Phase Separations in pH-Responsive Polyelectrolyte Multilayers

Department of Chemistry and Biochemistry, Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida ...
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Phase Separations in pH-Responsive Polyelectrolyte Multilayers: Charge Extrusion versus Charge Expulsion Zhijie Sui and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306-4390 Received February 16, 2004. In Final Form: April 13, 2004 Polyelectrolyte multilayers with continuously variable amounts of ionizable weak acid functionality were prepared by blending ionizable and nonionizable polyelectrolytes in the deposition solutions. Diluting ionizable groups in this way yielded multilayers that were more structurally stable, shown by thickness and atomic force microscopy measurements, as their internal polymer charge was varied by the pH of the external solution. Multilayers prepared with opposite surface charge to that appearing within the bulk (as a result of ionization) were more stable, as were thinner films, both results suggesting the extrusion of bulk charge to the surface. These multilayers were able to control the direction and magnitude of electroosmotic flow in microfluidics systems. Multilayers bearing only one, diluted layer of ionizable material were surprisingly effective in this respect.

Introduction Polyelectrolyte multilayers, PEMUs, are prepared by alternately adsorbing a pair of oppositely charged polyelectrolytes onto a substrate.1,2 Although extensive interpenetration of neighboring layers occurs, there is, nevertheless, “fuzzy” stratification within PEMUs over the range of a few “layer” thicknesses.3 This internal structuring is evidence for a limited range of motion of polyelectrolytes when they are complexed in such thin films and is consistent with the mechanism for buildup, whereby excess polymer charge is constrained near the surface, leading to a linear growth of polymer as a function of number of layers.4 There has been mounting evidence in recent years, however, for extensive polyelectrolyte mobility during or after PEMU construction under certain conditions.5-14 These internal rearrangements of polymer appear to be facilitated by conditions or treatments that minimize interactions between polyelectrolytes. For example, controlling the state of ionization of weak acid polyelectrolytes, such as polycarboxylates, within PEMUs, which has been * To whom correspondence should be addressed. E-mail: [email protected]. (1) Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003. (2) Decher, G. Science 1997, 277, 1232-1237. (3) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (4) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592-598. (5) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (6) Fery, A.; Scho¨ler, B.; Cassagneau, T.; Caruso, F. Langmuir 2001, 17, 3779-3783. (7) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 95509551. (8) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 37363740. (9) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368-5369. (10) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725-7727. (11) McAloney, R. A.; Dudnik, V.; Goh, M. C. Langmuir 2003, 19, 3947-3952. (12) Scho¨ler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889-897. (13) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575-1586. (14) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.-C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159-1162.

observed directly by infrared spectroscopy,7,15-19 leads to large-scale rearrangements of material.7,9 pH-induced phase separation, yielding pores, has been observed.5 At the extreme, complete dissociation of multilayers may be induced by pH control.7,9,16 Other evidence for enhanced polyelectrolyte mobility includes stripping from the surface during buildup12,20 and annealing of PEMU by salt, which loosens polyelectrolyte interactions.10,11 More recently, PEMU systems which grow exponentially, instead of linearly, provide evidence that one or both polyelectrolytes are able to access the entire film during growth, rather than simply a region close to the surface.13,14,21-25 In recent work on using pH-responsive PEMUs to control surface charge,26 employing a poly(acrylic acid) copolymer, we noticed that multilayers made with pure copolymer tended to be unstable with respect to morphology and properties. To circumvent this problem, the copolymer was coadsorbed with a homopolyelectrolyte to reduce the overall density of ionizable groups within the PEMU, yielding stable multilayers. Since pH-sensitive PEMUs have been proposed for many applications,26-30 it (15) Xie, A. F.; Granick, S. Macromolecules 2002, 35, 1805-1813. (16) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301310. (17) Kharlampieva, E.; Sukhishvili, S. A. Macromolecules 2003, 36, 9950-9956. (18) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 12351243. (19) Izumrudov, V.; Sukhishvili, S. A. Langmuir 2003, 19, 51885191. (20) Sui, Z.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 24912495. (21) Hu¨bsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J.-C.; Schaaf, P. Langmuir 2004, 20, 1980-1985. (22) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J.-C. Langmuir 2003, 19, 440-445. (23) Lavalle, Ph.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C.; Picart, C. Macromolecules 2002, 35, 4458-4465. (24) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J.-C.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635-648. (25) Scho¨ler, B.; Poptoshev, E.; Caruso, F. Macromolecules 2003, 36, 5258-5264. (26) Sui, Z.; Schlenoff, J. B. Langmuir 2003, 19, 7829-7831. This work was also presented at the 2003 Pittsburgh Conference, March 14, 2003, Orlando, Abstract 2590-3, and the 225th American Chemical Society Meeting, March 23, 2003, New Orleans, Abstract COLL-058. (27) Mo¨hwald, H.; Donath, E.; Sukhorukov, G. Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003; Chapter 13.

10.1021/la0495985 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/11/2004

Phase Separations in Polyelectrolyte Multilayers

is important to establish the boundaries between reversible and irreversible pH response, including morphological changes. In this work, we vary the total volume fraction of ionizable groups within a PEMU using the straightforward blending technique introduced earlier.26 We then examine the changes in morphology and surface charge to establish upper boundaries for the fraction of pHdependent ionizable groups. Experimental Section Material. Poly(styrene sulfonic acid), PSS (Scientific Polymer Products, Mw ∼ 70 000), was used as polyanion. Random copolymer poly(diallyldimethylammonium)-co-poly(acrylic acid) (PDADMA-co-PAA) (Calgon Inc., mole fraction of acrylic acid (AA) is 0.36, Mw ∼ 200 000) was used as polycation. Homopolymer PDADMA (Aldrich, Mw ) 370 000, Mw/Mn ) 2.1) was also used as polycation. Ultrapure water (Barnstead E-pure deionizing system) was used as the solvent for rinsing and preparing deposition solutions in all experiments. Deposition solutions of polymers were all 1 mM based on the repeated unit. Citric buffer series (10 mM, pH range of 2.78-7.30) was used to control pH. PEMUs Built on Silicon Wafers and Germanium Crystal. Silicon wafers (Si〈100〉, 0.5 mm thick, 1 in. diameter, undoped, double-side polished from Topsil Inc.) were cleaned in “piranha” solution (70% H2SO4 (concentrated)/30% H2O2(aq); caution: highly corrosive; prepare fresh and do not store in closed containers) and then in H2O2/ammonia/water 1:1:7 v/v, rinsed in distilled water, and dried with a stream of N2. To prepare multilayers on these Si wafers, they were affixed to a stainless steel shaft with Teflon tape. The shaft was rotated at 300 rpm by a small dc motor. A robotic platform (StratoSequence V, nanoStrata Inc.), accommodating eight 100 mL beakers, was programmed to expose the wafer alternately to the 1 mM polymer solutions, with acidic water rinses in between. The salt concentration (NaCl) in the deposition solutions was 1.0 M. The pHs of both deposition solutions and rinsing water were adjusted to 2 by HCl. Transmission IR spectra were recorded with a Nicolet Avatar 360 FTIR spectrometer. The thickness of the dried multilayers was measured with a Gaertner Scientific L116S ellipsometer, using 632.8 nm light at a 70° incidence angle. A refractive index of 1.55 was employed for multilayers. Atomic force microcopy, AFM (Dimension 3100, Digital Instruments), in Tapping Mode was used to track multilayer surface morphology changes. Attenuated total reflection (ATR) measurements were performed with a Nicolet Nexus 470 fitted with a 0.5 mL capacity flow-through ATR assembly (Specac Benchmark) using a 70 × 10 × 6 mm 45° germanium crystal. Each spectrum had 32 scans coadded at a resolution of 4 cm-1. Multilayers were deposited on the ATR crystal while it was loaded in the flow cell by passing polyelectrolyte and rinse solutions, in an alternating manner, through the cell. PEMUs Coated Inside Fused Silica Capillary Columns. Experiments were conducted on a Beckman Coulter P/ACE MDQ capillary electrophoresis system (Palo Alto, CA) with 254 nm UV detection operating at 15 kV and 25 °C. Fused silica capillary (Polymicro Technologies Inc., Phoenix, AZ), 50 µm i.d. × 31.2 cm (effective length, 21 cm), was employed. Sample injection was performed by pressure (0.5 psi, 5 s). Multilayer coatings were deposited using the rinse function on the Beckman CE system.26,31 Polymer deposition solutions contained 1 mM polymer and 1.0 M NaCl. The capillary was first conditioned by a 30 min rinse of 1 M NaOH. The first layer of polymer was deposited by rinsing the polymer solution through the capillary for 10 min followed by a 5 min pH 2.78 buffer rinse. All other layers were deposited with 5 min deposition time followed by a 5 min pH 2.78 buffer rinse. After the coating was (28) Balachandra, A. M.; Dai, J.; Bruening, M. L. Macromolecules 2002, 35, 3171-3178. (29) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (30) Mendelsohn, J. D.; Yang, S. Y.; Hiller, J.; Hochbaum, A. I.; Rubner, M. F. Biomacromolecules 2003, 4, 96-106. (31) Graul, T. W.; Schlenoff, J. B. Anal. Chem. 1999, 71, 4007-4013.

Langmuir, Vol. 20, No. 14, 2004 6027 finished, the tube was rinsed by pH 2.78 buffer for an hour to stabilize the coating. Acetone was used as a neutral electroosmotic flow marker. Electroosmotic mobility (µeo) is used here to quantify the electroosmotic flow (EOF) and is given as the velocity of solvent flow per unit electric field strength (cm2 V-1 s-1). We use the convention that the “normal” direction, from anode to cathode, is positive. Multilayer Terminology. We employ the following shorthand for multilayers: (A/B)x where A is the starting polyelectrolyte contacting the substrate, B is the terminating polyelectrolyte in contact with solution, and x is the number of layer pairs. In (A/B)xA, A would be the terminating polymer. Salt, MY (cation M and anion Y), has an important role in the buildup process and is represented by (A/B)x @c MY, where c is the molarity of the salt (MY) in the polymer solution. The pH can be included in the nomenclature when using pH-dependent PEMUs. For example, (PDADMA/PAA)2 @0.25M NaCl @pH 7.4 represents two layer pairs of PDADMA/PAA built at 0.25 M NaCl and a pH of 7.4. Since in this work a mixture of copolymer, PDADMA-co-PAA, and homopolymer, PDADMA, was used in the “cationic polyelectrolyte” solution for the PEMU buildup, the term for this polycation mixture is defined as copolymerφ-blend-PDADMA1-φ. The subscript φ denotes the mole fraction of copolymer in the cationic buildup solutions. For example, (copolymer0.2-blendPDADMA0.8/PSS)10 @1.0M NaCl @pH 2 represents 10 layer pairs made from a solution of 20 mol % copolymer and 80 mol % PDADMA homopolymer, and a solution of PSS, built at 1.0 M NaCl and a pH of 2.

Result and Discussion Internal pKs. To anticipate the solution pH required to induce ionization within a multilayer, some knowledge of the pKa of the carboxylate functionality is required. This is not trivial, as the pKa of weak acid/bases within a PEMU is not the same as their solution pKa, as described5,7,15-19 and rationalized32 previously: the “internal” pKa of a polycarboxylate within a PEMU is lower due to favorable ion-pair interactions with positively charged polyelectrolyte.32 Moreover, the pKa depends on the strength of the ion pairing.32 The copolymer system used here is unusual, as carboxylates have the opportunity to pair with (positive) DADMA units either on the same chain of the copolymer or on a different chain of copolymer or DADMA homopolymer (if added). In addition, permanently charged sulfonate groups are available to bind with PDADMA, possibly competing with AA groups. Unfortunately, pure PAA/PDADMA multilayers, while easily prepared at high pH, are not stable to pH cycling.8 However, the PDADMA-co-PAA copolymer itself is a good model for the interactions of ionized or nonionized PAA with DADMA units. This copolymer has a preponderance of DADMA units and is therefore soluble at all pHs. Figure 1 compares the titration of a solution of this copolymer with hompolymer PAA. The presence of positively charged units lowers the pKa, taken as the pH of half neutralization, from 5.6 to 3.7, an effect that is generally observed for polyelectrolyte complexation.33 The internal pKa of a PEMU comprising the copolymer was determined using the ATR-FTIR technique described by Granick7,15,16 and Sukhishvili.16-19 (Copolymer0.5-blendPDADMA0.5/PSS)7copolymer0.5-blend-PDADMA0.5 @1.0M NaCl @pH 1.97 was built and exposed to solutions of gradually increasing pH. In Figure 2, peak areas for protonated (1677-1756 cm-1) and unprotonated (1504(32) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263-8265. (33) Kabanov, V. Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; WileyVCH: Weinheim, 2003.; Chapter 2.

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Figure 1. Titration of the copolymer PDADMA-co-PAA (A) and homopolymer PAA (B), both 0.1 M in acrylic acid groups with 0.1 M NaCl, by 0.1 M NaOH.

Figure 3. A scheme for possible outcomes for a weak polyacid multilayer constructed at low pH and exposed to high pH. (a) The multilayer decomposes completely. (b) The excess charge that appears causes phases of high surface area which, in the absence of material loss, may be pH-reversible. Alternatively, polymer charge is expelled from the PEMU (c), or the bulk and surface are able to absorb the excess charge without major rearrangements (d). Figure 2. Normalized peak area of COO- (open squares with increasing pH, open circles with decreasing pH) and COOH (triangles) bands vs pH. Peak areas were measured from ATRFTIR spectra. (Copolymer0.5-blend-PDADMA0.5/PSS)7copolymer0.5-blend-PDADMA0.5 @1.0M NaCl @pH 1.97 was deposited on Ge crystal. The pKa of AA in the multilayer was determined to be 4.01. Inset: ATR-FTIR spectra of the multilayer film at pH 1.97, 4.01, and 6.10. The range 1660-1768 cm-1 corresponds to the COOH peak, while the range of 1504-1625 cm-1 corresponds to the COO- peak. The spectrum taken at pH 1.97 was used as the background.

1625 cm-1) carboxylate group (inset) indicated the relative abundance of each species. The internal pKa measured in this way was 4.01, somewhat more basic than that for the solution copolymer. Under the conditions of PEMU assembly, carboxylate groups of PDADMA-co-PAA are fully protonated and the multilayer is presumably “stitched” together by the PSS/PDADMA interactions, which are pH independent. In other words, as assembled, the AA units, being neutral, are not paired with DADMA units. The full protonation of multilayer PAA at pH 2 is in contrast with the behavior for PAA multilayered with PAH,34 where the pKa of PAA shifts to much more acidic values (ca. 2) leaving PAA partially ionized at pH 2.0. The reason for the greater acidity of PAA in the presence of PAH, compared to PDADMA, is that PAH forms a stronger ion pair with ionized PAA,10 making it energetically less favorable to protonate AA.32 In separate transmission FTIR experiments using copolymer0.5-blend-PDADMA0.5/ PSS on Si wafers, we found that exposure to pH ) 2 was sufficient to induce at least 95% protonation. It is interesting that the pKa of AA is still suppressed within the multilayer, which implies a certain amount of inter(34) Petrov, A. I.; Antipov, A. A.; Sukhorukov, G. B. Macromolecules 2003, 36, 10079-10086.

action with DADMA when AA is ionized. Presumably, the ionized AA units are competing with PSS units for a limited supply of DADMA, implying at least some mobility/ rearrangements within the PEMU as AA is titrated. Another interesting difference between PAA in multilayers with PDADMA and PAA coupled with PAH is that the PAA back-titration, shown in Figure 2, essentially overlies the forward titration, whereas considerable hysteresis is observed in the association and dissociation paths of PAA in PAH/PAA multilayers.35 Again, exceptionally strong ion pairing between PAH and PAA probably leads to kinetically irreversible internal rearrangements as PAA is protonated and then deprotonated. The copolymer used here is the same one that we used in combination with PSS to create pH-decomposable strata within multilayers.9 Because of this instability, reversible pH-responsive PEMUs could not be constructed with pure copolymer. Complete dissociation of multilayer components is one of several possible fates of a PEMU following internal ionization. Possible outcomes for a polycarboxylate system constructed at low pH and exposed to high pH are summarized in Figure 3. To remain intact, a PEMU must accommodate a large quantity of excess polymer negative charge that appears throughout the PEMU, both at the surface and within the bulk. If it cannot, polymer must be lost. In Figure 3a, the new charges that have appeared within the PEMU following ionization cause complete dissociation. This approach has been used to disrupt PEMUs held together by hydrogen bonding7 or ion pairing.8,9 The effect can be rationalized in terms of electrostatic repulsion between newly created charge or by a strong increase in the osmotic pressure within the PEMU, which draws water in and hyperswells the thin film. We favor the “osmotic explosion” explanation, since (35) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078-4083.

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Figure 4. Mole fraction of the copolymer PDADMA-co-PAA in the multilayer (φmultilayer) vs in the polycation solution (φsolution). (Copolymerφ-blend-PDADMA1-φ/PSS)10 @1.0M NaCl @pH 2 was deposited on the silicon wafer. The dotted line shows “ideal” multilayer blend formation, for the case where the mole fraction in the PEMU is the same as the mole fraction in solution.

counterions are known to be substantially paired with highly charged polyelectrolytes. Figure 3b shows how some of the excess negative charge appearing within the multilayer may be accommodated by rearrangement into a higher surface area. Excess charge is extruded to the surface of a highly porous structure, with or without concomitant loss of polyelectrolyte. An example of this type of behavior is provided by PAH/PAA multilayers exposed to low pH.5 Such morphological changes may be reversible in the absence of polymer expulsion. pH-stimulated reversible poration has been invoked as a possible mechanism27 for loading PEMU capsules. In Figure 3c, excess charge leads to loss of polymer from the surface, which is shown and discussed in more detail below, while in Figure 3d some bulk swelling occurs, along with reversal of the surface charge if it happens to be positive, but the PEMU remains intact and continuous. Diluting Ionizable Groups. To minimize physical disruption within the PEMU, the volume fraction of ionizable groups must be reduced. For example, Kharlampieva and Sukhishvili assessed the stabilizing effect of increasing alkylation of poly(vinylpyridines) layered with poly(methacrylic acid).17 We used an alternative method of diluting pH-sensitive groups: PDADMA-coPAA copolymer was mixed with PDADMA homopolymer and the blend was used as the “polycation” solution in multilayer buildup with PSS polyanion.26 Ideally, the PAA component would be incorporated into the multilayer in the same proportion as copolymer in the mixed polycation solution. To ascertain whether this was, indeed, the case, the amount of copolymer in the multilayer was evaluated by transmission FTIR. Mole fractions of copolymer in the PEMU are plotted as a function of mole fraction in the polycation solution (Figure 4). In the ideal case, there would be no preference for either copolymer or homopolyDADMA, giving a slope of 1.0 (dotted line). A positive deviation indicates some preference for copolymer. Assuming a “hit and stick” model for polyelectrolyte adsorption (all polymers that encounter an oppositely charged surface stick irreversibly), it is possible that copolymer, having a lower molecular weight and being more compact due to internal pairing of DADMA and AA units, has a higher diffusion coefficient and therefore a greater flux to the surface than the DADMA homopolycation. Notwithstanding the slight preference for copolymer, it is shown that PEMUs of continuously variable composition may be assembled by the blending strategy described.26

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Figure 5. Thickness loss (%) vs mole fraction of the copolymer in the polycation solution (φsolution). (Copolymerφ-blendPDADMA1-φ/PSS)10 @1.0M NaCl @pH 2 (diamonds) and (copolymer φ -blend-PDADMA 1-φ /PSS) 10 copolymer φ -blendPDADMA1-φ@1.0M NaCl @pH 2 (squares) were deposited on Si wafers and then exposed to pH 11 solution for 60 min.

Stability to Internal Ionization. To assess the stability of various combinations of PEMU, they were subjected to “pH shock” by immersion in pH 11 solution (adjusted by NaOH) for 60 min. Ellipsometric thicknesses were recorded before and after this treatment, and the loss in thickness is plotted in Figure 5 as a function of composition. At the extreme of pure copolymer, the entire multilayer was lost, as observed previously, PEMUs made with pure PDADMA, on the other hand, were stable. For the blends, intermediate behavior was observed, which showed an interesting dependence on the surface charge of the as-made PEMU: films terminated with the polycation mixture appeared to be more resilient to pH jumps than those terminated with PSS. Films with negative surface charges exhibited erosion of polymer with any copolymer present, while positive-capped PEMUs with up to about a nominal copolymer mole fraction of 0.2 were stable. Referring to Figure 3, regimes of polymer loss correspond to part a or part c. The differential stability of positive- versus negativecapped PEMUs sheds some light on how excess charge appearing within the multilayer is accommodated. The swing in pH between synthesis (pH 2) and soaking (pH 11) is wide enough to ionize all bulk and surface carboxylate groups. A key question is whether the new polymer charge appearing within the bulk of the PEMU, compensated by sodium cations (“extrinsic” charge36), remains where it is. Multilayers, as made, generally have very little extrinsic charge within their bulk,37 a consequence of favorable ion-pair interactions between oppositely charged polyelectrolyte segments. Excess polymer charge is restricted to the surface, where it is key to multilayer buildup4 (these assertions may not hold for “exponential growth” multilayers,14,21-25 where the interacting units seem to be weaker). These findings apply to layer-by-layer growth, where each adsorbing layer has an opportunity to penetrate somewhat into the existing PEMU to seek out all extrinsic charge. Creation of extrinsic charge within the bulk of a multilayer, postsynthesis, is a different matter. In order for this extrinsic charge to migrate to the surface, via small-scale rearrangements (charge extrusion) or large-scale loss of material (charge expulsion) some polymer mobility must be allowed.35 (36) Schlenoff, J. B. Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; WileyVCH: Weinheim, 2003; Chapter 4. (37) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626-7634.

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Ionization of an AA group draws highly solvated cations into the PEMU. The swelling that results decreases the net interaction energy between polyelectrolytes, leading to complete dissociation in the extreme. For lower degrees of swelling, a decrease in polyelectrolyte interaction translates to an increase in their mobility. An example is provided by the enhancement of polyelectrolyte interdiffusion by the addition of salt, which swells the PEMU.10,11 A scenario which fits the observation that positively capped PEMUs are more stable (Figure 5) is that the positive surface acts as a kind of charge “reservoir”, able to soak up at least part of the destabilizing negative polymer charge appearing within the PEMU bulk. For this to occur, the excess charge must be able to migrate to the surface. As pointed out previously,35 charge migration does not mean that entire polymer molecules have to move to the surface. Smaller scale, localized rearrangements of polyelectrolyte segments will be able to translate the extrinsic charge to the surface. If bulk extrinsic charge may be “buffered” by surface extrinsic charge, it is logical that thinner films will be more stable, since the total amount of extrinsic charge generated within the bulk PEMU is proportional to the film thickness, yet the surface charge remains constant. Indeed, it was found that thinner films are more resiliant under pH change than thicker ones. These findings were not provided by ellipsometry. Ellipsometric measurements sample an area on the millimeter length scale and are unable to reveal roughness or porosity at the micrometer or nanometer level. Thus, case b, assuming no loss of material, is not revealed by ellipsometry, although it is an example of ionization-induced instability and phase separation within multilayers. AFM was employed to evaluate multilayer morphology on a smaller scale. Figure 6A,B compares AFM images of 21- and 11-layer PEMUs with the same nominal copolymer composition (a mole fraction of 0.2 in polycation solution). While both multilayers are positive-capped, the thicker multilayer shows “craters” on the 1 µm scale and smaller pores of tens of nanometers in diameter. There is clearly larger scale reorganization of material for the thicker film. For certain applications, it would be advantageous to induce minimal perturbation of bulk structure while allowing the creation and control of surface charge under the influence of pH. For example, we have recently described the use of similar pH-dependent PEMUs to control the sign and magnitude of surface charge for microfluidics applications, including capillary zone electrophoresis and capillary electrochromatography.31 In the present work, to reduce perturbations within the bulk, we limited ionizable material to the surface of the PEMU by employing the copolymer only in the last layer. For example, the surface topology of (PDADMA/PSS)5copolymer0.2-blend-PDADMA0.8 @0.5M NaCl @pH 2, as seen in Figure 6C, shows very little evidence of disruption, yet it performs effectively in controlling surface charge (vide infra). Evidence for Charge Extrusion. Thickness and morphology studies provide information on material redistribution following internal ionization but yield no direct evidence of “charge management” by the PEMU. The sign and relative magnitude of the surface charge are provided by measurements such as electrophoresis (when the PEMU coats a particle), streaming potentials, or electroosmosis. The latter technique shows unambiguous reversal of surface charge on alternate layering31 or pH switching26 (for PEMUs comprising weak acids/bases). Figure 7 compares (copolymer0.2-blend-PDADMA0.8/PSS)10copolymer0.2-blend-PDADMA0.8 @1.0M NaCl @pH 2 with

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Figure 6. AFM images of PEMU topologies after exposure to pH 11 solution: (A) (copolymer0.2-blend-PDADMA0.8/PSS)10copolymer0.2-blend-PDADMA0.8 @1.0M NaCl @pH 2; (B) (copolymer0.2-blend-PDADMA0.8/PSS)5copolymer0.2-blend-PDADMA0.8 @1.0M NaCl @pH 2; (C) (PDADMA/PSS)5copolymer0.2blend-PDADMA0.8 @1.0M NaCl @pH 2. Root-mean-square roughness: (A) 6.4 nm; (B) 3.3 nm; (C) 2.3 nm.

(copolymer0.2-blend-PDADMA0.8/PSS)5copolymer0.2-blendPDADMA0.8 @1.0M NaCl @pH 2 at various pHs. Zero EOF corresponds to a net neutral effective surface charge. It is seen that surface charge can, indeed, be controlled, but for the microfluidics application only a slight improvement is obtained with the more structurally stable “thinner” (copolymer0.2-blend-PDADMA0.8/PSS)5copolymer0.2-blendPDADMA0.8 @1.0M NaCl @pH 2 system. Electroosmotic

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Figure 7. EOF mobility of PEMU coatings on fused silica capillary vs pH: (copolymer0.2-blend-PDADMA0.8/PSS)10copolymer0.2-blend-PDADMA0.8 (open squares), (copolymer0.2blend-PDADMA0.8/PSS)5copolymer0.2-blend-PDADMA0.8 (diamonds), and (PDADMA/PSS)5copolymer0.2-blend-PDADMA0.8 (triangles) @1.0M NaCl @pH 2.

Figure 8. EOF mobility of the PEMU coatings vs number of runs between pH 2.78 and 7.30. (Copolymer0.2-blend-PDADMA0.8/PSS)10copolymer0.2-blend-PDADMA0.8 (open squares), (copolymer0.2-blend-PDADMA0.8/PSS)5copolymer0.2-blend-PDADMA0.8 (diamonds), and (PDADMA/PSS)5copolymer0.2-blendPDADMA0.8 (triangles) @1.0 M NaCl @pH 2 were coated on the inside of the capillary column. Even-numbered runs were done at pH 7.30; a positive EOF indicated the surface charge was negative. Odd-numbered runs were done at pH 2.78; a negative EOF indicated the surface charge was positive. The hollow circle represents the EOF of bare capillary.

flow appears to be somewhat forgiving of the quality of the surface. A single “layer” of copolymer0.2-blend-PDADMA0.8 on top of (PDADMA/PSS)5 @1.0 M NaCl @pH 2 gives performance that is indistinguishable from that of the 21-layer film (which contains 11 layers of copolymer0.2blend-PDADMA0.8). This is a significant finding, as it shows how, by using only one pH-responsive layer on top of many non-pH-responsive layers, surface charge may be controlled without the drawback of major structural rearrangements (such as those in Figure 6). Figure 8 shows more detail of the performance of the multilayer coatings subjected to multiple pH switching. Two features may be noted here: first, quasistable switching of flow direction is possible. Second, the 21layer film shows deterioration in flow rate, while the 11layer and the (copolymer0.2-blend-PDADMA0.8)-terminated PEMUs are more stable. Since the strategy of terminating the multilayer with only one pH-responsive layer proved effective in EOF

Langmuir, Vol. 20, No. 14, 2004 6031

Figure 9. EOF mobility of the PEMU coatings vs pH. The coatings were (PDADMA/PSS)5copolymerφ-blend-PDADMA1-φ with φ ) 0.05 (squares), 0.2 (diamonds), and 1.0 (triangles).

control, we investigated the dependence of surface charge reversal on the composition of the top layer. In Figure 9, EOF versus pH response curves are shown for (PDADMA/ PSS)5copolymerφ-blend-PDADMA1-φ systems where φ ) 0.05, 0.2, and 1, respectively. It is seen that the higher the proportion of AA units, the lower the pH required for switching surface charge (flow direction); more carboxylate functionality requires less ionization (lower pH) to overwhelm surface positive charges. This surface charge “titration” experiment showed that a minimum of about 5 mol % AA is required for switching. It is surprising that such a low percentage of acrylic acid functionality on only the top layer is able to control or switch the surface charge (even taking into account the slight enrichment of AA within the PEMU shown in Figure 4). It may be that charges from the ionization of the silanol groups on the fused silica/PEMU interface are transmitted to the multilayer/solution interface. This might explain the pH dependence of EOF for PDADMA-terminated multilayers.31 This is an interesting way of transmitting excess charge over relatively long range (much longer than the electrostatic Debye length).15 Conclusions We have demonstrated the extent and the nature of the instabilities within complexed polyelectrolyte films, deposited by the multilayering technique, induced by internal ionization changes of weak acid/base functionality. As shown in our earlier work on surface charge switching for controlling electroosmotic flow, blending nonpH-sensitive polymers with pH-sensitive polymers decreases the net density of ionizable groups within the multilayer, yielding more stable films. The finding that thinner films are more stable, as are films terminated with the opposite charge to that created by ionization, was interpreted as evidence of excess polymer charge extrusion to the surface. Multilayers unable to accommodate internal excess charge by extrusion and/or bulk rearrangements are likely to lose material (charge expulsion). The introduction of chemical cross-links would serve to stabilize such multilayers. Acknowledgment. This work was supported, in part, by the National Science Foundation (Grants DMR9727717 and DMR0309441) and by the Center for Materials Research and Technology at FSU. LA0495985