Synthesis and Characterization of Organo-Soluble Polyimides Derived

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Synthesis and Characterization of Organo-Soluble Polyimides Derived from Alicyclic Dianhydrides and a Dihydroxyl-Functionalized Spirobisindane Diamine Mahmoud Abdulhamid, Xiao-Hua Ma, Bader S Ghanem, and Ingo Pinnau ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00036 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Polymer Materials

Synthesis and Characterization of Organo-Soluble Polyimides Derived from Alicyclic Dianhydrides and a Dihydroxyl-Functionalized Spirobisindane Diamine

Mahmoud A. Abdulhamid, Xiaohua Ma, Bader S. Ghanem, Ingo Pinnau*

Functional Polymer Membranes Group, Advanced Membranes and Porous Materials Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.

*E-mail: [email protected]

Keywords: alicyclic dianhydrides; polyimides; intrinsic microporosity; optical properties; membrane; gas separation

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ABSTRACT Two organo-soluble polyimides were synthesized by reaction of alicyclic bicyclo[2.2.2]oct-7ene-2,3,5,6-tetracarboxylic dianhydride (BC) or 1,4,7,8-tetrabromobicyclo[2.2.2]oct-7-ene2,3,5,6-tetracarboxylic anhydride (BCBr4) with 3,3,3’,3’-tetramethyl-1,1’-spirobisindane-5,5’diamino-6,6’-diol (SBIDA). BC-SBIDA and BCBr4-SBIDA showed thermal stability of up to ~420 and 352 °C and displayed microporosity as indicated by Brunauer-Emmett-Teller (BET) surface areas of 191 and 243 m2 g-1, respectively. The polyimides were solution processible in polar organic solvents and exhibited strong mechanical properties with tensile modulus of 1.151.4 GPa, tensile strength of 27-28 MPa, and elongation at break of 2-4%. Introducing alicyclic moieties disturbs the delocalization of π-electrons across the polyimide backbone that reduces formation of charge transfer complexes (CTCs) leading to formation of colorless and transparent polyimide films. A fresh film sample of the bromine substituted BCBr4-SBIDA showed oxygen permeability of 31 barrer and oxygen/nitrogen selectivity of 5.9. Long-term physical aging of BCBr4-SBIDA over 365 days resulted in decrease of O2 permeability to 17 barrer with a simultaneous boost in O2/N2 selectivity to 6.6, which demonstrated highly competitive performance compared to commercially available polymers for air separation.

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1. INTRODUCTION Aromatic polyimides (PIs) are utilized in various highly demanding applications due to their excellent mechanical, mechanical, electrical and chemical properties.1,2 Fully aromatic PIs are composed of conjugated structures with strong intra-and inter-chain interactions leading to charge transfer complex (CTC) formation.3-5 The CTCs originate from interactions of the electron-withdrawing groups in the aromatic dianhydride and the electron-donating moieties in the aromatic diamine building block. As a result, aromatic PI films often show deep yellow to brown colors with poor optical transmittance which restrict their use in optical and optoelectronic applications.7 High optical transparency in polyimide films can be achieved by several strategies, including introduction of: (i) electronegative groups, such as –CF3, (ii) alicyclic building blocks, and (iii) sterically hindered groups with twisted or bulky structures, which can influence the color, dielectric constant and LCD alignment of polyimides.6-14 4,4'(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) has been extensively investigated as an architectural building block for polyimide synthesis because it can yield organo-soluble, transparent and almost colorless polyimides.15-18 6FDA has also been employed for the design of linear polyimides of intrinsic microporosity (PIM-PIs) as advanced membrane materials for gas separation by reaction with diamines bearing bulky contortion sites based on spirobisindane-19,20 , spirobifluorene-21-23, triptycene-24-28, Tröger’s base-29-34 and carbocyclic pseudo Tröger’s base35 building blocks. Alternatively, PIM-PIs can also be designed from dianhydrides with various architectural motifs that induce microporosity in the polymer. Specifically, optimized PIM-PIs based on triptycene- and ethanoanthracene-derived dianhydrides36,37 combined with sterically hindered diamines, such as 2,3,5,6-tetramethyl-pphenylenediamine or 3,3’-dimethylnaphthidine, demonstrated excellent gas separation

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properties with performance defining the 2015 permeability/selectivity upper bound for O2/N2 separation.38 Here, we report solution-processable PIM-PIs synthesized from alicyclic dianhydrides, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic

dianhydride

(BC)

or

1,4,7,8-

tetrabromobicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic anhydride (BCBr4), reacted with a hydroxyl-functionalized PIM-motif diamine, 3,3,3’,3’-tetramethyl-1,1’-spirobisindane-5,5’diamino-6,6’-diol (SBIDA). The optical and gas transport properties of BC-SBIDA and BCBr4SBIDA polyimides are compared to those of a previously reported aromatic 6FDA-SBIDA polyimide analogue.19,20 The protocols for the preparation of the intermediates, monomers and BC-SBIDA and BCBr4-SBIDA polyimides are presented in Scheme 1.

Scheme 1. Synthetic Procedure for BCBr4 and SBIDA Monomers and PIM-PIs (BCSBIDA, BCBr4-SBIDA and 6FDA-SBIDA19,20).

Br

Br Br

S

Br

Br m-CPBA

Br

Br

ClCH2CH2Cl

O

S

O

Br

Maleic anhydride

O

Bromobenzene

Br

O O

HO

H 2N OH

OH

HO

NH2

NO2 III

4


O O

Br

I

O 2N

Br

Br II

O

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

H

H

O

O

O

O

H

OH O

O

H

HO

Br

Br

OH

HO

NH2

m-cresol

O

Isoquinoline

O

O N

H

n H

O

Br

O

BC-SBIDA

O

O

H 2N

H

N

BC

O

H

O Br

OH O

O

Br

N

BCBr4

200 oC

HO

O

SBIDA

O

F3C

Br

CF3

N Br

n Br

O

F3C

CF3

BCBr4-SBIDA

O

O

O

O

OH O

O 6FDA

N HO

O

O N

n

6FDA-SBIDA

O

2. EXPERIMENTAL SECTION 2.1. Materials. Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BC), 3,3,3’,3’tetramethyl-1,1’-spirobisindane-5,5’-dinitrol-6,6’-diol, tetrabromothiophene, 1,2-dichloroethane, 3-chloroperbenzoic acid (m-CPBA), maleic anhydride, bromobenzene, isoquinoline, palladium on carbon (loading 5 wt%), and hydrazine monohydrate were obtained from Sigma-Aldrich. mCresol was purified by distillation and stored over molecular sieves. Other solvents from various sources were used as received. 2.2. Characterization and Methods. The intermediates, monomers and polyimides were characterized as previously reported39 by: 1H NMR and

13C

NMR (Bruker AVANCE-III), gel

permeation chromatography (GPC, Agilent 1200), FT-IR (Varian 670-IR), and thermal gravimetric analysis (TGA, TA Instruments Q5000). N2 adsorption on powder polymer samples at -196 °C (Micrometrics ASAP 2020) was used for determination of BET surface area. The density of the polyimides was measured based on Archimedes’ principle using iso-octane as the

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reference liquid. Optical properties were assessed by fluorescence spectrosopy (PerkinElmer LS45) and UV-vis spectroscopy (Varian Lambda 1050). Dynamic mechanical analysis (DMA, TA instruments, Q-800) was used to determine mechanical properties of the polymers. 2.3. Synthesis of Tetrabromothiophene-1,1-dioxide (I). Tetrabromothiophene-1,1-dioxide was synthesized as previously reported.40 Tetrabromothiophene (4.5 g, 11.3 mmol) and 3chloroperbenzoic acid (8.4 g, 48.4 mmol) were refluxed in 1,2-dichloroethane (75 ml) for 5 days. The solution was cooled to ambient temperature and 3-chlorobenzoic acid was filtered off, washed twice with saturated sodium bicarbonate solution and dried over MgSO4. The crude solid was purified by column chromatography over silica gel with dichloromethane (DCM)/petroleum ether (1:1) as an eluent. The solvents were removed by rota-evaporation to yield tetrabromothiophene-1,1-dioxide as light yellow crystals. (3 g, 63% yield); mp = 201 °C; 13C

NMR (100 MHz, CDCl3, δ): 119.9, 128. FT-IR (powder, ν, cm-1): 1570 and 1530

(conjugated C=C, str), 1325 (asym S=O, str), 1311 (sym S=O, str). 2.4.

Synthesis

of

1,4,7,8-Tetrabromobicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic

Anhydride (BCBr4) (II). The product (I) (3.26 g, 7.55 mmol) was charged in a flask and reacted with maleic anhydride (1.85 g, 18.9 mmol) in bromobenzene (40 ml) under reflux for 5 days. After cooling the solution to ambient temperature, the product, 1,4,7,8-tetrabromobicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic anhydride, was filtered off and dried at 120 °C under vacuum. The product was recrystallized using acetic anhydride to obtain white crystals as a final product. (1.8 g, 42% yield); mp = 355 °C; 1H NMR (400 MHz, DMSO-d6, δ): 4.23 (s, 4H). 13C NMR (100 MHz, DMSO-d6, δ): 51.5, 114.6, 125.0, 165.2. 2.5. Synthesis of 3,3,3’,3’-Tetramethyl-1,1’-spirobisindane-5,5’-diamino-6,6’-diol (III) (SBIDA).19 Hydrazine hydrate (7 ml) was added under stirring to a mixture of 3,3,3’,3’-

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tetramethyl-1,1’-spirobisindane-5,5’-dinitrol-6,6’-diol (2.5 g, 6.3 mmol) and 5% Pd/C (0.6 g) in absolute ethanol (70 ml). After refluxing overnight, the mixture was filtered through Celite into water (600 ml). The white solid product was then dried under vacuum at 120 °C for 20 h. (2.25 g, 90 % yield); mp = 263 °C; 1H NMR (400 MHz, DMSO-d6, δ): 1.21 (s, 6H), 1.27 (s, 6H), 1.97 (d, J = 16 Hz, 2H), 2.15 (d, J = 16 Hz, 2H,), 4.31 (br s, 4H), 6.05 (s, 2H), 6.39 (s, 2H), 8.66 (br s, 2H). 13C NMR (100 MHz, DMSO-d6, δ): 31.1, 32.1, 42.7,56.7, 60.2, 107.5, 109.6, 135.9, 139.2, 142.6, 144.2. 2.6. Synthesis of BC-SBIDA. BC-SBIDA was prepared under previously reported polycondensation reaction conditions at 200 °C35 from the BC dianhydride (0.22 g, 0.88 mmol) and SBIDA (0.3 g, 0.88 mmol) (III) using 3 ml m-cresol as solvent and isoquinoline (0.1 ml) serving as catalyst. The polymer was precipitated in 300 ml methanol and obtained as an offwhite powder (90% yield) after re-precipitation and drying at 120 °C under vacuum. 1H NMR (400 MHz, DMSO-d6, δ): 1.26-1.31 (br d, J= 19.56 Hz, 12H), 2.15-2.32 (br m, 4H), 3.47 (br m, 6H), 6.21-6.21 (br m, 2H), 6.59 (br s, 2H), 6.9 (br s, 2H), 9.49 (br s, 2H). FT-IR (film, ν, cm-1): 3100-3500 (br, str O-H), 1775 (C=O asym), 1769 (C=O sym, str), 1385 (C−N, str), 780 (imide ring deformation); Mn = 95,400 g mol-1; PDI = 2.0; ρ = 1.26 g cm-3; SBET = 191 m2 g-1; TGA analysis: Td,5% = 420 °C. 2.7. Synthesis of BCBr4-SBIDA. Following the above reaction conditions, BCBr4-SBIDA was made from the BCBr4 dianhydride (0.167 g, 0.295 mmol) (II) and SBIDA (0.1 g, 0.295 mmol) (III) and obtained as a white powder (91% yield). 1H NMR (400 MHz, DMSO-d6, δ): 1.32-1.33 (br s, 12H), 2.21 (br m, 2H), 2.36 (br m, 2H), 3.82-4.12 (br m, 4H), 6.36 (br s, 2H), 6.54 (br m, 2H). FT-IR (film, ν, cm-1): 3100-3500 (br, str O-H), 1787 (C=O asym), 1717 (C=O

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sym, str), 1372 (C−N, str), 780 (imide ring deformation); Mn = 140,600 g mol-1; PDI = 1.7; ρ = 1.55 g cm-3. SBET = 243 m2 g-1; TGA analysis: Td,5%= 352 °C. 2.8. Film Preparation. Dense polymer films (~ 40 µm) used for permeation measurements were obtained from polyimide solutions in DMF made under the same conditions reported in our previous work.39 2.9. Gas Permeability Measurements. Hydrogen, oxygen, nitrogen, methane and carbon dioxide permeabilities (P) of the polyimide films were measured at 35 °C and 2 bar.19 Apparent diffusion coefficients (D) were deduced from permeation time lag measurements and apparent solubility coefficients (S) were then estimated from the solution/diffusion relationship: S = P/D.19

3. RESULTS AND DISCUSSION 3.1. Synthesis and Physical Properties. Substituted BCBr4 (II) was synthesized by a previously

reported

procedure

by

Pascoe

and

Harruna.40

The

key

intermediate,

tetrabromothiophene-1,1-dioxide (I) was prepared by reacting tetrabromothiophene with mchloroperbenzoic acid (m-CPBA) at 90 °C for 5 days. Thereafter, a double Diels-Alder reaction was carried out with maleic anhydride at 160 °C for 5 days. The chemical structure of BCBr4 was confirmed by 1H NMR (Figure S1) and 13C NMR (Figure S2). The alicyclic dianhydrides BC and BCBr4 were reacted with SBIDA in 3 ml m-cresol and 0.1 ml isoquinoline at 200 °C for 4 h under nitrogen. The structures of the two polyimides were confirmed by FT-IR (Figure 1) and 1H NMR spectroscopy (Figures S3 and S4). 1H NMR showed no signals above 10 ppm indicating complete imidization of the polyamic acid precursor. The spectra showed the typical

8


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imide group absorption bands at 1787 and 1717 cm-1 (imide asymmetric and symmetric C=O stretching), and 1372 cm-1 (C−N, stretching frequency).

Figure 1. FT-IR spectra of BC-SBIDA and BCBr4-SBIDA.

The two polyimides were soluble in polar solvents including dimethylformamide (DMF), Nmethylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and mcresol. A related previously reported 6FDA-SBIDA polyimide showed enhanced solubility in low-boiling solvents, such as acetone and tetrahydrofuran (THF), compared to the alicyclic dianhydride-based derivatives (Table 1).19,20 Robust BC-SBIDA and BCBr4-SBIDA films with good mechanical properties were obtained as shown in Table S1 and Figure S5. The tensile strength (27-28 MPa), tensile modulus (1.15-1.4 GPa) and elongation at break (2-4%) of the BC-SBIDA and BCBr4-SBIDA polyimides were similar, indicating that the bromine substitution in the alicyclic dianhydride did not significantly affect the rigidity of the PIM-PIs. The molecular weights of BC-SBIDA and BCBr4-SBIDA were determined by GPC in DMF (Table 2). BC-SBIDA and BCBr4-SBIDA showed number-average molecular weights of

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95,400 and 140,600 g mol-1 with polydispersity index (PDI) of 2.0 and 1.7, respectively. Excellent thermal stability was displayed by BC-SBIDA and BCBr4-SBIDA with 5% weight loss decomposition temperatures, Td5%, of 420 and 352 °C, respectively, as determined by TGA (Figure 2, Table 2). The lower Td5% of BCBr4-SBIDA at 352 °C resulted from the decomposition of the bromine substituents in the bicyclo moiety which led to a decrease in the thermal stability of BCBr4-SBIDA compared to BC-SBIDA.

Table 1. Solubility of BC-SBIDA, BCBr4-SBIDA and 6FDA-SBIDA in Organic Solvents. Polymer

CH3Cl Acetone THF

DMF

NMP

DMAc

DMSO

m-Cresol

BC-SBIDA

--

--

--

++

++

++

++

++

BCBr4-SBIDA

--

--

--

++

++

++

++

++

6FDA-SBIDA

--

++

++

++

++

++

++

++

Table 2. Basic Properties of BC-SBIDA, BCBr4-SBIDA and 6FDA-SBIDA Polyimides. Polymer BC-SBIDA

Mw (g mol-1) 190,000

Mn (g mol-1) 95,400

PDI (-) 2.0

Td,5%a (°C) 420

Density (g cm-3) 1.26

SBET (m2 g-1) 191

BCBr4-SBIDA

243,250

140,600

1.7

352

1.55

243

6FDA-SBIDAb

165,000

85,400

1.9

400

1.26

190

aDecomposition

temperature at 5% weight loss. bPolyimide denoted as PIM-6FDA-OH in

reference [19].

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Figure 2. TGA of BC-SBIDA and BCBr4-SBIDA under nitrogen atmosphere.

To shed some light on the microporosity of the polyimides, nitrogen adsorption isotherms were determined at -196 °C up to ~ 1 bar for BC-SBIDA, BCBr4-SBIDA and, for comparison, 6FDA-SBIDA19 (Figure 3). The calculated BET surface areas of BC-SBIDA, BCBr4-SBIDA and 6FDA-SBIDA were 191, 243 and 190 m2 g-1, respectively, indicating moderately high microporosity in this series of polyimides. It is suggested that the bromine substitution in BCBr4-SBIDA exhibited two effects on the molecular polymer chain packing: (i) tightening of micropores in the intrachain bicyclic structure (internal free volume) leading to higher gas-pair selectivity and (ii) increase in interchain spacing due to the bulky bromine atoms that resulted in moderately higher BET surface area and gas permeability than BC-SBIDA.

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Figure 3. N2 adsorption isotherms of BC-SBIDA (blue), BCBr4-SBIDA (red) and 6FDASBIDA (black) measured at -196 °C up to 1 bar.

3.2. Optical Properties. To compare the strength of the intra- and interchain charge transfer complexes (CTC) between the mostly aromatic 6FDA-SBIDA polyimide and semialicyclic/semi-aromatic BCBr4-SBIDA polyimide solid-state fluorescence spectroscopy experiments were performed. The emission spectra of the polyimide films are shown in Figure 4. Formation of CTCs typically occurs due to interactions between the nucleophilic heterocyclic rings and electrophilic aromatic rings in polyimides.4,41 Both polyimides showed emission spectra band peaks at ~545 nm when excited at 400 nm. However, the existence of the alicyclic moiety in the BCBr4-SBIDA polyimide structure resulted in a decrease in the emission peak intensity as compared to 6FDA-SBIDA. Consequently, the BCBr4-SBIDA featured a colorless film, whereas the 6FDA-SBIDA film was yellow colored which followed the qualitative trend in the strength of the charge transfer complex formation (Figure 4).

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Figure 4. Solid-state fluorescence emission spectra (excitation at 400 nm) of BCBr4-SBIDA and 6FDA-SBIDA.

Figure 5. Transmission UV-vis spectra of BC-SBIDA, BCBr4-SBIDA, and 6FDA-SBIDA polyimide films.

The transmission UV-vis spectra of 11-12 µm thick transparent polyimide films are shown in Figure 5. BC-SBIDA and BCBr4-SBIDA films were colorless and exhibited UV cutoffs (λo) at a wavelength of ~ 300-310 nm, whereas the yellow colored 6FDA-SBIDA film displayed a wavelength cutoff of 380 nm. The optical transparency of the polyimide films in the visible region was evaluated from 400 to 800 nm, as shown in Table 3.

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Table 3. Optical Transparency of BC-SBIDA, BCBr4-SBIDA and 6FDA-SBIDA Films. 0 (nm)a

T400 (%)b

T450 (%)b

T500 (%)b

BC-SBIDA

310

83

83

84

BCBr4-SBIDA

300

80

82

83

6FDA-SBIDA

380

49

70

76

Polymer

a UV

cut-off wavelength; b transmittance at 400, 450 and 500 nm, respectively.

Clearly, the two SBIDA-based polyimides derived from alicyclic dianhydrides, BC-SBIDA and BCBr4-SBIDA, exhibited significantly better transparency than the aromatic 6FDA-SBIDA analogue over the entire UV-vis spectrum. This resulted from the weaker electron withdrawing effect of the alicyclic fraction in the polyimide repeat units that reduced the intra- and intermolecular CTCs in the polymers.12

3.3. Gas Permeation Properties of the BC-SBIDA and BCBr4-SBIDA Polyimides. The gas permeation properties of freshly prepared and aged BC-SBIDA and BCBr4-SBIDA polyimide films are listed in Table 4. The gas transport properties of the previously reported aromatic 6FDA-SBIDA polyimide are also included in Table 4. Interestingly, the BC-SBIDA and 6FDA-SBIDA films displayed similar permeability and marginal difference in permselectivity for different gas pairs. For example, fresh 6FDA-SBIDA and BC-SBIDA films displayed hydrogen permeabilities of 181 and 206 barrer with H2/CH4 selectivities of 53 and 45, respectively. Bromine substitution in the polymer backbone can significantly alter the gas permeation properties of glassy polymers, as previously demonstrated for polycarbonate (PC),42,43 poly(phenylene oxide) (PPO)44, Matrimid polyimide45 and a ladder polymer of intrinsic 14


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microporosity46. For example, Muruganandam et al. demonstrated a notable rise in O2/N2 selectivity from 4.8 to 7.5 by 3,3’,5,5’-tetrabromo-substitution of the biphenol-A polycarbonate backbone with essentially no change in O2 permeability of ~ 1.4-1.5 barrer. It was suggested that the boost in O2/N2 selectivity resulted from stronger interchain cohesive forces induced by the bromine substitution leading to more pronounced size sieving properties.42 Chowdhury et al. investigated bromination of poly(phenylene oxide) and its effect on the gas transport properties.44 O2 permeability increased by ~ 50% from 16.7 barrer for the pristine PPO to 24.7 barrer for the 60% brominated PPO with a small increase in O2/N2 selectivity from 4.8 to 5.0. Guiver et al. demonstrated that bromination of Matrimid polyimide resulted in an increase in O2 permeability to 3 barrer from 1.9 barrer for the pristine polyimide with a small decrease in O2/N2 selectivity from 7.0 to 6.5.45 Halder et al. recently reported the gas permeation properties of a methylated PIM-1 ladder polymer (PIM-1-CO-100) and its bromine-substituted derivative polymers (PIM-1-COBr).46 Their study showed that the fractional free volume (FFV) of the pristine PIM-1-CO-100 decreased from 0.284 to 0.238 as the degree of bromination reached 95% in a series of systematically brominated polymers. Because of the reduction in free volume upon bromination, permeability of all gases dropped with commensurate increase in gas-pair selectivities, e.g., PIM-1-CO experienced a 10-fold loss in O2 permeability (2180 to 210 barrer) coupled with a 30% increase in O2/N2 selectivity from 2.6 to 3.4 relative to the polymer with 95% degree of bromination. In our study, BCBr4-SBIDA displayed higher gas-pair selectivity values and similar or higher gas permeabilities than the unsubstituted BC-SBIDA polyimide. For example, fresh BCBr4SBIDA and BC-SBIDA films exhibited oxygen permeabilities of 31 and 23 barrer , respectively, coupled with O2/N2 selectivities of 5.9 and 4.2. The increased O2 permeability of BCBr4-

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SBIDA resulted from a ~20% higher O2 diffusion coefficient and ~ 10% higher O2 solubility coefficient than observed in BC-SBIDA (Table S2). The enhanced size-sieving properties of BCBr4-SBIDA are evident from higher O2/N2 permselectivity due to increased O2/N2 diffusion selectivity (Table S3).

Table 4. Gas Permeability and Selectivity of BC-SBIDA, BCBr4-SBIDA and 6FDA-SBIDA Polyimides (p = 2 bar; 35 °C). Polymer

Permeability (barrer)

Ideal selectivity (αX/Y)

H2

N2

O2

CH4

CO2

BC-SBIDAa

206

5.5

23

4.6

130

37

Aged 365 d

127

3.0

14.6

2.4

79

BCBr4-SBIDAa

274

5.3

31.2

4.9

Aged 365 d

152

2.6

17.2

6FDA-SBIDAb

181

5.5

23.8

a

H2/N2 H2/CH4

O2/N2

CO2/CH4

45

4.2

28

43

54

4.9

33

138

52

55

5.9

28

2.5

81

58

61

6.6

33

3.4

119

33

53

4.3

35

Fresh films were treated in methanol for 24 h, dried at 120 °C under vacuum for 24 h; b fresh

films were treated in methanol for 24 h , dried at 250 °C under vacuum for 24 h; 1 barrer = 10-10 cm3(STP) cm cm-2 s-1 cmHg-1.

To check the effect of physical aging on the gas permeation properties of the polyimides, membrane samples were re-evaluated after 365 days of storage under ambient conditions. Permeabilities of BC-SBIDA and BCBr4-SBIDA dropped by ~40-50% for all gases with concurrent increase in selectivities. This is a typical general behavior observed for glassy polymers of intrinsic microporosity.47 As the chain conformation in freshly made glassy polymer films is confined in a non-equilibrium state, physical aging leads to more efficient, and, hence, tighter packing of polymer chains over time toward their equilibrium state.47

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Interestingly, the aged BC-SBIDA and BCBr4-SBIDA samples showed essentially the same gas permeation properties for CO2/CH4 separation with CO2 permeability of ~ 80 barrer and CO2/CH4 selectivity of 33. Similar to previous results obtained for brominated polycarbonate42,43, the positive effect of bromination on enhanced O2/N2 selectivity is also evident for BCBr4-SBIDA. The aged BCBr4-SBIDA demonstrated its potential as membrane material for air separation with O2 permeability of 17 barrer and O2/N2 selectivity of 6.6. To put this performance in prospective, the most selective commercially available membrane materials, Matrimid45 and tetrabromo-polycarbonate,43 exhibit slightly higher O2/N2 selectivity (7.0 and 7.5, respectively) but with ∼10-fold lower O2 permeability, as shown on the 2008 O2/N2 performance upper bound for air separation (Figure 6).48

Figure 6. Performance of BC-SBIDA and BCBr4-SBIDA compared to commercially available membrane materials for air separation relative to the 2008 O2/N2 trade-off upper bound.48

4. CONCLUSIONS Two novel organo-soluble and colorless polyimides of intrinsic microporosity were made from two alicyclic bicyclo dianhydrides (BC and BCBr4) and 3,3,3’,3’-tetramethyl-1,1’-

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spirobisindane-5,5’-diamino-6,6’-diol (SBIDA). The BC-SBIDA and BCBr4-SBIDA polyimides displayed microporous texture with BET surface areas of 191 and 243 m2 g-1, respectively. Better size-sieving properties were obtained with the bromine-substituted PI due to some intermolecular polar interactions between the polymer chains. Compared to the highest selective commercially available polymer membrane materials for O2/N2 separation (TB-PC and Matrimid), BCBr4-SBIDA showed ~10-fold higher O2 permeability with similar O2/N2 selectivity, making alicyclic BCBr4-SBIDA a potential candidate material for membrane-based air separation.

ASSOCIATED CONTENT Supporting Information  The Supporting Information is available free of charge on the ACS Publications website at DOI: ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (I.P.). ORCID  Ingo Pinnau: 0000-0003-3040-9088  Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by funding (BAS/1/1323-01-01) from King Abdullah University of Science and Technology (KAUST).

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Figure 1. FT-IR spectra of BC-SBIDA and BCBr4-SBIDA. 272x208mm (300 x 300 DPI)



Figure 2. TGA of BC-SBIDA and BCBr4-SBIDA under nitrogen atmosphere. 272x208mm (300 x 300 DPI)


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Figure 3. N2 adsorption isotherms of BC-SBIDA (blue), BCBr4-SBIDA (red) and 6FDA-SBIDA (black) measured at -196 °C up to 1 bar. 272x208mm (300 x 300 DPI)



Figure 4. Solid-state fluorescence emission spectra (excitation at 400 nm) of BCBr4-SBIDA and 6FDASBIDA. 305x127mm (150 x 150 DPI)


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Figure 5. Transmission UV-vis spectra of BC-SBIDA, BCBr4-SBIDA, and 6FDA-SBIDA polyimide films. 272x208mm (300 x 300 DPI)



Figure 6. Performance of BC-SBIDA and BCBr4-SBIDA compared to commercially available membrane materials for air separation relative to the 2008 O2/N2 trade-off upper bound.48 180x152mm (150 x 150 DPI)


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Synthesis and Characterization of Organo-Soluble Polyimides Derived

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