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Langmuir 1999, 15, 4682-4684
A Novel Fabrication of Ultrathin Poly(vinylamine) Films with a Molecularly Smooth Surface Takeshi Serizawa, Kazuya Yamamoto, and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Received November 24, 1998. In Final Form: March 22, 1999
Introduction Ultrathin polymer films, which are fabricated on a certain substrate by a suitable methodology, demonstrate a high potential for functional modifications of material surfaces, which may be utilized as molecular devices including molecular recognition processes.1 Although ultrathin films that are composed of certain polymers that are dissolved in an organic solvent can be easily fabricated by conventional methodologies, such as spin coating, solution casting, and dip coating, it seems to be difficult to apply them to water-soluble polymers. To prepare ultrathin films from water-soluble polymers, Decher et al.2 recently developed an alternate adsorption technique, in which a charged polymer was electrostatically adsorbed from its aqueous solution to the surface of an oppositely charged polymer on a substrate. Subsequently, other research groups extensively studied the methodology as well.3 The methodology could be applied only to charged polymers, although some research groups4 utilized other specific interactions between polymers. In our previous paper, we studied the stepwise fabrication of ultrathin films that were composed both of collagen5 * To whom correspondence should be addressed. Telephone: +81-99-285-8320. Fax: +81-99-255-1229. E-mail: akashi@ apc.eng.kagoshima-u.ac.jp. (1) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (b) Andrade, J. D., Ed. Polymer Surface Dynamics; Plenum Press: New York, 1988. (c) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Harcourt Brace Jovanovich: Boston, MA, 1991. (d) Tsukruk, V. V. Prog. Polym. Sci. 1997, 22, 247. (e) Grainger, D. W. Prog. Colloid Polym. Sci. 1997, 103, 2243. (2) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (c) Decher, G. Compr. Supramol. Chem. 1996, 9, 507. (d) Decher, G. Science 1997, 277, 1232. (3) (a) Lvov, Y.; Ichinose, I.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (b) Sano, M.; Lvov, Y.; Kunitake, T. Annu. Rev. Mater. Sci. 1996, 26, 153. (c) Sun, Y.; Hao, E.; Zhang, X.; Yang, B.; Gao, M.; Shen, J.; Chi, L.; Fuchs, H. Langmuir 1997, 13, 5168. (d) Caruso, F.; Kurth, D. G.; Volkmer, D.; Koop, M. J.; Mu¨ller, A. Langmuir 1998, 14, 3462. (e) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (f) Kotov, N. A.; Haraszti, T.; Turi, L.; Zavala, G.; Geer, R. E.; De´ka´ny, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821. (g) Fang, M.; Kaschak, D. M.; Sutorik, A. C.; Mallouk, T. E. J. Am. Chem. Soc. 1997, 119, 12184. (h) Sohling, U.; Schouten, A. J. Langmuir 1996, 12, 3912. (i) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (j) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178. (k) Serizawa, T.; Takeshita, H.; Akashi, M. Chem. Lett. 1998, 487. (l) Serizawa, T.; Takeshita, H.; Akashi, M. Langmuir 1998, 14, 4088. (m) Serizawa, T.; Goto, H.; Kishida, A.; Endo, T.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem., in press. (4) (a) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (b) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (c) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (5) Serizawa, T.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem., in press.
and poly(vinyl alcohol) (PVA)6 by the repetition of the adsorption and subsequent drying processes. The processes were quite simple and included (1) polymer adsorption on a substrate, possibly by the hydrophobic effect from their aqueous solutions, in the presence of sodium chloride and rinsing with water, (2) drying the adsorbed film in air in order to be stabilized, possibly by hydrogen bonding in the absence of water and salt. We easily controlled the film thickness by changing the number of times the process has repeated or by changing the salt concentration. The methodology significantly demonstrated that we could utilize even uncharged polymers for the ultrathin film fabrication. The basic concept of the stepwise film fabrication may be expanded to other polymers. In this study, we demonstrate the stepwise fabrication of ultrathin films that were composed of poly(vinylamine) (PVAm) (a simple vinyl polymer which has a primary amine group in each unit) by the repetition of its adsorption from an aqueous solution under a controlled pH condition at a lower temperature, and subsequent drying processes. PVAm will be assigned to a novel class of polymers that can be fabricated to the ultrathin films by the present methodology. The mechanism for PVAm deposition seems to be different to that for other systems. Experimental Section We synthesized poly(vinylamine) (PVAm) by the alkaline hydrolysis (2 N NaOH(aq), 60 °C, 24 h) of poly(N-vinylformamide) (polyNVF), which was prepared by the free radical polymerization (60 °C, 24 h) of NVF (donated by Mitsubishi Chemical Co., Japan) with a suitable amount of VA-044 as an initiator in the aqueous phase, following our unpublished data.7 The polymerization and the molecular weight were analyzed by 1H NMR and size exclusion chromatography with a poly(ethylene glycol) standard, respectively. The distributions of the molecular weights (Mw/ Mn) were around 2, indicating a relatively narrow distribution. The stepwise fabrication of ultrathin PVAm films was quantitatively analyzed by a quartz crystal microbalance (QCM), which was used as a substrate. We purchased an AT-cut quartz crystal with a parent frequency of 9 MHz from USI (Japan). A crystal (9 mm in diameter) was coated on both sides with silver electrodes (4.5 mm in diameter). An Iwatsu frequency counter (Model SC7201) monitored the frequency. The leads of the QCM were sealed and protected by a rubber gel in order to prevent degradation of the solvent contacts when it was immersed in an aqueous solution. The assembling amount of PVAm on a QCM, ∆m, could be analyzed by the frequency decrease of QCM, ∆F, by Sauerbrey’s equation8 as follows:
-∆F )
2Fo2 AxFqµq
× ∆m
where Fo is the parent frequency of QCM (9 × 106 Hz), A is the electrode area (0.159 cm2), Fq is the density of the quartz (2.65 g cm-3), and µq is the shear modulus (2.95 × 1011 dyn cm-2). This equation was reliable when we analyzed the material in air, as in this study, because it never detects the mass of the solvents as a frequency shift and the effect of the viscosity of the absorbent on the frequency could be ignored. The stepwise fabrication basically followed the processes in our preliminary papers.5,6 The fabrication processes are sche(6) Serizawa, T.; Hashiguchi, S.; Akashi, M. Langmuir, in press. (7) Yamamoto, K.; Serizawa, T.; Muraoka, Y.; Akashi, M. Manuscript in preparation. (8) Sauerbrey, G. Z. Phys. 1959, 155, 206.
10.1021/la9816398 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/15/1999
Notes
Langmuir, Vol. 15, No. 13, 1999 4683 Table 1. Data Analyses of the Stepwise PVAm Assemblya [NaOH]/mM
temp/°C
0 5 15
4
Mn 9700
10 15
9700 20 3200
15
4 240000
a
Figure 1. Schematic representation for fabricating ultrathin PVAm films.
Figure 2. Frequency shift against assembling step when the fabrication processes were performed at the PVAm (Mn 9700) concentration of 0.05 unit M at 4 °C in the presence of sodium hydroxide: (a) 15 mM; (b) 5 mM; (c) 0 M. matically shown in Figure 1. Before the fabrication, QCM electrodes were treated with a piranha solution (H2SO4:H2O2 ) 3:1) three times for 1 min in order to clean their surfaces. The QCM was immersed into the aqueous PVAm solution at the suitable concentrations for 15 min at an adequate temperature, taken out, rinsed with water for several tens of seconds, and dried with N2 gas; then its frequency decrease was measured. We found the immersing time was sufficient to the saturated assembly. We repeated the above processes for the stepwise fabrication of ultrathin PVAm films. The AFM images were obtained with a Digital Instruments NanoScope III that was operated with a tapping mode in air at an ambient temperature. No image processing was performed without flat leveling. The surface roughness (Ra) was analyzed from the following equation:
Ra )
∫∫
1 Lx Ly
Ly
0
Lx
0
|F(x,y)| dx dy
Here F(x,y) is the surface relative to the center plane that is a flat plane parallel to the mean plane, and Lx and Ly are the dimensions of the surface.
Results and Discussion Figure 2 shows the frequency shift against the assembling step when we performed the repetition of the PVAm adsorption to a QCM substrate from the aqueous solution with the concentration of 0.05 unit M in the presence of suitable concentrations of sodium hydroxide at 4 °C and subsequent drying processes. We observed the stepwise increase of the frequency shift with an increase in the assembling step in the presence of 15 mM sodium hydroxide, while we hardly observed the shift in the absence or presence of 5 mM sodium hydroxide. The slope at the initial steps was smaller than that at the later steps, indicating the direct influence of a silver
-frequency shift/Hz 17 ( 10 21 ( 18 160 ( 65 151 ( 70 63 ( 24 37 ( 13 841 ( 363
The concentration of PVAm was 0.05 unit M in all experiments.
substrate. The mean frequency shift at each step on the former conditions was 160 ( 65 Hz which corresponded to a 58 ( 23 Å thickness, assuming the film density was around 1.0, as it is listed in Table 1. Although we used the density for thickness estimation, we need to observe directly the thickness by other methodologies such as AFM or SEM for more accurate estimation. The assembly seems to be significantly affected by the charge density of PVAm. In fact, the pH value of the former solution was 12.2, which was significantly larger than the pKa value of PVAm (pKa ) 10.0),9 while those of the later solutions were 8.5 and 6.0, respectively. This means the absence of the charges, which correspond to the presence of free amine groups, in PVAm accelerates the stepwise assembly on a substrate. It might be reasonable if we consider that there are hydrogen bonds between the lone pairs in nitrogen (-NH2) and hydrogen bound to nitrogen (N-H) during adsorption processes. When we immersed a QCM for an amount of time equal to the total immersing time of many steps, the results did not differ from the results of only one step’s immersing. That means the drying process is necessary for the stepwise film assembly. To analyze the assembly further, we analyzed the effect of the solution temperature during the adsorption process. We also observed the stepwise frequency shift (data not shown). The mean frequency shifts at each step were increased with a decrease in the temperature, indicating the assembly was clearly dependent on the solution temperature as well as the concentration of sodium hydroxide, as were also listed in Table 1. This also supports that the driving force for the adsorption is hydrogen bonding between primary amines. The AFM analyses of the surfaces of the film and a bare QCM were shown in Figure 3. The film surface was significantly smooth, although we observed a dotted structure of a sputtered gold on the QCM surface. The Ra value of the film surface was 0.7 nm, which was smaller than that of a bare QCM (Ra ) 1.8 nm), although the values in the case of the collagen and PVA films was larger than it. This observation might show that ultrathin films with a lower surface roughness were fabricated from the present class of polymers. Significantly, when we immersed the film into water in the absence or presence of 15 mM sodium hydroxide at 4 °C or an ambient temperature, only 10-15% of the PVAm assembled was dissociated from a QCM substrate after 72 h. Although the collagen5 and PVA6 films were not dissociated from a substrate because of the lower solubility into an aqueous phase, solid or powder PVAm usually dissolves in an aqueous phase. However, it seems to take a long time to dissociate PVAm after it is assembled on a substrate by the present fabrication processes. PVAm (9) Sumaru, K.; Matsuoka, H.; Yamaoka, H. J. Phys. Chem. 1996, 100, 9000.
4684 Langmuir, Vol. 15, No. 13, 1999
Notes
accelerated with an increase in an NaCl concentration and/or solution temperature, indicating the adsorption possibly occurred as a result of the hydrophobic effect. On the other hand, PVAm’s might be possibly adsorbed by hydrogen bonds between the PVAm adsorbed and that from the aqueous solution. In fact, PVAm hardly adsorbed when we added 15 mM of NaCl concentration that was a concentration equal to the NaOH added here. Furthermore, stepwise depositions of collagen and PVA might be reasonable because their solids or films are hardly dissolved in an aqueous phase at an ambient temperature. However, PVAm solids are usually dissolved in an aqueous phase. It seems to be specific that ultrathin PVAm films are deposited even from its aqueous solution and are stable for immersing even in an aqueous phase. As a consequence, the mechanism for PVAm deposition is significantly different from that for other systems.5,6 To discuss the details, we should further analyze the in situ process of the adsorption by a QCM or infrared spectroscopic techniques.
Figure 3. AFM images that were measured at ambient temperatures in air: (a) a bare QCM; (b) an ultrathin PVAm film from Figure 2a.
might be adsorbed and stabilized on a QCM substrate by the entanglement between the polymers, in which water molecules cannot be inserted. The larger frequency shifts that were observed with the larger molecular weights of PVAm, as are also shown in Table 1, might suggest such a mechanism. The detail of the mechanisms should be analyzed soon. The effects of adsorption parameters in the present study were completely different from those of collagen5 and PVA6 systems (except for the long time immersion described above). In the later systems, the adsorption was
Conclusion We fabricated ultrathin PVAm films by the repetition of the adsorption from its aqueous solution with the suitable concentration of sodium hydroxide at low temperatures and subsequent drying processes. The surface of the film was sufficiently smooth, compared to that of a bare substrate. The PVAm that was assembled on a substrate was quite stable for the immersion into an aqueous phase under various conditions. The methodology for PVAm was similar to that for collagen5 and PVA,6 while the driving forces for the adsorption process and the stabilization mechanism seemed to be significantly different. PVAm will be one of novel polymers, of which ultrathin films are fabricated by the repetition of the adsorption and subsequent drying processes. Further research on adsorption and stabilization mechanism are in progress. Acknowledgment. We would like to acknowledge to Dr. A. Kishida (Kagoshima University, Japan) for his grateful discussions. This work was financially supported in part by a Grant-in-Aid for Scientific Research in the Priority Areas of “New Polymers and Their NanoOrganized Systems” (No.277/101266248) from the Ministry of Education, Science, Sports and Culture, Japan. LA9816398
