Effect of thermal treatment on gas transport properties of hyperbranched polyimide–silica hybrid membranes

Journal of Membrane Science 417–418 (2012) 193–200

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Effect of thermal treatment on gas transport properties of hyperbranched polyimide–silica hybrid membranes Tomoyuki Suzuki, Yasuharu Yamada n Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

a r t i c l e i n f o

abstract

Article history: Received 27 December 2011 Received in revised form 21 June 2012 Accepted 22 June 2012 Available online 4 July 2012

Physical and gas transport properties of thermotreated hyperbranched polyimide and its silica hybrid membranes were investigated. Thermal treatment at 450 1C resulted in partial degradation and rearrangement of molecular chains. Gas permeabilities of the hyperbranched polyimide membrane were remarkably enhanced by thermal treatment at 450 1C, compared to those of linear-type polyimide membrane with a similar chemical structure. The enhanced gas permeabilities were considered to be due to local deformation of molecular packing caused by thermal degradation and rearrangement of molecular chains, which provided an increase in free volume elements. In addition, for the hyperbranched polyimide–silica hybrid membrane treated at 450 1C, hybridization with silica brought about increases in gas permeabilities and improvement of CO2/CH4 separation ability. The prominent CO2/CH4 separation ability was attributed to the enhanced free volume elements induced by facilitated thermal degradation and rearrangement of molecular chains, as well as created around interfacial region between polymer and well-dispersed silica domains with a large surface area. & 2012 Elsevier B.V. All rights reserved.

Keywords: Hyperbranched polyimide Thermal treatment Silica hybrid Sol–gel Gas separation

1. Introduction Over the last decades, membrane-based gas separation processes have greatly been developed. Especially, polyimides have been of great interest in gas separation membranes because of their excellent thermal, mechanical, and gas transport properties [1–4]. It has recently been reported that hyperbranched polyimides (HBPIs) have different physical and gas transport properties from linear-type polyimides [5–7]. Fang et al. have studied physical and gas transport properties of HBPIs derived from a triamine, tris(4-aminophenyl)amine (TAPA), and commercially available dianhydrides, and it has been shown that the HBPIs have good gas separation performance, compared to linear-type polyimides [8,9]. Similarly, in our previous study, it has been found that HBPIs prepared by polycondensation of a triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and conventional dianhydrides exhibit high gas permeabilities and selectivities, arising from characteristic hyperbranched structures [10,11]. Organic–inorganic hybrids are attractive materials since they generally possess desirable organic and inorganic properties. Hybridization with inorganic compounds has also been focused on the modification of polyimides in order to improve their thermal, mechanical, and gas transport properties [12–16].

n

Corresponding author. Tel./Fax: þ 81 75 724 7932. E-mail address: [email protected] (Y. Yamada).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.06.035

We have also found out that CO2/CH4 selectivity of the HBPI–silica hybrid membranes prepared via a sol–gel reaction using alkoxysilanes are markedly increased with increasing silica content, suggesting characteristic distribution and interconnectivity of free volume elements created by the incorporation of silica [17–19]. As another promising candidate, polyimide membranes treated at high temperatures have shown great enhancements of gas permeabilities and selectivities owing to thermal rearrangement of molecular chains, which provide favorable molecular-sieving effects. Lee et al. have been demonstrated that thermal rearrangement of hydroxyl-containing polyimides forms microporous polybenzoxazoles, showing extraordinarily high gas separation abilities [20–22]. Additionally, it has been reported that conventional polyimide membranes treated at moderate temperature (400–500 1C) also show improved gas transport properties by thermal degradation and rearrangement of molecular chains [23,24]. Thus far, however, there are no reports about the gas transport properties of HBPIs and their hybrid membranes treated at the temperature range of 400–500 1C. In this study, physical and gas transport properties of the HBPI prepared by polycondensation of a triamine, TAPOB, and a dianhydride, 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and its silica hybrid membranes thermally treated at 450 1C were investigated, and compared with those of the lineartype polyimide system with a similar chemical structure. By simultaneous application of thermal treatment and hybridization techniques, synergetic enhancements of gas permeabilities

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and/or selectivities of the HBPI–silica hybrid membranes will be expected.

2. Experimental 2.1. Materials TAPOB was synthesized by reduction of 1,3,5-tris(4-nitrophenoxy)benzene with palladium carbon and hydrazine in methanol [25]. 6FDA was kindly supplied from Daikin Industries, Ltd. (Osaka, Japan). 1,3-bis(4-aminophenoxy)benzene (TPER) was purchased from Wakayama Seika Kogyo Co., Ltd. (Wakayama, Japan). 3-aminopropyltrimethoxysilane (APTrMOS) and tetramethoxysilane (TMOS) were obtained from AZmax Co., Ltd. (Tokyo, Japan). N,N-dimethylacetamide (DMAc) as a solvent was supplied from Nacalai Tesque, Inc. (Kyoto, Japan). Chemical structures of monomers are shown in Fig. 1. 2.2. Polymerization 2.2.1. 6FDA-TAPOB hyperbranched polyamic acid 3 mmol of 6FDA was dissolved in 30 ml of DMAc in a 100 ml three-neck flask under N2 flow at room temperature. To this solution, 1.6 mmol of TAPOB in 30 ml of DMAc was added dropwise through a syringe with stirring for 3 h. After that 0.4 mmol of APTrMOS (8 mol% as a monomer ratio) was added in the reaction mixture with stirring for 1 h to afford 6FDA-TAPOB hyperbranched polyamic acid (HBPAA(6FDA-TAPOB)). 2.2.2. 6FDA-TPER polyamic acid 6 mmol of 6FDA was dissolved in 30 ml of DMAc in a 100 ml three-neck flask under N2 flow at room temperature. To this solution, 5.5 mmol of TPER was added in the DMAc solution with stirring. The reaction mixture was further stirred for 3 h. After that 1.0 mmol of APTrMOS (8 mol% as a monomer ratio) was added in the reaction mixture with further stirring for 1 h, and 6FDA-TPER polyamic acid (PAA(6FDA-TPER)) was obtained. 2.3. Membrane formation The 6FDA-based polyimide–silica hybrid membranes were prepared by thermal imidization and sol–gel reaction with an alkoxysilane, TMOS. Appropriate amounts of TMOS and deionized water were added in the DMAc solutions of the polyamic acids, and stirred overnight. After that, the reaction mixtures were cast on PET sheets and dried at 85 1C for 3 h in a heating oven to form

thin membranes. The prepared membranes were peeled off and subsequently imidized and hybridized at 100 1C for 1 h, 200 1C for 1 h, and 300 1C for 1 h in a heating oven under N2 flow. Additional heating at 450 1C for 1 h was continuously carried out for the thermotreated membranes. Neat 6FDA-based polyimide membranes without silica were also prepared in a similar manner. Table 1 summarizes the preparation conditions of the 6FDA-based polyimides and silica hybrid membranes. 2.4. Measurements Thermogravimetric-differential thermal analysis (TG-DTA) experiments were carried out with a Seiko TG/DTA6300 at a heating rate of 10 1C/min and a temperature range of 25–800 1C under N2 or air flow. Thermal mechanical analysis (TMA) measurements were performed using a Seiko TMA/SS6100 at a heating rate of 5 1C/min under N2 flow. Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectra were recorded at a wavenumber range of 550–4000 cm  1 and a resolution of 1 cm  1 with a JASCO FT/IR-460 plus. Wide-angle X-ray scattering (WAXS) patterns were recorded by a Rigaku RINT2000 using Cu-Ka radia˚ The scan range was from 5 to tion with a wavelength of l ¼1.54 A. 601 under a voltage of 40 kV and a current of 50 mA. Average ˚ was determined based on Bragg’s law [26]; d-spacing, d (A), nl ¼ 2d sin

ð1Þ

where n is the integral number (1, 2, 3, y), l denotes the X-ray wavelength, and y indicates the diffraction angle. The densities of the membranes were measured by the floating method with a NaBr aqueous solution. CO2, O2, N2, and CH4 permeation measurements were carried out by a constant volume/variable pressure apparatus at 76 cm Hg and 25 1C. The permeability coefficient, P (cm3(STP)cm/cm2 s cm Hg), was determined by the following equation [27]: P¼

273 V 1 1 dp L T A p 76 dt

ð2Þ

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 P can be explained on the basis of the solution-diffusion mechanism [28,29]; P¼DS

ð3Þ 2

3

where D (cm /s) is the diffusion coefficient and S (cm (STP)/ cm3polym.cmHg) is the solubility coefficient. The D was calculated by the time-lag method [30]; D¼

L2 6t

ð4Þ

where t (s) is the time-lag. The ideal selectivity for the combination of gases A and B (a(A/B)) is defined by the following equation [31]:

aðA=BÞ ¼

PðAÞ DðAÞ SðAÞ ¼ ¼ aD ðA=BÞaS ðA=BÞ PðBÞ DðBÞ SðBÞ

ð5Þ

Table 1 Preparation of 6FDA-based polyamic acids and their hybrid membranes.

Fig. 1. Chemical structures of monomers.

Sample

DMAc solution of PAA (g)

PAA (g)

TMOS (g)

H2O (g)

HBPAA(6FDA-TAPOB) SiO2 ¼10 wt%

6.00 6.00

0.210 0.210

– 0.059

– 0.042

PAA(6FDA-TPER) SiO2 ¼10 wt%

3.00 3.00

0.409 0.409

– 0.115

– 0.082

T. Suzuki, Y. Yamada / Journal of Membrane Science 417–418 (2012) 193–200

where aD(A/B) is the diffusivity selectivity and aS(A/B) is the solubility selectivity. 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.

3. Results and discussion 3.1. Membrane characterization Fig. 2 shows photographs of 6FDA-TAPOB hyperbranched polyimide (HBPI(6FDA-TAPOB)) and 6FDA-TPER linear-type polyimide (PI(6FDA-TPER)) membranes treated at 300 and 450 1C. By thermal treatment at 450 1C, the color of the membranes is varied from pale yellow to dark brown, suggesting a partial degradation of molecular chains. Fig. 3 shows TG and DTG curves of the 6FDAbased polyimide membranes obtained by the TG-DTA measurements in the N2 flow, and resulting 5% weight-loss temperatures (T5ds) and residuals at 800 1C are summarized in Table 2. As sown in Fig. 3, thermal degradation of the

Fig. 2. Photographs of HBPI(6FDA-TAPOB) (a) and PI(6FDA-TPER) (b) membranes treated at 300 and 450 1C.

195

300 1C-treated HBPI(6FDA-TAPOB) membrane is initiated from lower temperature than that of the 300 1C-treated PI(6FDA-TPER) membrane, indicating thermal instability of a large number of molecular chain terminals in the HBPI(6FDATAPOB). From the DTG curves in Fig. 3, it is pointed out that the peak attributed to an initial degradation process, which is distinctively observed for the 300 1Ctreated membranes, is diminished by the 450 1C treatment. Additionally, the 450 1C-treated membranes have higher T5ds and residuals than corresponding 300 1C-treated membranes (Table 2). These facts suggest a prior degradation of thermally weak parts in the molecular chains such as –CF3 group, propylene group in APTrMOS moiety, and, in addition, unmodified carboxylic anhydride terminals in the HBPI(6FDA-TAPOB) by the long-term thermal treatment at 450 1C around which the thermal degradation is initiated. The thermal degradation behavior at a relatively low temperature region is supported by some literatures that isothermal degradation of polyimides is already initiated around 400–500 1C in an inert atmosphere [23,24,32]. Thermal mechanical properties of the membranes were examined by TMA measurements, and TG-DTA measurements in the air flow were also carried out to evaluate the silica content in the hybrid membranes. Obtained coefficients of thermal expansion (CTEs), glass transition temperatures (Tgs), T5ds, and residuals are listed in Table 2. It is confirmed from the residuals that all hybrid membranes contain appropriate amounts of silica as expected. The hybrid membranes have higher Tg and T5d values and lower CTEs than corresponding neat polyimide membranes, indicating an increase in thermal stability by hybridization with silica. From the comparison of 300 1C- and 450 1C -treated membranes, it is pointed out thermal treatment at 450 1C leads to increases in T5d and Tg and decrease in CTE. The increased T5d is, as mentioned above, resulted from the prior degradation of the molecular chains. It has been reported about thermal degradation behaviors of aromatic polyimides in an inert atmosphere that intermolecular cross-linking reactions caused by thermal radical species take place [33,34]. The increased Tg and decreased CTE, therefore, might be attributed to the formation of intermolecular cross-linking. It is worth noting that the Tgs of the HBPI(6FDATAPOB) system are remarkably increased by the 450 1C treatment, compared to those of the PI(6FDA-TPER) system. This result is considered to be due to the progressed intermolecular cross-linking induced by thermal degradation of the carboxylic anhydride terminals in the HBPI(6FDA-TAPOB). The viewpoint is supported by the fact that a carboxylic acid-containing 6FDA-based polyimide forms decarboxylation-induced cross-linking by thermal treatment at a moderate temperature range [35]. Fig. 4 gives ATR FT-IR spectra of the 6FDA-based polyimides and their hybrid membranes. The bands observed around 1783 cm  1 (C¼ O asymmetrical stretching), 1721 cm  1 (C ¼O symmetrical stretching), 1373 cm  1 (C–N stretching), and 720 cm  1 (C¼ O bending) are characteristic absorption bands of polyimides [9,36]. In contrast, the characteristic band of polyamic acids around 1680 cm  1 is not found. These facts indicate prepared membranes are well imidized. It is also found that the band around 1100 cm  1 assigned to Si–O–Si stretching is observed for the hybrid membranes, indicating the formation of a three-dimensional Si–O–Si network [13,37]. From the comparison of the spectra, slight decrease in the absorption intensity is recognized for the 450 1C-treated membranes. Although it is difficult to discuss in detail about molecular chains deformation and morphological change of silica domains, the decreased absorption intensity can be attributed to the partial degradation of the molecular chains by thermal treatment at 450 1C [23]. Especially for the HBPI(6FDA-TAPOB) system, the band observed around 1857 cm  1 assigned to the carboxylic anhydride groups [9] is diminished

Fig. 3. TG and DTG curves of HBPI(6FDA-TAPOB) (a) and PI(6FDA-TPER) (b) membranes treated at 300 and 450 1C.

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Table 2 Thermal properties of 6FDA-based polyimide membranes. Sample

Thermal treatment (1C)

T5d (1C)

Residual at 800 1C (wt%)

In N2

In air

In N2

In air

Tg1(1C)

CTE 2(ppm/1C)

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

300

503 –

457 490

53 –

0 10

282 305

54 47

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

450

523 –

508 513

55 –

0 10

400 –

48 –

PI(6FDA-TPER) SiO2 ¼ 10 wt%

300

516 –

494 505

55 –

0 10

247 249

59 55

PI(6FDA-TPER) SiO2 ¼ 10 wt%

450

524 –

515 519

56 –

0 9

267 295

48 46

1 2

Glass transition temperature determined from the TMA measurement. CTE at 100–150 1C.

Fig. 4. ATR FT-IR spectra of HBPI(6FDA-TAPOB) (a), HBPI(6FDA-TAPOB), SiO2 ¼ 10 wt% (b), PI(6FDA-TPER) (c), and PI(6FDA-TPER), SiO2 ¼10 wt% (d) membranes treated at 300 and 450 1C.

by the 450 1C treatment, supporting the thermal degradation of the carboxylic anhydride terminals. WAXS patterns of the 300 1C- and 450 1C-treated membranes are shown in Fig. 5. The d-spacing of intermolecular chain packing of the HBPI(6FDA-TAPOB) and PI(6FDA-TPER) systems treated at 300 1C is calculated using Eq. (1) to 5.8 A˚ (2y ¼ 15.31). Additionally, a broad shoulder peak (approximately from 151 to 301) for the hybrid membranes implies the existence of amorphous silica [38–40]. For the 450 1C-treated membranes, while the pristine d-spacing is essentially maintained, a small diffraction peak is appeared around 2y ¼24–251 (d-spacing of ˚ suggesting local deformation of molecular packing caused by about 3.6–3.7 A), thermal degradation and rearrangement of molecular chains. 3.2. Gas transport properties Gas permeability, diffusion, and solubility coefficients of the membranes are listed in Table 3. For the 300 1C-treated membranes, as described previously,

HBPI(6FDA-TAPOB) and its hybrid membranes show higher gas permeability than the PI(6FDA-TPER) system, arising from high fractional free volume and additional formation of free volume elements around polymer–silica interface [17–19]. Fig. 6 illustrates the effect of thermal treatment on CO2 permeability, diffusion, and solubility coefficients of the membranes. The membranes treated at 450 1C exhibit increased CO2 permeability mainly connected to increased CO2 diffusivity. The increased CO2 permeability is considered to be due to local deformation of molecular packing caused by thermal degradation and rearrangement of molecular chains, which induces an increase in free volume elements. Densities of the 300 1C-treated HBPI(6FDA-TAPOB) and PI(6FDA-TPER) membranes are, however, slightly increased from 1.433 g/cm3 to 1.436 g/cm3 and 1.417 g/cm3 to 1.424 g/ cm3, respectively, by the thermal treatment at 450 1C. The discrepancy might be caused by changes of pristine chemical structures by thermal degradation. As shown in Table 3 and Fig. 6, it is worth noting that gas permeabilities of the HBPI(6FDA-TAPOB) are remarkably enhanced by the thermal treatment at 450 1C, compared to those of the PI(6FDA-TPER). The remarkable enhancement of gas

T. Suzuki, Y. Yamada / Journal of Membrane Science 417–418 (2012) 193–200

197

Fig. 5. WAXS patterns of HBPI(6FDA-TAPOB) (a), HBPI(6FDA-TAPOB), SiO2 ¼10 wt% (b), PI(6FDA-TPER) (c), and PI(6FDA-TPER), SiO2 ¼10 wt% (d) membranes treated at 300 and 450 1C.

Table 3 Gas transport properties of 6FDA-based polyimide membranes at 76 cmHg and 25 1C. Sample

Thermal treatment (1C)

P  1010 (cm3(STP)cm/cm2 s cmHg)

D  108 (cm2/s)

S  102 (cm3(STP)/cm3polym cmHg)

CO2

O2

N2

CH4

CO2

O2

N2

CH4

CO2

O2

N2

CH4

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

300

7.4 10

1.5 2.0

0.23 0.31

0.098 0.13

0.30 0.34

1.4 1.5

0.24 0.29

0.028 0.026

25 30

1.1 1.4

0.92 1.1

3.5 5.1

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

450

46 134

8.2 22

1.4 4.2

0.74 2.4

1.1 2.8

3.9 8.2

0.83 1.8

0.11 0.29

40 48

2.1 2.6

1.7 2.3

6.9 8.4

PI(6FDA-TPER) SiO2 ¼ 10 wt%

300

5.5 5.9

1.1 1.1

0.16 0.17

0.085 0.088

0.30 0.26

1.3 1.1

0.24 0.21

0.032 0.024

18 23

0.88 0.99

0.66 0.79

2.7 3.6

PI(6FDA-TPER) SiO2 ¼ 10 wt%

450 1C

14 17

2.7 3.1

0.43 0.51

0.21 0.29

0.54 0.56

2.1 2.1

0.43 0.42

0.048 0.058

26 30

1.3 1.5

1.0 1.2

4.5 4.9

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Fig. 6. CO2 permeability (a), diffusion (b), and solubility (c) coefficients of 6FDA-based polyimide membranes treated at 300 and 450 1C.

Fig. 7. Cross-sectional SEM images of 300 1C-treated HBPI(6FDA-TAPOB), SiO2 ¼10 wt% (a), 450 1C-treated HBPI(6FDA-TAPOB), SiO2 ¼10 wt% (b), 300 1C-treated PI(6FDATPER), SiO2 ¼ 10 wt% (c), and 450 1C-treated PI(6FDA-TPER), SiO2 ¼ 10 wt% (d) membranes.

permeabilities of the 450 1C-treated HBPI(6FDA-TAPOB) membrane might be attributed to facilitated thermal degradation of molecular chain terminals and characteristic thermal rearrangement of molecular chains brought by a unique hyperbranched structure, which advantageously induces free volume elements. It is worth noting the hybridization with silica significantly accelerates increases in gas permeabilities for the 450 1C-treated HBPI(6FDA-TAPOB)–silica hybrid membrane. Fig. 7 shows cross-sectional SEM images of the 300 1C- and 450 1C-treated hybrid membranes. The PI(6FDA-TPER)–silica hybrid membranes have rugged cross-sectional morphology because of aggregated silica domains (Fig. 7 (c) and (d)). On the other hand, the HBPI(6FDA-TAPOB)–silica hybrid membranes show smooth cross-section (Fig. 7 (a) and (b)), indicating the existence of well-dispersed silica domains. The high homogeneity is considered to be brought by the unique hyperbranched structure of molecular chains, and the resulting well-dispersed silica domains, therefore, provide a large amount of polymer–silica interfacial region. It has been known about the preparation of silica by sol–gel reaction that residual surface silanol groups are gradually lost with increasing temperature [41]. The reduced silanol groups might provide vacant spaces around polymer–silica interface. From these facts, it can be said the remarkably

increased gas permeabilities of the 450 1C-treated HBPI(6FDA-TAPOB)–silica hybrid membrane are caused by enhanced free volume elements created around interfacial region between polymer and well-dispersed silica domains with a large surface area in addition to the free volume elements induced by facilitated thermal degradation and rearrangement of molecular chains. In near future, positron annihilation lifetime spectroscopy and solid-state 29Si NMR spectroscopy characterization of the membranes will be carried to obtain detailed morphological information. O2/N2 and CO2/CH4 selectivities of the membranes are summarized in Table 4. It is recognized that the ideal O2/N2 and CO2/CH4 selectivities of the membranes mainly depend on the diffusivity selectivities rather than the solubility selectivities. Fig. 8 gives ideal O2/N2 selectivity (a(O2/N2)) values of the membranes plotted against O2 permeability coefficient. The a(O2/N2) values of the membranes slightly decrease with increasing O2 permeability along with the upper bound trade-off line for O2/N2 separation demonstrated by Robeson in 1991 [42]. This behavior is consistent with the general understanding that polymers that are more permeable are less selective, and vice versa [31]. In any event, it can be seen the HBPI(6FDA-TAPOB) and its silica hybrid membranes treated at 450 1C have a favorable O2/N2 separation ability because these membranes show high a(O2/N2)

T. Suzuki, Y. Yamada / Journal of Membrane Science 417–418 (2012) 193–200

199

Table 4 O2/N2 and CO2/CH4 selectivities of 6FDA-based polyimide membranes at 76 cmHg and 25 1C. Sample

Thermal treatment (1C)

O2/N2 selectivity

CO2/CH4 selectivity

a(O2/N2)

aD(O2/N2)

aS(O2/N2)

a(CO2/CH4)

aD(CO2/CH4)

aS(CO2/CH4)

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

300

6.8 6.6

5.8 5.2

1.2 1.3

75 79

11 13

7.0 5.9

HBPI(6FDA-TAPOB) SiO2 ¼ 10 wt%

450

5.7 5.2

5.0 4.6

1.2 1.1

62 55

11 9.7

5.9 5.6

PI(6FDA-TPER) SiO2 ¼ 10 wt%

300

6.9 6.4

5.2 5.1

1.3 1.3

65 67

9.6 11

6.7 6.3

PI(6FDA-TPER) SiO2 ¼ 10 wt%

450

6.3 6.2

4.9 4.9

1.3 1.3

66 60

12 9.7

5.8 6.1

Fig. 8. Ideal O2/N2 selectivities (a(O2/N2)) of 6FDA-based polyimide membranes treated at 300 and 450 1C; attached numbers represent silica content (wt%) in the membrane.

values just below the upper bound in 1991. For CO2/CH4 separation, an attractive behavior is observed. Ideal CO2/CH4 selectivity (a(CO2/CH4)) values of the membranes are plotted against CO2 permeability coefficient in Fig. 9. CO2 permeabilities of the 6FDA-based polyimide membranes are increased toward the upper bound of CO2/CH4 separation, almost maintaining initial a(CO2/CH4) values, by thermal treatment at 450 1C. Especially for the HBPI(6FDA-TAPOB)– silica hybrid membrane, the a(CO2/CH4) reaches the updated upper bound in 2008 [43]. The prominent CO2/CH4 separation ability might be resulted from the enhanced free volume elements, which are favorable for CO2/CH4 separation, induced by facilitated thermal degradation and rearrangement of molecular chains, as well as created around polymer–silica interface.

4. Conclusions Physical and gas transport properties of thermotreated HBPI(6FDA-TAPOB) and its silica hybrid membranes were investigated, and compared with those of thermotreated linear-type PI(6FDA-TPER) system. Thermal treatment at 450 1C results in partial degradation and rearrangement of molecular chains. The 450 1C-treated membranes show increased gas permeabilities mainly connected to increased gas diffusivities. The increased gas

Fig. 9. Ideal CO2/CH4 selectivities (a(CO2/CH4)) of 6FDA-based polyimide membranes treated at 300 and 450 1C; attached numbers represent silica content (wt%) in the membrane.

permeabilities are considered to be due to increased free volume elements brought by thermal degradation and rearrangement of molecular chains. Especially, gas permeabilities of the HBPI(6FDATAPOB) are remarkably enhanced by thermal treatment at 450 1C, compared to those of the PI(6FDA-TPER). The remarkable enhancement of gas permeabilities of the 450 1C-treated HBPI(6FDATAPOB) membrane might be attributed to facilitated thermal degradation of molecular chain terminals and characteristic thermal rearrangement of molecular chains brought by a unique hyperbranched structure, which advantageously induces free volume elements. In addition, for the 450 1C-treated HBPI(6FDATAPOB)–silica hybrid membrane, hybridization with silica leads to increases in gas permeabilities and improvement of CO2/CH4 separation ability. The prominent CO2/CH4 separation ability is attributed to the enhanced free volume elements induced by facilitated thermal degradation and rearrangement of molecular chains, as well as created around interfacial region between polymer and well-dispersed silica domains with a large surface area. The thermotreated HBPI(6FDA-TAPOB) and its silica hybrid membranes are expected to apply to high-performance gas separation membranes.

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