Article pubs.acs.org/Macromolecules
Novel Spirobifluorene- and Dibromospirobifluorene-Based Polyimides of Intrinsic Microporosity for Gas Separation Applications Xiaohua Ma, Octavio Salinas, Eric Litwiller, and Ingo Pinnau* Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: Two series of novel intrinsically microporous polyimides were synthesized from 9,9′-spirobifluorene-2,2′-diamine (SBF) and its bromine-substituted analogue 3,3′-dibromo-9,9′-spirobifluorene-2,2′-diamine (BSBF) with three different dianhydrides (6FDA, PMDA, and SPDA). All polymers exhibited high molecular weight, good solubility in common organic solvents, and high thermal stability. Bromine-substituted polyimides showed significantly increased gas permeabilities but slightly lower selectivities than the SBF-based polyimides. The CO2 permeability of PMDA−BSBF (693 Barrer) was 3.5 times as high as that of PMDA− SBF (197 Barrer), while its CO2/CH4 selectivity was similar (19 vs 22). Molecular simulations of PMDA−SBF and PMDA− BSBF repeat units indicate that the twist angle between the PMDA and fluorene plane changes from 0° in PMDA−SBF to 77.8° in PMDA−BSBF, which decreases the ability of the polymer to pack efficiently due to severe steric hindrance induced by the bromine side groups.
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INTRODUCTION Polymeric gas separation membranes find important applications in hydrogen recovery, onsite nitrogen production from air, and natural gas sweetening and purification as well as carbon dioxide capture/separation due to their potential costeffectiveness and environmental benefits.1,2 Ideal membranes for industrial separations should be both highly permeable and selective. High permeability leads to reduced membrane area, and hence lower capital cost; high selectivity has the benefit of higher product recovery at the desired product purity. However, an undesirable trade-off between permeability (P) and selectivity (α) is typically observed for polymeric membranes, as illustrated by the Robeson upper bound relationship that is frequently updated according to the performance of state-ofthe-art membranes.3,4 Rational molecular design of polymeric materials is the key to creating new membranes with superior gas separation properties. One effective strategy is to introduce intrinsic micropores ( CO2 > O2 > N2 > CH4, similar to conventional polyimides, such as Matrimid.38 However, in the case of the relatively high BET surface area SPDA−SBF, the transport behavior changed to that of highly microporous polymers such as PIM-1 and PIM−PI-8,26 with the sequence of CO2 > H2 > O2 > CH4 > N2; (iii) introduction of bromine in the polymers increased the gas permeability, e.g., the CO2 permeability of 6FDA−BSBF
Figure 1. 1H NMR of the BSBF and SBF monomers.
other diamine monomers. The electron-rich diamine enhanced the reactivity of the nearby carbon atoms of the spirobifluorene molecule, and the substitution reaction finished selectively at the 3, 3′ position of the SBF (Figure 1). The polyimides were prepared by the cycloimidization reaction between SBF and BSBF with three types of dianhydrides, namely, 6FDA, PMDA, and SPDA. The bulky bromine substitution resulted in the chemical shift of all protons to the lower field (Figure 1), corresponding to the decrease in reactivity of the monomer (BSBF). As a result, the reaction time increased more than two times compared with the pristine SBF analogue. Polymer Properties. Wide-angle X-ray diffraction clearly indicated that all polyimides based on the spirobifluorene moiety exhibited amorphous structures (Figure S1, Supporting Information). The PIM−PIs exhibited very good solubility (Table 1) in solvents such as DMAc, DMF, m-cresol, THF, and chloroform. High molecular weight (Mn ranging from 4.4 to 9.4 × 104 g/mol) and narrow polydispersity index (PDI is around 1.6) was obtained for all polymers (Table 2). The spirobifluorene-based polyimides did not show any Tg up to 350 °C, and excellent thermal stability with the onset decomposition temperature (Td) ranging from 440 to 480 °C, as shown in Figure 2. PMDA−SBF shows the highest thermal
Table 1. Solubility of SBF- and BSBF-Based Polyimides in Organic Solvents Solvent polymers
m-cresol
NMP
DMF
THF
CHCl3
acetone
MeOH
6FDA−SBF PMDA−SBF SPDA−SBF 6FDA−BSBF PMDA−BSBF SPDA−BSBF
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ − − + − −
− − − − − −
D
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Figure 2. TGA of (a) SBF- and (b) BSBF-based polyimides.
Figure 3. Nitrogen sorption isotherms of (a) PMDA- and (b) SPDA-based spirobifluorene- and dibromospirobifluorene PIM−PIs at 77 K.
Table 3. Permeability and Selectivity of SBF- and BSBF-Based PIM−PI Membranes for Different Gases at 35 °C permeability (Barrer)a
a
ideal selectivity (α)
polymers
H2
N2
O2
CH4
CO2
H2/N2
O2/N2
CO2/CH4
CO2/N2
6FDA−SBF PMDA−SBF SPDA−SBF 6FDA−BSBF PMDA−BSBF SPDA−BSBF PIM−PI-3b
234 230 501 531 560 919 360
7.8 8.5 28.6 27.0 28.8 69.0 23.0
35.1 35.5 111 107 116 243 85
6.4 9.1 41.1 24.9 36.5 102 27.0
182 197 614 580 693 1340 520
30.0 27.1 17.5 19.7 19.4 13.3 16.0
4.5 4.2 3.9 4.0 4.0 3.5 3.7
27.3 21.6 14.9 23.3 19.0 13.1 19.0
23.3 23.2 21.5 21.5 24.1 19.4 22.6
1 Barrer =10−10 cm3 (STP) cm cm−2 s−1 cmHg−1 or 7.5 × 10−18 m3 (STP) m m−2 s−1 Pa−1. bData from ref 26
e.g., 6FDA−SBF (αCO2/CH4 = 27.3) and 6FDA−BSBF (αCO2/CH4 = 23.3). In summary, bromine substitution in microporous polyimides is a very efficient way to improve permeability without significantly sacrificing selectivity, as shown in Figure 4 for CO2/CH4 separation. Similar trends were observed for CO2/N2, O2/N2, and H2/N2 gas pairs; the information is provided in Figure S2. The ideal gas selectivity determined from pure-gas permeation measurements (αA/B) involves contributions from both the solubility selectivity (SA/SB) and diffusion selectivity
(580 Barrer) was 3-fold higher than that of 6FDA−SBF (182 Barrer). Meanwhile, the CO2/CH4 selectivity of 6FDA−BSBF was 23.3, which is higher than previously reported PIM−PIs with similar CO 2 permeability (PIM−PI-7, CO 2 /CH 4 selectivity is 18.9);27 (iv) when the bulky SPDA was used as dianhydride, the bromine-substituted polymer (SPDA−BSBF) exhibited a CO2 permeability of 1340 Barrer, among the highest permeability values for PIM−PIs, including PIM−PI-1 (2000 Barrer)26 and PIM−PI-9 (2180 Barrer);28 (v) the introduction of bromine resulted in a very small decrease in CO2/CH4 selectivity, E
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Figure 4. CO2/CH4 gas pair separation performance of SBF- and BSBF-based PIM−PIs; the green four point stars are previously reported data for PIM−PIs.
Figure 6. Calculated rotation energy of one repeat unit for PMDA− SBF and PMDA−BSBF.
(DA/DB). To better understand the effect of different dianhydrides and bromine substitution on membrane selectivity, diffusion coefficients (D) and solubility coefficients (S) of each PIM−PI type were determined using the time-lag permeation method. The results are summarized in Table 4. When bromine was introduced in the PIM−PIs, the diffusion and solubility coefficients increased for all gases; e.g., 6FDA− BSBF exhibited 2.3 times higher CO2 diffusion coefficient (CO2 increased from 7.20 × 10−8 cm2/s to 16.8× 10−8 cm2/s) and 1.4-fold higher CO2 solubility coefficient. This leads to an increase in CO2 permeability from 182 (6FDA−SBF) to 580 Barrer (6FDA−BSBF) with only a small reduction in CO2/CH4 selectivity. Geometric Modeling. The effect of bromine substitution on the structure of the polymers was investigated by molecular modeling using Materials Studio software. PMDA was chosen as dianhydride and one repeat unit was selected for PMDA−
SBF and PMDA−BSBF, and their geometric optimization conformations are shown in Figure 5. The introduction of bromine had no effect on the conformation of the spirobifluorene unit, that is, the spiro center exhibited a contortion angle of exactly 90° (Figure 5). However, the bromine atoms affect the twist angle between PMDA and the fluorene plane; in PMDA−SBF, the twist angle is 0°. Such a highly conjugated structure has rigidity and exhibits relatively high BET surface area (320 m2/g). On the other hand, introducing bulky bromine atoms in PMDA−BSBF changed the twist angle to 77.8°, which provides an additional contortion site. As a result, the surface area increased to 450 m2/g. Rotating these repeat units around the imide bond (highlighted in red in Scheme 1) from 0° to 360° (Figure 6) showed that the bromine-substituted polymer (PMDA−BSBF) exhibited a much higher energy barrier (over 12 kcal/mol) than the pristine polymer (PMDA−SBF,
Table 4. Diffusion Coefficient (D), Solubility Coefficient (S), Diffusion Selectivity (αD) and Solubility Selectivity (αS) for Different Gases of SBF- and BSBF-Based PIM−PIs D (10−8 cm2/s)a
a
S (10−2 cm3/cm3 cmHg)
CO2/CH4
polymers
N2
CH4
CO2
N2
CH4
CO2
αD
αS
6FDA−SBF PMDA−SBF SPDA−SBF 6FDA−BSBF PMDA−BSBF SPDA−BSBF
5.20 5.47 15.0 13.9 11.5 27.4
1.20 1.18 4.76 3.18 3.47 9.11
7.20 6.03 19.6 16.8 17.2 29.9
1.50 1.55 1.90 1.94 2.50 2.53
5.30 7.71 8.64 7.82 10.5 11.3
25.0 32.7 31.2 34.6 40.3 44.7
6.0 5.1 4.1 5.3 5.0 3.3
4.7 4.2 3.6 4.4 3.8 4.0
D was determined by time-lag method; S was deduced from: P = D × S.
Figure 5. Optimized repeat unit structure of PMDA−SBF (left) and PMDA−BSBF (right). F
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8.4 kcal/mol), further indicating much higher rigidity in the bromine-substituted SBF-based PIM−PIs.
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CONCLUSIONS A general method is reported for the synthesis of microporous polyimides using spirobifluorene diamine (SBF) and dibromospirobifluorene diamine (BSBF) with different dianhydrides (6FDA, PMDA, and SPDA, respectively). All polymers exhibited good solubility in common organic solvents, excellent thermal stability and prominent microporosity. The introduction of bromine resulted in increases in surface area and gas permeability with small decreases in gas pair selectivity. Enhanced stiffness as a result of hindered bond rotation because of introduction of bromine was evidenced by a twist angle of 77.8°. The increased rotation energy for the imide bond gave a more rigid and contorted structure that resulted in increased gas permeability with only a small reduction in selectivity.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray diffractograms and permeability/selectivity trade-off relationships for CO2/N2, O2/N2, and H2/N2 of the SBF- and BSBF-based polyimides. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +966 02 80821328. E-mail:
[email protected] (I.P.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by KAUST baseline funding for Prof. Ingo Pinnau. The authors thank Raja Swaidan for helpful discussions of geometric modeling.
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REFERENCES
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