alkyl groups as gas separation membranes

Journal of Membrane Science 285 (2006) 412–419

Synthesis and characterization of poly(diphenylacetylenes) containing both hydroxy and halogen/alkyl groups as gas separation membranes Yanming Hu, Toshikazu Sakaguchi, Masashi Shiotsuki, Fumio Sanda, Toshio Masuda ∗ Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Kyoto 615-8510, Japan Received 26 July 2006; received in revised form 25 August 2006; accepted 11 September 2006 Available online 15 September 2006

Abstract Diphenylacetylene containing siloxy and halogen/alkyl groups (P-t-BuMe2 SiO-C6 H4 C CC6 H4 -p-X, X = Cl, Br, I, Me, Et, 1b–f, respectively) were polymerized with TaCl5 -n-Bu4 Sn catalyst to provide high molecular weight polymers (2b–f) in good yields. These polymers afforded tough free-standing membranes by casting from toluene solution. Desilylation of these siloxy-containing polymer membranes was carried out with trifluoroacetic acid to give membranes of poly(diphenylacetylenes) having hydroxy and halogen/alkyl groups (3b–f). Polymers 2b–f were soluble in nonpolar solvents such as cyclohexane, toluene, and THF, while polymers 3b–f were insoluble in these solvents. The oxygen permeability coefficients (PO2 ) of membranes 2b–f and 3b–f were in the range of 140–280 and 12–190 barrers, respectively. The gas permeabilities of most membranes (2b–e and 3b–f) were higher than those of the corresponding polymer membranes without halogen/alkyl substituents (2a and 3a), suggesting that introduction of small spherical groups (halogen and methyl) into the polymers 2a and 3a enhances gas permeability. The incorporation of halogen atoms into the present membranes was more effective to improve gas permeability than methyl and ethyl groups. The points of 3b–e in the PCO2 versus PCO2 /PN2 and PCO2 /PCH4 plots were located obviously above Robeson’s upper bound. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(diphenylacetylene); Membrane; Gas permeability; Desilylation

1. Introduction Membrane separation of gases offers many advantages over traditional process such as its lower cost, higher energy saving, and better environment benefits [1–8]. Among various membrane materials, a very attractive characteristic of substituted polyacetylenes is their extremely high permeability to gases due to their stiff main chain composed of alternating double bonds and the steric repulsion of spherical pendent groups [9,10]. For instance, poly[1-(trimethylsilyl)-1-propyne] [poly(TMSP)] is by far the most permeable polymer, whose oxygen coefficient (PO2 ) is as large as 8000 barrers [11]. Triggered by this finding, a large number of substituted polyacetylenes have been synthesized in order to study the structure/permeability relationships of these polymers. Polymers from disubstituted acetylenes are highly gas-permeable and thermally stable compared to those from monosubstituted ones [9]. Therefore, the former polymers ∗

Corresponding author. Tel.: +81 75 383 2589; fax: +81 75 383 2590. E-mail address: [email protected] (T. Masuda).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.09.017

have attracted much attention as gas separation membrane materials applicable to practical use [9,12]. Desilylation of silylated poly(diphenylacetylenes) is useful to prepare a solvent-insoluble membrane of poly (diphenylacetylene), which cannot be fabricated into membrane by solution casting method due to its insolubility [13]. These studies give valuable information about the synthesis and properties of substituted polyacetylenes. In an attempt to create CO2 separation membranes, a precursor polymer membrane of poly(diphenylacetylene) containing siloxy groups (2a) was synthesized by polymerization of monomer 1a, and then membrane 2a was converted to a poly(diphenylacetylene) membrane having hydroxy groups (3a) by desilylation (Scheme 1) [14,15]. These membranes show outstanding separation performance for CO2 against methane and nitrogen. The separation of these gases is of interest in some areas, such as oil recovery, the treatment of landfill gases, sweetening nature gases, and reducing the green house effect [16,17]. However, their gas permeability is generally low compared to other poly(diphenylacetylene) analogues. Thus, improving the CO2 permeability while keeping the high

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Scheme 1. Synthesis of poly(diphenylacetylenes) having hydroxy groups.

permselectivity is important subjects to develop novel CO2 separation membrane materials. The beneficial effects of halogen substituents on enhancing fractional free volume (FFV) and permeability of polymer membrane have been reported. For example, the gas permeability of substituted polyacetylenes [18,19], polyarylates [20], polyimides [21], poly(phenylene oxide) [22,23], and polycarbonates [24] is enhanced by the introduction of halogen atoms into the polymer structure. Therefore, it is considered that the synthesis of poly(diphenylacetylenes) containing both hydroxy and halogen groups is of great interest for the development of novel gas separation membrane materials. The present study deals with the polymerization of diphenylacetylene monomers containing both siloxy and halogen groups (1b–d, in Scheme 1), and investigation of the effect of halogen atoms on the properties of the formed polymers (2b–d) including the gas permeability before and after desilylation. Analogous poly(diphenylacetylenes) having methyl and ethyl groups (2e and 2f) are also discussed for comparison.

diiodobenzene, 4-iodotoluene, and 1-ethyl-4-iodobenzene were purchased from Aldrich. p-(tert-Butyldimethylsiloxy)phenylacetylene was prepared according to the literature procedure [25]. Monomers 1b–f were synthesized according to Scheme 2, referring to the literature concerning ethynylation [26] and silylation [27]. Molecular weights of polymers were estimated by gel permeation chromatography (CHCl3 as eluent, polystyrene calibration). IR spectra were recorded on a JASCO FT/IR-4100 spectrophotometer. NMR spectra were observed on a JEOL EX-400 spectrometer. Elemental analysis of monomers was performed at the Microanalytical Center of Kyoto University. TGA was conducted in air with a Perkin-Elmer TGA7 thermal analyzer. Gas permeability coefficients of polymer membranes were measured with a Rikaseiki K-315-N gas permeability apparatus at 25 ◦ C. 2.2. Synthesis of 1-(4-chlorophenyl)-2-(4-tertbutyldimethylsiloxyphenyl)acetylene (1b)

2. Experimental 2.1. Materials and methods TaCl5 (Strem) was purchased and used without further purification. n-Bu4 Sn (Wako, Japan) was used after distillation. pIodophenol, trifluoroacetic acid (TFA), and common solvents (Wako, Japan) were used without further purification except toluene as polymerization solvent. tert-Butyldimethylsilyl chloride, 1-chloro-4-iodobenzene, 1-bromo-4-iodobenzene, 1,4-

A 300 mL three-necked flask was equipped with a threeway stopcock and a magnetic stirring bar. After the flask was flushed with nitrogen, 1-chloro-4-iodobenzene (6.0 g, 25 mmol), dichlorobis(triphenylphosphine)palladium (0.06 g, 0.08 mmol), cuprous iodide (0.09 g, 0.48 mmol), and triphenylphosphine (0.11 g, 0.40 mmol) were placed in the flask and dissolved in triethylamine (100 mL) at room temperature. Then, a solution of p(tert-butyldimethylsiloxy)phenylacetylene (4.5 g, 19 mmol) in triethylamine (20 mL) was added, and stirring was continued at

Scheme 2. Synthesis of diphenylacetylene monomers having a siloxy group and either a halogen or alkyl group.

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room temperature for 5 h. After that, triethylamine in the reaction mixture was evaporated off, and then ether (200 mL) was added to the residual mass. Solvent-insoluble solid was filtered off, and the filtrate was washed with 1N HCl aq. and then with water. The ethereal solution was dried over anhydrous magnesium sulfate. After evaporating the solvent, the crude product was purified by silica gel column chromatography (eluent: hexane) to give 1b (5.1 g, 78%) as a white solid; mp 82–83 ◦ C. IR (KBr, cm−1 ): 2930, 1603, 1509, 1255, 906, 851, 828, 803, 779, 521. 1 H NMR (CDCl3 ) δ 7.37 (m, 7.28–7.45, 6H, Ar), 6.81 (d, J = 8.0 Hz, 2H, Ar), 0.98 (s, 9H, SiC(CH3 )3 ), 0.21 (s, 6H, Si(CH3 )2 ). 13 C NMR (CDCl3 ) δ 157.0, 134.8, 133.9, 133.5, 129.5, 123.0, 121.2, 116.6, 91.3, 88.0, 26.6, 19.2, −3.4. Anal. Calad for C20 H23 ClOSi: C, 70.05; H, 6.76. Found: C, 69.98; H, 6.77. 2.3. Synthesis of 1-(4-bromophenyl)-2-(4-tertbutyldimethylsiloxyphenyl)acetylene (1c) This monomer was prepared by the same method as for 1b using 1-bromo-4-iodobenzene instead of 1-chloro-4iodobenzene to give a white solid; yield 78%, mp 87–88 ◦ C. IR (KBr, cm−1 ): 2929, 1603, 1509, 1254, 907, 850, 824, 778, 517, 405. 1 H NMR (CDCl3 ) δ 7.40 (m, 7.32–7.48, 6H, Ar), 6.81 (d, J = 8.0 Hz, 2H, Ar), 0.96 (s, 9H, SiC(CH3 )3 ), 0.21 (s, 6H, Si(CH3 )2 ). 13 C NMR (CDCl3 ) δ 156.2, 133.1, 132.9, 131.6, 122.6, 122.1, 120.3, 115.7, 90.7, 87.3, 25.8, 18.4, −4.3. Anal. Calcd for C20 H23 BrOSi: C, 62.01; H, 5.98. Found: C, 62.05; H, 6.05. 2.4. Synthesis of 1-(4-iodophenyl)-2-(4-tertbutyldimethylsiloxyphenyl)acetylene (1d) This monomer was prepared by the same method as for 1b using 1,4-diiodobenzene instead of 1-chloro-4-iodobenzene to give a white solid; yield 61%, mp 78–79 ◦ C. IR (KBr, cm−1 ): 2929, 1601, 1508, 1252, 1005, 909, 848, 821, 777, 511. 1 H NMR (CDCl3 ) δ 7.66 (d, J = 8.0 Hz, 2H, Ar), 7.39 (d, J = 8.0 Hz, 2H, Ar), 7.22 (d, J = 8.0 Hz, 2H, Ar), 6.81 (d, J = 8.0 Hz, 2H, Ar), 0.98 (s, 9H, SiC(CH3 )3 ), 0.21 (s, 6H, Si(CH3 )2 ). 13 C NMR (CDCl3 ) δ 156.0, 137.2, 132.8, 132.7, 122.9, 120.1, 115.4, 93.4, 90.7, 87.1, 25.5, 18.1, −4.5. Anal. Calcd for C20 H23 IOSi: C, 55.30; H, 5.34. Found: C, 55.16; H, 5.24. 2.5. Synthesis of 1-(4-methylphenyl)-2-(4-tertbutyldimethylsiloxyphenyl)acetylene (1e) This monomer was prepared by the same method as for 1b using 4-iodotoluene instead of 1-chloro-4-iodobenzene to give a white solid; yield 80%, mp 36.5–37.5 ◦ C. IR (KBr, cm−1 ): 2930, 1560, 1517, 1254, 908, 850, 818, 777, 526, 433. 1 H NMR (CDCl3 ) δ 7.39 (d, J = 8.0 Hz, 4H, Ar), 7.13 (d, J = 8.0 Hz, 2H, Ar), 6.80 (d, J = 8.0 Hz, 2H, Ar), 2.35 (s, 3H, ArCH3 ), 0.98 (s, 9H, SiC(CH3 )3 ), 0.20 (s, 6H, Si(CH3 )2 ). 13 C NMR (CDCl3 ) δ 155.7, 137.9, 132.9, 131.3, 129.0, 120.4, 120.1, 116.2, 88.6,

88.3, 25.6, 21.5, 18.2, −4.4. Anal. Calcd for C21 H26 OSi: C, 78.21; H, 8.13. Found: 78.51; H, 8.06. 2.6. Synthesis of 1-(4-ethylphenyl)-2-(4-tertbutyldimethylsiloxyphenyl)acetylene (1f) This monomer was prepared by the same method as for 1b using 1-ethyl-4-iodobenzene instead of 1-chloro-4-iodobenzene to give a colorless liquid; yield 85%. IR (KBr, cm−1 ): 2959, 2858, 1560, 1517, 1471, 1263, 1167, 911, 834, 782. 1 H NMR (CDCl3 ) δ 7.41 (m, 4H, Ar), 7.15 (d, J = 8.0 Hz, 2H, Ar), 6.80 (d, J = 8.0 Hz, 2H, Ar), 2.64 (q, J = 8.0 Hz, 2H, ArCH2 ), 1.23 (t, J = 8.0 Hz, 3H, ArCH2 CH3 ), 0.98 (s, 9H, SiC(CH3 )3 ), 0.20 (s, 6H, Si(CH3 )2 ). 13 C NMR (CDCl3 ) δ 155.3, 143.8, 132.4, 130.9, 127.4, 120.3, 119.7, 115.8, 88.2, 87.9, 28.4, 25.2, 17.8, 14.9, −4.8. Anal. Calcd for C22 H28 OSi: C, 78.51; H, 8.39. Found: C, 78.53; H, 8.43. 2.7. Polymerization Polymerizations were performed in a Schlenk tube equipped with a three-way stopcock at 80 ◦ C for 24 h using toluene as solvent under dry nitrogen at the following reagent concentrations: [TaCl5 ] = 20 mM, [n-Bu4 Sn] = 40 mM. The formed polymers were isolated by precipitation into a large amount of methanol, and the polymer yields were determined by gravimetry. 2.8. Membrane fabrication and desilylation The membranes (thickness ca. 40–80 ␮m) of polymers 2b–f were fabricated by casting from toluene solution of the polymer (concentration ca. 0.50–1.0 wt.%) onto a flat-bottomed Petri dish. Then, the dish was covered with a glass vessel to slow solvent evaporation (ca. 4–7 days). After membranes were prepared, they were immersed in methanol for 24 h and dried to constant weight at room temperature for 24 h. With reference to the method described in the literature [14,15], the desilylation reaction of polymer membranes 2b–f was carried out using TFA. A typical procedure of the desilylation reaction is as follows: a polymer membrane was immersed in a mixture of TFA and water (volume ratio 4:1) at room temperature for 24 h. The membrane was immersed in water for 24 h, then washed with water to remove residual impurities, and dried at room temperature for 24 h, then in vacuo for 2 h to constant weight, the weight of membrane decreased to the value anticipated for desilylation. 2.9. Density and fractional free volume of polymer membranes The membrane density was determined by hydrostatic weighing using a Mettler Toledo balance (model AG204, Switzerland) and a density determination kit [28]. In this method, a liquid with known density (ρ0 ) is needed, and the membrane density (ρ) is

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given by the following equations: ρ=

ρ0 MA MA − M L

where MA is membrane weight in air, and ML is membrane weight in the auxiliary liquid. Aqueous NaNO3 solution was used as the auxiliary liquid. FFV (cm3 of free volume/cm3 of polymer) is commonly used to estimate the efficiency of chain packing and the amount of space (free volume) available for gas permeation in a polymer matrix. It is defined as [29,30] FFV =

vsp − v0 vsp − 1.3vw ≈ vsp vsp

where vsp and v0 are the specific volume and occupied volume (or zero-point volume at 0 K) of the polymer, respectively. Typically, v0 is 1.3 times larger than the van der Waals volume (vw ), which is calculated by the group contribution methods [31]. 2.10. Measurement of gas permeabilities Gas permeability coefficients of polymer membranes were measured with a Rikaseiki K-315-N gas permeability apparatus equipped with a MKS Baratron detector at 25 ◦ C. The downstream side of the membrane was evacuated to 0.3 Pa, while the upstream side was filled with a gas at about 1 atm (105 Pa), and the increase of pressure in a downstream receiving vessel was measured. The P values were calculated from the slopes of time–pressure curves in the steady state where Fick’s law held. 3. Results and discussion 3.1. Synthesis of poly(diphenylacetylenes) Diphenylacetylene monomers 1b–f containing both siloxy and halogen/alkyl groups were polymerized with TaCl5 -nBu4 Sn catalyst in toluene. The results are listed in Table 1. The TaCl5 -n-Bu4 Sn catalyst is known to be effective for the polymerization of diphenylacetylenes [32,33], providing polymers with high molecular weight, which is essential for fabrication of free-standing membranes. The polymerization of chlorine-containing monomer 1b afforded a polymer with a high molecular weight (Mw > 6.0 × 106 ) in a good yield (81%). Table 1 Polymerization of 1b–f by TaCl5 -n-Bu4 Sn catalyst Monomer

1b 1c 1d 1e 1f

X

Cl Br I Me Et

Fig. 1. IR spectra of 2b and its desilylated product 3b (KBr pellet).

Monomers 1c–f also polymerized to give polymers in high yields of 65–71%, and the Mw values were also as high as 4.6 × 106 and above. Thus, all of the present monomers containing chlorine, bromine, iodine, methyl, and ethyl substituents were successfully polymerized. 3.2. Fabrication and desilylation of polymer membranes Free-standing membranes could be prepared by casting the polymers from toluene solution. The formed membranes were sufficiently tough, transparent, and yellow. The desilylation of polymer membranes 2b–f was carried out in a mixture of TFA and water (4:1 volume ratio) at room temperature for 24 h. In all cases, the silyl group was completely removed catalyzed by TFA to provide the membranes of hydroxy-containing polymers 3b–f. The completion of desilylation was confirmed by IR spectroscopy. Fig. 1 shows the IR spectra of polymer membranes 2b and 3b before and after desilylation. The absorption peak at 3396 cm−1 characteristic of the hydroxy group is observed in 3b, and the bands at 916 and 780 cm−1 indicating the presence of siloxy group in 2b have disappeared. 3.3. Physical properties of the polymers The processability for fabricating membranes depends on polymer solubility. Table 2 presents the solubility of the polymers. All of the newly synthesized silylated polymers 2b–f readily dissolved in nonpolar solvents such as cyclohexane, toluene, CHCl3 , and THF, while these polymers were insoluble Table 2 Solubility of the polymers

Polymera Yield (%)

Mw (×10−6 )b

Mw /Mn b

81 71 65 66 69

>6.0 >6.0 4.6 >6.0 >6.0

– – 6.3 – –

In toluene at 80 ◦ C for 24 h; [M]0 = 0.10 M, [TaCl5 ] = 20 mM, [nBu4 Sn] = 40 mM. a Methanol-insoluble product. b Measured by GPC.

Hexane Cyclohexane Toluene CHCl3 THF Methanol DMF DMSO

2b

2c

2d

2e

2f

3b

3c

3d

3e

3f

+ + + + + − − −

+ + + + + − − −

± + + + + − − −

± + + + + − − −

+ + + + + − − −

− − − − ± − ± ±

− − − − ± − ± ±

− − − − ± − + ±

− − − − − − – ±

− − − − − ± ± ±

Symbols: (+) soluble; (±) partly soluble; (−) insoluble.

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Table 3 Density and FFV of the polymer membranes Polymer

X

vw a of X (cm3 /mol)

2 Densityb

ad b c d e f a b c d

H Cl Br I Me Et

3.44 12.0 14.4 19.6 13.7 23.9

0.99 1.03 1.15 1.26 0.96 0.98

3 (g/cm3 )

FFVc

Densityb (g/cm3 )

FFVc

0.177 0.219 0.217 0.215 0.217 0.195

1.22 1.23 1.45 1.63 1.14 1.14

0.081 0.178 0.172 0.171 0.151 0.137

vw : van der Waals volume. Determined by hydrostatic weighing. FFV: fractional free volume. Estimated from membrane density. Data from Ref. [15].

Fig. 2. TGA curves of siloxy group-containing poly(diphenylacetylenes) 2b–f (in air, heating rate 10 ◦ C/min).

in polar solvents such as methanol, DMF, and DMSO. Chlorine, bromine- and ethyl-containing polymers 2b, 2c, and 2f were completely soluble even in hexane. On the other hand, desilylated polymers 3b–f were insoluble in nonpolar solvents, while partly soluble in highly polar solvents such as DMF and DMSO, which is reflective of incorporation of hydroxy groups. The thermal stability of polymers 2b–f and 3b–f was examined by TGA in air (Figs. 2 and 3). The onset temperatures of weight loss (T0 ) of 2b–d containing halogen substituents were 270 ◦ C, whereas the T0 values of 2e and 2f having alkyl groups (methyl and ethyl, respectively) were 360 ◦ C. When 2b–f were heated above 900 ◦ C in air, silica remained as residual ash, whose

amount agreed with the expected value in every case. The T0 values of desilylated polymers 3b–f were almost the same to one another (approximately 380 ◦ C). In the case of 3b–f, no SiO2 residue was detected, which confirms that the desilylation of the membranes proceeded quantitatively. Table 3 lists the densities and fractional free volumes of polymer membranes 2b–f and 3b–f along with the van der Waals volumes (vw ) of substituents X [31]. The FFV of each membrane was estimated from the experimental density. In every case, the FFV is increased by the substitution of halogen or alkyl groups on the phenyl ring of the polymer; i.e., the FFVs of polymer membranes 2b–f were in the range of 0.195–0.219, which were larger than that of unsubstituted 2a (0.177). Desilylation led to the shrinkage of the membranes, which resulted in the increase of densities. Accordingly, the FFVs of desilylated polymer membranes 3b–f were smaller than those of the corresponding polymer membranes 2b–f. Intermolecular hydrogen bonding seems to predominate over the formation of microvoids due to elimination of the bulky silyl groups in the solid state [15]. The FFVs of 3b–f were in the range of 0.137–0.178, and also larger than that of unsubstituted 3a (0.081). Quite interestingly, polymer membranes 3b–d containing halogen atoms exhibited higher FFVs than those of 3e and 3f having methyl and ethyl groups. These results cannot be explained simply in terms of the steric effect of substituents, as seen from their van der Waals radii shown in Table 3. This presumably results from both electronic and steric effects of the substituents. For example, membranes 2c and 2e containing bromine atoms and methyl groups, which had comparable sizes (vw = 14.4 and 13.7 cm3 /mol, respectively), exhibited the same FFV values as 0.217. In contrast, the FFV values of desilylated membranes of 3c and 3e were 0.172 and 0.151, respectively. The difference in FFVs of the desilylated membranes of bromine and methyl-containing polymers 3c and 3e can be related to the difference in electronegativity of the substituents. These results reveal that intersegmental packing is inhibited by the presence of the halogen, methyl, and ethyl groups, as estimated from FFV. 3.4. Gas permeability of the membranes

Fig. 3. TGA curves of hydroxy group-containing poly(diphenylacetylenes) 3b–f (in air, heating rate 10 ◦ C/min).

The permeability of membranes of siloxy-containing polymers 2b–f and hydroxy-containing ones 3b–f to various gases

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Table 4 Gas permeability (P) of polymer membranes Polymer

P (barrer) He

H2

O2

N2

CO2

CH4

2b 2c 2d 2e 2f

170 300 250 200 260 180

330 580 500 410 500 320

160 280 230 180 230 140

50 110 87 64 86 48

810 1600 1400 1200 1200 730

160 300 240 190 240 140

3aa 3b 3c 3d 3e 3f

38 300 250 160 150 47

56 690 580 380 300 75

8.0 190 150 96 62 12

2aa

2.4 57 44 26 17 2.5

110 1300 1100 810 440 81

2.3 98 79 60 34 4.0

PO2 /PN2

PCO2 /PN2

3.2 2.5 2.6 2.8 2.7 2.9

16 15 16 19 14 15

3.3 3.3 3.4 3.7 3.6 4.8

46 23 25 31 26 32

PCO2 /PCH4

5.1 5.3 5.8 6.3 5.0 5.2 48 13 14 14 13 20

At 25 ◦ C in units of 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg) (=1 barrer). a Data from Ref. [15].

was examined at 25 ◦ C (Table 4). It was found that the chemical structure of the polymers strongly influenced the gas permeability. Polymer membrane 2b containing chlorine atoms exhibited the highest oxygen permeability coefficient (PO2 ) among the present polymers, whose PO2 value was 280 barrers. This value is about twice as large as that of 2a (PO2 = 160 barrers). Polymer membranes 2c–e having other ring-substituents such as bromine, iodine, and methyl also showed higher oxygen permeability than 2a did, and their PO2 values were in the range of 180–230 barrers. The PO2 value of 2f containing ethyl groups was 140 barrers, which was slightly lower than that of 2a. The increase in permeability is attributable to the loosening of interchain packing by the incorporation of halogen/methyl groups into the polymer. The gas permeability of the membranes of desilylated polymers 3b–f decreased compared to those of siloxy-containing polymers 2b–f, and their PO2 values were in the range of 12–190 barrers. This behavior should be related to the decrease of FFVs upon desilylation. The PO2 values of the membranes having halogen atoms slightly decreased after desilylation. In contrast, those of the desilylated membranes having methyl and ethyl groups largely decreased. For instance, 2c and 2e showed the same PO2 values (230 barrers) before desilylation, but the PO2 values of the desilylated membranes 3c and 3e decreased to 150 and 62 barrers, respectively. The permeability of 3b–f to other gases such as He, H2 , N2 , CO2 , and CH4 exhibited similar tendencies to the case of O2 . The CO2 permeability coefficient (PCO2 ) of 3b was 1300 barrers and 12 times as large as that of 3a (110 barrers). The PCO2 values of 3c–e were 1100, 810, and 440 barrers, respectively, which are also much larger than that of 3a. Polymer 3f showed somewhat lower CO2 permeability (PCO2 = 81 barrers) than that of 3a. After leaving the samples for 7 days, no change in gas permeability was observed. The separation factors of oxygen against nitrogen (PO2 /PN2 ) of polymers 2b–f and 3b–f were 2.5–2.9 and 3.3–4.8, respectively, which are similar to one another among the same groups. The separation factors of CO2 against N2 (PCO2 /PN2 ) of 3b–f were 23–32, and larger than those of 2b–f (14–19), indicating

that the separation performance for CO2 was improved by the conversion of siloxy groups to hydroxy groups. The CO2 /N2 permselectivity of 3b–f was lower than that of unsubstituted analogue 3a, as expected from the well known “tradeoff” relationship; however, in some cases, the decrease in permselectivity was small compared to the increase in permeability. For instance, 3d had a 33% lower CO2 /N2 permselectivity (PCO2 /PN2 = 31) than that of 3a (PCO2 /PN2 = 46), while its CO2 permeability (810 barrers) was seven times higher than that of 3a (110 barrers) [15]. The polymer membranes with halogen atoms offered a favorable combination of permeability and permselectivity for CO2 and N2 . Fig. 4 shows the effects of halogen and alkyl (methyl and ethyl) substituents on the relationship between CO2 /N2 selectivity and CO2 permeability of the membranes. As is clear from this figure, all the membranes were located above the “upper bound” proposed by Robeson [34]. Fig. 5 shows a similar plot for the CO2 /CH4 permeability/selectivity. Polymer membranes 2b–d having halogen atoms were closer to the upper bound than did 2e and 2f having methyl and ethyl groups. Interestingly, the desilylated 3b–e exhibited gas separation performances above the upper bound line, while 3f was located on the upper bound line. Thus, desirable CO2 permeation characteristics, i.e., the

Fig. 4. Relationship between the CO2 /N2 permselectivity and CO2 permeability of the polymeric membranes before and after desilylation.

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Fig. 5. Relationship between the CO2 /CH4 permselectivity and CO2 permeability of the polymeric membranes before and after desilylation.

high permeability and relatively high permselectivity, can be achieved by the incorporation of small spherical groups such as halogen and methyl into 3a. The gas permeability of the polymer membranes bearing halogen atoms is larger than those bearing methyl and ethyl groups. 4. Conclusion Novel poly(diphenylacetylenes) containing both siloxy and halogen/alkyl (methyl and ethyl) groups were synthesized, which had high molecular weights, good solubility in organic solvents, and membrane-forming ability. The polymer membranes with hydroxy groups were obtained by desilylation of the precursor polymer membranes using TFA. The FFV values of the polymer membranes containing halogen/alkyl groups were larger than those of the corresponding polymer membranes without substituents irrespective of the presence and absence of the siloxy groups. The gas permeability could be improved by incorporating small spherical groups such as halogen atoms or methyl groups. Further, the gas permeability of poly(diphenylacetylenes) bearing halogen atoms was larger than that of their analogues having methyl or ethyl groups. The polymer membranes possessing hydroxy groups exhibited excellent separation performance for CO2 against N2 and CH4 . The PCO2 of chlorine- and hydroxy-containing membrane was 1300 barrers, 12 times as large as that of the unsubstituted counterpart, 3a. Thus, it has been found that the incorporation of halogen atoms into poly(diphenylacetylene) derivatives is effective to enhance the gas permeability of their membrane. Acknowledgements Y.M.H. acknowledges Scholarship from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This research was partly supported by a Programmed R&D for CO2 Sequestration from RITE. References [1] B.D. Freeman, Yu. Yampolskii, I. Pinnau (Eds.), Materials Science of Membranes for Gas and Vapor Separation, Wiley, Chichester, 2006. [2] R.W. Baker, Membrane Technology and Application, Wiley, Chichester, 2004.

[3] I. Pinnau, B.D. Freeman, Advanced materials for membrane separation, in: ACS Symposium Series 876, American Chemical Society, Washington, DC, 2004. [4] S.P. Nunes, K.V. Peinemann, Membrane Technology in the Chemical Industry, Wiley, New York, 2001. [5] M. Ulbricht, Advanced functional polymer membranes, Polymer 47 (2006) 2217–2262. [6] S. Damle, W.J. Koros, Permeation equipment for high-pressure gas separation membranes, Ind. Eng. Chem. Res. 42 (2003) 6389–6395. [7] Y. Tsujita, Gas sorption and permeation of glassy polymers with microvoids, Prog. Polym. Sci. 28 (2003) 1377–1401. [8] P. Pandey, R.S. Chauhan, Membranes for gas separation, Prog. Polym. Sci. 26 (2001) 853–893. [9] K. Nagai, T. Masuda, T. Nakagawa, B.D. Freeman, I. Pinnau, Poly[1(trimethylsilyl)-1-propyne] and related polymers: synthesis, properties and functions, Prog. Polym. Sci. 26 (2001) 721–798. [10] T. Aoki, Macromolecular design of permselective membranes, Prog. Polym. Sci. 24 (1999) 951–993. [11] T. Masuda, E. Isobe, T. Higashimura, K. Takada, Poly[1-(trimethylsilyl)-1propyne]: a new high polymer synthesized with transition-metal catalysts and characterized by extremely high gas permeability, J. Am. Chem. Soc. 105 (1983) 7473–7474. [12] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res. 41 (2002) 1393–1411. [13] M. Teraguchi, T. Masuda, Poly(diphenylacetylene) membranes with high gas permeability and remarkable chiral memory, Macromolecules 35 (2002) 1149–1151. [14] Y. Shida, T. Sakaguchi, M. Shiotsuki, K.B. Wagenar, T. Masuda, Preparation and properties of polytolan membranes bearing p-hydroxyl groups, Polymer 46 (2005) 1–4. [15] Y. Shida, T. Sakaguchi, M. Shiotsuki, F. Sanda, B.D. Freeman, T. Masuda, Synthesis and properties of poly(diphenylacetylenes) having hydroxyl groups, Macromolecules 38 (2005) 4096–4102. [16] B.D. Bhide, S.A. Stern, Membrane processes for the removal of acid gases from natural gas. I. Process configurations and optimization of operating conditions, J. Membr. Sci. 81 (1993) 209–237. [17] R. Rautenbach, K. Welschb, Treatment of landfill gas by gas permeation—pilot plant results and comparison to alternatives, J. Membr. Sci. 87 (1994) 107–118. [18] T. Sakaguchi, M. Shiotsuki, F. Sanda, B.D. Freeman, T. Masuda, Synthesis and properties of F-containing poly(diphenylacetylene) membranes, Macromolecules 38 (2005) 8327–8332. [19] Y. Shida, T. Sakaguchi, M. Shiotsuki, F. Sanda, B.D. Freeman, T. Masuda, Synthesis and properties of membranes of poly(diphenylacetylenes) having fluorines and hydroxyl groups, Macromolecules 39 (2006) 569– 574. [20] M.R. Pixton, D.R. Paul, Gas transport properties of polyarylates part II: tetrabromination of bisphenol, J. Polym. Sci. Part B: Polym. Phys. 33 (1995) 1353–1364. [21] Y. Xiao, Y. Dai, T.–S. Chung, M.D. Guiver, Effects of brominating matrimid polyimide on the physical and gas transport properties of derived carbon membranes, Macromolecules 38 (2005) 10042–10049. [22] F. Hamad, T. Matsuura, Performance of gas separation membranes made from sulfonated brominated high molecular weight poly(2,4-dimethyl-l,6phenyIene oxide), J. Membr. Sci. 253 (2005) 183–189. [23] G. Chowdhury, R. Vujosevic, T. Matsuura, B. Laverty, Effects of polymer molecular weight and chemical modification on the gas transport properties of poly(2,6-dimethyl-1,4-phenylacetylene oxide), J. Appl. Polym. Sci. 77 (2000) 1137–1143. [24] M.W. Hellums, W.J. Koros, G.R. Husk, D.R. Paul, Gas transport in halogen-containing aromatic polycarbonates, J. Appl. Polym. Sci. 43 (1991) 1977–1986. [25] E. Yashima, S. Huang, T. Matsushima, Y. Okamoto, Synthesis and conformational study of optically active poly(phenylacetylene) derivatives bearing a bulky substituent, Macromolecules 28 (1995) 4184–4193. [26] K. Sonogashira, Development of Pd–Cu catalyzed cross-coupling of terminal acetylenes with sp2 -carbon halides, J. Organomet. Chem. 653 (2002) 46–49.

Y. Hu et al. / Journal of Membrane Science 285 (2006) 412–419 [27] T. Suzuki, E. Sato, K. Unno, T. Kametani, Enantioselective synthesis of trans-octahydro-1,6-dioxoinden-4-ylpropionic acid from (R)-2,3O-isopropylideneglyceraldehyde by tandem orthoester Claisen rearrangement, J. Chem. Soc. Perkin Trans. 1 (1986) 2263–2268. [28] H. Lin, B.D. Freeman, Gas solubility, diffusivity and permeability in poly(ethylene oxide), J. Membr. Sci. 239 (2004) 105–117. [29] M.R. Pixton, D.R. Paul, Relationships between structure and transport properties for polymers with aromatic backbones, in: D.R. Paul, Y.P. Yampolskii (Eds.), Polymeric Gas Separation Membranes, CRC Press, Boca Raton, FL, 1994, pp. 83–153. [30] D.W. van Krevelen, Properties of Polymers: Their Correlation with Chemical Structure: Their Numerical Estimation and Prediction from Additive

[31]

[32] [33]

[34]

419

Group Contributions, 3rd ed., Elsevier Science, Amsterdam, 1990, p. 71–107. A. Bondi, in: R.F. Boyer (Ed.), Physical Properties of Molecular Crystals, Liquids and Glasses, John Wiley and Sons, New York, 1968, pp. 25– 97. T. Masuda, in: S. Kobayashi (Ed.), Catalysis in Precision Polymerization, Wiley, Chichester, 1997. H. Shirakawa, T. Masuda, K. Takeda, in: S. Paita (Ed.), The Chemistry of Triple-Bonded Functional Groups (supplement C2), Wiley, Chichester, 1999. L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165–185.