Two-Dimensional Correlation Analysis of Polyimide Films using Attenuated Total Reflection-Based Dynamic Compression Modulation Step-Scan Fourier Transform Infrared Spectroscopy YUJI NISHIKAWA,* TATSUHIKO NAKANO, and ISAO NODA Material Analysis Division, Material Technology R&D Laboratories, Konica Minolta Technology Center Inc., No. 1 Sakura-machi, Hino-shi, Tokyo, 191-8511, Japan (Y.N.); Thermo Fisher Scientific K.K., C-2F, 3–9 Moriya-cho, Kanagawa-ku, Yokohama, 221-0022, Japan (T.N.); and The Procter & Gamble Company, 8611 Beckett Road, West Chester, Ohio 45069 (I.N.) Attenuated total reflection (ATR)-based dynamic compression modulation two-dimensional (2D) correlation study of poly(p-phenylene biphenyl- tetracarboximide) film is carried out in combination with spectral simulation analysis by density functional theory (DFT). The dynamic 2D infrared (IR) correlation spectra in the region of imide I (C¼O stretching mode) show three distinct correlation peaks located around 1777, 1725, and 1708 cm 1 . The band at 1708 cm 1 is the lower wavenumber shift component of 1777 or 1735 cm 1 peaks and is attributed to the results from intermolecular interactions, according to the DFT analysis. The 1708 cm 1 band also shows the largest dynamic response, suggesting that these intermolecular interactions may enhance the dynamic response. The dynamic 2D IR correlation spectra in the region of imide II (C–N–C axial stretching mode) vibrations also show three correlation peaks located around 1335, 1355, and 1370 cm 1 , although the imide II band is shown to consist substantially of one component by the DFT analysis. These multiple peaks may be attributed to the compression-induced wavenum- ber shift of the band in the backbone structures. The sequential analysis of 2D correlation data show that, upon applying the dynamic compression, the response of the backbone regions (imide II) occurs first, followed by that of the side-chain regions (imide I, C¼O). Index Headings: Fourier transform infrared spectroscopy; Step-scan FT- IR spectroscopy; Dynamic spectroscopy; Compression modulation; Attenuated total reflection; Polarized ATR; Internal reflection; Poly(p- phenylene biphenyltetracarboximide); Molecular orbital simulation; Density functional theory; DFT. INTRODUCTION Polyimides have been widely used for many applications because of their excellent properties such as low thermal expansion, chemical inertness, ease of film formation, and high thermal and oxidative stability. 1 These applications include dielectric and insulating layers for microelectronic devices, 2 adhesive coatings for metals and composites, 3 surface alignment layers, which are essential for production of flat- panel liquid crystal displays (LCDs), 4 and optical materials. 5 For optical applications, it is important to control several optical properties such as optical transparency, refractive index, and birefringence. These parameters are closely connected with the stress. Thermal expansion or compression can also be considered as expansion or compression stress with respect to temperature. We expect that important insights may be obtained regarding optical properties, mechanical stress, and molecular level behaviors (e.g., phase and magnitude of stress- induced molecular response) by using a rheo-optical method, i.e., spectroscopy coupled with the presence of static or periodic stress. One of the well-known rheo-optical methods is dynamic infrared linear dichroism (DIRLD) spectroscopy, 6,7 where infrared (IR) dichroism measurement is coupled with a small-amplitude dynamic stress applied to samples. In the case of polyimides, however, it is difficult to apply the normal DIRLD method, which is usually performed under transmission mode. Polyimides have very strong infrared absorption due to imide I and imide II vibrations, so that only extremely thin film samples may be used for transmission- mode IR analysis. In order to overcome this problem, we have developed an attenuated total reflection (ATR)-based compres- sion modulation step-scan Fourier transform infrared spectro- scopic method 8 and have successfully applied it to uniaxially drawn poly(ethylene terephthalate) (PET) films. 9 The purpose of the paper is to demonstrate the utility of the ATR-based dynamic compression modulation spectroscopy. The emphasis of this paper is not placed on the specific spatial relationship between the applied stress (either compression or stretching) and reorientation responses. Rather, it deals with the relative phase delays of the transient responses of the constituents regardless of the reorientation directions with respect to the applied stress axis. In our previous paper, we revealed that molecular-level responses of uniaxially drawn (PET) are not uniform, i.e., the response of the side-chain regions (ester groups) occurred first, followed by that of the backbone regions (benzene ring), by applying two-dimensional (2D) correlation sequential analysis. 9,21 In the case of polyimides, different dynamic molecular responses can be expected. In the present paper, more detailed discussions on ATR- based dynamic compression modulation spectroscopy of polymers are given by using two-dimensional (2D) correlation spectroscopy for the analysis of poly (p-phenylene biphenyl- tetracarboximide) film. We further tried to draw a close connection between dynamic response spectra in the strong absorption region (imide I and imide II) of the film and simulated spectra generated by using density function theory (DFT)-based programs. EXPERIMENTAL Compression Modulation Attenuated Total Reflection System. A detailed experimental setup has been described previously. 8,9 A single reflection Harrick Seagullt ATR accessory equipped with a wire-grid polarizer was used to measure dynamic compression ATR spectra. A hemispherical silicon internal reflection element (IRE, radius ¼ 7.0 mm) with an incident angle of 35.08 was used in the ATR accessory in order to not exceed a maximum absorbance of 0.7 in the Received 10 December 2006; accepted 11 May 2007. * Author to whom correspondence should be sent. E-mail: yuji. nishikawa@konicaminolta.jp. Volume 61, Number 8, 2007 APPLIED SPECTROSCOPY 873 0003-7028/07/6108-0873$2.00/0 Ó 2007 Society for Applied Spectroscopy