Colorless Partially Alicyclic Polyimides Based on TrÃ¶ger's Base

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Colorless Partially Alicyclic Polyimides Based on Tröger’s Base Exhibiting Good Solubility and Dual Fluorescence/Phosphorescence Emission Yongbing Zhuang,*,†,‡ Ryoji Orita,† Eisuke Fujiwara,† Yu Zhang,‡ and Shinji Ando*,† †

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Department of Chemical Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan ‡ State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: This study proposes a molecular design and synthetic route to novel colorless, transparent polyimides that exhibit dual fluorescence/phosphorescence emission at long wavelengths applicable to solar energy conversion. Partially alicyclic polyimides (AcPI-TBs) based on Tröger’s base (TB) and denoted as Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 were synthesized by in situ TB formation. The resulting Ac-PI-TBs are readily soluble in common organic solvents and have good mechanical properties with tensile strengths of 72.5−102.3 MPa, elongations at breaks of 12.5−75.0%, low dielectric constants (∼2.66) and low thermal diffusivities (D⊥ ≤ 7.7 × 10−8 m2/s), and good thermal stability. The films are totally colorless and transparent with transmittances above 77% at 400 nm. The films also show dual fluorescence and phosphorescence emissions with Stokes shifts as large as 11 421 cm−1 at low temperatures. The results highlight the possible application of these films in the spectral conversion of unused UV solar radiation to useful visible light.

1. INTRODUCTION Polyimides (PIs) are widely used in advanced fields including aerospace, microelectronics,1 optoelectronics,2−7 and membrane separation8−10 because of their outstanding thermal stability, chemical resistance, and mechanical and electrical insulation properties. The design and development of functional PI films for application in areas such as opticalwavelength conversion (e.g., spectral downconverters) is currently a subject of intense interest.11−14 Wavelengthconversion materials absorb UV light and emit visible light, offering the possibility to prepare highly efficient and durable solar cells.15,16 Transparent PI films emitting phosphorescence are candidate materials for downconverting applications because of their large Stokes shift, which is the energy difference between the wavelengths of the absorption and emission peaks. However, the molecular design of novel PIs combining high transparency and phosphorescent emission is a technological challenge. Until now, only a few PIs are known to be transparent and phosphorescent.12,13,17 1,5-Methano-1,5-diazocine, Tröger’s base (TB), is a wellknown rigidly bridged, V-shaped, nonplanar heterobicyclic building block.18−21 The TB fragment has been effectively introduced to polymer backbones to construct soluble, highly permeable polymers.20−28 TB-based PIs are among the family of developed TB-based polymers.23,25,29−32 In previous reports, aromatic dianhydrides including 4,4′-(hexafluoroisopropyli© XXXX American Chemical Society

dene) diphthalic anhydride (6FDA), 3,3′,4,4′-tetracarboxybenzophenone dianhydride (BTDA), and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) have been used to prepare TBbased polyimide membranes for gas separation applications.29−32 These TB-based PIs exhibit a range of properties, including excellent solubility in common solvents, high thermal stability, acceptable mechanical strength and toughness, and high gas permeation, which are well-suited to their separation applications. Their free volume, chain orientation, and related physical properties such as dielectric constant, linear coefficient of thermal expansion (CTE), and out-of-plane thermal diffusivity have been investigated to probe structure−property relationships.33 TB-based PIs also have low dielectric constants and thermal diffusivity coefficients, which promotes their application in protective sunshades, heat insulation tape, and thermal barrier coatings.33 The Tröger’s base scaffold has been used to design and develop host materials for phosphorescent OLEDs (PhOLEDs).34 Its rigid, V-shaped, chiral, and C2-symmetric structure imparts an amorphous property to the resulting polymers. Until now, there has been no reported design and development of phosphorescent TB-based PIs. Previous Received: February 8, 2019 Revised: April 15, 2019

A

DOI: 10.1021/acs.macromol.9b00273 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. (a) Chemical Structures of Previously Reported PI-TB-1 and PI-TB-229 and (b) Synthesis of the TB-Based Partially Alicyclic PIs

research29−31 has shown TB-based PIs derived from aromatic dianhydrides to be deep yellow [e.g., PI-TB-2 in Scheme 1(a)], which limits their photoluminescence applications. However, colorless transparent PIs with high thermal stability can be prepared by introducing alicyclic structures and/or fluorine groups to the PI backbone.35 Exclusion of fluorine groups may be required to eliminate color from TB-based PIs, because PITB-1 [Scheme 1(a)] derived from 6FDA contains −C(CF3)2− groups in the main chain and is pale yellow.29 Thus, incorporation of alicyclic segments into PI backbones may succeed in preparing highly transparent TB-based PI films. In this report, we introduce TB units and alicyclic segments into chain backbones to develop novel optically transparent and phosphorescent PIs. Three commercial alicyclic dianhydrides, 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (H-PMDA), bicyclo-[2.2.2]oct-7-ene-2-exo,3-exo,5-exo,6-exo2,3:5,6-dianhydride (BTA), and 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), are used to synthesize new imide-containing partially alicyclic diamine monomers as shown in Scheme 1(b). The TB-based partially alicyclic PIs (Ac-PI-TBs) designated as Ac-PI-TB-1, Ac-PI-TB-2, and AcPI-TB-3 are synthesized by an in situ TB formation reaction. We investigated the chain structures and mechanical, thermal, and optical properties of the PIs including transparency in the UV−vis region, refractive index and in-plane/out-of-plane birefringence to clarify the relationship between structure and physical properties. These Ac-PI-TB films are characterized by high toughness, excellent thermal stability, low dielectric constant, and low thermal diffusivity. Their high transparency and dual fluorescence/phosphorescence emission with Stokes shifts up to 11 421 cm−1 at low temperature suggests their potential role as wavelength-conversion films in aerospace applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethoxymethane (DMM, 99.0%), toluene (99.8%), and N-methyl-2-pyrrolidinone (NMP, >99.0%) were purchased from Sigma-Aldrich (USA) and used as received. 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (H-PMDA, >98.0%) was kindly supplied by New Japan Chemical Co., Ltd. Bicyclo-[2.2.2]oct-7-ene-2-exo,3-exo,5-exo,6-exo-2,3:5,6-dianhydride (BTA, >98%) and 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA, >98.0%), 2,5-dimethyl-1,4-phenylenediamine (DPD, >98.0%), trifluoroacetic acid (TFA, >99.0%), and chloroform (>99.0%) were purchased from Tokyo Chemical Industry (TCI, Japan) and used as received. 2.2. Synthesis of Imide-Containing Diamine Monomers. The imide-containing partially alicyclic diamine monomers, Ac-D1, Ac-D2, and Ac-D3 [Scheme 1(b)], were synthesized by a previous method.29,30 Using Ac-D1 as an example, DPD (8.1720 g, 60 mmol) was dissolved in NMP (160 mL) in a three-neck flask, HPMDA (4.4834 g, 20 mmol) was added, and the mixture was stirred at room temperature under N2. After 12 h, 55 mL of toluene was added as an azeotropic agent. The system was heated to 180 °C and maintained at that temperature for at least 9 h. Water was removed during heating with a Dean−Stark trap by refluxing toluene. The resulting brownish solution was poured into a mixture of water and methanol (2 L, v/v = 1:1) under vigorous stirring. The resulting precipitate was filtered, washed with cold water (2 L), and dried to yield a white powder. The powder was recrystallized from DMF/H2O (v/v = 1:1) and dried at 80 °C in a vacuum oven before use. 2,6-Bis(4-amino-2,5-dimethylphenyl)hexahydropyrrolo[3,4-f ]isoindole-1,3,5,7(2H,6H)-tetraone (Ac-D1). Yield: 81.0%. 1H NMR (500 MHz, DMSO-d6): δ 1.88 (s, 6H), 2.01 (s, 6H), 2.11 (m, 4H), 3.05 (s, br, 2H), 3.17 (s, br, 2H), 5.04 (s, 4H), 6.51 (s, 2H), 6.62− 6.71 (t, 2H). FTIR (powder, ν, cm−1): 3460, 3380, 3237 (N−H stretching), 1772 (imide carbonyl symmetric stretching), 1704 (imide carbonyl asymmetric stretching), 1395 (imide −C−N). 2,6-Bis(4-amino-2,5-dimethylphenyl)-3a,4,4a,7a,8,8a-hexahydro-4,8-ethenopyrrolo[3,4-f ]iso-indole-1,3,5,7(2H,6H)-tetraone (Ac-D2). Yield: 83.1%. 1H NMR (500 MHz, DMSO-d6): δ 1.74 (s, 3H), 1.86 (s, 3H), 1.96−1.98 (d, 6H), 2.73 (s, 1H), 2.89 (s, 1H), 3.39 (s, br, 2H), 3.48 (s, br, 2H), 5.00 (s, 6H), 6.31(s, 2H), 6.45 (s, 2H), 6.61 (s, 2H). FTIR (powder, ν, cm−1): 3451, 3417, 3363, 3238 (N−H B


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Macromolecules Scheme 2. Synthesis of TB-Based Model Compound (TB-Model)

infrared (FT-IR) absorption spectra of imide-containing compounds and TB-based PI powders were measured at 650 to 4000 cm−1 using a Thermo Fisher Avatar-320 spectrometer. Molecular weight was determined by gel permeation chromatography (Waters GPC system, Milford, MA) with polystyrene as the external standard and DMF as the eluent. Mechanical properties were obtained using a Universal Testing Machine, UTM (AGS-J, Shimadzu, Kyoto, Japan), with specimens prepared according to ASTM D638-Type 5 recommendations. At least three specimens of each sample were examined. Thermogravimetric analysis (TGA) was performed using a TGAQ50 instrument (TA Instrument, Newcastle, DE, USA) at a heating rate of 10 °C/min under nitrogen. The Brunauer−Emmett−Teller (BET) surface areas (SBET) of the polymer powders were analyzed by nitrogen adsorption/desorption at 77 K in a Micromeritics ASAP 2020 analyzer (Micrometrics Instrument Corp., Norcross, GA, USA). Apparent surface areas were calculated from N2 adsorption data by multipoint BET analysis. Film density measurements were performed using an analytical balance (XSE105, Mettler Toledo, Columbus, OH, USA) fitted with a Mettler density kit in 2,2,4-trimethylpentane (Sigma-Aldrich, USA).29 Typically, the film samples were cut to a dimension of 1.5 × 1.5 cm with a thickness of 20−40 μm. The density (ρ) of film sample is determined by eq 1 mA1 ρ= (ρ − ρair ) + ρaux mA1 − mA2 aux (1)

stretching), 1771 (imide carbonyl symmetric stretching), 1706 (imide carbonyl asymmetric stretching), 1394 (imide −C−N). 2,5-Bis(4-amino-2,5-dimethylphenyl)tetrahydrocyclobuta[1,2c:3,4-c′]dipyrrole-1,3,4,6(2H, 5H)-tetraone (Ac-D3). Yield: 80.8%. 1H NMR (500 MHz, DMSO-d6): δ 1.88 (s, 3H), 1.90 (s, 3H), 1.96 (s, 6H), 3.63 (s, 4H), 5.09 (s, 4H), 6.54 (s, 2H), 6.68−6.73 (d, 2H), 6.84−6.90 (d, 2H). FTIR (powder, ν, cm−1): 3452, 3386, 3249 (N− H stretching), 1769 (imide carbonyl symmetric stretching), 1717 (imide carbonyl asymmetric stretching), 1388 (imide −C−N). 2.3. Synthesis of TB-Based Polyimides. The TB-based polyimides denoted as Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 [Scheme 1(b)] were prepared by polymerizing the imide-containing diamines (Ac-D1, Ac-D2, and Ac-D3) with DMM in trifluoroacetic acid (TFA).29,30 Using Ac-PI-TB-1 as an example, Ac-D1 (1.8421 g, 4.0 mmol) was dissolved in DMM (2.0 mL, 22.4 mmol) under a nitrogen atmosphere and cooled in an ice bath. TFA (80 mL) was added dropwise over 10 min. The mixture was stirred at room temperature for 48 h and then slowly poured into deionized water to precipitate a white fibrous solid. The solid product was washed with deionized water and methanol until the eluent was neutral. The product was purified by precipitation of a chloroform solution into methanol and dried at 120 °C for 24 h under vacuum to produce AcPI-TB-1 as a white powder. Ac-PI-TB-1. Yield: 91.9%. 1H NMR (500 MHz, DMSO-d6): δ 1.81 (s, 6H), 2.37 (m, 4H), 3.21 (s, 6H), 3.54 (s, 4H), 3.96 (s, br, 2H), 4.19 (s, br, 2H), 4.46 (s, br, 2H), 6.92−6.95 (m, 2H). FTIR (powder, ν, cm−1): 2950−2891 (C−Hx stretching), 1782 (imide carbonyl symmetric stretching), 1711 (imide carbonyl asymmetric stretching), 1384 (imide −C−N). Ac-PI-TB-2. Yield: 93.5%. 1H NMR (500 MHz, DMSO-d6): δ 1.77 (t, 6H), 2.18 (d, 6H), 2.69 (s, 2H), 3.45 (s, br, 4H), 3.88 (s, 2H), 4.17 (s, 2H), 4.41 (s, 2H), 6.34 (s, br, 2H), 6.52−6.86 (d, 2H). FTIR (powder, ν, cm−1): 2954−2886 (C−Hx stretching), 1774 (imide carbonyl symmetric stretching), 1697 (imide carbonyl asymmetric stretching), 1381 (imide −C−N). Ac-PI-TB-3. Yield: 92.9%. 1H NMR (500 MHz, DMSO-d6): δ 1.85 (s, 6H), 2.41 (s, 6H), 3.62 (s, br, 4H), 3.99 (s, 2H), 4.23 (s, 2H), 4.50 (s, 2H), 6.91−7.10 (d, 2H). FTIR (powder, ν, cm−1): 2955−2887 (C-Hx stretching), 1777 (imide carbonyl symmetric stretching), 1722 (imide carbonyl asymmetric stretching), 1377 (imide −C−N). 2.4. Synthesis of TB-Model. The TB-based model compound (TB-Model) was synthesized by using the imide-containing single amine (S-An) and DMM in trifluoroacetic acid as shown in Scheme 2. Details of the synthesis and characterization of TB-Model are contained in the Supporting Information (SI). The structures of S-An and TB-Model were confirmed by 1H NMR (Figure S1). 2.5. Film Preparation. The fibrous polymer powders were dissolved in NMP to form ∼5 wt % polymer solutions and cast onto fused silica substrates after filtering with 1.0 μm nylon (NY) filter cartridges. The solvent was evaporated by successive heating at 60 °C/3 h, 120 °C/9 h, and 200 °C/0.5 h in a vacuum oven. Freestanding films were obtained by peeling the materials from the substrates. 2.6. Characterization Methods. Solution-state 1H NMR spectra were measured with a JEOL AL-400 spectrometer operating at a frequency of 500 MHz. 1H chemical shifts are reported in ppm (δH) using tetramethylsilane (TMS) as reference. Fourier transform

where mA1 and mA2 are the sample’s dry mass in air and its buoyant mass in auxiliary liquid, respectively. ρaux is the density of auxiliary liquid, and ρair is air density (0.0012 g/cm3). Before a density measurement, film samples were dried in a vacuum oven at least 12 h at 120 °C. The corresponding errors for all measurements were less than 0.003 g/cm3. The fractional free volume (FFV, Vf) was calculated from measured density values as follows Vf =

V − V0 V

(2)

where V is the actual molar volume, which includes the occupied molar volume (V0) and the free volume. It is noted that V0 is larger than its van der Waals molar volume (Vw), because there is a limit for the molecular packing density achieved in a condensed phase.36 Typically, the V0 is 1.3 times larger than its Vw based on the packing density of a molecular crystal at 0 K.37 Therefore, eq 2 can be transformed as follows

Vf =

V − 1.3·Vw V ·ρ = 1 − 1.3· w V M

(3)

where M (g/mol) is the molecular weight of the repeat unit; ρ (g/ cm3) is film density determined by eq 1, and Vw (cm3/mol) is estimated by using a group contribution method38 (implemented in the Synthia module from Materials Studio 7.0). Linear thermal expansion coefficients (CTE) were measured in the film plane (x−y direction) of PI specimens (10 mm long, 2 mm wide) in the glassy region using thermomechanical analysis (TMA). An average value was determined in the 100−200 °C range at a heating rate of 5 °C min−1 with a thermomechanical analyzer (TMA-60, C


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Macromolecules Table 1. Molecular Weights and Packing Parameters of TB-Based PIs polymer

Mw × 10−3a

PDI (Mw/Mn)

ρb (g/cm3)

Vfc

d-spacingd (Å)

d-spacinge (Å)

SBET (m2/g)

Ac-PI-TB-1 Ac-PI-TB-2 Ac-PI-TB-3 PI-TB-129 PI-TB-229 Matrimid 521833,45,46

327 193 102 151 124 69.7

2.93 3.60 2.17 2.86 2.99 2.1

1.2288 1.2104 1.2134 1.2592 1.1876 1.2260

0.165 0.171 0.191 0.223 0.215 0.169

6.49 6.67 6.38 6.74 6.46 −

6.60 6.78 6.87 − − −

18 93 122 544 270 −

a

By gel permeation chromatography (DMF eluent, polystyrene standards). bFilm density measurements were performed with an analytical balance. From density measurements. dd-Spacing values in the solid film state. ed-Spacing values in the powder state.

c

Table 2. Solubility of Ac-PI-TBs in Organic Solvents solvent polymer

acetone

THF

DMSO

NMP

DMF

methanol

ethanol

chloroform

Ac-PI-TB-1 Ac-PI-TB-2 Ac-PI-TB-3

− − −

+ − −

+ + +

+ + +

+ + +

− − −

− − −

+ − −

+, soluble at room temperature; − , insoluble at room temperature.

a

Shimadzu Corp., Kyoto, Japan) under an appropriate load (typically 1.0 g for 20 μm thick films) in a dry nitrogen atmosphere. After preliminary heating to 300 °C and cooling to room temperature in the TMA chamber, data were collected from the second heating to avoid the influence of adsorbed water. Thermogravimetric analyses (TGA) were conducted with a DTG-60 analyzer (Shimadzu, Corp., Kyoto, Japan) at a heating rate of 10 °C/min. The out-of-plane thermal diffusivity (D⊥) of the PI films was determined at room temperature by the temperature wave analysis (TWA) method (Mobile-1, ai-Phase Co. Ltd., Tokyo, Japan) at a voltage of 1.8 V. Wide angle X-ray diffractometry (WAXD) was recorded on a Rigaku SmartLab diffractometer by using Cu Kα radiation (λ = 1.54 Å) at room temperature. The in-plane (nTE) and out-of-plane (nTM) refractive indices of PI membranes formed on fused silica substrates were measured using a prism coupler (PC-2010, Metricon Corp, Pennington, NJ, USA) at a wavelength of 1310 nm at room temperature. Average refractive indices (nav) were calculated using eq 4. The anisotropy of the refractive index, which is the difference between the in-plane and outof-plane birefringence (Δn), was calculated according to eq 5.

optical chopper, the detector shutter is opened following a 1 ms delay, and phosphorescence signals are collected for 23 ms until the next pulse occurs. Thus, the phosphorescence signal is collected from 1 to 24 ms after excitation. Photoluminescence quantum efficiencies (ΦPL) of powder and film samples were measured by another method using a calibrated integrating sphere (C9920, Hamamatsu) connected to a multichannel analyzer (C7473, Hamamatsu) via an optical fiber link. Samples in this measurement were excited at constant λex using a monochromated Xe light source. 2.7. Time-Resolved Luminescence Measurements. The decay component was recorded with excitation by flush LED light (λ = 340 nm). Phosphorescence lifetimes were measured by using a xenon flush lump unit (C11567-02, Hamamatsu). The decay component was recorded with excitation by using a band-pass filter (λ = 340 ± 10 nm). Phosphorescence decay curves were accumulated for 60 min. The emission decay was well-fitted by using a multiexponential decay function according to eq 6.40 I(t ) =

∑ Aiet/ τ i

nav =

2 2 + n TM 2n TE 3

Δn = n TE − n TM

(6)

where Ai and τi are the amplitudes and lifetimes of the individual components for multiexponential decay profiles, respectively. 2.8. Polarizability Anisotropy Calculations. Molecular polarizabilities (α) were calculated with the neglect of differential diatomic overlap (NDDO) approximation proposed by Martin et al.41 The method and procedures were the same as in our previous research.33 The VAMP module and AM1 method in Materials Studio 7.0 were used for the calculations, which were based on two repeating units with an extended conformation. The average polarizabilities (αav) were calculated from eq 7

(4) (5)

Dielectric constants (ε) of PI membranes were estimated from an empirical relationship using the average refractive index, εopt = 1.1nav2.39 UV−vis absorption spectra of film samples were recorded using a spectrophotometer (U-3500, Hitachi Hi-Technologies Corp, Tokyo, Japan). Photoluminescence excitation and emission spectra of solutions and of PI films were measured with a fluorescence spectrometer (F-7100, Hitachi Hi-Technologies Corp, Tokyo, Japan) equipped with a R928 photomultiplier tube (Hamamatsu Photonics, Hamamatsu, Japan). Fluorescence spectra of solutions were measured without degassing. The front-face method was adopted for film samples to reduce self-absorption of the emitted fluorescence. Emission spectra were measured with excitation at the peak wavelength (λex) of the corresponding excitation spectra. Excitation spectra were measured by monitoring the fluorescence intensity at the peak wavelength (λem) of the emission spectra. Spectra were not corrected for the sensitivity of the photomultiplier tube in the range of photoluminescence wavelengths. Phosphorescence spectra were measured in the phosphorescence mode with the same fluorescence spectrometer. In this measurement, a quasiexcitation pulse was generated from a continuous-wave (CW) light source by using an optical chopper. After the excitation beam is closed by the

αav =

αxx + αyy + αzz 3

(7)

where αzz, αxx, and αyy are the principal values of the polarizability tensor along the z-, x-, and y-axes, respectively. 2.9. Quantum Chemical Calculations. Optimized geometries, electronic structures, and spectroscopic properties of TB-Model and of the Ac-PI-TB and TB-PI model compounds were calculated using the Gaussian 16, Revision A.03 program package, which was installed at the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology.42,43 Model structures were optimized by density functional theory (DFT) at the B3LYP/6-311G(d,p) level followed by calculations of one-electron transitions using the timedependent DFT (TD-DFT) method with B3LYP and PBE0 functionals and the 6-311++G(d,p) basis set. Each calculated D


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Macromolecules Table 3. Calculated Structural Parameters for the Repeat Units of Ac-PI-TBs samples

Ma (g/mol)

Vw (cm3/mol)

Vvdw (Å3/mol)

αav (Å3)

αav/Vvdwb

α⊥/Vvdwb

Ac-PI-TB-1 Ac-PI-TB-2 Ac-PI-TB-3 PI-TB-129 PI-TB-229

496.567 520.589 468.513 716.641 594.628

259.492 274.137 240.243 340.390 302.338

430.897 455.216 398.932 565.231 502.045

55.8 58.7 52.0 69.2 65.3

0.129 0.129 0.130 0.122 0.130

0.124 0.122 0.123 0.115 0.122

a

M is the molecular weight of one repeat unit in chain backbone. bVvdw = Vw/NA is the van der Waals volume.

transition was replaced by a Gaussian broadening function with a width of 0.15 eV to reproduce the shapes of experimental spectra. The calculated absorbance was represented by the oscillator strengths divided by the van der Waals volumes of molecules, which were calculated from the optimized geometries based on Slonimski’s method.44 The van der Waals radii of atoms used in the calculation were reported by Bond.38

3. RESULTS AND DISCUSSION 3.1. Synthesis and Polymerization. A series of fully aromatic imide-containing diamines was synthesized previously

Figure 1. N2 adsorption (filled) and desorption (empty) isotherms for the Ac-PI-TB powders at 77 K.

Figure 2. WAXD curves of (a) Ac-PI-TB films and (b) Ac-PI-TB powders.

by using various aromatic dianhydrides with DPD.29,31 To prepare highly transparent PIs, the fully alicyclic H-PMDA, BTA, and CBDA dianhydrides were used as monomers to synthesize three partially alicyclic imide-containing diamines, Ac-D1, Ac-D2, and Ac-D3 [Scheme 1(b)]. These compounds were obtained in yields above 80%, and their chemical structures were confirmed by FTIR and 1H NMR as shown in Figures S2 and S3. The TB-based partially alicyclic PIs (AcPI-TBs), Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 [Scheme 1(b)], were synthesized in situ by a TB formation reaction, which has been described in detail.20 The chemical structures of the Ac-PI-TBs were confirmed by FTIR and 1H NMR (Figures S4 and S5). TB polymerization of imide-containing diamine monomers and DMM is readily performed to yield TB-based PIs as shown in Scheme 1(b). The 1H NMR signals of the −NH2 group in the imide-containing diamines are located near δ = 5 ppm (Figure S3). New 1H NMR signals assigned to the methylene protons of aliphatic C−Hx appear at δ = 3−6 ppm (Figure S5) after TB polymerization because of bridged bicyclic TB ring formation. Successful polymerization is indicated by polymer products with molecular weights of Mw = 1.02 × 105 − 3.27 × 105 g/mol and polydispersities of 2.17−3.60 as determined by GPC (Table 1). 3.2. Solubility of PI Powders. Table 2 contains the solubility of the Ac-PI-TB powders. All Ac-PI-TBs are soluble

Table 4. Mechanical Properties of Ac-PI-TB Films polymer code

tensile strength (MPa)

tensile modulus (GPa)

elongation at break (%)

76.1 72.5 102.3 59.4 ± 4.0 64.0 ± 2.8 72.28

0.82 0.91 1.22 1.58 ± 0.04 1.38 ± 0.04 1.41

75.0 32.0 12.5 5.1 ± 0.4 17.0 ± 3.2 19.4


Table 5. Thermal Properties of Ac-PI-TB Films sample Ac-PI-TB-1 Ac-PI-TB-2 Ac-PI-TB-3 PI-TB-129 PI-TB-229 Matrimid 521833,45,46 a

Td, 5%a (°C)

Td, 10%b (°C)

CTE (ppm/K)

thermal diffusivity, D⊥ (×10−8 m2/s)

Φ ⊥c

421 423 411 487 492 483

478 451 461 513 512 512

47.5 35.6 38.8 52.6 48.5 −

7.7 7.2 7.5 6.2 8.5 −

0.310 0.303 0.304 0.289 0.320 −

5% weight-loss temperature recorded by TGA. b10% weight-loss

temperature recorded by TGA. cVuks parameters Φ⊥ =

E

n⊥2 − 1 2 +2 nav

.


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Macromolecules

from CBDA has the largest Vf value. Greater chain rigidity increases Vf for polymers of intrinsic microporosity (PIM) including TB-based polymers.21 The sterically compact structure of the cyclobutyl ring in CBDA is more rigid than the cyclohexyl ring in H-PMDA and the bicyclo[2.2.2]oct-7ene moiety of BTA, which produces the largest Vf value for AcPI-TB-3. The Vf values of the TB-based PIs derived from alicyclic dianhydrides are much less than those of TB-based PIs derived from the aromatic dianhydrides 6FDA (PI-TB-1) and BTDA (PI-TB-2) (Table 1). Because greater chain rigidity and larger structural distortions increase the PI free volume, introduction of the alicyclic dianhydride segments from H-PMDA, BTA, and CBDA into the chain backbone results in smaller free volumes compared to those of the corresponding PIs with aromatic dianhydride segments (6FDA and BTDA). TB-based partially alicyclic PIs have greater Vf values than the corresponding fully aromatic PIs such as ODPA-PDA (Vf = 0.119)47 and BTDA-pp′-ODA (Vf = 0.124)48 and comparable or greater Vf values than the partially alicyclic commercial PI, Matrimid 5218 (Table 1). These results confirm the role of backbone TB units in enhancing Vf values as described in previous research.29,31,49 3.4. Nitrogen Adsorption/Desorption Behavior and Chain Aggregation. The nitrogen adsorption/desorption behavior was measured at 77 K to estimate the polymer porosity. Their resulting BET surface areas were in the range of 18−122 m2/g (Table 1), which were significantly smaller in comparison with those of PI-TB-1 (SBET = 544 m2/g) and PITB-2 (SBET = 270 m2/g) derived from aromatic dianhydrides, 6FDA and BTDA, respectively.29 As shown in Figure 1, only the partially alicyclic TB-based PI derived from CBDA exhibited typical microporosity behavior, which is confirmed by its rapid nitrogen uptake at very low relative pressure (p/ p0). The cyclobutyl rings of CBDA residues in chain backbones are more rigid than the cyclohexyl rings of H-PMDA residues and the bicyclo[2.2.2]oct-7-ene moieties of BTA residues, resulting in the intrinsic microporosity of AcPI-TB-3. WAXD measurements were conducted to investigate PI chain aggregation. TB-based PIs derived from aromatic dianhydrides such as PI-TB-1 and PI-TB-2 display obvious structural order in their chain aggregation.29 However, Ac-PITBs films [Figure 2(a)] exhibit only a single broad amorphous halo at 2θ near 13°, which suggests an almost amorphous morphology. The d-spacings of 6.49, 6.67, and 6.38 Å calculated from peaks at 2θ = 13.63, 13.27, and 13.87° for the Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 films, respectively, are assigned to the interchain distances of the main chains in the out-of-plane direction.50,51 However, d-spacings

Figure 3. TGA curves of Ac-PI-TB films under a nitrogen atmosphere.

Figure 4. Out-of-plane thermal diffusivity (D⊥) versus the Vuks parameter (Φ⊥) of various polymers including TB-based PIs from the alicyclic dianhydrides in this study (▲) and TB-based PIs with aromatic dianhydrides (●), conventional polyimides (Δ), and amorphous polymers (○).55

in common polar organic solvents such as DMSO, DMF, and NMP. Only Ac-PI-TB-1 is soluble in chloroform. Good solubility in common solvents facilitates processing in the formation of film/membrane materials. 3.3. Density and Free Volume. The film densities of the Ac-PI-TBs are 1.2104−1.2288 g/cm3 (Table 1). The Vf values of the TB-based PIs calculated from density measurements range from 0.165 to 0.191 based on film densities (Table 1) and structural parameters (Table 3). The Ac-PI-TB-3 derived

Table 6. Optical Properties of TB-Based PIs Derived from Alicyclic Dianhydrides polymers

n||

n⊥

Δna

navb

λEc (nm)

λcutd (nm)

T400e (%)

εoptf


1.5590 1.5594 1.5660 1.5353 1.6046 1.6076

1.5448 1.5451 1.5472 1.5163 1.5790 1.5952

0.0142 0.0143 0.0188 0.0189 0.0256 0.0125

1.5543 1.5546 1.5597 1.5290 1.5961 1.6035

303 304 305 / / /

297 303 310 / / /

83(96) 77(91) 71(92) (66) (52) /

2.66 2.66 2.68 2.57 2.80 2.83

a

In-plane/out-of-plane birefringence. bAverage refractive indices measured at 1310 nm. cAbsorption edges (λE) estimated as the point at which a linear extrapolation of the decrease in absorbance intersects the baseline. dUV−vis transmission cutoff wavelength. eTransmittance (%) at 400 nm (T400) for 25 ± 3 μm film thicknesses; the data in parentheses are for 2 μm films. fDielectric constants estimated as εopt = 1.1nav2. F


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Figure 6. Photoluminescence spectra of TB-Model crystallites. (a) Normalized excitation (dotted lines) and PL emission (solid lines) spectra and (b) phosphorescence spectra of TB-Model crystallites. Spectra recorded between 298 and 123 K at 25 K intervals. (c) Photoluminescence decay curves of TB-Model crystallites monitored at 298 and 77 K. λex = 340 nm. Figure 5. UV−vis absorption spectra (a), optical transmission spectra (b) of 25 μm thick Ac-PI-TB films, and photograph of their freestanding films (∼10 μm thick) (c).

the range of 76.1−102.3 MPa, initial moduli of 0.82−1.22 GPa, and elongations at breaks of 12.5−75.0% define the moderate mechanical properties of Ac-PI-TB films. Ac-PI-TB-1 and AcPI-TB-2 show an excellent elongation at break of ≥32.0%, which is significantly greater than values such as 5.1% (PI-TB1) and 17.0% (PI-TB-2) reported for TB-based PIs derived from aromatic dianhydrides and 19.4% reported for commercial Matrimid 5218. This result implies a degree of toughness for TB-based PIs because of the introduction of alicyclic segments into the chain backbone. BTA is known to have poor polymerizability, because all C=O groups exhibit an exo configuration as shown in Scheme 1.3,52,53 Once a functional group reacts with a terminal amino group, the surviving functionalities may experience steric hindrance in the

in the out-of-plane direction do not correspond to the accurate molecular packing in whole films because of the in-plane orientation of chains and a certain degree of order in chain aggregation. Chain aggregation is three-dimensionally homogeneous in the powder state, which provides a more reliable means of assessing the influence of chain structure on dspacing. Figure 2(b) shows the WAXD patterns of powder samples. The order of the average d-spacing (Ac-PI-TB-1 < Ac-PI-TB-2 < Ac-PI-TB-3) coincides well with the Vf values from density measurements (Table 1). 3.5. Mechanical Properties. The mechanical properties of the TB-based PI films are listed in Table 4. Tensile strengths in G


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Figure 7. Normalized photoluminescence (PL) spectra (short dot line) and phosphorescence spectra (solid line) of Ac-PI-TB films recorded at 123 K. Excitation wavelengths are listed in Table 7.

formation of extended polymer chains.52,53 Thus, there has been no report of successful tough PI-film preparation using only BTA dianhydride. In this research, a tough PI film derived from BTA is obtained by in situ TB polymerization, which effectively avoids the “self-steric hindrance” of the anhydride functional groups in BTA.3,54 3.6. Thermal Properties. Thermal analysis data of the AcPI-TB films are summarized in Table 5 and Figure 3. Weight losses of 5 and 10% were observed at 411−423 and 451−478 °C, respectively. The Ac-PI-TB films show poorer thermal stability than their PI-TB counterparts (e.g., PI-TB-1 and PITB-2) because of the introduction of the alicyclic segments. However, a 5% weight-loss temperature above 410 °C is sufficient for thermally stable optical materials. TMA was used to probe the linear thermal expansion coefficients (CTE) of Ac-PI-TB films. Their modest values of 35.6−47.5 ppm/K (Table 5) are less than those of films based on aromatic dianhydrides (PI-TB-1 and PI-TB-2) probably because of the improvement in chain linearity upon introduction of the alicyclic segments, resulting from less distorted conformation of Ac-PI-TB chains as compared to those of PI-TB-1 and PI-TB-2 (Figure S6). The out-of-plane thermal diffusivities (D⊥) listed in Table 5 range between 7.2 × 10−8 and 7.7 × 10−8 m2/s. The D⊥ values of Ac-PI-TBs are slightly greater than that of PI-TB-1 but are much smaller than those of fully aromatic PI films (e.g., D⊥ = 16.5 × 10−8 and 12.3 × 10−8 m2/s for BPDA-PDA and 6FDAODA,55 respectively). We previously reported that the Vuks parameter (Φ⊥) effectively assesses the thermal diffusivity of many polymer materials and qualitatively exhibits a positive, linear correlation with D⊥.55 Φ⊥ is estimated by use of eq 8

Figure 8. Photoluminescence decay curves of Ac-PI-TB films monitored at their phosphorescence wavelengths at 77 K. λex = 340 nm for all samples.

Φ⊥ =

n⊥2 − 1 nav2

+2

=

a 4π ·(1 − Vf ) · ⊥ 3.9 Vvdw

(8)

Figure 4 shows that Ac-PI-TBs have smaller Φ⊥ values (Φ⊥ ≤ 0.310) than fully aromatic PI films such as 6FDA-ODA (Φ⊥ = 0.321) and BPDA-PDA (Φ⊥ = 0.342) and certain

Table 7. Photophysical Data of TB-Model and TB-Based PI Films samples Ac-PI-TB-1 Ac-PI-TB-2 Ac-PI-TB-3 TB-Model

a,b λPL (nm) ex

302 275 275 274 274f

b λPL max (nm)

364, 454 354, 447 365, 458 324, 439 324, 439 (weak)f

a,b λPhos (nm) ex

b λPhos (nm) max

302 310 314 296 296f

461 461 458 439 439f

λvc (cm−1) 5640, 8115, 8966, 5632, 5632,

11 421 10 561 10 013 13 717 13 717

τavd (ms) g

326.0 211.1g 160.7g 184.2g 20.6f

τmaxe (ms)

ΦPL

918.0g 579.5g 508.8g 1039.3g 61.7f

0.035g 0.054g 0.092g 0.445g 0.051f

λex corresponds to the S0 → S2 transition wavelength. bAt 123 K. cStokes shifts, ν = 107 (1/λex − 1/λem). dAverage lifetime (τav) was calculated as ⟨τ⟩ = ΣAiτi2/ΣAiτi, where Ai is the pre-exponential for lifetime τi. eτmax is the longest lifetime of all component lifetimes for multiexponential decay profiles. fAt 298 K. gAt 77 K. a

H


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PI films and partially alicyclic PI films such as CBDA-BTPB and CBDA-BCTPB (∼322 nm).6 The introduction of Vshaped TB moieties and alicyclic segments to the PI chain backbones breaks the long-range intrachain conjugation of imide−phenyl and phenyl−phenyl bonds, which weakens the intra and interchain charge transfer (CT) interactions and results in a shorter λE. The transmission spectra of 25 μm thick Ac-PI-TB films in Figure 5 reveal a high transparency in the visible region (%T at 400 nm = 83 for Ac-PI-TB-1, 77 for Ac-PI-TB-2, and 71 for Ac-PI-TB-3). This property is attributed to the lower electron affinity of the alicyclic dianhydride segments and the presence of V-shaped TB moieties, which combine to reduce CT interactions. The high optical transparency and colorless feature [Figure 5(c)] of Ac-PI-TB films suggest their possible application as flexible substrates for electronic or micro-optical devices in the fields of displays, memory, lighting, solar cells, sensors, and waveguides. The TB-Model compound (Scheme 2) is helpful in understanding the optical properties of Ac-PI-TBs. Figure S7(a) shows the UV−vis absorption spectrum of 1.0 × 10−4 M TB-Model dissolved in CHCl3 at room temperature. The absorption maximum at 257 nm corresponds to the locally excited (LE) π−π* transition, which is calculated by TD-DFT to have a maximum absorption at 277 nm [Figure S7(b)] or 266 nm [Figure S7(c)] with an oscillator strength of 0.109 [(Table S1)] or 0.139 [(Table S2)] at the B3LYP and PBE0 levels of theory, respectively. The absorption arises from the HOMO → LUMO + 2 transition in the diamine-linked TB unit as shown in Figure S8. The small mismatch between experimental and calculated absorption maxima is likely caused by a solvent effect. Figure S9 shows the calculated absorption spectra of Ac-PITB model compounds in this study and of previous PI-TBs (Scheme S1) based on TD-DFT theory. Experimental λE and λcut values for the Ac-PI-TBs are well-reproduced by these calculations. The common peak at 270 nm corresponds to the LE π−π* transition of TB-Model observed at 257 nm [Figure S7(a)]. This indicates that the strong UV absorption of Ac-PITBs also is assignable to the diamine-linked TB unit in the main chain. The pale and darker yellow colors of PI-TB-1 and PI-TB-2 films are explained on the basis of model compounds as originating from weak and relatively strong π−π* CT transitions, respectively, in the visible region. 3.9. Photoluminescence (PL). Crystalline particles of TBModel were used to confirm the potential for phosphorescence by the Ac-PI-TBs. TB-Model crystallites have three-dimensional positional order, which suppresses local molecular motion at room temperature. Figure 6 shows the photoluminescence (PL) spectra of TB-Model at temperatures between 298 and 123 K at 25 K intervals. The phenomenon of dual emission at λ= 324 and 439 nm is observed even at 298 K. The PL intensities of the two emission peaks increase with decreasing temperature with those at 324 and 439 nm enhanced by 2.5 and 10 times, respectively, from 298 to 123 K. The intensity of the 324 nm emission changes less abruptly with temperature. Only a single band at 439 nm is observed in the phosphorescence mode [Figure 6(b)]. On the basis of these experimental results, we propose that the two emission signals in Figure 6 arise from dual fluorescence/phosphorescence emission. The emission lifetimes (τav) of TB-Model monitored at 439 nm are 20.6 and 184.2 ms at 298 and 77 K, respectively. The photoluminescence quantum yields, ΦPL, of

amorphous polymers such as PS (Φ⊥ = 0.335), PES (Φ⊥ = 0.356), and PC (Φ⊥ = 0.336). TB-based PIs from alicyclic dianhydrides exhibit a more narrow distribution of D⊥ versus Φ⊥ than PI-TBs from aromatic dianhydrides. The value of Φ⊥ is closely related to the parameters Vf and a⊥ in eq 8. The AcVvdw

PI-TB films exhibit small differences in Vf (Table 1) and almost identical a⊥ values (∼0.123 in Table 3), which results Vvdw

in similar D⊥ values (Table 5). 3.7. Refractive Indices and Birefringence. The in-plane (n||) and out-of-plane (n⊥) refractive indices of Ac-PI-TB films measured at 1310 nm are listed in Table 6. The n|| and n⊥ values range from 1.5590−1.5660 and 1.5448−1.5472, respectively. The average refractive indices (nav) of Ac-PITBs measured at 1310 nm and estimated by eq 9 range from 1.5543 to 1.5597. ÄÅ ÅÅ ÅÅ Å nav = ÅÅÅÅ ÅÅ ÅÅ 1 − ÅÅÇ

ÉÑ1/2 ÑÑ ÑÑ Ñ 3 − 2ÑÑÑÑ α ÑÑ 4π ÑÑ · V av ·(1 − Vf ) 3.9 ÑÑÖ vdw

( )

(9)

Here, αav is the average molecular polarizability, Vf is the fractional free volume of the polymer, and Vvdw is the van der Waals volume based on the group contribution.38 This relationship shows that an increase in Vf and a decrease in αav/Vvdw will decrease nav. Table 6 shows that the Ac-PI-TB-3 derived from CBDA exhibits a slightly larger nav value (1.5597) than Ac-PI-TB-1 and Ac-PI-TB-2 because of the slightly greater αav/Vvdw value (0.130 in Table 3) of the corresponding PI. Values of nav for all Ac-PI-TBs are greater than that of fluorinated PI-TB-1 (nav= 1.5290), because the fluorine substituents produce a very small αav/Vvdw (0.122 for the repeat unit of PI-TB-1 in Table 3). In-plane/out-of-plane birefringence, Δn, is an important property of colorless PI films that are used as substrates for flexible organic electroluminescent (EL) displays. Table 6 shows that the Ac-PI-TBs exhibit small, nonzero Δn values ranging from 0.0142 to 0.0188 because of the combined impact of high chain rigidity, small polarizability, and nonlinear V-shaped configurations that arise from the introduction of TB moieties into the chain backbones. The Ac-PI-TBs display slightly smaller Δn values (

Colorless Partially Alicyclic Polyimides Based on TrÃ¶ger's Base

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