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Investigation into an Alternating Multilayer Film of Poly(4-Vinylpyridine) and Poly(acrylic acid) Based on Hydrogen Bonding Liyan Wang, Yu Fu, Zhiqiang Wang, Yuguo Fan, and Xi Zhang* Key Lab of Supramolecular Structure and Spectroscopy, Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received September 8, 1998. In Final Form: November 25, 1998 A new approach for the fabrication of a multilayer film assembly is explored, which is based on the alternating assembling of poly(4-vinylpyridine) and poly(acrylic acid) via hydrogen bonding. The homogeneous multilayer films were characterized by UV-vis and X-ray diffraction measurements. The control of layer thickness was studied in detail via adjustment of the molecular weight and concentration of poly(4-vinylpyridine) solution. The nature of the interaction between the two polymers was identified as hydrogen bonding by IR spectroscopy.
Introduction Organized molecular films, fabricated mainly by the Langmuir-Blodgett method and self-assembling techniques, have various potential applications in molecular electronics, optical devices, and biomedical applications.1 Since Nuzzo and Allara showed the formation of selfassembled monolayers (SAMs) on gold, the field of SAMs has witnessed tremendous progress.2,3 The driving force for this self-assembly is based on chemisorption of an active surfactant on a solid surface, e.g., the formation of gold thiolate resulting in SAMs of organosulfur compounds and the in situ formation of polysiloxane directing SAMs of organosilicon derivatives.4,5 On the basis of coordination bonds between phosphate compounds and transitionmetal ions such as Zr4+, Mallouk discovered a new route to the simple preparation of self-assembled multilayers of diphosphates.6 Seven years ago, Decher et al. demonstrated the application of ionic attraction to construct multilayer polymer films.7 Up to now, this assembling technique has been developed far beyond in the polyelectrolyte system, and a wide range of application was found such as in the assembly of nanoparticles, biomacromolecules, and different functional organic pigments.8 Very recently, new approaches for the fabrication of an alternating multilayer film on the basis of hydrogen bonds were reported by Zhang et al. and Rubner et al. at almost the same time but in different journals.9,10 Herein we (1) Ulman, A. An introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. 1988, 100, 117; Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (5) Maoz, R.; Matlis, S.; Dimasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150. (6) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (7) Decher, G.; Hong, J. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (8) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. Kleinfield, E. R.; Ferguson, G. S. Science 1994, 265, 370. Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. Gao, M. Y.; Zhang, X.; Yang, B.; Shen, J. C. J. Chem. Soc., Chem. Commun. 1994, 2229. Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, Z. Q.; Shen, J. C.; Wang, D. J.; Li, T. J. J. Chem. Soc., Chem. Commun. 1996, 2739. (9) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (10) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717.
further investigate the multilayer film of poly(4-vinylpyridine) (PVP) and poly(acrylic acid) (PAA). Experimental Part As is well-known, pyridine and carboxylic acid are good hydrogen bonding complementary partners. Fre´chet et al. has built a new type of side-chain supramolecular liquid-crystalline aggregate via intermolecular hydrogen bonding between pyridine and carboxylic acid.11 The complementary partners we used here are poly(4-vinylpyridine) and poly(acrylic acid). Polymerization of 4-vinylpyridine was carried out in methanol with 2,2′-azobis(isobutyronitrile) (AIBN) at 56 °C for 20 h.12 After being cooled, the polymer solution was poured into toluene. The precipitate was dried under vacuum. Yield: 70%. Intrinsic viscosity [η] ) 48.5 (in ethanol at 25 °C); i.e., weight-average molecular weight (Mw) is about 6 × 104.13 A series of poly(4vinylpyridine) with different Mw’s were synthesized similarly, whose Mw’s are about 3.8 × 104, 6.2 × 104, 8.6 × 104, and 1.8 × 105, respectively. Polymerization of acrylic acid was carried out in THF with benzoylperoxide (BPO) at 67 °C for 1 h. The product was purified by fractional precipitation in chloroform and drying under vacuum. Yield: 85%. [η] ) 11.5 (in 1,4-dioxane at 30 °C); i.e., Mw is about 2.3 × 104.13 Multilayer films were assembled on a variety of substrates, including quartz, silicon, and CaF2 plates. In the case of quartz and silicon, their surfaces were modified with (4-aminobutyl)dimethylmethoxysilane in advance, having a NH2-tailoring surface.14 In the case of CaF2, the surface was coated by adsorption of a single layer of poly(ethyleneimine) (PEI). UV-vis spectra were obtained on a Shimadzu UV-3100. FTIR spectra were performed on a Bruker IFS 66V instrument. X-ray diffraction (XRD) was carried out on a Rigaku X-ray diffractometer (D/max γA, using Cu KR radiation of a wavelength of 1.542 Å). Atomic force microscopy (AFM) images were taken with an atomic force microscope (nanoscope D 3000, Digital Instrument, Santa Barbara, CA) under ambient conditions. AFM was operated in the tapping mode with an optical readout using Si cantilevers. The construction of the multilayer film is simply shown in Chart 1. The (4-aminobutyl)-dimethylsilanized substrate was first immersed in a PAA methanolic solution (0.252 g/L) for 5 min. In this way, the substrate was covered with a PAA layer, (11) Kumar, U.; Kato, T.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1992, 114, 6630. (12) Strauss, U. P.; Williams, B. L. J. Phys. Chem. 1961, 65, 1390. Martin, V.; Ringsdorf, H.; Ritter, H.; Sutter, W. Angew. Chem., Int. Ed. Engl. 1973, 12, 432. (13) Brandrup, J.; Immergut, E. H. Polymer Handbook; John Wiley & Sons: New York, 1989. (14) Haller, I. J. Am. Chem. Soc. 1978, 100, 8050.
10.1021/la981181+ CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999
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Figure 2. Growth in the absorbance at 256 nm of PVP as a function of the adsorption time with different concentrations of the PVP solution. Figure 1. UV-vis absorption spectra of the multilayer film of PAA and PVP with different numbers of bilayers. Chart 1
i.e., a hydrogen bonding donors (carboxyl groups) tailored surface. After being rinsed by dipping into three beakers with pure methanol for 1 min each, the substrate was transferred into a PVP methanolic solution (0.256 g/L) for 5 min, thus adding a PVP layer, i.e., a hydrogen bonding acceptors (pyridine groups) tailored surface. After being rinsed, the substrate covered with hydrogen bonding acceptors was again dipped into a PAA solution for 5 min to get another PAA layer. Thus, an alternating multilayer film was obtained by repeating the above steps in a cyclic fashion.
Results and Discussion UV-Vis absorption spectroscopy was used to follow the assembling process of the multilayer film. Figure 1 shows the UV-vis absorption spectra of the multilayer film of PAA and PVP with different numbers of bilayers on a quartz slide. The absorption band appearing at 256 nm in the UV region was assigned to the contribution of PVP. The linear increase of the optical density at 256 nm of the films with increasing numbers of bilayers indicated a process of uniform assembling. Furthermore, UV-vis spectroscopy was employed to monitor the adsorption process of the polymers in methanolic solutions. Figure 2 shows the growth in the absorbance at 256 nm of PVP as a function of the adsorption time with different concentrations of PVP solution. The absorbance increased rapidly until a saturation state was reached. As for different concentrations of PVP solution,
Figure 3. Growth in the absorbance of PVP as a function of the adsorption time with different Mw’s of PVP.
Figure 4. X-ray diffraction pattern of a nine-bilayer film of PAA and PVP.
the saturation times were almost the same, about 60 s, while the saturation absorbance increased with an increase in the concentration of the PVP solution. Figure 3 shows the growth in the absorbance of PVP as a function of the adsorption time with different Mw’s of PVP. As for different Mw’s of PVP, similarly, the saturation times were almost the same, about 60 s, while the saturation absorbance increased with an increase of the Mw of PVP.
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Figure 5. Growth of the thickness of a nine-bilayer film as a function of the concentration of a PVP methanolic solution.
Figure 6. Growth of the thickness of a nine-bilayer film as a function of the molecular weight of PVP.
So, the immersing time was set as 5 min in most of the cases in this paper. The structure of the multilayer film was studied by X-ray diffraction experiments. Figure 4 shows the X-ray diffraction pattern of a nine-bilayer film of PAA and PVP,
Wang et al.
which was incorrectly assigned as a series of Bragg diffraction peaks in our previous paper.9 The Kiessig fringes suggested a good and smooth film with constant thickness and low surface roughness, without regular electron density modulation across the film.15 The total thickness of the film was calculated to be about 18.2 nm; i.e., the thickness of one bilayer of PAA and PVP was 2.1 nm. The smoothness of this film was about 0.6 nm, which is consistent with the result of the AFM experiment.9 As mentioned before, the saturation absorbance is dependent on the concentration of the PVP solution. Does this mean different thicknesses of PVP layers deposited? For confirming this, we attempted to fabricate a series of films in different concentrations of solutions of PVP (Mw ) 6 × 104) while maintaining the concentration of the PAA solution at 0.252 g/L. Figure 5 shows the growth of the thickness of the nine-bilayer film as a function of the concentration of the PVP methanolic solution. The total thickness of the film was calculated from the X-ray diffraction pattern. The thickness increased gradually from 14.9 to 22.7 nm with variation of the concentration of the PVP solution from 0.008 to 0.256 g/L. This is reasonable since the PVP chain conformations were more extended in dilute solution. Thus, PVP in solutions of low concentration adsorbed as a thinner layer onto the substrate. This means that the thickness of the layer can be controlled in the range of nanometer scale by changing the concentration of the polymer solution. Multilayer films of nine bilayers were constructed by alternating deposition of PVP with different Mw’s and PAA. The solutions of PVP with different Mw’s were prepared by dissolving the polymers in methanol at the same concentration of 0.256 g/L, and the concentration of the PAA solution was maintained at 0.252 g/L. Figure 6 shows the growth of the thickness of the nine-bilayer film as a function of the molecular weight of PVP. It was found that the thickness increased with an increase of the Mw of PVP, which was calculated from the X-ray diffraction pattern. It can be rationalized that the hydrodynamic radius of PVP molecules in solutions increased with an increase of the Mw of PVP. These results are consistent
Figure 7. IR spectrum of a cast film of pure PAA and that of pure PVP on CaF2 plates.
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Figure 8. IR spectrum of a five-bilayer PAA/PVP film on a CaF2 plate.
with the results that the saturation absorbance increased with an increase of the Mw or concentration. This offers another possibility to control the thickness of the basic bilayer building block at the molecular level. The nature of the interaction between the polymers was established by IR spectroscopy. Figure 7 shows the IR spectrum of a cast film of pure PAA and that of pure PVP on CaF2 plates. The broad absorption band around 3000 cm-1 and the absorption appearing at 1709 cm-1 indicated that the carboxyl group in pure PAA was not in a free but in an associated state. The absorption peaks at 1595, 1556, and 1450 cm-1 can be assigned to the ring vibration of pyridine of PVP. Figure 8 shows the IR spectrum of a five-bilayer PAA/PVP film on a CaF2 plate. On the one hand, a CdO stretching vibration appeared at 1718 cm-1, which showed that the carbonyl group was in a less associated state than that in pure PAA. On the other hand, a O-H stretching vibration appeared at 2530 and 1945 cm-1, which showed that the hydroxyl group formed stronger hydrogen bonds than those in pure PAA.16 Both (15) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. Tippmann-Krayer, P.; Mohwald, H.; Lvov, Y. Langmuir 1991, 7, 2298.
suggest that hydrogen bonds formed between PAA and PVP in adjacent layers.11,16 Furthermore, there is almost no change of position of the absorption peaks from 1650 to 1300 cm-1 in Figure 8 in comparison with Figure 7, which implies that the carboxyl group of PAA in the film as well as the pyridine group of PVP was not ionized.17 These results support the concept that the multilayer film was not assembled via electrostatic attraction but hydrogen bonding. Acknowledgment. We thank L. F. Chi and H. Fuchs for the AFM observations. This work was supported by FOK YING TUNG Education Foundation, State Education Commission of P. R. China, and Natural Science Foundation of China. LA981181+ (16) Odinokov, S. E.; Mashkovshky, A. A.; Glazunov, V. P.; Iogansen, A. V.; Rassadin, B. V. Spectrochim. Acta 1976, 32A, 1355. Kato, T.; Kihara, H.; Uryu, T.; Fujishima, A.; Fre´chet, J. M. J. Macromolecules 1992, 25, 6836. (17) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000.
