Layer-by-Layer Deposition of Polyelectrolyte−Polyelectrolyte

1004

Langmuir 2009, 25, 1004-1010

Layer-by-Layer Deposition of Polyelectrolyte-Polyelectrolyte Complexes for Multilayer Film Fabrication Yongmei Guo, Wei Geng, and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed October 20, 2008. ReVised Manuscript ReceiVed NoVember 13, 2008 Positively charged poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were layer-by-layer (LbL) assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce multilayer films. The film structure and deposition behavior of the PAH-PAA/ PSS films were influenced by the structure of PAH-PAA complexes in solution. For the PAH-PAA complexes with a low ratio of PAA to PAH the PAH-PAA complexes have low-level cross-linking and are flexible. The resultant PAH-PAA/PSS films have a thin film thickness and smooth surface and exhibit a nonlinear deposition behavior where the amount of PAH-PAA complexes and PSS deposited in each deposition cycle are larger than in its previous cycle. The PAH-PAA complexes with a high ratio of PAA to PAH have high-level cross-linking and are rigid. The PAH-PAA/ PSS films constructed from the rigid PAH-PAA complexes have a large film thickness and rough surface and exhibit a linear deposition behavior. Deposition of the PAH-PAA/PSS films was well characterized by quartz crystal microbalance, atomic force microscopy, and scanning electron microscopy. The thermally cross-linked PAH-PAA/ PSS films can be released from substrate to form stable free-standing films by an ion-triggered exfoliation method. Meanwhile, positively charged PAH-PAA complexes can be LbL assembled with negatively charged PAH-PAA complexes with a molar excess of PAA to produce multilayer films. Use of polyelectrolyte-polyelectrolyte complexes as building blocks for LbL fabrication provides a facile way to tailor the structures of the resultant films by simply changing the structure of the complexes in solution.

Introduction In the past decade, the layer-by-layer (LbL) assembly technique has developed as a group of versatile and convenient methods for construction of composite films with precise control of film structures and composition.1,2 Advanced multilayer film materials with components such as synthetic linear polymers, polymeric microgels,3 biomacromolecules,4 particles,5 dentritic molecules,6 organic components,7 block copolymers,8 and polyelectrolyte* To whom correspondence should be addressed. Phone: 0086-43185168723. Fax: 0086-431-85193421. E-mail: [email protected]. (1) (a) Decher, G. Science 1997, 277, 1232. (b) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (c) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (2) (a) Hammond, P. T. AdV. Mater. 2004, 16, 1271. (b) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (c) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. ReV. 2007, 36, 707. (d) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (3) (a) Nolan, C. M.; Serp, M. J.; Lyon, L. A. Biomacromolecules 2004, 5, 1940. (b) Serizawa, T.; Matsukuma, D.; Nanameki, K.; Uemura, M.; Kurusu, F.; Akashi, M. Macromolecules 2004, 37, 6531. (c) Kharlampieva, E.; Erel-Unal, I.; Sukhishvili, S. A. Langmuir 2007, 23, 175. (d) Wang, L.; Wang, X.; Xu, M. F.; Chen, D. D.; Sun, J. Q. Langmuir 2008, 24, 1902. (4) (a) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405. (b) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (c) Lvov, Y.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (d) Serizawa, T.; Yamaguchi, M.; Akashi, M. Macromolecules 2002, 35, 8656. (e) Johnston, A. P. R.; Read, E. S.; Caruso, F. Nano Lett. 2005, 5, 953. (f) Haynie, D. T.; Zhang, L.; Rudra, J. S.; Zhao, W.; Zhong, Y.; Palath, N. Biomacromolecules 2005, 6, 2895. (5) (a) Gao, M. Y.; Gao, M. L.; Zhang, X.; Yang, Y.; Yang, B.; Shen, J. C. Chem. Commun. 1994, 2777. (b) Schmitt, J.; Decher, G. AdV. Mater. 1997, 9, 61. (c) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (6) (a) He, J.-A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (b) Huo, F. W.; Xu, H. P.; Zhang, L.; Fu, Y.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2003, 874. (7) (a) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. Chem. Commun. 1994, 1055. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (c) Saremi, F.; Tieke, B. AdV. Mater. 1998, 10, 388. (d) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (e) Advincula, R. C.; Fells, E.; Park, M. K. Chem. Mater. 2001, 13, 2870.

stabilized micelles9 have been successfully fabricated. The driving force for LbL film fabrication is diverse, including electrostatic interaction, hydrogen-bond,10 halogen-bond,11 coordinationbond,12 charge-transfer interactions,13 biospecific interaction (e.g., sugar-lectininteractions),14 guest-hostinteraction,15 cation-dipole interaction,16 the synergetic interaction of the above forces, etc. These LbL-deposited films have multiple applications in areas such as antireflection coatings,8b,17 controlled releasing coatings,3,8d,18 biosensors,19 nonlinear optics,20 solid-state ionconducting materials,21 solar-energy conversion,22 and separation membranes.23 Extension of materials for LbL film fabrication will certainly enrich the structures and therefore functionalities (8) (a) Ma, N.; Zhang, H.; Song, B.; Wang, Z.; Zhang, X. Chem. Mater. 2005, 17, 5065. (b) Cho, J.; Hong, J.; Char, K.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9935. (c) Qi, B.; Tong, X.; Zhao, Y. Macromolecules 2006, 39, 5714. (d) Nguyen, P. M.; Zacharia, N. S.; Verploegen, E.; Hammond, P. T. Chem. Mater. 2007, 19, 5524. (9) Liu, X. K.; Zhou, L.; Geng, W.; Sun, J. Q. Langmuir 2008, 24, 12986. (10) (a) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (b) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (11) Wang, F.; Ma, N.; Chen, Q.; Wang, W.; Wang, L. Langmuir 2007, 23, 9540. (12) (a) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (b) Xiong, H. M.; Cheng, M. H.; Zhou, Z.; Zhang, X.; Shen, J. C. AdV. Mater. 1998, 10, 529. (13) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (14) (a) Anzai, J.; Kobayashi, Y.; Nakamura, N.; Nishimura, M.; Hoshi, T. Langmuir 1999, 15, 221. (b) Anzai, J.; Kobayashi, Y. Langmuir 2000, 16, 2851. (15) (a) Suzuki, I.; Egawa, Y.; Mizukawa, Y.; Hoshi, T.; Anzai, J. Chem. Commun. 2002, 164. (b) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594. (16) Ogawa, Y.; Arikawa, Y.; Kida, T.; Akashi, M. Langmuir 2008, 24, 8606. (17) (a) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (b) Cebeci, F. C.; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856. (c) Zhang, L. B.; Li, Y.; Sun, J. Q.; Shen, J. C. J. Colloid Interface Sci. 2008, 319, 302. (d) Zhang, L. B.; Li, Y.; Sun, J. Q.; Shen, J. C. Langmuir 2008, 24, 10851. (18) (a) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. (b) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammond, P. T. Langmuir 2005, 21, 1603. (c) Ren, K. F.; Ji, J.; Shen, J. C. Bioconjugate Chem. 2006, 17, 77.

10.1021/la803479a CCC: $40.75  2009 American Chemical Society Published on Web 12/23/2008

Deposition of Polyelectrolyte-Polyelectrolyte Complexes

of the LbL-deposited film materials. Polymeric building blocks possessing versatile structures in solution are helpful to obtain polymeric films with well-tailored structures as well as functionalities, as exemplified by LbL deposition of weak polyelectrolytes.17a,24 Polyelectrolyte-polyelectrolyte complexes (PECs) represent a special class of polymeric compounds consisting of polycations and polyanions.25-27 By mixing solutions of polyanions and polycations, PECs form spontaneously under release of the counterions. The driving force for PECs formation is mainly electrostatic interactions between the oppositely charged groups in the involved polyelectrolyte chains, but other weak interactions such as hydrogen bonding, van der Waals, or hydrophobic ones could also play an additional role.25 Depending on their composition and preparative conditions, PECs can be divided into two categories: those that are insoluble but swellable or dispersible in water solution28 and those that are water soluble. PECs are “living systems”, which may respond very sensitively to changes of their environment.25 The structures of PECs are diverse and can be tailored by changing parameters such as mixing ratio, concentration, solution pH, ionic strength, temperature, etc., during or after PECs formation. Meanwhile, the detailed structure of the polyelectrolytes, such as chain rigidity/flexibility, topography, charge density, and molecular weight of the polyelectrolytes, also plays an important role on defining the structure of the PECs.25-27,29 PECs have been under intense investigation for various potential applications, such as nanocarriers for the biomedical and pharmaceutical purpose, water purification, cosmetic use, etc., benefiting from their versatility (19) (a) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, B.; Shen, J. C. Macromol. Chem. Phys. 1996, 197, 147. (b) Lvov, Y.; Caruso, F. Anal. Chem. 2001, 73, 4212. (c) Wang, Y.; Joshi, P. P.; Hobbs, K. L.; Johnson, M. B.; Schmidtke, D. W. Langmuir 2006, 22, 9776. (d) Calvo, E. J.; Danilowicz, C.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 2452. (20) (a) Lvov, Y.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (b) Balasubramanian, S.; Wang, X. G.; Wang, H. C.; Yang, K.; Kumar, J.; Tripathy, S. K.; Li, L. Chem. Mater. 1998, 10, 1554. (c) Van Cott, K. E.; Guzy, M.; Neyman, P.; Brands, C.; Heflin, J. R.; Gibson, H. W.; Davis, R. M. Angew. Chem., Int. Ed. 2002, 41, 3236. (d) Kang, E.-H.; Jin, P. C.; Yang, Y. Q.; Sun, J. Q.; Shen, J. C. Chem. Commun. 2006, 4332. (e) Kang, E.-H.; Bu, T. J.; Jin, P. C. ; Sun, J. Q.; Yang, Y. Q.; Shen, J. C. Langmuir 2007, 23, 7594. (21) Lowman, G. M.; Tokuhisa, H.; Lutkenhaus, J. L.; Hammond, P. T. Langmuir 2004, 20, 9791. (22) (a) Guldi, D. M.; Zilbermann, I.; Anderson, G.; Kotov, N. A.; Tagmatarchise, N.; Prato, M. J. Mater. Chem. 2005, 15, 114. (b) Guldi, D. M.; Rahman, G. M. A.; Prato, M.; Jux, N.; Qin, S.; Ford, W. Angew. Chem., Int. Ed. 2005, 44, 2015. (23) (a) Leva¨salmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (b) Krasemann, L.; Tieke, B. J. Membr. Sci. 1998, 150, 23. (c) Bruening, M. L.; Sullivan, D. M. Chem. Eur. J. 2002, 8, 3833. (d) Park, M.-K.; Deng, S.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723. (e) Ball, V.; Voegel, J.-C.; Schaaf, P. Langmuir 2005, 21, 4129. (f) Kang, E.-H.; Liu, X. K.; Sun, J. Q.; Shen, J. C. Langmuir 2006, 22, 7894. (24) (a) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (b) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (c) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (d) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354. (25) Thu¨nemann1, A. F.; Mu¨ller, M.; Dautzenberg, H.; Joanny, J.-F.; Lo¨wen, H. AdV. Polym. Sci. 2004, 166, 113. (26) Kabanov, V. In Multilayer Thin Films: Sequential Assembly of Nanocomposite Material; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003; pp 47-86. (27) Philipp, B.; Dautzenberg, H.; Linow, K.-J.; Koetz, J.; Dawydoff, W. Prog. Polym. Sci. 1989, 14, 91. (28) (a) Kabanov, V. A.; Zezin, A. B. Makromol. Chem. 1984, 6, 259. (b) Biesheuvel, P. M.; Cohen-Stuart, M. A. Langmuir 2004, 20, 2785. (29) (a) Parthasarathy, M.; Kakade, B. A.; Pillai, V. K. Macromolecules 2008, 41, 3653–3658. (b) Ga¨rdlund, L.; Wågberg, L.; Norgren, M. J. Colloid Interface Sci. 2007, 312, 237. (c) Sui, Z. J.; Jaber, J. A.; Schlenoff, J. B. Macromolecules 2006, 39, 8145. (d) Chelushkin, P. S.; Lysenko, E. A.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. J. Phys. Chem. B 2007, 111, 8419. (e) Pergushov, D. V.; Babin, I. A.; Plamper, F. A.; Zezin, A. B.; Mu¨ller, A. H. E. Langmuir 2008, 24, 6414.

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in chemical composition and structures.30 However, less attention has been paid to use PECs for LbL-assembled multilayer fabrication.17d,31 We believe that LbL deposition of PECs will provide a facile way to fabricate advanced film materials with well-tailored film structures as well as functionalities because of the easily tailored structures of PECs in solution. In our previous work we showed that water-soluble poly(diallyldimethylammonium chloride) (PDDA)-sodium silicate complexes (noted as PDDA-silicate) with an average size of 13.2 nm could be alternatively assembled with poly(acrylic acid) (PAA) to produce mechanically stable antireflection and antifogging coatings after burning out of the polymeric components.17d Use of PDDA-silicate complexes increased the ratio of organic components in the asprepared PDDA-silicate/PAA films and porosity density of the calcined films. Therefore, highly antireflection coatings of a low refractive index with a superhydrophilic surface were finally obtained. In this paper, we demonstrated that highly aggregated PECs of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) (noted as PAH-PAA) can be used as building blocks for LbL film fabrication. The film structures are highly related to the mixing molar ratio of PAA to PAH in the PAH-PAA complexes. The mechanism for film formation was discussed. The LbL-assembled polymeric films of PAH-PAA complexes can be thermally cross-linked and subsequently released from the substrate by an ion-triggered exfoliation method to produce robust free-standing films with large areas.

Experimental Section Materials. Poly(acrylic acid) (PAA, Mw ca. 2000), poly(allylamine hydrochloride) (PAH, Mw ca. 70 000), poly(sodium 4-styrenesulfonate) (PSS, Mw ca. 70 000), and poly(diallyldimethylammonium chloride) (PDDA, Mw ca. 100 000-200 000) were purchased from Sigma-Aldrich. All chemicals were used without further purification. Deionized water was used for all experiments. Preparation of PAH-PAA Complexes. The PAH-PAA complexes were prepared by dropping aqueous PAA solution into aqueous PAH solution under intense stirring. Four kinds of PAH-PAA complexes were prepared with the feed monomer molar ratio of PAH to PAA being 1:0.25, 1:0.5, 1:0.75, and 0.5:1. For simplicity, they were noted as PAH-PAA0.25, PAH-PAA0.5, PAH-PAA0.75, and PAH0.5-PAA, respectively. The pH value of the aqueous dispersions was adjusted to 9.5 for the PAH-PAA0.25, PAH-PAA0.5, and PAH-PAA0.75 complexes and 3.5 for PAH0.5-PAA complexes with addition of either diluted NaOH or HCl. For aqueous dispersions of PAH-PAA0.25, PAH-PAA0.5, and PAH-PAA0.75 complexes the concentration of PAH is 10.7 × 10-3 M, while that of PAA is 2.7 × 10-3, 5.3 × 10-3, 8.0 × 10-3 M, respectively. The dispersion of PAH0.5-PAA complexes is comprised of 13.9 × 10-3 M PAA and 6.9 × 10-3 M PAH. Film Preparation. Quartz and silicon wafers were immersed in piranha solution (1:3 mixture of 30% H2O2 and 98% H2SO4) and heated until no bubbles were released. Caution: Piranha solution reacts Violently with organic material and should be handled carefully. Ag-coated quartz crystal microbalance (QCM) resonators with both sides coated with Ag (F0 ) 9 MHz) were sonicated slightly in ethanol and water and dried by N2 flow. The cleaned quartz and silicon wafers were immersed in PDDA aqueous solution (1.0 mg/ mL) for 20 min to obtain a cationic ammonium-terminated surface. The positively charged PAH-PAA complexes of PAH-PAA0.25, PAH-PAA0.5, and PAH-PAA0.75 were alternatively deposited with PSS to fabricate multilayer films of PAH-PAA/PSS. The PDDA(30) (a) Ouyang, W.; Mu¨ller, M. Macromol. Biosci. 2006, 6, 929. (b) Buchhammer, H.-M.; Petzold, G; Lunkwitz, K. Colloid Polym. Sci. 2000, 278, 841. (c) Petzold, G.; Buchhammer, H. M.; Lunkwitz, K. Colloids Surf. A 1996, 119, 87. (d) Takagishi, T.; Kuroki, N.; Mitsuishi, M. J. Polym. Sci. 1985, 23, 3021. (31) (a) Schuetz, P.; Caruso, F. Colloids Surf., A 2002, 207, 33. (b) Reihs, T.; Mu¨ller, M.; Lunkwitz, K. Colloids Surf., A 2003, 212, 79.

1006 Langmuir, Vol. 25, No. 2, 2009 modified quartz or silicon wafer was first immersed into an aqueous PSS solution (1.0 mg/mL, pH 9.5) for 20 min, rinsed with water (pH 9.5) three times for 1 min each time, and blown dry with N2 flow. The substrate was then transferred to an aqueous dispersion of PAH-PAA for 20 min to obtain a layer of PAH-PAA complexes followed by rinsing with water (pH 9.5) three times for 1 min each and drying with N2 flow. By repetition of the above deposition processes in a cyclic fashion a multilayer film of PSS/PAH-PAA can be fabricated. By alternative deposition of the PAH0.5-PAA complexes with PAH-PAA0.5 complexes multilayer films of PAH0.5-PAA/PAH-PAA0.5 can be fabricated in a similar way to that of PSS/PAH-PAA films. The pH of the aqueous dispersions of PAH0.5-PAA and PAH-PAA0.5 complexes was 3.5 and 9.5, respectively. Deionized water without pH adjustment (pH of ∼6.5) was used for rinsing the PAH0.5-PAA/PAH-PAA0.5. The films were dried with N2 flow after each layer deposition. For film deposition on a QCM resonator, a precursor film of (PDDA/PSS)*3 was deposited to eliminate the surface difference among different resonators.3d The PDDA/PSS film was prepared by immersing the resonator in aqueous PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) solutions for 20 min alternatively with intermediate water washing and N2 drying. Thermal cross-linking of the PSS/PAH-PAA0.5 film was conducted by heating the film at 180 °C for 2 h with a heating rate of 2 °C/min from room temperature to 180 °C. The thermally cross-linked free-standing film of PSS/PAH-PAA0.5 was obtained by immersing the film in an aqueous NaOH solution (0.1 M) to release the film from the substrate. Characterization. QCM measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Dynamic light scattering (DLS) studies and ζ-potential measurements were carried out on a Malvern Nano-ZS zetasizer at room temperature. The measurements were made at a scattering angle of θ ) 173° at 25 °C using a He-Ne laser with a wavelength of 633 nm. Scanning electron microscopy (SEM) images were obtained on a XL30 ESEM FEG field emission scanning electron microscope. Atomic force microscopy (AFM) images were performed in a dry state on a commercial instrument, Veeco Co. Nanoscope IIIa. AFM measurement was operated in the tapping mode using silicon cantilevers with a force constant of 40 N/m. Fourier transform infrared (FT-IR) spectra were collected on a Bruker IFS 66V instrument.

Results and Discussion Preparation of PAH-PAA Complexes. The as-prepared aqueous dispersions of the PAH-PAA complexes are turbid but homogeneous, indicating complexation of PAH and PAA in all cases. The sizes of the PAH-PAA complexes were characterized by DLS measurements. As shown in Figure 1, DLS measurements reveal that PAH-PAA0.25, PAH-PAA0.5, and PAH-PAA0.75 complexes exhibit a polydisperse but monomodal distribution with an average hydrodynamic diameter of 145, 177, and 735 nm, respectively. The DLS spectrum of the PAH0.5-PAA complexes has two peaks with the weak one at 85 nm and the strong one at 164 nm. The driving force for complex formation is based mainly on electrostatic interaction and hydrogen bonding between the carboxylic acid groups of PAA and the amine groups of PAH. The absence of peaks below 50 nm in DLS measurements for the as-prepared dispersion of PAH-PAA complexes indicates that there are almost no free polyelectrolyte chains in dispersions of PAH-PAA complexes. The strong interaction between PAA and PAH favors complete complexation between them. Electrophoresis measurements show that PAH-PAA0.25, PAH-PAA0.5, and PAH-PAA0.75 complexes are positive with ζ potentials of +39.2, +33.2, and +16.8 mV, respectively. The extent of complexation between PAH and PAA increases with increasing mixing molar ratio of PAA to PAH, leading to an increased size of the complexes. Meanwhile, the amount of free

Guo et al.

Figure 1. Hydrodynamic diameter distribution curves of PAH-PAA0.25 (a), PAH-PAA0.5 (b), PAH-PAA0.75 (c), and PAH0.5-PAA (d) complexes in deionized water.

Figure 2. QCM frequency decrease (-∆F) of alternative deposition of PAA-PAA complexes (hollow symbols) and PSS (solid symbols): PAH-PAA0.25/PSS(squares),PAH-PAA0.5/PSS(triangles),andPAH-PAA0.75/ PSS (circles) films.

amine groups decreases and results in gradually decreasing ζ potentials. The PAH0.5-PAA complexes have a ζ potential of -52.2 mV, indicating that PAA is excessive and the surface of the complexes is full of carboxylic acid groups. Fabrication of PAH-PAA/PSS Multilayer Films. The electrophoretic measurements show that PAH-PAA complexes with an excessive PAH component are positively charged in aqueous dispersions. Therefore, the positively charged PAH-PAA complexes can be LbL assembled with polyanion PSS based on electrostatic interaction as the driving force to prepare PAH-PAA/ PSS multilayer films. QCM measurements were employed to monitor the deposition process of PAH-PAA/PSS multilayer films. Figure 2 shows the decreases of frequency as a function of the layer number for PAH-PAA complexes of different mixing ratios and PSS. In all cases, the QCM frequency decreases with the number of film deposition cycles because of the successive deposition of PAH-PAA/PSS multilayers on the resonators. Within the examined 10 deposition cycles, the PAH-PAA0.75/ PSS films show a nearly linear deposition behavior starting from the fourth deposition cycles. Nonlinear film deposition in the primary stage is influenced by the substrate. For PAH-PAA0.25/ PSS and PAH-PAA0.5/PSS films the amount of PAH-PAA complexes and PSS deposited increases more than that in their previous step, resembling the exponential growth behavior

Deposition of Polyelectrolyte-Polyelectrolyte Complexes

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Figure 4. SEM image of the (PSS/PAH-PAA0.75)*n films with n ) 4 (a), 8 (b), 16 (c), and 20 (d).

Figure 3. AFM images of the PSS/PAH-PAA films: (a) (PSS/ PAH-PAA0.25)*19.5, (b) (PSS/PAH-PAA0.25)*20, (c) (PSS/ PAH-PAA0.5)*19.5, (d) (PSS/PAH-PAA0.5)*20, (e) (PSS/ PAH-PAA0.75)*19.5, and (f) (PSS/PAH-PAA0.75)*20 films. Size: (a-d) 5 × 5 µm and (e, f) 10 × 10 µm. Table 1. Root-Mean-Square Roughness of the PSS/PAH-PAA Films with PAH-PAA Complexes of Different Mixing Ratios PSS/PAH-PAA0.25 PSS/PAH-PAA0.5 PSS/PAH-PAA0.75 bilayers 9.5 10 19.5 20 9.5 10 19.5 20 19.5 rms/nm 12.1 2.2 31.1 1.9 27.6 4.7 42.8 3.0 380

20 382

reported previously.32 The PAH-PAA0.25/PSS and PAH-PAA0.5/ PSS multilayer films show a quite similar deposition behavior with a slightly increased amount of PAH-PAA complexes and PSS deposited in the (PAH-PAA0.5/PSS)*10 film than in the (PAH-PAA0.25/PSS)*10 film. The quite similar deposition behavior for PAH-PAA0.25/PSS and PAH-PAA0.5/PSS multilayer films is reasonable because the PAH-PAA0.25 and PAH-PAA0.5 complexes have quite similar sizes and surface charge density. In contrast, the increased size of PAH-PAA0.75 complexes leads to more PAH-PAA complexes and PSS deposition when the number of film deposition layers is equal to those of PAH-PAA0.25/PSS and PAH-PAA0.5/PSS films. Structural Characterization of PSS/PAH-PAA Films. The surface morphology of the PSS/PAH-PAA films with different mixing ratios was investigated by AFM and SEM. The representative AFM images are shown in Figure 3, and the rootmean-square (rms) roughness of the films is summarized in Table 1. For PSS/PAH-PAA0.25 and PSS/PAH-PAA0.5 films the films have a rough surface when PSS is the outmost layers. When PAH-PAA complexes are the outmost layers the films have rather smooth surfaces. Detailed investigation shows that the surface roughness of the PSS/PAH-PAA0.25 and PSS/ (32) (a) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (b) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C. Langmuir 2001, 17, 7414. (c) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458. (d) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J. C.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635. (e) Podsiadlo, P.; Michel, M.; Lee, J.; Verploegen, E.; Kam, N. W. S.; Ball, V.; Lee, J.; Qi, Y.; Hart, A. J.; Hammond, P. T.; Kotov, N. A. Nano Lett. 2008, 8, 1762. (f) Porcel, C.; Lavalle, P.; Ball, V.; Decher, G.; Senger, B.; Voegel, J.-C.; Schaaf, P. Langmuir 2006, 17, 4376.

Figure 5. (a-c) Cross-sectional SEM images of a (PSS/ PAH-PAA0.25)*20 (a), (PSS/PAH-PAA0.5)*20 (b), and (PSS/ PAH-PAA0.75)*20 (c) film. (d) Dependence of the thickness of the PSS/PAH-PAA films as a function of deposition cycles.

PAH-PAA0.5 films increases gradually with increasing number of film deposition cycles when PSS is the outmost layer. When PAH-PAA complexes are the outmost layer the surface roughness of the PSS/PAH-PAA0.25 and PSS/PAH-PAA0.5 films decreases slightly with increasing number of deposition cycles, as exemplified in (PSS/PAH-PAA0.5)*10 and (PSS/ PAH-PAA0.5)*20 films. The (PSS/PAH-PAA0.75)*19.5 and (PSS/PAH-PAA0.75)*20 films have a bumpy surface structure (Figure 3e and 3f) with their rms roughness being 380 and 382 nm, respectively. SEM allows large area observation of film morphology. Therefore, SEM was employed to characterize the surface morphology of PSS/PAH-PAA0.75 films of different deposition cycles. The surface of the (PSS/PAH-PAA0.75)*4 film is relatively smooth with a few embedded particles observed (Figure 4a). The embedded particles are vertically compressed and much larger than the individual PAH-PAA0.75 complexes. These particles should correspond to the aggregates of several PAH-PAA0.75 complexes or PAH-PAA0.75 complexes glued by PSS. With further increasing number of film deposition cycles the density of particles on the surface of the films increases too (Figure 4b-d). For the (PSS/PAH-PAA0.75)*20 film the particles aggregated together and the surface of the film becomes rugged (Figure 4d). The thickness of the PAH-PAA/PSS films with different deposition cycles was determined by their corresponding cross-sectional SEM images. As shown in Figure 5a and 5b the cross-sectional SEM images of the (PSS/PAH-PAA0.25)*20 and (PSS/PAH-PAA0.5)*20 films reveal a constant film thickness of

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853.5 ( 29.2 and 966.1 ( 15.5 nm, respectively. The crosssectional SEM image of the (PSS/PAH-PAA0.75)*20 film (shown in Figure 5c) indicates an uneven film structure with an average film thickness of 2048 ( 676 nm (thickness averaged by three individual cross-sectional SEM images, and for each image 40 equidistant positions were measured). The film thickness of the PSS/PAH-PAA films with PAH-PAA complexes of different mixing ratios was plotted as a function of the number of film deposition cycles and is shown in Figure 5d. The PSS/ PAH-PAA0.25 and PSS/PAH-PAA0.5 films show a nonlinear deposition behavior with their film thickness increasing more rapidly than in the previous deposition cycles. The PSS/ PAH-PAA0.25 and PSS/PAH-PAA0.5 films have a quite similar deposition behavior with the thickness of the PSS/PAH-PAA0.5 films being slightly larger than that of the PSS/PAH-PAA0.25 films when the number of deposition cycles exceeds 16. The PSS/PAH-PAA0.75 films show an almost linear deposition behavior with a slightly rapid thickness increase starting from 16 deposition cycles because of the more porous film structures. The film deposition behavior of PAH-PAA/PSS films detected on silicon wafers and Ag-coated QCM resonators is consistent with each other. The difference in the film deposition behavior and film structure of the PSS/PAH-PAA films with PAH-PAA complexes of different mixing ratios originates from the structural difference of PAH-PAA complexes in solution. When the mixing ratio of PAA to PAH in the PAH-PAA complexes is below 0.5, the PAH-PAA complexes are comprised of low-level cross-linked PAH and PAA chains. The PAH-PAA complexes are flexible and contain free amine groups in their interior and periphery. PSS has a rigid structure because of the phenyl and high density of charged sulfonate groups in the side chains. Deposition of a PSS layer leads to a rough surface because of the rigid structure of PSS as the (PSS/PAH)*19.5 film has a rough surface with rms being 18.2 nm when PSS is the outmost layer. When PAH-PAA0.25 and PAH-PAA0.5 complexes are deposited the flexible PAH-PAA complexes make themselves self-adjustable to form PSS/PAH-PAA films with a flat surface. As a comparison, the PAH-PAA0.75 complexes are rigid and have larger sizes than that of PAH-PAA0.25 and PAH-PAA0.5 complexes because of the heavy cross-linking of the PAH and PAA chains. When PAH-PAA0.75 complexes are deposited the spherical structure of PAH-PAA0.75 complexes with slight deformation was kept in the films because their rigidity prevents flattening of the PAH-PAA0.75 complexes. The rigid and highly charged PSS cannot make itself adjustable to produce a flat surface when PSS is deposited on top of the rough PAH-PAA0.75 surface. Therefore, rough PSS/PAH-PAA0.75 films were obtained whether PAH-PAA0.75 complexes or PSS is the outmost layer. With the increase of film deposition cycles the film surface roughness of the PSS/PAH-PAA0.25 and PSS/PAH-PAA0.5 films increases when PSS is the outmost layer and decreases when PAH-PAA complexes as the outmost layer. This phenomenon of surface roughness changes is consistent with the gradually increased amount of deposited PAH-PAA complexes and PSS when the film deposition proceeds. The nonlinear deposition of the PSS/ PAH-PAA0.25 and PSS/PAH-PAA0.5 films can be explained by the diffusion “in” and “out” mechanism.32 The free amine groups in the slightly cross-linked PAH-PAA complexes with lower PAA ratios allow for diffusion of PSS into the previous layers of PAH-PAA complexes during deposition of the PSS layer. Then when the film is put in contact with a solution of PAH-PAA complexes the excess PSS will diffuse out and form complexes with PAH-PAA complexes, which leads to a larger

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Figure 6. QCM frequency decrease (-∆F) of alternative deposition of PAH-PAA0.5 PAH-complexes (b) and PAH0.5-PAA (O).

Figure 7. (a, b) Top-down SEM image of a (PAH0.5-PAA/ PAH-PAA0.5)*n film with n ) 4 (a) and 20 (b). (c) Cross-sectional SEM image of a (PAH0.5-PAA/PAH-PAA0.5)*20 film. (d) Dependence of the thickness of the PAH0.5-PAA/PAH-PAA0.5 films as a function of deposition cycles.

amount of PAH-PAA complexes deposited than in the previous layer. In turn, the larger amounts of PAH-PAA complexes permit more PSS to be diffused in and deposited. In this way, the PSS/ PAH-PAA0.25 and PSS/PAH-PAA0.5 films exhibit a nonlinear deposition behavior with larger amounts of PAH-PAA complexes and PSS deposited than in the previous deposition cycles. The highly cross-linked PAH-PAA0.75 complexes contain a lower amount of free amine groups in their interior, and diffusion in and out of PSS during PAH-PAA0.75/PSS film fabrication were heavily suppressed. Therefore, an almost linear deposition of PAH-PAA0.75/PSS films was observed. LbL Deposition of Oppositely Charged PAH-PAA Complexes. The negatively charged PAH0.5-PAA complexes were alternatively deposited with positively charged PAH-PAA0.5 complexes to fabricate PAH0.5-PAA/ PAH-PAA0.5 multilayer films. As shown in Figure 6, the QCM frequency decreases because of the alternative deposition of PAH0.5-PAA and PAH-PAA0.5 complexes, confirming successful fabrication of PAH0.5-PAA/PAH-PAA0.5 films. Starting with the fourth deposition cycles the QCM frequency decreases linearly with the number of film deposition cycles with an average frequency decrease of 918.1 ( 250.6 and 1414.5 ( 282.1 Hz for each layer of PAH0.5-PAA and PAH-PAA0.5 complexes, respectively. The surface morphology of the PAH0.5-PAA/PAH-PAA0.5 films resembles that of PSS/PAH-PAA0.5 films. The SEM image shown in Figure 7a indicates that the (PAH0.5-PAA/ PAH-PAA0.5)*4 film has a flat surface. With the increasing number of film deposition cycles the surface of the PAH0.5-PAA/

Deposition of Polyelectrolyte-Polyelectrolyte Complexes

PAH-PAA0.5 filmsbecomesgraduallyrough.Forthe(PAH0.5-PAA/ PAH-PAA0.5)*20 film (Figure 7b) its surface contains aggregated particles, which are aggregates of PAH0.5-PAA and PAH-PAA0.5 complexes. Compared with the PSS/PAH-PAA0.5 films the self-adjustability of PAH0.5-PAA and PAH-PAA0.5 complexes to form flat films decreased because of the involvement of two kinds of polymeric complexes. The cross-sectional SEM image of a (PAH0.5-PAA/PAH-PAA0.5)*20 film shown in Figure 7c indicates that the film has an average thickness of 1112 ( 586 nm. The uneven film structure of the (PAH0.5-PAA/ PAH-PAA0.5)*20 film can be evidenced from the cross-sectional SEM image. As indicated in Figure 7d, the thickness of the PAH0.5-PAA/PAH-PAA0.5 films almost increases linearly with the number of film deposition cycles. Replacing PSS with PAH0.5-PAA complexes diffusion in and out of the complexes during fabrication of PAH0.5-PAA/PAH-PAA0.5 films become impossible because of the large dimensions of the PAH-PAA complexes than PSS. Therefore, a nearly linear film increase versus the number of film deposition cycles was obtained for PAH0.5-PAA/PAH-PAA0.5 films. This result also supports the assumption that PSS diffusion in and out of the films leads to the nonlinear deposition of PSS/PAH-PAA0.25 and PSS/ PAH-PAA0.5 films. Fabrication of Free-Standing PSS/PAH-PAA0.5 Films. Free-standing films refer to films existing without solid substrates either in solution or in a dried state. Recently, fabrication of free-standing films especially LbL-assembled free-standing multilayer films have attracted much attention because of the potential application of these films as separation membranes, sensors, catalytic film, micromechanical devices, or artificial organs.33 Many efforts have been devoted to find ways to release LbL-assembled films from substrates to produce free-standing films.34 In our previous study we reported that LbL-assembled films can be released from the substrate to obtain free-standing films by an ion-triggered exfoliation method.35 Release of the LbL-assembled films from the substrate was achieved by breaking the interaction of the first layer of the films with the underlying substrate while keeping the integrity of the resultant films. Herein, we further extend the ion-triggered exfoliation method to the preparation of free-standing PSS/PAH-PAA0.5 films. The thermal cross-linking of the PSS/PAH-PAA0.5 film was conducted by heating the film at 180 °C for 2 h. Thermal cross-linking leads to formation of amide bonds between the amine groups of PAH and the acid groups of PAA, as confirmed by the appearance of the amine I peak at 1640 cm-1 and amine II peak at 1554 cm-1 in the FT-IR spectrum of the thermally cross-linked (PSS/ PAH-PAA0.5)*20 film deposited on the CaF2 substrate.35,36 The thermally cross-linked (PSS/PAH-PAA0.5)*20 film deposited on a silicon wafer was immersed into an aqueous solution containing 0.1 M NaOH to exfoliate the film from the substrate. To facilitate exfoliation of the film the edges of the sample were cut off with a knife.35 As indicated in Figure 8a, a defect-free free-standing PSS/PAH-PAA0.5 film with a size of 2 × 2 cm2 was obtained after being immersed in the exfoliation solution for (33) (a) Ono, S. S.; Decher, G. Nano Lett. 2006, 6, 592. (b) Jiang, C. Y.; Markutsya, S.; Oikus, Y.; Tsukruk, V. V. Nat. Mater. 2004, 3, 721. (c) Zimnitsky, D.; Shevchenko, V. V.; Tsukruk, V. V. Langmuir 2008, 24, 5996. (d) Vendamme, R.; Onoue, S.-Y.; Nakao, A.; Kunitake, T. Nat. Mater. 2006, 5, 494. (34) (a) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530. (b) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368. (c) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413. (d) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. J. Am. Chem. Soc. 2005, 127, 17228. (e) Mallwitz, F.; Laschewsky, A. AdV. Mater. 2005, 17, 1296. (35) Ma, Y.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2007, 19, 5058. (36) (a) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (b) Balachandra, A. M.; Dai, J. H.; Bruening, M. L. Macromolecules 2002, 35, 3171.

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Figure 8. (a) Photograph of a thermally cross-linked PSS/PAH-PAA0.5 free-standing film in air. (b, c) Cross-sectional SEM images of a crosslinked (PSS/PAH-PAA0.5)*20 film before (b) and after exfoliation (c).

2 min. The free-standing multilayer film keeps its integrity upon transferring to a solid substrate, indicating the good stability of the resultant free-standing film. The thermally cross-linked (PSS/ PAH-PAA0.5)*20 film deposited on PDDA-modified silicon wafer is comprised of electrostatically deposited 1-bilayer PDDA/ PSS film and covalently attached (PAH-PAA0.5/PSS)*19.5 film. The interpenetration of polymeric chains makes the thermally cross-linked (PAH-PAA0.5/PSS)*19.5 film more stable than that of the ionically attached PDDA/PSS layers. Therefore, the interaction of the PSS layer in the first bilayer of PDDA/PSS with the first PAH-PAA layer could be easily broken under an aqueous solution of high concentration of NaOH. As a result, the (PAH-PAA0.5/PSS)*19.5 film was released to form freestanding films. The as-prepared (PSS/PAH-PAA0.5)*20 film has a constant film thickness of 966 nm, as measured by its crosssectional SEM image in Figure 5b. After thermal cross-linking the film has a thickness of 787 nm. Obviously, thermal crosslinking leads to shrinkage of the film. The free-standing film after drying has a thickness of ca. 660 nm. The further decrease of the film thickness for the free-standing film is due to partial dissolution of PSS from the film during the exfoliation process. UV-vis absorption spectra reveal that, compared with the thermally cross-linked (PSS/PAH-PAA0.5)*20 film before exfoliation, the absorbance of the free-standing film has a ca. 13% decrease at 226 nm, which corresponds to the characteristic absorbance of PSS in the film. Part of the PSS in the free-standing PAH-PAA0.5/PSS films can be dissolved in aqueous solution with high ionic strength. The stable PAH-PAA/PSS free-standing films can be potentially useful as a separation membrane.

Conclusions In the present study we demonstrate that the nonstoichiometric PAH-PAA complexes can be used as building blocks for LbL film fabrication. The positively charged PAH-PAA complexes with a molar excess of PAH can be LbL assembled with either polyanion PSS or negatively charged PAH-PAA complexes with a molar excess of PAA to produce multilayer films. The structure and deposition behavior of the LbL-assembled PSS/ PAH-PAA films can be tuned by controlling the structure of the PAH-PAA complexes in solution, which is determined by the mixing ratio of PAH to PAA. We further proved that the thermally cross-linked PAH-PAA/PSS films can be released

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from substrate to produce stable free-standing films with large areas by an ion-triggered exfoliation method. PECs have diverse structures in solution compared with their corresponding simplex linear polymers, and their structures in solution can be well tailored by parameters such as solution pH, ionic strength, temperature, mixing ratio, molecular weight of the polymers, etc. We believe that the diversity in structures and abundance in species of PECs will provide a facile way to fabricate advanced film materials with well-tailored film structures as well as functionalities.

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Acknowledgment. This work was supported by the National Natural Science Foundation of China (NSFC Grant No. 20774035),theNationalBasicResearchProgram(2007CB808000), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant No. 200323), the Program for New Century Excellent Talents in University (NCET), and the Jilin Provincial Science and Technology Bureau of Jilin Province (20070104). LA803479A