Langmuir 1999, 15, 5389-5395
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Surface Hardness of Pristine and Modified Polyaniline Films E. T. Kang,*,† Z. H. Ma,‡ and K. L. Tan‡ Departments of Chemical Engineering and Physics, National University of Singapore, Kent Ridge, Singapore 119260
O. N. Tretinnikov, Y. Uyama, and Y. Ikada Institute for Frontier Medical Sciences, Kyoto University, 53, Kawaharacho, Shogoin, Sankyo-ku, Kyoto 606, Japan Received December 14, 1998. In Final Form: April 7, 1999 The surface microhardness of conventional thermoplastics lie between 0.1 and 0.6 GPa. The as-cast emeraldine (EM-25) base film of polyaniline (PANi) exhibits a surface microhardness of about 1 GPa. The hardness is increased to about 4 GPa in the highly cross-linked EM (EM-150) film. This hardness value is further enhanced to about 6 GPa after 1 cycle of acid-base treatment. The hardness of the EM-150 film can also be enhanced to about 8 GPa through protonation or through reduction to the leucoemeraldine state. Surface modification of the EM-150 films via graft copolymerization with acrylic acid or styrenesulfonic acid readily gives rise to hard-surfaced PANi films having microhardness values approaching 20 GPa. This surface hardness is comparable to many of those reported for the very hard-surfaced conventional polymers from high-energy ion-beam bombardments. The surface hardness of the surface-modified EM-150 film arises from the additional covalent bonding of the graft chains on the highly cross-linked EM-150 film surface and the charge-transfer interaction between the graft and the substrate chains.
Introduction The synthesis and characterization of electroactive conjugated polymers have become one of the most important areas of research in polymer and materials science during the past 2 decades.1-4 Among all the electroactive polymers, the century-old family of aniline polymers5,6 have been of particular interest because of their environmental stability,7-10 controllable electrical conductivity,11-12 and interesting redox properties associated with the chain nitrogens.13-16 The aniline polymers have the general formula [(B-NH-B-NH)y(B-NdQdN)1-y]x, * To whom correspondence should be addressed. Fax: (65)7791936. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Physics. (1) Chiang, C. K.; Fincher, C. R., Jr.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; MacDiarmid, A. J. Phys. Rev. Lett. 1977, 93.1098. (2) Billingham, N. C.; Calvert, P. D. Adv. Polym. Sci. 1980, 90, 2. (3) Skotheim, T., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vols. 1 and 2. (4) Nalwa, H. S., Ed. Handbook of Organic Conductive Molecules and Polymers; John Wiley & Sons: Chichester, 1997; Vols. 1-4. (5) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1910, 1117. (6) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1910, 2388. (7) Neoh, K. G.; Kang, E. T.; Khor, S. H.; Tan, K. L. Polym. Degrad. Stab. 1990, 27, 107. (8) Amano, K.; Ishikawa, H.; Kobayashi, A.; Satoh, M.; Hasegawa, E. Synth. Met. 1994, 62, 229. (9) Pyo, M.; Reynolds, J. R.; Warren, L.; Marcy, H. O. Synth. Met. 1994, 68, 71. (10) Thieblemount, J. C.; Plache, M. F.; Petrescu, C.; Bouvier, J. M.; Bidan, G. Synth. Met. 1993, 59, 81. (11) Ray, A.; Asturias, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid, A. F.; Epstein, A. J. Synth. Met. 1989, 29, E141. (12) Khor, S. H.; Neoh, K. G.; Kang, E. T. J. Appl. Polym. Sci. 1990, 40, 2015. (13) Kang, E. T.; Neoh, K. G.; Tan, K. L. Surf. Interface Anal. 1992, 19, 33. (14) Goff, A. H.-L.; Bernar, M. C. Synth. Met. 1993, 60, 115. (15) Tan, K. L.; Kang, E. T.; Neoh, K. G. Polym. Adv. Technol. 1994, 5, 171.
where B represents a benzenoid ring while Q represents a quinonoid ring. The intrinsic redox state of the polymer can be controllably varied from the fully oxidized pernigraniline state (PNA, y ) 0), through the 50% intrinsically oxidized emeraldine state (EM, y ) 0.5), to the fully reduced leucoemeraldine state (LM, y ) 1).5,6,16 The aniline polymers also exhibit crystallinity17,18 and solution- or counterion-induced processability.19-25 Furthermore, the electrical properties of aniline polymers can be substantially improved through secondary doping.26 The presence of a number of stable intrinsic redox states and the excellent processability have substantially enhanced the potential of the aniline polymers in practical applications, such as in light-emitting devices,27-29 materials for electrodes and sensors,30-32 and corrosion protection of (16) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277. (17) Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. Macromolecules 1991, 24, 779. (18) Nicolau, Y. F.; Durado, D. Synth. Met. 1993, 55-57, 394. (19) Angelopoulous, M.; Asturius, G. E.; Ermer, S. P.; Ray, A.; Scherr, E. M.; MacDiarmid, A. G.; Akhtar, M.; Kiss, Z.; Epstein, A. J. Mol. Cryst. Liq. Cryst. 1988, 160, 151. (20) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91. (21) Beyer, G.; Steckenbiegler, B. Synth. Met. 1993, 60, 169. (22) Osterholm, J. E.; Cao, Y.; Klaveffer, F.; Smith, P. Synth. Met. 1994, 65, 2901. (23) Kang, E. T.; Neoh, K. G.; Tan, K. L. Polymer 1994, 35, 3193. (24) Chen, S. A.; Hwang, G. W. J. Am. Chem. Soc. 1994, 116, 7939. (25) Wang, Y. Z.; Joo, J.; Hsu, C. H.; Epstein, A. J. Synth. Met. 1995, 68, 207. (26) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. (27) Wang, H. L.; MacDiarmid, A. G.; Wang, Y. Z.; Gebler, D. D.; Epstein, A. J. Synth. Met. 1996, 78, 33. (28) Wang, Y. Z.; Gebler, D. D.; Lin, L. B.; Blatchford, J. W.; Jessen, S. W.; Wang, H. L.; Epstein, A. J. Appl. Phys. Lett. 1996, 68, 894. (29) Chen, S. A.; Chuang, K. R.; Chao, C. I.; Lee, H. T. Synth. Met. 1996, 82, 207. (30) Heeger, A. J.; Yang, Y.; Westerweele, E.; Zhang, C.; Cao, Y.; Smith, P. In The Polymeric Materials Encyclopedia, Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press Inc., Boca, Raton, FL, 1996; p 5500.
10.1021/la981717r CCC: $18.00 © 1999 American Chemical Society Published on Web 06/16/1999
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metals.33,34 With respect to most practical applications, materials modification, and functionalization, in particular those aiming at the surface and interface, will be crucial. The pristine and modified surfaces of polyaniline (PANi) have been of great research interest. Most of the studies, however, have been centered on the chemistry and chemical states of the PANi surface.35-38 Less attention has been paid to the physical and morphological states of the polymer surfaces. An earlier study39 has shown that the surface of the EM film cast from N-methylpyrrolidinone solution is rather smooth and dense. A subsequent scanning tunneling microscopic (STM) and atomic force microscopic (AFM) study of similar EM film has also revealed featureless and dense surface nanostructures before and after protonation.40 Accordingly, it should be interesting and timely to explore the surface morphology and microstructure of the polymer film further. In the present work, we report on the measurements of the surface microhardness of pristine, bulk-modified, and surface-modified polyaniline films. The rigid conjugated backbones of PANi and their susceptibility to cross-linking reactions, together with the dense nanostructure of the film surface, would suggest the potential of PANi films to exhibit hard-surfaced properties. Thermoplastic polymers are inherently soft and have poor abrasion resistance. Thermosets are harder and more rigid because of their highly cross-linked state. On one hand, the hardness of most thermoplastics lies in the range of 0.1-0.6 GPa. The hardness of thermosets, such as ebonite, on the other hand, is about 1 GPa.41 Super-hardsurfaced polymers have been prepared under drastic conditions, for example, under the high-energy ion-beam bombardment of the surface. Ion implantation of polymer surfaces can produce surface hardness values of 10-20 GPa.41-49 In the present work, it will be demonstrated that the surface microhardness of PANi films can be substantially increased under the relatively mild conditions of bulk and surface chemical modifications. It will be further demonstrated that the observed surface mi(31) MacDiarmid, A. G.; Yang, L. S.; Huang, W. S.; Humphrey, B. D. Synth. Met. 1987, 18, 393. (32) Epstein, A. J.; Yue, J. U.S. Patent 5,208,301, assigned to Ohio State University Research Fund. (33) Lu, W. K.; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163. (34) Ahmad, N.; MacDiarmid, A. G. Synth. Met. 1996, 78, 103. (35) Kang, E. T.; Neoh, K. G.; Tan, K. L.; Uyama, Y.; Morikawa, N.; Ikada, Y. Macromolecules 1992, 25, 1959. (36) Li, Z. F.; Kang, E. T.; Neoh, K. G.; Tan, K. L.; Huang, C. C.; Liaw, D. J. Macromolecules 1997, 30, 3354. (37) Lazzaroni, R.; Cregoire, C.; Chtaib, M.; Pireaux, J. J. In PolymerSolid Interfaces; Pireaux, J. J., Bertrand, P., Bre´das, J. L., Eds.; Institute of Physics Publications: Bristol, 1992; p 213. (38) Van Dyke, L. S.; Brumlik, C. J.; Liang, W.; Lei, J.; Martin, C. R.; Yu, Z.; Li, L.; Collins, G. J. Synth. Met. 1994, 62, 75. (39) Chen, S. A.; Lee, H. T. Macromolecules 1993, 26, 3254. (40) Porter, T. L.; Caple, K.; Caple, G. Synth. Met. 1993, 60, 211. (41) Lee, E. H.; Rao, G. R.; Mansur, L. K. TRIP 1996, 4, 229. (42) Stanishevsky, A. V.; Tochitsky, E. I. J. Chem. Vapor Deposit. 1996, 4, 297. (43) Lee, E. H.; Hembree, D. M., Jr.; Rao, G. R.; Mansure, L. K. Phys. Rev. B 1993, 48, 15540. (44) Lee, Y. C.; Lee, E. H.; Mansur, L. K. Surf. Coating Technol. 1993, 51, 267. (45) Ray, A.; Asturies, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989, 29, E141. (46) Li, Z. F.; Kang, E. T.; Neoh, K. G.; Tan, K. L. Synth. Met. 1997, 87, 45. (47) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Macromol. Sci. Chem. 1992, A29, 401. (48) Tzou, K.; Gregory, R. V. Synth. Met. 1993, 55-57, 983. (49) Kang, E. T.; Neoh, K. G.; Woo, Y. L.; Tan, K. L.; Huan, C. H. A.; Wee, A. T. S. Synth. Met. 1993, 53, 333.
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crohardness can be correlated with the chemical structure of the polymer surface. Experimental Section Polymer Synthesis. Polyaniline (PANi) in its conductive emeraldine (EM) salt form was first prepared by the oxidative polymerization of aniline by ammonium persulfate in 1 M HCl according to the method reported in the literature.45 The EM salt was then converted to the neutral EM base by treatment with excess 0.5 M NaOH. The pristine “non-cross-linked” EM film (EM-25) was obtained by dissolving the EM base powder in dilute N-methylpyrrolidinone (NMP) solution (4 wt % or less) and then casting that into thin films of 10-20 µm thickness on glass substrates at room temperature (25 °C).46 It should be noted, however, that even for EM films cast from a dilute NMP solution at room temperature, a small degree of cross-linking might have been introduced.47 The cross-linked EM film (EM-150) of about 20-30 µm thickness was prepared by heating a 8 wt % NMP solution of EM base in an air-circulated oven at the temperature of 150 °C for 6 h. The solution first formed a gel which eventually led to the formation of cross-linked EM films.48 The residual NMP in both types of polymer films was removed by exhaustive pumping at room temperature. On one hand, the non-cross-linked nigraniline (NA-25) and cross-linked nigraniline (NA-150) films were obtained by subjecting the respective EM-25 and EM-150 films to 1 cycle of acid (1 M HCl)-base (0.5 M NaOH) treatment.16,49 The cross-linked leucoemeraldine (LM-150) film, on the other hand, was prepared by treating the EM-150 film with hydrazine for about 4-5 h.46 Surface Modification. Surface modification of EM base film was carried out via thermally induced surface graft copolymerization with a functional monomer, such as acrylic acid (AAc), Na salt of styrenesulfonic acid (NaSS), and N,N-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS), according to the method reported earlier.35,36 On one hand, the AAc and NaSS were purchased from Aldrich Chemical Co. of Milwaukee, WI. The DMAPS, on the other hand, was prepared according to the method reported in the literature.50 The amphoteric DMAPS molecule has the following chemical structure: CH2dC(CH3)COOC2H4N+(CH3)2C3H6SO3-. EM film strips of about 2.0 cm × 4.0 cm were used in all graft polymerization experiments. Each EM film was immersed in an aqueous monomer solution of a predetermined concentration in a Pyrex glass tube. The reaction mixture was thoroughly deoxygenated with argon before being kept in an 80 °C bath for about 1 h. After each graft copolymerization experiment, the EM film was removed from the viscous homopolymer solution and washed with a jet of doubly distilled water. It was then immersed in a room-temperature water bath with continuous stirring for at least 48 h to remove the residual homopolymer. Surface Characterization. The chemical compositions of the PANi films after surface modification were characterized by X-ray photoelectron spectroscopy (XPS). XPS measurements were made on a VG ESCALAB MKII spectrometer with a Mg KR X-ray source (1253.6 eV photons) at a constant retard ratio of 40. The core-level signals were obtained at the photoelectron takeoff angle of 75° (R, with respect to sample surface). The X-ray source was run at a reduced power of 120 W (120 kV and 10 mA). All binding energies (BEs) were referenced to the C(1s) neutral carbon peak at 284.6 eV. In peak synthesis, the line width (full width at halfmaximum or fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak-area ratios, after being corrected with the experimentally determined sensitivity factors, and were accurate to within (10%. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. The subsurface regions were examined by the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The measurements were carried out on a Shimadzu FTIR-8100 spectrophotometer equipped with a Shimadzu 8000 ATR attachment. Internal reflection elements were Ge and KRS-5 prisms (50) Liaw, D. J.; Lee, W. F.; Whuang, Y. C.; Lin, M. C. J. Appl. Polym. Sci. 1987, 34, 999.
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When a material surface is modified to produce an increase in hardness, the size of indentation deformation can often be too small to be measured by the conventional optical techniques of hardness measurement. This problem has been overcome by introducing the so-called dynamic hardness measurement, which analyzes the force indentation function of the test specimen to calculate the hardness under an applied normal force.51,52 For comparison purposes, the surface microhardness values for a number of common polymers, diamonds and their high-energy ion-beam irradiated counterparts are shown in Table 1.41,43 Thus, ion implantation can produce surface hardness values in the order of 10-20 GPa. The thickness of the ion-beam modified layer is fairly shallow, typically in the range of a few tenths of a micrometer to several micrometers, depending on the energy and mass of the ion. The increase in surface microhardness has been attributed mainly to cross-linking, as a result of electronic energy transfer and ionization.41 (1) Surfaces of Bulk-Modified Polyaniline Films. Earlier XPS studies53 have shown that the quinonoid imine
(dN-), benzenoid amine (-NH-), and the positively charged nitrogen in a PANi complex correspond respectively to N(1s) peak components with binding energies (BEs) at 398.2, 399.4, and >400 eV in a properly curvefitted N(1s) core-level spectrum. Thus, XPS provides a unique and convenient tool for the study of the intrinsic structure and oxidation states of the aniline polymers. Figure 1 summarizes the schemes for the interconversions among the various intrinsic oxidation states and protonation levels of PANi. Figure 2a-d, on the other hand, shows the N(1s) core-level spectra of the EM-25 (noncross-linked EM), EM-150 (cross-linked EM), NA-150 (from EM-150), and LM-150 (from EM-150) films used in the present work. In the case of EM-25 or the non-cross-linked EM film, the N(1s) core-level spectrum is best curve-fitted with two Gaussian components at BEs of 398.2 and 399.4 eV, which are attributable respectively to the imine and amine nitrogen moieties. The N(1s) line shape is consistent with the intrinsic redox state of the EM base, although the [dN-]/[-NH-] ratio deviates somewhat from the ideal value of unity. Earlier studies49 have demonstrated that the surface [dN-]/[-NH-] ratio for EM film is dependent on the history and processing condition of the film. The high-BE tail above 400 eV may have resulted in part from surface oxidation products or weakly charge-transfer complexed oxygen. The contribution of the imine satellite structure to the high-BE tail has also been suggested.54 The cross-linked EM film or EM-150 has a lower [dN-]/[NH-] ratio than that of the non-cross-linked EM-25 film. This reduction in the ratio can be attributed to the crosslinking reaction resulting from a coupling of two neighboring NdQdN groups to give two N-B-N groups through the linkage of the N with its neighboring quinonoid ring, as suggested by Scheer et al.55 On one hand, one protonation-deprotonation cycle has led to the increase in the intrinsic oxidation state of the cross-linked EM-150 film to almost that of NA (Figure 2c). On the other hand, after treatment of the cross-linked EM film with hydrazine, the N(1s) core-level spectrum of the film is reduced to almost a single peak with a BE of 399.4 eV, characteristic of PANi in the LM state. The dynamic surface microhardness as a function of the indentation depth for the non-cross-linked EM-25 film is shown in Figure 3. A difference in the surface microhardness between the glass side and the air side of the as-cast EM-25 film is discernible, with the air side of the film exhibiting a higher microhardness value at the outermost surface. For surfaces from both sides, the hardness decreases rapidly with an increasing indentation depth up to about 1 µm. Beyond this depth, the hardness of the film is only weakly dependent on the indentation depth and exhibits values comparable to those of the thermoplastics. These microhardness data clearly indicate that a thin, hard layer, especially on the air side, was produced on the as-cast EM-25 film, probably as a result of an increase in the extent of cross-linking at the film surface. The postulation is consistent with the fact that a lower intrinsic redox state or [dN-]/[-NH-] ratio is always observed at the outermost surface of the as-cast EM film rather than in the subsurface layer, in the angleresolved XPS studies.46,49 Other factors, such as the amount of residual NMP solvent which can act as a
(51) Pethica, J. B.; Hutchings, R.; Oliver, W. C. Philos. Mag. A. 1983, 48, 593. (52) Kulkarni, A. V.; Bhushan, B. Thin Solid Films 1996, 206, 290. (53) Kang, E. T.; Neoh, K. G.; Tan, K. L. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: Chichester, 1997; Vol. 3, Chapter 3, pp 121-181.
(54) Snauweart, P.; Lazzaroni, R.; Riga, J.; Verbist, J. J.; Gonbeau, D. J. Chem. Phys. 1990, 92, 2187. (55) Scheer, E. M.; MacDiarmid, A. G.; Manohar, S. K.; Masters, J. G.; Sun, Y.; Tang, X.; Druy, M. A.; Glatkowski, P. J.; Cajipe, V. B.; Fischer, J. E.; Cremack, K. R.; Jozefowicz, M. E.; Ginder, J. M.; McCall, R. P.; Epstein, A. J. Synth. Met. 1993, 41-43, 735.
Table 1. Surface Micro-Hardness of Some Common Polymers, Diamond, and Their High-Energy Ion-Beam Irradiated Counterparts41,43 materials
ion
energy (keV)
fluence (ions/m2)
hardness (GPa)
polyethylene polyethylene polystyrene polystyrene Nylon 6,6 Nylon 6,6 polycarbonate polycarbonate PMMA PMMA PEEK PEEK PTFE PTFE diamond diamond
nonea Ar+ none Ar+ none Ar+ none O+ none Ar+ none Ar+ none Fe2+ none Ar+
0 1000 0 1000 0 1000 0 2000 0 2000 0 1000 0 2000 0 1000
0 5 × 1018 0 1 × 1021 0 5 × 1018 0 5 × 1018 0 1 × 1019 0 5 × 1018 0 all fluence 0 2 × 1020
0.15 3.4 0.40 20.2 0.60 4.6 0.32 15.0 0.49 10.80 0.38 10.0 0.20 decompose 222 39
a
Pristine polymer/material.
with a face angle of 45°. Each spectrum was obtained after at least 400 scans and averaged at a resolution of 4 cm-1. Surface Microhardness Measurements. Surface microhardness of PANi films was measured using a Shimadzu DUH200 dynamic ultra-microhardness tester (Shimadzu Corp., Kyoto, Japan). The dynamic hardness (H) was obtained from a load (P) applied to the film surface through a trianglar microindenter with an apex angle of 115° and the resulting indent depth (D). During the measurements, the indenter is pressed against the sample surface by an electromagnetic force. The pressing force is increased linearly from zero to the preset point. While the indenter penetrates the specimen, the indentation depth is continuously measured. The dynamic hardness is calculated from the formula H ) P/RD, where R is a constant associated with the shape of the indenter and RD represents the projected contact area of the indenter under the load P. Thus, with measurements at various test loads, the hardness can be obtained as a function of indentation depth. All the values reported in this work are the average of at least eight readings measured at different locations of the film surface.
Results and Discussion
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Figure 1. Interconversions among the various intrinsic oxidation states and protonation levels of polyaniline.
Figure 2. N(1s) core-level spectra of the (a) non-cross-linked EM (EM-25) film, (b) cross-linked EM (EM-150) film, (c) crosslinked NA (NA-150) film, and (d) cross-linked LM (LM-150) film.
plasticizer,39 can also exert an effect on the observed hardness and may account, to some extent, for the disparity in microhardness values of the two surfaces (see below). For comparison purposes, the dynamic surface microhardness values of a poly(tetrafluoroethylene) (PTFE) film is also shown in Figure 3. Thus, the hardness of a PTFE film is independent of the indentation depth over the whole range of depth studied (0.2-6 µm). The effect of cross-linking on the microhardness of the polymer is best demonstrated by the result obtained for the cross-linked EM-150 film. Figure 4 shows the surface microhardness of both sides of the EM-150 film as a function of indentation depth. Thus, the surface hardnesses of the cross-linked EM film at the attained depth of 0.1 µm was found to be about 3 and 4 GPa, respectively, for the glass side and the air side. These hardness values represent a 3-fold increase over the corresponding values observed for the non-cross-linked EM-25 film surfaces. More importantly, although the surface microhardness
Figure 3. Dynamic surface microhardness of the non-crosslinked EM-25 film as a function of the indentation depth.
of the cross-linked film also decreases rapidly with the indentation depth, the hardness values of the bulk of crosslinked films are also correspondingly higher (compare Figures 3 and 4). Again, disparity in surface microhardness is observed for the two film surfaces, with the surface exposed to air during film preparation exhibiting a significantly higher hardness. Although the disparity in surface microhardness can be attributed to the different degree of cross-linking at the two surfaces, the plasticizing effect of the residual NMP trapped in the film must also be taken into account. As the penetration depth of the typical ATR-FTIR is in the order of 100-1000 nm,56 the presence of trapped residual NMP in the subsurface layer is best revealed by this technique. Figure 5, parts a and b, show, respectively, the ATR-FTIR spectra obtained from the glass side and air side of the cross-linked EM-150 film used for the surface hardness measurement. A weak absorption band at about 1675 cm-1 is discernible in each spectrum. This absorption (56) Harrick, N. J. International Reflection Spectroscopy; WileyInterscience: New York, 1967; p 6.
Surface Hardness of Polyaniline Films
Figure 4. Dynamic surface microhardness of the cross-linked EM-150 film as a function of the indentation depth.
Figure 5. ATR-FTIR absorption spectra of (a) the glass side and (b) the air side of the EM-150 film and of (c) the glass side and (d) the air side of the EM-150 film after 1 cycle of acidbase treatment.
mode is attributable to the carbonyl group39 of the residual NMP solvent in the subsurface region. The higher intensity of the carbonyl peak observed on the film surface from the glass side is consistent with the diffusion limitation of the solvent evaporating from this surface. It is also consistent with the lower microhardness value obtained from this surface than that from the air-side surface (Figure 4). The ATR-FTIR results thus suggest that the surface microhardness of the EM film can be further enhanced by the removal of the residual NMP solvent in the subsurface region and from the bulk of the polymer film. Earlier studies49 have suggested that 1 cycle of acid-base treatment can result in the removal of the residual NMP solvent and also an increase in the intrinsic oxidation state of the polymer to that of the nigraniline (NA) base. The N(1s) core-level spectrum of the so-obtained NA-150 film is shown in Figure 2c. Figure 5, parts c and d, show, respectively, the ATR-FTIR spectra obtained from the glass side and the air side of the cross-linked NA-150 film, obtained from 1 cycle of acid-base treatment of the crosslinked EM-150 film. Earlier IR studies on PANi have assigned the 1600 cm-1 peak to the quinonoid ring and
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Figure 6. Dynamic surface microhardness as a function of the indentation depth of the non-cross-linked EM-25 and the crosslinked EM-150 films after 1 cycle of acid-base treatment.
the 1500 cm-1 peak to the benzenoid ring of the polymer.16 The increase in the intrinsic oxidation state of the polymer film from the EM state to the NA state after 1 cycle of the acid-base treatment is thus clearly indicated by the significant increase in the intention ratio of the 1600: 1500 cm-1 peaks for the NA film. Furthermore, 1 cycle of the acid-base treatment has resulted in the complete removal of the residual NMP solvent and the disappearance of the carbonyl absorption band at about 1675 cm-1. Furthermore, the surface microhardness of the NMP-free, cross-linked NA film has been substantially enhanced to a value of about 6 GPa, as shown in Figure 6. More importantly, the disparity in microhardness between the two surfaces of the polymer film, arising from the differential residual NMP content at the two surfaces, has been removed almost completely in the NMP-free, cross-linked NA film. Similar results are obtained for the non-cross-linked EM-25 film after 1 cycle of acid-base treatment (Figure 6). Further increase in surface microhardness is achieved when the cross-linked EM-150 film is converted to the fully reduced LM state. Reduction of almost all the imine nitrogen (dN-) to the amine nitrogen (-NH-) (see Figure 2d) and the removal of the residual NMP solvent during hydrazine treatment have resulted in the increase in surface microhardness to about 8 GPa, as shown in Figure 7. Again, the surface microhardness decreases rapidly with indentation depth, although the subsurface or bulk of the film still exhibits a hardness characteristic of that of a cross-linked film. A similar extent of enhancement in surface microhardness is observed when the cross-linked EM-150 film is protonated by an acid, such as 1 M H2SO4. The surface microhardness of the H2SO4 protonated EM film, at a protonation level of about 0.40, as a function of indentation depth is also shown in Figure 7. The protonation level was determined from the [S]/[N] and [N+]/[N] ratios, which in turn were derived from the XPS corelevel spectral area ratios.53 In Figure 7, only the microhardness of the air side of the LM and EM-H2SO4 films are reported, as the disparity in microhardness between the two surfaces of each film has become less distinctive after the reduction and protonation treatments. The sharp increase in hardness at the submicrometer indentation depth for all the present films tends to suggest that a limiting value of surface hardness has not been reached in the present measurement. The hardness values of 6-8 GPa obtained for the various forms of the cross-
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Figure 7. Dynamic surface microhardness of the cross-linked EM-150 film after protonation by H2SO4 and after reduction to the leucoemeraldine state.
Figure 8. C(1s) and N(1s) core-level spectra of the EM-150 film after graft copolymerization in (a) and (b) 10 wt % and (c) and (d) 20 wt % AAc solution.
linked PANi films at the lowest attainable indentation depth of about 0.1 µm, although are much higher than most of the conventional thermosets, may still be regarded as an underestimate of the actual hardness of the outermost surface of each PANi film. The surface microhardness data, nevertheless, clearly indicate that the dense surface morphology of the PANi film cast from NMP solution, together with the rigidity of the conjugated polymer chains and the ability of the polymer to undergo spontaneous interchain cross-linking, have given rise to a very hard-surfaced polymer. It should be emphasized that this extraordinary surface hardness is obtained in the complete absence of any external surface treatment or modification. (2) Surface-Modified Polyaniline Films. In this section, it is demonstrated that the surface microhardness of the cross-linked PANi film can be further enhanced through simple chemical modifications of the surface to achieve very hard-surfaced properties. Figure 8, parts a-d, show the respective C(1s) and N(1s) core-level spectra of two cross-linked EM-150 films after surface modification by thermal graft copolymerization36 in 10 and 20 wt % AAc solutions. The presence of a surfacegrafted AAc polymer is readily indicated by the appearance of a small but distinct high-BE C(1s) component at about 288.7 eV, which is associated with the carboxylic acid group
Kang et al.
Figure 9. Dynamic surface microhardness as a function of the indentation depth of the surface-modified EM-150 films from graft copolymerization with AAc.
of the AAc polymer. The graft concentration, expressed as the ratio of carboxylic acid group per EM repeating unit or [COOH]/[N] ratio, can be readily determined from the corrected area ratio of the C(1s) peak component at 288.7 eV and the total N(1s) spectral area. Thus, the graft concentrations for the two resulting EM film surfaces are about 0.16 and 0.22, respectively. Graft copolymerization with AAc also gives rise to a self-protonated EM surface.35 The presence of self-protonation at the surface is indicated by the reduction in the amount of the imine component and the appearance of the high-BE tail above 400 eV in the N(1s) core-level spectra, attributable to the positively charged nitrogen. The surface composition and microstructure of modified EN film from surface graft copolymerization with AAc have been reported in detail earlier.35 Tethering or covalent bonding of the grafted AAc chains on the cross-linked EM film surface and the accompanied charge-transfer interaction (protonation) between the grafted and the substrate chains have resulted in a further increase in surface microhardness. Figure 9 shows the dynamic surface microhardness of the two AAc surface graft copolymerization EM films as a function of the indentation depth. For the EM film with a graft concentration ([COOH]/[N] ratio) of about 0.22, the surface microhardness at the lowest attainable depth of about 0.05 µm has achieved a value of about 19 GPa. This hardness value is comparable to that of the very hardsurfaced polystyrene41 obtained only by bombardment with a 1000 keV Ar+ ion beam at a high fluence of 6 × 1021 ions/m2! The fact that fairly high hardness values are also observed in the immediate subsurface layer of the AAc graft copolymerized EM film is consistent with the presence of a dense surface state. A further increase in surface graft concentration results in the more extensive and macroscopic coverage of the EM film surface by the AAc polymer, and thus also a more hydrophilic EM film surface. As a consequence, the surface microhardness can no longer be measured with reliable consistency. The effect of surface grafting and the associated chargetransfer interaction between the graft and the substrate chains on the surface microhardness is further demonstrated in EM film after surface modification by graft copolymerization with Na salt of styrenesulfonic acid (NaSS). As in the case of EM film with surface-grafted AAc polymers, the EM film with a surface-grafted styrenesulfonic acid (SSAc) polymer (as no sodium was
Surface Hardness of Polyaniline Films
Langmuir, Vol. 15, No. 16, 1999 5395
Figure 10. Dynamic surface microhardness as a function of the indentation depth of the surface-modified EM-150 films from graft copolymerization with SSAc.
Figure 11. Dynamic surface microhardness as a function of the indentation depth of the surface-modified EM-150 films from graft copolymerization with DMAPS.
detected after the graft copolymerization) also acquires a self-protonated surface structure.35 The amount of surface grafting in this case can be simply expressed as the [S]/[N] ratio and determined directly from the sensitivity factorscorrected S(2p) and N(1s) core-level spectral area ratios. Figure 10 shows the surface microhardness as a function of the indentation depth for two cross-linked EM-150 base films after surface modification by thermal graft copolymerization in 10 and 20 wt % NaSS aqueous solutions. The graft concentrations ([S]/[N] ratios) for the two resulting EM film surfaces are in the order of 0.08 and 0.12, respectively. Again, a super-hard-surfaced EM base film with a hardness value exceeding 19 GPa is obtained after surface modification via graft copolymerization with NaSS under relatively mild conditions. As in the case of the AAc graft-copolymerized EM film surface, a further increase in the surface graft concentration results in the more extensive and macroscopic coverage of the EM film surface by the SSAc polymer. Thus, the surface microhardness values are not reported for EM film surfaces with higher graft concentrations. The effect of excess surface graft on the microhardness is demonstrated in surface-modified EM-150 films from graft copolymerization with DMAPS. The amphoteric monomer undergoes efficient thermal graft copolymerization on an EM film surface. Earlier studies36 have shown that, for graft copolymerization carried out with a DMAPS monomer concentration greater than 1 wt %, the surface of the EM film can become completely covered by a layer of the DMAPS polymer with a thickness exceeding the probing depth35,36 (in the order of about 7.5 nm at the photoelectron takeoff angle of 75°) of the XPS technique. Similar surface graft copolymerization results are obtained in the present work. In this case, the amount of the grafted DMAPS polymer on the EM substrate is determined from the N(1s) core-level spectral component areas and expressed as the [N+]/([dN-] + [-NH-]) ratio. At total surface coverage, the N(1s) spectrum is dominated by the N(1s) component (N+ species) of the DMAPS polymer at about 401.7 eV. Figure 11 shows the surface microhardness as a function of indentation depth for two cross-linked EM-150 base films after surface thermal graft copolymerization in 1 and 10 wt % aqueous DMAPS solution. Thus,
at total surface coverage by the DMAPS polymer, the surface microhardness of the EM-150 film does not exhibit a significance increase over that of the pristine EM-150 film. In fact, a substantial enhancement in surface microhardness is observed only for the EM film with the lower extent of DMAPS polymer coverage from graft copolymerization in 1 wt % DMAPS solution. The surface microhardness of this DMAPS graft copolymerized EM film reaches about 12 GPa. This hardness value, although is significantly higher than that of the pristine EM-150 film, is lower than the hard-surfaced EM films obtained from graft copolymerization with AAc or NaSS, probably due to the lack of strong charge-transfer interaction between the grafted DMAPS polymer and the substrate EM chains, and the presence of a higher concentration of the graft chains at the surface. Conclusion Arising from its rigid aromatic ring-containing conjugated backbone, susceptibility to interchain cross-linking, and dense surface nanostructure, polyaniline (PANi) films have the potential of exhibiting hard-surfaced properties. In this work, the dynamic surface microhardness values of pristine, bulk-modified, and surface-modified PANi films were measured. Bulk cross-linking and removal of the residual NMP solvent resulted in more than a 5-fold increase in the surface microhardness of the as-cast emeraldine (EM) film to a value of about 6 GPa. This hardness value can be further increased to about 8 GPa through protonation or through reduction of the crosslinked EM film to the leucoemeraldine (LM) state. Surface modification of the highly cross-linked EM films via graft copolymerization with acrylic acid or styrenesulfonic acid readily gives rise to super-hard-surfaced PANi films having microhardness values approaching 20 GPa. The present study thus demonstrated that a very hard-surface property can be readily achieved in PANi films under the relatively mild conditions of chemical modification, in comparison with the vigorous conditions of high-energy ion-beam bombardment generally required for conventional polymers. LA981717R
