Surface Characterization of Polyamic Acid and Polyimide Films

Dec 1, 2001 - Figure 1 Structural formulas and schematic representation of the reaction of ... 25 ps pulse width and 10 Hz repetition rate was used as...
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Langmuir 2001, 17, 8125-8130

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Surface Characterization of Polyamic Acid and Polyimide Films Prepared by Vapor Deposition Polymerization by Using Sum-Frequency Generation Takayuki Miyamae,† Kiyomi Tsukagoshi,† Osamu Matsuoka,‡ Sadaaki Yamamoto,‡ and Hisakazu Nozoye*,† Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan, and Mitsui Chemicals Inc., 580-32 Nagaura, Sodegaura-city, Chiba 299-0265, Japan Received April 26, 2001. In Final Form: September 12, 2001 The surface structures of polyamic acid and polyimide films made by vapor deposition polymerization (VDP) technique were investigated using infrared-visible sum-frequency generation (SFG). From the SFG spectra of VDP-made polyamic acid films, the polymer chains have a broad orientational distribution, and residual monomers are observed even for a sample that was kept in air for more than 3 months. The amidization reaction of surface residual monomers is quite slow at room temperature, and the reaction proceeds preferentially in the bulk film. In the early stage of curing, surface species become parallel to the surface plane. After the surface reorientation, the imidization reaction proceeds.

1. Introduction Polyimide is one of the most valuable polymers; it has extremely good resistance to high temperature, radiation rays, and mechanical stress. Therefore, it has been widely used as an insulating material in various fields, such as space applications, microelectronics, liquid crystal alignment in display devices, and membrane films. Polyimides are formed by the condensation reaction of polyimide precursors, dianhydride, and diamine, via the formation of polyamic acid and subsequent high-temperature imidization. The most popular polyimides are those formed by the reaction of 4,4′-diaminodiphenyl ether (oxydianiline, ODA) and 1,2,4,5-benzenetetracarboxylic anhydride (pyromellic dianhydride, PMDA) (Figure 1). Polyimide films are usually prepared by heating spincast films of the corresponding polyamic acid for thermal imidization. This solution-based process suffers from several drawbacks in processing, e.g., residual solvent, microporosity induced by solvent evaporation, and difficulties in controlling the film thickness and uniformity, because of the rheological properties of polymer solution. In contrast, vapor deposition polymerization (VDP) process, in which the polymerization reaction proceeds in under vacuum condition without solvent and catalyst, has a number of potential advantages over solvent-based processes.1 Several works have dealt with VDP-made polyimide films and reported preparation conditions,1-3 electrical properties,4 interaction of metal with ultrathin polyimide films,5-12 phthalocyanine dispersed in the * To whom correspondence should be addressed. Telephone: 81 298 61 4527. Fax: 81 298 61 4504. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Mitsui Chemicals Inc. (1) Salem, J. R.; Sequeda, F. O.; Duran, J.; Lee, W. Y. J. Vac. Sci. Technol. A 1986, 4, 369. (2) Takahashi, Y.; Iijima, M.; Inagawa, K.; Itoh, A. J. Vac. Sci. Technol. A 1987, 5, 2253. (3) Ito, Y.; Hikita, M.; Kimura, T.; Mizutani, T. Jpn. J. Appl. Phys. 1990, 29, 1128. (4) Iida, K.; Nohara, T.; Kotani, K.; Nakamura, S.; Sawa, G. Jpn. J. Appl. Phys. 1989, 28, 2552. (5) Grunze, M.; Lamb, R. N. Surf. Sci. 1988, 204, 183.

Figure 1. Structural formulas and schematic representation of the reaction of PMDA and ODA to form polyimide.

polymer films,13,14 and preparation of hybrid films of aromatic polyimides.15 Whereas the structure of VDPpolyimide films has been the subject of a number of (6) Jordan-Sweet, J. L.; Kovac, C. A.; Goldberg, M. J.; Morar, J. F. J. Chem. Phys. 1988, 89, 2482. (7) Hahn, C.; Strunskus, T.; Frankel, D.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 1123. (8) Ashton, M. R.; Jones, T. S.; Richardson, N. V.; Mack, R. G.; Unertl, W. N. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55 1133. (9) Mack, R. G.; Grossman, E.; Unertl, W. N. J. Vac. Sci. Technol. A 1990, 8, 3827. (10) Grunze, M.; Buck, M.; Dressler, Ch.; Langpape, M. J. Adhes. 1994, 45, 227. (11) Plank, R. V.; DiNardo, N. J.; Vohs, J. M. J. Vac. Sci. Technol. A 1996, 14, 3174. (12) Young, J. T.; Tsai, W. H.; Boerio, F. J. Macromolecules 1992, 25, 887. (13) Sakakibara, Y.; Iijima, M.; Tsukagoshi, K.; Takahashi, Y. Jpn. J. Appl. Phys. 1993, 32, L332. (14) Sakakibara, Y.; Matsuhata, H.; Tani, T. Jpn. J. Appl. Phys. 1993, 32, L1688. (15) Iijima, M.; Takahashi, Y.; Kakimoto, M.; Imai, Y. J. Photopolym. Sci. Technol. 1991, 5, 351.

10.1021/la0106135 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2001

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Figure 2. Schematic diagram of the experimental system for vapor deposition polymerization.

investigations, the surface structure of polyimide films has been less extensively examined. Recently, sum-frequency generation (SFG) vibrational spectroscopy has been demonstrated to be an effective surface and interface analytical probe.16-18 Since SFG is second-order nonlinear optical process, the process is electric-dipole forbidden in bulk with inversion symmetry but allowed at interfaces where the inversion symmetry is broken. Therefore, it is particularly sensitive to the interfacial structure between two centrosymmetric media. The resonant enhancement of the SF signal occurs when the input IR frequency agrees with a vibrational mode that is both IR- and Raman-active. Furthermore, input/ output polarization dependence of the spectra provides information about average orientation of surface molecules. Thus this spectroscopic technique is ideal for studies of chemical composition and structure of a polymer surface.19-23 In this work, we report the SFG spectra of VDP polyamic acid and polyimide films to obtain the structural information of these polymer surfaces. By using SFG, the surface molecular orientational difference between VDP polyamic acid and spin-cast polyamic acid film is also investigated. We also investigated the influence of the oxygen-radical beam exposure to the polyamic acid and polyimide surface. 2. Experimental Section 2.1. Sample Preparations. The VDP system used in the present study is shown in Figure 2. A pumping system was composed of a 2000 L/s turbomolecular pump and a 500 L/min rotary pump for a VDP chamber and a 50 L/s turbomolecular pump and a 90 L/min rotary pump for a load-lock chamber. The base pressure of the VDP chamber was 1 × 10-7 Pa, and the pressure during deposition was about 6 × 10-5 Pa. Pyromeric (16) Shen, Y. R. The Principle of Nonliniear Optics; John Wiley: New York, 1984. (17) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327. (18) Domen, K.; Hirose, C. Appl. Catal. 1997, 160, 153. (19) Zhang, D.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. Lett. 1997, 281, 394. (20) Zhang, D.; Ward, R. S.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 9060. (21) Gracias, D. H.; Zhang, D.; Lianos, L.; Ibach, W.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. 1999, 245, 277. (22) Wei, X.; Zhuang, X.; Hong, S.; Goto, T.; Shen, Y. R. Phys. Rev. Lett. 1999, 82, 4256. (23) Kim D.; Shen, Y. R. Appl. Phys. Lett. 1999, 74, 3314.

Miyamae et al. dianhydride (PMDA) and oxydianiline (ODA), both monomers were supplied from Tokyo Kasei, were used without further purification. To keep the pressure as low as possible during the polymerization and control the flux of the monomers, these monomers were evaporated separately from thermostatically controlled pulse molecular evaporation sources, which were actuated by solenoid valves. The distance between sources and substrate was about 100 mm. The typical evaporation temperatures of ODA and PMDA were kept constant at 160 and 180 °C, respectively. Each monomer can be easily vaporized without decomposition. The vapor streams of each monomer were impinged on a Si(100) substrate covered with native oxide (SiOx) at room temperature. These monomers were polymerized there to form polyamic acid. The typical deposition rate was about 3 nm/min, which was measured by a quartz thickness monitor and was controlled by evaporation temperatures and opening times of the solenoid valves. The thickness of deposited polyamic acid film was about 100 nm. After deposition, thermal imidization was performed at 150-200 °C for 1 h in situ or in air. By this curing treatment, residual monomers were desorbed from the film or further polymerized to polyamic acid, and moreover, polyamic acid was dehydrated to polyimide.3 For the sake of comparison, spin-cast films of polyamic acid were prepared by 14 ( 1 wt % 1-methyl-2-pyrrolidine/aromatic hydrocarbon solution of polyamic acid (Aldrich Chemical Co.). It was dropped onto a grass plate rotating at 3500 rpm and then dried in dry nitrogen atmosphere for 1 day. An oxygen-radical beam source, which was attached to the system, was composed of a quartz rf discharge tube, in which the rf coil was set to induce oxygen plasma. Details of the oxygenradical beam source was described elsewhere.24 The distance between the oxygen radical beam source and the sample was about 120 mm. The flux of oxygen radicals from the oxygen radical beam source was 1 × 1014 radicals cm-2 s-1, which was determined by monitoring the change of the resonant frequency of Ag precoated on the quartz crystal.25 Infrared measurements were carried out by a Bio-Rad Fourier transform IR spectrometer. 2.2. IR-visible SFG Spectroscopy. In IR-visible SFG experiments, a mode-locked Nd:YAG laser (PL2143D, EKSPLA) at 1064 nm with 25 ps pulse width and 10 Hz repetition rate was used as a master light source. An IR beam, tunable from 1200 to 4300 cm-1, was generated by a AgGaS2 crystal by different frequency mixing of the fundamental of the Nd:YAG laser with the output of an optical parametric oscillator/amplifier (OPO/ OPA) system, which was composed of a LiB3O5 crystal, pumped by the third harmonics of the laser. The frequency of the tunable IR source was calibrated using a standard polystyrene film. The visible and IR beams were overlapped at a sample surface spatially and temporally with the incidence angles of 70° and 50°, respectively. Pulse energies and beam sizes were about less than 1 mJ and 0.4 mm for the visible input and 50-200 µJ and 0.5-1.0 mm for the infrared input, respectively. The spectral bandwidth of the IR pulses was ∼10 cm-1. When the sample was irradiated by an intense infrared beam, the spectrum changed rapidly due to the radiation damage. To minimize this, the infrared beam was slightly defocused. The absence of the damage effect was confirmed by repeating the SFG spectra measurements. The SF output signal in the reflected direction was filtered with a monochromator (Oriel MS257) and then detected by a photomultiplier tube (Hamamatsu R649). The visible light scattered from a sample surface was suppressed by spatial filters and a holographic notch-plus filter installed in front of the monochromator. The signal was then averaged over 100 pulses by a gated integrator (Stanford Research Systems, SR250) for every data point taken at a 5 cm-1 interval and was stored in a personal computer. In this experiment, all measurements were performed in air. In the frequency region between 2000 and 1500 cm-1, a significant portion of the infrared beam is expected to be absorbed by water vapor in the optical path. To minimize this, the optical path of the infrared beam was purged by dry nitrogen gas. The SFG spectra were normalized for infrared and visible intensity variations, respectively. The SFG spectra were recorded with (24) Imai, F.; Kunimori, K.; Nozoye, H. J. Vac. Sci. Technol. A 1995, 13, 2508. (25) Matijasevic, V.; Garwin, E. L.; Hammond, R. H. Rev. Sci. Instrum. 1990, 61, 1747.

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Figure 3. IR transmission spectra of polyamic acid (a) and polyimide (b) VDP films. various polarization combinations of the incident light pulses and SFG signals with respect to the plane of incidence. We introduce a set of abbreviations, “s” or “p”, to specify the polarization directions of the SFG, visible, and infrared beams and describe the combination of polarization in the order stated above. For example, the abbreviation ssp indicates that the detected SFG and the input visible beams were s-polarized, and the input infrared beam was p-polarized.

3. Results and Discussion 3.1. IR Spectra of Polyamic Acid and Polyimide Films. We first characterized the VDP polyamic acid and polyimide films by IR spectroscopy. Figure 3a shows an IR transmission spectrum of an as-deposited polyamic acid film over the range from 1300 to 2000 cm-1. The spectrum is almost identical with published results of VDP-made polyamic acid films.1,7,12 Peaks at 1850 and 1776 cm-1 are due to the anhydride carbonyl stretching modes and one at about 1650 cm-1 is due to amide coupling. These absorption peaks indicate that the as-deposited polyamic acid film was the admixed state of nonreacted monomers and polyamic acid.1,2 On curing the film, the IR transmission spectrum is changed as shown in Figure 3b. These peaks at 1850, 1776, and 1650 cm-1 disappeared and new peaks at 1710 and 1380 cm-1, which are assigned to the imide CdO stretching and imide C-N stretching modes, appeared. Imidization reaction is completed during curing at 200 °C for 1 h in a vacuum. Nonreacted monomers are desorbed from the film or further polymerized to polyamic acid, and a stoichiometric polyimide film is finally formed.4 The peak at 1500 cm-1 is derived from the aromatic CdC stretching mode of the ODA moiety. This peak remained unchanged or changed only a little (excess ODA), indicating that almost all residual ODA monomers are also polymerized. The IR spectrum of polyimide film is also consistent with the published results of VDP-made polyimide films.1,7,12 3.2. SFG Spectra of Polyamic Acid Films. Figure 4 shows SFG surface vibrational spectra for these films in the 1300-2000 cm-1 region. The SFG spectra for these films are quite different from the corresponding IR spectra. In the ssp polarization spectrum of the polyamic acid film shows the bands at 1330, 1420, 1510, 1570, 1610, 1660, and 1850 cm-1 and a broad band around 1800 cm-1. The assignments are summarized in Table 1. The band at 1330 cm-1 corresponds to the amide C-N stretching mode of polyamic acid, and the band at 1570 cm-1 is derived from the amide N-H bending mode.7 The band at 1610 cm-1 is derived from the symmetric stretching mode of O-Cd

Figure 4. (a) SFG spectra of polyamic acid and polyimide films. Filled square (9) and open circles (O) are the experimental data obtained in ssp and sps polarization combinations, respectively. The solid line is a guide for the eye through the symbols. The inset shows the schematics of normal modes of ODA. (b) The SFG spectra of a PMDA cast film (0), an ODA vacuum deposited film (b), and a PMDA vacuum deposited film (2) in ssp polarization combination. Table 1. Vibrational Assignments for Polyamic Acid Film SFG (cm-1) 1330 1420 1510 1570 1610 1660 1800 1850 a

infrareda (cm-1)

Ramanb (cm-1)

assignments

1325 1340-1345 CN str 1410 1412 sym. COO str and C6H2 str 1500 1513, 1517 CdC str C6H4, B1u 1540-1550 1574 amide NH bend 1610 1620 asym COO str 1670 1662 amide CdO str 1780-1830 1790 residual PMDA CdO sym str 1830-1870 1872 residual PMDA CdO asym str

From refs 1, 7, and 12. b From refs 26 and 35.

O of polyamic acid. The band at 1420 cm-1 is due to combination of the tangential stretching mode of C6H2 and the asymmetric stretching mode of O-CdO of the polyamic acid.12,26 The band at 1660 cm-1 corresponds to the amide CdO stretching mode. There are two possible origins of the band at 1510 cm-1, one is the phenyl group CdC stretching mode of the ODA moiety, and the other is derived from residual ODA monomer. The peak intensity of 1510 cm-1 also does not change after the oxygen radical beam exposure, as described later. This result indicates that the band at 1510 cm-1 corresponds to the phenyl group CdC stretching mode of ODA moiety, as displayed in the inset of Figure 4.7,27 The SFG spectra for the CdC ring stretching modes of ODA moiety can be allowed only if the infrared polarization has a component parallel to the 1,4-bonding axis of the ODA phenyl ring. This mode is prominent in ssp polarization combination and hardly detectable in the (26) Mack, R. G.; Patterson, H. H.; Cook, M. R.; Carlin, C. M. J. Polym. Sci.: Polym. Lett. Ed. 1989, 27, 25. (27) Tobin, M. C. J. Phys. Chem. 1957, 61, 1392.

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sps spectrum, indicating that the phenyl rings of ODA moiety have a broad orientational distribution with its average along the surface normal. We also measured SFG spectra in sps polarization combination, as shown in Figure 4a. The SFG signals from the polyamic acid film are prominent in the ssp polarization spectrum but not discernible in sps. This indicates that polymer chains have a broad orientational distribution with its average along the surface normal, similar to the case of methanol.28 Unfortunately, the very weak sps spectrum of the polyamic acid film prevents us from deducing further quantitative information about the orientation of the molecules. However, the result indicates that the amide moiety and the phenyl ring of the ODA moiety at the surface are pointing outward along the surface normal with a broad orientation distribution. 3.3. Surface Residual Monomers. The band at 1850 cm-1 and the broad band around 1800 cm-1 in Figure 4 originate from the CdO stretching modes of unreacted PMDA monomers. In Figure 4b, we show SFG spectra of an ODA vacuum-deposited film and a PMDA/methanol solution cast film.29 The ssp SFG spectrum of the ODA vacuum deposited film exhibits a pronounced peak at 1630 cm-1, which is assigned to the CdC phenyl ring stretching vibration, as displayed in the inset of Figure 4. In the case of the vacuum deposited PMDA film, the SFG signal is quite weak and featureless in this region, as shown in Figure 4b. This might indicate that the PMDA molecular plane lies almost parallel to the surface plane and has an isotropic azimuthal distribution in the case of the vacuum deposited film. Actually, an isolated PMDA molecule has D2h symmetry, which has the inversion center. If the molecular plane is nearly parallel to the surface, little SFG signal is expected from the surface because the surface has the inversion symmetry. In contrast, in the SFG spectrum of the cast PMDA film, two peaks at 1800 and 1850 cm-1, which are attributed to the asymmetric and symmetric CdO stretch modes of PMDA, are clearly observed.30 The asymmetric and symmetric CdO stretch modes can be excited only if the infrared polarization has a component parallel to the transition moments of these modes, respectively. Since the surface PMDA molecules must be randomly oriented in the case of the solution cast film, both symmetric and asymmetric CdO stretching modes are observed in SFG. In IR spectroscopic studies of polyamic acid films made by the VDP method, these carboxylic acid anhydride peaks almost disappear when the films are kept in air for 10 h, because the amidization reaction proceeds in the film even at room temperature.2 In the SFG spectrum, however, the peaks that originated from the residual PMDA monomer are observed even for a sample that is kept in air for more than 3 months. If the amidization reaction completely finished, the peaks originating from residual monomers should disappear. These findings indicate that the amidization reaction of the residual monomers is quite slow at the surface region. Thus, the reaction of the residual monomers proceeds preferentially in the bulk of the polymer film. 3.4. The Effects of O-Radical Exposure. To obtain more understanding about the surface structure and the surface component of the polyamic acid film, we performed (28) Superfine, R.; Huang, J. Y.; Shen, Y. R. Phys. Rev. Lett. 1991, 66, 1066. (29) To make a random-oriented PMDA film, PMDA/methanol solution was dropped onto a grass plate and then dried in dry nitrogen atmosphere. (30) Hahn, C.; Strunskus, T.; Grunze, M. J. Phys. Chem. 1994, 98, 3851.

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Figure 5. IR transmission spectra of a polyamic acid film (a) before oxygen-radical beam exposure, (b) after oxygen-radical beam exposure, and (c) the difference spectra of b - a. The asterisks (*) are artifacts due to water vapor absorption.

oxygen radical beam exposure. IR transmission spectra of the polyamic acid film before and after oxygen radical exposure for 30 min are shown in Figure 5. From the difference IR spectrum, the polyamic acid film is shown to be etched by the oxygen radical beam. A similar etching effect is also observed in the case of polyimide film after oxygen radical beam exposure. Another reaction should be expected, e.g. oxygen insertion and degradation of the polymer chain; however, there are no additional bands in the IR spectra of the polyamic acid film after the oxygen radical beam exposure. In the difference IR spectrum, it should be noticed that the peaks originating from residual PMDA monomer are still observed. In contrast, the decrease of the SFG intensity at 1630, 1660, 1800, and 1850 cm-1 caused by the oxygen-radical beam exposure is clearly observed, as shown in Figure 6. The bands at 1800 and 1850 cm-1 originated from residual PMDA monomers, as mentioned above. The band at 1630 cm-1 originated from the CdC phenyl ring stretching mode of residual ODA monomer, as mentioned in the preceding section. Thus, we conclude that the oxygen radical beam exposure induces the removal of the residual monomers from the polymer surface by surface etching. This indicates that the SFG spectra in our experiment indeed come from the polymer surface. This is the result of the high surface specificy of the SFG technique. The band derived from the amide CdO (1660 cm-1) is also decreased after the oxygen radical beam exposure. Similar trends were also reported by the XPS spectra of oxygen-radical-treated polyimide31 and oxygen-plasma-treated polyimide films.32 This result indicates that the chemical reaction of oxygen radical degradation of polyamic acid films proceeds the same way as the case of the polyimide film. The intensity and the peak position of another peak are almost the same between the spectra measured before and after oxygen irradiation. 3.5. SFG Spectra of Polyimide Films. In contrast to the case of the polyamic acid films, the SFG intensity in (31) Grossman, E.; Lifshitz, Y. J. Spacecraft Rockets 1999, 36, 75. (32) Nakamura, Y.; Suzuki, Y.; Watanabe, Y. Thin Solid Films 1996, 290-291, 367.

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Figure 7. The changes of the SFG spectra of a polyamic acid film in ppp and ssp polarization combinations with curing time. The solid line is a guide for the eye through the symbols.

Figure 6. SFG spectra of a polyamic acid film (a) before oxygen radical beam exposure, (b) after oxygen radical beam exposure, and (c) the difference spectrum of b - a, in ssp polarization combination. The solid line is a guide for the eye through the symbols.

ssp polarization combination of a cured polyimide film, which is formed by curing the polyamic acid film at 200 °C for 1 h under a vacuum condition, is quite weak and featureless, as shown in Figure 4a. PMDA-ODA polyimide films are known to have a semicrystalline-like structure, but it is macroscopically random. X-ray diffraction reveals a unit cell that is orthorhombic with lattice parameters of 6.31, 3.97, and 32 Å along the a, b, and c axes, respectively.33 Each unit cell is composed of two monomer units in a zigzag configuration along the c axis. At the surface, if the a-c plane is nearly parallel to the surface, then because each PMDA-ODA monomer unit has the inversion symmetry, little SFG signal is expected from the surface. The PMDA-ODA polyimide film made by spincast method shows the in-plane molecular orientation.34 The weak intensity of the SFG signal from VDP polyimide film indicates that the molecular chain lies almost parallel to the surface plane and has an isotropic azimuthal distribution. This observation can be further tested by examining the SFG spectra with an increase of curing time. The polyamic acid films are cured at 150 °C in air. In Figure 7, we show the changes of the SFG spectra of the polyamic acid film with curing time in ppp and ssp polarization combinations. Although the curing condition is different from that for the above-mentioned polyimide films, no significant difference is observed in SFG spectra of the films cured under vacuum and in air. We also check, by the IR measurements, that the curing temperature of 150 °C is sufficient for the imidization. Johnson and Wunder reported that the imide conversion at this temperature is more than 80% after 20 min curing at 150 °C.35 The SFG (33) Kazaryan, L. G.; Tsvankin, D. Ya.; Ginsburg, B. M.; Tuichiev, Sh.; Korzhavin, L. N.; Frenkel, S. Ya. Vysokomol. Soed. A 1972, 14, 1199 [Polym. Sci. USSR 1972, 14, 1344]. (34) Sakamoto, K.; Arafune, R.; Ito, N.; Ushioda, S.; Suzuki, Y.; Morokawa, S. J. Appl. Phys. 1996, 80, 431. (35) Johnson, C.; Wunder, S. L. J. Polym. Sci. Part B: Polym. Phys. 1993, 31, 677.

signals from the polyamic acid films in ppp combination are about 1.5 times as strong as that in ssp spectra. After 5 min curing, the SFG intensities of the peak located at 1510 cm-1 decreased in ppp combination. With increase of curing time, the intensities of other peaks are also reduced both in ppp and ssp combinations. The different behaviors of the peak at 1510 cm-1 in ssp and ppp polarizations are due to the change of the molecular orientation. In the case of an azimuthally isotropic surface, ppp polarization spectra are more sensitive to the change of the molecular tilt angle than those in the ssp polarization combination.36 Therefore, reduction in the SFG signal intensity of the CdC stretching derived from the ODA moiety (1510 cm-1) indicates that tilted ODA phenyl rings become nearly parallel to the surface plane, because the CdC ring stretching modes of ODA moiety can be allowed only if the infrared polarization has a component parallel to the 1,4-bonding direction of the ODA phenyl ring. No significant difference is observed in the IR transmission spectra until 10 min curing under this condition. After 15 min curing, the imide CdO stretching and imide C-N stretching modes appeared in the IR spectra, while amide peaks are still observed. After 130 min curing, the IR spectrum is completely the same as that of the polyimide film, although a broad band at around 1700 cm-1 is observed for the SFG spectrum in the ppp polarization combination. Presumably, this band is derived from the imide CdO stretching of the polyimide moiety, which is tilted away from the surface plane. In the early stage of curing, we could not also observe this imide peak in the SFG spectra, while intensities of the peaks derived from the amide carbonyl stretching at 1660 and 1740 cm-1, the amide C-N stretching at 1330 cm-1, and the N-H bending at 1570 cm-1 decreased upon increasing the curing time. Since the imide CdO stretching band is not observed and the intensity of the amide CdO mode is decreased at the early stage of curing, amide CdO also becomes nearly parallel to the surface plane. From the SFG spectral changes, we can conclude the reorientation of polymer chains at the surface during curing. At first, the phenyl ring of the ODA moiety and the amide CdO become nearly parallel to the surface plane. After the surface reorientation, imidization reaction proceeds. (36) Wei, X. Ph. D thesis, University of California, Berkeley, 2000.

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observed in the expanded spectra of spin-cast polyamic acid film. Thus the molecular chain is almost parallel to the surface plane in the case of the spin-cast film. After the curing of the spin-cast film, little SFG signal is observed, indicating that the molecular chain is almost parallel to the surface plane. This is in good agreement with the polarized IR measurement and SHG of polyimide films made by the spin-cast method.34,37 The IR transmission spectra and XPS spectra show no significant difference between the spin-cast films and VDP-made films (data are not shown). These results indicate that surface structures are different between the spin-cast polymer films and the VDP-made polymer film. 4. Conclusion

Figure 8. SFG spectra of (a) a VDP-polyamic acid film and (b) a spin-cast polyamic acid film, in ssp polarization combination.

3.6. Different Surface Structures between the VDP and Spin-Cast Polyamic Acid Film. Next we will discuss the difference of the surface structures between a spin-cast polyamic acid film and a VDP-made polyamic acid film. In Figure 8, we show an SFG spectrum of a spin-cast film of polyamic acid. For the purpose of comparison, we also show the SFG spectrum of a VDPmade polyamic acid film again in Figure 8a. Spectral features of the spin-cast film are quite different from the VDP-made polyamic acid. The SFG signal intensity is quite weak in the SFG spectrum of the spin-cast polyamic acid film, while the peaks derived from amide moieties are

The surface structures of the VDP-made polyamic acid and polyimide films are investigated with SFG vibrational spectroscopy. From the SFG spectrum of the VDP-made polyamic acid film, the phenyl rings of the ODA moiety have a broad orientational distribution. The amidization reaction of surface residual monomers is quite slow at room temperature, and the reaction proceeds preferentially in the bulk film. In the early stage of the curing, surface moieties become parallel to the surface plane. After the surface reorientation, the imidization reaction proceeds. The surface structures of spin-cast films and VDPmade polyamic acid films are quite different, while IR and XPS measurements show no significant difference between them. LA0106135 (37) Oh-e, M.; Hong, S.-C.; Shen, Y. R. J. Phys. Chem. B 2000, 104, 7455.