Synthesis, Multilayer Film Assembly, and Capsule Formation of

Jul 19, 2008 - The University of New South Wales. Cite this:Langmuir ... Khine Y. Mya , Esther M. J. Lin , Chakravarthy S. Gudipati , Lu Shen and Chao...
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Langmuir 2008, 24, 8981-8990

8981

Synthesis, Multilayer Film Assembly, and Capsule Formation of Macromolecularly Engineered Acrylic Acid and Styrene Sulfonate Block Copolymers Heng Pho Yap,† Xiaojuan Hao,‡ Elvira Tjipto,† Chakravarthy Gudipati,‡ John F. Quinn,† Thomas P. Davis,‡ Christopher Barner-Kowollik,‡ Martina H. Stenzel,‡ and Frank Caruso*,† Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Melbourne, Victoria 3010, Australia, and Centre for AdVanced Macromolecular Design, School of Chemical Engineering and Industrial Chemistry, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia ReceiVed April 8, 2008. ReVised Manuscript ReceiVed May 18, 2008 We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAAx-b-SSy copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA37-b-SS63, PAA55-b-SS45, and PAA73-b-SS27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAAx-b-SSy copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAAxb-SSy/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAAx-b-SSy. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.

* To whom correspondence should be addressed E-mail: fcaruso@ unimelb.edu.au. † The University of Melbourne. ‡ The University of New South Wales.

promise in biomedicine,12–17 optics,18,19 and catalysis.20,21 The LbL technique most frequently involves the alternating adsorption of oppositely charged polymers (i.e., polyelectrolytes, PEs), although it is possible to assemble LbL materials from other species (including uncharged materials) using interactions such as hydrogen bonding,22,23 DNA hybridization,24–27 or covalent reactions.28–30 Nevertheless, the overwhelming majority of reports in the area have focused on commercially available PEs, such as poly(sodium 4-styrenesulfonate) (PSS), poly(allylamine hydrochloride) (PAH), poly(diallyldimethylammonium chloride)

(1) Decher, G.; Schlenoff, J. B. Multilayer Thin Films; Wiley-VCH: Weinheim, 2003. (2) Caruso, F.,Ed. Colloids and Colloid Assemblies. Wiley-VCH: Weinheim, 2004. (3) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (4) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762. (5) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319. (6) De Geest, B. G.; Sanders, N. N.; Sukhorukov, G. B.; Demeester, J.; De Smedt, S. C. Chem. Soc. ReV. 2007, 36, 636. (7) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848. (8) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. J. Am. Chem. Soc. 2005, 127, 17228. (9) Jaber, J. A.; Schlenoff, J. B. Chem. Mater. 2006, 18, 5768. (10) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (11) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (12) Cortez, C.; Tomaskovic-Crook, E.; Johnston, A. P. R.; Radt, B.; Cody, S. H.; Scott, A. M.; Nice, E. C.; Heath, J. K.; Caruso, F. AdV. Mater. 2006, 18, 1998. (13) Berg, M. C.; Yang, S. Y.; Hammond, P. T.; Rubner, M. F. Langmuir 2004, 20, 1362. (14) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammond, P. T. Langmuir 2005, 21, 1603. (15) Yoo, P. J.; Nam, K. T.; Qi, J. F.; Lee, S. K.; Park, J.; Belcher, A. M.; Hammond, P. T. Nat. Mater. 2006, 5, 234.

(16) Volodkin, D.; Arntz, Y.; Schaaf, P.; Moehwald, H.; Voegel, J. C.; Ball, V. Soft Matter 2008, 4, 122. (17) Schneider, A.; Vodouhe, C.; Richert, L.; Francius, G.; Le Guen, E.; Schaaf, P.; Voegel, J. C.; Frisch, B.; Picart, C. Biomacromolecules 2007, 8, 139. (18) Zhai, L.; Nolte, A. J.; Cohen, R. E.; Rubner, M. F. Macromolecules 2004, 37, 6113. (19) Wang, D. Y.; Li, J. S.; Chan, C. T.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M.; Romanov, S.; Caruso, F. Small 2005, 1, 122. (20) Kidambi, S.; Dai, J. H.; Li, J.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (21) Dotzauer, D. M.; Dai, J. H.; Sun, L.; Bruening, M. L. Nano Lett. 2006, 6, 2268. (22) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (23) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (24) Johnston, A. P. R.; Read, E. S.; Caruso, F. Nano Lett. 2005, 5, 953. (25) Johnston, A. P. R.; Mitomo, H.; Read, E. S.; Caruso, F. Langmuir 2006, 22, 3251. (26) Johnston, A. P. R.; Caruso, F. Angew. Chem., Int. Ed. 2007, 46, 2677. (27) Johnston, A. P. R.; Zelikin, A. N.; Caruso, F. AdV. Mater. 2007, 19, 3727. (28) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655. (29) Liu, Y. L.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114. (30) Such, G. K.; Quinn, J. F.; Quinn, A.; Tjipto, E.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9318.

Introduction In recent times, the layer-by-layer (LbL) assembly method has become the focus of intense research interest as an excellent means of preparing tailored thin films and colloids,1–7 including free-standing films,8,9 and capsules.10,11 These materials have

10.1021/la8011074 CCC: $40.75  2008 American Chemical Society Published on Web 07/19/2008

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(PDADMAC), and poly(acrylic) acid (PAA). While a host of interesting properties can be achieved by assembling these PEs under different conditions (i.e., pH,31,32 temperature,33,34 and ionic strength35,36), the advent of advanced polymer synthesis techniques (such as the living radical techniques like nitroxidemediated polymerization (NMP),37 atom transfer radical polymerization (ATRP)38,39 and reversible-addition fragmentation chain transfer (RAFT) polymerization40) has greatly increased the number of high-functionality polymers that could potentially be used in LbL assembly. In particular, the use of RAFT polymerization is quite attractive because of the ability to synthesize well-defined, water-soluble polymers without the need to utilize protecting group chemistry.41–45 Thus, the use of RAFT polymerization in tandem with LbL assembly holds great promise for the preparation of high-functionality films, colloidal coatings, and capsules for advanced applications. Weak (i.e., pH-dependent) PEs are of particular interest in LbL assembly because the films assembled can exhibit unique morphological and optical properties, depending on both the pH of assembly and the final pH to which the films are exposed.13,46–50 In addition to work where weak PEs have been used as both the polyanion and polycation in the assembly, research has examined cases in which a weak polyelectrolyte is blended with a strong PE, with this blend then used in the assembly process.51–54 By using this approach, the final films display properties of both the weak PEs (pH dependence) and strong PEs (high stability). An alternative method for incorporating both weak and strong PE functionality into films is to use copolymers with both pHdependent and pH-independent moieties. To this end, we previously examined the assembly of multilayer thin films from a copolymer of styrene sulfonate (SS) and maleic acid (MA) (poly(styrene sulfonic acid-co-maleic acid) (PSSMA)).55,56 In this polymer, the maleic acid carboxylates are either protonated (31) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (32) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (33) Gopinadhan, M.; Ahrens, H.; Gunther, J. U.; Steitz, R.; Helm, C. A. Macromolecules 2005, 38, 5228. (34) Quinn, J. F.; Caruso, F. Langmuir 2004, 20, 20. (35) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (36) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (37) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Macromolecules 1993, 26, 2987. (38) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. (39) Wang, J. S.; Matyjaszewski, K. Macromolecules 1995, 28, 7901. (40) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (41) Sumerlin, B. S.; Donovan, M. S.; Mitsukami, Y.; Lowe, A. B.; McCormick, C. L. Macromolecules 2001, 34, 6561. (42) Arotcarena, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc. 2002, 124, 3787. (43) Donovan, M. S.; Sanford, T. A.; Lowe, A. B.; Sumerlin, B. S.; Mitsukami, Y.; McCormick, C. L. Macromolecules 2002, 35, 4570. (44) Sumerlin, B. S.; Lowe, A. B.; Thomas, D. B.; McCormick, C. L. Macromolecules 2003, 36, 5982. (45) Thomas, D. B.; Sumerlin, B. S.; Lowe, A. B.; McCormick, C. L. Macromolecules 2003, 36, 1436. (46) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (47) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Chem. Mater. 2003, 15, 299. (48) Yang, S. Y.; Mendelsohn, J. D.; Rubner, M. F. Biomacromolecules 2003, 4, 987. (49) Berg, M. C.; Choi, J.; Hammond, P. T.; Rubner, M. F. Langmuir 2003, 19, 2231. (50) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078. (51) Cho, J. H.; Quinn, J. F.; Caruso, F. J. Am. Chem. Soc. 2004, 126, 2270. (52) Hubsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P Langmuir 2004, 20, 1980. (53) Yap, H. P.; Quinn, J. F.; Ng, S. M.; Cho, J. H.; Caruso, F. Langmuir 2005, 21, 4328. (54) Quinn, A.; Tjipto, E.; Yu, A. M.; Gengenbach, T. R.; Caruso, F. Langmuir 2007, 23, 4944. (55) Tjipto, E.; Quinn, J. F.; Caruso, F. Langmuir 2005, 21, 8785. (56) Tjipto, E.; Quinn, J. F.; Caruso, F. J. Polym. Sci. Pol. Chem. 2007, 45, 4341.

Yap et al.

(uncharged) or deprotonated (charged) depending on the solution pH, while the sulfonate groups remain charged across most of the pH values studied. Initial studies examined the sequential assembly of PSSMA and a weak PE, PAH,55 to form thin films, whereas subsequent work investigated multilayers assembled from PSSMA and a strong PE, PDADMAC.56 In both cases, FTIR results showed that the carboxylic acids in the multilayer films were highly ionized, even in instances where the films were assembled from acidic conditions. This is in contrast to PAA/PAH multilayers, where a substantially lower ionization of carboxylic acids in the multilayers was observed when PAA was deposited at acidic conditions.32 The difference in the ionization behavior of the carboxylic acids may stem from the fact that each maleic acid group (in PSSMA) consists of two neighboring carboxylic acids (which therefore have the ability to form a hydrogen bond with each other), while the carboxylic acids in PAA are separated by a CH2 group in the polymer backbone. Building on such work, we sought to study a copolymer consisting of block domains of styrene sulfonate (SS) and acrylic acid (AA) groups, and to investigate if the films assembled exhibit properties that are intermediate between PSS and PAA. This work differs from that conducted using blends of the two polymers53,57 in that the AA and SS moieties are actually contained within the one polymer chain. Therefore, the possibility of displacement effects or desorption of a particular species during the film assembly can be abrogated, and the film composition will be dictated by the composition of the polymer initially adsorbed. Since block copolymers of AA and SS are not available commercially, these materials were synthesized using RAFT polymerization. An advantage of using RAFT polymerization for this synthesis is that it can be used with a wide range of monomers and reaction conditions, and importantly, the polymers produced have controlled molecular weight and very narrow molecular weight distributions.40–45 To date, the only study of multilayer films constructed from RAFT-synthesized copolymers was reported by Morgan et al.58 These authors used block copolymers of poly(sodium 3-acrylamido-3-methylbutonate-block-sodium-2-acrylamido-2-methylpropanesulfonate) (P(AMBA-b-AMPS), which contain pendant groups with either carboxylic acid (i.e., weak PE) or sulfonate (strong PE) moieties, in varying proportions. It is important to note that the chemical structure of P(AMBA-b-AMPS) is substantially different to the PAAx-b-SSy copolymers utilized in our current study. Morgan et al. deposited P(AMBA-b-AMPS) in alternation with a strong polycation, quarternized poly(N[(dimethylamino)ethyl] acrylamide) P(DMAEA). The resultant multilayer films were shown to be pH-responsive, and film (in)stability and swelling properties were dependent on the ratio of AMBA to AMPS groups in the copolymer. In the current study, the block copolymer PAAx-b-SSy (x and y denotes the % mass of acrylic acid (AA) and styrene sulfonate (SS) domains) was assembled in alternation with the weak PE PAH. The film assembly and post treatment conditions were specifically chosen to be in the region where PAA/PAH films have been shown to produce optimal film buildup and induce substantial pH-induced porosity.31 The resulting PAAx-b-PSSy/ PAH films were characterized with ellipsometry and atomic force microscopy (AFM). PSS/PAH and PAA/PAH films were also assembled and treated at the same conditions used for PAAxb-SSy/PAH films to enable a comparison between the properties of the homopolymers and the copolymers. The results indicated (57) Yap, H. P.; Quinn, J. F.; Johnston, A. P. R.; Caruso, F. Macromolecules 2007, 40, 7581. (58) Morgan, S. E.; Jones, P.; Lamont, A. S.; Heidenreich, A.; McCormick, C. L. Langmuir 2007, 23, 230.

Macromolecularly Engineered Block Copolymers

that PAA55-b-SS45 films were the most pH-responsive. Further, in instances where a substantial change in film morphology was observed, the thickness of PAAx-b-SSy/PAH films deviated only slightly from the original film thickness, unlike PAA/PAH films. In the homopolymer case, the film thickness increased markedly when the films underwent rearrangement in acidic pH conditions. Subsequently, films were formed on particle substrates (SiO2) via the assembly of PAA37-b-SS63 or PAA55-b-SS45 in alternation with PAH under various adsorption conditions. Surface charge variation of the particles, depending on the assembly pH, was observed. Furthermore, we report the successful fabrication of PAAx-b-SSy/PAH capsules, and investigated the effect of the proportion of AA groups on the growth profile of the capsules.

Experimental Section Materials. Poly(styrene sulfonate) (PSS, Mw 70 000 g mol-1), poly(acrylic acid) (PAA, Mw 30 000 g mol-1), poly(allylamine hydrochloride) (PAH, Mw 70 000 g mol-1), and poly(ethyleneimine) (PEI, Mw 20 000 g mol-1, water free) were obtained from Aldrich and were used without further purification. Silica (SiO2) particles (1.0 and 5.3 µm diameter) were purchased from Microparticles GmbH, Germany. Oxidized monocrystalline silicon wafers were purchased from MMRC Pty Ltd. (Melbourne, Australia). An inline Millipore RiOs/Origin system was used to produce deionized water with a resistivity greater than 18 MΩ · cm. 4,4′-azobis-4-cyanopentanoic acid (ACPA), styrene sulfonate (SS), and acrylic acid (AA) were all purchased from Sigma-Aldrich. ACPA and SS were used as received. AA was distilled under vacuum and used immediately. The synthesis of the RAFT agent, 3-benzylsulfanylthiocarbonylsulfanyl propionic acid, has been described elsewhere.59 Polymer Synthesis. The block copolymers PAAx-PSSy were synthesized by chain extension from PAA macromolecular chain transfer agents (RAFT-PAA) in water at 70 °C, using the water soluble thermal initiator 4,4′-azobis-4-cyanopentanoic acid (ACPA). The two-step procedure is outlined below for the example of PAA55b-SS45. PAA73-b-SS27 and PAA37-b-SS63 were synthesized using the same approach, with the exception of varying the concentrations and reaction times to control the ultimate molecular weight of the polymer synthesized. The RAFT-PAA used in the case of PAA37PSS63 was synthesized using the thermal decomposition of ACPA, rather than γ-radiation, as the source of initiating radicals. SynthesisofMacromolecularChainTransferAgent(RAFT-PAA). RAFT-PAA was prepared in the presence of the 3-benzylsulfanylthiocarbonylsulfanyl propionic acid RAFT agent. Polymerization was conducted in water at ambient temperature using a 60Co γ-source to initiate radicals. AA (2.16 g, 0.03 moles) and the RAFT agent (16.3 mg, 0.06 mmoles) were dissolved in deionized water (13 mL). A small amount of acetone was added to help dissolve the RAFT agent. The mixture was degassed under vacuum using a Schlenk line, after which the vessel was blanketed with nitrogen. The polymerization vessel was then exposed to the γ-source at ambient temperature for 5 h. The polymer was purified by extensive dialysis against water, followed by freeze-drying. Approximately 75% conversion was achieved. The Mn determined via 1H NMR was 27 000 g mol-1, which was the same as the molecular weight calculated using the initial concentrations of the RAFT agent and monomer (Mn(th) ) 27 000 g mol-1). The aqueous GPC-determined Mn ) 37 000 g mol-1 and PDI ) 1.1. Synthesis of PAA55-b-SS45. Sodium 4-styrenesulfonate (1.033 g, 0.005 moles), RAFT-PAA, (406.8 mg, 0.015 mmoles), and the initiator, ACPA (0.8 mg, 0.003 mmoles), were added to a glass vial. Deionized water (10 mL) was added to dissolve all of the components. The mixture was then degassed under vacuum using a Schlenk line, and blanketed with nitrogen immediately prior to polymerization. The polymerization was initiated by immersing the glass vial in an oil bath set at 70 °C for 4.5 h. The polymer was purified by (59) Stenzel, M. H.; Davis, T. P. J. Polym. Sci. Pol. Chem. 2002, 40, 4498. (60) Quinn, J. F.; Davis, T. P.; Barner, L.; Barner-Kowollik, C. Polymer 2007, 48, 6467.

Langmuir, Vol. 24, No. 16, 2008 8983 precipitating it into acetone, filtering, and drying overnight. Close to 89% conversion was achieved. The Mn determined via 1H NMR of the second block (PSS) was 63 000 g mol-1, which was close the theoretical Mn(th) of 62 000 g mol-1. The water GPC-determined Mn of the polymer was 73 000 g mol-1 with a PDI ) 1.1. Substrate Preparation. Silicon wafers, and quartz and glass slides were cleaned with Piranha solution (70:30 v/v% sulfuric acid/30% hydrogen peroxide). Caution must be exercised as Piranha solution is highly corrosiVe. Extreme care should be taken when handling Piranha solution and only small quantities should be prepared. The RCA protocol was then applied to further clean and hydrophilize the substrates. This involved sonication in a 1:1 mixture of water and isopropanol for 15 min, followed by heating at 60 °C for 15 min in a 5:1:1 mixture of water, aqueous hydrogen peroxide (30% v/v) and aqueous ammonia solution (25% v/v). Preparation of Multilayer Films. Aqueous PE solutions were prepared to a concentration of 1 mg mL-1, with no added NaCl. NaOH (1 M) and HCl (1 M) were used to adjust the solution pH of PSS, PAA, PAAx-b-SSy, and PAH. The polyanion solutions were prepared to pH 3.5, 4.0, 4.5 and 7.5, and the PAH solutions were prepared to pH 3.5 and 7.5. The pH assembly conditions are indicated throughout the text and figures as A/B (e.g., 3.5/7.5, indicating polyanion assembly pH of 3.5 and PAH assembly pH of 7.5). A precursor layer of PEI (1 mg mL-1, 0.5 M NaCl) was always deposited as the first layer. A 15 min adsorption time and 3 × 1 min water rinses were used for each deposition step. The films were gently blow-dried with nitrogen after each layer. Post Assembly Treatment of Multilayer Films. After assembly, multilayer films (all of which had a total of 20 layers) were exposed to aqueous solutions of different pH to the assembly pH for 24 h. These films were then rinsed with water for 3 s and gently air-dried before characterization. Particle Coating. SiO2 particles were dispersed in a small quantity of water (50 µL) using ultrasound and vortex mixing. The deposition solution was then added (1 mL, PAH or PAAx-b-SSy), and the mixture was agitated for 15 min during adsorption. In order to remove excess PE, the dispersion was either centrifuged or allowed to sediment, after which the supernatant was removed and replaced by water. Centrifugation conditions were optimized to prevent particle aggregation, and ranged from 400 to 27g on different stages of polymer adsorption and washing. In general, the centrifugation speed was decreased as more layers were deposited to prevent particle aggregation. The particles were redispersed after washing via vortex mixing. Ultrasonication was used intermittently to aid dispersion of the particles. The rinsing process was repeated twice in order to ensure complete removal of the PE, after which the next PE adsorption solution was added. As the SiO2 particles used had a negative charge, the first layer adsorbed was PAH, followed by PAAx-b-SSy. The process was continued until the desired layer number was reached. Core Dissolution. The cores of PAAx-b-SSy/PAH-coated SiO2 particles were dissolved by exposure to 1 M HF/4 M NH4F. Briefly, 10 µL of 5 wt % core-shell particles (with a negatively charged (i.e., PAAx-b-SSy) outermost layer) were adsorbed to a planar substrate coated with poly(ethyleneimine) (PEI) and air-dried. The SiO2 particles were then dissolved by exposure to 10 µL of 1 M HF/4 M NH4F for 30 s. The PEI-coated substrate with adhered capsules was then washed three times by dipping it in deionized water (30 s each), and was then air-dried. Microelectrophoresis. ζ-potentials were measured using a Malvern Zetasizer 2000. Measurements were taken on samples with a mass concentration of 1 × 10-4 %. The quoted values were calculated by taking the average of five successive measurements. The pH dependence of the ζ-potential was measured by adding 5 µL of the particle dispersion to dilute aqueous NaOH or HCl with a predetermined pH no more than 3 min before measurement. Atomic Force Microscopy (AFM). Planar Films. AFM images were taken on air-dried films with a Nanoscope IIIa microscope and an MFP-3D Asylum Research instrument in noncontact mode using silicon cantilevers with a resonance frequency of ca. 290 kHz (Budget Sensors BSTop300). Image processing (first-order flattening and plane fitting) was carried out with a Nanoscope 4.43r8 and Igor Pro

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Table 1. Composition and Molecular Weight of PAAx-b-SSy

a

copolymer

% SSa

% AAa

Mn (GPC)b

PDI (GPC)b

Mn (NMR)a

Mn (calcd)c

PAA37-b-SS63 PAA55-b-SS45 PAA73-b-SS27

63 45 27

37 55 73

96 000 73 000 116 000

1.424 1.114 1.253

110 000 90 000 122 000

106 000 89 000 108 000

Determined using 1H NMR.

b

Determined using aqueous GPC. c Calculated according to the molar ratio of monomer to RAFT in the feed.

5.04B for images acquired using the Nanoscope IIIa and MFP-3D, respectively. To measure the film thickness, a scalpel blade was used to scratch the films in several areas, and images were taken at several points on the edge of each scratch. The horizontal distance between two peaks in the height distribution analysis was determined as the film thickness. Root-mean-squared (rms) roughnesses were determined from 5 × 5 µm2 images. Capsules. The thickness and morphology of the capsules were examined with an MFP-3D Asylum Research instrument operated in AC mode. PEI (1 mg mL-1, 0.5 M NaCl) was deposited onto gold-coated silicon wafer substrates with a 15 min exposure time, followed by three washes in deionized water (30 s each). Ten microliters of the core-shell dispersion was then evaporated onto the silicon wafer substrate before undergoing core dissolution (see Core Dissolution section). The air-dried samples were then imaged in air with BS-Tap300 (Bulgaria) cantilevers. Spectroscopic Ellipsometry. Measurements were performed on a UVISEL model ellipsometer from Horiba Jobin Yvon. Spectroscopic data was acquired between 340 and 825 nm with a 5 nm increment, and thicknesses were extracted with the integrated software by fitting with a classical wavelength dispersion model. Transmission Electron Microscopy (TEM). TEM images were taken using a Philips CM 120 BioTWIN microscope operated at 120 kV. Copper grids coated with Pioloform film were first exposed to PEI (1 mg mL-1, 0.5 M NaCl) for 15 min, followed by three washes in deionized water (30 s each), after which they were air-dried. Ten microliters of the core-shell dispersion was evaporated onto the film. Core dissolution was performed, as described earlier. Gel Permeation Chromatography (GPC). GPC measurements were performed using a Shimadzu modular system comprising a DGU-12A solvent degasser, an LC-10AT pump, a SIL-10AD autoinjector, and both an RID-10A refractive index and an SPD10A UV detector. MilliQ water was used as eluent at a flow rate of 1 mL min-1. The system was equipped with a Polymer Laboratories 5.0 mm bead-size guard column (50 × 7.8 mm2) followed by three 300 × 7.8 mm2 linear PL columns (103, 104, and 105 Å). Nuclear Magnetic Resonance Spectroscopy. 1H NMR spectra were recorded on a 300 MHz (Bruker ACF300) spectrometer using D2O as a solvent, unless otherwise stated.

Results and Discussion PAAx-b-SSy Copolymer Synthesis. Three copolymers, each consisting of varying proportions of block domains of acrylic acid (AA) and styrene sulfonate (SS) groups, were prepared using the RAFT process. Table 1 shows that these copolymers contain 37%, 55%, and 73% AA groups by mass (denoted herein as PAA37-b-SS63, PAA55-b-SS45, and PAA73-b-SS27, respectively), each with molecular weight of ∼100 kDa. This is the first time, to our knowledge, that such polymers have been synthesized. A macromolecular chain transfer agent of poly(acrylic acid) (PAA) was first prepared by polymerizing AA under either thermal initiation using an azo initiator, or by using a γ-radiation source (60Co) as the source of radicals. The RAFT agent applied in these polymerization reactions was 3-benzylsulfanylthiocarbonylsulfanyl propionic acid (BPATT), which is sparingly soluble in aqueous systems due to the hydrophobic benzyl moiety. To abrogate this limitation, a small amount of acetone was added to the polymerization vessel in order to dissolve the agent. γ-radiation has previously been shown to be an effective source of initiation for living radical polymerizations in the presence of thiocarbonylthio compounds.60 In particular, the method is

highly efficient for the generation of well-defined polymers in aqueous solution. Similar results have been observed when using UV radiation as a source of initiating radicals.61 Interestingly, the macromolecular RAFT agents prepared via thermal initiation have substantially broader molecular weight distributions than those prepared using γ-initiation, possibly because side reactions are minimized at these lower temperatures. These results highlight the importance of ionizing radiation as an initiation source in living radical polymerization. Following the isolation of the macromolecular RAFT agent, chain extension experiments were then performed using 4-styrenesulfonate as a monomer. These experiments were performed using thermal initiation, due to the relatively slow rate of polymerization of this monomer at low temperatures. In all cases, the molecular weight of the SS domain, as determined by NMR spectroscopy, was shown to agree very closely with that predicted through the ratio of the monomer to the macromolecular transfer agent (see Table 1). This behavior is entirely consistent with what would be predicted for a living radical polymerization. However, Table 1 also revealed a variation of ∼10 kDa when comparing the molecular weight determined using gel permeation chromatography with that obtained theoretically. This discrepancy may arise because of the use of different calibrants in the GPC to the molecular structure of the polymer synthesized. Nevertheless, the data is useful in that it demonstrates that the polymers are relatively unimodal, with little evidence of side reactions resulting in dead polymer. It is not possible to completely prevent some amount of dead polymer forming, as radical-radical termination will invariably occur to some extent. PAAx-b-SSy/PAH Multilayer Formation. Weak PE multilayer films, such as those formed from PAA/PAH, have generated significant interest because they are pH-responsive. Rubner and co-workers reported that multilayer films assembled from PAA at pH 3.5 and PAH at pH 7.5 undergo large-scale film rearrangement when exposed to pH 2.5. However, multilayers from weak polyacids are also susceptible to disassembly in alkaline conditions.62 Therefore, using a copolymer (PAAx-bSSy), instead of PAA, should provide an extra degree of control over the pH-responsiveness and stability of the multilayer films, preserving a pH dependence on multilayer buildup while improving the stability of the films to pH extremes. Effect of Assembly pH on Film Thickness. The pH of assembly for PAAx-b-SSy and PAH plays an important role on the final multilayer film thickness. This is evident in Figure 1, which shows the ellipsometric thickness of (PAAx-b-SSy/PAH)10 films for the three different copolymers assembled under various conditions. We determined the apparent dissociation constant (pKa(app)) of all three copolymers to be ∼4.7. This value is in close agreement with the pKa(app) of homopolymeric PAA (ranging from 4.7 to 6.8),63,64 and suggests that SS moieties do not significantly affect the pKa(app) of the copolymers. Importantly, the thickness trend with the assembly pH observed is similar for all three PAAx-b-SSy copolymers. Notably, the thinnest films (61) Muthukrishnan, S.; Pan, E. H.; Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P.; Lewis, D.; Barner, L. Macromolecules 2007, 40, 2978. (62) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (63) Gregor, H. P.; Frederick, M. 1957, 23, 451. (64) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116.

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Figure 1. Thickness of (PAAx-b-SSy/PAH)10 multilayers determined using spectroscopic ellipsometry. The assembly pH is denoted in the legend, with the pH of the PAAx-b-SSy solution listed first, followed by the pH of the PAH solution. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. All films were rinsed with water between layers. The error in the thickness values is (10%.

were obtained at an assembly pH of 7.5/7.5 (ca. 20 nm for all three copolymers). At these conditions, PAAx-b-SSy is fully charged and PAH is highly charged. As such, intramolecular charge repulsion causes both the polyanion and polycation to assume an extended conformation and thus be adsorbed as thin, flat layers. This is consistent with the findings of Shiratori and Rubner.32 In that work, a layer thickness of only ∼0.3 nm was reported for PAA/PAH (7.5/7.5) films (a corresponding 20-layer film would have a thickness of ca. 6 nm). Comparatively, the PAAx-b-SSy/PAH films are considerably thicker, likely due to the presence of SS groups, which may enhance film thickness as they form very strong electrostatic linkages with PAH. Further, drying between layers (as used in this work but not in the previous study32) can increase film thickness, as reported by Lvov and co-workers.65 In that work, drying between adsorption steps influenced the bilayer thickness of PSS/PAH films significantly (5.6 nm (no drying) vs 9.6 nm (drying every step)) when the PE solutions contain 0.5 M NaCl. The use of a precursor layer of PEI in the current study may also enhance multilayer growth by providing a highly charged surface for subsequent layers to adsorb onto, although this effect is more likely to be evident in the early stages of film assembly and is unlikely to be a major contributor to the film thickness at higher layer numbers. The thickest PAAx-b-SSy/PAH films (∼100 nm for 20-layer films) were produced when deposited at pH 3.5/7.5 (Figure 1). Again, this is the same pH combination that Shiratori and Rubner found to result in the thickest and roughest PAA/PAH films.32 The enhanced thickness was attributed to the charge mismatch between PAA and PAH at pH 3.5 and 7.5, respectively. At pH 3.5, only 10% of AA groups in the PAA (in solution) are ionized, and this would cause PAA to adsorb in a conformation rich in tails and loops, which extend into the solution.32 When subsequently exposed to the pH 7.5 PAH solution, the AA segments extending into the solution become ionized, and are then able to form electrostatic links with PAH (at pH 7.5, ca. 55% of PAH (in solution) is ionized).64 Conversely, the nonionized functional groups of PAH (when adsorbed at pH 7.5) would become ionized when exposed to pH 3.5 conditions. This induces the ionization of weak polybases and polyacids in multilayers, and has been frequently documented in LbL literature.64 This ionization leads to the formation of extremely thick multilayer films under certain pH combinations. The PAAx-b-SSy/PAH films assembled at pH 3.5/3.5 were approximately half as thick as those assembled at pH 3.5/7.5 (65) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337.

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(Figure 1). This is due to both PE adsorption steps being conducted at pH 3.5, where PAH is fully charged, but where the PAAxb-SSy is considerably less ionized. Therefore, less PAH is required to reverse the charge of the previous PAAx-b-SSy layer. Further, the PAH will also be adsorbed as thinner and flatter layers due to intramolecular charge repulsion effects. In all other experiments, the pH of the PAH solution was kept constant at 7.5 while the pH of PAAx-b-SSy was varied. Figure 1 showed that the film thickness decreased as the pH of PAAx-b-SSy was increased from 3.5 to 7.5. An increase in the assembly pH of PAAx-b-SSy corresponds to a higher degree of ionization of AA groups and should consequently result in thinner layers, as was observed. For comparison, the thicknesses of (PSS/PAH)10 and (PAA/ PAH)10 films at different assembly pH are shown in Figure S1 (Supporting Information). We note that large differences in film thickness for the three copolymers were not observed, despite their different block ratios (Figure 1). There was a small increase in film thickness (∼18% on average) between PAA37-b-SS63 and PAA55-b-SS45. On the other hand, the difference in thickness between PAA55-b-SS45 and PAA73-b-SS27 films was negligible. It might be expected that the copolymers with higher proportions of AA groups would produce thicker films, especially in conditions where the AA groups are ionized to a low extent, as in all cases except the pH 7.5/7.5 combination. Closer analysis of the composition of the PAAx-b-SSy copolymers reveals that there is a 49% increase in AA groups between PAA37-b-SS63 and PAA55-b-SS45 (Table 1), which is consistent with thicker PAA55-b-SS45 films compared to PAA37-b-SS63 films. However, between PAA55-b-SS45 and PAA73-SS27, the proportion of AA groups increased by 33% with no measurable increase in the thickness of PAA73-b-SS27 films (Table 1). It is evident that the presence of SS moieties in the PAAx-b-SSy copolymers, regardless of the precise composition, leads to the films having a relatively consistent thickness. This provides an interesting contrast to studies conducted using blends of homopolymeric PAA and PSS, where variations in the proportion of the two components has a profound impact on the film thickness.51 However, when the PAA and PSS domains are contained within the one polymer molecule, the proportion of the moieties has little effect on the thickness over the range investigated. This highlights the unusual cooperative adsorption effects that are evident in blended PE multilayers.51,57 Influence of Acidic Post Treatment. Film Thickness. (PAAxb-SSy/PAH)10 films were immersed into acidic solutions to investigate their stability in low pH environments and their propensity to rearrange under these conditions, as has been observed for PAA/PAH films.31 These films were characterized with both ellipsometry and AFM. Figure 2 shows the film thickness as assembled, and also after separate treatments in pH 2.0, 2.2, and 2.5. For all of the (PAAx-b-SSy/PAH)10 films, there is only a slight decrease in thickness after the acidic post treatments (Figure 2). This result indicates that these films are very stable at acidic pH conditions and that they do not exhibit the large swelling and morphological variation commonly observed in PAA/PAH films when subjected to the same acidic treatment. Figure 3 shows the thickness of PSS/PAH films and PAA/PAH films at the same assembly and treatment conditions as those used to assemble the PAAx-b-SSy/PAH films. This data enables a comparison of the properties of the copolymer and its corresponding homopolymers when they are utilized in multilayer assemblies. By comparing Figure 3a and 3b, it is first evident that the PAA/PAH films (as assembled) were substantially thicker than PSS/PAH films. PSS/PAH films exhibit characteristics typical of strong PE films, even though PAH is a weak PE. That

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Figure 3. Film thickness of (A) (PSS/PAH)10 and (B) (PAA/PAH)10 films, as determined using spectroscopic ellipsometry. Films were assembled using pH combinations as listed, with the pH of the PAA or PSS solution listed first, followed by the pH of the PAH solution. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. In each case, the film thickness was measured as assembled and after separate treatments in pH 2.0, 2.2 and 2.5 for 24 h. Bars marked with an asterisk (*) were measurements made with AFM. All other films were measured with ellipsometry. The errors in the measurements are (10%.

Figure 2. Thickness of (A) (PAA37-b-SS63/PAH)10, (B) (PAA55-b-SS45/ PAH)10, and (C) (PAA73-b-SS27/PAH)10 films, as determined using spectroscopic ellipsometry. Films were assembled using pH combinations as listed, with the pH of the PAAx-b-SSy solution listed first, followed by the pH of the PAH solution. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. In each case, the film thickness was measured as assembled and after separate 24 h treatments in pH 2.0, 2.2 and 2.5 conditions. The errors in the measurements are (10%.

is, the film thickness is constant across the assembly pH range of 3.5-4.5. Further, the PSS/PAH films are completely resistant to changes in pH in the range studied, retaining their original thickness when they were subjected to the acidic post-treatments at pH 2.0-2.5. A similar thickness (∼375 ( 15 nm) was observed for PAA/ PAH films assembled at pH 3.5/7.5 and 4.0/7.5. Films assembled at pH 4.5/7.5 were approximately 30% thicker. In contrast to PSS/PAH films, the PAA/PAH films showed a large increase in film thickness when post-treated in acidic conditions. As some of the films turned opaque and therefore caused a substantial

level of light scattering, thus preventing accurate thickness determination with ellipsometry, the film thickness for these samples was measured with AFM (bars marked with an asterisk in Figure 3b). We have previously observed excellent agreement between the thickness of films determined via AFM step change measurements and ellipsometry.56 For films assembled at pH 3.5/7.5 and pH 4.0/7.5, the most significant change occurred when the films were treated at pH 2.0 (the film thickness increased by approximately ∼90%). For films assembled at pH 4.5/7.5, the film thickness more than doubled after the films were treated at pH 2.0 and pH 2.2. From AFM, pores were observed in the films after post-treatment. However, it is noted that the films (especially those assembled at pH 4.5/7.5) were not very stable after post treatment. Aside from the evident opacity of the samples, some films also began to separate from the surface at the edges of the silicon wafers, indicating that the bond between the film and the silicon wafer was possibly weakened by the treatment. Therefore, gentle handling was required during the drying procedures before characterization; excessive air flow can remove the films from the wafers. Interestingly, the large increase in the thickness of PAA/PAH films occurred over the narrow range of pH 2.0-2.2. When treated at pH 2.5, the increase over the original film thickness was greatly diminished, with the pH 4.0/7.5 PAA/ PAH film having a lower thickness after acidic treatment compared with when assembled. From Figure 3b, the optimal pH combination of assembly to effect the most substantial change in film thickness was pH 4.5/7.5. This was the same pH region

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Figure 4. AFM images of (PAA55-b-SS45/PAH)10 films. The PAA55-b-SS45 adsorption solution was at pH 3.5, while the PAH adsorption solution was at pH 7.5. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. AFM images were captured (A) as assembled and after separate treatments in pH (B) 2.0, (C) 2.2 and (D) 2.5 for 24 h.

that Rubner and co-workers found to be optimal for producing a pH-induced microporous film.31 A comparison of Figures 2 and 3 yields several interesting observations. PAAx-b-SSy/PAH films are thinner than the PAA/ PAH films (ca. 85 ( 15 nm vs 420 ( 70 nm) but considerably thicker than PSS/PAH films (∼20 nm). This indicates that the copolymer films exhibit adsorption properties that are intermediate between those of PSS and PAA. In terms of the swelling behavior of the films after post-treatment, the copolymer films resembled PSS/PAH films more closely than PAA/PAH films. No swelling was observed for the copolymer films; instead, a slight decrease in film thickness was observed. This suggests that the SS groups had a strong stabilizing influence on the properties of the copolymer films, which is likely a result of the strong intermolecular complexation that occurs between the protonated allylamine groups and the sulfonate groups.66 These interactions are clearly dominant in determining the thickness and swelling/ shrinkage properties of the PAAx-b-SSy/PAH films. Surface Morphology. One of the most interesting features of weak PE multilayers is their ability to change their film morphology/structure in response to the pH environment. As discussed previously, PAA/PAH films (especially those assembled in pH 3.5/7.5) have shown a propensity to develop (66) Gao, C.; Leporatti, S.; Moya, S.; Donath, E.; Mo¨hwald, H. Langmuir 2001, 17, 3491.

micropores throughout the film when treated at pH 2.5.31 Therefore, we examined the surface morphology of the copolymer films before and after acidic treatment. All of the copolymer films studied have low root-mean-square (rms) roughness values after assembly (∼1 nm), and the surface morphology (like the thickness) is similar for films made of the copolymers of varying composition. However, these films behaved very differently when subjected to treatment in acidic conditions postassembly. PAA37b-SS63/PAH films were largely resistant when treated (separately) in pH 2.0, 2.2, and 2.5; no significant change in surface morphology was observed after the pH treatment. This result was observed for PAA37-b-SS63/PAH films assembled from pH 3.5/7.5, 4.0/7.5, and 4.5/7.5, indicating that varying the film assembly conditions in this range is not sufficient to render the films susceptible to film rearrangement. PAA55-b-SS45/PAH films assembled at pH 4.0/7.5 and 4.5/7.5 showed similar behavior; that is, their morphology did not change significantly after the acidic post treatment (pH 2.0, 2.2 or 2.5). However, PAA55-bSS45/PAH films assembled at pH 3.5/7.5 developed ridges all over the surface of the films. AFM images of these films are shown in Figure 4. For films treated at pH 2.0 and 2.2, the “wrinkles” are finer and their heights are substantially lower than the ridges observed for films treated at pH 2.5, which showed regular discrete “rings” on top of a considerably porous surface. This surface structure is distinctly different from the more random

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Figure 6. ζ-potential of (PAA37-b-SS63/PAH)4-coated silica particles (1 µm diameter) as a function of (A) layer number and (B) sample dispersion pH. Block copolymer PAA37-b-SS63 adsorption solutions of various pH (as stated in the legend) containing 0 M NaCl is used in alternation with a PAH adsorption solution of pH 7.5 containing 0.5 M NaCl.

Figure 5. AFM images of (PAA73-b-SS27/PAH)10 films. The PAA73b-SS27 adsorption solution was at pH 3.5, while the PAH adsorption solution was at pH 7.5. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. AFM images were captured (A) as assembled and (B) after treatment in pH 2.5 for 24 h.

structures seen on films treated at pH 2.0 and 2.2. It is evident that, similar to PAA/PAH systems,32 subtle variations in the pH can have a profound impact on the response of the film and the final film morphology observed. Interestingly, while PAA73-b-SS27/PAH films contained the greatest proportion of AA groups, they did not show any substantial change in morphology after acidic treatment. The only exception to this was the case of the PAA73-b-SS27/PAH film (assembled at pH 3.5/7.5) treated at pH 2.5. AFM images depicting the phenomenon observed are shown in Figure 5. Prior to the post treatment, this film was very smooth (Figure 5A). Once treated, the film structure appeared very similar to the PAA55-b-SS45/PAH film assembled and treated at the same pH conditions (Figure 4). These results indicate that the specific pH required to trigger a change in film morphology is affected by the specific block ratios in the PAAx-b-SSy copolymer used and the assembly conditions. The PAA55-b-SS45/PAH films showed a pH-induced transformation between pH 2.0 to 2.5, whereas this change only occurred at pH 2.5 for the PAA73-b-SS27/PAH film. Further, the different morphologies observed for the PAA55b-SS45/PAH films treated at pH 2.0-2.5 provide evidence of the

ability to use block copolymer composition, pH of assembly, and pH of post-treatment to target a desired surface structure. Preparation of PAAx-b-SSy/PAH Core-Shell Particles and Capsules. In many applications, including drug delivery and photonics, it is desirable to form multilayer thin film coatings on colloidal interfaces, rather than planar substrates. The use of PAAx-b-SSy copolymers is an alternative strategy for the incorporation of weakly charged moieties into core-shell particles and/or capsules. The incorporation of such weakly charged moieties has previously been achieved using blends of weak and strong PEs in the adsorption solution,53,57 by adsorbing weak polyelectrolytes in the presence of Cu2+,67 or under conditions of relatively high ionization.68 The use of block copolymers, however, offers several key advantages: (i) the composition of the film is known, based on the composition of the homogeneous polymer population, (ii) it eliminates the possibility of polymer displacement/substitution effects, and (iii) the films display properties that are intermediate between weak and strong polymers, thus avoiding the sensitivity of weak PE multilayers (such as PAA/PAH) to solution extremes, while being more flexible and responsive than strong PEs (such as PAH/PSS). To this end, we investigated the formation of multilayers of PAA37b-SS63 and PAA55-b-SS45 adsorbed in alternation with PAH on SiO2 particles. Figure 6A shows the alternation of ζ-potential with layer number for PAA37-b-SS63/PAH films assembled on SiO2 particles. The pH of the PAA37-b-SS63 adsorption solutions was varied between 3.5 and 4.5, while the pH of the PAH solution was 7.5. In all cases, the ζ-potential varied between positive and negative, depending on whether the final adsorbed layer was the polycation (67) Schuetz, P.; Caruso, F. AdV. Funct. Mater. 2003, 13, 929. (68) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780.

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(PAH) or polyanion (PAA37-b-SS63). Interestingly, the variation was greatest when the PAA37-b-SS63 was adsorbed at pH 3.5, where PAA37-b-SS63 should be less charged. The variation likely stemmed from the measurement of ζ-potential, which was performed at approximately pH 5.8. At this pH, AA groups that were uncharged during the film assembly (due to being protonated) would subsequently deprotonate, leading to a greater surface charge. Figure 6B shows the pH-dependence of the surface charge, and found that the profile resembles that previously found for blends incorporating PAA and PSS,53 although the observed point of zero charge is slightly higher (at pH 4-5, depending on the pH of assembly). This suggests that the interfacial properties are influenced more strongly by the PAA domains as opposed to the PSS domains, given that a PSS coating would typically lead to a much lower point of zero charge (pH ∼2).53,68 Similar ζ-potential measurements were performed using PAA55-b-SS45, and again, the ζ-potential was shown to alternate with the layer number (see Supporting Information, Figure S2A). However, in this case, the pH of assembly did not have a substantial influence on the ζ-potential measured. This may be due to conformational differences in the adsorbed polymer, meaning that the AA moieties are unable to be recharged at the pH of measurement. Measurement of the pH dependence of the ζ-potential of these particles indicated a point of zero charge between pH 3-5 (see Supporting Information, Figure S2B). Although this range is somewhat broader than that determined for PAA37-b-SS63, and the dependence appears to be less obviously related to the pH of assembly, it is still clear that the interfacial properties are determined primarily by the AA groups, giving a point of zero charge comparable to that observed for PAA-terminated assemblies in previous studies.53,69 Further, we sought to characterize capsules formed after dissolution of the silica cores of the (PAAx-b-SSy/PAH)4-coated particles. SiO2 core dissolution by exposure to NH4F-buffered HF yielded intact (PAAx-b-SSy/PAH)4 capsules (see Figure 7). Importantly, these results demonstrated that stable capsules can be prepared from PAA37-b-SS63 and PAA55-b-SS45 copolymers, The ability to prepare stable capsules is crucial in potential future applications, such as encapsulation and drug delivery. In addition, AFM studies were conducted to examine the increase in capsule wall thickness with increasing layer number. The height of capsules where they were folded only once was used to determine the wall thickness. As in the studies on planar substrates, only films prepared from PAAx-b-SSy solutions containing 0 M NaCl were investigated. Figure 8 shows that the capsules became progressively thicker with increasing layer number, eventually reaching a film thickness of 80 nm for (PAA55b-SS45/PAH)4 capsules and 60 nm for (PAA37-b-SS63/PAH)4 capsules. Thicker film formation in the PAA55-b-SS45/PAH system is likely the result of a greater proportion of AA groups, which has been reported to result in thicker capsules in studies conducted using blends of homopolymeric PAA and PSS.57 Moreover, the total thickness of these capsules is substantially greater than the analogous films prepared on planar substrates. We attribute this observation to the presence of NaCl (0.5 M) in the PAH adsorption solution. NaCl is added for intramolecular charge shielding on the stretched PAH chains at pH 7.5 (∼79% charged).70 Thus, the incorporation of NaCl into PAH solution enabled PAH chains to adopt a more flexible conformation. The flexible PAH conformation likely leads to the formation of thicker films. This (69) Yang, W. J.; Trau, D.; Renneberg, R.; Yu, N. T.; Caruso, F. J. Colloid Interface Sci. 2001, 234, 356. (70) Itano, K.; Choi, J. Y.; Rubner, M. F. Macromolecules 2005, 38, 3450.

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Figure 7. AFM images of (A) (PAA37-b-SS63/PAH)4 and (B) (PAA55b-SS45/PAH)4 capsules obtained from 5.3 µm diameter silica particles. The PAAx-b-SSy adsorption solution was at pH 3.5, while the PAH adsorption solution was at pH 7.5. All adsorptions were performed from solutions with a polymer concentration of 1 mg mL-1, with an adsorption time of 15 min. The PAAx-b-SSy adsorption solution contained 0 M NaCl while PAH solutions contained 0.5 M NaCl.

Figure 8. Thickness changes for (PAAx-b-SSy/PAH)4 capsules obtained from 5.3 µm diameter silica particles, as determined from AFM images. The capsules were prepared from various copolymer (as stated in the diagram) and PAH adsorption solutions of pH 3.5 and 7.5, respectively. All copolymer solutions contained 0 M NaCl. PAH solutions contained 0.5 M NaCl.

effect is reflected in the greater thickness of the capsule walls, in comparison to analogous planar films (assembled using PAH in 0 M NaCl). More importantly, the presence of NaCl in the PAH solution facilitated the formation of an even coating on the

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particle surface and to prevent particle aggregation.57 In the absence of NaCl, we observed that the particles tended to aggregate and adhere to the vessel wall when PAH was the outer layer, leading to a considerable loss of particles. Figure 8 also revealed nonlinear growth of the PAAx-b-SSy/ PAH capsules. This is also likely the result of the greater proportion of AA groups in PAAx-b-SSy/PAH systems, and the coiled conformation in pH 3.5 adsorption solution leading to the formation of layers that contain loops and tails. These loops and tails contribute to an increased surface area that facilitates the adsorption of more PE. As the multilayer builds up, the increase in surface area, and consequently the increase in amount of PE adsorbed, is amplified. In this manner, the thickness of newly formed layers may increase exponentially as more layers are adsorbed.57 This effect is evident by comparing the growth of (PSS/PAH) with (PAA37-b-SS63/PAH) and (PAA55-b-SS45/PAH) capsules (see Figure 8). In the absence of AA groups, (PSS/ PAH) capsules followed a linear growth, while the presence of AA groups in both (PAAx-b-SSy/PAH) systems resulted in an exponential increase in the capsule thickness as more layers were adsorbed. Importantly, the nonlinearity was particularly pronounced in (PAA55-b-SS45/PAH) capsules, which contained a greater proportion of AA groups. This observation verified the role of AA groups in imparting tailored properties to the (PAAxb-SSy/PAH) systems.

Conclusions Multilayer thin films were successfully assembled using the pH-sensitive RAFT-synthesized copolymers PAAx-b-SSy, adsorbed in alternation with the weak PE, PAH. Increasing the assembly pH of the copolymers (maintaining the assembly of pH of PAH constant) resulted in a decrease in film thickness. This trend in film thickness was observed for all of the copolymers studied. However, comparison between films assembled using PAAx-b-SSy copolymers with different block ratios under the same conditions produced films with a relatively consistent

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thickness, likely due to the presence of strongly charged SS groups. Post-treatment experiments demonstrated that all the copolymer films investigated are stable in acidic conditions. They do not exhibit the large swelling typical in PAA/PAH films when treated at the same acidic pH. AFM studies showed that two of the copolymer films, (PAA55-b-SS45 and PAA73-b-SS27) assembled from pH 3.5 PAAx-b-SSy solutions and pH 7.5 PAH solutions, demonstrated pH-induced morphological changes when treated in acidic conditions. The treatment pH range to effect this change is wider for PAA55-b-SS45 (pH 2.0-2.5) than PAA73b-SS27 films (only at pH 2.5). Films could also be formed on colloidal substrates (such as SiO2) under various adsorption conditions. Additionally, these core-shell particles are suitable as precursors to capsules that incorporate weak PE functionality. Moreover, we found that the incorporation of weak PE functionality (i.e., AA groups) contributed toward the nonlinear growth profile of PAAx-b-SSy/PAH capsules. In conclusion, using copolymers synthesized with specific block ratios of weakly and strongly charged groups enables the preparation of stable pH-responsive multilayers. Acknowledgment. The authors acknowledge support from the Australian Research Council in the form of an Australian Postdoctoral Fellowship (J.F.Q.), an Australian Professorial Fellowship (C.B.K.) and Federation Fellowships (F.C. and T.P.D.). Funding through the Discovery Project Scheme is also acknowledged, along with infrastructure support from the Particulate Fluids Processing Centre at the University of Melbourne. Supporting Information Available: Ellipsometric thickness of (PSS/PAH)10 and (PAA/PAH)10 films assembled from different pH combinations, and the pH-dependence of surface charge for PAA55-bSS45/PAH films. This material is available free of charge via the Internet at http://pubs.acs.org. LA8011074