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Langmuir 2007, 23, 230-240
Layer-by-Layer Assembly of pH-Responsive, Compositionally Controlled (Co)polyelectrolytes Synthesized via RAFT† Sarah E. Morgan,* Paul Jones, Andrew S. Lamont, Andrew Heidenreich, and Charles L. McCormick* Department of Polymer Science, UniVersity of Southern Mississippi, Hattiesburg, Mississippi 39406 ReceiVed June 7, 2006. In Final Form: October 30, 2006 Homo- and block copolyelectrolytes that have well-defined structures and are responsive to pH were synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization and employed to produce layer-by-layer (LBL) films. Acrylamido monomers with carboxylate, sulfonate, and amine functionality were utilized to provide both strong and weak homopolyelectrolytes and mixed strong/weak copolyelectrolyte systems. Multilayer films were prepared under specified conditions of pH and ionic strength and analyzed via atomic force microscopy and ellipsometry to study the effects of changes in the local molecular environment on film morphologies. The pH responsiveness and integrity of the multilayer assemblies were investigated by exposing films to solutions of varying pH in a fluid cell and performing in situ AFM analysis. The multilayer dimensions, morphology, and integrity were found to depend on the molecular architecture of the polyelectrolytes, with changes in segmental type and repeating unit distribution producing dramatic differences in film characteristics. These results suggest the possibility of producing LBL assemblies of precisely controlled dimensions and properties by specifically tailoring copolymer structure. To our knowledge, this is the first report of LBL assembly of RAFT-synthesized homo- and copolyelectrolyte multilayer complexes.
Introduction Layer-by-layer (LBL) assembly, first introduced by Decher and co-workers,1,2 has become an increasingly important method for the creation of structural and functional thin films deposited on solid substrates of almost any composition or topology. Initially, this technique was developed to prepare multilayers via electrostatic interactions of oppositely charged polyelectrolytes. However, more recently the technique has been extended to take advantage of additional forces including hydrogen bonding,3-5 charge transfer,6,7 acid-base pairing,8 metal ion coordination,9,10 and covalent bonding.10 LBL assembly has produced functional multilayer films that are responsive to various stimuli (e.g., light,11 pH,12,13 salt,14 and temperature15-17). Multilayer films thus have potential as optical and electrochemical devices, biosensors, separation membranes, cell-adherent and cell-repellent surfaces, † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue.
(1) Decher, G. Science 1997, 277, 1232-1237. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210-211, 831-835. (3) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717-2725. (4) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Macromolecules 2005, 10523-10531. (5) DeLongchamp, D. M.; Hammond, P. T. Langmuir. 2004, 20, 5403-5411. (6) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir. 1997, 13, 1385-1387. (7) Wang, X.; Naka, K.; Itoh, H.; Uemura, T.; Chujo, Y. Macromolecules 2003, 36, 533-535. (8) Li, D.; Jiang, Y.; Wu, Z.; Chen, X.; Li, Y. Thin Solid Films 2000, 360, 24-27. (9) Hatzor, A.; Van der Boom-Moav, T.; Yochelis, S.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. Langmuir 2000, 16, 4420-4423. (10) Hatzor, A. M., T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469-13477. (11) Angelatos, A. S.; Radt, B.; Caruso, F. J. Phys. Chem. B 2005, 109, 30713076. (12) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078-4083. (13) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235-1243. (14) Antipov, A. A.; Sukhorukov, G. B.; Mohwald, H. Langmuir 2003, 19, 2444-2448. (15) Jaber, J. A.; Schlenoff, J. B. Macromolecules 2005, 38, 1300-1306. (16) Quinn, J. F.; Caruso, F. Langmuir 2004, 20, 20-22. (17) Quinn, J. F.; Caruso, F. Macromolecules 2005, 38, 3414-3419.
as catalytic systems, and in controlled delivery, surface lubricity, and adhesion.18 The mechanism of electrostatic LBL assembly has been attributed to the formation of a sufficiently stable interpolyelectrolyte complex at the interface but also requires charge overcompensation leading to charge reversal as subsequent polyelecrolyte layers are added.18 The adsorption behavior of polyelectrolytes is influenced by multiple factors including ionic strength19,20 and/or the pH21,22 of the polymer solution, solvent quality,19,21-23 and charge density.24,25 Experimental26,27 and theoretical28-30 studies indicate that nonelectrostatic, short-range interactions also play a vital role in multilayer film formation. Some of the short-range interactions that have been reported are van der Waals, hydrogen bonding, and secondary hydrophobic interactions.28-31 Many of the past experimental and theoretical studies have focused on strong polyelectrolytes (often referred to as “quenched” polyelectrolytes) for which the charge density does not change over the pH range examined. The electrostatic interactions and film properties are mainly affected by the charge density and conformational behavior of such polyelectrolytes. Recently, weak polyelectrolytes (“unquenched”) have been utilized, and the pH (18) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2003. (19) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153-8160. (20) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592-598. (21) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116-124. (22) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213-4219. (23) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829-834. (24) Schoeler, B.; Sharpe, S.; Hatton, T. A.; Caruso, F. Langmuir 2004, 20, 2730-2738. (25) Serizawa, T.; Nakashima, Y.; Akashi, M. Macromolecules 2003, 36, 20722078. (26) Cochin, D.; Laschewsky, A. Macromol. Chem. Phys. 1999, 200, 609615. (27) Lojou, E.; Bianco, P. Langmuir 2004, 20, 748-755. (28) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789-796. (29) Messina, R. Macromolecules 2003, 37, 621-629. (30) Park, S. Y.; Rubner, M. F.; Mayes, A. M. Langmuir 2002, 18, 96009604. (31) Guyomard, A.; Muller, G.; Glinel, K. Macromolecules 2005, 38, 57375742.
10.1021/la061638c CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006
Layer-by-Layer Assembly of (Co)polyelectrolytes
of the deposition solution determines the ionization of the polymer chains21,22,32-38 and thus the extent of ion pairing and the morphological features of the final multilayered film. A widely studied weak polyelectrolyte system is that of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). Quite dramatic changes in film properties are elicited by small changes in pH, enabling greater control over film morphology,34 thickness,21,22 and surface and internal composition.36 Recently poly(4-styrenesulfonic acid-co-maleic acid), with varying ratios of the respective weak and strong acids, was utilized in multilayer film formation with PAH.39 The authors showed the advantages of selecting a specific anionic block composition for deposition, morphology, and film responsiveness to pH changes. A survey of the existing literature of both strong and weak water-soluble polyelectrolytes utilized in LBL reveals that little work has been conducted with well-defined homopolymers and copolymers. Instead, multilayered films have been largely limited to biologically produced or synthetic polyelectrolyte pairs with limited control of key macromolecular parameters including microstructure, molecular weight, molecular weight distribution, charge type, charge distribution, and conformation. The rapid development of controlled/“living” radical polymerization (CLRP) techniques40 including the versatile reversible additionfragmentation chain-transfer (RAFT) polymerization process41,42 now allows the preparation of well-controlled polyelectrolyte and polyzwitterionic homopolymers as well as statistical, block, graft, and star copolymers from a variety of monomers directly in water43-52 without the necessity of post-polymerization modification required in classical living polymerization. The objectives of the work reported here are to (a) synthesize via RAFT a series of well-defined strong (SA), weak (WA), and mixed anionic polyelectrolyte homopolymers and copolymers and their strong (SC) and weak (WC) cationic polyelectrolyte counterparts utilizing acrylamido monomers with sulfonate, carboxylate, or ammonium groups (Figure 1); (b) prepare LBL (32) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780-9787. (33) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458-4465. (34) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (35) Mermut, O.; Barrett, C. J. Phys. Chem. B 2003, 107, 2525-2530. (36) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 43094318. (37) Kujawa, P.; Moraille, P.; Sanchez, J.; Badia, A.; Winnik, F. J. Am. Chem. Soc. 2005, 127, 9224-9234. (38) Kujawa, P.; Sanchez, J.; Badia, A.; Winnik, F. M. J. Nanosci. Nanotechnol. 2006, 6, 1565-1574. (39) Tjipto, E.; Quinn, J. F.; Caruso, F. Langmuir 2005, 21, 8785-8792. (40) For an extensive presentation of controlled/living radical processes, see AdVances in Controlled/LiVing Radical Polymerization; Matyjaszewski, K., Ed.; ACS Symposium Series 654; American Chemical Society: Washington, DC, 2003. (41) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379-410. (42) Favier, A.; Charreyre, M. T. Macromol. Rapid Commun. 2006, 27, 653692. (43) McCormick, C. L.; Lowe, A. B. Acc. Chem. Res. 2004, 37, 5, 312-325. (44) Vasilieva, Y. A.; Thomas, D. B.; Scales, C. W.; McCormick, C. L. Macromolecules 2004, 37, 2728-2737. (45) Sumerlin, B. S.; Lowe, A. B.; Thomas, D. B.; Convertine, A. J.; Donovan, M. S.; McCormick, C. L. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 17241734. (46) Sumerlin, B. S.; Lowe, A. B.; Thomas, D. B.; McCormick, C. L. Macromolecules 2003, 36, 5982-5987. (47) Donovan, M. S.; Lowe, A. B.; Sanford, T. A.; McCormick, C. L. J. Polym. Sci.,Part A: Polym. Chem. 2003, 41, 1262-1281. (48) Donovan, M. S.; Sumerlin, B. S.; Lowe, A. B.; McCormick, C. L. Macromolecules 2002, 35, 8663-8666. (49) Lowe, A. B.; McCormick, C. L. Chem. ReV. 2002, 102, 4177-4190. (50) Lowe, A. B.; McCormick, C. L. Aust. J. Chem. 2002, 55, 367-379. (51) Sumerlin, B. S.; Donovan, M. S.; Mitsukami, Y.; Lowe, A. B.; McCormick, C. L. Macromolecules 2001, 34, 6561-6564. (52) Mitsukami, Y.; Donovan, M. S.; Lowe, A. B.; McCormick, C. L.; Macromolecules 2001 34, 2248-2256.
Langmuir, Vol. 23, No. 1, 2007 231
Figure 1. Structures of polymers prepared for LBL assembly: (SA) poly(sodium 2-acrylamido-2-methylpropanesulfonate) P(AMPS), (WA) poly(sodium 3-acrylamido-3-methylbutonate) P(AMBA), (B) poly(sodium 3-acrylamido-3-methylbutonate-b-sodium-2-acrylamido-2-methylpropanesulfonate) P(AMBA-b-AMPS), (R) poly(sodium 3-acryla-mido-3-methylbutonate-r-sodium-2-acrylamido2-methylpropanesulfonate) P(AMBA-r-AMPS), (WC) protonated poly(N-[(dimethylamino)ethyl]acrylamide) P(DMAEA), and (SC) quaternized poly(N-[(dimethylamino)ethyl]acrylamide) P(DMAEA). S or W represents the strong or weak nature of each polyelectrolyte whereas A or C represents the anionic or cationic character. B and R are block and random copolymers containing both strong and weak anionic segments.
films from these series and assess the effects that pH and ionic strength have on film thickness and morphology as measured by ellipsometry and AFM; and (c) evaluate the response of multilayer assemblies to changes in pH in situ using AFM techniques. The attributes of precise control of the molecular weight and molecular weight distribution by RAFT, the similar reactivities of the acrylamido functional monomers, the specific charge spacing along the resulting backbones, and the anticipated responses to changes in pH simplify a number of issues related to polymer solubility and charge registry during film formation. To our knowledge, this is the first report of LBL assembly of RAFTsynthesized homo- and copolyelectrolyte multilayer complexes. Experimental Section Materials. All reagents were purchased from Aldrich at the highest purity available and used as received unless otherwise stated. Sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) was recrystallized twice from methanol prior to use, and sodium 3-acrylamido-3methylbutanoate (AMBA) was synthesized as previously reported53 and recrystallized twice from methyl ethyl ketone. A procedure reported by Chang et al.54 was followed to produce N-[(dimethylamino)ethyl]acrylamide (DMAEA). 4,4′-Azobis(4-cyanopentanoic acid) (V-501), from Wako Chemicals, was recrystallized from methanol prior to use; 2-(1-carboxy-1-methyl-ethylsulfanylthiocarbonylsulfanyl)-2-methyl-propionic acid (CMP) was donated by Noveon and used as received. 4-Cyanopentanoic acid dithiobenzoate (CTP) was synthesized and purified as previously reported.52 Polymer Synthesis. AMPS and AMBA Polymers. Homo and block copolymers of AMPS and AMBA (Figure 1) were prepared using a method similar to that previously reported.46,51 Briefly, homopolymers of AMPS and AMBA were prepared in water at 70 °C with V-501 as the initiator and CTP as the RAFT chain-transfer agent. The [CTA]/[V-501] ratio was a 5:1 mole basis, with [CTA] ) 2.54 × 10-4 mol and [V-501] ) 5.07 × 10-5 mol. To ensure that the acid functional group on the monomer was neutralized, the pH of the polymerization solution was adjusted to 8.4 ( 0.1 by the addition of NaOH. The monomer/chain-transfer agent, [M]/[CTA], ratios were chosen such that, at a quantitative conversion, number average (53) Hoke, D. I.; Robins, R. D. J. Polym. Sci., Part A: Polym. Chem. 1972, 10, 3311-3315. (54) Chang, Y.; McCormick, C. L. Macromolecules 1993, 26, 6121-6126.
232 Langmuir, Vol. 23, No. 1, 2007
Morgan et al. Table 1. Characterization Data for RAFT Polymerizations
conv (%)a
Mn (theory)b,c
Mn (expt)c,d
Mw/Mnd
DPn AMBA
DPn AMPS
molar comp ASEC (AMBA/AMPS)d
molar comp NMR (AMBA/AMPS)e
38/62
sample
des.
P(DMAEA) P(AMPS) P(AMBA) P(AMBA)macro-CTA P(AMBA40-bAMPS60) P(AMBA50-bAMPS50) P(AMBA60-bAMPS40) P(AMBA50-rAMPS50)
f
C SA WA
19 85 78 79
19 100 17 000 18 200 13 600
17 000 16 200 17 500 13 800
1.20 1.15 1.20 1.08
B1
93
40 100
38 200
1.17
70
106
40/60
B2
57
29 900
28 400
1.09
68
70
50/50
B3
37
24 400
26 400
1.09
70
50
58/42
R
95
33 400
35 500
1.12
79
77
50/50
59/41
a Determined from the residual monomer concentration obtained from the RI detector during SEC. b Determined using Mn(theory) ) ([M][MWmon] × conversion)/([CTA] + MW(macro)CTA). c Values in g/mol. d Calculated by ASEC. e Determined using 1H NMR spectroscopy. f In quaternized form, SC; in protonated form, WC.
degrees of polymerization (DPn) of 73 and 93 would be attained, with [M] ) 0.022 and 0.026 mol, respectively. Polymerizations were conducted under nitrogen in a 22 mL reaction vial equipped with a magnetic stir bar. Polymers were purified by dialysis against deionized water and isolated by lyophilization. The RAFT-produced AMBA homopolymer, DPn 73, was used as a macro-CTA for the synthesis of AMBA/AMPS block copolymers. All polymerizations were performed in water (pH 8.0 ( 0.1) at 70 °C with V-501 as the initiator and at 1 M [M]. The [macro-CTA]/ [V-501] ratio was a 5:1 mole basis, with [macro-CTA] ) 2.58 × 10-4 mol and [V-501] ) 5.17 × 10-6 mol. An equimolar amount of NaOH was added prior to polymerization to ensure the neutrality of the acid functional groups. The [M]/[macro-CTA] ratio was held constant for all polymerizations at 1:125, [M] ) 0.0041 mol, and the block lengths were varied by stopping the polymerizations at predetermined times. A statistical copolymer was also prepared via RAFT using equimolar quantities of AMPS and AMBA monomers. Conversions, theoretical and experimental molecular weights, polydispersity indices, segmental degrees of polymerization, and molar compositions for all homo- and copolymer systems are given in Table 1. DMAEA Homopolymer. The polymerization of DMAEA was conducted in DMF at 70 °C with V-501 as the initiator and CMP as the RAFT chain-transfer agent. The [CTA]/[V-501] ratio was a 5:1 mole basis with [V-501] ) 1.00 × 10-5 mol. DMAEA was vacuum distilled prior to polymerization to ensure the purity of the monomer. The target DPn of 118 was achieved by calculating the [M]/[CTA] ratio such that a quantitative conversion could be reached, [M] ) 0.035 mol. Polymerization was conducted in a 50 mL roundbottomed Schlenk flask equipped with a magnetic stir bar and degassed by three freeze/pump/thaw cycles. To form polycationic species, the product was then either protonated or quaternized with hydrochloric acid and iodomethane, respectively, and purified by dialysis against deionized water followed by isolation via lyophilization. Polymer Characterization. MALLS/SEC Analysis. The molecular weights of P(AMPS) and P(AMBA) (co)polymers were determined by aqueous size-exclusion chromatography (ASEC) at 25 °C using Super AW 3000 and 4000 columns. The mobile phase consisted of 20% acetonitrile/80% 0.05 M Na2SO4 with a flow rate of 0.35 mL/min maintained with an Agilent 1100 series isocratic pump. Detectors included a Wyatt Optilab DSP interferometric refractometer, a Wyatt DAWN EOS multiangle laser light scattering (MALLS) detector operating at λ ) 690 nm, and a Polymer Labs LC1200 UV/vis detector. The molecular weight and polydispersity data were determined using the Wyatt ASTRA SEC/LS software package. Values of dn/dc in the mobile phase were determined to be 0.121, 0.123, and 0.122 for the P(AMPS), P(AMBA), and P(AMPS)/P(AMBA) polymers, respectively. SEC traces are shown in supplementary Figure S-1 (Supporting Informa-
tion) for the parent AMBA CTA and the AMBA/AMPS block copolymers. Molecular weights of P(DMAEA) were determined prior to protonation or quaternization. Molecular weight analysis was performed by gel permeation chromatography at 60 °C utilizing DMF as the mobile phase with a flow rate of 0.5 mL/min. A Polymer Labs PL gel 5-µm mixed C column was used along with a ViscotekTDA (302 RI, viscosity, 7 mW, 90 and 7° true low-angle light scattering detectors, λ ) 670 nm). The molecular weight and polydispersity index data were determined using the OmniSEC 2.02 analysis software package and summarized in Table 1. A dn/dc value of 0.124 was determined for P(DMAEA) in DMF. NMR Spectroscopy. All 1H and 13C NMR spectra were recorded with a Bruker AC-200 NMR spectrometer. Polymer samples were prepared as 2% w/w solutions in D2O with HOD as the internal reference, and DMAEA monomer samples were prepared in CDCl3 with TMS as the internal reference. Dynamic Light Scattering. Dynamic light scattering studies of the block copolymers in aqueous solution, 1% w/v, were conducted using a Malvern Instruments Zetasizer Nano Series instrument equipped with a 22 mW He-Ne laser operating at λ ) 632.8 nm, an avalanche photodiode detector with high quantum efficiency, an ALV/LSE-5003 multiple tau digital correlator electronics system, and an MPT-2 multi purpose titrator. Online particle-size measurements were taken every 0.5 ( 0.1 pH units between pH 9.0 and 6.0 and every 0.25 ( 0.1 pH units from pH 6.0 to 2.0. Multilayer Film Assembly. Substrate Preparation. Silicon wafers, approx 1 × 1 cm2, were cleaned with piranha solution (70: 30 concd H2SO4/30% H2O2) for 4 h at 80 °C to ensure a clean and uniform oxide surface. Wafers were then rinsed with DI water, ethanol, and methylene chloride and dried with a gentle stream of nitrogen. The clean silicon wafers were used immediately. Preparation of Multilayered Films. Polyelectrolyte deposition solutions were prepared by dissolving the polymer in 0.1 M sodium chloride or HPLC-grade water with a repeat unit molar concentration of 1.27 × 10-4 mol mL-1. The pH of the solution was adjusted to 5.5 ( 0.1 for cationic polymers and to 5.5 ( 0.1 or 7.0 ( 0.1 for anionic polymers by adding appropriate amounts of 0.1 M HCl or 0.1 M NaOH. Multilayer films were produced by alternately exposing the substrate to the respective solutions of polycation and polyanion for 20 min. The substrate was rinsed between each deposition by dipping 10 times in 3 different beakers containing HPLC-grade water. A gentle stream of nitrogen was used to dry the multilayers after each anionic polyelectrolyte adsorption step. Five thickness measurements were then conducted via ellipsometry. Note throughout this work that a layer is defined as one polyanion or polycation layer and a bilayer is defined as the layer pair of a polyanion and a polycation. Multilayer Film Characterization. Ellipsometry. Ellipsometry measurements were performed on a Gaertner Scientific LSE-Stokes
Layer-by-Layer Assembly of (Co)polyelectrolytes ellipsometer with an angle of incidence of 70° using a 632.8 nm He-Ne laser and Gaertner GEMP software after each anionic polyelectrolyte deposition step. An estimated refractive index value of 1.5 was utilized. The value was estimated by creating spin-coated films at 200 rpm on clean silicon substrates of all polymers utilized for this study. These films were then analyzed by ellipsometry using the software provided to estimate a refractive index value. Because of the interpenetration of layers, a single refractive index value was used by averaging all experimentally found values. Atomic Force Microscopy (AFM) Imaging. Atomic force microscopy imaging and film thickness measurements were made with a Nanoscope IIIa MultiMode AFM (Digital Instruments Inc.). The film morphology of dried assembled multilayers was investigated in tapping-mode operation in air. A DNISP, steel cantilever mounted with a diamond tip (Veeco Probes, Santa Barbara, CA) was used for dry imaging and dry film thickness measurements. The AFM was calibrated using a platinum reference grid, and the deflection sensitivity of the DNISP probe was found each time an adjustment was made to the laser. The deflection sensitivity was found from indentation on a sapphire surface at forces used for film thickness measurements. Force curves generated during the indentions were used to find the film thickness. Displacement of the piezoelectric actuator (∆zp) and tip deflection (∆zt) are related to the indentation displacement (∆zi) by ∆zi ) ∆zp - ∆zt cos(10°) where ∆zt is multiplied by cos(10°) to account for the angle of the probe relative to the horizontal.55 Initial contact with the surface was determined as the point where the oscillation of the cantilever ceased, and tip displacement began to increase with movement of the z piezo. Contact with the silicon substrate was determined as the point where ∆zt and ∆zp began to increase at a linear rate, reflecting the deformation of the silicon substrate. The difference between zi at initial contact and contact with the substrate was taken as the thickness of the multilayer. To analyze film response as a function of changes in pH, fluid imaging studies were conducted with RTESP, etched silicon probes (Veeco Probes, Santa Barbara, CA) in a multimode fluid cell. Samples were first immersed in DI water in the fluid cell for a period of 1 h and imaged in tapping mode. Aqueous solutions were then adjusted to pH 2.5 or 9.0 by the addition of 1 M HCl or 1 M NaOH. Films were then reimaged after 1 h of exposure to the pH 2.5 or 9.0 aqueous solutions. To obtain thickness measurements of the fluidswollen films, portions of the film were removed using the edge of a razor blade. The edge of the remaining film and substrate was then imaged under aqueous conditions to determine the thickness of the film. Image processing was performed using Veeco version 5.30R3.Sr2 software. The image root-mean-square roughness (rms) is calculated as the root-mean-square average of the height deviations taken from the mean data plane. The errors reported for the film thickness are the standard deviations of all of the measurements taken. Fluid thickness values were recorded using average cross-section analysis from different areas of several samples.
Results and Discussion Rationale for Structural Selection and Synthesis. The model homopolymers and copolymers shown in Figure 1 were specifically targeted and synthesized on the basis of the strong/weak characteristics of the side-chain functional groups and our ability to control sequence length and random or block character precisely in a facile manner utilizing aqueous RAFT polymerization techniques. In these series of (co)polymers, all charged mer units, designated as strong or weak anions or cations in Figure 1, occur at identical spacing from the polymer backbone. The carboxy (55) Van Landingham, M. R.; McKnight, S. H.; Palmese, G. R.; Elings, J. R.; Huang, X.; Bogetti, T. A.; Eduljee, R. F.; Glillespie, J. W. J. Adhes. 1997, 64, 31-59.
Langmuir, Vol. 23, No. 1, 2007 233
Figure 2. Hydrodynamic diameter as a function of pH: [ P(AMBA60-b-AMPS40) B1 and 9 P(AMBA40-b-AMPS60) B3. (See Table 1.)
groups (WA) and protonated amine moieties (WB), as contrasted with strong sulfonate (SA) anions and strong (SC) cations of the quarternary ammonium groups, are expected to respond to pH changes. Additionally, the strength of the cation/anion complex formed during LBL deposition and the integrity of the generated film will be affected by ionic strength. Copolymers B1-B3, composed of strong sulfonate and weak carboxylate groups, are of particular interest because they undergo pH/reversible micellization in aqueous solution as demonstrated by dynamic light scattering (Figure 2). Above pH 5, these hydrophilic/hydrophobic anionic block copolymers behave as unimers in solution. Below pH 5, protonation of the weak AMBA carboxylate groups occurs resulting in hydrophobic/hydrophobic blocks that organize into micellelike structures. Thus films assembled with these block copolymers at pH 7 may exhibit pH-responsive reorganization on lowering pH to values below 5. Block Copolymer Synthesis and Characterization. Table 1 lists characterization data including the structural composition, molecular weight, polydispersity, and segmental molecular weight for each of the RAFT-synthesized homopolymers, block copolymers, and random copolymer. Excellent agreement was found between predicted theoretical molecular weights and those found experimentally utilizing aqueous size-exclusion chromatography/ multiangle laser light scattering (SEC/MALLS) analysis of aliquots taken at specific reaction conversions. For simplicity, designations for each polymer are listed in Figure 1 and Table 1. The strong and weak polyanions generated from AMPS and AMBA are designated SA and WA, mixed AMBA/AMPS block copolymers B1-B3, and the random copolymer R. The strong (SC) and weak (WC) designations are given to the quaternized and protonated cationic derivatives of P(DMAEA), respectively. Assembly and Characterization of Multilayer Films. To explore the stimuli-responsive behavior of these systems, LBL films were produced under three sets of conditions: deionized water solution, saline solution, and solutions of varying pH. The response of selected systems to changes in pH was evaluated via in-situ AFM studies. All multilayer films were prepared as detailed in the Experimental Section by first allowing the silicon substrate to contact aqueous polycation solutions in the quaternized (SC) or protonated (WC) form. Following an extensive rinsing cycle in deionized water, the strong (SA), weak (WA), block (B), or random (R) polyanion solutions were applied at pH values of 5.5 or 7.0. The process was repeated with alternate exposure to cationic and anionic polyelectrolytes with intermediate rinsing until the desired multilayers were prepared. Keeping the pH
234 Langmuir, Vol. 23, No. 1, 2007
Morgan et al.
Table 2. Compositional Data for Multilayer Films Formed from the Deposition of Anionic Polyelectrolyte Solutions at pH 7.0 in Deionized Water entry number
a
multilayer filma
anionic polymerb
degree of ionization (%)c
cationic polymerb
degree of ionization (%)c
quaternized P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) quaternized P(DMAEA)
100
2.1
SA-SC
P(AMPS)
100
2.2
WA-SC
P(AMBA)
82
2.3
SA-WC
P(AMPS)
100
2.4
WA-WC
P(AMBA)
82
2.5
B2-SC
AMBA50-b-AMPS50
91
2.6
R-SC
AMBA50-r-AMPS50
91
100 95 95 100 100
Twenty layers. b See Figure 1 for chemical structures. c Estimated from pKa values in the deposition solutions.
Scheme 1. Idealized LBL Deposition Depicting Electrostatically Driven Layer-by-Layer Assemblya
Table 3. Structures and Degrees of Ionization for Anionic Polyelectrolyte Solutions Utilized for LBL Assembly with Strong (SC) Polycations from Deionized Water at pH 5.5 entry number
multilayer filma
anionic polymerb
degree of ionization (%)c
3.1 3.2 3.3 3.4 3.5 3.6
SA-SC WA-SC B1-SC B2-SC B3-SC R-SC
P(AMPS) P(AMBA) AMBA40-b-AMPS60 AMBA50-b-AMPS50 AMBA60-b-AMPS40 AMBA50-r-AMPS50
100 50 80 75 71 75
a Twenty-four layers. b See Figure 1 for structures. c Determined from pKa values.
a
Polymers with a higher effective charge density, SC and SA, deposit in more extended conformations than those with lower charge densities.
value above 5.5 assured that the homopolymer and copolymers containing P(AMBA) segments would remain soluble and not phase separate into micelles (Figure 2). It should also be noted that the sulfonated polyanion (SA) segments and the quaternized cationic polyelectrolyte (SC) should not be affected by pH whereas pH determines the degree of ionization of anionic AMBA (WA) and protonated ammonium (WC) cationic segments of the applied solutions. Films Prepared in Deionized Water Solution. DI Water, pH 7. The LBL deposition behavior of cationic and anionic polyelectrolytes is generally discussed in terms of the simple electrostatically driven model shown in Scheme 1, with thinner multilayer films formed from strong anionic (SA)/strong cationic (SC) polyelectrolytes as compared to their weak polyanionic (WA)/polycationic (WC) counterparts. In Table 2, we present pertinent information for multilayer films (entries 2.1-2.6) formed by utilizing weak and strong polycations with a series of weak, strong, and mixed (block, random) anionic polyelectrolytes, with the latter deposited in their most extended states (solution pH of 7.0). As can be seen in Figure 3, the SA-SC
combinations, as might be expected from their respective degrees of ionization, are adsorbed onto the cationic surfaces in very thin layers with total thickness increasing in an approximately linear fashion for successive layers, consistent with current models for single-layer polyelectrolyte adsorption.56 Thicker films result from partially charged systems, with increasing film thickness in the order of SA-SC < WA-SC < SA-WC < WA-WC. This behavior is attributed to the lower-charge-density polyelectrolytes adopting a more coiled conformation (as a result of the diminished intramolecular repulsive forces).57-60 The adsorption of these coiled chains with fewer anionic groups onto the cationic surface leads to an increase in the number of loops and tails present, allowing more chains to adsorb onto the surface before charge reversal occurs, creating thicker layers. Tapping-mode AFM images of these systems reveal raised spherical structures on the surfaces, which are presumably isolated areas of polymer aggregation37,38 and indicate the presence of poorly coalesced films (inset of Figure 3). DI Water, pH 5.5. The amphoteric effect is clearly seen for films assembled at pH 5.5 utilizing the strong polycation (SC) and the respective strong (SA), weak (WA), and mixed (B, R) polyanions (Tables 3 and 4, Figure 4). At pH 5.5, the films with weak (WA) P(AMBA) segments exhibit thicknesses that are an order of magnitude higher than those assembled at pH 7 and in general exhibit greater roughness. These systems also display nonlinear deposition. Significant increases in slope are apparent after the deposition of three to four bilayers. AFM images of these pH 5.5 assemblies reveal more continuous structures than (56) Van de Steeg, H. G. M.; Stuart, M. A. C.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538-2546. (57) Schoeler, B. Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889-897. (58) Schoeler, B.; Poptoshev, E.; Caruso, F. Macromolecules 2003, 36, 52585264. (59) Steitz, R.; Jaeger, W.; von Klitzing, R. Langmuir 2001, 17, 4471-4474. (60) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823-833.
Layer-by-Layer Assembly of (Co)polyelectrolytes
Langmuir, Vol. 23, No. 1, 2007 235
Figure 3. Multilayers assembled in DI water (pH 7.0) utilizing poly(AMBA) (WA) and poly(AMPS) (SA) polyanions with protonated (WC) and quaternized (SC) P(DMAEA) polycations. (See Table 2.) [, Entry 2.4; 9, entry 2.3; ], entry 2.2; 0, entry 2.1. Assembly thickness was measured via ellipsometry. Solid lines are a guide to the eye, and y error bars represent the range of thicknesses between samples. AFM images represent entries 2.2 (top) and 2.1 (bottom), with an image area of 5 µm2.
Figure 4. Average thickness vs layer number for films assembled from anionic polyelectrolyte solutions at pH 5.5 or 7 and strong (quaternized) polycationic solutions. (See Tables 2 and 3.) Thickness values were determined via ellipsometry. 2 Entry 3.2 (WA-SC), pH 5.5; [ entry 3.6 (R-SC), pH 5.5; 9 entry 3.4 (B2-SC), pH 5.5; ] entry 2.6 (R-SC), pH 7; 0 entry 2.5 (B2-SC), pH 7; 4 entry 2.2 (WA-SC), pH 7; × entry 2.1 (SA-SC), pH 7. Lines are a guide to the eye. Error bars represent one standard deviation for the samples measured. AFM images show entry 3.2 (WA-SC) at 4-bilayer thickness (bottom) and 12-bilayer thickness (top). The image area is 5 µm2 with a z range of 50 nm.
those observed at pH 7. These continuous structures appear to form as the number of bilayers increases. As reported by Winnik’s group,37,38 at a low number of deposited bilayers, spherical “islands” are observed, apparently because of isolated molecules or aggregates on the surface. As the number of bilayers is increased, these islands appear to coalesce into larger structures, and at higher numbers of deposited bilayers, a smooth, continuous surface is revealed. Choi and Rubner21 also observed thickness changes with weak polyelectrolytes. They suggested that when the degree of ionization falls below a critical value, dramatic increases in bilayer thickness occur driven by the less densely charged polymer in contact with a high charge density polymer of opposite charge. The degree of ionization of the adsorbing weak polyelectrolyte increases because of a change in the local environment. However, if the increased charge density is still below the critical value, then the polymer chains experience a thermodynamic energy barrier described by Park and co-workers.30,61 The enthalpic gain (61) Park, S. Y.; Barrett, C.; Rubner, M. F.; Mayes, A. M. Macromolecules 2001, 34, 3384-3388.
Table 4. Thicknesses and Roughness Values for LBL Films Assembled Using Strong, Weak, and Mixed Anionic Polyelectrolyte Solutions and Strong Cationic Polyelectrolyte Solutions thickness entry multilayer no. film 4.1b WA-SC 4.2c B3-SC 4.3c B1-SC 4.4d B2-SC 4.5d R-SC 4.6e WA-SC 4.7e SA-SC d
anionic polymer P(AMBA) AMBA60-bAMPS40 AMBA40-bAMPS60 AMBA50-bAMPS50 AMBA50-rAMPS50 P(AMBA) P(AMPS)
AFM ellipsometry pH (nm) SD (nm) SD rmsa 5.5 89.2 7.9 5.5 60.8 9.6
99.7 67.6
4.1 6.8 2.5 4.5
5.5 28.5 6.7
31.2
2.2 2.3
5.5
124
19 37
5.5
246
7
8.7 6.3
0.7 2.7 0.5 1.6
7 7
10.3 2.5 10.7 1.6
24
a Determined by AFM. b Eight-layer film. c Twelve-layer film. Twenty-four-layer film. e Twenty-layer film.
of free energy for the polymer to adsorb electrostatically onto the cationic surface in an extended state in such case is not high
236 Langmuir, Vol. 23, No. 1, 2007
Figure 5. Tapping-mode AFM images of multilayer films of polycation (SC) with (left) block copolymer (B), entry 3.4, and (right) random copolymer (R), entry 3.6 (Table 3). The image area is 5 µm2 with a z range of 200 nm.
enough to compensate for the loss in conformational entropy; therefore, the polymer chains adsorb with poorer interchain registry and possess a larger number of loops and tails. The degree of ionization of the polyelectrolyte is a key factor in determining the multilayer thickness and morphology, but the nature of the functional groups and their respective placement along the polymer backbone are also important. For example, random (R) and block (B) copolymers composed of equal numbers of AMBA (WA) and AMPS (SA) units possess equivalent degrees of ionization (75%), similar molecular weights and molecular weight distributions, and repeating structures with equivalent distances of pendent anionic groups from the polymer backbone. The thickness of the film built from the random copolymer (R) (entry 3.6), however, is double that produced from the block copolymer (entry 3.4) (Table 3, Figure 4). Because the total number of ionized carboxyl groups is similar at this pH, conformational differences arising from charge-charge and hydrogen bonding interactions in the “pure” block and mixed anionic segments appear to be important. The morphology of these multilayer films is also notably different (Figure 5). As another example of this phenomenon, block copolymers B1-B3 of differing block lengths yield films of different thicknesses (Table 4). Multilayers Prepared in Saline Solution. The effects of the addition of 0.1M NaCl to the respective polyanionic and polycationic solutions were investigated. The same experimental protocol as described for deionized water was otherwise followed (Table 5). The addition of 0.1 M NaCl to the assembly solutions results in partial shielding of the charges along the polymer backbone, enabling the polymer chain to adopt a less extended, more random coil conformation, thus creating thicker films. As expected, the weak polycation (protonated, WC) yields multilayers of greater thickness than the strong polycation (quaternized, SC) when assembled with both strong and weak polyanions in 0.1 M saline solution (Figure 6). This is hypothesized to arise from changes in the degree of ionization and thus the conformation of the (protonated, WC) P(DMAEA) chains as dictated by the local molecular environment and is consistent with findings of other researchers.21,22,24,62 In deionized water, deprotonation (and loss of the chloride anion) occurs along the polymer backbone to reach a more entropically favored state. The resulting chains with fewer cationic charges thus adsorb onto the anionic surface with more loops and tails. The ionic strength (0.1 M NaCl) also reduces electrostatic interactions. Both effects enable more chains to be adsorbed onto the surface prior to charge reversal, creating thicker films. Unlike protonated P(DMAEA) (WA), the quaternized P(DMAEA) (SC) has permanently fixed charges along the polymer (62) Dautzenberg, H.; Hartmann, J.; Grunewald, S.; Brand, F. Ber. BunsenGes. Phys. Chem. 1996, 100, 1024-1032.
Morgan et al.
backbone. These polymer chains cannot reduce their charge density and thus deposit in a more extended state in deionized water. Therefore, the more rodlike structures with better “registry” of the resulting interpolyelectrolyte complexes create thinner layers compared to multilayers assembled with protonated P(DMAEA). As would be expected on the basis of the calculated degrees of ionization, films of the weak polyanion P(AMBA) from pH 5.5 solutions are thicker than those from pH 7 solutions (Figure 6B and Table 6). AFM images are shown in Figure 7 and can be rationalized on the basis of polyelectrolyte structure. In general, LBL films produced from the protonated P(DMAEA) (WC) polymer exhibit less distinct morphological features and appear to be more continuous than the SC films. Presumably, as the films become thicker, areas between isolated raised loop and tail structures are filled with polyelectrolyte layers, and the films appear to be more continuous. The wormlike “vermiculate” morphology reported by other researchers37,38,63 is observed for strong (SA) polyanion P(AMPS) assemblies with both strong and weak polycations, whereas the mixed (strong/weak) block copolymer and weak (WA) polyanion P(AMBA) multilayers display raised spherical features. Figure 8 depicts the LBL assembly behavior of the various films prepared in deionized (DI) water or 0.1 M NaCl at either pH 5.5 or 7.0. LBL addition of either strong (SA) or weak (WA) anionic polyelectrolytes at pH 7 to the strong (SC) polycations in water or in 0.1 M NaCl gives the expected results of linear film growth with increasing number of layers (Figure 8A,C). However, the exponential increase in layer thickness utilizing pH 5.5, low ionic strength solutions of the weak (WA) poly(AMBA) and the strong (SC) polycation is unexpected. This contrasts with the linear LBL growth at pH 5.5 of the same WA-SC combination in 0.1 M NaCl (Figure 8B). This behavior is a strong indication that during deposition not only do the partially ionized AMBA (WA) segments interact with the cationic surface electrostatically but also strong hydrogen bonds likely form between multiple (WA) chains via carboxylate and possibly amide groups as idealized in Scheme 2. These strong interactions not only prevent deposition in an extended state but also lead to changes in water structuring. (An increase in the number of protonated carboxyl groups eventually leads to phase separation as discussed previously and shown in Figure 2.) It should be noted that other groups have also reported multilayer adsorption caused or enhanced by hydrogen bonding.5,64,65 It is interesting that the addition of 0.1 M NaCl is enough to disrupt these carboxylic acid‚‚‚carboxylate and/or carboxylic acid‚‚‚ amide hydrogen bonding interactions allowing linear growth. Multilayer Film Response to pH. The pH responsiveness and integrity of selected multilayer films were investigated via tappingmode AFM imaging in a fluid cell. Multilayer films were exposed to pH 2.5 or 9.0 solutions for 1 h prior to in situ AFM analysis. Results are summarized in Table 7. Multilayer films prepared with the strong (SA) polyanion, P(AMPS), remain intact when exposed to both high and low pH, as evidenced by the consistency of measured thickness and roughness (Table 7). This behavior is expected for the tightly bound SA-SC electrostatic complexes, which are insensitive to changes in pH. By contrast, films produced from the weak (WA) anionic P(AMBA) solution of pH 7.0 quickly lose integrity when exposed to both pH 2.5 and 9 solutions (entry 7.2). Significant (63) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655- 6663. (64) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301-310. (65) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Macromolecules 2005, 10523-10531.
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Langmuir, Vol. 23, No. 1, 2007 237
Figure 6. Average thickness vs layer number for films described in Table 5. Panel A shows the LBL deposition behavior of strong, weak, and block (mixed) polyanions onto weak (WC) and strong (SC) polycations at pH 7.0 as follows (top to bottom): 9 entry 5.2 (SA-WC), [ entry 5.4 (WA-WC), 2 entry 5.6 (B3-WC), 0 entry 5.1 (SA-SC), 4 entry 5.5 (B3-SC), ] entry 5.3 (WA-SC). Panel B shows the deposition behavior at pH 7.0 or 5.5 of weak anionic polyelectrolyte P(AMBA) onto the respective weak (WC) or strong (SC) polycation layers. Slopes decrease top to bottom: 9 entry 5.8 (WA-WC), [ entry 5.4 (WA-WC), 0 entry 5.7 (WA-SC), ] entry 5.4 (WA-SC). Scheme 2. Idealized Depiction of the Effects of Hydrogen Bonding on Weak Anionic Polyelectrolyte P(AMBA) Complexation with a Cationic Polyelectrolyte Layer at pH 5.5 in Deionized Water
Figure 7. AFM images of 12-layer films (Table 5) prepared on Si substrates in 0.1 M NaCl solutions: (A) Entry 5.1 (SA-SC, pH 7), (B) entry 5.2 (SA-WC, pH 7), (C) entry 5.3 (WA-SC, pH 7), (D) entry 5.4 (WA-WC, pH 7), (E) entry 5.7 (WA-SC, pH 5.5), (F) entry 5.8 (WA-WC, pH 5.5), (G) entry 5.5 (B3-SC, pH 7), (H) entry 5.6 (B3-WC, pH 7). The image area is 5 µm2 and the vertical scale is 50 nm for all images.
film deconstruction results as indicated by roughness values approaching that of a clean silicon wafer. At the low pH, the carboxylate anions are protonated, resulting in the loss of their ionic nature. This, in turn, removes the electrostatic interpolymer complex forces required to hold the film together. When the film is exposed to the pH 9.0 solution, a progressive loss of
stoichiometric charge balance results from an increasing number of carboxy anions in the AMBA segments. The excess negative charges cause the destruction of the multilayer. The weak polyanion assemblies prepared from pH 5.5 solution (entry 7.3) exhibit a third type of behavior. Films exposed to low pH disintegrate, whereas films exposed to high pH remain intact. Such films do not change significantly when subjected to high pH because of the conditions at which the multilayer was assembled. During assembly, the weak AMBA monomer units are only about 50% ionized in the pH 5.5 solution. When the film is exposed to the pH 9.0 solution, additional carboxylic acid groups are converted to their ionic form, but these appear to enhance electrostatic interlayer complexation and result in a stable multilayer assembly. At pH 2.5, the ionized AMBA monomer units are protonated, resulting in the loss of electrostatic complexing and disintegration of the film. Films assembled from anionic block copolymers and the strong cationic homopolymer SC exhibit yet another type of behavior. The 40/60 WA/SA block copolymer (AMBA40-b-AMPS60) assembled from pH 5.5 solution shows behavior opposite that of the AMBA (WA) homopolymer. It is likely that major film deconstruction occurs at pH 9.0 as a result of the buildup of excess negative charges. The multilayer becomes thicker and rougher when exposed to pH 2.5 solution, which is likely the result of electrostatic stoichiometry loss and possible aggregation of protonated AMBA moieties. Again, as shown earlier in Figure 2, the AMBA/AMPS (WA/SA) copolymers form micelles below
238 Langmuir, Vol. 23, No. 1, 2007
Morgan et al.
Table 5. Compositional Data for Multilayer Films Formed from the Deposition of Weak, Strong, and Mixed Polyanionic Solutions at pH 7 and 5.5 in 0.1 M NaCl entry number
multilayer film
anionic polymer
degree of ionization (%)
cationic polymer
degree of ionization (%)
5.1
SA-SC
P(AMPS)
7
100
100
P(AMPS)
7
100
WA-SC
P(AMBA)
7
82
5.4
WA-WC
P(AMBA)
7
82
5.5
B3-SC
AMBA60-b-AMPS40
7
90
5.6
B3-WC
AMBA60-b-AMPS40
7
90
5.7
WA-SC
P(AMBA)
5.5
50
5.8
WA-WC
P(AMBA)
5.5
50
quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA)
5.2
SA-WC
5.3
pH
95 100 95 100 95 100 95
Table 6. Thickness Measurements and Roughness Values for Films Prepared in 0.1 M NaCl Saline Solution and Deionized Watera entry number
multilayer film
anionic polymer
6.1c
SA-SC
P(AMPS)
6.2c
SA-WC
P(AMPS)
6.3c
WA-SC
P(AMBA)
6.4c
WA-WC
P(AMBA)
6.5c
B3-SC
AMBA60-b-AMPS40
6.6c
B3-WC
AMBA60-b-AMPS40
6.7d
SA-SC
P(AMPS)
6.8d
SA-WC
P(AMPS)
6.9d
WA-SC
P(AMBA)
6.10d
WA-WC
P(AMBA)
cationic polymer
NaCl conc (M)
AFM (nm)
SD
ellipsometry (nm)
SD
RMSb
quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA) quaternized P(DMAEA) protonated P(DMAEA)
0.1
15.3
5.7
11.9
0.60
6.27
0.1
26.6
4.4
28.4
1.1
3.74
0.1
9.0
3.0
6.4
0.31
3.43
0.1
34.1
6.1
22.7
1.7
3.03
0.1
13.7
2.9
7.6
0.72
5.67
0.1
19.5
4.7
19.0
0.95
2.52
0
10.7
1.6
5.9
0.48
1.58
0
14.0
2.3
9.9
0.20
1.49
0
10.3
2.5
8.7
0.69
2.67
0
30.5
3.0
15.1
0.28
0.93
a Polycation P(DMAEA) solutions were applied at pH 5.5 in quaternized (SC) and protonated (WC) forms. Anionic solutions were applied at pH 7. b Determined by AFM. c Twelve-layer films. d Twenty-layer films.
Scheme 3. Idealized Illustration of the pH Response for Strong, Mixed Block, and Weak Polyanion/Strong Polycation Bilayers
a pH of 5.0 in solution, a result of the hydrophobic interaction of the nonionized AMBA (WA) repeat units. Any hydrophobically driven aggregation of the hydrophobic blocks would likely cause phase separation and an increase in apparent thickness and a less uniform surface. Because the AMPS (SA) block is permanently
charged, the system maintains the minimum sufficient electrostatic complexation required to prevent film deconstruction, at least over the time frame of these experiments. The 60/40 WA/SA block copolymer, however, shows increased film stability over the entire pH range, with only partial deconstruction after exposure to pH 2.5 solution and swelling on exposure to pH 9 solution. The swelling at pH 9 may be an indication of the start of film deconstruction when a slight excess of negative charges develops; however, this is not great enough over the time frame of these studies to cause an observable change in film properties. An illustration of the maintenance of electrostatic interactions between SA and SC segments and a loss of interactions between WA and SC segments is idealized in Scheme 3. Such a mechanism appears to be operative for the respective AMPS, AMBA, and AMBAb-AMPS anionic polyelectrolytes of this study. These initial film integrity studies with weak, strong, and mixed polyanions and a strong polycationic system indicate the possibility of precisely tailoring assemblies for responsiveness to pH. These five films (Table 7) demonstrate five different types of pH responsiveness: (1) complete insensitivity to pH 2.5-9.0 for the SA/SC assembly, (2) complete disintegration for the WA/SC assembly prepared at pH 7, (3) stability at high pH but disintegration at low pH for the WA/SC assembly prepared
Layer-by-Layer Assembly of (Co)polyelectrolytes
Langmuir, Vol. 23, No. 1, 2007 239
Figure 8. Average thickness vs layer number. (A) Multilayer films assembled with WA and SC at pH 7.0. [ Entry 5.3, WA-SC, saline; 9 entry 2.3, WA-SC, DI water. (B) Multilayer films assembled with P(AMBA) and quaternized P(DMAEA) at pH 5.5. [ Entry 3.2, WA-SC, DI water; 9 entry 5.7, WA-SC, saline. (C) Multilayer films assembled with P(AMPS) and quaternized P(DMAEA) at pH 7.0. [ Entry 5.1, SA-SC, saline; 9 entry 2.1, SA-SC, DI water. Best fit linear regression lines are drawn for A and C. Part B lines are a guide to the eye. DI - deionized water; NaCl (0.1 M). Table 7. Root-Mean-Square Values and Thickness Measurements of Multilayers Exposed to pH 2.5 or 9.0 Solutions in an AFM Fluid Cella after film treatment pH 2.5
before film treatment entry no.
multilayer film
pH
7.1 7.2 7.3 7.4
SA-SC WA-SC WA-SC B1-SC
7 7 5.5 5.5
7.5
B3-SC
5.5
after film treatment pH 9.0
anionic polymer
rms (nm)b
thickness (nm)c
SD
rms (nm)b
thickness (nm)c
SD
rms (nm)b
thickness (nm)c
SD
P(AMPS) P(AMBA) P(AMBA) AMBA40-bAMPS60 AMBA60-bAMPS40 silicon wafer
0.9 2.9 5.6 1.8
8.0 11.2 76.7 24.5
0.3 1.0 4.3 4.7
1.0 0.3 0.3 7.9
7.8 ∼0 ∼0 42.1
0.7 NAd NAd 9.7
1.1 0.2 3.2 0.4
8.1 ∼0 70.1 ∼0
1.0 NAd 5.6 NAd
3.3
49.4
7.9
7.3
36.9
4.2
3.1
58.4
7.9
0.18
0.18
0.18
a Films were assembled in deionized water, the polycation is in the quaternized (SC) form, and polycation solution was applied at pH 5.5. Anionic solutions were applied at pH 5.5 and 7 as noted. b Determined by AFM. c Determined by AFM via the scratch method. d Not available.
at pH 5.5, (4) swelling at low pH but disintegration at high pH for the 40/60 WA/SA block copolymer, and (5) stability at low pH but swelling at high pH for the 60/40 WA/SA block copolymer.
Conclusions RAFT polymerization was employed to create well-defined (co)polymers of P(AMPS), P(AMBA), P(AMBA-b-AMPS), and P(DMAEA) as shown by SEC/ MALLS and NMR spectroscopy. The sulfonated, carboxylated, and amine-substituted monomers utilized in the synthesis of homo- and copolyelectrolytes allowed an investigation of the complexation of weak, strong, and mixed multilayer films created under specified conditions of pH and ionic strength. The multilayer films were characterized by ellipsometry and atomic force microscopy, and their integrity was evaluated as a function of change in pH. The degree of ionization of the polyelectrolytes in solution during assembly
proves to be an important factor in determining the thickness of the multilayer films, as previously demonstrated. More important, however, is the role that the architecture of the polyelectrolyte molecules plays in determining film dimensions, morphology, and stimuli-responsive behavior. For example, random and block copolymers possessing equivalent degrees of ionization, molecular weights, molecular weight distributions, and repeating structures yield dramatically different film dimensions and morphology. Responsiveness to pH is determined by block copolymer composition and structure. These results indicate the potential of producing multilayer assemblies with controlled dimensions by simply altering the block composition of strong and weak segments in a controlled fashion, for example, with block, statistical, alternating, or tapered structures. These initial studies suggest a plethora of new opportunities in the area of stimuli-responsive assemblies, and we have clearly
240 Langmuir, Vol. 23, No. 1, 2007
explored only a very small number of possibilities. However, methodologies for precise control of MW, MWD, block length, and anionic/cationic registry have only recently become available for functional water-soluble polymers. Thus RAFT and other controlled/living polymerization methods should allow the preparation of specific combinations of multilayer assemblies with designed responsiveness to external stimuli. Potential applications include controlled activity pharmaceuticals, “smart” cosmetic formulations, surfaces with enhanced lubricity, responsive microfluidic channels, and shape-changing hydrogels for biomedical applications.
Morgan et al.
Acknowledgment. This work is supported by the MRSEC Program of the National Science Foundation under award number DMR 0213883. C.L.M. gratefully acknowledges the support of the U.S. Department of Energy (DE-FC26-01BC15317). Supporting Information Available: RI traces of block copolymers of 3-acrylamido-3-methylbutanoate (AMBA) and 2-acrylamido-2-methylpropanesulfonate (AMPS) obtained by aqueous sizeexclusion chromatography. This material is available free of charge via the Internet at http://pubs.acs.org. LA061638C
