Microporous Polyimides with Rationally Designed Chain Structure

Article pubs.acs.org/Macromolecules

Microporous Polyimides with Rationally Designed Chain Structure Achieving High Performance for Gas Separation Zhenggong Wang,†,‡ Dong Wang,*,† and Jian Jin*,† †

Nano-Bionics Division and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Polyimides of intrinsic microporosity are important polymers for gas separation. Achieving polyimides with high permeability and high selectivity relies on rationally designing their chain structure. In this work, new ladder-like diamines, Tröger’s Base (TB) derived diamines, are designed, and two microporous polyimides are constructed by polymerizing TB-derived diamines and spirobisindane based dianhydride, aiming at enhancing the stiffness of the whole backbone and thus achieving improved performance by taking advantage of the stiffness of both diamines and dianhydride. The two polyimides present excellent separation performance, surpassing the 2008 Robeson upper bound for gas pairs of H2/N2 and O2/N2 and approach the 2008 Robeson upper bound for gas pairs H2/CH4 and CO2/CH4. Our results show that the designed polyimides have great potential for application especially for oxygen (O2/N2) and hydrogen (such as H2/N2 and H2/CH4) related separation.

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of O2/N2 around 1.4).4 In terms of backbone stiffness, polymers of intrinsic microporosity (PIMs) possessing a stiff backbone constituted of ladder-like repeat units were recently fabricated.5 PIMs exhibit excellent gas separation performance, representing the present upper bounds.6 Improved performance was also achieved by post-treatment of PIM-1 with functional groups such as carboxyl, tetrazole, thioamide, amidoxime, amine, etc.7 Aromatic polyimides (PIs) are a class of important polymer for gas separation owing to their good thermal and chemical stability, mechanical strength, processability, and structural tenability. Traditional PIs possess high gas selectivity. However, their permeability still needs to be improved continuously to meet the rapid development of practical application. To address this issue, PIM−PIs have been fabricated by integrating ladderlike structures such as spiro center into polyimides.8 Besides, other ladder-like structures such as spirobisindane, spirobifluorene, and triptycene were also incorporated into polyimdes.9 By such design, the gas permeability of these polyimides is greatly enhanced in comparison with traditional polyimides. However, their selectivity is reduced more or less correspondingly due to less stiffness of backbone of these polymers. Therefore, the performance of most of polyimides is still below the 2008 Robeson upper bound. Very recently,

INTRODUCTION Polymeric membranes have become more and more important in industrial applications for gas separation, such as hydrogen recovery (H2/N2, H2/CH4), nitrogen generation (O2/N2), carbon dioxide separation/removal (CO2/N2, CO2/CH4), and so on, as they provide a cost-effective, energy-efficient, environmentally benign avenue to accomplish these processes.1 The achievement of high-efficiency gas separation desires highperformance membrane which possesses both good permeability and good selectivity. However, there exists a trade-off between gas permeability and selectivity; that is to say, highly permeable polymer membranes always display poor selectivity and vice versa. This trade-off has been described by the Robeson upper-bound relationship which has been regarded as an empirical criterion to judge the comprehensive performance of membranes.2 Therefore, overcoming the limit of the Robeson upper bound has been the main objective for the study of membranes. A theory analysis proposed by Freeman points out that high permeability and selectivity could be obtained by designing shape persistent polymers containing stiffer backbone coupled with interchain separation.3 To reach this goal, many attempts have been done in the past decades. In terms of interchain separation, a series of glassy disubstituted polyacetylenes containing bulky side-chain substituents in the main chain have been designed to prevent efficient chain packing. As a representative polymer, poly(1-trimethylsilyl-1propyne) (PTMSP) displays a high oxygen permeability up to 10 000 barrer but with a relatively low selectivity (the selectivity © XXXX American Chemical Society

Received: August 25, 2014 Revised: October 21, 2014

A

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radiation of wavelength (λ) 1.54 Å. The value of d-spacing was calculated by means of Bragg’s law (d = nλ/2 sin θ). Synthesis of TBDA1-SBI-PI and TBDA2-SBI-PI. TBDA1 (0.51 g, 1.83 mmol) was dissolved in 10 mL of m-cresol, and then SBI (1.15 g, 1.83 mmol) was added in portions, occurring in a dried 25 mL threeneck flask equipped with a Dean−Stark trap and reflux condenser under a nitrogen atmosphere. The reaction mixture was continuously stirred for hours until the mixture became clear, and then quinoline (0.1 mL) and anhydrous toluene (2 mL) were added to the mixture. The temperature was raised gradually to 200 °C and held for 8−10 h under nitrogen protection. During this period, water was removed from the reaction mixture by azeotropic distillation. Chloroform was added to the resulting viscous solution on cooling; afterward, the solution was poured slowly to 300 mL of methanol. The resulting thread-like, elastic solid precipitate was collected by filtration. Purification was achieved by reprecipitation from chloroform (20 mL) into methanol (200 mL) twice and drying in a vacuum oven at 120 °C for 24 h, giving TBDA1-SBI-PI as a yellow powder (1.5 g, 93% yield). 1H NMR (400 MHz, CDCl3) δ: 7.32 (s, 1H), 7.23 (s, 1H), 7.06 (s, 1H), 6.81 (s, 1H), 6.70 (s, 1H), 6.36 (s, 1H), 4.63 (d, J = 16.0 Hz, 2H), 4.30 (s, 2H), 4.05 (d, J = 20.0 Hz, 2H), 2.43 (s, 6H), 2.33 (d, J = 12.0 Hz, 2H), 2.18 (d, J = 12.0 Hz, 2H), 1.34 (d, J = 24.0 Hz, 12H). ATR-FTIR (film, ν, cm−1): 2955 (str, aromatic C−H), 2920, 2850 (str, aliphatic C−H), 1778 (asymstr of imide CO), 1716 (sym str of imide CO), 1353 (str of imide C−N), 743 (str, imide ring C−N− C). Molecular mass (GPC, eluent CHCl3, against polystyrene standards): Mn = 23 000, Mw = 41 000 g mol−1, PDI = 1.8. BET surface area = 560 m2/g. The synthesis procedure of TBDA2-SBI-PI was the same as that of TBDA1-SBI-PI. Yield: 90%. 1H NMR (400 MHz, CDCl3) δ: 7.36 (d, J = 12.0 Hz, 1H), 7.28 (s, 1H), 6.91 (s, 1H), 6.73 (s, 1H), 6.39 (s, 1H), 4.64 (d, J = 16.0 Hz, 2H), 4.32 (s, 2H), 4.05 (d, J = 20.0 Hz, 2H), 2.34 (s, J = 16.0 Hz, 2H), 2.30−2.17 (br m, 8H), 1.36 (d, J = 24.0 Hz, 12H). ATR-FTIR (film, ν, cm−1): 2955 (str, aromatic C−H), 2920, 2850 (str, aliphatic C−H), 1778 (asym str of imide CO), 1716 (sym str of imide CO), 1353 (str of imide C− N), 743 (str, imide ring C−N−C). Molecular mass (GPC, eluent CHCl3, against polystyrene standards): Mn = 54 000, Mw = 69 000 g mol−1, PDI = 1.3. BET surface area = 615 m2/g. Membrane Preparation. 3 wt % chloroform solution of TBDA1SBI-PI or TBDA2-SBI-PI was filtered by 0.45 μm PTFE filter into a 9 cm circular Teflon mold. After slow evaporation of solvent about 3 days, dense membranes were obtained. Then, the membranes were placed in a high-vacuum, oil-free oven at 120 °C for 24 h to remove residual solvent, following immersing them in methanol for 24 h, dried in air, and placed in a high-vacuum oven at 120 °C for 24 h again. Gas Permeation Test. The test of gas permeation was performed at 35 °C and at a feed pressure of 1 bar with pure gas (99.999%) in the order of He (2.69 Å), H2 (2.89 Å), O2 (3.46 Å), N2 (3.64 Å), CH4 (3.87 Å), CO2 (3.3 Å) using a fixed-volume pressure increase instrument time-lag apparatus, starting with an oil-free vacuum. The permeability coefficient, P, was calculated from the slope in the steady state region by using eq 1 and apparent diffusion coefficient, D, from the time lag θ, using eq 2:

McKeown et al. and Pinnau et al. reported novel highperformance polyimides (PIM-PI-EA and KAUST-PIs) by introducing more rigid dianhydride containing ethanoanthracene and bridgehead-substituted triptycene into the polymer backbone. These polyimides exhibited superior selectivity and permeability exceeding the 2008 Robeson upper bound.10 These results demonstrate the importance of rational constructing the chain structure of polyimides with high stiffness to achieve improved performance. In general, PIM-PIs are composed of two segments: dianhydride and diamine. Besides dianhydride segment, the stiffness of the diamine segment is also important to affect the stiffness of the whole backbone of polyimide. Up to date, the design of microporous polyimides mainly focuses on dianhydride segment, and less attention is paid on the structure of diamine. In this work, a new ladder-like diamine, Tröger’s Base (TB) derived diamine, is designed, aiming at enhancing the stiffness of the whole backbone and thus achieving improved performance by taking advantage of the stiffness of both diamines and dianhydride. TB groups have been proved to be excellent shape persistent building blocks to construct molecular sieve materials where TB-based PIMs have exhibited outstanding gas separation performance.11 The shape persistent nature of TB were also verified by our previous report.12 Herein, two microporous polyimides are constructed by polymerizing TB-derived diamines and spirobisindane based dianhydride. The two rigid and kinked building blocks endow the polyimides stiff backbone structure and large free volume. The two polyimides thus present excellent separation performance, surpassing the 2008 Robeson upper bound for gas pairs of H2/N2 and O2/N2.

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EXPERIMENTAL SECTION

Materials. Spirobisindane-based dianhydride (SBI) was synthesized according to a previously reported method8a and dried at 120 °C under vacuum before use. m-Cresol and quinoline were obtained from Alfa Aesar and used as received. 2,8-Diamino-4,10-dimethyl-6H,12H5,11-methanodibenzo[1,5]diazocine (TBDA1) and 3,9-diamino-4,10dimethyl-6H,12H-5,11-methanodibenzo[1,5]diazocine (TBDA2) were synthesized according to our previous report.12b Characterization Methods. 1H NMR (400 MHz) and 13C NMR (100 MHz) of the synthesized monomers and resulting polymers were recorded on a Varian nuclear magnetic resonance (NMR) spectroscopy. Molecular weights and polydispersity index (PDI) of the polymers were determined by Varian (PL-GPC 50) gel permeation chromatography (GPC) using Ultrastyragel columns and chloroform eluent for TBDA1-SBI-PI and TBDA2-SBI-PI at a flow rate of 1 mL/ min at 40 °C. The values were determined by comparison with a series of polystyrene standards. ATR-FTIR spectra were measured on a Nicolet 6700 FTIR spectrometer. Temperatures of polymer thermal degradation were obtained from TG/DTA 6200. For TG measurement, samples were initially heated to 120 °C under nitrogen gas and maintained at that temperature for 1 h for removal of moisture and then heated to 800 °C at 5 °C/min for measurement of degradation temperature (Td). Glass transition temperatures (Tg) were observed from differential scanning calorimetry (DSC) (DSC 6220). An Accelerated Surface Area and Porosimetry (ASAP) 2020 system (Micromeritics Instrument Corporation) was used to study N2 adsorption/desorption of the samples. Apparent surface areas were calculated from N2 adsorption data by the multipoint Brunauer− Emmett−Teller (BET) method. Pore width distributions were calculated from the N2 adsorption data by the Horvath−Kawazoe method, assuming a slit-pore geometry and the original H−K carbon− graphite interaction potential. Wide-angle X-ray diffraction (WAXD) was conducted on a Bruke D8 instrument utilized with Cu Kα

P=

273.15 × 1010 Vl ⎛ dp ⎞ ⎜ ⎟ 760 AT Δp ⎝ dt ⎠

(1)

D=

l2 6θ

(2)

where P is the permeability represented in barrer (1 barrer = 10−10 [cm3 (STP) cm]/(cm2 s cmHg)), dp/dt is the rate of pressure rise under the steady state, V (cm3) is the downstream volume, l (cm) is the membrane thickness, Δp (cmHg) is the pressure difference between the two sides, T (K) is the measurement temperature, and A (cm2) is the effective area of the membrane. The solubility of the gas in the polymer matrix was determined indirectly, via eq 3:

S = P /D B

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Figure 1. (a) Structure formula of two TB-based polyimides. (b) Photograph of a solvent-cast membrane of TBDA1-SBI-PI. (c) Energy-minimized molecular model of TBDA1-SBI-PI. All gas permeation tests were performed more than three times. The ideal selectivity for pure gas A and B is defined as eq 4: αA/B =

PA PB

(4)

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RESULTS AND DISCUSSION Polymer Characterization and Their Physical Properties. The chemical formulas of the two polyimides, TBDA1SBI-PI and TBDA2-SBI-PI, are shown in Figure 1a. Their synthesis via a high-temperature, one-step solvothermal azeotropic cycloimidization reaction between the mole equivalent TB-derived diamine and anhydride monomer (SBI) is illustrated in Figure S1. Two shape persistent diamine monomers, 2,8-diamino-4,10-dimethyl-6H,12H-5,11methanodibenzo[1,5]diazocine (TBDA1) and 3,9-diamino4,10-dimethyl-6H,12H-5,11-methanodibenzo[1,5]diazocine (TBDA2), with different position of methyl relative amine are obtained via two steps: the dimerization of substituted aniline and the reduction of nitro groups (Figure S2); their chemical structures are confirmed by 1H NMR as shown in Figure 2. The synthesis of SBI is summarized in Figure S3, and its chemical structure is examined by 1H NMR (Figure S4). The molecular structures of the two polyimides have been fully characterized by 1H NMR and ATR-IR (Figures 2 and 3). The symbolic absorption band of imide at 1778, 1716, 1353, and 743 cm−1 and the disappearance of absorption band at ∼3300 cm−1 as given by ATR-IR indicate the full imidization of the polyamic acid. The molecular weights (Mw) of TBDA1-SBI-PI and TBDA2-SBI-PI are 41 000 and 69 000, respectively, and the polydispersity indices (PDI, Mw/Mn) of them are 1.8 and 1.3 as tested through gel permeation chromatography (Figure S5 and Table S1). The two polyimides show good solubility in common solvents such as chloroform, dichloromethane, mcresol, and NMP as summarized in Table S2. The homogeneous and transparent membranes of TBDA1-SBI-PI and TBDA2-SBI-PI with thickness of ∼80 μm were subsequently obtained by simply casting their chloroform solutions on a circular Teflon mold (Figure 1b). The good solubility of the two polymers is ascribed to their highly contorted and double-ladder chain structure (Figure 1c). The

Figure 2. 1H NMR for diamine monomers and resulting polyimides.

two membranes exhibit excellent thermal stability up to 480 °C (Td, 10% mass loss), and no apparent glass transition temperature (Tg) could be observed up to 350 °C (Figure 4, Figure S6 and Table S1). The good thermal stability of the two membranes makes them potentially applicable to high-temperature applications such as hydrogen separation during ammonia synthesis industry and petrochemical engineering. Nitrogen adsorption/desorption plots of the two polyimides measured in 77 K and the pore size distributions (PSD) calculated from the N2 adsorption data by the Horvath− Kawazoe (H−K) method are shown in Figure 5a,b. Both of them possess high BET surface areas. The BET surface area of TBDA1-SBI-PI is 560 cm3/g, and that of TBDA2-SBI-PI is 615 cm3/g. TBDA2-SBI-PI displays a larger BET surface area than TBDA1-SBI-PI, which is caused by their different steric hindrances derived from methyl position in terms of imide bond. When methyl is adjacent to imide bond in the case of TBDA2-SBI-PI, it hinders the rotation of imide bond and causes less efficient chain packing. The open hysteresis loops formed by the irreversible adsorption and desorption branches C

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Figure 3. ATR-IR spectra of TBDA1-SBI-PI and TBDA2-SBI-PI membranes.

Figure 5. (a) Nitrogen adsorption/desorption isotherms of TBDA1SBI-PI and TBDA2-SBI-PI measured at 77 K. (b) Pore width distributions calculated from low-pressure N2 adsorption data by the Horvath−Kawazoe method, assuming a slit-pore geometry.

Figure 4. Thermogravimetry (TG) analysis of TBDA1-SBI-PI and TBDA2-SBI-PI.

are both observed in the two samples. The similar hysteresis loops were also observed in other reported PIM-PIs.8a The adsorption isotherms show a high uptake at relatively low pressure, which indicates the presence of micropores in the polymers. The pore width of the two polymers is in the region of 5−7 Å, showing ultramicroporous character (Figure 5b). The concentrated distribution of ultramicroporosity enables a diffusion-dominated separation process for gas molecule to penetrate with less interaction with TBDA2-SBI-PI and TBDA2-SBI-PI. To further investigate their microstructures, the wide-angle X-ray diffraction (WAXD) is measured for the two membranes. The broad peaks around 13° as observed in WAXD indicate the amorphous property of the two polyimides (Figure 6). The position of the peak corresponds to the dspacing of 6.7 Å for TBDA1-SBI-PI and 6.9 Å for TBDA2-SBIPI. The d-spacing is always considered to represent the distance between segments of different chains and related to gas permeability of polymers. Fractional free volume (FFV) is also calculated to identify the different d-spacing of the two polymers. As given in Table S1, TBDA2-SBI-PI has a bit

Figure 6. WAXD patterns of TBDA1-SBI-PI and TBDA2-SBI-PI membranes.

larger FFV than TBDA1-SBI-PI, which supports the WAXD results further. D

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Table 1. Gas Permeability (P) and Ideal Selectivity (α) of TBDA1-SBI-PI, TBDA2-SBI-PI, and Representative PIM-PIsa permeability (barrer)

a

ideal selectivity (α)

polymers

H2

He

CO2

O2

N2

CH4

H2/N2

O2/N2

H2/CH4

CO2/N2

CO2/CH4

TBDA1-SBI-PI TBDA2-SBI-PI PIM-PI-18a,b PIM-PI-88a,b

915 1155 530 1600

398 530 260 660

895 1213 1100 3700

190 240 150 545

35 49 47 160

45 65 77 260

27 24 11.3 10

5.4 4.9 3.2 3.4

20.3 17.7 6.9 6.2

25.6 24.8 23.4 23.1

19.7 18.7 14.3 14.2

1 barrer = 10−10 [cm3 (STP) cm]/(cm2 s cmHg).

Figure 7. Relationship between gas permeability (P) and gas pair selectivity (α) with Robeson upper bound (red ★: TBDA1-SBI-PI; blue ★: TBDA2-SBI-PI): (a) for O2/N2, (b) for H2/N2, (c) for H2/CH4, and (d) for CO2/CH4. Other data points stand for representative polymers with high permeability reported since the upper bounds were updated in 2008.

Gas Transport Properties. To test the gas permeation properties of TBDA1-SBI-PI and TBDA2-SBI-PI, the asprepared membranes are soaked in methanol for about 20 h and dried in a high-vacuum oven at 120 °C for 24 h to remove any residual solvents. The pure-gas permeation experiments of TBDA1-SBI-PI and TBDA2-SBI-PI membranes for H2, He, N2, O2, CH4, and CO2 gases are carried out on a fixed-volume pressure increase time-lag apparatus with gas feed at 35 °C. The measured permeability (P) and calculated ideal selectivity (α) are summarized in Table 1. The permeation data reveal that TBDA1-SBI-PI and TBDA2-SBI-PI both show high permeability and good selectivity. The gas permeabilities of TBDA1SBI-PI are 915 barrer for H2, 398 barrer for He, 895 barrer for CO2, 190 barrer for O2, 35 barrer for N2, and 45 barrer for CH4. The gas permeabilities of TBDA2-SBI-PI are 1155, 530, 1213, 240, 49, and 65 barrer for H2, He, CO2, O2, N2, and CH4, respectively. The permeability of TBDA2-SBI-PI is higher than that of TBDA1-SBI-PI, which is consistent with the FFV results as discussed above. The ideal selectivity of TBDA1-SBI-PI and

TBDA2-SBI-PI is also excellent and has been largely improved as compared to previously reported PIM-PI-1 and PIM-PI-8 (representative polymers among PIM-PIs).8a The ideal selectivities of TBDA1-SBI-PI and TBDA2-SBI-PI are 5.4 and 4.9 for O2/N2 (PIM-PI-1, αO2/N2 = 3.2 and PIM-PI = 8, αO2/N2 = 3.4), 27 and 24 for H2/N2 (PIM-PI-1, αH2/N2 = 11.3 and PIM-PI = 8, αH2/N2 = 10), and 20.3 and 17.7 for H2/CH4 (PIM-PI-1, αH2/CH4 = 6.9 and PIM-PI = 8, αH2/CH4 = 6.2), respectively. Robeson plots of P versus α for gas pairs of O2/N2, H2/N2, H2/ CH4, and CO2/CH4 are shown in Figure 7. The outstanding separation performance of the two polyimides has been apparently demonstrated by comparison to the latest Robeson upper bound, among which the performance of the two polyimides for gas pairs O2/N2 and H2/N2 exceeds the 2008 Robeson upper bound and approaches the 2008 Robeson upper bound for gas pairs H2/CH4 and CO2/CH4. In the Robeson plots, TBDA1-SBI-PI and TBDA2-SBI-PI show obviously improved separation performance as compared to E

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Table 2. Diffusion Coefficient (D), Solubility Coefficient (S), Diffusion Selectivity (αD), and Solubility Selectivity (αS) for Different Gases (CH4, N2, CO2) of TBDA1-SBI-PI, TBDA2-SBI-PI, and Representative PIM-PIs D (10−8 cm2/s)

S (cm3/cm3 cmHg−1)

O2/N2

CO2/CH4

CO2/N2

polymers

N2

O2

CH4

CO2

N2

O2

CH4

CO2

αD

αS

αD

αS

αD

αS

TBDA1-SBI-PI TBDA2-SBI-PI PIM-PI-18a,b PIM-PI-88a,b

20.6 27.0 20 41

94 114 56 130

5.4 7.5 7 14

14 23 17 45

0.017 0.018 0.024 39

0.020 0.021 0.028 41

0.083 0.087 0.110 180

0.63 0.53 0.620 810

4.56 4.22 2.8 3.2

1.18 1.17 1.17 1.05

2.6 3.1 2.4 3.2

7.6 6.1 5.6 4.5

0.68 0.85 0.85 1.1

37.0 29.3 25.8 20.8

PIM-PI-1and PIM-PI-8.8a The excellent performance of our polyimides is attributed to the combination of both chain stiffness and interchain separation in the polymer. Different from PIM-PIs where CO2 permeability is much higher than H2, chain stiffness of TBDA1-SBI-PI and TBDA2-SBI-PI gives rise to the high H2 permeability while maintaining the high CO2 permeability (PH2 ≈ PCO2). This result further confirms the existence of stiffer polymer chain. The rigid polymer backbone enhances the segment mobility selectivity, which enables the smaller H2 molecule (kD = 2.89 Å) to penetrate faster than the larger CO2 molecule (kD = 3.3 Å). The position of methyl relative to imide bond has a certain effect on polyimide permeability due to steric hindrance influence. The permeability of TBDA2-SBI-PI where methyl is beside the imide bond is slightly higher than that of TBDA1SBI-PI where methyl is away from the imide bond. The CO2 permeability of TBD2-SBI-PI is 1.35 times that of TBDA1-SBIPI, and the O2 permeability of TBD2-SBI-PI is 1.26 times that of TBDA1-SBI-PI. It indicates that the improvement of gas permeability by utilizing steric hindrance is not obvious. Previous reports showed that immobilizing substitution groups (such as −CH3 or −Br) beside the imide bond could largely enhance the polyimde permeability under the effect of steric hindrance which contributes to the increase of polymer’s free volume.9c,13 However, the increase of free volume in PIMs is mainly ascribed to the rigidity and the contorted structure of the polymer backbone itself. Because of their rigid chain structure, TBD1-SBI-PI and TBDA2-SBI-PI have much larger free volume than traditional dianhydrides. The effect of steric hindrance on the increase of free volume is thus not so obvious. To further understand the separation process, the solubility selectivity (αS) and diffusion selectivity (αD) of gas pairs O2/N2 and CO2/CH4 have been analyzed and are summarized in Table 2. For the gases with less interaction with polymers such as O2 and N2, the αS of TBDA1-SBI-PI and TBDA2-SBI-PI for gas pair O2/N2 are 1.18 and 1.17, which are similar to that of PIM-PI-1 (αS = 1.17) and PIM-PI-8 (αS =1.05). However, the αD of TBDA1-SBI-PI and TBDA2-SBI-PI is increased largely to 4.56 and 4.22 in comparison with 2.8 of PIM-PI-1 and 3.2 of PIM-PI-8. This result is consistent with our conclusion that the increased stiffness of whole chain structure is aroused by the introduction of TB units. The whole backbone constituted of rigid dianhydride, diamine, and imide linkages endows the polymers with molecular sieve properties and thus exhibits high diffusion selectivity. These results indicate that for the gases with weak interaction with polymers diffusion selectivity dominates their penetrating process as occurred in TBDA1SBI-PI and TBDA2-SBI-PI. For the gases with strong interaction with polymer such as CO2, the two polyimides exhibit much larger solubility selectivity (αS) as observed for gas pairs CO2/CH4 and CO2/N2. It is ascribed to the solubility effect of the TB unit on CO2. The nitrogen atoms of tertiary

amine in TB structure could strongly interact with CO2 driven by Lewis acid−base interaction and/or affinity between polymer and CO2 molecules arising from the quadrupole moment of the CO bond and thus could quickly adsorb CO2.14 However, the enhanced αS does not influence the gas pair selectivity largely as shown in Figure S7. As has been clarified in our previous report, the high CO2 solubility effect in the polymer might impede the CO2 molecular diffusion and had no effect on increasing the gas permeability. Hence, the solution-dominated separation mechanism is considered to play the main role during penetrating CO2 gas molecules.

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CONCLUSIONS In conclusion, aiming at achieving high-performance polyimides with both high permeability and selectivity for gas separation, two microporous polyimides containing stiff whole backbone have been designed and synthesized. The stiff backbone of the polyimides includes two important moieties: anhydride and diamine. The shape persistent diamine could increase the stiffness of the whole backbone, and the rigid anhydride could enhance the free volume of polymers. The two moieties are combined together through imide linkage that is rigid, too. Such a design endows the polyimides with excellent gas separation performance with improved selectivites and simultaneously maintaining higher permeability as compared to most of other polyimides reported so far. The gas separation performance of the two polyimides for gas pairs O2/N2 and H2/N2 both exceeded the 2008 Robeson upper bound, representing a top-level performance. The analysis on separation process demonstrates that diffusion selectivity dominates penetrating process for the gases with weak interaction with polymers, reflecting a molecular sieve effect, and the solution-dominated separation mechanism plays the main role for the gases with strong interaction with polymers. Taking their good processability and high thermal stability into account, our polyimides might have potential for practical application, especially for oxygen (O2/N2) and hydrogen (such as H2/N2 and H2/CH4) related separation in the industry field.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures including dianhydride monomer (SBI) synthesis procedure, SBI 1H NMR, synthesis of the polyimides, GPC and DSC curve of result polymers, relationship between gas permeability (P) and gas pair selectivity (α) with Robeson upper bound for gas pair CO2/N2, and physical properties of the polyimides. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.J.). *E-mail [email protected] (D.W.). F

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Project of National Natural Science Foundation (No. 21433012), the National Basic Research Program of China (No. 2013CB933000), the National Natural Science Foundation of China (Grant No. 21473239), the Key Development Project of Chinese Academy of Sciences (No. KJZD-EW-M01-3), and the Natural Science Foundation of Jiangsu Province (No. BK20130007).

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dx.doi.org/10.1021/ma5017506 | Macromolecules XXXX, XXX, XXX−XXX