Relationship between Molecular Aggregation Structures and Optical

Macromolecules 2011, 44, 349–359

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DOI: 10.1021/ma101765k

Relationship between Molecular Aggregation Structures and Optical Properties of Polyimide Films Analyzed by Synchrotron Wide-Angle X-ray Diffraction, Infrared Absorption, and UV/Visible Absorption Spectroscopy at Very High Pressure Kazuhiro Takizawa, Junji Wakita, Shohei Azami, and Shinji Ando* Department of Chemistry & Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan Received August 3, 2010; Revised Manuscript Received November 29, 2010

ABSTRACT: The relationship between the molecular aggregation structures and the optical properties of fully aromatic and semialiphatic polyimide (PI) films were analyzed by synchrotron wide-angle X-ray diffraction (WAXD), infrared absorption, and UV/visible absorption spectroscopy at very high pressures up to 8 GPa. The PIs showed significant reduction in the interchain distances in the first stage of compression up to 1 GPa, which resulted in an appreciable decrease in the interchain free volume. In addition, reduction in the C-C bond lengths of aromatic rings by ca. 0.7% was confirmed by the pressure-induced high wavenumber shift of the infrared stretching vibration of the PIs. Furthermore, pressure-induced bathochromic shifts were observed in the locally excited (LE) absorption band of PIs, which are related to the enhanced van der Waals interaction caused by the reduced interchain distances. The intensity of the charge transfer (CT) absorption band of s-BPDA/PDA poly(p-phenylene biphenyltetracarboximide) PI was reduced up to 0.3 GPa, indicating that conformational changes affect the intramolecular CT interactions. In contrast, the CT absorptions of PMDA/ODA (poly(4,40 -oxidiphenylene pyromelliticimide) and PMDA/DCHM (poly(4,40 -diaminocyclohexylmethane pyromelliticimide) PIs were enhanced by increasing the pressure, which was caused by an enhancement of intermolecular CT interactions. The significant variations observed in the LE and intermolecular CT bands below 1 GPa accord with the significant decrease in the interchain distance, as indicated by synchrotron WAXD.

*Corresponding author. Telephone: þ81-3-5734-2137. Fax: þ81-35734-2889. E-mail: [email protected].

the electron-donating property of diamine and the electronaccepting property of dianhydride.9 The smaller energy gaps of the CT transitions displace the absorption edges of PI films to a longer wavelength region (λ > 400 nm, visible region), which is the essential origin of the significant coloration of Ar-PI films from pale yellow to deep brown. Furthermore, it was also found that intra- and intermolecular CT transitions can occur in PI films. Previously, Ishida and coworkers6 reported that an intramolecular CT absorption band was observed near the absorption edge at around 400 nm in a thin film of PMDA/ODA PI, and the band could be assigned using the absorption spectrum of a corresponding model compound: N,N0 -bis(phenoxyphenyl) pyromelliticimide. In particular, they concluded that the observed CT absorption band was attributable to an intramolecular CT transition due to the fact that the absorbance of the shoulder peak at 371 nm was linearly correlated with the concentration of the model compound. On the other hand, Hasegawa and co-workers7,8 reported that the intensity of a fluorescent emission from an excited CT state (CT fluorescence) was enhanced by high-temperature annealing of a thin film of s-BPDA/PDA PI, which is due to the enhancement of intermolecular CT interactions by changes in molecular aggregation structures. The above results clearly indicate that not only the chemical and electronic structures of the repeating unit but also the intermolecular electronic interactions significantly affect the optical properties of PI films, and therefore, knowledge of the influence of aggregation structures is a prerequisite for a detailed understanding and precise control of the optical properties of PIs.

r 2010 American Chemical Society

Published on Web 12/23/2010

1. Introduction The fully aromatic polyimide (PI) derived from pyromelliticdianhydride and bis(4-aminophenyl) ether (PMDA/ODA) and that from 3,30 ,4,40 -biphenyltetracarboxylic dianhydride and p-phenylene diamine (s-BPDA/PDA) are known as high-performance engineering plastics exhibiting high thermal and chemical stability, flame resistance, radiation resistance, mechanical strength, and good flexibility.1 Recently, semialiphatic PIs have attracted much interests as a new class of thermally stable electronic and optical materials.2-5 For example, they have been applied to optical waveguides with low optical losses in the visible and near-IR regions.4 For further development of novel thermally stable and optically functional PIs, it is mandatory to understand the relationship between the molecular aggregation structures and the optical properties of PIs at the molecular level. It has been reported that two kinds of UV/visible absorption bands are observed for fully aromatic polyimides (Ar-PIs).5-8 The first one is “locally excited” (LE) transition that occurs between the molecular orbitals (MOs) located around the dianhydride moiety of PIs. The second one is “charge transfer” (CT) transition originating from the CT complexes formed between the dianhydride and diamine moieties. The lowest energy CT transition generally occurs between the highest occupied MO (HOMO) located around the diamine moiety and the lowest unoccupied MO (LUMO) located around the dianhydride moiety. The energy gap of this transition is essentially determined by

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Macromolecules, Vol. 44, No. 2, 2011

Chart 1. Molecular Structure of PIs and Optimized Geometries of PI Trimers Obtained by DFT Calculations at the Level of [B3LYP/6311G(d)]

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pressures, which indicates that the intramolecular CT band was not enhanced. The Sda of intramolecular CT band could be enhanced by an increase in the coplanarity between the benzene rings of the dianhydride and diamine moieties of PIs. Hence, such a conformational change at the imide-phenyl (>N-Ph) bond was not induced in s-BPDA/PDA by pressure. Recently, we have reported variations in the aggregation structures of PI chains in the ordered domains up to 8 GPa as a Communication.23 Though, the relationship between pressure-induced variations in the molecular aggregation structures and optical properties of PI films have not been examined so far. In this paper, the influence of variations in the molecular aggregation structures of PI chains to the UV/visible absorption properties, which were induced at very high pressures up to 8 GPa, are extensively examined using synchrotron-radiation WAXD, infrared absorption (IR), and UV/visible absorption spectroscopy. 2. Experimental Section

The molecular aggregation structures of PI chains have been mainly investigated using wide-angle X-ray diffraction (WAXD).10-19 For instance, Russell et al.11 reported that the aggregation structures of PMDA/ODA films ranged from amorphous structures to ordered crystalline structures, depending on the film thickness and preparation conditions. In general, PI films do not exhibit definitive crystalline diffraction peaks, which indicates the absence of large domains with three-dimensional positional order. Such ordered domains with mesomorphic order between crystalline and amorphous phase in the film were formed during thermal imidization and frozen at room temperature, which can be interpreted as liquid crystalline-like (LC-like) ordered domains.19 Pressure is a more suitable factor than temperature in perturbing the intermolecular interactions of polymer chains in the solid state. X-ray diffraction,20-23 infrared absorption (IR) spectra,24-26 UV/visible absorption and the fluorescence spectra27-31 of polymers have been measured under high pressure to examine the compression effects on their crystalline structure, and on their physical and optical properties. For instance, Lorenzen et al.21 reported variations in the bulk compressibility of crystalline poly(tetrafluoroethylene) (PTFE) by increasing the pressure using X-ray diffraction patterns. Convington et al.25,26 measured the pressure-induced shifts of the infrared absorption modes of poly(methyl methacrylate) (PMMA) and polycarbonate (PC), and they concluded that most of the free volume was removed after compression up to 2 and 1 GPa, respectively. Drickermer et al.28,30,31 reported variations in the steady-state and timedependent emission properties of neat polymer and polymer blends. Erskine et al.29 measured pressure-induced variations in the optical transmission spectra of PMDA/ODA PI up to 12 GPa, and concluded that intermolecular CT transitions exist in the PI because a significant bathochromic shift of the absorption edges was caused by applying pressure. Very recently, we reported pressure-induced variations in the absorption spectra of aromatic and semialiphatic PIs up to 400 MPa.32 A pressureinduced bathochromic shift was observed in the LE band, which is related to increases in the van der Waals interactions due to a decrease in interchain distances. In addition, a pressure-induced increase in absorbance was observed for the intermolecular CT absorption band of PMDA/ODA. The intensity of the CT absorption band depends on the overlap integral (Sda) between occupied MOs of electron-donating diamine moiety and unoccupied MOs of electron-accepting dianhydride moiety.32 Thereby, the increase in the intermolecular CT absorption can be readily explained by an increase in Sda owing to a decrease in the intermolecular distance. In contrast, in the case of s-BPDA/PDA, in which the intramolecular CT interaction is dominant, no increase in the CT absorption band was observed at elevated

Materials. Pyromellitic dianhydride (PMDA) purchased from Kanto Chemical Co., Inc. was dried and purified by sublimation under reduced pressure. 3,4,30 ,40 -Biphenyltetracarboxylic dianhydride (s-BPDA) from Wako Pure Chemical Industries, Ltd. was dried at 170 °C for 12 h under reduced pressure. p-Phenylenediamine (PDA) and 4,4-diaminodiphenyl ether (ODA) from Wako Pure Chemical Industries, Ltd. were recyrstallized from tetrahydrofuran, followed by sublimation under reduced pressure. 4,40 -Diaminocyclohexylmethane (DCHM), purchased from Tokyo Kasei Kogyo Co. Ltd. was recrystallized from n-hexane, followed by sublimation under reduced pressure. The content of trans-trans isomer in purified DCHM was estimated as 94% by 1H NMR spectrum.23 N,O-Bis(trimethylsilyl) trifluoroacetamide (99þ%, BSTFA) and N, N-dimethylacetamide (anhydrous, DMAc), purchased from Aldrich, were used without further purification. Preparation of Polyimide Films. The molecular structures of the PIs used in this study are shown in Chart 1. To investigate the effects of the rigidity of PI chains on their compressibility, three PI films, s-BPDA/PDA, having a pseudo-rigid-rod molecular structure, PMDA/ODA, having a rigid and bent molecular structure, and PMDA/DCHM, having a semiflexible structure, were studied. The precursors of aromatic PIs, poly(amic acid)s (PAAs), were prepared by mixing equimolar amounts of dianhydride and diamine in a DMAc solution under dry nitrogen. The PAA solutions were stirred at room temperature for 48 h. The precursors of PMDA/DCHM, poly(amic acid) silyleter (PASE), were prepared by the in situ silylation method reported by Matsumoto33 and Oishi.34,35 For instance, DCHM was dissolved in DMAc, stirred for a few minutes, and then a 1.05 molar amount of BSTFA was slowly added. An equimolar amount of dianhydride was then added and stirred at room temperature for 48 h to give a PASE solution. The trimethylsilylated amino groups of diamines can avoid salt formation between unreacted amino groups and the carboxyl groups of the dianhydride moieties. In general, PAA and PASE solutions become viscous after stirring for several hours, depending on the degree of polymerization and the rigidity of the molecular structures. The PI films were prepared by thermal imidization of the corresponding PAA or PASE precursors. The solutions were spin-coated onto silica substrates, followed by soft-baking at 70 °C for 1 h and subsequent thermal imidization in a one-step imidization procedure: the final curing conditions were 300 °C/ 1.5 h for the PMDA/DCHM thick film (thickness: ca. 20 μm) and 350 °C/1.5 h for the s-BPDA/PDA and PMDA/ODA thin films (thickness: ca. 1-2 μm). The heating rate was 4.6 °C/min from 70 °C to the final curing temperatures, and all curing procedures were conducted under nitrogen flow. Most PMDA/ DCHM chains should have an extended trans-trans configuration due to the high trans-trans content of DCHM, as described above. The PI thick films of s-BPDA/PDA and PMDA/ODA

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Figure 1. (a) Transmission X-ray diffraction patterns of s-BPDA/PDA, PMDA/ODA, and PMDA/DCHM PI thick films (thickness: ca. 20-25 μm) at atmospheric pressure. (b) Magnified representations of transmission X-ray diffraction patterns of s-BPDA/PDA, PMDA/ODA, and PMDA/ DCHM PI thick films at atmospheric pressure. Thin solid and dotted lines respectively represent the diffraction peaks from the ordered domains and the amorphous matrix fitted by Gaussian broadening functions.

(product names: Upilex-S and Kapton-V; thickness: ca. 25 μm, Chart: 1), were kindly supplied by Ube Industries, Ltd. and DuPont-Toray Co., Ltd., respectively. The PI thin films were used for measurements of the micro-IR and the UV/visible absorption spectra, and the thick films for measurements of the transmission X-ray diffraction patterns and UV/visible absorption spectra. Measurements. To generate pressure up to 8 GPa, the PI films were loaded into a diamond anvil cell (DAC, Syntech Co. Ltd.) equipped with 600-μm culet synthetic diamond anvils (Sumicrystal type-IIa, Sumitomo Electric Hardmetal Corp.). Type-II diamonds (Sumitomo Denko Co., Ltd.) were used because of the high transparency in the range between the ultraviolet region and the infrared region. After a 200 μm thick stainless steel gasket was indented to a thickness of 50 μm, a 230 μm diameter hole was drilled in the center of the indentation using an laser beam (JL-SP0100, DEOS., Ltd.). The standard ruby fluorescence technique was used to determine the pressure inside the sample room.36 Ruby fluorescence spectra were measured with HR4000 spectrometer (Ocean Optics Co., Ltd.; grating, H11; slit width, 5 μm) in the range of 650-740 nm. Diode Pumped Solid State laser (Shanghai Dream Lasers Technology Co., Ltd.) was used as an excitation light source. Because the size of the sample chamber was only 230 μm in diameter, the transmission X-ray diffraction measurements were performed with a BL40B2 beamline at the Japan Synchrotron Radiation Research Institute (SPring-8) using an image-plate as the detector, and silicone oil as the pressure medium. The wavelengths of the X-ray were 0.7 A˚ for s-BPDA/PDA and 0.8 A˚ for PMDA/ODA and PMDA/DCHM. The diffraction peaks assignable to silicone oil and diamond anvil cell were removed from the diffraction patterns by subtracting a reference pattern measured without a sample. The UV/visible absorption spectra of PI films at atmospheric pressure were measured by a Hitachi U-3500 spectrophotometer. The UV/visible absorption spectra of the PI films loaded in a DAC at elevated pressures were measured with a multichannel CCD spectrometer (PMA-11, Hamamatsu Photonics Co., Ltd.) in the range of 200-950 nm (C7473-36), using objective lens (cutoff wavelength: 320 nm) designed for measurement in the ultraviolet region to focus the light on the sample room, an Xe lamp (Hamamatsu Photonics Co., Ltd.) as a light source, a pinhole for adjustment to light intensity, and

silicone oil as the pressure medium. Resolution of the UV/visible absorption spectra was ∼0.2 nm. The infrared absorption spectra of the PI films loaded in a DAC at elevated pressures were measured by an IRT-3000 infrared microscope (JASCO Co., Ltd.). A liquid nitrogencooled mercury-cadmium-tellurium (MCT) detector was used to detect the infrared radiation. The scans were performed with a nominal resolution of 2 cm-1 over the range of 4004000 cm-1. KBr was used as the pressure medium. Quantum Chemical Calculation. The density functional theory (DFT) with the three-parameter Becke-style hybrid functional (B3LYP)37-39 was adopted for the calculations of the electronic structures and spectroscopic properties of the PIs and their source materials. The 6-311G(d) basis set was utilized for the geometry optimizations and the IR spectra of the imide compounds, and the 6-311þþG(d,p) basis set was used for calculating the oscillator strengths. The calculated wavenumbers were scaled down by multiplying by a single factor of 0.98 for correcting vibrational anharmonicity, basis set truncation, and the neglected part of the electron correlation. For reproducing the shapes of the experimental absorption spectra, each calculated transition was replaced by a Gaussian broadening function with a half-width at half-maximum (HWHM) of 0.3 eV. The calculated absorbance was represented by an oscillator strength divided by a van der Waals volume (nm3) of molecules.40-42 The van der Waals volumes were calculated based on Slonimski’s method43 using the optimized geometries, in which van der Waals radii of atoms reported by Bondi44 were used. All calculations were performed with the Gaussian-03 D.0245 program package installed in the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology.

3. Results and Discussion 3.1. Variations in the Aggregation Structure of PIs. A. Transmission X-ray Diffraction Patterns at Atmospheric Pressure. Figure 1a shows the transmission X-ray diffraction patterns of three kinds of 20-25 μm thick PI films measured at atmospheric pressure, and Figure 1b shows the results of peak fitting using Gaussian broadening functions. The PI film of s-BPDA/PDA, having a pseudo-rigid-rod molecular structure, exhibits peaks at

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Figure 3. (a) Variations in the strains (ε) of chain packing peaks by applying pressure. (b) Variations in the linear compressibilities (κ) along the (110) peak of s-BPDA/PDA and the ch-pack peak of PMDA/ DCHM by applying pressure.

Figure 2. Variations in the X-ray diffraction patterns for (a) s-BPDA/ PDA, (b) PMDA/ODA, and (c) PMDA/DCHM PI thick films by applying pressure. The negative peaks indicated with asterisks (/) arise from nonperfect spectral subtraction of the pressure medium (silicone oil).

q = 7.9 nm-1 (d = 7.9 A˚) and 19.8 nm-1 (3.2 A˚). This PI was reported to form a crystalline structure based on an orthorhombic unit cell, and these peaks are respectively attributable to the diffractions of (004) and (0010) which represent the periodic structure along the PI main chains (c-axis).14,16 In addition, the peaks observed at 12.7 nm-1 (4.9 A˚) and 17.8 nm-1 (3.5 A˚) were indexed as (110) and (210), respectively. These represent the intermolecular ordering in the directions of a- and b-axes of the PI chain packing, which are perpendicular to the PI main chains (c-axis). Because s-BPDA/PDA exhibited the diffraction peaks representing orderings along a-, b- and c-axes, this PI should have a highly ordered crystalline-like domain.14,16 Also, PMDA/ODA, having a bent ether (-O-) linkage in the main chain, exhibits diffraction peaks at q = 4.0 nm-1 (d = 15.6 A˚) and 7.9 nm-1 (7.9 A˚). These peaks were respectively indexed as (002) and (004), in accordance with previous X-ray measurements,10 which indicate that a curved structure consisting of two repeating units corresponds to the periodic structure along the PI chains of PMDA/ODA (see Chart 1). The peaks observed at 10.0 nm-1 (d = 6.3 A˚), 10.7 nm-1 (5.9 A˚), 11.6 nm-1 (5.4 A˚), 15.6 nm-1 (4.0 A˚), and 18.7 nm-1 (3.4 A˚) were indexed as (101), (102), (103), (010), and (110), respectively.10 Because PMDA/ODA exhibited the diffraction peaks which represent the ordering along a-, b-, and c-axes, this PI also should have a crystalline-like ordered domain. In addition, PMDA/ DCHM exhibits a sharp peak at q = 3.9 nm-1 (d = 16.1 A˚)

and a broad peak at 12.4 nm-1 (5.1 A˚). The former diffraction peak was indexed as (002),19,23 and one pitch of the curved molecular structure consists of two repeating units similar to PMDA/ODA. Although no diffraction peaks assignable to the crystalline region were observed in PMDA/DCHM, the bandwidth of the scattering at 12.4 nm-1 is much narrower than those of amorphous halos.11 Thereby, this peak can be characterized as scattering representing the PI chain packing (ch-pack) in the LClike ordered region.19 As shown in Figure 1b, the diffraction peaks of the three PIs are significantly overlapped by amorphous halos, as indicated by the dotted lines, which indicates the relatively low proportions of the ordered regions. B. Variations in Interchain Distance. Figure 2 shows the pressure-induced variations in the diffraction patterns of 20-25 μm thick PI films. Figure 3a and 4a show the variations in strains (ε) of each peak observed in Figure 2, and Figure 4b shows the magnified representation of the strains of the (00l ) peaks of aromatic PIs. In this study, strain is defined as ε(hkl ) = Δd/d0,(hkl), where d0,(hkl) is the d-spacing of the (hkl ) plane at 0 GPa and Δd the variation in d-spacing at elevated pressures. By applying pressure up to 6 GPa, the d-spacing of the (110) peak, (d(110)), of s-BPDA/PDA and the d(ch-pack) of PMDA/DCHM were significantly decreased by 11 and 14%, respectively, whereas the d(004) of s-BPDA/PDA and the d(002) of PMDA/ODA were slightly decreased by 0.8 and 1.0%, respectively. This clearly indicates that the periodic structures perpendicular to the PI main chains are much more easily compressed than those along the PI main chains. Figure 3b depicts the variations in the linear compressibilities (κ) for the diffraction peak of (110), κ110 of s-BPDA/PDA and those for the peak of the chpack, κch-pack of PMDA/DCHM. The κ values were estimated as the numerical first derivatives of strain with respect to pressure (κ = ∂ε/∂P). It should be noted that the κch-pack of PMDA/DCHM are significantly larger than those of κ110 of s-BPDA/PDA below 2 GPa. Dlubek et al.46 reported the pressure dependence of compressibility for fluoroelastomeric copolymers of tetrafluoroethylene and perfluoro(methyl vinyl

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Figure 4. (a) Variations in the strains of the (00l ) peaks of PIs by applying pressure. (b) Magnified representation of the strains of the (00l ) peaks of aromatic PIs.

ether) as well as vinylidene fluoride and hexafluoropropylene by pressure-volume-temperature experiment (PVT, P = 0.1-200 MPa at 22.5 °C) and positron annihilation lifetime spectroscopy (PALS, P = 0.1-448 MPa at 22.5 °C). They reported that the compressibility of free volume is 1 order of magnitude larger than that of the total volume, and that specific occupied volume showed a definite compressibility with a value similar to that of polymer crystals. Hence, the large κch-pack values observed below 2 GPa indicate that semialiphatic PMDA/DCHM contains larger amounts of interchain free volume in the LC-like ordered regions, and that free volume is more compressible at lower pressures. In contrast, the much smaller κ110 values observed below 2 GPa indicate that fully aromatic s-BPDA/PDA contains a smaller amount of compressible free volume in its crystalline-like ordered region. Furthermore, the trend of significant decrease in the κch-pack in the former was completed at around 2 GPa, and the values of κch-pack and κ110 come close to each other above that pressure. This suggests that the easily compressible free volume in PMDA/DCHM almost disappeared at this pressure, and both the semialiphatic and aromatic PIs possess similar and dense molecular packing structures at the higher pressures. Comparable phenomena were also observed in the variations of free volume in PMMA and PC at high pressures.25,26 C. Variations in Periodic Length and Bond Length of PI Chains. In this section, the molecular structure dependence of pressure-induced variations in the periodic length of PI chains is investigated. As shown in Figure 4b, the d(004) value of s-BPDA/PDA was decreased by only 0.8% up to 6 GPa. Since s-BPDA/PDA has a pseudo-rigid-rod structure without bent linkages, the conformational changes should have little effect on the variation of d(004). Hence, the gradual decrease in d(004) could be mainly attributable to the shrinkage of bond lengths. For several kinds of polymers, their IR and Raman spectra have been measured at high pressures to examine the variations in intramolecular structures caused by compression.24-26 The most noticeable changes in those spectra were the pressure-induced higherfrequency shifts of the stretching vibration peaks, and they

Figure 5. (a) Calculated IR peak shifts of model compound of s-BPDA/PDA (m(BP/PD)) for the rocking vibration of the aromatic ring of the diamine moiety as a function of reduction in bond lengths. (b) Variations in the IR spectra for the vibration of the aromatic ring of the diamine moiety at elevated pressure for s-BPDA/PDA thin film. (c) Comparison of the strains estimated from the peak shift of (002) for s-BPDA/PDA thick film (2) and the shortening of bond lengths estimated from IR peak shift for s-BPDA/PDA thin film (1) by applying pressure.

were attributed to the shrinkage of bond lengths associated with the anharmonicity of molecular vibrations. In this study, the variations in bond lengths were evaluated from the pressure-induced IR peak shifts observed for PI thin films by comparison with the IR peak shifts calculated for model compounds with shortened bond lengths. For simplicity, all the bond lengths of the model compounds were reduced at a constant proportion. The C-C stretching vibration of the benzene ring in the diamine moiety (Phstretch) was used for the evaluation of bond length shrinkage because of its insensitivity to variations in bond angles and dihedral angles. As shown in Figure 5a, the calculated IR peak position of Ph-stretch (νcal) of a model compound for s-BPDA/PDA (m(BP/PD)) is linearly shifted to higher wavenumbers with decreases in the bond lengths. The shrinkage of bond lengths by 1% induces a peak shift of 23 cm-1. On the other hand, the experimental IR peak position of Phstretch (νexp) of a 2 μm thick film of s-BPDA/PDA was shifted to a higher wavenumber by 18.6 cm-1 by applying pressure up to 6.3 GPa (see Figure 5b). The variations in IR spectra in a wider range for s-BPDA/PDA are shown in Figure S1 (Supporting Information). By combining parts a and b of Figure 5, the variations in bond lengths, i.e., the strains caused by compression, εIR, were estimated by the

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Figure 6. (a) Calculated IR peak shifts of model compound of PMDA/ ODA (m(PM/OD)) for the rocking vibration of the aromatic ring of the diamine moiety as a function of reduction in bond lengths. (b) Variations in the IR spectra for the vibration of the aromatic ring of the diamine moiety at elevated pressure for PMDA/ODA thin film. (c) Comparison of strains estimated from the peak shift of (002) for PMDA/ODA thick film (2) and the shortening of the bond lengths estimated from the IR peak shift for PMDA/ODA thin film (1) by applying pressure.

following equation: εIR ¼ Δω=k

ð1Þ

where Δω is the pressure-induced peak shift for the Phstretch vibration, and k is the gradient of the fitted line depicted in Figure 5a. The k value corresponds to the peak shift (cm-1) caused by variation in bond length (%). As shown in Figure 5c, the εIR value of s-BPDA/PDA was decreased by 0.7% by applying pressure up to 6 GPa, which coincides well with the experimental strain estimated for the d(004) peak (ε004 = 0.8%) of s-BPDA/PDA. Note that the slope of the linear pressure-εIR relation below 1 GPa (-0.072 10-2 GPa-1) is significantly smaller than that directly estimated from the pressure-ε(004) relation (-0.173 10-2 GPa-1). Hence, in the lower pressure range, the shrinkage of C-C bond lengths is smaller than that of the periodic lengths along the c-axis by ca. 60%, which indicates that some structural changes in the bond angles and/or dihedral angles in the pseudo rigid-rod molecular structure of s-BPDA/PDA were induced by pressure. Moreover, the slope of the pressure-εIR relation below 1 GPa is slightly smaller than that at above 1 GPa (-0.093 10-2 GPa-1). This suggests that the compression stress induced on the

skeletal structure of the PI could be partly released by the reduction of free volume and some structural changes in the bond angles and/or dihedral angles below 1 GPa, which could circumvent the ’stress concentration’ on the phenyl C-C bonds in the PI chains. In contrast, the slopes of both the pressure-strain plots estimated from the IR band and the d(004) values are close to each other (-0.093 10-2 GPa-1 for ε(004) and -0.103 10-2 GPa-1 for εIR) above 2 GPa. This indicates that most of the pressured-induced structural changes along the c-axis are attributable to the shrinkage of the covalent bonds in s-BPDA/PDA. The ab initio allelectron periodic Hartree-Fock study for the hydrostatic compression of pentaerythritol tetranitrate indicated that the C-C bond lengths decreased by 1.12% by applying pressure up to 6.5 GPa,47 which is close to the variation in the aromatic C-C bond length estimated from our results. In the case of PMDA/ODA, the d(002) value was not decreased but increased by 0.3% by applying pressure up to 0.4 GPa (see Figure 4b). At a glance, the increase in the periodic length along the PI main chains looks curious, but it should be noted that this PI includes a bent diphenyl ether (-O-) linkage in the main chain. The extended chain of this PI forms not a rigid-rod structure but a zigzag structure. When the bond angle at the ether group (θC-O-C) was increased by pressure, the projected length of the repeating unit along the main chain could be elongated (see Chart: 1). In other words, when compression stress is concentrated on the ether linkage, an increase in θC-O-C led to a stretch of the pitch consisting of two repeating units. With further compression up to 6 GPa, the d(002) value was decreased by 1.0%, as expected. Figure 6a shows the calculated IR peak positions for the Ph-stretch (νcal) of a model compound for PMDA/ODA (m(PM/OD)) caused by the shortening of all the bond lengths. The νcal values are linearly shifted to higher wavenumbers by decreasing the bond lengths, the same as the case of m(BP/PD) (Figure 5a). However, as shown in Figure 6b, the experimental peak position (νexp) observed for a 2 μm thick PMDA/ODA film exhibited little changes below 1 GPa, under which the slope of the pressure-εIR is as small as -0.03 10-2 GPa-1, whereas remarkable broadening was observed for the IR band. This indicates that the first stage of compression up to 1 GPa did not effectively shorten the average C-C bond lengths, but inhomogeneous structural and conformational changes were induced in addition to the reduction of interchain free volume. With further pressure up to 6 GPa, the IR peak was shifted to a higher wavenumber by 13.6 cm-1. As shown in Figure 6c, the slope of the pressure-εIR relation above 1 GPa (-0.092 10-2 GPa-1) is almost the same as that of the s-BPDA/PDA above 1 GPa (-0.093 10-2 GPa-1), which suggests a similar amount of shrinkage for aromatic C-C bond lengths in PMDA/ODA. In contrast, the former is significantly smaller than that directly estimated from the pressure-ε(002) relation (-0.196 10-2 GPa-1). This is explainable as follows. First, in contrast to the pressure range below 0.4 GPa, a narrowing of the bond angle, θC-O-C, was induced by pressure, and thus the projected length of the repeating unit was shortened up to 6 GPa. Second, the compressibility of the covalent bond varies depending on the strength of each bond, and the ether linkage in PMDA/ODA could be easily compressed compared with the aromatic rings. This is supported by the fact that the IR band of asymmetric stretching vibration of the C-O-C bond (1240 cm-1) is observed at a smaller wavenumber than that of the Ph-stretch (1500 cm-1), as shown in Figure S2 (Supporting Information). In contrast to these aromatic PIs, the d(002) of semialiphatic PMDA/DCHM was significantly decreased by 5.3%

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Figure 7. UV/visible absorption spectra of PIs at atmosphere pressure: (a) thin films (thickness: ca. 1 μm); (b) thick films (thickness: ca. 20-25 μm).

by increasing pressure up to 6 GPa, as shown in Figure 4a. The large slope of the variation in ε(002) gradually decreased above 2 GPa, but it did not become constant, even at the highest pressure of 8.2 GPa. Moreover, as shown in Figures S3 and S4 (Supporting Information), one of the C-N stretching vibration peaks at 1376 cm-1 of PMDA/DCHM thin film disappeared by increasing the pressure, which may indicates some structural and/or conformational changes by compression at elevated pressures. Christian et al.48 reported that effective conversion occurs for halogenated cyclohexanes from equatorial to axial forms by applying pressure. Thereby, such a significant decrease in d(002) could be accompanied by drastic conformational changes around the -CH2- linkage and/or the six-member cyclohexane structure in the diamine moiety of PMDA/DCHM. 3.2. Variations in the UV/Visible Absorption Properties of PIs. A. Absorptions at Atmospheric Pressure. As stated in the introduction, the optical properties of PI films are affected not only by the chemical and electronic structures of repeating units but also by intermolecular interactions. Parts a and b of Figure 7 show the UV/visible absorption spectra of ca.1 μm thick films of s-BPDA/PDA and PMDA/ ODA, and 20-25 μm thick films of s-BPDA/PDA, PMDA/ ODA and PMDA/DCHM PIs measured at atmospheric pressure. The former and the latter films are called as ‘thin films’ and ‘thick films’, respectively. The absorption bands in the spectra were assigned in accordance with the previous results.5,32 The thin film of s-BPDA/PDA shows a strong LE absorption at wavelengths (λ) shorter than 400 nm without apparent tailing near the absorption edge, whereas the thick film exhibits extensive tailing from 500 to 800 nm. Since the TD-DFT calculations indicate that a CT absorption band should appear at around 400 nm in s-BPDA/PDA,5 the observed tailing is assignable to CT absorption. In contrast, the PMDA/ODA thin film shows a strong LE absorption at shorter wavelengths (λ < 360 nm) accompanied by extensive tailing to 500 nm, which is attributable to CT absorption. Furthermore, the PMDA/ODA thick film shows strong CT absorbance at longer wavelengths (λ > 520 nm), which should be due to apparent bathochromic effect caused by

Figure 8. Pressure dependence of (a) UV/visible absorption spectra and (b) absorption differential spectra for s-BPDA/PDA thin film. (c) Pressure dependence of UV/visible absorption spectra for s-BPDA/ PDA thick film. (d) Pressure-induced variations in color of s-BPDA/ PDA thick film.

the thickening of the film. Furthermore, the semialiphatic PMDA/DCHM thick film shows strong absorption at λ < 380 nm with weak tailing to 520 nm, as indicated by the arrow in Figure 7b. Lee et al.4 have reported that PMDA/ DCHM exhibited a weak fluorescent peak at 520 nm with an excitation peak at around 410 nm, which is assignable to CT fluorescence. We have demonstrated that intramolecular CT complexes are very unlikely to be formed in this PI based on TD-DFT calculations, and the fluorescence peak observed at 495 nm with an excitation peak at 426 nm is attributable to emission from the excited intermolecular CT complexes.5 Therefore, the tailing observed at longer wavelengths could be assignable to intermolecular CT absorption band. B. Variations in UV/Visible Absorption Spectra at High Pressure. The influence of the variations in the aggregation structures of PI chains on their optical properties under high pressure were investigated using UV/visible absorption spectroscopy (Figures 8-10). The appearance of the thick films of s-BPDA/PDA, PMDA/ODA and PMDA/DCHM at elevated pressures are shown in Figures 8d, 9d, and 10c, respectively. Apparent densification in color from pale yellow through reddish brown to blackish brown was observed

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Figure 10. Pressure dependence of (a) UV/visible absorption spectra and (b) absorption differential spectra for PMDA/DCHM thick film. (c) Pressure-induced variations in color of PMDA/DCHM thick film.

Figure 9. Pressure dependence of (a) UV/visible absorption spectra and (b) absorption differential spectra for PMDA/ODA thin film. (c) Pressure dependence of UV/visible absorption spectra for PMDA/ ODA thick film. (d) Pressure-induced variations in color of PMDA/ ODA thick film.

for fully aromatic PIs, but no apparent color changes were observed for the semialiphatic PI. Figure 8 illustrates the pressure-induced variations in (a) the absorption and (b) differential spectra of s-BPDA/PDA thin film and (c) the absorption spectra of s-BPDA/PDA thick film. The reference for the differential spectra is the spectrum measured at atmospheric pressure (0 GPa). As seen in Figure 8, pressureinduced bathochromic shifts were clearly observed in the LE absorption band of s-BPDA/PDA. The intercept wavelengths of the LE absorption bands at an optical density (O.D.) of 1.0 (λOD1) for the thin and thick films were respectively shifted by 67 nm (0.46 eV) and 83 nm (0.45 eV) to longer wavelengths by increasing pressure from 0 to 7 GPa. One possible cause for the bathochromic shifts is a reduction in the band gap between the ground and excited states due to enhanced van der Waals interactions.5,32 The other cause is an apparently longer wavelength shift caused by absorption enhancement. For the thin film of s-BPDA/ PDA, no significant enhancement of the CT absorption band at around 400 nm was observed in the differential spectra (Figure 8b). Moreover, it is interesting to note that the absorbance of the tailing of the s-BPDA/PDA thick film

Figure 11. Calculated absorption spectra of a model compound of PI (N-phenyl phthalimide) as a function of the dihedral angles at the imidephenyl bond (ω).

from 500 to 800 nm was decreased by applying pressure up to 0.3 GPa (Figure 8c), which indicates that the CT interactions were weakened under the pressure. We have reported that intramolecular CT interactions are dominant in this PI rather than intermolecular CT.32 Figure 11 shows the calculated absorption spectra of N-phenyl-phthalimide with different dihedral angles at the imide bond (ω). By increasing ω, which apart from the coplanarity at the imide-phenyl (>N-Ph) bond, the intramolecular CT absorption band appearing at 400 nm was gradually shifted to shorter wavelengths by reducing its intensity. Hence, the intramolecular CT interaction could be weakened by conformational changes, such as an increase in the dihedral angle at the imide-phenyl bond, and thereby the decrease in CT absorption in s-BPDA/PDA is explainable by conformational changes caused by pressure. Furthermore, the absorption tailing exhibited only bathochromic shifts without enhancement of the absorbance above 0.3 GPa. These facts indicate

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Figure 12. Pressure-induced energy shifts estimated from the intercept wavelength (optical density (O.D.) at 1.0) of the LE absorption of s-BPDA/PDA and PMDA/DCHM thick films.

that the pressure-induced variations in the intra- and intermolecular CT interactions had little influence on the λOD1 value, and the observed bathochromic shift of λOD1 is mainly due to enhancement of the van der Waals interactions. Figure 9 illustrates the variations in (a) the absorption and (b) differential spectra of PMDA/ODA thin film, and (c) the absorption spectra of PMDA/ODA thick film by increasing pressure up to 7 GPa. The λOD1 of the LE absorption of the PMDA/ODA thin film (Figure 9a) was shifted to longer wavelengths by 55 nm (0.47 eV), and the CT absorption band of the PMDA/ODA thick film was shifted by 154 nm (0.64 eV) from 0 GPa to the highest pressure (Figure 9c). In the differential spectra of the PMDA/ODA thin film (Figure 9b), a significant increase in absorbance was observed at around 450 nm, which corresponds to the enhancement of CT absorption. For PMDA/ODA, the WAXD and IR spectra demonstrated that the bond angle at the C-O-C linkage could be increased by applying pressure up to 0.4 GPa, but decreased at higher pressures. However, the calculated absorption spectra suggested that variations in the ether (C-O-C) bond angle have little effect on the intramolecular CT interactions (Figure S5, Supporting Information). Moreover, if a conformational change similar to s-BPDA/PDA occurred in this PI, an increase in ω should weaken the intramolecular CT interactions.32 Thereby, the pressure-induced increase in the CT absorption appearing at around 450 nm is attributable to the enhancement of the intermolecular CT interactions. Figure 10 illustrates the pressure-induced variations in (a) the absorption and (b) differential spectra of PMDA/ DCHM thick film. The λOD1 of the LE absorption band of PMDA/DCHM thick film was shifted by 21 nm (0.19 eV) by increasing pressure up to 6 GPa (Figure 10a). In the differential spectra of the same film (Figure 10b), the pressureinduced enhancement of the intermolecular CT absorption band at about 440 nm was 2 orders of magnitude smaller than that of the CT absorption of PMDA/ODA thick film. Thereby, the observed bathochromic shift of λOD1 should originate from the enhancement of the van der Waals interactions in the same way as for s-BPDA/PDA, not by the enhancement of CT absorption like PMDA/ODA. C. Pressure-Induced Shifts of the LE Absorption Bands at High Pressure. Figure 12 illustrates the pressure-induced energy shifts (in eV) of the λOD1 values, which represent the LE absorption of s-BPDA/PDA and PMDA/DCHM thick films. As shown in the figure, both bands are significantly shifted to longer wavelengths up to 1 GPa, whereas the slopes of the variations slightly decrease above 1 GPa. In particular, this trend is remarkable for PMDA/DCHM. This phenomenon accords with the trend of the decrease in the interchain distance occurring in the LC-like ordered regions of PMDA/ DCHM below 1-2 GPa, as observed by synchrotron

Figure 13. Pressure dependence of optical absorbance at λ=450 nm in the differential spectra for (a) PMDA/ODA thin film and (b) PMDA/ DCHM thick film.

WAXD (see Figure 3). The WAXD patterns in Figure 1b also indicate that these PIs contain significant amounts of amorphous regions having large free volumes, as represented by the amorphous halos (dotted lines). As mentioned above, Dlubek et al.46 reported that the free volume of amorphous polymers was preferentially decreased by applying pressure, compared to the specific occupied volume. In addition, Covington et al.25,26 reported that most of the free volumes in PMMA and PC were removed after compression up to 1 GPa. Therefore, the significant bathochromic shift of s-BPDA/ PDA and PMDA/DCHM below 1 GPa (Figure 12) is attributable to the compression of interchain free volume in the amorphous region as well as in the LC-like ordered region. The slope of the variations in PMDA/DCHM dropped by 60% around 1 GPa, whereas that of s-BPDA/ PDA dropped by 40%. The smaller decrease in the latter is attributable to the smaller variation in the interchain distance of the crystalline-like structure in s-BPDA/PDA than that of the LC-like ordered structure of PMDA/DCHM, which is characterized by smaller linear compressibility along the (110) plane of s-BPDA/PDA below 2 GPa (see Figure 3b). D. Pressure-Induced Increase in CT Absorption at High Pressure. The magnitude of the increase in CT absorption at elevated pressures also depends on the decrease in interchain distances. Parts a and b of Figure 13 illustrate the pressureinduced increase in absorbance at 450 nm estimated from the differential spectra PMDA/ODA thin film (Figure 9b) and PMDA/DCHM thick film (Figure 10b), respectively. The slope values below 1 GPa are larger than those above that pressure, which is attributable to the enhancement of the intermolecular interactions via compression of interchain free volume in the amorphous and LC-like ordered regions below 1 GPa, as described above. The slopes of the increase in absorbance at 450 nm in the differential spectra go down above 1 GPa, which suggests that the easily compressible components of free volume in PMDA/ODA and PMDA/DCHM almost disappeared at 1 GPa, which is consistent with the variations in the d(ch-pack) of PMDA/DCHM (Figure 3).

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As seen above, the pressure-induced variations in the LE absorption bands and intermolecular CT absorption bands in PI films below 1 GPa are closely related to the enhancement of the intermolecular interactions via the significant decrease in the interchain free volume, as indicated by WAXD. In consequence, the pressure-induced variations in the optical properties accord well with those in the molecular aggregation structure of PIs.

Takizawa et al.

Acknowledgment. The synchrotron radiation experiments were performed with a BL40B2 beamline with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.2008B-1434, 2009A-1348, 2009B-1306). The authors thank Kei Hirose and Ryosuke Sinmyo at the Department of Earth and Planetary Sciences, Tokyo Institute of Technology, for advice on the diamond anvil cell. We also thank Kazuyuki Horie at JASRI and Masaki Kakiage at the Department of Chemistry and Materials Science, Saitama University, for helpful discussions.

4. Conclusion The wide-angle X-ray diffraction patterns, the infrared absorption (IR) and the UV/visible absorption spectra of three kinds of fully aromatic and semialiphatic polyimides (PIs) were measured under high pressure up to 8 GPa using a diamond anvil cell. Semialiphatic PMDA/DCHM exhibited a significant decrease in d(ch-pack) below 2 GPa, which is attributable to an appreciable decrease in the interchain free volume. In contrast, fully aromatic s-BPDA/PDA showed a much smaller decrease in the interchain distance, d(110), which is due to the highly ordered packing structure of the PI chains. The slope of the pressure-strain plots of s-BPDA/PDA, estimated from d(004) above 1 GPa, is almost the same as that from the C-C stretching vibration of the aromatic ring of the diamine moiety (ring C-C stretch), which indicates that the decrease in d(004) above 1 GPa is attributable to the reduction in bond lengths. In contrast, the d(002) of PMDA/ ODA, having an ether linkage in the main chain, abruptly increased up to 0.4 GPa, whereas the ring C-C stretch peak exhibited no shift up to 1 GPa. This is explainable by a widening of the bond angle by concentrated compression stress at the ether linkage without significant reduction in the bond lengths. On the contrary, d(002) exhibited a pronounced decrease above 1 GPa as compared to the strain estimated from the ring C-C stretch peak shift of PMDA/ODA thin film. This should be due to a narrowing of the bond angle at the ether group by further compression accompanied by a slight reduction in the bond lengths. Moreover, the d(002) of PMDA/DCHM, having a methylene linkage and two cyclohexane rings in the main chain, significantly decreased at elevated pressures, which should be due to the structural (conformational) change in the flexible diamine moiety. The locally excited (LE) absorption bands of fully aromatic and semialiphatic PIs shifted to longer wavelength by applying pressure, which is attributable to an enhancement of the van der Waals interactions caused by a decrease in the interchain distances. The absorbance of charge transfer (CT) band of s-BPDA/ PDA decreased up to 0.3 GPa, which originated from a decrease in the intramolecular CT interaction caused by some structural changes, such as the decrease in the coplanarity between the dianhydride and diamine moieties. On the other hand, the CT absorption of PMDA/ODA was significantly enhanced, and the CT absorption of PMDA/DCHM was slightly enhanced by increasing pressure, both of which are caused by the enhancement of the intermolecular CT interactions. In addition, more pronounced bathochromic shifts of LE bands was observed for s-BPDA/PDA and PMDA/DCHM, and large enhancement of intermolecular CT absorption were observed for PMDA/ODA and PMDA/DCHM below 1 GPa, which is attributable to the significant decrease in interchain distance via compression of free volume as indicated by WAXD. Overall, it has been indicated from very high pressure experiments that the optical properties and the aggregation structure of PIs are related in a linear manner. These experimental results lead to a guiding principle to reduce the CT interactions in PIs: the incorporation of bent and/or bulky structures in the main chain, which effectively enlarge the interchain distance, and thus lower the degree of molecular aggregation. This is beneficial for developing a new class of thermally stable and highly transparent/colorless aromatic polyimides.

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Relationship between Molecular Aggregation Structures and Optical

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