (MikroMasch, USA). Several images were taken on macroscopically separated areas of the films to ensure representative AFM images of the samples. Image processing (first-order flattening and plane fitting) was carried out with Nanoscope 4.43r8 software. To measure the film thickness, a scalpel blade was used to scratch the films in several places, and images were taken at several points on the edge of each scratch. Film thickness analysis was carried out using the Nanoscope 4.43r8 software, and the thickness values obtained were averaged. Titration of PSSMA. To determine the pKa values of PSSMA 1:1 and 3:1, titrations were carried out using the polyelectrolyte solutions. PSSMA (50 mg mL-1) without salt (pH adjusted with HCl to pH 1) was titrated with 0.5 M NaOH. The pKa value was determined from the titration curve at the point where half of the amount of base necessary for reaching the equivalence point was added (see Supporting Information, Figure S1). Copolymers of MA are known to dissociate in two steps, hence two pKa values were obtained.32-35 This is because, when one of the carboxylic acid groups of MA is ionized, it can hydrogen bond with the proton of the adjacent un-ionized acid group, thus stabilizing the halfdissociated state. The pKa values for PSSMA 1:1 were found to be 2.9 and 8.8, while the pKa values for PSSMA 3:1 were slightly lower (2.7 and 8.3). It has been reported that pKa values of polyelectrolytes in multilayer films are different from the bulk solution values by as much as several units.17,36-38 This difference
Formation of PSSMA/PAH Multilayers. The growth of multilayers from PSSMA and PAH solutions containing 0.5 M NaCl was monitored using QCM and UV-vis. QCM allows determination of the mass adsorbed and, therefore, the average thickness of the multilayer, after each deposition step, while UV-vis provides information on the amount of PSSMA incorporated into the film, as SS absorbs at 226 nm (Figure 2). Multilayers deposited from different pH solutions of PSSMA show different growth behavior (Figure 2). Mass loadings were found to decrease with increasing pH for both PSSMA 1:1 and 3:1. The film formed from PSSMA 1:1 at pH 2 is thick (ca. 140 nm for a 20-layer film) (Figure 2a) and begins to become cloudy after 10 layers, resulting in strong scattering and, therefore, unreliable absorbance values at higher layer numbers (Figure 2c). At pH 5.8 and 11, the absorbance of PSSMA 3:1 (Figure 2d) was slightly higher than that for PSSMA 1:1, although this is likely to be due to the higher proportion of SS groups in PSSMA 3:1 (QCM data in Figure 2a and b indicate there is less mass for the PSSMA 3:1 films). The observed growth behavior of PSSMA/PAH multilayer films can be rationalized as follows. At high pH, a greater proportion of the carboxylic acid groups in the PSSMA become ionized, leading to an increase in the effective charge density of the polymer. As the charge density of the polyelectrolytes increase, repulsive forces between the charged groups increase, causing the polyelectrolytes to adopt a flatter conformation, adsorbing as thinner layers. This accounts for the lower mass loadings observed at higher pH. Conversely, at low pH, an increasing proportion of the carboxylic acid groups become protonated, leading to a decrease in the charge density. This, in turn, causes the polyelectrolyte to adopt a more coiled conformation (due to diminished intramolecular repulsive forces) and to adsorb as thicker layers. Further, greater amounts of lower charge density polyelectrolytes are required to compensate the charge on the multilayer surface. Similar effects have been observed in other systems where the charge density of polyelectrolytes is varied, either by altering pH (with weak polyelectrolytes) or by adjusting the molecular structure of the polyelectrolyte chain.19,26,39-42 Further, the ionic strength of polyelectrolyte solutions is known to significantly affect the growth of multilayer films because it influences both the conformation and charge density of polyelectrolytes.37,43-47 As such, the experiments described above were repeated using solutions with no added salt (see Supporting Information, Figure S2). The same techniques (UV-vis, QCM, and AFM) were used to characterize the resulting films. The absorbance values from UV-vis are significantly lower
(30) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (31) Polymer Handbook, Part 5; Brandrup, I., Immergut, E., Eds.; John Wiley & Sons: New York, Chichester, Brisbane, Toronto, 1975. (32) Kawaguchi, S.; Kitano, T.; Ito, K. Macromolecules 1992, 25, 1294. (33) Kitano, T.; Kawaguchi, S.; Ito, K.; Minakata, A. Macromolecules 1987, 20, 1598. (34) Osaki, T.; Werner, C. Langmuir 2003, 19, 5787. (35) Schultz, A. W.; Strauss, U. P. J. Phys. Chem. 1972, 76, 1767. (36) Petrov, A. I.; Antipov, A. A.; Sukhorukov, G. B. Macromolecules 2003, 36, 10079. (37) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263. (38) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116.
(39) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889. (40) Steitz, R.; Jaeger, W.; v. Klitzing, R. Langmuir 2001, 17, 4471. (41) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (42) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823. (43) Izumrudov, V.; Sukhishvili, S. A. Langmuir 2003, 19, 5188. (44) Fery, A.; Schoeler, B.; Cassagneau, T.; Caruso, F. Langmuir 2001, 17, 3779. (45) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Stuart, M. A. C. Langmuir 2002, 18, 5607. (46) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (47) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655.
d (nm) ) -(1.89 × 10-2) ∆F (Hz) for Kyushu Dentsu QCMs, and d (nm) ) -(3.79 × 10-2) ∆F (Hz) for Uko Dentsi QCMs
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Figure 2. Thickness of PSSMA/PAH multilayers calculated from QCM for (a) PSSMA 1:1 and (b) PSSMA 3:1, and UV-vis measurements for (c) PSSMA 1:1 and (d) PSSMA 3:1 deposited from solutions containing 0.5 M NaCl. Odd layer numbers correspond to PSSMA, even layer numbers to PAH.
than when salt is used in the polyelectrolyte solutions. When no salt is added, the polyelectrolyte chain assumes a more extended conformation and contains less loops and tails because charges on the chain are less screened. Consequently, the polyelectrolyte adsorbs as thinner layers. Nevertheless, the same growth trend with pH is observed as when salt is present in the deposition solutions. The absorbance of PSSMA/PAH films is lower when PSSMA adsorption occurs from higher pH solution, and the films prepared with PSSMA 3:1 again have a slightly higher absorbance than those prepared with PSSMA 1:1. Multilayer growth is also less regular when no salt is added (see Supporting Information, Figure S2), indicating that salt enhances the stability of films by inducing more entanglements of the polyelectrolyte chains. This, in turn, prevents desorption during the next adsorption step. Supporting this contention, Kolarik et al. proposed that, in multilayers assembled from polyelectrolytes with molecular weights less than 105 Da, salt promotes interpenetration of adsorbing polyelectrolyte into preceding layers, thus enabling more effective charge compensation and facilitating multilayer formation.48 Multilayer growth (from solutions with no salt) continues for 20 layers, except for PSSMA deposited at pH 11, with the absorbance plateauing after the first layer. This could be due to little adsorption occurring and/or growth proceeding via a series of adsorption-desorption steps, with loosely bound polyelectrolytes desorbing during the adsorption of the next layer. Further, secondary interactions between protonated carboxylic acid groups and PAH at lower pH may help to ensure regular multilayer growth when salt is absent. This observation also indicates that, when both PSSMA and PAH are adsorbed at conditions where they are highly charged, salt is necessary to promote regular multilayer growth. Tapping mode AFM was performed on air-dried 10 bilayer films assembled from 0.5 M NaCl solutions onto silicon wafers to study the film surface morphology (Figure (48) Kolarik, L.; Furlong, D. N.; Joy, H.; Struijk, C.; Rowe, R. Langmuir 1999, 15, 8265.
Figure 3. AFM images of 20-layer PSSMA 1:1/PAH (a-c) and PSSMA 3:1/PAH (d-f) films from 0.5 M NaCl solutions at various PSSMA pH on silicon substrates. Scan size: 1 µm × 1 µm for all images except (a), which is 50 µm × 50 µm.
3). The film thickness, as measured by AFM (Table 1), shows the same trend as the UV-vis and QCM data, that is, the thickness decreases with increasing pH for both PSSMA 1:1 and PSSMA 3:1 films. Also, PSSMA 3:1 films are thinner than PSSMA 1:1 films. This is attributed to the higher charge density of PSSMA 3:1 (caused by the increased proportion of strong SS pendants). At higher charge density, charge compensation requires a smaller amount of PSSMA 3:1 to reverse the surface charge of the previous layer. Films deposited from PSSMA solutions at pH 2 have intriguing surface morphology. Both 1:1 and 3:1 films appear colored (blue and yellow, respectively), although their surface roughness is significantly different.
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Table 1. Roughness and Thickness (d) of Air-Dried 20-Layer Films Prepared from 0.5 M NaCl Aqueous Solutions, as Measured by AFM and QCM rms (nm)a (AFM)
d (nm)b (AFM)
d (nm)c (QCM)
PSSMA pH
PSSMA 1:1
PSSMA 3:1
PSSMA 1:1
PSSMA 3:1
PSSMA 1:1
PSSMA 3:1
2 5.8 11
80.2 1.3 2.4
0.5 0.5 1.8
136.4 37.1 16.3
50.8 34.8 13.3
147.2 33.1 19.2
45.6 27.6 16.9
a,b The error in these values is estimated as 10%. c The error is estimated as 10%; this is due to the variations in the frequency values and errors in the assumed polyelectrolyte film density.
Table 2. Roughness and Thickness (d) of Air-Dried 20-Layer Films Prepared from Aqueous Solutions Containing No Added Salt, as Measured by AFM and QCM rms (nm)a(AFM)
d (nm)b(AFM)
d (nm)c(QCM)
PSSMA pH
PSSMA 1:1
PSSMA 3:1
PSSMA 1:1
PSSMA 3:1
PSSMA 1:1
PSSMA 3:1
2 5.8 11
1.0 0.5 0.3
0.5 0.9 0.4
61.1 7.0
39.4 7.2
60.0 9.1 1.7
27.8 7.8 3.0
a,b The error in these values is estimated as 10%. c The error is estimated as 10%; this is due to the variations in the frequency values and errors in the assumed polyelectrolyte film density.
The 1:1 film is very thick compared to other films and appears rough to the naked eye, whereas the 3:1 film is quite smooth (rms roughness ∼0.5 nm). Increasing the pH of PSSMA deposition solutions does not give a clear trend in the surface roughness of films. Increasing the pH of PSSMA solution from 2 to 5.8 results in a much smoother film for PSSMA 1:1, while no significant difference was observed for PSSMA 3:1. This may be due to the larger number of strong polyelectrolyte pendants (SS groups) in the PSSMA 3:1, leading to a diminished impact of pH variation. Films deposited at pH 11 have greater rms values, and feature sizes are larger, compared to those of films deposited at pH 5.8. This could be due to polyelectrolyte chains adopting a more rigid conformation as repulsive forces increase at higher charge densities. Film thickness measurements from AFM confirm that the multilayer films deposited from solutions with no added salt are much thinner than films deposited from 0.5 M NaCl solutions (Table 2). The thicknesses of multilayers deposited at pH 11 are too thin to be reliably measured by AFM; however, QCM data still suggests that multilayers are thinner when deposited at higher pH. The surface morphology of films prepared without salt at different assembly pH conditions is similar (Figure 4). Surface roughness is lower for multilayers prepared without salt (Table 2), and this is especially pronounced for the case where PSSMA 1:1 is deposited at pH 2. The addition of salt in the deposition solutions induces charge screening and enables PSSMA to assume a more coiled conformation, resulting in rougher films. The difference in surface morphologies between multilayers assembled from 0.5 M NaCl solutions and no added salt solutions are more significant for multilayers assembled from pH 11 compared to multilayers assembled from pH 5.8 (Table 1 and Figure 3; Table 2 and Figure 4). Films deposited at pH 11 without salt have less globular surfaces than films deposited at the same pH with salt, while films deposited at pH 5.8 have similar morphologies, regardless of whether salt is present in the polyelectrolyte solutions. PSSMA has a higher charge density at pH 11 compared to pH 5.8 (carboxylic acid groups are more ionized at higher pH) and thus is likely to have a more dramatic effect when 0.5
Figure 4. AFM images of 20-layer PSSMA 1:1/PAH (a-c) and PSSMA 3:1/PAH (d-f) films from solutions with no added salt at various PSSMA pH on silicon substrates. Scan size: 1 µm × 1 µm for all images.
M NaCl solutions are used. As such, the difference between films assembled from 0.5 M NaCl and no added salt solutions would be greater at pH 11 than films assembled at pH 5.8. Ionization of PSSMA in Solution and in Multilayer Films. Cast films of PSSMA and multilayer films of PSSMA/PAH were measured using FTIR to study the ionization of the carboxylic acid groups in the film (Figure 5). The peaks at 1720, 1580, and 1412 cm-1 are due to the CdO stretching of COOH groups and asymmetric and symmetric stretching of COO-, respectively. The peaks at approximately 1200, 1130, 1041, and 1011 cm-1 are due to the SS groups of PSSMA. For (neat) cast films of PSSMA (Figure 5a and b), almost all the carboxylic acid is protonated at pH 2, as indicated by the absence of COOpeaks. As the pH increases, the intensity of the COOH band decreases, while the intensities of the COO- bands at approximately 1580 and 1412 cm-1 increase. This indicates an increase in the number of ionized acid groups. Conversely, the extent of ionization of the acid groups in the multilayer films (Figure 5c and d) is quite similar for all assembly pHs. The COOH band has all but disappeared in the spectra of multilayer films; instead, the most prominent bands are at 1570-1540 cm-1 and 1405 cm-1. Previous findings indicate that the pKa of weak polyacids in multilayers tend to be lower than that in solution7,14,15,17,21,36 due to induced ionization of the polyacids in the multilayer as they adjust to maintain charge neutrality when responding to changes in the local electrostatic environment. Choi and Rubner reported that the degree of ionization of PAA in multilayers was significantly higher than the solution value in the pH range of 2-7. They also reported that this effect was most dramatic when PAA was assembled with PAH compared to when other fully charged polycations, such as poly(diallyldimethylammonium chloride) (PDADMAC), were used.38 However, it is intriguing that the ionization state in the multilayer remains largely the same regardless of the pH of the PSSMA deposition solutions. In the FTIR spectra of multilayer films assembled at pH 2 and 5.8 from solutions containing no added salt, there is an
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Figure 5. FTIR spectra of cast films of (a) PSSMA 1:1 and (b) PSSMA 3:1 and multilayer films of (c) (PSSMA 1:1/PAH)20 and (d) (PSSMA 3:1/PAH)20 deposited at various pH values. The cast films were deposited from PSSMA solutions at various pH (1 M HCl and NaOH are used for pH adjustments), while the multilayers were deposited from 0.5 M NaCl solutions. The spectra were offset in the positive direction for clarity.
adsorption band around 1710 cm-1 (which appears as a shoulder), indicating that some carboxylic acid remains protonated (see Supporting Information, Figure S3). This is despite the multilayers being subjected to the same washing step with water as when 0.5 M NaCl polyelectrolyte solutions were used. Hence, the similarity of the FTIR spectra of the multilayers assembled at different pHs from 0.5 M NaCl polyelectrolyte solutions is likely due to the salt in the deposition solutions promoting more effective complexation between the carboxylate groups and ammonium groups of the PAH. Notably, the FTIR spectra of the multilayer films deposited at different pH from 0.5 M NaCl solutions are similar, even though their growth behavior is quite different, as evidenced by QCM and UV-vis. Exposure of Preassembled PSSMA/PAH Films to Various pH Solutions. PSSMA/PAH multilayer films were immersed into solutions of different pH to the assembly pH to investigate the integrity and pH responsiveness of the films. Films were immersed into water adjusted by 1 M HCl or NaOH to pH 2 and 11, respectively,
for 24 h. From Table 3, it can be seen that multilayer films assembled from 0.5 M NaCl solutions comprising PSSMA 3:1/PAH are more stable after exposure to different pH compared to PSSMA 1:1/PAH films. This can be rationalized by PSSMA 3:1 having more SS groups, which provide enhanced electrostatic interaction, and thus enhanced multilayer stability. Films exposed to pH 11 showed little variation from their original morphology and thickness. This is because the carboxylic acid groups in the multilayer are significantly more ionized than the solution state ionization value at the assembly pH (see Figure 5). Therefore, exposure to a pH 11 solution is unlikely to induce major changes in the ionization of the carboxylic groups, as they are already highly ionized. Previous studies have found that local pKa values of polyacids incorporated in multilayers tend to be lower by as much as 4 pH units.17,21 As a result, exposing preassembled films at a lower pH to more alkaline pH may not significantly impact film structure. Multilayers assembled at pH 5.8 and 11 show vastly different responses when they are exposed to pH 2
Multilayer Films from Polyelectrolytes
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Table 3. Roughness and Percentage of PSSMA Remaining for pH-Treated 20-Layer Films Assembled from Aqueous 0.5 M NaCl Solutions, as Measured by AFM and UV-Vis, Respectively after pH treatmenta
before pH treatment assembly pH/ treatment pH 2/11 5.8/2 5.8/11 11/2 silicon wafer a
rms roughness 1:1d 80.2 1.3 1.3 2.4 0.2
(nm)b 3:1 0.5 0.5 0.5 1.8
rms roughness (nm)b 1:1 3:1
% PSSMA remainingc 1:1 3:1
85.0 0.4 1.3 1.7 0.2
89 7 99 84
0.6 0.7 0.8 2.3
Films treated for 24 h at room temperature. b Determined using AFM. c Determined using UV-vis.
Figure 6. AFM images of 20-layer PSSMA 1:1/PAH (a-d) and PSSMA 3:1/PAH (e-h) films from 0.5 M NaCl solutions exposed to various pH conditions. Scan size: 1 µm × 1 µm for all images.
solutions (Table 3 and Figure 6). Films prepared at pH 5.8 are largely destroyed, while those prepared at pH 11 lose only 10-20% of the adsorbed amount, as assessed by UV-vis. Rubner and co-workers have postulated that, at different pH regimes, weak polyelectrolytes adopt a variation between two extreme molecular organizations: a “ladder-like” architecture with cooperatively stitched chain segments, and a “scrambled salt” architecture where chains adopt a more statistical distribution of ion pairs with many different chains.16 When the weak polyelectrolytes are fully charged, the chains are linked together with a large fraction of cooperatively stitched chain segments, hence a “ladder-like” architecture. However, when the polyelectrolytes have low charge density, they form loopy structures where ionic stitching is substantially more random, hence “scrambled salt”, architecture.16 In PSSMA/PAH films assembled at pH 11, exposure to lower pH protonates the carboxylic acid groups and reduces the number of ionic cross-links within the film, but not sufficiently to free large segments of chains and result in multilayer destruction. In PSSMA/PAH films assembled at pH 5.8, the polyelectrolytes may assume a more entangled arrangement, and as such, a reduction of the COO- ionic links may be sufficient to allow large segments
d
99 10 100 92
Ratio of SS/MA.
of the polymer chains to be released into the solution. This may explain the different behavior of the multilayer films prepared from pH 5.8 and 11 upon treatment with pH 2 solution. AFM studies show that the surface morphology of PSSMA/PAH multilayer films was not altered significantly by pH variation (Figure 6 and Table 3). This indicates that interchain cross-links within the films are quite robust and thus prevent rearrangement of the bulk film upon pH variation. Films assembled at pH 5.8 are the exception because most of the film is removed by exposure to acidic solution (see above). When films assembled from solutions with no added salt were exposed to different pH environments, they show very different behavior compared with films assembled from aqueous 0.5 M NaCl solutions. In addition to playing a role in increasing film thickness, salt also influences pH stability of films by way of controlling polyelectrolyte conformation during adsorption. In this system, films deposited from solutions with no added salt were observed to be less robust to pH variation. Further, PSSMA 1:1 films lose significantly less material when treated compared to PSSMA 3:1 films (Table 4). Notably, films assembled from pH 5.8 solutions were stable upon exposure to pH 2 solution, but not on exposure to pH 11 solution, which is the opposite trend observed for films deposited from 0.5 M NaCl solutions. By measuring the pH of polyelectrolyte solutions at different ionic strengths, it was found that adding salt decreases the pH of PSSMA solutions (data not shown). This is consistent with the findings of Petrov et al., who have demonstrated that the pKa of PAA is ∼1 pH unit lower when the solution contains 0.5 M NaCl.36 As such, multilayers deposited from PSSMA solutions with no added salt are most likely to have a greater propensity for hydrogen bonding at acidic pH because un-ionized carboxylic acid groups are available, as seen from the FTIR spectra of multilayer films assembled from solutions containing no added salt (see Supporting Information, Figure S3). When multilayers deposited at pH 2 and 5.8 were exposed to pH 11 solutions, most of the films disassembled (Table 4). This is due to un-ionized carboxylic groups in the assembly (which are not involved in electrostatic complexation) becoming ionized, resulting in an excess of negative charges within the film and the subsequent release of PSSMA into the solution. Another observation supporting this contention is the fact that multilayers built at pH 5.8 are quite stable when exposed to pH 2. This shows that the ionization state within the films is not altered significantly in the pH 2-5.8 range. AFM studies were performed to examine the surface morphology of films prepared without added salt in the deposition solutions after post-treatment (Figure 7 and Table 4). Surfaces of multilayers deposited at pH 2 and exposed to pH 11 become smoother, and the grain sizes
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Table 4. Roughness and Percentage of PSSMA Remaining for pH-Treated 20-Layer Films Assembled from Aqueous Solutions Containing No Added Salt, as Measured by AFM and UV-Vis, Respectively after pH treatmenta
before pH treatment
a
assembly pH/ treatment pH
rms roughness 1:1d
2/11 5.8/2 5.8/11 11/2 silicon wafer
1.0 0.5 0.5 0.3 0.2
(nm)b 3:1 0.5 0.9 0.9 0.4
rms roughness (nm)b 1:1 3:1
% PSSMA remainingc 1:1 3:1
0.3 0.6 1.0 0.5 0.2
39 92 34 100
0.2 0.8 1.3 0.6
Films treated for 24 h at room temperature. b Determined using AFM. c Determined using UV-vis.
d
17 100 24 100
Ratio of SS:MA.
Conclusions
Figure 7. AFM images of 20-layer PSSMA 1:1/PAH (a-d) and PSSMA 3:1/PAH (e-h) films from solutions with no added salt, exposed to various pH conditions. Scan size: 1 µm × 1 µm for all images.
are smaller. These films have roughness values approaching that of a clean silicon wafer, indicative of significant film deconstruction. On the other hand, the surfaces of multilayer films deposited at pH 5.8 and immersed in pH 11 become rougher, possibly because multilayer deconstruction is slower in this case (as the pH change was not as large). The surfaces of multilayers grown at pH 5.8 and exposed to pH 2 do not show any discernible change, consistent with the UV-vis measurements, which also show very little change. With multilayers grown at pH 11 and exposed to pH 2, rms values increase slightly, although the morphology is still very similar. Nevertheless, these films are rougher than bare silicon wafers, confirming that the film remains intact after pH variation.
Multilayers of PSSMA/PAH were deposited at various pHs and characterized using UV-vis spectrophotometry, QCM, and AFM. There is a systematic trend between multilayer thickness and pH, with multilayer thickness decreasing with increasing pH. This is rationalized by the polyelectrolyte chains having a reduced charge density at lower pH, thereby adopting a more coiled conformation and adsorbing in greater amounts (because more polyelectrolyte is needed for charge overcompensation). The ionic strength of the deposition solutions was found to significantly influence multilayer growth, with multilayers being thinner, smoother, and growing less regularly when no salt was present in the deposition solutions. Despite this, the same trend between multilayer thickness and pH was still observed. The ionic strength of the solution also influenced the ionization state within the multilayer film, resulting in different pH stability regimes. When salt was present in the deposition solutions, multilayers deposited at pH 2 and 5.8 were stable when placed into pH 11 solutions. The same multilayers prepared without salt were unstable at pH 11. This is due to the complex interplay of polyelectrolyte conformation and ionization within the bulk multilayer films as the environmental pH is varied. Ongoing work in our laboratory will examine the assembly of these multilayers on colloidal particles, where the carboxylate groups are likely to give pH responsive surface properties, while the permanently charged SS groups should enhance the colloidal stability. Acknowledgment. This work was supported by the Australian Research Council under the Federation Fellowship and Discovery Project schemes, and by the Victorian State Government under the STI Initiative. Provision of infrastructure by the Particulate Fluids Processing Centre is also acknowledged. Supporting Information Available: Titration curves for PSSMA 1:1 and 3:1, QCM and UV-vis data for films assembled with no added salt, and FTIR spectra for films assembled with no added salt. This material is available free of charge via the Internet at http://pubs.acs.org. LA051197H
