Pressure-Induced Changes in Crystalline Structures of Polyimides

Jun 2, 2014 - ABSTRACT: Variations in the crystalline structures of polyimides (PIs) were analyzed under high pressures up to 8. GPa using wide-angle ...
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Pressure-Induced Changes in Crystalline Structures of Polyimides Analyzed by Wide-Angle X‑ray Diffraction at High Pressures Kazuhiro Takizawa, Hiroshi Fukudome, Yukiko Kozaki, and Shinji Ando* Department of Chemistry & Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Variations in the crystalline structures of polyimides (PIs) were analyzed under high pressures up to 8 GPa using wide-angle X-ray diffraction. The compressibilities along the polymer chain axis (c-axis) of rigid-rod PIs increased with an increase in the number of phenyl rings in the diamine moiety (PMDA/PPD < PMDA/BZ < PMDA/DATP). This could be due to an increased shrinkage of the C−C bond lengths between the phenyl rings and/or a pressure-induced deformation of the periodic structure associated with changes in bond angles and dihedral angles. In contrast, PMDA/ODA, having an ether linkage, showed an increase in the lattice parameter along the c-axis up to 0.8 GPa, which could be due to a widening of the ether bond angle. Moreover, PMDA/PPD showed isotropic compression along interchain directions, whereas PMDA/DATP and PMDA/ODA showed anisotropic compression along the cofacial stacking direction, which resulted in the larger volumetric shrinkages of the latter PIs.

1. INTRODUCTION Fully aromatic polyimides (PIs) are well-known high-performance engineering plastics that exhibit outstanding physical and chemical properties: thermal and chemical stability, flame resistance, radiation resistance, mechanical strength, and flexibility.1 PI films prepared from pyromellitic dianhydride and 4,4′-diaminodiphenyl ether (PMDA/ODA) are widely commercialized as Kapton. The aggregation structures and orientation of PI chains in solid films have been investigated because the thermal, mechanical, electric, and optical properties of PIs are significantly influenced by these structural characteristics. For example, PI films with appreciable chain orientation along the in-plane direction exhibit extraordinarily large inplane/out-of-plane birefringence (Δn)2 and anisotropy in linear thermal expansion.3 Hasegawa et al.4 reported that the fluorescence intensity of a semialiphatic s-BPDA/CHDA PI prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and trans-1,4-cyclohexanediamine decreased upon increasing the imidization temperature (Ti), which suggests that the aggregation structures accompanying intermolecular interactions such as π−π stacking and charge transfer (CT) interactions significantly affect the optical properties of PIs. The molecular aggregation structures of PI chains have been mainly investigated using wide-angle X-ray diffraction (WAXD).5−23 Russell et al.8 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. For example, the positional order along the interchain direction is higher, and the © 2014 American Chemical Society

d-spacing corresponding to the chain axis is larger at the surface than in the bulk, which indicates that the PMDA/ODA chains have a more ordered structure with a more planar zigzag conformation near the surface. 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 nematic and smectic liquid crystalline-like (LC-like) ordered domains.7,20 On the other hand, several research groups have examined the crystalline or ordered structures of PIs having high crystallinity. Martin et al.24 examined the morphology of PMDA/ODA single crystals grown from a 1.4 wt % N-methyl-2-pyrrolidone (NMP) solution of a precursor poly(amic acid) (PAA) using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Several defect regions were found in the orthorhombic lattice of PMDA/ODA with γ ranging from 81° to 99°. This indicates that the crystallites exhibit local fluctuations in the packing geometry, which leads to a pseudo-orthorhombic lattice. Okuyama et al.11 analyzed highly crystalline fibers of poly(4,4′-diphenylene pyromellitimide) (PMDA/BZ) by wet-spinning NMP solutions of PAA, followed by thermal and chemical imidization. Kajiyama et al.12 obtained Received: January 31, 2014 Revised: March 27, 2014 Published: June 2, 2014 3951

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highly crystalline particles of aromatic PIs by heating NMP solutions of PAAs at 180 °C and then washing with acetone and water, followed by heat treatments at 100 and 400 °C. More recently, Kimura et al.17,20,21,23 reported that interesting morphologies were formed for aromatic PIs by applying phase separation techniques during solution polymerization. For perturbing the intermolecular interactions of polymer chains in the solid state, pressure is inherently more ideal than temperature. X-ray diffraction,25−29 infrared absorption (IR) spectra,30−32 UV/vis absorption, and fluorescence spectra33−42 of polymers have been measured under high pressure to examine the compression effects on their crystalline structures and their physical and optical properties. Recently, we reported on the relationship between pressure-induced variations in the molecular aggregation structures and the UV/vis absorption and fluorescence spectra of PIs up to 8 GPa.41,42 Significant variations in the local excitation (LE) and intermolecular CT absorption bands and fluorescence intensity were separately observed for nonfluorescent and highly fluorescent PI films up to 1 GPa. The observed variations agreed well with the significant decrease in the interchain distance, as indicated by synchrotron WAXD patterns. However, pressure-induced variations in the ordered structures, such as changes in the lattice parameters along the a- and b-axes and volumetric shrinkage of the ordered region of PIs, have not been examined because these PIs did not have high crystallinity, and they exhibited a limited number of diffraction peaks under high pressure. The insight into the pressure−volume−temperature (PVT) behaviors of crystalline PIs is particularly valuable to investigate the responses to pressures and temperatures of the PIs and to predict the limiting values of compressibilities and coefficients of thermal expansion of PIs, which leads to a guiding principle to reduce or erase the residual stress at interfacial surfaces with inorganic/metallic layers.43 In this study, we examined the variations in the WAXD patterns of PI powder and fiber up to 8 GPa to clarify the influence of the molecular structure on the ordered structure of PIs. The highly crystalline PI samples enabled us to obtain several diffraction peaks even at high pressures.

Chart 1. Chemical Structures of the PIs Used in This Study

benzidine is commercially unavailable and its production is prohibited owing to its carcinogenic character. Further, an as-received film of PMDA/ODA (Kapton-V film, 25 μm thick), supplied by DuPontToray Co., Ltd. (Tokyo, Japan), was annealed in NMP at 202 °C for 6 h to increase the crystallinity. Measurements. The experimental procedures using a highpressure diamond anvil cell (DAC) have been described elsewhere.41 The ruby fluorescence technique was used to estimate the pressure inside the sample room,44 and a mixture of three kinds of silicone oils was used as a pressure medium. Although strong crystalline peaks attributable to silicone oil were observed when only one type of silicone was used as a pressure medium, crystallization was effectively suppressed by using a mixture of three kinds of silicone oils (see Figure S1 in the Supporting Information). Ruby fluorescence spectra at elevated pressures are also shown in Figure S2 in the Supporting Information. Because of the small size of the sample chamber (only 200 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. The wavelength of the X-ray was set at 0.8 Å. The diffraction peaks assignable to the silicone oil were removed from the diffraction patterns by subtracting a reference pattern measured without a sample.

3. RESULTS AND DISCUSSION 3.1. Wide-Angle X-ray Diffraction Patterns at Atmospheric Pressure. Figure 1 shows the transmission X-ray diffraction patterns of PMDA/PPD powder, PMDA/BZ fiber, and PMDA/DATP and PMDA/ODA powders at atmospheric pressure measured at 50 °C. The crystal structures of the PIs were assigned as orthorhombic, and the diffraction peaks were indexed on the basis of previously reported X-ray measurements.5,6,12 Because these PIs exhibit (00l) peaks representing periodic structures along the PI main chains and several (hk0) peaks representing intermolecular ordering along the a- and baxes of the PI chains, the lattice parameters of the a-, b-, and caxes were quantitatively estimated. Table 1 shows the lattice parameters and crystallinities (Xc) of the PI samples at atmospheric pressure. The values of Xc were calculated from the ratio of the crystalline diffraction peaks to the amorphous halos, which were both fitted using Gaussian broadening functions (see Figures S3−S6 in the Supporting Information). It should be noted that, because fully aromatic PIs used in study have stiff polymer chains, and they were thermally treated at high temperatures, most of their noncrystalline region could form smectic liquid crystalline-like structure. To avoid overestimation of Xc values due to the intense (00l) diffractions from the smectic layer structure, the (00l) peaks were excluded from the estimation of Xc. As shown in the table, the lattice parameters of the rigid-rod PIs along the a-axis (da‑axis) decreased with an increasing number of phenyl rings in the diamine moiety (PMDA/PPD > PMDA/BZ > PMDA/DATP).

2. EXPERIMENTAL SECTION Materials. PMDA, purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), was dried and purified by sublimation under reduced pressure. p-Phenylenediamine (PPD) and ODA were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and recrystallized from tetrahydrofuran, followed by sublimation under reduced pressure. 4,4′-Diamino-p-terphenyldiamine (DATP) was supplied by Japan Carlit Co., Ltd. (Tokyo, Japan), and was recrystallized from tetrahydrofuran and n-hexane, followed by sublimation under reduced pressure. Preparation of PIs. The molecular structures of the PIs used in this study are shown in Chart 1. A highly crystalline powder of PMDA/DATP was prepared by the in situ thermal imidization techniques reported by Kajiyama et al.12 For instance, DATP was dissolved in N-methyl-2-pyrrolidone (NMP), and an equimolar amount of PMDA was added and stirred at room temperature for 48 h to give a solution of the precursor of the PI: PAA. The PI powder was subsequently prepared by stirring the PAA solution at reflux for 4 h under a N2 atmosphere. The PI powder thus obtained was washed with NMP and distilled water several times, followed by drying in vacuo at 100 °C for 1 h and subsequently annealing at 400 °C under a N2 atmosphere. The PMDA/PPD and PMDA/ODA powders were kindly supplied by Prof. Kunio Kimura at Okayama University. Crystalline fibers of PMDA/BZ kindly supplied by Toray Research Center, Inc., were used for the WAXD measurements because 4,4′3952

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Figure 1. Transmission X-ray diffraction patterns of PMDA/PPD powder, PMDA/BZ fiber, and PMDA/DATP and PMDA/ODA powders at atmospheric pressure measured at 50 °C.

Figure 2. (a) Transmission X-ray diffraction patterns of PMDA/ODA powder at atmospheric pressure measured at 50 °C and those of an asreceived film and a film annealed in NMP measured at room temperature. (b) Magnified patterns of the transmission X-ray diffractions.

Table 1. Lattice Parameters and Crystallinity of the PIs lattice parameter (Å)

polyimide sample PMDA/PPDa PMDA/BZa PMDA/DATPa PMDA/ODAa PMDA/ODAb PMDA/ODAb

powder fiber powder powder as-received film film annealed in NMP

a-axis

b-axis

c-axis

8.32 8.17 8.13 6.17 6.07

5.53 5.72 5.46 3.92 4.03

12.32 16.46 20.90 31.96 31.42

6.09

4.00

31.59

degree of crystallinityc (%)

powder. Figure 2b shows a magnified representation of the transmission X-ray diffraction patterns of the PMDA/ODA samples. The (hk0) diffraction peaks became distinct in the following order: the as-received film < the film annealed in NMP < highly crystalline powder, which indicates that the crystallinity Xc increased in this order. Moreover, the values of the dc‑axis of these samples increased with the increasing Xc, which suggests that the PI chains are more elongated in the crystalline domain than in the amorphous domain. Because PMDA/ODA has a bent and rotatable ether (−O−) linkage and a curved chain structure consisting of two repeating units, corresponding to the periodic structure along the PI chains (see Figures S8 in the Supporting Information), the projected length of the repeating unit along the main chain can be elongated when the bond angle at the ether group (θC−O−C) increases. Moreover, the values of da‑axis increased, and those of db‑axis decreased with increasing Xc, which could also be due to the variations in the bond angles and/or dihedral angles. These phenomena could result from the enhanced ordered structure of the PI chains. Comparable results were also observed by Russell and co-workers.8 They reported that the surface of the PMDA/ODA film was more ordered than the bulk of the film and that the surface PMDA/ODA molecules assumed a planar zigzag conformation. 3.2. Variations in the Aggregation Structure of PIs. Figures 3a−3d show the pressure-induced variations in the diffraction patterns of the PI powders and PI fiber. From the pressure-induced peak shifts, the strains ε along the a-, b-, and c-axes were quantitatively estimated. Here, ε is expressed as Δd/d0, where d0 is the lattice parameter at atmospheric pressure (0 GPa) and Δd is the variation in the lattice parameter at elevated pressures. For the PMDA/BZ fiber, only

55 24 27 19

Measured at 50 °C. bMeasured at room temperature. cIntense (00l) reflections were not used for the calculation.

a

Guha and co-workers45 reported that the da‑axis and db‑axis of oligophenyls decreased with an increasing number of phenyl rings, which indicates that the crystallographic packing in the ab-plane is more dense in lager oligophenyls. Moreover, it has been suggested from theoretical calculations that the torsional angle between neighboring phenyl rings is smaller in lager oligophenyls.45 Therefore, the decrease in the da‑axis of the rigidrod PIs with an increasing number of phenyl rings in the diamine moiety could be associated with a decrease in the torsional angle between neighboring phenyl rings. Figure 2a shows the transmission X-ray diffraction patterns of PMDA/ODA powder at atmospheric pressure measured at 50 °C, along with those of an as-received film and the film annealed in NMP measured at room temperature. The PMDA/ ODA films exhibited much stronger (00l) peaks compared to the powder sample. This should be due to the in-plane orientation of the PI chains of the PMDA/ODA films, as indicated by the reflection X-ray diffraction patterns (see Figure S7 in the Supporting Information), whereas the crystalline domains should be randomly orientated in PMDA/ODA 3953

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Figure 3. Variations in the X-ray diffraction patterns for (a) PMDA/PPD powder, (b) PMDA/BZ fiber, and (c) PMDA/DATP and (d) PMDA/ ODA powders as a function of applied pressure. The diffraction peak indicated with an asterisk is indexed as the diffraction of a ruby particle.

εc‑axis was estimated because this sample exhibited no distinct (hkl) peaks. A. Pressured-Induced Variations in Rigid-Rod PIs. First, the relationship between the number of phenyl rings in the repeating units of PIs and the pressure-induced variations in the periodic intermolecular distances of the PI chains, da‑axis and db‑axis, was investigated. Figure 4a shows the variations in the ε values along the a- and b-axes for PMDA/PPD and PMDA/ DATP powders. The former PI, with the shortest repeating unit with a rigid-rod structure, exhibited isotropic compressions along both the a- and b-axes by 5.9% upon applying pressure up to 6 GPa. In contrast, the decrease in the da‑axis (8.7%) of the latter PI containing a terphenyl unit was significantly larger than that in the db‑axis (6.2%) at 6 GPa. The a-axis of PMDA/BZ was reported to represent a periodic structure along the cofacial stacking direction of the phenyl and imide rings (see Figure 5).12 Therefore, if PMDA/DATP has a similar crystalline packing structure as PMDA/BZ, it should exhibit a larger compressibility along the cofacial stacking direction that is the a-axis. Figure 4b shows the variations in the linear compressibilities (κ) along the a- and b-axes of these PIs. The κ values were estimated by fitting the strain values in Figure 4a using formulas which exhibited the best fit, ε = −p/(k1 + k2p + k3p1/2), where p is pressure and k1, k2, and k3 are coefficients, followed by numerical calculation of the first derivatives of the strain with respect to the pressure (κ = ∂ε/∂P). The lattice parameter of PMDA/DATP along the a-axis exhibited a significantly large κ value compared to that of PMDA/PPD along the a-axis at atmospheric pressure. We have previously reported that a semialiphatic PMDA/DCHM PI having a mesomorphic order between the crystalline and amorphous phases exhibited a higher compressibility than a fully aromatic s-BPDA/PPD PI with a higher crystallinity, which indicates that the aggregation

Figure 4. Variations in (a) the strains (ε) and (b) the linear compressibilities (κ) along the a- and b-axes of PMDA/PPD and PMDA/DATP as a function of applied pressure.

structures having positional disorder along the interchain directions, such as amorphous and liquid crystalline-like ordered domains, contain a larger amount of interchain compressible free volume.41 In the cases of PMDA/PPD and 3954

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Figure 6. Variations in the strain along the c-axis of PMDA/PPD and PMDA/DATP powders and PMDA/BZ fiber as a function of applied pressure.

Figure 5. Schematic illustration of the packing structure of PMDA/BZ viewed along the c-axis.

PMDA/DATP, both PIs contain highly ordered structures. The larger interchain compressibility of the latter along the a-axis indicates that an increase in the number of phenyl rings in the diamine moiety also leads to an enhancement of compressible volume. Guha and co-workers45 measured the Raman spectrum of p-hexaphenyl under high pressure, and they reported that the torsional angles between the phenyl rings decreased at elevated pressures up to 1.5 GPa; i.e., more planar phenylene structures were generated at high pressures. The significantly large compressibility along the a-axis of PMDA/DATP could be associated with the planarization of the terphenyl unit in the diamine moiety. In contrast, we have previously reported that the intensity of the charge transfer (CT) absorption band of sBPDA/PPD PI film was reduced by applying pressure up to 0.3 GPa, which originated from a decrease in the intramolecular CT interaction caused by some structural changes, such as a decrease in the coplanarity between the dianhydride and diamine moieties. Therefore, in contrast to the terphenyl unit in the diamine moiety, the torsional angle between the imide and phenyl planes of PIs could be increased by applying pressure, which could be reflected on the pressure-induced variations in the da‑axis and db‑axis. Moreover, the κ value of each lattice parameter exhibited a significant decrease up to 2−3 GPa (see Figure 4b), and those of PMDA/PPD and PMDA/DATP became close above the pressures. This may indicates that the variations in the dihedral angles and/or bond angles were terminated at around 3 GPa, and both PIs formed the dense molecular packing structures with least free volume. Next, the influence of the number of phenyl rings on the pressure-induced variations in the periodic length of PI chains (dc‑axis) was investigated. As shown in Figure 6, the dc‑axis of PMDA/PPD decreased by only 0.55% up to 6 GPa. Because PMDA/PPD has the simplest rigid-rod structure with the fewest degrees of freedom, possible structural changes are shrinkage of the bond lengths, out-of-plane bending at the imide nitrogen, and uniaxial rotation of the pyromellitic and phenyl rings. We have previously reported the reduction in the C−C bond lengths of aromatic rings by ca. 0.7% as confirmed by the pressure-induced high wavenumber shift of the infrared stretching vibration of the PIs up to 6 GPa.41 Moreover, a quantum chemical study of the hydrostatic compression of pentaerythritol tetranitrate indicated that the C−C bond lengths could be decreased by 1.12% by applying pressure up to 6.5 GPa,46 which is a more appreciable decrease compared to the shrinkage of dc‑axis observed for PMDA/PPD. Hence, the gradual decrease in the dc‑axis of PMDA/PPD could be mainly attributable to the decrease in the bond lengths. On the other

hand, PMDA/BZ and PMDA/DATP, containing diphenyl and terphenyl units, respectively, exhibited much larger decreases in the dc‑axis of 0.95% and 1.7% up to 6 GPa, respectively, which indicates that the compressibility along the PI main chains increases with an increasing number of phenyl rings. This could be due to the increasing variation in the C−C bond lengths between phenyl rings in the diamine moiety. Further, because the pressure-induced decreases in the dc‑axis of PMDA/PPD and PMDA/DATP were different by ca. 1% up to 6 GPa, it is possible that an increased distortion due to structural changes, including the pressure-induced deformation of the periodic structures caused by changes in the bond angles and/or dihedral angle, also affected the variations in the dc‑axis of PMDA/BZ and PMDA/DATP. B. Variations in a Bent PI, PMDA/ODA. In this section, the pressure-induced variations for PMDA/ODA having a bent and rotatable ether (−O−) linkage are discussed. Figure 7a shows the variations in the ε values along the a-, b-, and c-axes for the crystalline powder of PMDA/ODA with applied pressure. The db‑axis of PMDA/ODA was significantly decreased by 15% compared to the da‑axis (5%) up to 6 GPa. The fact that the baxis corresponds to the cofacial stacking direction of the phenyl rings5 indicates that the crystalline lattice is more compressible along the cofacial stacking direction. As stated above, PMDA/ DATP also exhibited anisotropic compression between the aand b-axes, which indicates that the enhanced structural freedom caused by the incorporation of p-phenylene linkage or −O− linkage increases the compressibility along the cofacial stacking direction. In other words, compressible volume is preferentially generated at the spaces wedged between the cofacially stacking benzene and imide rings. Figure 7b shows the magnified representations of the strains at lower pressures. Very interestingly, the da‑axis of PMDA/ODA was increased by applying pressure up to 0.2 GPa. At a glance, this increase in the interchain distance along the a-axis is unusual, but it should be noted that the db‑axis exhibited a significant decrease by 1.7% at the same time. The increase in the da‑axis could be caused by the compensation of the large decrease in the db‑axis. Simultaneously, the dc‑axis also increased in the first compression stage up to 0.8 GPa. The initial increase in the periodic length of the main chains could be mainly caused by a widening of the bond angle (θC−O−C) by a concentrated compressive stress at the ether linkage, which effectively elongated the projected length along the PI chain.41 As mentioned above, the dc‑axis of PMDA/ODA at atmospheric pressure varied depending on the crystallinity (see Table 1). To investigate the influence of the 3955

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the repeating unit was significantly shortened up to 6 GPa.41 In contrast, the film annealed in NMP and the powder sample with higher crystallinities respectively exhibited variations by 0.6% and 0.7% from 1 GPa up to 6 GPa, which was smaller than the as-received film. These results indicates that, in contrast to the variations in the first stage of the compression, the PIs with highly ordered structures exhibit much smaller pressure-induced variations against additional compression. This can be attributed to the high modulus and the minimized free volume involved in the crystalline region. C. Volumetric Variations in the Ordered Region. As stated in the Introduction, the insight into the PVT behaviors of crystalline PIs is valuable to predict the responses to pressures and temperatures of PIs and to investigate the limiting values of compressibility and thermal expansion coefficients of PIs. Because all the PIs prepared in this study were reported to form crystalline structures based on the orthorhombic unit cell,6 the volumetric variations at elevated pressures in the ordered region were calculated as a summation of the variations in the lattice parameters along the a-, b-, and c-axes. Figure 9 shows

Figure 7. (a) Variations in the strain along the a-, b-, and c-axes of PMDA/ODA powder as a function of applied pressure. (b) Magnified representation of the strains at lower pressures.

ordered structure, the pressure-induced variations in compressibility were compared among PI samples with different crystallinities. Figure 8 shows the variations in strain ε along Figure 9. Volumetric variations in the ordered region of the PI powders as a function of applied pressure.

the volumetric variations in the ordered region of the PMDA/ PPD, PMDA/DATP, and PMDA/ODA powders. By applying pressure up to 6 GPa, PMDA/DATP and PMDA/ODA exhibited significantly large volumetric compression, by 16.7% and 19.1%, respectively, compared to PMDA/PPD (12.1%). This indicates that the increases in rotational and deformational freedom and in the flexibility of structure, such as the increase in the number of phenyl rings and bent ether linkages in the diamine moiety, enhanced the volumetric compressibility of PIs, as was observed with the compressibility along the cofacial stacking direction. The fact that the rigid-rod PI having the shortest repeating unit exhibited the smallest volumetric compressibility indicates that the effective reduction of compressible free volume and rotatable substituents should be an essential molecular design concept for polymers exhibiting minimized volumetric expansion.

Figure 8. Variations in the strain along the c-axis of PMDA/ODA powder, the as-received film, and the film annealed in NMP as a function of applied pressure.

the c-axis for three kinds of PMDA/ODA samples with applied pressure. The dc‑axis of the as-received film with a low Xc increased by 0.33% up to 0.4 GPa, whereas those of the film annealed in NMP and the crystalline powder increased by 0.36% up to 1.3 GPa and 0.51% up to 0.8 GPa, respectively. The enhanced pressure-induced elongation of dc‑axis with increased crystallinity suggests that a stronger stress is concentrated at the −O− linkage with an increase in the projected length of the repeating unit in the crystal lattices (see Table 1). At higher pressures from 0.4 to 6 GPa, the as-received film exhibited a large decrease in dc‑axis by 1.0%, which is attributable to narrowing of the bond angle, θC−O−C, induced by compression along the c-axis, and thus the projected length of

4. CONCLUSIONS Wide-angle X-ray diffraction patterns of powder and fiber samples of four kinds of fully aromatic PIs were investigated at high pressure up to 8 GPa. The dc‑axis of PMDA/PPD having a rigid-rod structure and the shortest repeating unit slightly decreased with applied pressure, which is mainly attributable to a decrease in the C−C and C−N bond lengths. The compressibility along the c-axis of the rigid-rod PIs increased with an increasing number of phenyl rings in the diamine 3956

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and Planetary Sciences, Tokyo Institute of Technology, for advice on the diamond anvil cell.

moiety (PMDA/PPD < PMDA/BZ < PMDA/DATP). This could be due to an increase in the variations in the bond lengths of the C−C linkages between phenyl rings in the diamine moiety and/or due to the increase in the degree of freedom of the structure, which enabled the pressure-induced deflection of the periodic structure associated with changes in the bond angles and/or dihedral angles. In contrast, the dc‑axis of PMDA/ ODA having an ether linkage in the main chain increased up to 0.8 GPa, which is attributed to a widening of the bond angle by concentrating the compression stress at the bent ether linkage; however, it decreased with further compression. By comparing the pressure-induced variations of the three PMDA/ODA samples with different crystallinity, it was found that highly crystalline samples containing more elongated polymer chains at atmospheric pressure exhibited a larger increase in the dc‑axis in the first stage of compression, whereas the less ordered sample exhibited a larger pressure-induced decrease in the dc‑axis at further compression. The pressure-induced variations along the interchain a- and b-axes depend on the chemical structures of the PIs. PMDA/ PPD exhibited an isotropic reduction along both axes, whereas PMDA/DATP and PMDA/ODA exhibited an anisotropic decrease along the a- and b-axes with a larger compression along the cofacial stacking direction of the benzene and imide rings. Moreover, PMDA/DATP and PMDA/ODA exhibited larger volumetric variations in the ordered region than PMDA/ PPD. This indicates that the increases in the freedom of structure resulting from the inclusion of the phenyl rings and ether linkages enhance the compressibility along the cofacial stacking direction, which increases the volumetric compressibility in the ordered region. Overall, the compression behavior of these PIs at elevated pressures provides valuable information regarding their aggregation structures, which can be closely related to their thermal expansion at elevated temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing the pressure-induced variations in the transmission WAXD patterns of silicone oils, the pressureinduced variations in ruby fluorescence spectra, the transmission WAXD patterns of the PIs fitted by Gaussian broadening functions to calculate the degree of crystallinity, the reflection WAXD patterns of PMDA/ODA films, and the periodic structure of PMDA/ODA polyimide. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel +81-3-5734-2137, Fax +81-3-5734-2889, e-mail sando@ polymer.titech.ac.jp (S.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science (25288096, 24-7535). The synchrotron radiation experiments were performed using a BL40B2 beamline with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2012B-1307, 2013A-1077). The authors thank Kei Hirose and Shigehiko Tateno at the Department of Earth 3957

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