Synthesis and gas transport properties of hyperbranched polybenzoxazole – silica hybrid membranes

Journal of Membrane Science 521 (2017) 10–17

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Synthesis and gas transport properties of hyperbranched polybenzoxazole – silica hybrid membranes Tomoyuki Suzuki a,n, Mikako Takenaka a, Yasuharu Yamada b a b

Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Institute of Technological Research, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan

art ic l e i nf o

a b s t r a c t

Article history: Received 8 June 2016 Received in revised form 18 August 2016 Accepted 26 August 2016

Gas transport properties of hyperbranched polybenzoxazole (PBO) – silica hybrid membranes were investigated. The hyperbranched PBO – silica hybrid membranes were prepared via in-situ silylation method and sol-gel reaction. Thermal decomposition temperature of the hybrid membranes increased with increasing silica content, indicating improved thermal stability brought by the hybridization with silica. The hyperbranched PBO – silica hybrid membranes showed higher gas permeability than corresponding linear-type PBO – silica hybrid membranes with similar chemical structure. It was worth noting that both CO2 permeability and CO2/CH4 selectivity of the hyperbranched PBO – silica hybrid membranes were increased with increasing silica content across the upper bound trade-off line for CO2/CH4 separation. The prominent CO2/CH4 separation ability of the hyperbranched PBO – silica hybrid membranes might be achieved by large amounts of free volume holes, that were fundamentally brought by a characteristic hyperbranched structure and additionally crated around polymer/silica interfacial area, equipped with unique distribution and interconnectivity advantageous for a size-selective CO2/CH4 separation. & 2016 Elsevier B.V. All rights reserved.

Keywords: Polybenzoxazole Silica Hybrid Gas separation

1. Introduction The separation of gases by polymeric membranes has remarkably attracted the attention during several decades [1–4]. Compared to conventional separation processes, membrane-based gas separation provides many advantages such as low capital and operating costs, high energy efficiency, and ease of operation [5–7]. In this regard, a large number of polymeric membranes with high gas permeation and separation abilities have been developed. Especially, a class of rigid-rod aromatic polymers including polyimides (PIs) [8–11] and polybenzoxazoles (PBOs) [12,13] are promising candidates for applications to high-performance gas separation membranes because of their high gas permeation and separation abilities as well as high thermal and mechanical properties. Recently, Lee and his coworkers have reported about gas transport properties of PBOs prepared via thermal rearrangement process of polyimides containing ortho-positioned hydroxyl group, and the thermally rearranged PBOs, named TR-PBOs, have shown extraordinarily high gas permeability and selectivity [7,14– 17]. As another way to synthesize PBOs, dehydration reaction of aromatic polyamides containing ortho-positioned hydroxyl group, n

Corresponding author. E-mail address: [email protected] (T. Suzuki).

http://dx.doi.org/10.1016/j.memsci.2016.08.051 0376-7388/& 2016 Elsevier B.V. All rights reserved.

poly(o-hydroxy amide)s (PHAs), is also available, and gas transport properties of the PBOs prepared from corresponding PHA precursors have been investigated [18–21]. In our previous study, novel hyperbranched PIs and their silica hybrids have been prepared, and their gas transport properties have been investigated [22–24]. It has been found that the hyperbranched PIs and their silica hybrids exhibit outstanding gas transport and separation behaviors, owing to synergic effects of hyperbranched structure and hybridization with silica. However, to our knowledge, there are no experimental reports about gas transport properties of hyperbranched PBO and its silica hybrid membranes. In this study, hyperbranched PBO – silica (SiO2) hybrid membranes were prepared via in-situ silylation method [25,26] and solgel reaction, and their gas transport properties were investigated. In addition, obtained gas transport properties of the hyperbranched PBO – silica hybrid membranes were compared with those of linear-type PBO – silica hybrid membranes with similar chemical structure. Generally, for the synthesis of hyperbranched polymers, two different methods are known; the one is the self-polymerization of an AB2-type monomer and the other is the polymerization of an A2-type monomer and a B3-type monomer [27]. For the former method, Hong et al. have reported about the synthesis of a hyperbranched polybenzoxazole and its application for the

T. Suzuki et al. / Journal of Membrane Science 521 (2017) 10–17

chemically amplified materials [28]. On the other hand, for the latter method, Kudo et al. have successfully synthesized hyperbranched polybenzoxazoles through the polymerization of commercially available dihydroxyamine as an A2-monomer and tricarbonylchloride as a B3-monomer [29]. The polymerization with A2- and B3-type monomers is affected by many factors such as order of monomer addition, molar ratio of monomers, and concentration of reaction system [30]. [Manner 1] When a solution of A2-type monomer is added dropwise to a solution of B3-type monomer in a stoichiometric monomer feed ratio of A2:B3 ¼1:1 (the molar ratio between functional A and B groups of the corresponding monomers is 2/3), B-terminated hyperbranched polymer is obtained. [Manner 2] When a solution of B3-type monomer is added dropwise to a solution of A2-type monomer in a stoichiometric monomer feed ratio of A2:B3 ¼2:1 (the molar ratio between functional A and B groups of the corresponding monomers is 4/3), A-terminated hyperbranched polymer is obtained. The objective hyperbranched PHA precursor for this study was synthesized on the basis of the Manner 2.

11

The precipitated polymer was collected and washed thoroughly with distilled water, followed by vacuum drying at 90 °C for 4 h. Finally, polymer solid of 6FAHP-BTC hyperbranched poly(o-hydroxy amide), PHA(6FAHP-BTC), was obtained. Schematic representation of the synthesis of the PHA(6FAHP-BTC) is shown in Fig. 2. 2.2.2. 6FAHP-OBC poly(o-hydroxy amide) 5 mmol of 6FAHP was dissolved in 10 ml of DMAc in a 50 ml three-neck flask under N2 flow. 20 mmol of BSA was added to this solution with stirring at room temperature, and the solution was kept stirring for 1 h. After that, the solution was cooled with an ice-ethanol bath at 0–5 °C, and 4.85 mmol of OBC was added with stirring. The mixture was kept stirring at 0–5 °C for 1 h, and then, at room temperature for 3 h under N2 flow. The resulting polymer solution was poured into distilled water. The precipitated polymer was collected and washed thoroughly with distilled water, followed by vacuum drying at 90 °C for 4 h. Finally, polymer solid of 6FAHP-OBC poly(o-hydroxy amide), PHA(6FAHP-OBC), was obtained. 2.3. Membrane formation

2. Experimental 2.1. Materials 1,3,5-benzenetricarbonyl trichloride (BTC) was obtained from Sigma-Aldrich Japan (Tokyo, Japan). 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAHP), 4,4′-oxybis(benzoic acid chloride) (OBC), and tetraethoxysilane (TEOS) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). N,O-bis (trimethylsilyl)acetamide (BSA) as an amide-type silylation agent was obtained from Sigma-Aldrich Japan (Tokyo, Japan). 3-(triethoxysilyl)propylsuccinic anhydride (TEOSPSA) as a coupling agent and N,N-dimethylacetamide (DMAc) as a solvent were supplied by AZmax Co. (Tokyo, Japan) and Nacalai Tesque, Inc. (Kyoto, Japan), respectively. These regents and solvent were used as received. Chemical structures of monomers are shown in Fig. 1.

0.5 g of PHA(6FAHP-BTC) or PHA(6FAHP-OBC) were dissolved in 5 ml of DMAc, and 0.050 g of TEOSPSA were added with stirring. To this solutions, appropriate amounts of TEOS and distilled water (TEOS:distilled water¼1:6 as a molar ratio) and catalytic amount of diluted hydrochloric acid were added, and the reaction mixture was stirred overnight. Preparation condition of corresponding reaction mixtures is summarized in Table 1. After that, the mixtures were cast on PET sheets and dried at 85 °C for 1 h in a heating oven to form thin membranes. The prepared membranes were peeled off and fixed between two window-opened metal frames and subsequently cyclized and hybridized at 100 °C for 1 h, 200 °C for 1 h, and 400 °C for 1 h in a heating oven under N2 flow. Finally, hyperbranched PBO, PBO(6FAHP-BTC), or linear-type PBO, PBO (6FAHP-OBC), – silica hybrid membranes were obtained. 2.4. Measurements

2.2. Polymerization 2.2.1. 6FAHP-BTC hyperbranched poly(o-hydroxy amide) 3 mmol of 6FAHP was dissolved in 10 ml of DMAc in a 50 ml three-neck flask under N2 flow at room temperature. To this solution, 12 mmol of BSA was added with stirring at room temperature, and the solution was kept stirring for 1 h. After that, the solution was cooled with an ice-ethanol bath at 0–5 °C, and 1.65 mmol of BTC dissolved in 15 ml of DMAc was added dropwise through a syringe with stirring. The mixture was kept stirring at 0– 5 °C for 1 h, and then, at room temperature for 3 h under N2 flow. The resulting polymer solution was poured into distilled water.

Attenuated total reflection Fourier transform infrared (ATR FTIR) spectra were recorded by a FT/IR-4100 (JASCO Corp.) equipped with an ATR PRO ONE (ZnSe prism) (JASCO Corp.) at a wavenumber range of 700–4000 cm  1 and a resolution of 1 cm  1. UV/ VIS optical transmittances were investigated by a JASCO V-530 UV/ VIS spectrometer at a wavelength of 200–800 nm. Scanning electron microscopy (SEM) images were acquired using a Hitachi S-3400N variable pressure scanning electron microscope at an accelerating voltage of 10 kV. Samples for the SEM analyses were coated using a Hitachi E-1010 ion sputter coater with a platinum target. Thermogravimetric – differential thermal analysis (TG-DTA)

Fig. 1. Chemical structures of monomers.

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T. Suzuki et al. / Journal of Membrane Science 521 (2017) 10–17

Fig. 2. Schematic representation of the synthesis of PHA(6FAHP-BTC).

Table 1 Preparation condition of PBO – silica hybrid membranes. Polymer (g)

TEOSPSA (g)

Target SiO2 content (wt%)

TEOS (g)

H2O (g)

0.5

0.050

0 10 20 30

– 0.212 0.477 0.818

– 0.110 0.248 0.424

where θ (s) is the time-lag. A floating method was used for measuring density of pristine PBO(6FAHP-BTC) and PBO(6FAHP-OBC) membranes with NaBr aqueous solution at 25 °C. According to the group contribution method, fractional free volume (FFV) of a polymer can be estimated by the following equations [35,36];

FFV =

Vsp =

Vsp − 1.3Vw Vsp

M ρ

(4)

experiments were performed with a Seiko TG/DTA6300 at a heating rate of 10 °C/min and a temperature range of 25–800 °C under N2 or air flow. CO2, O2, N2, and CH4 permeation measurements were carried out by a constant volume/variable pressure apparatus at 76 cmHg and 25 °C. The permeability coefficient, P (barrer, 1 barrer ¼1  10  10 cm3(STP)cm/cm2 s cmHg), was determined by following equation [31];

where Vsp (cm3/mol) is the specific molar volume, Vw (cm3/mol) is the van der Waals volume of the repeat unit, M (g/mol) is the molecular weight of the repeat unit, and ρ (g/cm3) is the experimental density.

⎡ 273 V 1 1 dp ⎤ P=⎢ ⋅ ⋅L⋅ ⋅ ⋅ ⎥ × 1010 ⎣ T A p 76 dt ⎦

3. Results and discussion

(1)

where T is the absolute temperature (K), V is the downstream volume (cm3), A is the membrane area (cm2), L is the membrane thickness (cm), p is the upstream pressure (cmHg), and dp/dt is the permeation rate (cmHg/s). The gas permeability coefficient can be explained on the basis of the solution-diffusion mechanism, which is represented by following equation [32,33];

(2)

P=D×S 2

3

where D (cm /s) is the diffusion coefficient and S (cm (STP)/ cm3polym.cmHg) is the solubility coefficient. The diffusion coefficient was calculated by the time-lag method represented by following equation [34];

D=

L2 6θ

(5)

(3)

3.1. Membrane characterization Fig. 3 shows ATR FT-IR spectra of PHA(6FAHP-BTC) and PBO (6FAHP-BTC) – silica hybrid membranes. For the PHA(6FAHP-BTC), the bands assigned to amide linkage are found around 1510 (N–H bending) and 1650 cm  1 (CQO stretching), and the broad band around 2800–3400 cm  1 (O–H stretching of hydroxyl group in the backbone and N–H stretching) is also observed [19,37]. On the other hand, for the PBO(6FAHP-BTC), the bands attributed to amide linkage and hydroxyl group mentioned above are disappeared, and new absorption bands are appeared around 849 (benzoxazole ring), 1480 (benzoxazole ring), and 1628 cm  1 (CQN stretching), that are typical bands of PBOs [15,19,29,38]. Additionally, for the PBO(6FAHP-BTC) – silica hybrid membranes, strong absorption band around 1100 cm  1 assigned to Si–O–Si asymmetric stretching appears, and the intensity of the band

T. Suzuki et al. / Journal of Membrane Science 521 (2017) 10–17

13

Fig. 3. ATR FT-IR spectra of (a) PHA(6FAHP-BTC) and (b)–(e) PBO(6FAHP-BTC) – silica hybrid membranes; silica content ¼(b) 0, (c) 10, (d) 20, and (e) 30 wt%.

Table 2 Physical properties of PBO – silica hybrid membranes. Sample

Optical transmittance at 600 nm (%)

Tg (°C) Td5 (°C) Residual (wt%)

PBO(6FAHP-OBC) SiO2 ¼ 10 wt% SiO2 ¼ 20 wt% SiO2 ¼ 30 wt%

66 61 16 5

308 312 313 314

525 530 532 535

0 10 24 33

PBO(6FAHP-BTC) SiO2 ¼ 10 wt% SiO2 ¼ 20 wt% SiO2 ¼ 30 wt%

66 71 75 74

n.d.a n.d.a n.d.a n.d.a

513 517 522 527

0 10 18 29

a

Fig. 4. Cross-sectional SEM images of (a) PBO(6FAHP-OBC) – and (b) PBO(6FAHPBTC) – silica hybrid membranes (silica content ¼ 20 wt%).

Not detected.

increases with increasing silica content [39,40]. This result indicates the formation of robust three-dimensional Si–O–Si network in the hybrid membranes. Similar behaviors were observed for PHA(6FAHP-OBC) and PBO(6FAHP-OBC) – silica hybrid membranes (data not shown). Optical transmittances at the wavelength of 600 nm for the PBO – silica hybrid membranes are listed in Table 2. The PBO (6FAHP-OBC) – silica hybrid membranes show decreased transmittance with increasing silica content, whereas the PBO (6FAHP-BTC) – silica hybrid membranes maintain high transparency. Fig. 4 represents cross-sectional SEM images of PBO(6FAHPOBC) – and PBO(6FAHP-BTC) – silica hybrid membranes (silica content¼20 wt%). The PBO(6FAHP-OBC) – silica hybrid membrane has rugged cross-sectional morphology because of approximately 1 mm of aggregated silica domains (Fig. 4(a)). This fact indicates the decreased transmittance of the PBO(6FAHP-OBC) – silica hybrid membranes is caused by the aggregated silica domains. On the other hand, the PBO(6FAHP-BTC) – silica hybrid membrane shows smooth cross-section (Fig. 4(b)), indicating the existence of welldispersed silica domains. The high homogeneity is considered to be brought by a characteristic hyperbranched structure of

Fig. 5. Thermogravimetric and differential thermogravimetric curves of (a) PHA (6FAHP-OBC) and (b) PBO(6FAHP-OBC) membranes obtained under N2 flow condition.

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T. Suzuki et al. / Journal of Membrane Science 521 (2017) 10–17

molecular chains as similar to the hyperbranched polyimide – silica hybrid membranes reported elsewhere [41], that provides high transparency of the PBO(6FAHP-BTC) – silica hybrid membranes. Thermal properties of the membranes were investigated by TGDTA measurements. Figs. 5 and 6 show thermogravimetric and differential thermogravimetric curves of pristine PHA and PBO membranes obtained under N2 flow condition. Before the TG-DTA measurements, the PHA membranes were additionally dried at 200 °C for 1 h to remove residual DMAc in the membranes [20]. For PHA(6FAHP-OBC) (Fig. 5(a)) and PHA(6FAHP-BTC) (Fig. 6(a)), two-step weight loss is observed. The first step in the range of 220–330 °C provides 6.4 and 7.3 wt% of weight losses for PHA (6FAHP-OBC) and PHA(6FAHP-BTC), respectively. The experimental weight losses are in roughly agreement with expected weight losses caused by the removal of H2O during

cyclodehydration reaction of PHAs into corresponding PBOs; 6.1 wt% for PHA(6FAHP-OBC) and 6.6 wt% for PHA(6FAHP-BTC). On the other hand, for PBO(6FAHP-OBC) (Fig. 5(b)) and PBO(6FAHPBTC) (Fig. 6(b)), the first step weight loss is disappeared, and thermal decomposition initiates around 500 °C which corresponds to the second step weight loss for the PHAs. From the results of TG-DTA measurements, it is again confirmed that the PHAs are successfully converted into the corresponding PBOs by the established cyclization protocol for preparing PBO membranes. Next, glass transition temperatures (Tgs) determined from DTA curves and 5% weight-loss temperatures (Td5s) of the hybrid membranes obtained under air flow condition are summarized in Table 2 in addition to the residuals at 800 °C. It is confirmed from the residuals that all hybrid membranes contain appropriate amounts of silica as expected. Tg value of the PBO(6FAHP-OBC) – silica hybrid membranes increases with increasing silica content, suggesting the formation of robust three-dimensional Si–O–Si network. However, for the PBO(6FAHP-BTC) system, Tg of the membranes cannot be detected from corresponding DTA curves. This behavior might be attributed to rigid molecular structure of the PBO(6FAHP-BTC) and increased restriction of molecular mobility by hybridization with silica. On the other hand, Td5s of the hybrid membranes are increased with increasing silica content (Table 2), indicating increased thermal stability by the hybridization with silica. 3.2. Gas transport properties

Fig. 6. Thermogravimetric and differential thermogravimetric curves of (a) PHA (6FAHP-BTC) and (b) PBO(6FAHP-BTC) membranes obtained under N2 flow condition.

Gas permeability, diffusion, and solubility coefficients of the hybrid membranes are summarized in Table 3 together with the results of related polymeric membranes. Unfortunately, we could not carry out the gas permeation measurement for the PBO (6FAHP-BTC) – silica hybrid membrane containing 30 wt% of silica because of its mechanical brittleness. As shown in Table 3, Gas permeability coefficients of pristine PBO(6FAHP-BTC) are higher than corresponding those of pristine PBO(6FAHP-OBC), primarily attributed to higher gas diffusivity. Additionally, it is worth noting the gas permeability of the PBO(6FAHP-BTC) is approximately one order of magnitude higher than that of a class of fluorine-contained hyperbranched polyimides [41,42] and comparable lineartype PBOs prepared via corresponding PHA precursors [18,21]

Table 3 Gas permeability, diffusion, and solubility coefficients of PBO – silica hybrid membranes and related polymers. Sample

P (barrer)a

D  108 (cm2/s)

S  102 (cm3 (STP)/cm3polym.cmHg)

FFV

CO2

O2

N2

CH4

CO2

O2

N2

CH4

CO2

O2

N2

CH4

PBO (6FAHP-OBC) SiO2 ¼10 wt% SiO2 ¼20 wt% SiO2 ¼30 wt%

67.7 103 168 206

12.4 18.5 28.6 33.6

2.30 3.52 6.16 6.85

1.71 2.53 4.09 4.23

2.36 3.61 4.99 6.42

6.76 9.06 14.6 18.0

1.60 2.19 3.96 4.35

0.324 0.453 0.713 0.825

28.7 28.5 33.8 32.0

1.83 2.04 1.96 1.87

1.44 1.61 1.56 1.58

5.27 5.60 5.73 5.13

0.182

PBO (6FAHP-BTC) SiO2 ¼10 wt% SiO2 ¼20 wt%

407 558 713

69.5 89.7 112

16.6 22.1 27.5

10.2 14.1 17.5

8.53 9.64 13.9

23.2 25.7 33.3

6.46 7.71 9.90

1.14 1.34 1.89

47.7 57.9 51.2

3.00 3.49 3.34

2.56 2.87 2.77

8.90 10.5 9.25

0.193

6FDA-TAPAb HBPI(6FDA-TAPB)c FPBO-1d TR-pPBOe

65 25 66 53

11 4.6 19.4 11

2.16 0.79 4.5 2.3

1.59 0.44 2.25 1.4

2.7 0.76 9.0 4.5

9.4 2.8 19 5.2

2.06 0.57 5.8 1.3

0.31 0.091 1.3 0.46

24 33 7.3 12

1.15 1.7 1.0 2.1

1.05 1.4 0.77 1.7

5.1 4.8 1.7 3.0

Ref.

[41] [42] [18] [21]

1 barrer¼1  10  10 cm3(STP)cm/cm2 s cmHg. Hyperbranched polyimide synthesized with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and tris(4-aminophenyl)amine (TAPA) measured at 76 cmHg and 35 °C. c Hyperbranched polyimide synthesized with 6FDA and 1,3,5-tris(4-aminophenyl)benzene (TAPB) measured at 76 cmHg and 25 °C. d Linear-type PBO synthesized with 6FAHP and 4,4′-(1,1,1,3,3,3-hexafluoroisopropylidene)dibenzoyl chloride measured at 760 cmHg and 50 °C. e Linear-type PBO synthesized with 6FAHP and terephthaloyl chloride (TPC) measured at 76 cmHg and 35 °C. a

b

T. Suzuki et al. / Journal of Membrane Science 521 (2017) 10–17

reported previously. It is well known that gas permeability and/or diffusivity of glassy polymers including rigid-rod aromatic polymers strongly depends on the fractional free volume (FFV) [36,43]. The FFV values of the pristine PBOs calculated from Eqs. (4) and (5) are listed in Table 3. It is found the PBO(6FAHP-BTC) has higher FFV than the PBO(6FAHP-OBC), and the high gas permeability of the PBO(6FAHP-BTC) is, therefore, attributed to its high FFV. The high FFV of the PBO(6FAHP-BTC) might be brought by a characteristic hyperbranched structure which brings about loose packing of molecular chains. It should be noted that gas permeability coefficients of the PBO – silica hybrid membranes increase with increasing silica content mainly in connection with increased gas diffusivity. Similar enhancements of gas permeability have been reported by Merkel et al. for high-free-volume, glassy polymer – nano-sized silica composites, and they have concluded that the nano-sized silica particles yield polymer/particle interfacial area and provide disruption of polymer chain packing and affect gas transport behavior [44,45]. The increased gas permeability of the PBO – silica hybrid membranes, as a consequence, might be caused by additional formation of free volume holes around polymer/silica interfacial area. From these facts, it can be said that the specifically high gas permeability of the PBO(6FAHP-BTC) – silica hybrid membranes is resulted in both fundamentally high FFV brought by a characteristic hyperbranched structure and free volume holes additionally created around polymer/silica interfacial area. The ideal gas selectivity for the combination of gases A and B (α (A/B)) is defined by the following equation [46];

α(A/B) =

P (A) D(A) S(A) = × = α D(A/B) × α S (A/B) P (B) D(B) S(B)

15

Fig. 7. Ideal O2/N2 selectivity of PBO – silica hybrid membranes plotted against O2 permeability; attached number represents silica content (wt%) in the membrane.

(6)

where α (A/B) is the diffusivity selectivity and α (A/B) is the solubility selectivity. The O2/N2 and CO2/CH4 selectivities of the hybrid membranes are summarized in Table 4. It is recognized the ideal selectivity (α(A/B)) for given gas pairs essentially depends on the diffusivity selectivity (αD(A/B)) rather than the solubility selectivity (αS(A/B)), which is consistent with a general D

S

Table 4 O2/N2 and CO2/CH4 selectivities of PBO – silica hybrid membranes and related polymers. Sample

O2/N2 separation

CO2/CH4 separation

α (O2/ N2)

αD (O2/ N2)

αS (O2/ N2)

α (CO2/ αD CH4) (CO2/ CH4)

αS (CO2/ CH4)

PBO(6FAHP-OBC) SiO2 ¼ 10 wt% SiO2 ¼ 20 wt% SiO2 ¼ 30 wt%

5.4 5.3 4.6 4.9

4.2 4.1 3.7 4.1

1.3 1.3 1.3 1.2

40 41 41. 49

7.3 8.0 7.0 7.8

5.5 5.1 5.9 6.2

PBO(6FAHP-BTC) SiO2 ¼ 10 wt% SiO2 ¼ 20 wt%

4.2 4.1 4.1

3.6 3.3 3.4

1.2 1.2 1.2

40 40 41

7.5 7.2 7.4

5.4 5.5 5.5

6FDA-TAPAa HBPI(6FDA-TAPB)b FPBO-1c TR-pPBOd

5.0 5.8 4.3 4.8

4.5 4.9 3.3 3.9

1.1 1.2 1.3 1.2

41 58 29 38

8.9 8.3 6.9 9.8

4.7 7.0 4.3 3.9

Ref.

Fig. 8. Ideal CO2/CH4 selectivity of PBO – silica hybrid membranes plotted against CO2 permeability; attached number represents silica content (wt%) in the membrane.

[41] [42] [18] [21]

a Hyperbranched polyimide synthesized with 6FDA and TAPA measured at 76 cmHg and 35 °C. b Hyperbranched polyimide synthesized with 6FDA and TAPB measured at 76 cmHg and 25 °C. c Linear-type PBO synthesized with 6FAHP and 4,4′-(1,1,1,3,3,3-hexafluoroisopropylidene)dibenzoyl chloride measured at 760 cmHg and 50 °C. d Linear-type PBO synthesized with 6FAHP and TPC measured at 76 cmHg and 35 °C.

understanding of gas separation behavior of glassy polymers [46]. Figs. 7 and 8 show α(O2/N2) and α(CO2/CH4) values plotted against O2 and CO2 permeability coefficients, respectively. In Fig. 7, the α (O2/N2) values of the hybrid membranes slightly decrease with increasing O2 permeability along with the upper bound trade-off line for O2/N2 separation demonstrated by Robeson in 1991 [47]. This behavior is in accord with the general trend that glassy polymers that are more permeable are less selective, and vice versa [46]. In any event, it can be seen the PBO – silica hybrid membranes have a favorable O2/N2 separation ability because they show high α(O2/N2) values just below the upper bound in 1991. For CO2/CH4 separation, the α(CO2/CH4) values of the hybrid membranes increase with increasing CO2 permeability, or silica

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content, tending to across the upper bound for CO2/CH4 separation (Fig. 8) [47,48]. The increased CO2/CH4 separation behavior is considered to be brought by unique distribution and interconnectivity of free volume holes created around polymer/silica interfacial area that accelerates a size-selective CO2/CH4 separation ability. Similarly, He et al. have reported that vapor/gas separation properties of high-free-volume, glassy polymers with rigid molecular chains are improved by incorporation of nano-sized inorganic fillers that disrupt molecular chain packing [49]. Especially, the PBO(6FAHP-BTC) and its silica hybrid membranes possess high degree of CO2/CH4 separation ability which exceeds the upper bound updated in 2008 and is essentially superior to the ability for the related hyperbranched polyimides and linear-type PBOs. The prominent CO2/CH4 separation ability of the PBO (6FAHP-BTC) system might be achieved by large amounts of free volume holes, that are fundamentally brought by a characteristic hyperbranched structure and additionally crated around polymer/ silica interfacial area, equipped with unique distribution and interconnectivity advantageous for a size-selective CO2/CH4 separation. Although, at the present state, the PBO(6FAHP-BTC) system cannot exceed the advanced trade-off line for TR-polymers including TR-PBOs [14,48,50], the PBO(6FAHP-BTC) system has comparable potential to TR-PBOs for CO2/CH4 separation.

4. Conclusions

[5]

[6]

[7]

[8]

[9]

[10] [11]

[12]

[13]

[14]

[15] [16]

Gas transport properties of hyperbranched PBO, PBO(6FAHPBTC), – silica hybrid membranes were investigated. Thermal decomposition temperature of the PBO(6FAHP-BTC) – silica hybrid membranes increases with increasing silica content, indicating improved thermal stability brought by the hybridization with silica, as similar to the linear-type PBO, PBO(6FAHP-OBC), – silica hybrid membranes with similar chemical structure. The PBO (6FAHP-BTC) shows higher gas permeability than the PBO(6FAHPOBC). The high gas permeability of the PBO(6FAHP-BTC) is attributed to its high FFV which might be brought by a characteristic hyperbranched structure brings about loose packing of molecular chains. Both CO2 permeability and CO2/CH4 selectivity of the PBO (6FAHP-OBC) – and PBO(6FAHP-BTC) – silica hybrid membranes increase with increasing silica content beyond the upper bound trade-off line for CO2/CH4 separation. This fact suggests a sizeselective CO2/CH4 separation ability is enhanced by the incorporation of silica. In particular, the PBO(6FAHP-BTC) – silica hybrid membranes possess high CO2/CH4 separation ability which exceeds the upper bound updated in 2008. The prominent CO2/ CH4 separation ability of the PBO(6FAHP-BTC) – silica hybrid membranes might be achieved by large amounts of free volume holes, that are fundamentally brought by a characteristic hyperbranched structure and additionally crated around polymer/silica interfacial area, equipped with unique distribution and interconnectivity advantageous for a size-selective CO2/CH4 separation. From these facts, it can be said the PBO(6FAHP-BTC) and its silica hybrid membranes have high thermal stability and excellent CO2/ CH4 selectivity, and are expected to apply to high-performance gas separation membranes.

[17]

[18]

[19]

[20]

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