Permeability of Dry Gases and Those Dissolved in Water through

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Permeability of Dry Gases and Those Dissolved in Water through Hydrophobic High Free-Volume Silicon- or Fluorine-Containing Nonporous Glassy Polymer Membranes Kaoru Nakamura, Takeharu Kitagawa, Suguru Nara, Toshihiro Wakamatsu, Yusuke Ishiba, Shinji Kanehashi, Shuichi Sato, and Kazukiyo Nagai* Department of Applied Chemistry, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan ABSTRACT: This study systematically investigates the oxygen and carbon dioxide permeability of dry gases and those dissolved in water through hydrophobic high free-volume silicon- or fluorine-containing nonporous glassy polymers including poly(trimethylsilylmethylmethacrylate) (PTMSMMA), poly(1-trimethylsilyl-1-propyne), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride-4,4′-(9-fluorenylidene) dianiline, and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride-2,3,5,6-tetramethyl1,4-phenylene-diamine. The dry state gas permeability coefficient was almost higher than that of the wet state because the effect of the boundary layer, which is the water resistant layer, on the membrane surface depends on the gas permeability in the wet state. In addition, the gas permeability difference between the dry and wet states depends on the combination of competitive sorption between water and gas species and plasticization by water. Carbon dioxide/oxygen permselectivity increased with the decrease in gas permeability, except for PTMSMMA. The gas permeability in the wet state of PTMSMMA with low glass transition temperature was approximately equal to that in the dry state because plasticization by water increased in the wet state. Therefore, the mobility of the polymer segment depends on the plasticization by water.

1. INTRODUCTION Oxygen and carbon dioxide concentrations in water depend on living organisms such as plankton, fishes, and shellfishes. These concentrations are also important in the potability of drinking water. The removal of dissolved gas in water is desirable in industrial settings such as boiler plumbing. Dissolved gaspermeable membranes are required for membrane degasifying because oxygen (e.g., oxidation) and carbon dioxide influence material degradation. Dissolved gas-permeable degasifying membranes were developed to solve the problems caused by dissolved oxygen and carbon dioxide. Higher oxygen and carbon dioxide permeation and nonwater permeation (i.e., hydrophobic) properties are required for membrane-degasifying application. However, exposure to conditions such as moisture and water content significantly influences membrane permeability.1−4 Gas permeability under dry conditions is different from that under wet conditions. In such applications, a hydrophobic nonporous glassy polymer membrane with water resistance, fouling resistance, and good durability is required. A porous membrane exhibits high permeability for all species and will not be selective if the pore size is large. Moreover, the Knudsen diffusion is dominant. Similarly, a nonporous membrane can be gas or water selective. Therefore, high freevolume nonporous polymer membranes with higher gas permeable and lower water permeable properties are required. Silicon- and fluorine-containing polymers with bulky substituents in polymer chains exhibit higher fractional free volume (FFV) values in polymer membranes. The FFV values of general polymers range from 0.06 to 0.16. However poly(1trimethylsilyl-1-propyne) (PTMSP) has the highest value (0.29) among existing polymers (Table 1). Fluorine-containing polyimide also has a higher free volume among the polymers according to a previous study.5 These polymers show high gas © 2012 American Chemical Society

diffusivity and permeability, with high FFV, high hydrophobicity, and contact angles of more than 90°. This study is the first to investigate the permeability of dry oxygen and carbon dioxide, and those dissolved in water through hydrophobic high free-volume silicon-containing nonporous polymer membranes, including poly(trimethylsilylmethylmethacrylate) (PTMSMMA), PTMSP, and fluorinecontaining nonporous polymer membranes such as 4,4′(hexafluoroisopropylidene) diphthalic anhydride-4,4′-(9-fluorenylidene) dianiline (6FDA-FDA) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride-2,3,5,6-tetramethyl-1,4-phenylene-diamine (6FDA-TeMPD).

2. EXPERIMENTAL SECTION 2.1. Preparation of Membranes. The PTMSMMA, PTMSP, 6FDA-FDA, and 6FDA-TeMPD polymers used in this study were synthesized according to the literature.6−9 1H nuclear magnetic resonance (JNM-ECA500, JEOL Ltd., Tokyo, Japan) and Fourier transform infrared (FT-IR 460+. JASCO Co., Tokyo, Japan) analyses confirmed the chemical structures of these polymers (Figure 1). The PTMSMMA and PTMSP membranes were prepared by casting 3 wt % toluene solutions of each solvent onto a flatbottomed Petri dish in a glass bell-type vessel at room temperature. The 6FDA-based polyimide membranes were prepared with the same conditions, but 3.5 wt % dichloroSpecial Issue: Baker Festschrift Received: Revised: Accepted: Published: 1133

March 25, 2012 July 23, 2012 July 25, 2012 July 25, 2012 dx.doi.org/10.1021/ie300793t | Ind. Eng. Chem. Res. 2013, 52, 1133−1140

Industrial & Engineering Chemistry Research

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Table 1. Physical Properties of Silicon- or Fluorine-Containing Polymer Membranes polymer

Tg (°C)

ρ (g/cm3)

FFV

CED (MPa)

δ (MPa1/2)

PTMSMMA PTMSP 6FDA-FDA 6FDA-TeMPD

79 ± 1 >250 376 ± 1 422 ± 2

1.00 0.75 1.33 1.33

± ± ± ±

0.17 0.29 0.18 0.19

313 251 743 679

17.7 15.8 27.3 26.1

0.01 0.01 0.01 0.01

water contact angle (deg) 95 96 90 90

± ± ± ±

water content (wt %)

2 2 2 1

0.6 0.3 2.9 5.9

± ± ± ±

0.1 0.1 0.1 0.4

of the endothermic transition using the data from the second heating scan. Membrane density (ρ) was determined by the flotation of small membrane samples in a density gradient column maintained at 23 °C, except for PTMSP. The density of the PTMSP membrane was determined from the membrane weight and volume at 23 °C. FFV was estimated using the following equation: FFV =

V − 1.3VW V

(1)

where V is the polymer-specific volume and VW is the van der Waals volume, which is calculated using the group contribution method of van Krevelen.10 The cohesive energy density of polymer (CED) and the solubility parameter (δ) were calculated from the group contribution method of Fedors.11 Wide angle X-ray diffraction (WAXD) patterns were obtained using a RINT-1200 X-ray diffractometer (Rigaku Corp., Tokyo, Japan) with a scanning speed of 2°/min, using Cu Kα radiation at 40 kV and 20 mA in a dispersion angle of 3−30°. Water contact angle was measured at least three times per membrane using a contact angle meter IMC-159D (Imoto Machinery Co., Ltd., Kyoto, Japan). Contact angle changes were recorded within 5 s at 23 ± 1 °C. Water content was calculated using the following equation: water content (wt %) = Figure 1. Chemical structures of silicon- or fluorine-containing polymers: PTMSMMA, PTMSP, 6FDA-FDA, 6FDA-TeMPD.

WS − WD × 100 WD

(2)

where WS (g) and WD (g) are the weight of the water-swollen and completely dry membranes, respectively. The waterswollen membrane was blotted to remove excess water. WS measurement was repeated until a constant weight was obtained. 2.3. Measurements of Gas Permeation Properties. Oxygen and carbon dioxide permeability in the dry state (i.e., both sides of the membrane were in the gas phase) was determined through the constant volume-variable pressure method at 25 ± 1, 30 ± 1, 35 ± 1, and 40 ± 1 °C, according to the literature.5 Downstream pressure was maintained under vacuum during experiments. The permeability coefficient, P (cm3(STP) cm/(cm2 s cmHg)), was determined through the following equation:

methane solutions were used instead. Each solvent was allowed to evaporate slowly in the vessel for 7 to 10 days. The membrane thickness of the dry membranes used for the experiments ranged from 30 to 180 μm for PTMSMMA, 100 to 540 μm for PTMSP, 50 to 250 μm for 6FDA-FDA, and 50 to 280 μm for 6FDA-TeMPD (standard deviation of membrane thickness, ±2 μm). These results were obtained because thickness dependence on gas permeability can be measured through each polymer membrane between these thickness ranges. Among the polymers in this study, PTMSP was specifically influenced by aging and was thus used for subsequent experiments.7 The PTMSP membrane was immersed in methanol before several measurements were taken to prevent membrane hysteresis for aging phenomena. 2.2. Characterization Analysis. All of the characterization data were determined in the membrane state of at least three samples to confirm the reproducibility of the experimental results. Differential scanning calorimeter (DSC) analysis was performed to determine the glass transition temperature (Tg) using a Diamond DSC (Perkin-Elmer Inc., Shelton, USA) at a heating rate of 10 °C/min. Tg was determined at the midpoint

PG =

dp 273V 1 1 S dt 760(273 + T ) A Δp

(3)

where dp/dt (mmHg) is the pressure increase in time, V (cm3) is the downstream volume, T (°C) is the temperature, A (cm2) is the membrane area, Δp (cmHg) is the upstream pressure, and S (cm) is the membrane thickness. Dissolved oxygen permeability in the wet state (i.e., both sides of the membrane were in the liquid phase) was determined through the oxygen electrode method at 25 ± 1, 1134

dx.doi.org/10.1021/ie300793t | Ind. Eng. Chem. Res. 2013, 52, 1133−1140


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30 ± 1, 35 ± 1, and 40 ± 1 °C, according to the literature.1 This measurement was performed by bubbling through the liquid on the feed side. The gas feed side was at the air side during membrane preparation. The apparent dissolved oxygen permeability coefficient, PL (cm3(STP) cm/(cm2 s cmHg)), was determined using the following equation: c PL = Ki∞S 1 c2 (4) where K (cm3(STP)/(A cm s cmHg)) is the peculiar electrode constant of the electrode rod, i∞ (A) is the amount of current in the steady state of the permeate side, and c1 and c2 are the theoretical and experimental feed oxygen concentrations, respectively. Dissolved carbon dioxide permeability in the wet state (i.e., both sides of the membrane were in the liquid phase) was determined through the carbon dioxide electrode method at 25 ± 1, 30 ± 1, 35 ± 1, and 40 ± 1 °C, according to the literature.12 This measurement was performed by bubbling through the liquid on the feed side. The gas feed side was at the air side during membrane preparation. The apparent dissolved carbon dioxide permeability coefficient, PL (cm3(STP) cm/ (cm2 s cmHg)), was determined using the following equation: PL =

dC V S dt Ap′

Figure 2. Wide-angle X-ray diffraction patterns of silicon- or fluorinecontaining polymers.

difference was observed in the size and distribution of the space in the membrane. The F molecule with higher molecular weight than Si depends on the density, FFV, and membrane density. All of the polymer membranes in this study showed high hydrophobic properties with contact angles higher than 90°. Water content increases when the δ value of the water (i.e., δ = 47.9 MPa1/2) is near that of the polymer. Hence, water content increased as the δ value of the polymers increased. The water content of all the polymer membranes in this study was below 6 wt %. Interestingly, the water content of 6FDA-TeMPD was much higher than that of 6FDA-FDA, whereas the solubility parameter of the latter is higher. The ranking of water content was different from that of δ. Relatively, the solubility parameter of 6FDA-TeMPD and 6FDA-FDA was higher than that of PTMSMMA and PTMSP. Moreover, the value of 6FDATeMPD was near that of 6FDA-FDA with a high solubility parameter. 3.2. Permeability of Dry Gases. Table 2 summarizes the PG(O2) and PG(CO2) of the silicon- and fluorine-containing polymers in the dry state at 25, 30, 35, and 40 °C, respectively. Data obtained from this study were similar to the literature values.8,9,14−16 All of the polymer membranes used in these measurement temperatures were in the glassy states. Neither hysteresis nor thickness dependence was observed in the membranes during the permeation experiments. The temperature dependence on PG(O2) (Figure 3a) and PG(CO2) (Figure 3b) of the silicon- and fluorine-containing polymers is presented in Figure 3. A linear relationship was observed between PG and the reciprocal of temperature in all of the polymer membranes, which can be represented as an Arrhenius equation:

(5)

where dC/dt (cm 3 (STP) cm) is the carbon dioxide concentration increase in time (i.e., c1 − c2), V (cm3) is the cell volume of the permeate side, and p′ (cmHg) is the partial carbon dioxide pressure. All of the permeation data were determined in at least three membrane samples to confirm the reproducibility of the experimental results. Samples were changed at each permeation condition.

3. RESULTS AND DISCUSSION 3.1. Membrane Characterization. The properties of silicon- and fluorine-containing polymers are summarized in Table 1. The Tg value of the PTMSP was not determined, but previous studies indicated that it is higher than 250 °C.13 No thermal change was observed up to 250 °C during the DSC measurements. The permeation temperature measurements indicated that all of the polymers in this study were glassy. The WAXD patterns of the polymer membranes in this study are presented in Figure 2. PTMSP and all 6FDA-based polyimides showed one broad peak, whereas PTMSMMA exhibited two broad peaks, indicating that all polymer membranes have amorphous structures. The 2θ value of PTMSP, 6FDA-FDA, and 6FDA-TeMPD exhibited a distance between their polymer segments. The peak in the small angle in PTMSMMA appeared at a distance between polymer segments, whereas it was attributed to the intermolecular distance between the long side chains in the large angle.5 In this regard, all of the membranes had a wide distribution in the distance between polymer segments, which could contribute to gas diffusion in the membrane. The d-spacing value is estimated as the distance between polymer segments based on the maximum 2θ intensity value. Relatively, the d-spacing ranking of the maximum 2θ intensity value was PTMSMMA (between polymer segments) > PTMSP > 6FDA-TeMPD > 6FDAFDA > PTMSMMA (between long side chains). This order was different from the ranking of ρ and FFV, wherein a

⎛ E ⎞ PG = PG0 exp⎜ − GP ⎟ ⎝ RT ⎠ 3

(6) 2

where PG0 (cm (STP) cm/(cm s cmHg)) is the preexponential factor of permeability coefficient, EGP (kJ/mol) is the permeation activation energy, and R is the gas constant (i.e., R = 8.314 J/(mol K)). The PG0 and EGP of all the polymer membranes are summarized in Table 2. The PG0 (i.e., the intercept of the lines of PG) order was different from the PG ranking at each temperature. The EGP values (i.e., the slope of the lines of PG) are the interaction 1135



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Table 2. Oxygen and Carbon Dioxide Permeability Coefficients in the Dry State through Silicon- or Fluorine-Containing Polymer Membranes at 1 atm PG × 1010 (cm3(STP) cm/(cm2 s cmHg)) polymer

25 °C

30 °C

35 °C

40 °C

PG0 × 107 (cm3(STP) cm/(cm2 s cmHg))

EGP (kJ/mol)

PTMSMMA PTMSMMA8 PTMSP PTMSP14 6FDA-FDA 6FDA-FDA9 6FDA-TeMPD PTMSMMA PTMSMMA8 PTMSP PTMSP14 6FDA-FDA 6FDA-FDA15 6FDA-TeMPD

33 ± 2

35 ± 2 31 10800 ± 100

37 ± 2

40 ± 2

1.47 ± 0.22

10100 ± 700

9330 ± 100

0.132 ± 0.013

9.40 ± 0.52 2.4a −10.2 ± 0.6

18.0 ± 1

19.0 ± 1

20.0 ± 1

0.506 ± 0.065

7.77 ± 0.60

157 ± 1 142 ± 1 120 36200 ± 200

162 ± 1 141 ± 1

168 ± 3 139 ± 1

1.07 ± 0.23 0.176 ± 0.024

34000 ± 800

32300 ± 200

1.09 ± 0.02

4.49 ± 0.55 0.580 ± 0.387 0.7a −9.42 ± 0.10

136 ± 1

137 ± 1

142 ± 1

0.361 ± 0.002

2.45 ± 0.02

994 ± 6

992 ± 21b

993 ± 16

1.74 ± 0.54

1.42 ± 1.05

gas O2

CO2

a

11600 ± 100 9700 17.0 ± 1 11.7 153 ± 1 139 ± 1 37700 ± 200 34000 135 ± 1 98 979 ± 1

These values were not listed in the original articles. We estimated them using eq 6. bData from ref 16.

Figure 3. Temperature dependence on dry (a) oxygen and (b) carbon dioxide permeability coefficient through silicon- or fluorine-containing polymer membranes. Polymers: PTMSMMA (●), PTMSP (■), 6FDA-FDA (▲), 6FDA-TeMPD (⧫).

Table 3. Apparent Oxygen Permeability Coefficients Dissolved in Water in Various Thicknesses of Silicon- or FluorineContaining Polymer Membranes PL (O2) × 1010 (cm3(STP) cm/(cm2 s cmHg)) polymer PTMSMMA

PTMSP

6FDA-FDA

6FDA-TeMPD

membrane thickness (μm) 34−45 90−95 108−123 176−181 90−200 300−366 418−512 55−61 101−139 170−182 254−256 57−70 84−128 188−189 248−279

25 °C 13.5 18.6 23.0 27.7 145 228 261 5.40 6.70 6.80 7.30 8.40 11.5 16.1 17.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.6 0.6 1.1 1.6 20 30 40 0.1 0.2 0.5 0.6 0.1 0.5 0.2 0.1

30 °C 14.7 20.9 25.2 29.6 159 234 290 5.90 7.30 7.40 8.00 9.30 12.7 17.6 19.6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.8 0.4 1.4 1.9 30 30 40 0.2 0.2 0.5 0.7 0.2 0.5 0.6 0.5

35 °C 14.8 22.3 26.0 31.0 171 237 290 6.30 8.10 8.20 8.80 9.90 14.0 18.8 22.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.8 0.2 1.7 2.3 30 40 40 0.2 0.1 0.5 0.8 0.1 0.7 0.2 0.4

40 °C 15.1 22.6 26.3 32.1 179 245 284 6.60 8.60 8.80 9.50 10.1 14.9 20.1 24.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.8 0.7 0.1 3.0 20 30 30 0.2 0.2 0.5 0.8 0.1 0.7 0.3 0.7

temperature and EGP. This result indicates that PG does not depend on PG0 or EGP, but on the balance between them.

standard between the gas molecules and the polymer segment. The PG0 order was different from the PG ranking at each 1136



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Table 4. Apparent Carbon Dioxide Permeability Coefficients Dissolved in Water in Various Thicknesses of Silicon- or FluorineContaining Polymer Membranes PL (CO2) × 1010 (cm3(STP) cm/(cm2 s cmHg)) polymer PTMSMMA

PTMSP

6FDA-FDA

6FDA-TeMPD

membrane thickness (μm) 34−45 90−95 108−123 176−181 90−200 300−366 418−512 55−61 101−139 170−182 254−256 57−70 84−128 188−189 248−279

25 °C 51.7 67.0 73.0 84.6 1210 3240 3250 58.1 66.1 73.1 82.6 91.3 114 172 208

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 2.4 8.4 4.3 90 410 320 2.1 2.4 0.5 3.1 3.3 8 3 24

30 °C 53.9 72.3 75.8 91.1 1150 2610 2450 58.3 69.4 78.0 84.8 96.8 127 192 223

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.7 4.5 1.6 7.2 80 100 240 0.7 1.9 3.2 2.2 4.9 4 9 7

35 °C 54.4 76.4 85.2 95.9 1030 2270 2350 64.1 71.7 83.6 90.0 105 137 211 258

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.2 6.1 17.4 3.1 50 110 140 2.6 3.7 1.4 7.3 4 7 9 25

L 2L W L = LS + PL PLS PW

3.3. Permeability of Gases Dissolved in Water. Tables 3 and 4 summarize the PL(O2) and PL(CO2) of the silicon- and fluorine-containing polymers in the wet state at 25, 30, 35, and 40 °C, respectively. All of the polymer membranes used to measure temperature was in their glassy states. Hysteresis was not observed in the permeation data in all the membranes during the permeation experiments. PG is independent of membrane thickness, whereas PL is dependent because the boundary layer, which is the water resistant layer of the membrane surface, depends on the gas permeability in the wet state, as observed in this study.1−4 The boundary layer depended on the dissolved oxygen and carbon dioxide permeability coefficient for all polymer membranes. The PL that affected the boundary layer was measured (Figure 4). Hence, the true dissolved gas permeability was determined using the following equation:

40 °C 57.5 77.3 88.3 99.1 1050 2730 2710 68.9 76.6 84.9 97.8 112 147 226 274

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.4 5.8 2.3 2.1 30 950 200 3.1 2.4 4.7 2.1 5 7 13 14

(7)

where L (i.e., LLS + 2LW) is the total membrane thickness including the boundary layer, PL is the apparent dissolved gas permeability coefficient that affects the water boundary layer, LLS is the polymer membrane thickness, PLS is the true dissolved gas permeability coefficient, LW is the thickness of the water boundary layer, and PW is the permeability coefficient of the water boundary layer. PL and LLS can be measured, but PLS, PW, LW, and L cannot be determined directly. Therefore, eq 8, which shows the relationship between dissolved gas flux and membrane thickness, can be rewritten as eq 9: PL = R × L

(8)

L L 2L W 1 1 = LS + = LS + R PLS PW PLS RW

(9)

where R (cm3(STP)/(cm2 s cmHg)) is the dissolved gas flux, 1/R is the total resistance, and 1/RW (i.e., 2LW/PW) is the resistance of the boundary layer. Based on eq 9, a linear relationship was noted between the reciprocal R and LLS. Hence, the reciprocal slope shows the true dissolved gas permeability coefficient (PLS) and the intercept shows the boundary layer resistance (1/RW). Table 5 summarizes the 1/RW values of all the polymer membranes. The resistance of the oxygen boundary layer was higher than that of carbon dioxide. The ranking of 1/RW for oxygen and carbon dioxide was 6FDA-TeMPD > 6FDA-FDA > PTMSMMA > PTMSP in the entire temperature range. The contact angles of all the polymer membranes were 96° for PTMSP, 95° for PTMSMMA, 90° for 6FDA-FDA, and 90° for 6FDA-TeMPD. The water contents of all the polymer membranes were 0.3 wt % for PTMSP, 0.6 wt % for PTMSMMA, 2.9 wt % for 6FDA-FDA, and 5.9 wt % for 6FDA-TeMPD. The 1/RW order was similar to that of the contact angle and the water content. These results indicate that both surface and bulk hydrophobicities depend on the boundary layer resistance of all the polymer membranes. Boundary layer resistance decreased as hydrophobicity increased.

Figure 4. Schematic diagram of the boundary layer and gas permeable membrane layer dissolved in water. 1137



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Table 5. Resistance of Oxygen and Carbon Dioxide Boundary Layer in Silicon- or Fluorine-Containing Polymer Membranes 1/RW × 10−5 (cm2 s cmHg/(cm3(STP))) gas

25 °C

polymer

O2

PTMSMMA PTMSP 6FDA-FDA 6FDA-TeMPD PTMSMMA PTMSP 6FDA-FDA 6FDA-TeMPD

CO2

24.7 9.84 28.5 61.0 3.79 0.569 4.18 4.72

± ± ± ± ± ± ± ±

30 °C

3.0 2.1 5.6 0.4 0.66 0.403 0.20 0.17

21.3 9.44 27.3 58.9 3.74 0.491 4.00 4.34

± ± ± ± ± ± ± ±

2.1 1.9 5.1 1.2 0.93 0.347 0.13 0.11

35 °C 21.5 8.04 27.4 57.0 3.69 0.601 3.70 4.24

± ± ± ± ± ± ± ±

40 °C

1.8 1.5 5.0 0.8 0.30 0.347 0.51 0.25

21.8 6.83 27.9 54.6 3.54 0.635 3.72 3.93

± ± ± ± ± ± ± ±

1.9 1.3 5.1 1.6 0.21 0.367 0.23 0.13

Figure 5. Temperature dependence on true (a) oxygen and (b) carbon dioxide permeability coefficient in the wet state in silicon- or fluorinecontaining polymer membranes. Polymers: PTMSMMA (●), PTMSP (■), 6FDA-FDA (▲), 6FDA-TeMPD (⧫).

Table 6. True Oxygen and Carbon Dioxide Permeability Coefficients Dissolved in Water in the Silicon- or Fluorine-Containing Polymer Membranes PLS × 1010 (cm3(STP)cm/(cm2 s cmHg)) gas O2

CO2

25 °C

polymer PTMSMMA PTMSP 6FDA-FDA 6FDA-TeMPD PTMSMMA PTMSP 6FDA-FDA 6FDA-TeMPD

43.8 541 8.12 29.6 99.2 6450 93.4 328

± ± ± ± ± ± ± ±

4.9 45 0.10 0.3 4.4 246 1.9 20

30 °C 44.9 612 8.92 33.7 107 3720 97.1 359

± ± ± ± ± ± ± ±

4.7 50 0.17 0.6 12 361 1.6 8

35 °C 49.2 522 9.93 40.4 117 3730 103 440

± ± ± ± ± ± ± ±

40 °C

5.3 15 0.17 0.8 4 280 9 62

± ± ± ± ± ± ± ±

6.8 21 0.21 2.8 2 466 4 47

1.81 0.280 3.99 416 6.39 0.0411 3.27 804

± ± ± ± ± ± ± ±

0.67 0.153 0.71 264 3.67 0.0226 2.80 641

ELSP (kJ/mol) 9.30 −10.4 15.1 23.7 10.3 −11.6 8.84 19.4

± ± ± ± ± ± ± ±

1.24 4.5 0.5 3.1 2.5 2.2 1.83 3.7

true dissolved gas permeation. The PLS0 and ELSP of all the polymer membranes are summarized in Table 6. The PLS0 order was different from the PLS ranking at each temperature and ELSP. This result indicates that PLS does not depend on either PLS0 or ELSP(CO2), but on the balance between them. The eliminated boundary layer resistance by PLS leads the high flux of the membranes in the liquid phase. However, PG was not simply equal to PLS. The PLS changes are caused by a combination of competitive sorption between the water and the gas species as well as the plasticization by water. Therefore, the relationship between the permeability coefficient in the dry state (PG) and that in the wet state (PLS) was systematically investigated. 3.4. Relationship between Permeability of Dry Gases and Those Dissolved in Water. PG as a function of the PLS of all the polymer membranes is presented in Figure 6. PLS(O2)

The temperature dependence on PLS(O2) (Figure 5a) and PLS(CO2) (Figure 5b) are presented in Figure 5. Based in Figure 3, the order of PTMSMMA and 6FDA-TeMPD was reversed in PG(O2) and PLS(O2), whereas the order of PG(CO2) was not different from PLS(CO2). A linear relationship was observed between the true dissolved gas permeability coefficient and the reciprocal of temperature in all the polymer membranes, which can be represented as an Arrhenius equation: ⎛ E ⎞ PLS = PLS0 exp⎜ − LSP ⎟ ⎝ RT ⎠

51.8 455 10.9 46.3 120 5070 111 464

PLS0 × 107 (cm3(STP)cm/(cm2 s cmHg))

(10)

where PLS0 (cm3(STP) cm/(cm2 s cmHg)) is the preexponential factor of the true dissolved gas permeability coefficient and ELSP (kJ/mol) is the activation energy of the 1138



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oxygen solubility in water was 50 times higher than that of carbon dioxide. Oxygen permeability in the wet state of PTMSMMA with low Tg was approximately equal to that in dry state because plasticization by water increased in the wet state. Therefore, high mobility of the polymer segment depends on the plasticization by water. 3.5. Selectivity. The CO2/O2 permselectivity as a function of the carbon dioxide permeability is presented in Figure 7.

Figure 6. True gas permeability coefficient in the wet state in siliconor fluorine-containing polymer membranes as a function of that in the dry state. The dashed lines represent equal values of PLS and PG. Gases: oxygen (●), carbon dioxide (□).

increased as PG(O2) increased, except for PTMSMMA. The PLS(O2) of PTMSMMA was equal to PG(O2). In addition, PLS(CO2) increased as PG(CO2) increased in all of the polymer membranes. The gradients of the lines were similar. This result indicates that the difference between PG and PLS in a highly gas-permeable polymer membrane is remarkable because gas solubility in water depends on dissolved gas permeability. Gas molecules in a wet state permeate to water and polymer molecules. Thus, gas permeability in water should be considered, unlike the case of the dry state (i.e., not including water molecules). Dissolved gas permeability was considered as restrained by the gas solubility in water. Table 7 summarizes the gas solubility and diffusivity in water according to the literature.17,18 In the given temperature range,

Figure 7. Ideal CO2/O2 permselectivity for silicon- or fluorinecontaining polymer membranes as a function of carbon dioxide permeability coefficient. Measurement state: dry gases (▲), gases dissolved in water (◊).

PG(CO2)/PG(O2) and PLS(CO2)/PLS(O2) showed that the carbon dioxide-permselective behavior is higher than 1. PLS(CO2)/PLS(O2) was lower than PG(CO2)/PG(O2) in PTMSMMA, whereas other polymers showed the opposite result. Comparing the carbon dioxide permeability (i.e., PG(CO2) and PLS(CO2)) and CO2/O2 permselectivity (i.e., PG(CO2)/PG(O2) and PLS(CO2)/PLS(O2)), CO2/O2 permselectivity increased as carbon dioxide permeability decreased, except for PTMSMMA. The degree of the plot shift from the dry to the wet state increased as gas permeability increased. However, PLS(CO2)/PLS(O2) and PLS(CO2) were lower than PG(CO2)/PG(O2) and PG(CO2) in PTMSMMA. A linear tradeoff relationship exists between gas permeability and selectivity in the separation of polymer membranes;19 however, this is not true for PTMSMMA. Interestingly, the PG and PLS of 6FDAFDA were similar to those of PTMSMMA, whereas the PG(CO2)/PG(O2) and PLS(CO2)/PLS(O2) of 6FDA-FDA were higher than those of PTMSMMA. Only PTMSMMA did not follow this relationship, but the exact reason was not clear. The mobility of the polymer segment depends on gas permeability. PTMSMMA showed low Tg with lower mobility among these polymer membranes. PLS changes are caused by a combination of competitive sorption between the water and gas species and the plasticization by water. Plasticization occurred only in PTMSMMA; thus PLS(CO2)/PLS(O2) was less than PG(CO2)/ PG(O2). Meanwhile, plasticization did not occur in other polymer membranes; thus, CO2/O2 permselectivity increased in the wet state.

Table 7. Temperature Dependence on Gas Solubility and Diffusivity Coefficients in Water gas O2

CO2

a

temperature (°C) 25 30 35 40 25 30 35 40

solubilitya (mg/L)

S (cm3(STP)/(cm3 cmHg))

40.8 37.8 35.3 33.3 2080 1840 1640 1480

3.76 3.48 3.25 3.06 1.39 1.23 1.10 9.91

× × × × × × × ×

−4

10 10−4 10−4 10−4 10−2 10−2 10−2 10−3

Db (cm2/s) 2.42 2.76 3.10 3.43 1.84 2.08 2.35 2.49

× × × × × × × ×

10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5

Data from ref 17. bData from ref 18.

the oxygen solubility coefficient decreased as temperature increased. The value varied from 3.76 × 10−4 to 3.06 × 10−4 cm3(STP)/(cm3 cmHg). Oxygen diffusivity coefficient increased as temperature increased; the value varied from 2.42 × 10−5 to 3.43 × 10−5 cm2/s. Meanwhile, the carbon dioxide solubility coefficient decreased as temperature increased; the value varied from 1.39 × 10−2 to 9.91 × 10−3 cm3(STP)/(cm3 cmHg). The carbon dioxide diffusivity coefficient increased as temperature increased, with the value ranging from 1.84 × 10−5 to 2.49 × 10−5 cm2/s. The oxygen diffusivity in water was approximately equal to that of carbon dioxide; however, the

4. CONCLUSIONS Oxygen and carbon dioxide permeability of dry gases and those dissolved in water through hydrophobic high free-volume 1139



Article

silicon- or fluorine-containing nonporous glassy polymer membranes was systematically investigated in this study. Both surface and bulk hydrophobicities depend on the boundary layer resistance of all the polymer membranes. The boundary layer resistance decreased as hydrophobicity increased; thus, PLS was less than PG. However, the PLS(O2) of PTMSMMA was equal to PG(O2). The gas permeability difference between PLS and PG depended on the combination of competitive sorption between water and gas species and the plasticization by water. Plasticization occurred only in PTMSMMA; thus, PLS(CO2)/ PLS(O2) was less than PG(CO2)/PG(O2). The PTMSMMA with low glass transition temperature depended on the plasticization by water. Therefore, gas permeability in the wet state was approximately equal to that in the dry state.

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(12) Nakagawa, T.; Naruse, A.; Higuchi, A. Permeation of dissolved carbon dioxide in synthetic membranes. J. Appl. Polym. Sci. 1991, 42, 383. (13) Nagai, K.; Masuda, T.; Nakagawa, T.; Freeman, B. D.; Pinnau, I. Poly[1-(trimethylsilyl)-1-propyne] and related polymers: synthesis, properties and functions. Prog. Polym. Sci. 2001, 26, 721. (14) Toy, L. G.; Nagai, K.; Freeman, B. D.; Pinnau, I.; He, Z.; Masuda, T.; Teraguchi, M.; Yampolskii, Y. P. Pure-Gas and vapor permeation and sorption properties of poly[1-phenyl-2-[p(trimethylsilyl)phenyl]acetylene] (PTMSDPA). Macromolecules 2000, 33, 2516. (15) Kazama, S.; Teramoto, T.; Haraya, K. Carbon dioxide and nitrogen transport properties of bis(phenyl)fluorene-based cardo polymer membranes. J. Membr. Sci. 2002, 207, 91. (16) Kanehashi, S.; Nakagawa, T.; Nagai, K.; Duthie, X.; Kentish, S.; Stevens, G. Effects of carbon dioxide-induced plasticization on the gas transport properties of glassy polyimide membranes. J. Membr. Sci. 2007, 298, 147. (17) The Chemical Society of Japan. Kagaku Binran, basic ed.; Maruzen: Tokyo, 1980. (18) Kellogg, V. International Critical Tables; McGraw-Hill: New York, 1933. (19) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-44-934-7211. Fax: +81-44-934-7906. E-mail: nagai@ meiji.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by a Grant-in-aid for Scientific Research C (24560862) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, the Japanese Society of the Promotion of Science and Research Project Grant B (3) from the Institute of Science and Technology, Meiji University, Japan.

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REFERENCES

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Permeability of Dry Gases and Those Dissolved in Water through

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