Layer-by-layer assembly of weak-strong copolymer polyelectrolytes: A route to morphological control...

Layer-by-Layer Assembly of Weak-Strong CopolymerPolyelectrolytes: A Route to Morphological Control ofThin Films

ELVIRA TJIPTO, JOHN F. QUINN, FRANK CARUSO

Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering,The University of Melbourne, Victoria 3010, Australia

Received 13 March 2007; accepted 23 April 2007DOI: 10.1002/pola.22179Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Multilayer films were assembled from a strong polyelectrolyte (poly(dial-lyldimethylammonium chloride), PDADMAC) and a copolymer containing bothstrongly charged styrene sulfonate moieties and weakly charged maleic acid moieties(poly(4-styrenesulfonic acid-co-maleic acid), PSSMA). Growth of PSSMA/PDADMACmultilayers was linear, as characterized by UV-vis spectroscopy and quartz crystalmicrogravimetry. The influence of both the pH of the PSSMA adsorption solutionsand the ratio of SS:MA in the PSSMA on multilayer properties was investigated.Fourier transform infrared spectroscopy results showed that the ionization ofcarboxylic acid groups in PSSMA/PDADMAC multilayers did not vary significantlywith changes in the PSSMA assembly pH. However, the multilayers showed differentthicknesses, surface morphologies, and stability to post-assembly pH treatment. Wealso demonstrate that PSSMA/PDADMAC multilayers are significantly more stablethan PSSMA/PAH multilayers after post-assembly pH treatment (i.e. the filmsremain intact when exposed to pH extremes). In addition, the surface morphology oftwo films (PSSMA 1:1 assembled at pH 5.8, post-treated at pH 2 and PSSMA 3:1assembled at pH 5.8, post-treated at pH 11) changed significantly when the filmswere exposed to solutions of different pH and, in the former case, this change in filmmorphology was reversible. The porous morphology after treatment at pH 2 could bereversed to give a significantly smoother film after subsequent exposure to water for24 h. Our results demonstrate that by the rational choice of the assembly pH ofPSSMA, stable and pH-responsive films can be obtained via the sequential assemblyof PSSMA and PDADMAC. These films have potential in controlled release applica-tions where film stability and pH-responsive behavior are essential. VVC 2007 Wiley Peri-

odicals, Inc. J Polym Sci Part A: Polym Chem 45: 4341–4351, 2007

Keywords: adsorption; atomic force microscopy (AFM); films; nanolayers; polyelec-trolytes; stimuli-sensitive polymers

INTRODUCTION

The layer-by-layer (LbL) technique for assem-bling thin films has, in the last decade, attracted

interest because of its versatility.1 A vast arrayof materials can be incorporated into multilayerassemblies, resulting in ultrathin films in whichproperties such as thickness, composition, mor-phology, and chemical functionality can be finelycontrolled via judicious choice of the startingmaterials and assembly conditions.2 Weak poly-electrolytes are increasingly being used in multi-layer thin films because greater control over

Correspondence to: F. Caruso (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 4341–4351 (2007)VVC 2007 Wiley Periodicals, Inc.

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film properties can be achieved by varying theionization of the weakly charged groups throughpH adjustments. Changes in solution pH affectthe charge density and conformation of theweak polyelectrolytes. Hence, multilayers com-posed of weak polyelectrolytes can be designedto be stimuli-responsive.3 For example, Rubnerand coworkers have shown that a simple pHtreatment after assembly of poly(acrylic acid)(PAA) and poly(allylamine hydrochloride) (PAH)multilayer films can result in a drastic changein film morphology.4 In addition, the pH-respon-siveness can also be exploited to trigger film dis-solution. This applies in particular to multi-layers containing carboxylic acid groups thatparticipate in hydrogen bonding to drive multi-layer assembly. At high pH, the deprotonation ofthe carboxylic acid groups disrupts hydrogenbonding, resulting in film disassembly.5 How-ever, when applied to colloids, weak poly-electrolyte multilayers are also more prone toinduce colloidal aggregation than strong poly-electrolyte multilayers.6 When weak polyelectro-lytes are adsorbed at pH conditions where theyhave a low charge density, the tendency of col-loids to aggregate increases, as the repulsiveforces are weaker when compared with instan-ces where a strongly charged polyelectrolyte isused.6

Our previous study involved multilayer as-sembly from a copolymer, poly(4-styrenesulfonicacid-co-maleic acid) (PSSMA), which consists ofstrongly charged styrene sulfonate (SS) groupsand weakly charged maleic acid (MA) groups,deposited in alternation with PAH.7 We postu-lated that using such a copolymer would resultin multilayer films with enhanced stability; SSgroups would form electrostatic interactionswhile the weakly charged MA groups wouldremain pH-responsive. The influence of pH, co-polymer composition (SS:MA) in PSSMA, andthe ionic strength of the polyelectrolyte solutionson multilayer assembly were investigated. Inparticular, the results showed that variation inthe assembly pH led to different pH stabilityregimes when the multilayers, post-assembly,were exposed to different pH solutions. Multi-layers assembled from polyelectrolyte solutionscontaining 0.5 M NaCl were found to be morestable to pH variations post-assembly (comparedto films assembled from solutions with no addedsalt). The exception was formultilayers assembledat pH 5.8, where only 7–10% of the film remainedafter post-treatment in pH 2 solutions. Further,

atomic force microscopy (AFM) studies on theeffect of the post-assembly pH treatment on thestructure of the films revealed that post-assem-bly treatment in solutions of varying pH did notresult in significant changes in film morpholo-gies. PSS and PAH are known to form very sta-ble electrostatic links because charges along thepolymer chains are well-matched.8 It is likelythat these moieties also form very stable bondsin PSSMA, thus forming multilayer films thatare resistant to large-scale film reorganizationthat might otherwise be induced by post-assem-bly pH treatments.

In this study, multilayers were assembled fromPSSMA and a strongly charged polycation, poly(diallyldimethylammonium chloride) (PDADMAC).This system is particularly interesting becausePSSMA is susceptible to variation in the degreeof ionization with changes in pH, but the polyca-tion (PDADMAC) remains charged over theentire pH range studied. This is substantiallydifferent to the case of using PAH as the polyca-tion, where variation of the pH affects both theionization of the polycation and polyanion.Chemical structures of PSSMA and PDADMACare shown in Scheme 1. The results demon-strated that PSSMA/PDADMAC multilayers aremore stable than PSSMA/PAH multilayers afterpost-assembly pH treatment. Furthermore,PSSMA/PDADMAC multilayers can be designedto be pH-responsive by tailoring the assemblypH of PSSMA. Significant changes in film mor-phology were also observed and, in one particu-lar case, the change in film morphology was re-versible. These films have potential in controlledrelease applications where the film needs to beengineered to be responsive to pH changes,without simultaneously compromising the integ-rity of the films.

Scheme 1. Chemical structures of the polyelectrolytesused in this study.

4342 TJIPTO, QUINN, AND CARUSO

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

EXPERIMENTAL

Materials

Poly(4-styrenesulfonic acid-co-maleic acid, 1:1SS: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 molecularweights of 20,000 g mol�1, were obtained fromAldrich. Poly(diallyldimethylammonium chlo-ride) (PDADMAC) with a molecular weight ofless than 100,000 g mol�1 (40 wt % in water) andpoly(ethyleneimine) (PEI) (Mw ¼ 20,000 g mol�1

,

water free), were also obtained from Aldrich. Allpolyelectrolytes were used as supplied. Quartzslides for UV-Vis measurements were obtainedfrom Hellma Optik GmbH (Jena, Germany) andHerbert A. Groisse and Co. (Melbourne, Aus-tralia). Oxidized monocrystalline silicon waferswere purchased from MMRC Pty (Melbourne,Australia) and quartz crystal microgravimetry(QCM) electrodes were from Kyushu Dentsu(Nagasaki, Japan). An inline Millipore RiOs/Ori-gin system was used to produce high puritywater with a resistivity greater than 18 MX cm.

Substrate Preparation

Silicon wafers, quartz, and glass slides werecleaned with Piranha solution (70/30 v/v% sulfuricacid:30% hydrogen peroxide). Caution! Piranha so-lution is highly corrosive. Extreme care should betaken when handling Piranha solution and onlysmall quantities should be prepared. The RCA pro-tocol was then applied to further clean and hydro-philize the substrates. This involved sonication ina 1:1 mixture of water and isopropanol for 15 min,followed by heating at 60 8C for 15 min in a 5:1:1mixture of water, hydrogen peroxide (30%), andammonia solution (29%).

Preparation of Multilayer Films

Aqueous polyelectrolyte solutions were preparedto a concentration of 1 mg mL�1 in 0.5 M NaCl,unless otherwise indicated. 1 M NaOH was usedto adjust the solution pH of PSSMA to pH 11and 1 M HCl was used to adjust the PSSMA so-lution pH to either 5.8 or 2. No acid or base wasadded to the PDADMAC solutions (pH of PDAD-MAC in 0.5 M NaCl is 5.3). The substrates weredipped for 15 min in PEI solutions to establish ahighly charged precursor layer on the surface.The PEI-modified substrates were then dippedthree times in Millipore water for 1 min each,

and dried with a gentle stream of nitrogen. Theywere then immersed in the anionic PSSMA solu-tion for 15 min, followed by further washing anddrying. This procedure was repeated (alternatingbetween PSSMA and PDADMAC) until 20 layerswere deposited (i.e. 10 bilayers).

Post-Assembly Treatment of Multilayer Films

After assembly, multilayer films were exposed toaqueous solutions of different pH to the assem-bly pH for 24 h, to ensure that the rearrange-ment process was complete. These films werethen rinsed with water for 20 s and air-driedbefore characterization.

QCM Measurements

An in-house-built QCM device with a frequencycounter from Agilent was used to determine thefilm mass after each adsorption step. The piezo-electric quartz crystal changes its fundamentaloscillation frequency, F0, as mass is depositedonto (or depleted from) the surface. According tothe Sauerbrey equation, the resonant frequencyshift, DF, of a QCM is proportional to the masschange, Dm9:

DF ¼ � 2F20

AðlqqqÞ1=2Dm ð1Þ

where lq is the shear modulus of the quartz(2.947 3 1013 g m�1 s�2), qq is the density of thequartz (2.648 3 106 g m�3), Fo is the resonantfrequency of the crystals (9 3 106 Hz), and A isthe piezoelectric area of the electrode. Substitut-ing these values into eq 1 gives:

DF ¼ �ð1:833104ÞDmA

where DmA is the mass change per piezoelectricarea in g m�2. The QCM electrodes were gold-coated on both sides and have a diameter of 4.5mm. By taking into account that the effectivesurface area is approximately 20% larger thanthat calculated directly from the diameter of theelectrode (because of surface roughness),10 themass adsorbed can be converted into thickness(assuming the density of the polyelectrolyte filmis 1.2 3 106 6 0.1 g m�3)11:

dðnmÞ ¼ �ð1:89310�2ÞDFðHzÞ;

where d is the thickness of the polyelectrolytefilm.

pH-RESPONSIVE MULTILAYER FILM 4343


UV-Vis Measurements

UV-Visible spectra were collected using anAgilent 8453 single beam UV-Visible spectro-photometer. An air blank was taken beforeeach measurement. While PDADMAC does notabsorb in the region of interest (200–400 nm),PSSMA has an absorption peak at 226 nmbecause of the sulfonated phenyl ring in the SSmoiety.

Spectroscopic Ellipsometry

Measurements were performed on a UVISELmodel ellipsometer from Horiba Jobin Yvon.Spectroscopic data was acquired between 340and 825 nm with a 5 nm increment, and thick-nesses were extracted with the integrated soft-ware by fitting with a classical wavelength dis-persion model. Refractive indices were quoted ata wavelength of 632 nm.

Atomic Force Microscopy

Atomic force microscopy (AFM) images weretaken on air-dried films with a Nanoscope IIIamicroscope and a MFP-3D Asylum Researchinstrument in noncontact mode using siliconcantilevers with a resonance frequency of about290 kHz (Budget Sensors BSTop300). Imageprocessing (first-order flattening and plane fit-ting) was carried out with Nanoscope 4.43r8 andIgor Pro 5.04B for images acquired using theNanoscope IIIa and MFP-3D, respectively. Tomeasure the film thickness, a scalpel blade wasused to scratch the films in several areas, andimages were taken at several points on the edgeof each scratch. The distance between two peaksin the height distribution analysis was deter-mined as the film thickness. Root-mean-squared(rms) roughnesses were determined from 5 mm3 5 mm images.

FTIR Measurements

Measurements were taken using a Varian 7000FTIR spectrometer with a variable angle reflec-tance attachment. The incident angle for themeasurements was 348. Films were deposited onhighly reflective substrates (glass slides coatedwith chromium (10 nm) and then gold (150 nm)using an Edwards Auto 306 thermal depositionchamber).

RESULTS AND DISCUSSION

Formation of PSSMA/PDADMAC Multilayers

The growth of PSSMA/PDADMAC multilayerfilms at various pH and two different SS/MAratios was monitored using QCM and UV–visspectroscopy (Fig. 1). QCM was used to quantifythe mass adsorbed (and hence film thickness) ongold electrodes after each deposition step, whileUV–vis spectroscopy provides information on theamount of PSSMA in the multilayers (by meas-uring the absorbance of the SS moiety at 226nm). Multilayer growth was near linear in allcases except for the PSSMA 3:1/PDADMAC mul-tilayer film assembled at pH 5.8. In this case,the increase in film mass was greater at higherlayer numbers (Fig. 1). This film was the thick-est (92 nm for a 20-layer film) and roughest (rmsroughness 22.1 nm) compared to all other filmsinvestigated. It was also observed to becomecloudy after 16 layers, which is likely to havecaused scattering during UV-Vis measurements,therefore leading to some uncertainty in themeasured absorbance values. In addition toQCM, the final film thicknesses were also deter-mined using AFM and spectroscopic ellipsome-try. This was to ensure that accurate values offilm thickness were obtained, since the differentmeasurement techniques rely on different as-sumptions. In QCM measurements, an assumedfilm density is required to convert mass adsorbedinto film thickness. AFM measures the stepchange of localized areas of a film next to anarea where film has been removed, and thereforesome variability may be involved, depending onthe homogeneity of the film. Spectroscopic ellips-ometry makes no assumption on the refractiveindex of the films, unlike single-wavelengthellipsometry. The film thicknesses obtained fromthe three different techniques (ellipsometry,AFM, and QCM) are presented in Figure 2.Although there are slight discrepancies betweenthe thickness values determined for the samefilm (<20%) using different techniques, the sametrend between film thickness and assembly pHof PSSMA was observed. The largest variation indata was observed for the PSSMA 3:1/PDAD-MAC multilayer assembled from PSSMA at pH5.8 [Fig. 2(b)]. In this case, the film thicknessesdetermined are 110, 92, or 78 nm, using ellips-ometry, AFM, or QCM, respectively. We attrib-uted this difference to the extremely rough sur-face topology of this film (see later).



We previously found that the thickness ofPSSMA/PAH multilayers decreases with increas-ing pH of the PSSMA solutions.7 More carboxy-lic acid groups are ionized at high pH,12 thusincreasing the effective charge density ofPSSMA. The increase in repulsive forces bet-ween the charged groups would cause the poly-mer to adopt a more extended confirmation andto adsorb as thinner layers. This trend was notobserved for the PSSMA/PDADMAC multilayers(see Fig. 2). In this case, the thinnest films werestill those assembled from PSSMA at pH 11. ForPSSMA 1:1, lowering the assembly pH ofPSSMA from 11 to 5.8 resulted in a twofoldincrease in the film thickness (from 24 to 47nm). However, decreasing the assembly pH evenfurther to pH 2 did not cause a substantialincrease in film thickness (from 47 to 53 nm)[Fig. 2(a)]. Similarly, a dramatic increase in film

thickness from 32 to 92 nm was observed whenthe assembly pH of PSSMA 3:1 was loweredfrom 11 to 5.8 [Fig. 2(b)]. In addition, decreasingthe assembly pH of PSSMA 3:1 further to pH 2resulted in a decrease in film thickness from 93to 44 nm [Fig. 2(b)]. This result is interesting asan increase in film thickness is typicallyobserved when the charge density of the poly-mer is decreased.13,14 As the pH of the PSSMAsolution is lowered, a greater proportion of thecarboxylic acids in PSSMA are protonated (pKa1

and pKa2 of PSSMA 1:1 in solution are 2.9 and8.8, respectively, while pKa1 and pKa2 of PSSMA3:1 in solution are 2.7 and 8.3).7 Therefore, low-ering the pH leads to a decrease in the chargedensity, and causes the polyelectrolyte to adopta more coiled conformation. Furthermore,greater amounts of the lower charge densitypolyelectrolyte are required to reverse the

Figure 1. QCM measurements of PSSMA/PDADMAC multilayers for (a) PSSMA1:1 and (b) PSSMA 3:1, and UV-Vis measurements for (c) PSSMA 1:1 and (d) PSSMA3:1. Odd layer numbers correspond to PSSMA, even layer numbers to PDADMAC.Polyelectrolyte solutions: 1 mg mL�1 and contain 0.5 M NaCl.



charge on the surface of the multilayer.15–17

Hence, it is generally observed that lowering thecharge density of a polyelectrolyte results in theformation of thicker films. The fact that we didnot observe a larger film thickness for the PSSMA1:1 and PSSMA 3:1 films assembled at pH 2 com-pared to pH 5.8 suggests that the ionization of thePSSMAwithin the films did not vary significantlybetween these cases. In support of this, the FTIRspectra of the films (assembled at different pHconditions) were remarkably similar (Fig. 3). Theadsorption band at 1720 cm�1 (corresponding toC¼¼O stretching of COOH groups) was notobserved, whereas the most prominent bandswere observed at 1580 and 1474 cm�1 (corre-sponding to asymmetric and symmetric stretchingof COO�, respectively). This indicates that almost

all of the carboxylic acid groups in the films(assembled at pH 2 and 5.8) were ionized. Thisphenomenon was previously observed withPSSMA/PAH multilayers; the ionization of car-boxylic acid groups in the PSSMA/PAH films aresignificantly higher than in solution.7 Previousfindings indicate that the pKa of weak polyacidsare very much dependant on their local en-vironment. The pKa values of weak polyacids inmultilayer films tend to be lower than in solu-tion,13,14,18–21 as the positive charges from thepolycations in the multilayer induce ionization ofthe carboxylic acids. Choi and Rubner reportedthat the degree of ionization of PAA in multi-layers was significantly higher than in solution,and that this effect was most dramatic whenPAA was assembled with PAH compared to when

Figure 3. FTIR spectra of (a) (PSSMA 1:1/PDADMAC)10 and (b) (PSSMA 3:1/PDADMAC)10 multilayers assembled from various PSSMA pHs. The inset in (b)shows an enlargement of (PSSMA 3:1/PDADMAC)10 multilayers assembled fromPSSMA at pH 2 and 11. [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Figure 2. Thickness of (a) (PSSMA 1:1/PDADMAC)10 and (b) (PSSMA 3:1/PDAD-MAC)10 multilayer films determined from ellipsometry, AFM, and QCM measure-ments. See text for details. [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]



other fully charged polycations, such as PDAD-MAC, were used.12 It is important to note thatthe degree of ionization of PAA in PAA/PAH andPAA/PDADMAC multilayers did decrease as thepH of the assembly solutions was lowered (81–52% for PAA/PDADMAC and 82–78% for PAA/PAH when moving from pH 6.5 to 5.5), eventhough it is still considerably higher than thedegree of ionization of PAA in solution at theequivalent pH (52–21%).12 In contrast, the car-boxylic acids in PSSMA/PDADMAC multilayersremained ionized to largely the same extent forfilms assembled at pH 2, 5.8, and 11. The sameresult was observed for PSSMA/PAH multi-layers; the extent of ionization of the carboxylicacids in the films deposited from PSSMA at dif-ferent pHs was similar, in spite of the observeddifferences in multilayer thickness and morphol-ogy.7 Comparisons between PAA and PSSMAmultilayers indicate that the ionization behaviorof carboxylic acid groups were indeed differentwhen incorporated into a copolymer (as is thecase in PSSMA), as opposed to in a homopoly-mer (PAA). This could also be due to differentbehaviors of maleic acid groups (in PSSMA) andacrylic acid groups (in PAA). In addition, theionization of PSSMA in the multilayer is alsodependant on the type of polycation used andthe pH of the polycation solutions. Both PAHand PDADMAC ionize almost all the carboxylicgroups of PSSMA in the multilayer film, evenwhen PSSMA is deposited at acidic pH. How-ever, our previous studies also showed thatPSSMA can be assembled into multilayerswith poly(N-isopropylacrylamide) (PNiPAAm) atacidic conditions.22,23 In the latter case, multi-layer growth occurs through hydrogen bondingbetween the protonated maleic acid groups andthe amide groups of PNiPAAm. Consideringthese results, we infer that the induced ioniza-tion of PSSMA in electrostatic multilayers iscaused by the presence of positive charges fromeither PAH or PDADMAC. Our results showedthat the resulting films have different properties(in terms of stability, morphology, etc) despitehaving a similar degree of ionization of the car-boxylic acids in the dry film. This suggests thatthe conformation of the polymers is differentwhen adsorbed from solutions with various pHs.It is also possible that the presence of positivecharges from PAH or PDADMAC causes PSSMAchains to re-arrange to different extents(depending on the adsorption pH) to achieve thesame optimal degree of ionization in the dry film.

Noncontact AFM was used to characterize thesurface morphology of 20-layer air-dried films(Fig. 4). The root-mean-squared (rms) roughnessof the films was larger for PSSMA 3:1 films com-pared to the PSSMA 1:1 films, but the rough-ness of both PSSMA 1:1 and PSSMA 3:1 filmsfollowed the same trend with assembly pH. Mul-tilayer films assembled from PSSMA at pH 5.8show the highest roughness, followed by multi-layer films assembled from PSSMA at pH 2 andpH 11. The thinner, smoother films assembled atpH 11 are likely the result of the polymer chainsadopting a more extended conformation duringadsorption under these pH conditions. It is note-worthy that the roughest films were thoseassembled at the intermediate pH of 5.8, ratherthan at the extreme pH of 2 or 11. In fact, therms roughness of the PSSMA 3:1 film assembledat pH 5.8 (22 nm) was much larger than otherfilms, and the film surface had an unusual mor-phology [see Fig. 4(e)], with a series of intercon-nected ridges evident on the film surface. These

Figure 4. AFM images of (PSSMA 1:1/PDAD-MAC)10 multilayers (a–c) and (PSSMA 3:1/PDAD-MAC)10 multilayers (d–f) assembled from variousPSSMA pHs on silicon substrates. Scan size: 5 lm 35 lm for all images except (e), which is 50 lm 3 50lm. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]



features are also on a much larger scale than inany of the other films. The rough surface madereliable estimation of the film thickness in thiscase quite difficult, and contributed to the varia-tions in thickness values obtained from ellipsom-etry, AFM and QCM (see Fig. 2). The high sur-face area of this film is also likely to contributeto the nonlinear film growth at higher layernumbers, since the amount of polymer adsorbedgenerally increases with an increase in availablesurface area for adsorption [Fig. 1(b)].24 Thisdata suggests that this film may be significantlyswollen during polymer adsorption, and thelarge folds evident on the air-dried film mayoccur as the film ‘‘collapses’’ when it is dried. Inaddition, the fact that this film is dramaticallythicker and rougher than all other films investi-gated herein (despite the nearly identical degreeof ionization of the carboxylic acids in all thefilms) suggests that two competing effects maybe operative. On one hand, decreasing thecharge density of PSSMA (by either loweringthe pH of PSSMA or using PSSMA with lowerSS:MA ratio) would cause it to assume a morecoiled conformation (as repulsive forces betweencharged groups are diminished) and conse-quently adsorb as thicker layers. Conversely, thelower charge density of PSSMA means that lessPDADMAC chains are required to compensateand reverse the surface charge of the precedingPSSMA layer; hence the PDADMAC layerswould be thinner. We observed this for PSSMA/PDADMAC films assembled at pH 5.8: thePSSMA 3:1 film was much thicker than thePSSMA 1:1 film, even though the latter had alower charge density due to its lower SS:MA ra-

tio (Fig. 2). This suggests that the higher chargedensity of PSSMA 3:1 results in more PDAD-MAC adsorbing on the surface on PSSMA 3:1 toneutralize and reverse the surface charges. Fur-ther, it is likely that PSSMA 3:1 forms a stron-ger complex than PSSMA 1:1 with PDADMAC,as the former contains more SS moieties in eachpolymer chain. More SS moieties provide stron-ger electrostatic linkages, which prevent subse-quent polyelectrolyte desorption into solutions,contributing to thicker films.

Exposure of Preassembled PSSMA/PDADMAC Filmsto Various pH Solutions

(PSSMA/PDADMAC)10 films were immersedinto solutions of extreme pH to investigate theirintegrity and pH-responsiveness. Quartz slideswere immersed for 24 h into water adjusted by1 M HCl or NaOH to pH 2 and 11, respectively.The amount of PSSMA lost was determinedfrom UV-Vis measurements. From Table 1, itcan be seen that the films are intact after expo-sure to extreme pH. The loss of material in allcases was quite minor except for the (PSSMA3:1/PDADMAC)10 film assembled at pH 5.8and treated at pH 11; about 70% of PSSMAremained after the pH-treatment (see later).AFM imaging revealed that two of the filmsunderwent significant rearrangement because ofpH-treatment: (a) PSSMA 1:1/PDADMAC filmassembled at pH 5.8 and treated at pH 2, and(b) PSSMA 3:1/PDADMAC film assembled at pH5.8 and treated at pH 11. Further experimentswere also performed to determine if the changein film morphology only occurred over a narrow

Table 1. Roughness and Percentage of PSSMA Remaining for pH-treated (PSSMA/PDADMAC)10 Films, asMeasured by AFM and UV-vis, Respectively

AssemblypH/Treatment

pH

Before pH Treatment After pH Treatmenta

rms Roughness (nm)b rms Roughness (nm)b % PSSMA Remainingc

1:1d 3:1 1:1 3:1 1:1 3:1

2/11 1.9 2.4 3.7 1.0 90 965.8/2 4.4 22.1 9.9 20.4 100 925.8/11 2.1 11.6 99 7011/2 2.5 4.6 4.3 4.3 100 99

aFilms treated for 24 h at 22 8C.bDetermined using AFM.cDetermined using UV-vis.d Ratio of SS:MA.



pH range (see Figs. 5 and 6). Figure 5 showsthe surface morphology of the PSSMA 3:1/PDADMAC film assembled at pH 5.8 before andafter pH treatment between pH 10.5 and 11.5.The alkaline conditions caused a dramatic changein the topology of the films. The rms values ofthe pH-treated films decreased considerably,even though these films were still quite rough(rms 7–15 nm). Between the treatment pH rangeof 10.8–11.5, the resulting film morphologies werequite similar to each other. Ellipsometry resultsindicated that the thickness of these films onlyvaried by about 15% after treatment in alkalineconditions, despite UV-Vis measurements show-ing that �30% of PSSMA was lost from the filmafter the same treatment (Table 1). We attributethe change in film morphology to increased ioni-zation of the carboxylic acids at pH 11. FTIRresults revealed an increase in the absorption

band at 1581 cm�1 (corresponding to asymmet-ric stretching of COO� groups) after the filmwas treated at pH 11 [Fig. 7(b)]. At the sametime, there is a corresponding flattening in thesignal around 1710 cm�1, indicating conversionof the few remaining carboxylic acid groups tocarboxylate. This increase in the amount of neg-ative charges in the film would lead to a chargeimbalance, which may then cause the release ofPSSMA chains into the solution, facilitating filmrearrangement.

For the PSSMA 1:1/PDADMAC film as-sembled at pH 5.8, a gradual change in filmmorphology was observed between pH 1.5 and2.5, with the most dramatic change occurring toa film that was treated at pH 2.2 (Fig. 6). Thistreatment yielded small pores that extendedthroughout the cross section of the film (the

Figure 5. AFM images of (PSSMA 3:1/PDAD-MAC)10 multilayers assembled from PSSMA solutionat pH 5.8 and exposed to various pH conditions. Scansize: 50 lm 3 50 lm. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com.]

Figure 6. AFM images of (PSSMA 1:1/PDAD-MAC)10 multilayers assembled from PSSMA solutionat pH 5.8 and exposed to various pH conditions. Scansize: 3 lm 3 3 lm. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]



z-scale for Fig. 6 was 85 nm despite the filmhaving a thickness of only 62.5 nm). No obviousfilm loss was observed for these films over thetreatment pH range (1.5–2.5), as measured byUV-Vis and ellipsometry. Instead, slightincreases in film thickness were observed (ca. 2–5 nm), which may be attributed to film rear-rangement, as they became more porous. Wealso found that the refractive indices of the filmsdecreased from 1.50 to 1.46 after post-treatment(obtained from ellipsometry), indicating that thefilm density decreased, which is consistent withthe films becoming more porous. The change infilm morphology was due to protonation of car-boxylic acids in the film when immersed in anacidic environment. This was confirmed by FTIRmeasurements [Fig. 7(a)]. After pH 2 treatment,an absorption band was observed at 1716 cm�1

(corresponding to C¼¼O stretching of COOHgroups). These results demonstrated that proto-nation of a small amount of carboxylic acidgroups (strong absorption bands were stillobserved at 1588 and 1474 cm�1 correspondingto stretching bands of COO�) was sufficient toinduce film rearrangement. The majority of thecarboxylic acid groups remained ionized, sug-gesting that the ionic crosslinks within the mul-tilayer film are strong enough to prevent poly-mer desorption even while the film rearrangesin response to the more acidic local environ-ment. In addition, the change in film morphol-ogy for this particular film was reversible fortwo treatment cycles. For this experiment, the

film was alternately immersed in pH 2 solutionand water for 24 h each time. AFM imagingrevealed that after the first cycle, the rmsroughness of the film decreased from 10 nm (af-ter pH 2 treatment) to 2 nm (after immersion inwater) (Fig. 8). Subsequent cycles showedsmaller changes in film morphology, with thefilm roughness reaching a steady value afterthree cycles. This is an important result, as itshows that the properties of the resulting multi-layer film can be designed by careful selection of

Figure 7. FTIR spectra of (a) (PSSMA 1:1/PDADMAC)10 and (b) (PSSMA 3:1/PDADMAC)10 multilayers assembled from PSSMA solution at pH 5.8 and exposed tovarious pH conditions. The spectra were offset in the positive direction for clarity.[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 8. RMS roughness of a (PSSMA 1:1/PDAD-MAC)10 multilayer film assembled from PSSMAsolution at pH 5.8 after assembly (?) and after alter-nate immersions in pH 2 solution (~) and water (*).The pH of Milli-Q water is � 5.7.



assembly and treatment pH (and the number oftreatment cycles).

CONCLUSIONS

Multilayer thin films were successfully assembledusing the pH-sensitive copolymer PSSMA, inalternation with the strong polyelectrolyte PDAD-MAC. The effect of the pH of PSSMA solutionsand the ratio of SS:MA in PSSMA on multilayerproperties was investigated. FTIR resultsshowed that the ionization of carboxylic acids inPSSMA/PDADMAC multilayers did not vary sig-nificantly with changes in the assembly pH ofPSSMA. In all cases, only the ionized forms ofthe carboxylic acid groups of PSSMA weredetected by FTIR spectroscopy. Despite this sim-ilarity, the multilayers had different thicknesses,surface morphologies, and stability to postassem-bly pH treatment. This suggests that PSSMAchains assume different conformations (dependingon the adsorption pH) during adsorption in anaqueous environment to achieve the same optimaldegree of ionization in the dry film.We also showedthat PSSMA/PDADMAC multilayers are signifi-cantly more stable than PSSMA/PAH multilayersafter postassembly pH treatments. The loss ofmaterial was negligible in all cases except the par-ticularly thick (PSSMA 3:1/PDADMAC)10 film (93nm). In addition, the surface topology of two films(PSSMA 1:1 and PSSMA 3:1 assembled at pH 5.8)was vastly different when these films were exposedto pH 2 and 11, respectively. In the former case, thechange in film morphology is reversible. Ourresults demonstrate that by careful choice of the as-sembly conditions, stable and pH-responsive filmscan be obtained from PSSMA/PDADMAC multi-layers. These films can find application in areaswhere fine control over the film morphology, andrearrangement with variation in pH are desirable,such as in the controlled release of therapeutics.

We thank Anthony W. Quinn for gold deposition ofFTIR substrates and technical assistance with ellips-ometry. This work was supported by the AustralianResearch Council under the Federation Fellowshipand Discovery Project schemes, by the Victorian State

Government under the STI Initiative. The ParticulateFluids Processing Centre at the University of Mel-bourne is acknowledged for infrastructure support.

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