Pressure-Induced Variations of Aggregation Structures in Colorless

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

pubs.acs.org/JPCB

Pressure-Induced Variations of Aggregation Structures in Colorless and Transparent Polyimide Films Analyzed by Optical Microscopy, UV−Vis Absorption, and Fluorescence Spectroscopy Eisuke Fujiwara, Hiroshi Fukudome, Kazuhiro Takizawa, Ryohei Ishige, and Shinji Ando*

J. Phys. Chem. B Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/13/18. For personal use only.

Department of Chemical Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Pressure-induced variations in the main chain and aggregation structures of colorless and transparent semialiphatic polyimide (PI) films were investigated by optical microscopy, UV−vis absorption, and fluorescence spectroscopy up to 8 GPa. Upon application of pressures up to 2 GPa, a gradual volumetric compression was clearly observed by microscopy, and definite bathochromic shifts of locally excited (LE) absorption bands were detected, which was attributed to the compression of interchain free volume and enhanced intermolecular interactions. In addition, a significant reduction in fluorescence intensity was observed for PIs with quasilinear structures below 2 GPa due to enhanced energy transfer in the excited states caused by the densification of PI chain packing. In contrast, the volumetric compression of the PI films and bathochromic shifts of the LE absorption bands were gradually reduced at pressures above 2 GPa. The former is closely correlated with the bulkiness and flexibility of the alicyclic diamine structure. The latter reflects the intense compression stress generated around the dianhydride moiety, associated with the deformability and in-plane orientation of the main PI chains. High-pressure experiments on PI films are beneficial to investigate variations in aggregation structures and local electronic structures of PI chains induced by dense molecular packing and enhanced intermolecular interactions.

1. INTRODUCTION Fully aromatic polyimides (Ar-PIs) are high-performance engineering plastics that exhibit high thermal and chemical stability, radiation resistance, and mechanical strength.1,2 ArPIs have been widely used in the aerospace, electric, electronic, and optical industries.3 Recently, semialiphatic polyimides (AlPIs) have attracted significant attention because they show high optical transparency in the visible region due to effective suppression of inter- and intramolecular charge transfer interactions, originating from the weak electron-donating properties of the alicyclic diamine.4−6 For instance, Al-PIs have been used in flat panel and bendable display applications owing to their high transparency, low refractive indices, and low birefringence.4,6,7 Since a repeating unit of PI consists of alternating dianhydride and diamine moieties, two major kinds of electronic transitions are observed in PIs.7−11 The first one is called a “locally excited” (LE) transition that occurs between the occupied and unoccupied molecular orbitals (MOs), both of which are located at the dianhydride moiety. The ultraviolet−visible (UV−vis) absorption spectra of model PI compounds indicated that the LE transitions are π → π* transitions occurring within the aromatic and imide rings of the dianhydride moiety.7 The second transition is called a “charge transfer” (CT) transition that occurs between the dianhydride and diamine moiety. The lowest-energy CT transition in Ar© XXXX American Chemical Society

PIs occurs between the highest occupied MO (HOMO) located around the diamine moiety and the lowest unoccupied MO (LUMO) located around the dianhydride moiety. Thus, the CT transition is significantly influenced by the electrondonating properties of the diamine and electron-accepting capabilities of the dianhydride.8 The relatively small energy gap of the CT transition causes a bathochromic shift of the absorption edge accompanied by tailing toward the visible region (λ > 400 nm), which is the origin of the coloration of conventional Ar-PI films, ranging from pale orange to deep brown. Ishida et al.9 demonstrated that the absorption edge observed in poly(4,4′-oxidiphenylene pyromellitimide) (PMDA/ODA) film at approximately 400 nm can be attributed to a CT transition based on a study of model compounds. Single electron CT transitions in PI films can occur both intra- and intermolecularly. Hasegawa et al.12 reported that the fluorescence intensity of poly(trans-1,4-cyclohexyl-3,3′,4,4′diphenyldiimide) (sBPDA/CHDA) films decreased with increasing curing temperature (Ti). This indicated that the aggregation structures and intermolecular interactions, such as π−π stacking and CT interactions, are strongly dependent on Received: July 5, 2018 Revised: August 22, 2018

A

DOI: 10.1021/acs.jpcb.8b06423 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B the final curing temperature. In addition, we reported the pressure-induced variations in the UV−vis absorption spectra of Ar-PI films up to 400 MPa caused by the reduction in intermolecular distance.13 A significant increase in absorbance was observed for PMDA/ODA and poly(2,2′-bis(trifluoromethyl)-4,4′-biphenylene pyromellitimide) (PMDA/ TFDB) films, indicating that their CT absorption bands could be assigned to intermolecular CT transitions. In contrast, since this increase was not observed for poly(p-phenylene biphenyltetracarboximide) (sBPDA/PPD), its CT band was assigned to an intramolecular CT transition. Thus, a detailed investigation of structure−property relationships among the optical properties, chemical structures, electronic structures, and molecular aggregation structures is required for developing novel functional PIs for optical applications. Molecular aggregation structures are mainly investigated using wide-angle X-ray diffraction (WAXD).14−26 Russell et al. reported that the aggregation structures of PMDA/ODA films change from amorphous to ordered crystalline structures, depending on the film thickness and preparation conditions.17 PI films generally do not exhibit definitive crystalline diffraction peaks due to the lack of large domains with threedimensional positional order. Hence, polymer domains with mesomorphic order between the amorphous and crystalline phases in PI films spontaneously form during thermal imidization above the glass transition temperature (Tg) and are frozen below Tg during cooling.25 However, semicrystalline powder samples can be prepared from aromatic PIs containing rigid-rod and/or rotatable symmetrical structures. We have recently evaluated the coefficients of linear and volumetric thermal expansion (CTE and CVE, respectively) of the crystal lattice for 13 different PIs.26 It was suggested that the CVE of the PIs can be predicted from the weight density of the crystallites, regardless of the chemical structure and extent of the second-kind disorder. To stimulate and perturb the aggregation structures of polymer chains in the solid state, applying high pressure is more effective than varying temperature. Compression effects on the crystalline and/or amorphous structures of polymers and their physical properties have been examined by WAXD,27−32 UV−vis absorption, fluorescence spectroscopy,13,33−42 infrared (IR) absorption spectroscopy,43−45 and Brillouin scattering spectroscopy46−51 under high pressure. Recently, we reported the relationship between the optical properties and molecular aggregation structures of PIs with ordered structure, as indicated by the pressure-induced variations in the UV−vis, IR absorption, and florescence spectra, as well as synchrotron WAXD patterns up to 8 GPa.38,39 These results demonstrated that significant variations in the LE, intermolecular CT absorption bands, and the florescence intensity up to 1−2 GPa agree well with the reduction in the interchain distance caused by pressure application. In addition, we reported pressure-induced variations in the ordered structures, such as anisotropic changes in the lattice parameters along the a-, b-, and c-axes as well as volumetric shrinkage, through investigations of synchrotron WAXD patterns of the crystalline PI powder.31 These results indicated that the increase in rotational and deformational freedom due to structure flexibility, such as the increase in the number of phenyl rings and bent ether linkages in the chain structure, enhanced compressibility along the main-chain direction (c-axes) and interchain direction (a- and b-axes). However, most PI films generally exhibit an

amorphous nature without highly ordered domains, and precise WAXD analysis is only applicable to semicrystalline PIs. Thus, the compression behaviors of amorphous PI films have yet to be examined in detail. Recently, Amin et al.52 reported the pressure-induced compression behaviors of the glassy amorphous solids GeSe2 and As2O3 using optical microscopy with a few key assumptions. For example, amorphous solids undergo isotropic compression under hydrostatic conditions, which differs from anisotropic compression in crystalline solids. This simple technique relies on two-dimensional image acquisition and analysis to quantify changes in the sample area, which can be used to investigate the volumetric compression of amorphous PI films. In this study, the relationships between the compression behavior and aggregation structures of semialiphatic PI films derived from isomeric biphenyltetracarboxylic dianhydrides were investigated using optical microscopy, UV−vis absorption, and fluorescent emission/excitation spectroscopy at elevated pressures up to 8 GPa. The PIs with quasilinear or bent structures are totally colorless and transparent.

2. EXPERIMENTAL SECTION 2.1. Materials. The starting materials 3,3′,4,4′-biphenyltetracarboxylic dianhydride (sBPDA) and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (aBPDA) were provided by Ube Industries Ltd. (Tokyo, Japan) and purified by sublimation under reduced pressure. The other chemicals including 4,4′diaminodicyclohexylmethane (DCHM) and 1,4-diaminocyclohexane (CHDA) were purchased from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan) and recrystallized from n-hexane, followed by sublimation under reduced pressure. The 3,3′-dimethyl4,4′-diaminodicyclohexylmethane (DMDHM) was purchased from Aldrich and purified by distillation under reduced pressure. N,O-Bis(trimethylsilyl)trifluoroacetamide (99+%, BSTFA) and N,N-dimethylacetamide (anhydrous, 99.8%, DMAc) were purchased from Sigma-Aldrich (Tokyo, Japan) and used without further purification. 2.2. Sample Preparation. The chemical structures of the PIs derived from sBPDA dianhydride with a quasilinear structure (sBPDA-PI) and those from aBPDA dianhydride with a bent structure (aBPDA-PI) are shown in Chart 1. The precursors of the semialiphatic PIs, poly(amic acid)silylether Chart 1. Molecular Structures of Colorless and Transparent Polyimides (PIs)

B


Article

The Journal of Physical Chemistry B

2.4. High-Pressure Measurements. The experimental procedures using a high-pressure diamond anvil cell (DAC) have been described in detail in a previous report.38 Briefly, the ruby fluorescence technique was used to estimate the pressure inside the sample room.56 The UV−vis absorption and fluorescence spectra of the PI films loaded in a DAC at elevated pressure were measured using a multichannel charge coupled device (CCD) spectrometer (PMA-11, Hamamatsu Photonics Co., Ltd.) from 200 to 950 nm using an objective lens (cutoff wavelength: 320 nm) to focus the light on the sample. The resolution of the UV−vis absorption and fluorescence spectra was 0.2 nm. A xenon lamp (Hamamatsu Photonics Co., Ltd.) was used as a light source for the UV−vis absorption measurements. A UV LED light (λmax; 365 nm, Hamamatsu Photonics Co., Ltd.) was used as an excitation light source with a single 354−366 nm band-pass filter (Semrock FF01-360/12-25, IDEX Health & Science, LLC, New York) for fluorescence measurements. The fluorescence spectra were measured by using the backward-scattering geometry to suppress self-absorption of emitted fluorescence. The fluorescence emitted from the sample in the backwardscattering geometry was collected by a binary fiber equipped with a long pass filter (ITY430, 430 nm cutoff, Isuzu Glass Ltd., Japan). For precise detection of fluorescence intensity at each pressure, the geometry of the measurement system was tightly fixed during the measurements at all the pressure points for one sample. The volumetric compression of the PI films at elevated pressures was estimated from area variations in the 2D images taken with a 5.1 megapixel digital camera (C-5060, Olympus Corp., Japan) attached to the optical stereomicroscope (SZY-12, Olympus Corp., Japan). 2.5. Quantum Chemical Calculations. The density functional theory (DFT) was used to calculate the optimized geometry, electronic structures, and one electron transitions of model compounds of semialiphatic PIs. In addition, the linear polarizability tensor (a11, a22, and a33) and van der Waals volume (Vvdw) per repeating unit of PI were calculated on the basis of DFT theory, in which Vvdw was used to estimate the packing coefficients (Kp) of the PIs, as described below, and the model compounds used for the calculations are shown in Chart S1 (Supporting Information). The Vvdw value was calculated on the basis of the method reported by Slonimski et al.57 using the van der Waals radii reported by Bondi.58 The long-range correlated three-parameter Becke-style hybrid functional (CAM-B3LYP) was used. The 6-311G(d) basis sets were utilized for geometry optimization,59−61 and the 6311++G(d,p) basis sets was used for calculating the MOs, oscillator strength of the one electron transitions (f), and polarizability of the tensor calculations. All calculations were performed using the Gaussian 09 (Rev. C.01) software package, available via the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology. 2.6. Degree of Molecular Packing in PI Films. The molecular packing coefficient (Kp) of each PI film was estimated on the basis of the Lorentz−Lorenz equation62,63

(PASE), were prepared using the in situ silylation method.53−55 Alicyclic diamine was dissolved in DMAc with a 1.05 mol equiv of BSTFA, and stirred for approximately 0.5 h, and an equimolar amount of dianhydride was added thereafter. The PASE solution was stirred at room temperature for 72 h. Trimethylsilylation of the amino groups by BSTFA can avoid salt formation between the unreacted amino and carboxyl groups of the dianhydride moieties. PI films were prepared by thermal imidization of the corresponding PASE precursors. The PASE solutions were spin-coated onto silica substrates, followed by soft-baking at 70 °C for 1 h. Subsequent thermal imidization was performed following a one-step procedure: the final curing condition was 300 °C for 1.5 h under dry nitrogen flow. The thickness of all PI films was controlled at approximately 10 μm. 2.3. Measurements at Atmospheric Pressure. Decomposition temperatures with 5% weight loss (Td5) of the PI films were measured using a differential thermal analysis (DTG-60, Shimadzu, Kyoto, Japan). The heating rate was 10 °C/min from 30 to 600 °C under a nitrogen purge. Glass transition temperatures (Tg) of the PI films were determined using a differential scanning calorimeter (DSC-60, Shimadzu, Kyoto, Japan). The heating rate was 10 °C/min from 30 to 360 °C under a nitrogen purge. In-plane (nTE) and out-of-plane (nTM) refractive indices of the PI films were measured with a prism coupler (Metricon PC-2010) at 1310 nm. The average refractive index, nav, was calculated using the following equation, nav2 = (2nTE2 + nTM2)/3, and the in-plane/out-ofplane birefringence (Δn) was defined as Δn = nTE − nTM. UV− vis absorption spectra of the PI films were measured using a Hitachi F-3500 spectrophotometer (Hitachi High-Technology Corp., Japan). The film thickness of the Al-PIs was measured using a sensing-pin-type surface profilometer (DEKTAK-III, Ulvac, Japan). The optical anisotropy of the PI films was evaluated by conoscopic observation using an Olympus polarizing microscope (BX51, Olympus Co. Ltd., Japan). Transmission WAXD measurements of the PI films were performed at the Photon Factory of the High Energy Accelerator Research Organization, Tsukuba, Japan, at the BL-6A beamline. The X-ray’s wavelength was set to 1.5 Å. The two-dimensional (2D) WAXD patterns were exposed on a PILATUS-100 K detector for 1 min, and these patterns were circularly averaged to obtain intensity profiles. The camera length was calibrated using the 00l diffractions from lead stearate as a standard. Fluorescence spectra were measured using a Hitachi U-4500 fluorescence spectrometer (Hitachi High-Technology Corp., Japan) equipped with a photomultiplier tube (Hamamatsu R928), and the front-face method was adopted to reduce self-absorption of the emitted fluorescence. A high-speed two-dimensional (2D) fluorescence spectrum with variable excitation and emission wavelengths was measured to determine the proper excitation (λex) and emission wavelengths (λem) for the 1D excitation spectra with higher resolution. The measured spectra were not corrected for the sensitivity of the photomultiplier tubes to different fluorescence wavelengths. The photoluminescence quantum yields (Φ) of the PI films formed on silica substrates were measured using a calibrated integrating sphere (Hamamatsu C9920) coupled to a multichannel analyzer (Hamamatsu C7473) via an optical fiber link. In these measurements, the PI films were excited at a tunable wavelength (λex) using a monochromated xenon light source.

nav2 − 1 nav2

+2

=

4π ρNA αav 3 M

(1)

where nav is the average refractive index, ρ is the density of the material, NA is Avogadro’s number, M is the molecular weight of the repeating unit, and αav is the average molecular C


Article


0.1772, indicating an enhanced orientation of the PI chains in the film plane. The sBPDA/DCHM PI film with a kinked structure (−CH2−) in the main chain showed a smaller Δn (0.0301, 17% smaller), though aBPDA/DCHM showed an order of magnitude smaller Δn (0.0037). In addition, the sBPDA/DMDHM and aBPDA/DMDHM PI films with bulky −CH3 side groups showed smaller Δn values of 0.0048 and 0.0033, respectively. The Δn values of the PIs clearly indicate that the incorporation of aBPDA or DMDHM moieties restricts the in-plane orientation of the PI chains and affords PI films with isotropic orientation and physical properties. This implies that the kink structure (−CH2−) in the DCHM moiety does not completely disorder the in-plane orientation, but the bent and rigid structure of the aBPDA dianhydride and the bulky −CH3 side groups in the DMDHM diamine totally disorder the in-plane orientation of the PI chains. This is beneficial for developing a novel class of thermally stable and highly colorless and transparent PIs with very small birefringence. The packing coefficients (Kp) of the PI films estimated from their average refractive indices (nav) and polarizabilities (αav) are listed in Table 2. The calculated values of αav and Vvdw are also listed in Table S1 (Supporting Information). Large Kp values correspond to dense PI chain packing with a small free volume.64 In a comparison of the Kp values of the sBPDA-PI films, sBPDA/CHDA with the most rigid and quasilinear structure exhibited the largest Kp (0.655) due to highly dense chain packing, and the Kp value decreased upon incorporating a kink linkage (−CH2−, sBPDA/DCHM, 0.636) and bulky −CH3 groups (sBPDA/DMDHM, 0.606). The Kp values of aBPDA/DCHM and aBPDA/DMDHM with bent structures at the dianhydride moieties were 0.613 and 0.611, respectively. Since the Kp value of aBPDA/DCHM (0.613) is smaller than that of the sBPDA/DCHM film (0.636), it can be concluded that the bent structure of the dianhydride moiety leads to loose molecular packing. The conoscopic images of the PI films are shown in Figure 1, and were used to investigate the optical anisotropy of the PI films. Only the sBPDA/CHDA film with a large Δn exhibited a definite interference pattern that could be attributed to uniaxial optical anisotropy with negative birefringence (nTE > nTM) caused by the strong in-plane orientation of the PI chains. In contrast, the other films exhibited no clear interference patterns, which is consistent with their small Δn values. The WAXD intensity profiles of the PI films are presented in Figure 2. The profile of the sBPDA/DCHM film exhibited sharp diffraction peaks at 3.3 nm−1 (19.0 Å) and 6.7 nm−1 (9.4 Å), which can be readily indexed as (001) and (002), respectively, because the calculated length of the repeating unit of sBPDA/DCHM PI (20.1 Å)25 is close to 19.0 Å. In addition, both sBPDA/DCHM and sBPDA/CHDA films showed relatively broad diffraction peaks at 12.3 nm−1 (5.1 Å) which can be assigned to the ordering of interchain packing.25,38,39 On the other hand, the other PIs exhibited no specific diffraction peaks. Thereby, the sBPDA/DCHM and sBPDA/CHDA PIs formed partially semicrystalline structures, whereas the other PI films are amorphous. 3.2. Volumetric Compression Behavior of the PI Films. The volumetric compression behavior of the PI films was analyzed by measuring their area in the 2D microscopic images taken at elevated pressures (Figure 3 and Figure S2 in the Supporting Information). This analysis was performed on the basis of two assumptions: (1) that the PI films are

polarizability. The intrinsic volume (Vint) can be defined as follows: Vint =

M ρNA

(2) 64

Kp can be defined as Kp =

Vvdw Vint

(3)

where Vvdw is the van der Waals volume of the repeating unit. Thus, eqs 1−3 can be rewritten as follows: Kp =

2 3 Vvdw nav − 1 2 4π αav nav + 2

(4)

3. RESULTS AND DISCUSSION 3.1. Characterization of the PI Films. The decomposition temperatures with 5% weight loss (Td5) determined by DTG and the glass transition temperatures (Tg) measured by DSC are listed in Table 1. The DTG and DSC curves of the Table 1. Decomposition Temperatures with 5% Weight Loss (Td5) and the Glass Transition Temperatures (Tg) of the PI Films polyimide

Td5/°C

Tg/°C

sBPDA/CHDA sBPDA/DCHM sBPDA/DMDHM aBPDA/CHDA aBPDA/DCHM aBPDA/DMDHM

463 461 400 424 405 402

306 228 239 308 305 310

PI films are shown in Figure S1 in the Supporting Information. Each PI film exhibited good thermal stability without significant weight loss below 400 °C and with the Tg values higher than 200 °C. As previously reported, the aBPDA-PI films seemed to have lower Td5 values than the sBPDA-PI films.65 On the other hand, the aBPDA-PI films exhibited higher Tg values than the sBPDA-PI films, due to the high rotational barrier at the aBPDA moiety and a relatively larger sweep volume required for conformational change through crank-shaft-like motion in the former PIs.65 The in-plane (nTE) and out-of-plane (nTM) refractive indices of the PI films measured at 1310 nm, the average refractive indices (nav), and the in-plane/out-of-plane birefringence (Δn) are listed in Table 2. The sBPDA/CHDA PI film with the most rigid and quasilinear structure exhibited the largest Δn of Table 2. Refractive Indices (nTE, nTM, nav), Packing Coefficients (Kp), and In-Plane/Out-of-Plane Birefringence (Δn) of the PI Films polyimide

nTE

nTM

nav

Kp

Δn

sBPDA/CHDA sBPDA/DCHM sBPDA/DMDHM aBPDA/CHDAa aBPDA/DCHM aBPDA/DMDHM

1.7190 1.6186 1.5651

1.5418 1.5885 1.5603

1.6620 1.6086 1.5635

0.655 0.636 0.606

0.1772 0.0301 0.0048

1.5690 1.5567

1.5653 1.5534

1.5678 1.5556

0.613 0.611

0.0037 0.0033

Unmeasurable due the fragility of the film causing many cracks.

a

D


Article


isotropically compressed by applying hydrostatic pressure along the in-plane and out-of-plane directions and (2) that the rectangular parallelepiped shapes of the PI samples were maintained under high pressures. Since the first assumption may not be applicable to anisotropic PI films exhibiting large Δn values, the sBPDA/CHDA film was excluded from the measurements and discussion. On the other hand, the second assumption was validated from the microscopy images shown in Figure 3 and Figure S2. The pressure-induced volume changes of the PI films are shown in Figure 4. A normalized volumetric compression ratio of the PI films at each pressure is given by Vp V0

=

apbp cp a0b0 c0

(5)

where ap and bp are the length of each side of the film in the image at pressure p, and cp/c0 is described by cp/c0 = (ap + bp)/ (a0 + b0). The pressure-dependent normalized volumetric compression ratio was fitted using the Tait equation, which is given by Py i = 1 − c lnjjj1 + zzz V0 b{ k

Vp

(6)

where P is the pressure, Vp/V0 is the normalized volumetric compression ratio, and b and c are the fitted parameters.46,66,67 The ratio of b/c is equivalent to the isothermal bulk modulus (B0) at atmospheric pressure (0.1 MPa). The B0 values for the sBPDA/DCHM, sBPDA/DMDHM, aBPDA/DCHM, aBPDA/DMDHM, and aBPDA/CHDA films were estimated to be 3.07, 2.40, 2.15, 2.17, and 2.14 GPa, respectively. The significantly larger B0 value of sBPDA/DCHM indicates its higher hardness, which is attributable to its dense aggregation structure, as indicated by a high refractive index and large Kp. As shown in Figure 3, all PI films exhibited significant volumetric compression by 12−16% from 0.1 MPa to 2 GPa. We reported that a semialiphatic poly(4,4′-diaminocyclohexylmethane pyromellitimide) (PMDA/DCHM) film contained larger amounts of interchain free volume in liquid crystal (LC)like ordered regions. In addition, large linear compressibility in the crystalline region was observed below 2 GPa by synchrotron WAXD analysis, indicating that the free volume is more compressible at lower pressures.38 Convington et al.44,45 also demonstrated that most of the free volume in poly(methyl methacrylate) (PMMA) and polycarbonate (PC) is removed after compression up to 2 and 1 GPa, respectively. Thus, the significant volumetric compression observed below 2 GPa is likely caused by a significant reduction in the interchain free volume. On the other hand, the rate of change in the volumetric compression ratio (Vp/V0) of the PI films gradually decreased above 2 GPa, coinciding with the compression behavior of the crystalline lattice of semicrystalline PIs as determined by highpressure WAXD analysis.38 Comparing the three completely amorphous aBPDA-PI films, the reduction in Vp/V0 for aBPDA/CHDA (approximately −19% from 0.1 MPa to 8 GPa) was much smaller than that of the other two PIs (ranging from −27 to −28%). We previously reported that the main reason for the reduced compressibility above 1−2 GPa is the structural and/or conformational changes in the periodic structure of the PI main chans.31,38 In addition, the compressibility of crystalline lattices in aromatic PIs with rigid-rod structures increases with increasing numbers of

Figure 1. Conoscopic images of the PI films observed using a polarized microscope: (a) sBPDA/CHDA, (b) aBPDA/CHDA, (c) sBPDA/DCHM, (d) aBPDA/DCHM, (e) sBPDA/DMDHM, and (f) aBPDA/DMDHM.

Figure 2. WAXD intensity profiles of the PI films.

E


Article


Figure 3. (a) Two-dimensional images of the sBPDA/DCHM films at 0.1 MPa and (b) variations in the two-dimensional images of the sBPDA/ DCHM films upon pressure application.

3.3. Relation between the Optical Properties and Aggregation Structures under High Pressure. As mentioned previously, the optical properties of solid PI films are closely related to the aggregation structures of the PI chains as well as the chemical and electronic structures of repeating units. The UV−vis absorption and fluorescence spectra for investigating the pressure-induced variations in aggregation structures in PI films should be discussed.38,39 First, the nature of the electronic transitions corresponding to the UV−vis absorption and fluorescence properties of each semialiphatic PI film at atmospheric pressure (0.1 MPa) are discussed. The pressure-induced variations in aggregation structures are subsequently discussed on the basis of the wavelength shifts in the LE absorption bands and suppressed fluorescence intensity. 3.3.1. Nature of the UV−Vis Absorption at Atmospheric Pressure. Before examining the nature of the electronic transitions in Al-PIs, it is beneficial to discuss the UV−vis absorption spectra of the PI films. Figure 5 shows the UV−vis absorption spectra of ∼10-μm-thick films of semialiphatic PIs, which exhibit strong absorption at wavelengths (λ) shorter

Figure 4. Variations in the volume ratio of the PI films at each pressure normalized using the results at atmospheric pressure.

phenyl rings in the diamine moiety.38 This is likely caused by increasing rotational and deformational freedom due to the flexibility of the PI structure.31 The aBPDA/CHDA structure contains only one cyclohexyl (c-hex) ring, while those of aBPDA/DCHM and aBPDA/DMDHM have two c-hex rings connected by a bent methylene linkage. These results are partly analogous to the thermal expansion behaviors of aromatic PIs with phenylene, biphenylene, and p-terphenylene structure in their diamine units.26 The CVE and CTE of these PIs increase with the number of phenyl rings in the chain and with decreasing density of the crystal lattice. In addition, the volumetric compression ratios of sBPDA/DCHM and sBPDA/ DMDHM, containing the same diamine moieties, were estimated to be between −27% and −28%. Accordingly, the larger compressibility of the PIs derived from DCHM and DMDHM, regardless of dianhydride structure, can be attributed to the deformation of the diamine repeating unit, resulting in variations in conformation and/or bond angles, due to enhanced flexibility at the diamine moieties. Christian et al.68 reported that halogenated cyclohexanes exhibited effective conformational conversion from equatorial to axial forms upon pressure application based on axial/equatorial ratios estimated by FT-IR band analysis. Thus, the pressure-induced deformation at the flexible diamine moiety is reflected by the volumetric compression behavior of the PI films above 2 GPa regardless of the rigid dianhydride moiety.

Figure 5. Optical absorption spectra of the semialiphatic PIs at atmospheric pressure. The inset shows a magnified representation at approximately 400 nm. F


Article


and spatial distributions of the contributing MOs calculated for m(sBP/Ch). Accordingly, the absorption bands below 400 nm in sBPDA/DMDHM and sBPDA/CHDA films can be assigned to the LE(π−π*) absorptions. In the same manner, the absorptions of the aBPDA-PI films below 400 nm were also assigned to the LE(π−π*) bands. The spatial distributions of the contributing MOs of m(aBP/Ch) are illustrated in Figure 6. Here, HOMO − m and LUMO + m denote the (m +

than 400 nm. The strong absorption of the PIs was assigned on the basis of the TD-DFT calculations of the model compounds m(sBP/Ch)69 and m(aBP/Ch) for sBPDA-PIs and aBPDAPIs, respectively (Chart 2). According to time-dependent Chart 2. Structure of the Model Compounds, m(sBP/Ch) and m(aBP/Ch), Used for the DFT Calculations

perturbation theory, the wave function of the system, Ψ, can be expressed as Ψ = ∑kck exp(−iEkt/ℏ)ψk, where ck denotes the expansion coefficient and ψk represents the eigenfunctions of the time-independent Schrödinger equation with eigenvalues of Ek. The contribution of each ψk to the electronic transitions can be approximated using the normalized expansion coefficient, defined as ci2/(∑ick2), using the ck obtained by TD-DFT calculations with values larger than 0.1. Table 3 lists the vertical excitation wavelengths, oscillator strengths (f), contributing MOs, assignments of electronic transitions from the ground (S0) to excited states (Si), and contributions of each transition calculated for m(sBP/Ch) and m(aBP/Ch). Wakita et al.69 reported that the LE(π−π*) bands mainly contribute to the strong absorption below 400 nm in the sBPDA/DCHM film by considering one electron transition

Figure 6. Calculated molecular orbitals of m(aBP/Ch) (TD-DFT method at the B3LYP/6-311G(d) level). HOMO − m and LUMO + m denote the (m + 1)th highest occupied orbital and the (m + 1)th lowest unoccupied orbital, respectively.

Table 3. Transition Wavelengths, Oscillator Strengths, and Assignment of S0 → Si Transitions for the Model Compounds m(sBP/Ch) and m(aBP/Ch) state 1 2

3

transition wavelength/nm

oscillator strength

335.8 329.9

329.3

0.2073 0.0024

0.0053

1

338.5

0.0151

2

326.6

0.0019

3

323.7

0.0162

6

295.4

0.0355

orbitals Model m(sBP/DC)a HOMO → LUMO HOMO − 1 → LUMO HOMO − 3 → LUMO HOMO − 2 → LUMO HOMO − 3 → LUMO HOMO − 2 → LUMO HOMO − 3 → LUMO HOMO − 3 → LUMO HOMO − 2 → LUMO HOMO − 2 → LUMO HOMO − 2 → LUMO Model m(aBP/DC) HOMO → LUMO HOMO − 2 → LUMO HOMO − 4 → LUMO HOMO − 3 → LUMO HOMO − 3 → LUMO HOMO − 2 → LUMO HOMO → LUMO + 1 HOMO → LUMO HOMO − 2 → LUMO HOMO − 2 → LUMO HOMO − 8 → LUMO HOMO − 5 → LUMO

+1 +1 +1 +1 +1

+1 +1

assignment of transition

contribution

LE(π−π*) LE(π−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(n−π*) LE(π−π*)

0.95 0.05 0.51 0.21 0.15 0.13 0.47 0.21 0.15 0.14 0.02

LE(π−π*) LE(π−π*) LE(π−π*) LE(n−π*) LE(n−π*) LE(π−π*) LE(π−π*) LE(π−π*) LE(π−π*) LE(π−π*) LE(π−π*) LE(π−π*)

0.49 0.32 0.19 0.61 0.32 0.07 0.45 0.42 0.13 0.40 0.30 0.30

a

The calculated values were taken from ref 66. G


Article

The Journal of Physical Chemistry B 1)th highest occupied MOs and (m + 1)th lowest unoccupied MOs, respectively. The HOMO, HOMO − 2, HOMO − 4, LUMO, and LUMO + 1 were localized over the dianhydride moiety. It should be noted that all these MOs are regarded as π-orbitals. Thus, the calculated transitions at 338.5 nm (S0 → S1), 323.7 nm (S0 → S3), and 295.4 nm (S0 → S6) can be attributed to LE(π−π*) transitions. On the other hand, the HOMO − 3 is localized around the lone pairs of the carbonyl oxygens and imide nitrogen atoms and can be assigned to lone pair (n) orbitals, indicating that the calculated transition at 326.6 nm (S0 → S 2) is an LE(n−π*) transition. Because the calculated oscillator strengths of the LE(π−π*) transitions (f > 0.015) are much larger than those of the LE(n−π*) transition (f < 0.002), it can be concluded that the LE(π−π*) bands mainly contribute to the strong absorption below 400 nm in aBPDA-PI films. Consequently, the strong absorption at shorter wavelengths of the Al-PIs can be assigned to LE(π−π*) bands. 3.3.2. Nature of the Fluorescence Spectra at Atmospheric Pressure. Figure 7 shows the normalized fluorescence

Table 4. Excitation and Emission Wavelengths and Photoluminescence Quantum Efficiency (Φ) of the PI Films polyimide

λex/nm

λem/nm

Φ

sBPDA/CHDA sBPDA/DCHM sBPDA/DMDHM aBPDA/CHDA aBPDA/DCHM aBPDA/DMDHM

368 369 367 366 354 357

398 399 402 401 395 392

0.066 0.104 0.066 0.005 0.001 0.005

LE(π−π*, S0 → S 1) transition of m(sBP/Ch). In the same manner, sBPDA/DMDHM and sBPDA/CHDA films exhibited fluorescence peaks at (λ ex/λ em) = (367/402) and (368/398), respectively, which could also be assigned to LE(π ← π*) fluorescence. In addition, the fluorescence peaks of the aBPDA/DCHM, aBPDA/DMDHM, and aBPDA/CHDA films were observed at (λ ex/λ em) = (354/395), (367/392), and (366/401), respectively. Since the λ ex values of these aBPDA-PIs are close to the calculated wavelength for the LE(π−π*, S0 → S 1) transition of m(aBP/Ch) at 339 nm, the fluorescence of aBPDA-PI films was also assigned to LE(π ← π*) fluorescence. 3.3.3. Variations in UV−Vis Absorption and Fluorescence Spectra under High Pressure. The pressure dependences of UV−vis absorption spectra for all Al-PI films are shown in Figure 8 and Figure S3. A gradual bathochromic (long wavelength) shift occurred by applying pressure for the LE absorption bands of the sBPDA/DCHM and aBPDA/DCHM

Figure 7. Normalized fluorescence excitation and emission spectra of the (a) sBPDA-PI and (b) aBPDA-PI films at atmospheric pressure.

excitation and emission spectra of the ∼10 μm thick Al-PI films at atmospheric pressure. The excitation and emission wavelengths as well as photoluminescence quantum yields (Φ) are summarized in Table 4. The sBPDA/DCHM film shows excitation and emission peaks at 369 and 399 nm, respectively. In the following section, the excitation and fluorescence peaks will be expressed as (λex/λem) = (369/399). Wakita et al.69 reported that this fluorescence could be attributed to the LE(π ← π*) transition because the excitation wavelengths correspond to the calculated wavelength (336 nm) for the

Figure 8. Pressure dependence of the UV−vis absorption spectra of the (a) sBPDA/DCHM and (b) aBPDA/DCHM films. H


Article


polyimides, which are not π-conjugated polymers, is caused by an increase in intermolecular interaction.13,38,39 In general, molecules in the excited state (S1) are more polarizable and stabilized than those in the ground state (S0) by enhancement of van der Waals interactions caused by reduction in intermolecular distances.71,72 For example, Bakhshiev et al.73 reported that the polarizability of benzene increased by 25− 30% in the excited state. In the case of sBPDA-PIs and aBPDAPIs, the π-conjugation at the biphthalimide moieties in the former PIs and that at the phthalimide moieties in the latter PIs are localized without extension to the diamine moieties. This view is supported by our recent report based on far-IR analyses which indicates that the biphenyl structures in sBPDA-PIs have nearly coplanar conformations, whereas the phthalimide-N-phenyl structures have highly twisted conformations.74 In addition, Figure 8 and Figure S3 show that the sBPDA-PIs having nearly coplanar biphenyl structures exhibited larger pressure-induced bathochromic shifts than those of the aBPDA-PIs having bent dianhydride structures. These facts indicated that the pressure-induced red-shifts were not mainly caused by the increase in intramolecular interaction due to conformational changes which extended the πconjugation at the dianhydride moiety. Accordingly, the pressure-induced bathochromic shifts of the LE absorption bands and fluorescence peaks of the Al-PI films can be attributed to the lowered energy gaps between S0 and S1 due to the reduced interchain distance. 3.3.4. Pressure-Induced Energy Shifts of the LE Bands in the UV−Vis Absorption Spectra. Variations in the aggregation structures of the PI films can be discussed on the basis of the degree of relative energy shifts in the λOD0.5 of the LE absorption band. Owing to the nature of the LE(π−π*) transitions as described above, the pressure-induced bathochromic shift of the LE absorption band mainly reflects the variations in local electronic states at the dianhydride (sBPDA or aBPDA) moieties of the PI chains. The LE band of each PI film exhibited larger energy shifts at pressures up to 2 GPa compared to those at above 2 GPa, as shown in Figure 10. This phenomenon corresponding to the significant volumetric compression was observed by optical microscopy below 2 GPa (Figure 3). Therefore, the bathochromic shifts observed below 2 GPa are mainly caused by the decreased energy gap associated with the compression of interchain free volume. Furthermore, all Al−PI films exhibited smaller pressureinduced energy shifts in the LE band above 2 GPa, mainly due to the variations in electronic states at the dianhydride moiety caused by the conformational changes or structural deformation of the repeating unit. In comparison to the aBPDA-PIs with bent structures, the relative energy shifts of λOD0.5 between 0.1 MPa and 8 GPa of the aBPDA/CHDA (−0.53 eV) are larger than those of the aBPDA/DCHM (−0.46 eV) and aBPDA/DMDHM (−0.43 eV) films. This suggests that the structural deformation caused by compression stress around the dianhydride moiety is more significant in aBPDA/CHDA than that of the other PIs above 2 GPa. As the compression behaviors of these PI films indicated (Figure 3), the larger conformational freedom at the diamine moieties in aBPDA/DCHM and aBPDA/DMDHM can absorb or release compression stress at the aBPDA moiety. Moreover, as seen in Figure 10, the sBPDA-PI films exhibited larger pressure-induced energy shifts than those of the aBPDA-PI films above 2 GPa. This indicates that the quasilinear structure of the sBPDA-PI chains is more sensitive

films. The wavelengths, λOD0.5, at which the absorbance (optical density, o.d.) was 0.5 were shifted by 72 nm for sBPDA/DCHM and by 58 nm for aBPDA/DCHM when the pressure was raised to 8 GPa. Similarly, the λOD0.5 of the LE absorption bands for the sBPDA/DMDHM, aBPDA/ DMDHM, sBPDA/CHDA, and aBPDA/CHDA films exhibited bathochromic shifts by 58, 54, 101, and 69 nm, respectively (Figure S3). On the other hand, the pressure-induced variations in the fluorescence spectra of the Al−PI films were investigated at pressures up to 2 GPa, where the PI films exhibited detectable fluorescence intensity. The fluorescence spectra of the aBPDAPI films were hardly observed under high pressure because of insufficient emission intensity. The fluorescent quantum yields of the aBPDA-PI films (Φ = 0.001−0.005) at 0.1 MPa were much smaller than those of the sBPDA-PIs (Φ = 0.066−0.104, Table 3). This is supported by the calculated oscillator strength of the S0 → S1 transition of m(aBP/DC) (f = 0.0151) which is only 7.3% of that of m(sBP/DC) ( f = 0.2073). This phenomenon originates from the limited conjugation between the two phthalimide rings in the aBPDA moiety with a distorted conformation due to steric effects (Table 3 and Figure 5). Hence, only the sBPDA-PI films were used to investigate the relationships between fluorescence and aggregation structures. Figure 9 and Figure S4 show the

Figure 9. Pressure dependence of the fluorescence spectra of sBPDA/ DCHM.

pressure dependence of the fluorescence spectra of the sBPDAPI films, with the fluorescence intensity normalized using the 0.1 MPa spectrum. The fluorescence peaks of the sBPDA/ DCHM, sBPDA/DMDHM, and sBPDA/CHDA films exhibited bathochromic shifts by 16, 8, and 15 nm, respectively, when the pressure was raised to 2 GPa. Both the bathochromic shifts in the absorption and fluorescence spectra were discussed in relation to the decrease in energy gap between the ground and excited states.13,32,70−72 The pressure-induced decrease in band gap can be interpreted by an enhancement of intermolecular or intramolecular interactions. Guha et al.40,41 have reported that the pressure-induced decrease in the band gap energy in π-conjugated molecules and polymers is mainly induced by an increase in intramolecular interaction due to a planarization of the backbone, resulting in a higher degree of the overlap of the π-electron wave functions. On the other hand, we have reported that the pressure-induced decrease in the band gap energy in fully aromatic and semialiphatic I


Article


relative energy shift increases with increasing degree of inplane orientation of the PI chains (section 3.1). We previously reported that the main chains of sBPDA-PIs incorporated in the highly oriented LC-like ordered region are densely packed in a coplanar conformation at the biphenyldiimide moieties.74 Thus, the sBPDA-PIs exhibiting higher degrees of in-plane orientation and densely packed aggregation structures are more significantly affected by intense compression stress at the dianhydride moiety. On the other hand, no appreciable differences were observed in the energy shifts of the sBPDAPIs below 2 GPa, which supports the assumption that the pressure-induced variations in the LE bands are mainly caused by the compression of interchain free volumes at lower pressures. 3.3.5. Pressure-Induced Reduction in LE Fluorescence Intensity. Figure 11 shows the pressure-induced variations in

Figure 11. Pressure-induced variations in fluorescence intensities of the sBPDA-PI films.

the LE(π ← π*) fluorescence intensities of the sBPDA/ DCHM, sBPDA/DMDHM, and sBPDA/CHDA films, which decreased by −26.1%, −29.3%, and −39.1%, respectively, with increasing pressure from 0.1 MPa to 2 GPa. These significant reductions and accompanied bathochromic shifts of the fluorescence peaks were mainly caused by the increased rate of nonradiative transitions rather than decreased emission efficiency.30,75 The pressure-induced reduction in the LE(π ← π*) fluorescence intensity could also be related to variations in the local electronic states at the dianhydride moiety caused by decreasing interchain distances. In addition, the reduction in fluorescence intensity of the films below 0.5 GPa was slightly smaller than that observed above 0.5 GPa. The behaviors of the films differed from those of pressure-induced energy-shifts of the LE(π → π*) absorption bands. Since the rate of nonradiative transition exponentially increased with decreasing energy gaps between the ground and excited states (energy-gap law),76 the LE(π ← π*) fluorescence was not efficiently quenched at lower pressures (

Pressure-Induced Variations of Aggregation Structures in Colorless

Recommend Documents