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Gas transport properties of 6FDA-TAPOB hyperbranched polyimide membrane
Polymer 45 (2004) 7167–7171 www.elsevier.com/locate/polymer
Gas transport properties of 6FDA-TAPOB hyperbranched polyimide membrane Tomoyuki Suzukia, Yasuharu Yamadaa,*, Yoshiharu Tsujitab b
a Center for Cooperative Research, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Department of Materials Science and Engineering, Polymeric Materials Course, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Received 11 March 2004; received in revised form 25 June 2004; accepted 10 August 2004 Available online 28 August 2004
Abstract Physical and gas transport properties of the hyperbranched polyimide prepared from a triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and a dianhydride, 4,4 0 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA), were investigated and compared with those of linear-type polyimides with similar chemical structures prepared from diamines, 1,4-bis(4-aminophenoxy)benzene (TPEQ) or 1,3-bis(4aminophenoxy)benzene (TPER), and 6FDA. 6FDA-TAPOB hyperbranched polyimide exhibited a good thermal stability as well as lineartype analogues. Fractional free volume (FFV) value of 6FDA-TAPOB was higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. It was found that increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB, in spite of the higher FFV value. However, 6FDA-TAPOB exhibited considerably high gas solubility, resulting in high gas permeability. It was suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDA-TAPOB provide effective O2/N2 selectivity. It is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high permeability and O2/N2 selectivity, and is expected to apply to a highperformance gas separation membrane. q 2004 Elsevier Ltd. All rights reserved. Keywords: Hyperbranched polyimide; Gas transport properties; O2/N2 Selectivity
1. Introduction Polyimide membranes have been of great interest in gas separation applications because of their high gas selectivities and excellent thermal and mechanical properties. In this regard, a large number of polyimides have been investigated about their gas transport properties [1–7]. However, most of previous experimental studies of the gas transport properties have been focused on linear-type polyimides. Hyperbranched polymers, as well as dendrimers, have unique properties, e.g. multifunctionality arising from multiple end groups, good solubilities, and reduced * Corresponding author. Tel.: C81-52-735-5537; fax: C81-52-7355542. E-mail address: [email protected] (Y. Yamada). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.08.025
viscosities compared to linear-type polymers with similar chemical structures [8–12]. In recent years, novel triaminebased hyperbranched polyimides have been synthesized and characterized. Fang et al. have synthesized hyperbranched polyimides derived from a triamine, tris(4-aminophenyl)amine (TAPA), and commercially available dianhydrides and investigated their physical and gas transport properties [13,14]. On the other hand, hyperbranched polyimides polymerized with another triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB) have been prepared and characterized for the first time by Chen et al. [15]. At present, however, there are no data for gas transport properties of the TAPOB-based hyperbranched polyimides. In this study, physical and gas transport properties of the hyperbranched polyimide prepared from TAPOB as a triamine and 4,4 0 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA) as a dianhydride were investigated. In
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addition, the physical and gas transport properties were discussed comparing with those of linear-type polyimides with similar chemical structures.
TAPOB was synthesized by reduction of 1,3,5-tris(4nitrophenoxy)benzene with palladium carbon and hydrazine in methanol [16]. 1,4-Bis(4-aminophenoxy)benzene (TPEQ) and 1,3-bis(4-aminophenoxy)benzene (TPER) were obtained from Wakayama Seika Kogyo Co., Ltd, and 6FDA was purchased from Aldrich. Chemical structures of the monomers are shown in Fig. 1. All the monomers were dried at 80 8C for 24 h in vacuo before used.
was measured by floating method with NaBr aqueous solution at 25 8C. Gas permeability coefficient, P (cm3 STPcm/cm2 s cmHg), was determined by the vacuum-pressure method. A polyimide membrane, which was held in the permeation cell sealed with O-rings, was evacuated for 24 h or more in the permeation apparatus. Desired pressure of a gas was introduced on the upstream side, and downstream side of the membrane was held at high vacuum. The pressure on the upstream side was monitored using Baratron127AA pressure transducer (MKS Instruments, Inc.). The permeability coefficient was obtained from the permeation rate that was monitored as the variation of pressure on the downstream side at steady state using another Baratron127AA pressure transducer. The gas permeability coefficient can be explained on the basis of the solution-diffusion mechanism, which is represented by the following equation [17–20].
2.2. Polymerization
P Z DS
2. Experimental 2.1. Materials
2.2.1. 6FDA-TPEQ and 6FDA-TPER polyamic acids Diamine (TPEQ or TPER) (10 mmol) was dissolved in 30 ml of N-methyl-2-pyrrolidone (NMP) in a 100 ml threeneck flask under N2 flow at room temperature. After that 10 mmol of 6FDA was added in the NMP solution with stirring. The reaction mixture was further stirred for 3 h to afford 6FDA-TPEQ or 6FDA-TPER polyamic acid and finally stored in a refrigerator. 2.2.2. 6FDA-TAPOB hyperbranched polyamic acid TAPOB (2.5 mmol) was dissolved in 25 ml of NMP in a 100 ml three-neck flask under N2 flow at room temperature. To this solution, 2.5 mmol of 6FDA in 25 ml of NMP was added dropwise through a syringe with stirring. After the addition, the reaction mixture was further stirred for 3 h to afford 6FDA-TAPOB hyperbranched polyamic acid and finally stored in a refrigerator. 2.3. Membrane formation The polyamic acids solutions were cast on glass dishes and dried at 80 8C for 2 h. The prepared membranes were peeled off and subsequently imidized at 100 8C for 1 h, 200 8C for 1 h, and 300 8C for 1 h in a heating oven under N2 flow. 2.4. Measurements Inherent viscosity, hinh (dl/g), of a polyamic acid was determined in an Ubbelohde viscometer using 0.5 g/dl NMP solution at 25 8C. Infrared (IR) spectrum was recorded on a JASCO FT/IR-460 plus. Differential scanning calorimetry (DSC) was measured with a Perkin–Elmer DSC7 at a heating rate of 10 8C/min under N2 flow. Thermogravimetric-differential thermal analysis (TG-DTA) was performed with a Seiko TG/DTA6300 at a heating rate of 10 8C/min under N2 flow. Density of a polyimide membrane
(1)
where D (cm2/s) is the diffusion coefficient and S (cm3 STP/ cm3polym. cmHg) is the solubility coefficient. The diffusion coefficient was calculated by the time-lag method represented by the following equation [21]; DZ
L2 6q
(2)
where L (cm) is the thickness of the membrane and q (s) is the time-lag.
3. Results and discussion 3.1. Polymer characterization Inherent viscosity (hinh) of 6FDA-based polyamic acids is listed in Table 1. The hinh value of 6FDA-TAPOB hyperbranched polyamic acid is lower than those of lineartype polyamic acids with similar chemical structures, corresponding to a feature of general hyperbranched polymers. It is also found that prepared polyamic acids have moderate viscosities. This fact suggests that molecular weights of the polyamic acids are high enough to possess good membrane formability and to discuss physical properties of their imidized membranes. FT-IR measurements for the 6FDA-based polyimide membranes were carried out to confirm a progress of thermal imidization. The bands observed around 1784 cmK1 (CaO asymmetrical stretching), 1725 cmK1 (CaO symmetrical stretching), 1378 cmK1 (C–N stretching), and 723 cmK1 (CaO bending) are the characteristic absorption bands of a polyimide [13,15]. On the other hand, no characteristic band of a polyamic acid around 1680 cmK1 was found. These results indicate that the polyimide membranes are well imidized. Thermal properties of the polyimide membranes were
T. Suzuki et al. / Polymer 45 (2004) 7167–7171
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Fig. 1. Chemical structures of monomers. Table 1 Inherent viscosity of 6FDA-based polyamic acids and physical properties of the polyimide membranes Sample
hinha (dl/g)
Tg (8C)
Td5 (8C)
r (g/cm3)
FFV
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
0.60 0.80 1.06
297 275 247
510 517 517
1.405 1.406 1.417
0.171b 0.160 0.153
a b
Inherent viscosity of polyamic acids using 0.5 g/dl NMP solution at 25 8C. FFV value based on the fully branched structure.
investigated. DSC measurement was carried out to determine glass transition temperature (Tg) of the polyimides and to test for residual solvent or crystallinity in the polyimides. No evidence of solvent or crystallinity was revealed for all polyimide membranes. Observed Tg value of the polyimides is listed in Table 1. For linear-type polyimides, Tg of 6FDATPEQ as para-connected isomer is higher than that of 6FDA-TPER as meta-connected isomer. This behavior is considered to be due to an isomer effect, and similar phenomena have been observed in other polyimide systems [22,23]. In any event, it can be confirmed that these three polyimides are amorphous and lie in a glassy state at ambient temperature (25 8C). Fig. 2 shows TG curves of the polyimide membranes. The TG curves also demonstrate that the polyimide membranes are well imidized because no weight-loss attributed to dehydration by imidization all through the measurement is observed. The 5% weight-loss temperature ðTd5 Þ of the polyimides was determined and summarized in Table 1. It is found that all polyimides show Td5 of above 500 8C. This fact indicates that the 6FDATAPOB hyperbranched polyimide exhibits a good thermal stability as well as the linear-type analogues.
diffusivity of a polymer depend on the free volume in the polymer [24,25]. According to the group contribution method [26], FFV of a polymer can be estimated by the following equation; FFV Z
Vsp K 1:3Vw Vsp
3.2. Gas transport properties of 6FDA-based polyimide membranes It is generally interpreted that gas permeability and/or
Fig. 2. TG curves of 6FDA-based polyimide membranes.
(3)
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Table 2 Permeability coefficients and O2/N2 selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C P (!1010 cm3 STPcm/ cm2 s cmHg)
CO2
O2
N2
a (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
12.8 12.6 5.13
2.25 2.24 1.04
0.363 0.371 0.156
6.21 6.03 6.65
where Vsp (cm3/mol) is the specific molar volume which can be calculated from the experimental density, r (g/cm3), and Vw (cm3/mol) is the van der Waals volume of the repeat unit. Obtained r and FFV of the polyimides are listed in Table 1. For linear-type analogues, the FFV value of 6FDA-TPER is lower than that of 6FDA-TPEQ, indicating tighter packing of molecular chains of 6FDA-TPER. On the other hand 6FDA-TAPOB hyperbranched polyimide has higher FFV than the linear-type analogues. This fact indicates looser packing of molecular chains of the 6FDA-TAPOB attributed to the characteristic hyperbranched structure. Gas permeability, diffusion, and solubility coefficients of the polyimide membranes are summarized in Tables 2–4, respectively. From the comparison of permeability coefficients of linear-type analogues listed in Table 2, it is found that 6FDA-TPEQ has higher permeability than 6FDATPER. It is also pointed out from Tables 3 and 4 that the difference in the permeability of these two polyimides is mainly due to the difference in the diffusivity rather than the solubility. One factor contributes to the lower diffusivity of 6FDA-TPER is the tighter packing of molecular chains, that is, the lower FFV value described above. As another factor, it has been reported that the segmental mobility of molecular chains also affects the diffusivity [22,23,27–29]. Rotational motion of a central phenyl ring in TPER moiety is forbidden because of the meta-bonding between adjacent phenoxy groups. This increased resistance to the segmental mobility brings about the lower diffusivity of 6FDA-TPER. Therefore, the lower diffusivity of 6FDA-TPER can be attributed to both lower FFV and increased inhibition to the segmental mobility.
6FDA-TAPOB hyperbranched polyimide shows intermediate diffusivity between 6FDA-TPEQ and 6FDA-TPER although 6FDA-TAPOB has higher FFV value than the linear-type analogues. This fact is presumably ascribed to the lower segmental mobility mentioned above. Rotational motion of a central tri-substituted phenyl ring in TAPOB moiety is forbidden because of meta-bondings among adjacent phenoxy groups. This increased resistance to the segmental mobility similarly to the case of 6FDA-TPER brings about the lower diffusivity of 6FDA-TAPOB in spite of the higher FFV value. It is, however, worth noting that the 6FDA-TAPOB has considerably high solubility and, as a result, shows high permeability approximately equal to the permeability of 6FDA-TPEQ. Considering that 6FDATAPOB and the linear-type analogues lie in the glassy state at ambient temperature (25 8C), the increased solubility of 6FDA-TAPOB is presumed to be due to an increase in the Langmuir sorption site [2]. O2/N2 selectivity, a(O2/N2), is defined by the following equation [30–32]; aðO2 =N2 Þ Z
PðO2 Þ DðO2 Þ SðO2 Þ Z PðNÞ2 DðN2 Þ SðN2 Þ
Z aD ðO2 =N2 Þas ðO2 =N2 Þ
(4)
where aD(O2/N2) is the diffusivity selectivity and aS(O2/N2) is the solubility selectivity. These selectivities are listed in Tables 2–4, respectively. From Tables 2–4, it is recognized that the polyimide membranes show the O2/N2 selectivity depends on the diffusivity selectivity rather than the solubility selectivity. For linear-type analogues, 6FDA-
Table 3 Diffusion coefficients and O2/N2 diffusivity selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C D (!108 cm2/s)
CO2
O2
N2
aD (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
0.493 0.562 0.298
1.80 2.09 1.18
0.369 0.457 0.237
4.88 4.58 4.96
Table 4 Solubility coefficients and O2/N2 solubility selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C S (!102 cm3 STP/cm3polym. cmHg)
CO2
O2
N2
aS (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
26.0 22.5 17.2
1.25 1.07 0.881
0.986 0.811 0.658
1.27 1.32 1.34
T. Suzuki et al. / Polymer 45 (2004) 7167–7171
TPER has higher a(O2/N2) than 6FDA-TPEQ as shown in Table 2. This result is consistent with the general understanding that polymers, which are more permeable, are less selective and vice versa [30–32]. However, it should be noted that 6FDA-TAPOB hyperbranched polyimide has higher aD(O2/N2) and, as a result, higher a(O2/N2) than 6FDA-TPEQ although O2 and N2 permeabilities of the two polyimides are almost equal. This anomalous result is considered to be due to differences in the size and distribution of free volume holes [33–38]. Jean et al. have been reported that gas separation performance of polymers strongly depends on size and distribution of free volume holes [39]. Furthermore, Shimazu et al. have been pointed out that molecular sieving ability of polyimides tends to strong with decreasing local chain mobility [40]. From these facts, it is suggested that low segmental mobility and unique size and distribution of free volume holes arising from characteristic hyperbranched structure of 6FDA-TAPOB provide effective O2/N2 selectivity in comparison with 6FDA-TPEQ.
4. Conclusions Physical and gas transport properties of the 6FDATAPOB hyperbranched polyimide have been investigated. 6FDA-TAPOB exhibits a good thermal stability as well as linear-type polyimides, 6FDA-TPEQ and 6FDA-TPER. FFV value of 6FDA-TAPOB is higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. Rotational motion of a central tri-substituted phenyl ring in TAPOB moiety is forbidden because of metabondings among adjacent phenoxy groups, and the increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB in spite of the higher FFV value. However, 6FDA-TAPOB exhibits considerably high gas solubility and, as a result, shows high gas permeability. It is suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDATAPOB provide effective O2/N2 selectivity. From these results, it is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high gas permeability and selectivity, and is expected to apply to a high-performance gas separation membrane.
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Gas transport properties of 6FDA-TAPOB hyperbranched polyimide membrane Tomoyuki Suzukia, Yasuharu Yamadaa,*, Yoshiharu Tsujitab b
a Center for Cooperative Research, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Department of Materials Science and Engineering, Polymeric Materials Course, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Received 11 March 2004; received in revised form 25 June 2004; accepted 10 August 2004 Available online 28 August 2004
Abstract Physical and gas transport properties of the hyperbranched polyimide prepared from a triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and a dianhydride, 4,4 0 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA), were investigated and compared with those of linear-type polyimides with similar chemical structures prepared from diamines, 1,4-bis(4-aminophenoxy)benzene (TPEQ) or 1,3-bis(4aminophenoxy)benzene (TPER), and 6FDA. 6FDA-TAPOB hyperbranched polyimide exhibited a good thermal stability as well as lineartype analogues. Fractional free volume (FFV) value of 6FDA-TAPOB was higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. It was found that increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB, in spite of the higher FFV value. However, 6FDA-TAPOB exhibited considerably high gas solubility, resulting in high gas permeability. It was suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDA-TAPOB provide effective O2/N2 selectivity. It is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high permeability and O2/N2 selectivity, and is expected to apply to a highperformance gas separation membrane. q 2004 Elsevier Ltd. All rights reserved. Keywords: Hyperbranched polyimide; Gas transport properties; O2/N2 Selectivity
1. Introduction Polyimide membranes have been of great interest in gas separation applications because of their high gas selectivities and excellent thermal and mechanical properties. In this regard, a large number of polyimides have been investigated about their gas transport properties [1–7]. However, most of previous experimental studies of the gas transport properties have been focused on linear-type polyimides. Hyperbranched polymers, as well as dendrimers, have unique properties, e.g. multifunctionality arising from multiple end groups, good solubilities, and reduced * Corresponding author. Tel.: C81-52-735-5537; fax: C81-52-7355542. E-mail address: [email protected] (Y. Yamada). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.08.025
viscosities compared to linear-type polymers with similar chemical structures [8–12]. In recent years, novel triaminebased hyperbranched polyimides have been synthesized and characterized. Fang et al. have synthesized hyperbranched polyimides derived from a triamine, tris(4-aminophenyl)amine (TAPA), and commercially available dianhydrides and investigated their physical and gas transport properties [13,14]. On the other hand, hyperbranched polyimides polymerized with another triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB) have been prepared and characterized for the first time by Chen et al. [15]. At present, however, there are no data for gas transport properties of the TAPOB-based hyperbranched polyimides. In this study, physical and gas transport properties of the hyperbranched polyimide prepared from TAPOB as a triamine and 4,4 0 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA) as a dianhydride were investigated. In
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T. Suzuki et al. / Polymer 45 (2004) 7167–7171
addition, the physical and gas transport properties were discussed comparing with those of linear-type polyimides with similar chemical structures.
TAPOB was synthesized by reduction of 1,3,5-tris(4nitrophenoxy)benzene with palladium carbon and hydrazine in methanol [16]. 1,4-Bis(4-aminophenoxy)benzene (TPEQ) and 1,3-bis(4-aminophenoxy)benzene (TPER) were obtained from Wakayama Seika Kogyo Co., Ltd, and 6FDA was purchased from Aldrich. Chemical structures of the monomers are shown in Fig. 1. All the monomers were dried at 80 8C for 24 h in vacuo before used.
was measured by floating method with NaBr aqueous solution at 25 8C. Gas permeability coefficient, P (cm3 STPcm/cm2 s cmHg), was determined by the vacuum-pressure method. A polyimide membrane, which was held in the permeation cell sealed with O-rings, was evacuated for 24 h or more in the permeation apparatus. Desired pressure of a gas was introduced on the upstream side, and downstream side of the membrane was held at high vacuum. The pressure on the upstream side was monitored using Baratron127AA pressure transducer (MKS Instruments, Inc.). The permeability coefficient was obtained from the permeation rate that was monitored as the variation of pressure on the downstream side at steady state using another Baratron127AA pressure transducer. The gas permeability coefficient can be explained on the basis of the solution-diffusion mechanism, which is represented by the following equation [17–20].
2.2. Polymerization
P Z DS
2. Experimental 2.1. Materials
2.2.1. 6FDA-TPEQ and 6FDA-TPER polyamic acids Diamine (TPEQ or TPER) (10 mmol) was dissolved in 30 ml of N-methyl-2-pyrrolidone (NMP) in a 100 ml threeneck flask under N2 flow at room temperature. After that 10 mmol of 6FDA was added in the NMP solution with stirring. The reaction mixture was further stirred for 3 h to afford 6FDA-TPEQ or 6FDA-TPER polyamic acid and finally stored in a refrigerator. 2.2.2. 6FDA-TAPOB hyperbranched polyamic acid TAPOB (2.5 mmol) was dissolved in 25 ml of NMP in a 100 ml three-neck flask under N2 flow at room temperature. To this solution, 2.5 mmol of 6FDA in 25 ml of NMP was added dropwise through a syringe with stirring. After the addition, the reaction mixture was further stirred for 3 h to afford 6FDA-TAPOB hyperbranched polyamic acid and finally stored in a refrigerator. 2.3. Membrane formation The polyamic acids solutions were cast on glass dishes and dried at 80 8C for 2 h. The prepared membranes were peeled off and subsequently imidized at 100 8C for 1 h, 200 8C for 1 h, and 300 8C for 1 h in a heating oven under N2 flow. 2.4. Measurements Inherent viscosity, hinh (dl/g), of a polyamic acid was determined in an Ubbelohde viscometer using 0.5 g/dl NMP solution at 25 8C. Infrared (IR) spectrum was recorded on a JASCO FT/IR-460 plus. Differential scanning calorimetry (DSC) was measured with a Perkin–Elmer DSC7 at a heating rate of 10 8C/min under N2 flow. Thermogravimetric-differential thermal analysis (TG-DTA) was performed with a Seiko TG/DTA6300 at a heating rate of 10 8C/min under N2 flow. Density of a polyimide membrane
(1)
where D (cm2/s) is the diffusion coefficient and S (cm3 STP/ cm3polym. cmHg) is the solubility coefficient. The diffusion coefficient was calculated by the time-lag method represented by the following equation [21]; DZ
L2 6q
(2)
where L (cm) is the thickness of the membrane and q (s) is the time-lag.
3. Results and discussion 3.1. Polymer characterization Inherent viscosity (hinh) of 6FDA-based polyamic acids is listed in Table 1. The hinh value of 6FDA-TAPOB hyperbranched polyamic acid is lower than those of lineartype polyamic acids with similar chemical structures, corresponding to a feature of general hyperbranched polymers. It is also found that prepared polyamic acids have moderate viscosities. This fact suggests that molecular weights of the polyamic acids are high enough to possess good membrane formability and to discuss physical properties of their imidized membranes. FT-IR measurements for the 6FDA-based polyimide membranes were carried out to confirm a progress of thermal imidization. The bands observed around 1784 cmK1 (CaO asymmetrical stretching), 1725 cmK1 (CaO symmetrical stretching), 1378 cmK1 (C–N stretching), and 723 cmK1 (CaO bending) are the characteristic absorption bands of a polyimide [13,15]. On the other hand, no characteristic band of a polyamic acid around 1680 cmK1 was found. These results indicate that the polyimide membranes are well imidized. Thermal properties of the polyimide membranes were
T. Suzuki et al. / Polymer 45 (2004) 7167–7171
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Fig. 1. Chemical structures of monomers. Table 1 Inherent viscosity of 6FDA-based polyamic acids and physical properties of the polyimide membranes Sample
hinha (dl/g)
Tg (8C)
Td5 (8C)
r (g/cm3)
FFV
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
0.60 0.80 1.06
297 275 247
510 517 517
1.405 1.406 1.417
0.171b 0.160 0.153
a b
Inherent viscosity of polyamic acids using 0.5 g/dl NMP solution at 25 8C. FFV value based on the fully branched structure.
investigated. DSC measurement was carried out to determine glass transition temperature (Tg) of the polyimides and to test for residual solvent or crystallinity in the polyimides. No evidence of solvent or crystallinity was revealed for all polyimide membranes. Observed Tg value of the polyimides is listed in Table 1. For linear-type polyimides, Tg of 6FDATPEQ as para-connected isomer is higher than that of 6FDA-TPER as meta-connected isomer. This behavior is considered to be due to an isomer effect, and similar phenomena have been observed in other polyimide systems [22,23]. In any event, it can be confirmed that these three polyimides are amorphous and lie in a glassy state at ambient temperature (25 8C). Fig. 2 shows TG curves of the polyimide membranes. The TG curves also demonstrate that the polyimide membranes are well imidized because no weight-loss attributed to dehydration by imidization all through the measurement is observed. The 5% weight-loss temperature ðTd5 Þ of the polyimides was determined and summarized in Table 1. It is found that all polyimides show Td5 of above 500 8C. This fact indicates that the 6FDATAPOB hyperbranched polyimide exhibits a good thermal stability as well as the linear-type analogues.
diffusivity of a polymer depend on the free volume in the polymer [24,25]. According to the group contribution method [26], FFV of a polymer can be estimated by the following equation; FFV Z
Vsp K 1:3Vw Vsp
3.2. Gas transport properties of 6FDA-based polyimide membranes It is generally interpreted that gas permeability and/or
Fig. 2. TG curves of 6FDA-based polyimide membranes.
(3)
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Table 2 Permeability coefficients and O2/N2 selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C P (!1010 cm3 STPcm/ cm2 s cmHg)
CO2
O2
N2
a (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
12.8 12.6 5.13
2.25 2.24 1.04
0.363 0.371 0.156
6.21 6.03 6.65
where Vsp (cm3/mol) is the specific molar volume which can be calculated from the experimental density, r (g/cm3), and Vw (cm3/mol) is the van der Waals volume of the repeat unit. Obtained r and FFV of the polyimides are listed in Table 1. For linear-type analogues, the FFV value of 6FDA-TPER is lower than that of 6FDA-TPEQ, indicating tighter packing of molecular chains of 6FDA-TPER. On the other hand 6FDA-TAPOB hyperbranched polyimide has higher FFV than the linear-type analogues. This fact indicates looser packing of molecular chains of the 6FDA-TAPOB attributed to the characteristic hyperbranched structure. Gas permeability, diffusion, and solubility coefficients of the polyimide membranes are summarized in Tables 2–4, respectively. From the comparison of permeability coefficients of linear-type analogues listed in Table 2, it is found that 6FDA-TPEQ has higher permeability than 6FDATPER. It is also pointed out from Tables 3 and 4 that the difference in the permeability of these two polyimides is mainly due to the difference in the diffusivity rather than the solubility. One factor contributes to the lower diffusivity of 6FDA-TPER is the tighter packing of molecular chains, that is, the lower FFV value described above. As another factor, it has been reported that the segmental mobility of molecular chains also affects the diffusivity [22,23,27–29]. Rotational motion of a central phenyl ring in TPER moiety is forbidden because of the meta-bonding between adjacent phenoxy groups. This increased resistance to the segmental mobility brings about the lower diffusivity of 6FDA-TPER. Therefore, the lower diffusivity of 6FDA-TPER can be attributed to both lower FFV and increased inhibition to the segmental mobility.
6FDA-TAPOB hyperbranched polyimide shows intermediate diffusivity between 6FDA-TPEQ and 6FDA-TPER although 6FDA-TAPOB has higher FFV value than the linear-type analogues. This fact is presumably ascribed to the lower segmental mobility mentioned above. Rotational motion of a central tri-substituted phenyl ring in TAPOB moiety is forbidden because of meta-bondings among adjacent phenoxy groups. This increased resistance to the segmental mobility similarly to the case of 6FDA-TPER brings about the lower diffusivity of 6FDA-TAPOB in spite of the higher FFV value. It is, however, worth noting that the 6FDA-TAPOB has considerably high solubility and, as a result, shows high permeability approximately equal to the permeability of 6FDA-TPEQ. Considering that 6FDATAPOB and the linear-type analogues lie in the glassy state at ambient temperature (25 8C), the increased solubility of 6FDA-TAPOB is presumed to be due to an increase in the Langmuir sorption site [2]. O2/N2 selectivity, a(O2/N2), is defined by the following equation [30–32]; aðO2 =N2 Þ Z
PðO2 Þ DðO2 Þ SðO2 Þ Z PðNÞ2 DðN2 Þ SðN2 Þ
Z aD ðO2 =N2 Þas ðO2 =N2 Þ
(4)
where aD(O2/N2) is the diffusivity selectivity and aS(O2/N2) is the solubility selectivity. These selectivities are listed in Tables 2–4, respectively. From Tables 2–4, it is recognized that the polyimide membranes show the O2/N2 selectivity depends on the diffusivity selectivity rather than the solubility selectivity. For linear-type analogues, 6FDA-
Table 3 Diffusion coefficients and O2/N2 diffusivity selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C D (!108 cm2/s)
CO2
O2
N2
aD (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
0.493 0.562 0.298
1.80 2.09 1.18
0.369 0.457 0.237
4.88 4.58 4.96
Table 4 Solubility coefficients and O2/N2 solubility selectivity of 6FDA-based polyimide membranes at 76 cmHg and 25 8C S (!102 cm3 STP/cm3polym. cmHg)
CO2
O2
N2
aS (O2/N2)
6FDA-TAPOB 6FDA-TPEQ 6FDA-TPER
26.0 22.5 17.2
1.25 1.07 0.881
0.986 0.811 0.658
1.27 1.32 1.34
T. Suzuki et al. / Polymer 45 (2004) 7167–7171
TPER has higher a(O2/N2) than 6FDA-TPEQ as shown in Table 2. This result is consistent with the general understanding that polymers, which are more permeable, are less selective and vice versa [30–32]. However, it should be noted that 6FDA-TAPOB hyperbranched polyimide has higher aD(O2/N2) and, as a result, higher a(O2/N2) than 6FDA-TPEQ although O2 and N2 permeabilities of the two polyimides are almost equal. This anomalous result is considered to be due to differences in the size and distribution of free volume holes [33–38]. Jean et al. have been reported that gas separation performance of polymers strongly depends on size and distribution of free volume holes [39]. Furthermore, Shimazu et al. have been pointed out that molecular sieving ability of polyimides tends to strong with decreasing local chain mobility [40]. From these facts, it is suggested that low segmental mobility and unique size and distribution of free volume holes arising from characteristic hyperbranched structure of 6FDA-TAPOB provide effective O2/N2 selectivity in comparison with 6FDA-TPEQ.
4. Conclusions Physical and gas transport properties of the 6FDATAPOB hyperbranched polyimide have been investigated. 6FDA-TAPOB exhibits a good thermal stability as well as linear-type polyimides, 6FDA-TPEQ and 6FDA-TPER. FFV value of 6FDA-TAPOB is higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. Rotational motion of a central tri-substituted phenyl ring in TAPOB moiety is forbidden because of metabondings among adjacent phenoxy groups, and the increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB in spite of the higher FFV value. However, 6FDA-TAPOB exhibits considerably high gas solubility and, as a result, shows high gas permeability. It is suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDATAPOB provide effective O2/N2 selectivity. From these results, it is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high gas permeability and selectivity, and is expected to apply to a high-performance gas separation membrane.
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