Crosslinking and stabilization of high fractional free volume polymers for gas separation

international journal of greenhouse gas control 2 (2008) 492–501

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Crosslinking and stabilization of high fractional free volume polymers for gas separation Lei Shao a, Jon Samseth b,c, May-Britt Ha¨gg a,* a

Department of Chemical Engineering, Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology, Sem Saelandsvei 4, N-7491 Trondheim, Norway b SINTEF Materials and Chemistry, N-7465 Trondheim, Norway c Akershus University College, N-2001 Lillestrøm, Norway

article info

abstract

Article history:

Crosslinkable poly(4-methyl-2-pentyne) (PMP) membranes were cast from carbon tetra-

Received 12 December 2007

chloride solutions containing PMP and either 4,40 -diazidobenzophenone or 4,40 -(hexafluor-

Accepted 1 April 2008

oisopropylidene)diphenyl azide. The composite membranes were transparent and

Published on line 20 May 2008

homogeneous and were crosslinked by UV irradiation at room temperature or thermal treatment at 180 8C. Low levels of the bis(aryl azide) (1–4.5 wt%) were effective in rendering

Keywords:

the membranes insoluble in cyclohexane and carbon tetrachloride, both are good solvents

Poly(4-methyl-2-pentyne)

for PMP, thus PMP can easily be converted to mechanically stable membranes with perme-

Crosslinking

abilities and selectivities comparable or higher than those of the well-known poly(dimethyl-

Membranes

siloxane) (PDMS). The permeabilities of O2, N2, H2, CH4 and CO2 were measured. Compared to

Gas separation

pure PMP, the crosslinked membranes containing bis(aryl azide) had lower permeabilities and higher selectivities, consistent with a reduction in free volume. # 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

The use of polymeric materials for membrane gas and vapor separation has been on the market for more than 25 years, and the potential use is steadily increasing. High permeability and high selectivity in favor of the permeating gas as well as good durability and high mechanical strength of the material are the important properties for a commercial gas separating membrane. Poly(4-methyl-2-pentyne) (PMP) is an amorphous, disubstituted acetylene-based high free volume glassy polymer. It is one of the most permeable purely hydrocarbonbased polymer known (Morisato and Pinnau, 1996). The high permeation property and hence also low gas selectivity of PMP, results from very poor polymer chain packing due to the stiffness of the polymer chain as can be understood with reference to its chemical structure (Fig. 1). The unique

permeation property has been documented by several authors (Merkel et al., 2002, 2003; He et al., 2002; Nagai et al., 2004, 2005). However, it has also been documented that the gas permeability is not stable over time, and that it is sensitive to processing history. PMP undergoes significant physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers (Merkel et al., 2003; Nagai et al., 2004). For example, nitrogen permeability coefficient in PMP has been reported to decrease by 25% over a period of 29 days (Merkel et al., 2003). PMP is also soluble in some organic compounds, leading to potential dissolution of the membrane in process streams where its separation properties are of greatest interest. These phenomena compromise the practical utility of PMP. Polymer modification by crosslinking has attracted interest in membrane-based separation of gases or vapors. Today

* Corresponding author. Tel.: +47 73594033. E-mail address: [email protected] (M.-B. Ha¨gg). 1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.04.005

international journal of greenhouse gas control 2 (2008) 492–501

Fig. 1 – Chemical structure of PMP.

crosslinked poly(dimethylsiloxane) (PDMS) is used commercially as a vapor separation membrane material (Baker, 2004). Crosslinked polymer films offer obvious advantages as membranes, particularly in terms of stability. Without crosslinking the material will tend to swell when exposed to certain gas mixtures, and hence separation properties are affected. There is a lot of published literature on crosslinked membranes for gas and liquid separation (Paul and Yampolskii, 1994; Lin et al., 2005; Staudt-Bickel and Koros, 1999; Staudt-Bickel et al., 2007; Kita et al., 1994; Hsu et al., 1993; Wright and Paul, 1997; Chung et al., 2004; DeForset, 1975; Liu et al., 2001), with the most attractive processes being those initiated thermally or photochemically. Jia and Baker (1998) reported that crosslinking poly(1-trimethylsilyl-1-propyne) (PTMSP) with bis(aryl azides) has been shown to increase the

493

chemical and physical stability. Crosslinked PTMSP membranes are insoluble in common PTMSP solvents such as toluene, and the permeability of the crosslinked membranes are reported to be constant over time. These results are very interesting for the current work, as PTMSP and PMP are both high free volume polymers. All these results encouraged us to adapt the technique of crosslinking PMP membranes in order to increase its stability. A plausible mechanism for the crosslinking reactions is shown in Fig. 2 (Jia and Baker, 1998), under photochemical irradiation or thermal treatment of the bis(aryl azide) decomposes to nitrogen gas and reactive nitrenes, the resulting nitrenes can add to double bonds to form aziridines or insert into carbon–hydrogen bonds in PMP to form substituted amines. In the current study, the procedure of crosslinking, characterization and the permeability of crosslinked PMP membranes are described, the implications of these results for the mechanism of permeability decline in PMP are discussed as crosslinking inevitably will lead to a reduced gas flux, while selectivity will increase.

2.

Background

2.1.

Transport in polymers

The permeation of gases and vapors through a dense polymeric membrane is generally described as a solutiondiffusion process, and the permeability, P, of a penetrant molecule through a membrane is the product of its diffusivity, D, and solubility, S, i.e. P ¼ DS

Fig. 2 – Illustration of crosslinking reaction of PMP.

(1)

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The ability of a membrane to separate two molecules, for example, A and B, is the ratio of their permeabilities and is called the membrane ideal selectivity aA/B:

aA=B ¼

PA ¼ PB

   DA SA DB SB

(2)

where the first term on the right-hand side is the diffusivity selectivity and the second is the solubility selectivity. The balance between the solubility selectivity and the diffusivity selectivity determines whether a membrane material is selective for molecule A or molecule B in a feed mixture (Toshima, 1992; Stern, 1994; Mulder, 1996). The temperature dependency of solubility, diffusivity and permeability may be expressed as the van’t Hoff–Arrhenius relationships (Toshima, 1992; Stern, 1994; Mulder, 1996):   DHS S ¼ S0 exp RT

(3)

  Ed D ¼ D0 exp RT

(4)

  Ep P ¼ P0 exp RT

(5)

spectroscopy (PALS) (Kobayashi et al., 1994; Yampolskii et al., 1993; Consolati et al., 1996). PALS is one of the most widely applied techniques and provides the most direct and detailed information on the size and concentration of free volume elements in the materials.

3.

Experimental

3.1.

Instrumental characterization

NMR spectra were recorded on Bruker Avance DPX 400 with chemical shifts referenced to tetramethylsilane for deuteriochloroform. FT-IR spectra were recorded on a Thermo Nicolet FT-IR Nexus spectrometer. NMR and FT-IR spectra were used to confirm chemical structure. UV–vis spectra were recorded on a Varian Cary 50 UV–vis spectrophotometer. Differential scanning calorimetry (DSC) was done by a Q500 (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere at a heating rate of 10 8C/min. DSC measures the heat flow associated with transitions in the materials as a function of time and temperature in a controlled atmosphere.

3.2.

Sorption measurement

where S0, D0 and P0 are pre-exponential factors, DHS is the enthalpy of sorption, Ed is the activation energy of diffusion, Ep is the activation energy of permeation, R is the ideal gas constant, and T is the absolute temperature. The activation energy, Ep, of permeation is the sum of the activation energy of diffusion, Ed, and the enthalpy of sorption, DHS:

The solubility of gases in uncrosslinked PMP and crosslinked PMP membranes were measured with a sorption apparatus. The experimental method and setup is described elsewhere (Lindbra˚then, 2005; Lindbra˚then and Ha¨gg, 2005). After the membrane was loaded into the sample chamber, the sorption system was evacuated for at least 24 h prior to each test. The sorption was measured as cm3 (STP) gas adsorbed per cm3 material. The tests were performed at 35 8C with the pressure range of 1–4 bar.

Ep ¼ Ed þ DHS

3.3.

2.2.

(6)

Free volume

Molecular diffusion through a dense polymer depends strongly on the amount of free volume that a material possesses. A quantity frequently used to compare the amount of free volume in polymers is the fractional free volume (FFV), which is usually estimated according to Bondi’s method:

FFV ¼

V  1:3VW V

(7)

where V is the polymer specific volume (i.e. reciprocal of geometric density) and Vw is the specific van der Waals volume, which can be calculated by group contribution methods (Ghosal and Freeman, 1994; Bondi, 1968; van Krevelen, 1997). The FFV of PMP is 0.28, which is one of the highest values of any known polymer. A number of techniques have been using to study free volume in polymers, including spin probe methods (Yampolskii et al., 1999), molecular modelling (Hofmann et al., 2002, 2003), inverse gas chromatography (Yampolskii et al., 1986), smallangle X-ray scattering (Roe and Curro, 1983), 129Xe NMR (Golemme et al., 2003), and positron annihilation lifetime

Polymer synthesis

PMP was synthesized as described in literature (Morisato and Pinnau, 1996; Khotimsky et al., 2003; Pinnau and Morisato, 1998). The monomer, 4-methyl-2-pentyne (Lancaster, Inc.) was dried over calcium hydride for 24 h, and was then distilled in an atmosphere of high-purity nitrogen. The catalysts, niobium pentachloride (NbCl5) and triphenyl bismuth (Ph3Bi) (Aldrich Chemicals) used without further purification. A solution of 0.33 g of NbCl5, and 0.54 g Ph3Bi in 47 mL cyclohexane was stirred at 80 8C for 10 min under dry nitrogen. Then the monomer solution of 5 g 4-methyl-2-pentyne in 7 mL cyclohexane was added dropwise to the catalyst solution, and the mixture was reacted at 80 8C for 4 h. The viscosity of the solution increased very rapidly. The resulting gel was precipitated in methanol, filtered to recover the precipitated polymer, and dried under vacuum. The polymer was dissolved in cyclohexane and reprecipitated twice from methanol to remove excess monomer, oligomers and catalysts. The polymer yield was 85%. 1 H and 13C NMR and FT-IR analyses of the polymer confirmed the chemical structure of PMP (Fig. 1).

3.4.

Synthesis of crosslinking agents

The syntheses and structures of bis(aryl azide) crosslinking agents are shown in Fig. 3. 4,40 -Diazidobenzophenone (BAA),

international journal of greenhouse gas control 2 (2008) 492–501

495

Fig. 3 – Syntheses of 4,40 -diazidobenzophenone (BAA) and 4,40 -(hexafluoroisopropylidene)diphenyl azide (HFBAA).

and 4,40 -(hexafluoroisopropylidene)diphenyl azide (HFBAA) were obtained by diazotization of the corresponding amines (Aldrich Chemicals) followed by nucleophilic displacement of the diazonium salt with NaN3. The synthetic procedure follows that reported in the literature (Ling et al., 1992; Wiley and Sons, 1973; Hepher and Wagner, 1958; Coe, 1967).

3.4.1.

4,40 -Diazidobenzophenone

4,40 -Diaminobenzophenone (0.50 g, 2 mmol) was dissolved in 2 mL of water containing 1.1 mL of concentrated HC1, and cooled to 0 8C, then treated dropwise with a solution of sodium nitrite (0.34 g, 5 mmol) in 1.2 mL of water. After the addition, the reaction was maintained at 0–5 8C for 1.5 h. To the resultant clear orange solution was added dropwise 0.31 g (5 mmol) of sodium azide in 1.2 mL of water. The solution was stirred for 15 min, as a white precipitate formed. The solid was collected, washed with water, allowed to dry, dissolved in dichloromethane, and heated with activated charcoal. Filtration and solvent evaporation gave 0.51 g (84%) of pale yellow 4,40 -diazidobenzophenone. 1H and 13C NMR and FT-IR analyses of the product confirmed the chemical structure of BAA (see Fig. 3).

3.4.2.

4,40 -(Hexafluoroisopropylidene)diphenyl azide

4,40 -(Hexafluoroisopropylidene)dianiline (0.70 g, 2 mmol) was dissolved in 2 mL of water containing 1.1 mL of concentrated HC1, and cooled to 0 8C, then treated dropwise with a solution of sodium nitrite (0.34 g, 5 mmol) in 1.2 mL of water. After the addition, the reaction was maintained at 0–5 8C for 1.5 h. To the resultant clear solution was added dropwise 0.31 g (5 mmol) of sodium azide in 1.2 mL of water. The solution was stirred for 15 min, as a white precipitate formed. The solid was collected, washed with water, allowed to dry, dissolved in dichloromethane, and heated with activated charcoal. Filtration and solvent evaporation gave 0.63 g (82%) of 4,40 (hexafluoroisopropylidene)diphenyl azide. 1H and 13C NMR and FT-IR analyses of the product confirmed the chemical structure of HFBAA (see Fig. 3).

3.5.

solvent evaporation. The membrane was dried at ambient temperature for 5 days and then placed in a vacuum oven at room temperature for at least 24 h to remove any residual solvent. The final as-cast membrane thicknesses varied from 40 to 50 mm. To prepare modified membranes, PMP and a small amount of the appropriate bis(aryl azide) were codissolved in carbon tetrachloride, and membranes were then cast from the solution after filtration. Crosslinking of the bis(aryl azide) containing membranes was induced by either UV irradiation at room temperature or thermal treatment at 180 8C. In both cases, the crosslinking reactions were performed under an inert atmosphere (see Fig. 2). The temperature used to initiate thermal crosslinking, 180 8C, was determined from DSC measurements (Fig. 4) as the onset of N2 loss from PMP/azide composites (see Fig. 2). The irradiating wavelength for photochemical crosslinking was set to correspond to the peak of the absorption band for the azide. For fluorinated azide HFBAA, the peak of the absorption is near 254 nm while the absorption spectrum of the benzophenonebased azide BAA is red-shifted, with a peak near 302 nm (Fig. 5). Thus 254 and 302 nm light was used for crosslinking PMP/ HFBAA and PMP/BAA composite membranes, respectively. Photochemical crosslinking of membranes with bis(aryl azide), BAA, was performed at 302 nm for 60 min using a lamp in a vacuum oven. Photochemical crosslinking of membranes

Membrane preparation and modification

The PMP was dissolved in carbon tetrachloride to form a 1.2 wt% polymer solution, poured into a casting ring placed on a glass plate and covered with a funnel to allow for slow

Fig. 4 – DSC heating and cooling curves for PMP/3 wt% HFBAA. The dip in the heating curve corresponds to the decomposition of HFBAA and the loss of N2. Conditions: heating and cooling, 10 8C/min under N2 atmosphere.

496

international journal of greenhouse gas control 2 (2008) 492–501

with LabView1 data logging. The experimental method and equipment was described elsewhere (O’Brien et al., 1986; Lie, 2005). The membrane thickness was measured by an electronic Mitutoyo 2109F thickness gauge (Mitutoyo Corp., Kanagawa, Japan). The gauge was a non-destructive drop-down type with a resolution of 1 mm. Flat sheet membrane was scanned at a scaling of 100% (uncompressed tiff-format) and analyzed by Scion Image (Scion Corp., MD, USA) software. This image tool is available from http://www.scioncorp.com/ at no cost. The effective area was sketched with the draw-by-hand tool both clockwise and counter-clockwise several times.

Fig. 5 – UV–vis spectra of the bis(aryl azide) crosslinking agents used for crosslinking PMP membranes.

with fluorinated bis(aryl azide), HFBAA, was performed at 254 nm using a lamp for 30 min in a vacuum oven. After photochemical treatment, membranes were slightly curled toward the side that was exposed to UV light. Thermal crosslinking of the membranes was achieved by heating flat membranes in a vacuum oven at 180 8C for 90 min. Thermally treated membranes remained flat after crosslinking.

3.6.

Gas permeability

The membranes were masked using an impermeable aluminum tape, leaving open a defined permeation area. Epoxy was then applied along the interface of the tape and the membrane. A sintered metal disc covered with a filter paper was used as support for the membrane in the test cell. Single gases (O2, N2, H2, CH4 and CO2) were measured at 35 8C with feed pressure of 2.0 bar using a constant volume/ variable-pressure method in a standard pressure-rise setup (MKS Baratron1 pressure transducer, 0–134 mbar range)

4.

Results and discussion

The bis(aryl azide)s dissolved easily in PMP to form homogeneous mixtures. At high loadings (>4.0 wt% BAA and >4.5 wt% HFBAA), the membranes became cloudy and optical microscopy confirmed phase separation of crosslinker and polymer. All crosslinking studies reported here were performed on clear membranes which showed no apparent signs of phase separation. The dried membranes were clear, and UV–vis and FT-IR spectra show that the spectra of the asprepared membranes are simply the linear combination of the spectrum of PMP and that of the azide crosslinker. The stretching vibration for the azide at 2110 cm1 is easily monitored in the FT-IR, and the loss in its intensity can be correlated with the progress of the crosslinking reaction (Fig. 6). Double bonds on the PMP backbone and methyl groups on the side chains are two possible crosslinking sites, but the latter is more likely since access to the double bonds is sterically hindered. A preliminary indication of significant crosslinking is the lack of solubility of crosslinked membranes in known solvents for PMP. In this regard, crosslinked PMP was insoluble in cyclohexane and carbon tetrachloride, which are known to be

Fig. 6 – FT-IR spectra of PMP/3 wt% HFBAA composite membranes: (A) as cast, (B) after irradiation and (C) after heating.

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Table 1 – Gas permeabilities (Barrer)a of uncrosslinked and photochemically crosslinked PMP membranes at 35 8C and feed pressure of 2.0 bar Crosslinking agent Azide None

b

wt% azide

Before crosslinking

After crosslinking

N2

O2

H2

CH4

CO2

N2

O2

H2

CH4

CO2

0

950

1780

3970

1790

6700

870

1670

3810

1560

6420

BAA

1.0 2.0 3.0

810 750 680

1570 1530 1440

3680 3650 3520

1530 1420 1180

6230 6190 5820

450 340 280

1340 1250 1130

3580 3480 3410

890 710 630

5130 4910 4450

Nonec

0

950

1780

3970

1790

6700

490

1490

3640

920

4860

HFBAA

1.1 2.0 3.0

790 730 690

1580 1540 1420

3660 3610 3490

1500 1430 1070

6250 6100 5710

380 290 230

1190 1080 990

3420 3380 3300

810 630 510

4710 4320 3950

a b c

Permeability is in unit of Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1). Irradiated at 302 nm for 60 min. Irradiated at 254 nm for 30 min.

good solvents for uncrosslinked PMP (Morisato and Pinnau, 1996). It was found that when the bis azides concentration were about BAA 0.9 wt% and HFBAA 1.0 wt% or higher, PMP membranes were insoluble in carbon tetrachloride. The insolubility of crosslinked PMP membranes is defined when there is less than 0.5% weight loss in a dry membrane before and after soaking in carbon tetrachloride for 24 h.

4.1.

Photochemical crosslinking

Permeability data in Table 1 show membranes with different amounts of crosslinker. For all gases considered, the addition of bis(aryl azide)s to PMP decreased the permeability only slightly before photo-irradiation, with hardly any improvement of selectivity (Table 2). The crosslinked membranes show a significant decrease in permeabilities (Table 1), steadily decreasing as crosslinker content increase. The selectivities of O2/N2, H2/N2, CO2/N2, CO2/CH4 and H2/CH4 increased with increasing crosslinker. Higher degrees of crosslinking resulted in a lower gas permeability and higher selectivity. Blanks were run for photochemical reactions, to study the effects of irradiation on the properties of pure PMP. When irradiated at 302 nm for 60 min (the condition used for the

irradiation of PMP/BAA composites), PMP membranes showed only a slight change in permeabilities and selectivities; a more noticeable change can however be seen for N2 and partly CH4. When membranes were irradiated at 254 nm for 30 min (the condition used for the irradiation of PMP/HFBAA composites), the selectivities of O2/N2, H2/N2, CO2/N2, CO2/CH4 and H2/CH4 improved and the permeabilities decreased. Again, there is a large reduction in permeation for N2 and CH4. As mentioned above, PMP is a disubstituted acetylenebased glassy polymer, its chain consists of conjugated double bonds in the cis or trans configurations, the content of which depends on the temperature of polymerization. The trans isomer is the thermodynamically stable form. Under UV irradiation cis ! trans isomerization of PMP readily takes place (Rolland et al., 1980; Cernia et al., 1984), this strongly reduced chain mobility of polymer, intersegmental packing may become much denser and intrasegmental mobility smaller than in non-irradiated polymer, which causes the change of the free volume in polymer membranes (decrease), hence the gas permeability is decreased. For photochemical crosslinking, the permeability changes in going from pure PMP, to PMP with azide additives, to the crosslinked membrane were predictable. For example, the

Table 2 – Selectivitiesa of various gas pairs in uncrosslinked and photochemically crosslinked PMP membranes Crosslinking agent

Selectivity (before crosslinking)

Selectivity (after crosslinking)

O2/N2

H2/N2

CO2/N2

CO2/CH4

H2/CH4

O2/N2

H2/N2

CO2/N2

CO2/CH4

0

1.9

4.2

7.1

3.7

2.2

1.9

4.4

7.4

4.1

2.4

BAA

1.0 2.0 3.0

1.9 2.0 2.1

4.5 4.9 5.2

7.7 8.3 8.6

4.1 4.4 4.9

2.4 2.6 3.0

3.0 3.7 4.0

7.9 10.2 12.2

11.4 14.5 15.9

5.7 6.8 7.1

4.0 4.9 5.4

Nonec

0

1.9

4.2

7.1

3.7

2.2

3.0

7.4

9.9

5.3

4.0

HFBAA

1.1 2.0 3.0

2.0 2.1 2.1

4.6 4.9 5.0

7.9 8.4 8.3

4.2 4.3 5.3

2.4 2.5 3.3

3.1 3.8 4.3

9.0 11.7 14.4

12.3 14.9 17.2

5.8 6.9 7.8

4.2 5.4 6.5

Azide None

a b c

b

wt% azide

Selectivity is the ratio of the permeabilities for the pure gases. Irradiated at 302 nm for 60 min. Irradiated at 254 nm for 30 min.

H2/CH4

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Table 3 – Gas permeabilities (Barrer)a of uncrosslinked and thermally crosslinked PMP membranes at 35 8C and feed pressure of 2.0 bar Crosslinking agent Azide b

wt% azide

Before crosslinking

After crosslinking

N2

O2

H2

CH4

CO2

N2

O2

H2

CH4

CO2

None

0

950

1780

3970

1790

6700

890

1720

3820

1650

6640

BAA

1.0 2.0 3.0

810 750 680

1570 1530 1440

3680 3650 3520

1530 1420 1180

6230 6190 5820

610 540 460

1510 1470 1390

3620 3560 3510

980 810 720

5780 5310 4920

HFBAA

1.1 2.0 3.0

790 730 690

1580 1540 1420

3660 3610 3490

1500 1430 1070

6250 6100 5710

570 480 390

1490 1380 1260

3640 3570 3420

950 770 570

5690 5210 4320

a b

Permeability is in unit of Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1). Thermal treatment at 180 8C for 90 min.

Table 4 – Selectivitiesa of various gas pairs in uncrosslinked and thermally crosslinked PMP membranes Crosslinking agent Azide

wt% azide

Selectivity (before crosslinking)

Selectivity (after crosslinking)

O2/N2

H2/N2

CO2/N2

CO2/CH4

H2/CH4

O2/N2

H2/N2

CO2/N2

CO2/CH4

H2/CH4

None

0

1.9

4.2

7.1

3.7

2.2

1.9

4.3

7.5

4.0

2.3

BAA

1.0 2.0 3.0

1.9 2.0 2.1

4.5 4.9 5.2

7.7 8.3 8.6

4.1 4.4 4.9

2.4 2.6 3.0

2.5 2.7 3.0

5.9 6.5 7.6

9.5 9.8 10.7

5.9 6.6 6.8

3.7 4.4 4.9

HFBAA

1.1 2.0 3.0

2.0 2.1 2.1

4.6 5.0 5.1

7.9 8.4 8.3

4.2 4.3 5.3

2.4 2.5 3.3

2.6 2.9 3.2

6.3 7.4 8.8

10.0 10.9 11.1

6.0 6.8 7.6

3.8 4.6 6.0

a

Selectivity is the ratio of the permeabilities for the pure gases.

azide additive in composite membranes is expected to occupy much of the free volume in the polymer and thus the permeability is lower compared to pure PMP membranes. Crosslinking connects adjacent chains, increases the local segment density, and causes a further decline in the permeability.

Blank was run for thermal reaction, to study the effect of thermal treatment on the property of pure PMP. When treated at 180 8C for 90 min (the condition used for the thermal treatment of PMP/azide composite membrane), PMP membrane showed very slight change in permeabilities or selectivities.

4.2.

4.3.

Thermal crosslinking

After crosslinking, the membranes were insoluble in good solvents for PMP such as cyclohexane and carbon tetrachloride. Permeability data in Table 3 show membranes with different amounts of crosslinker. For all gases considered, the addition of bis(aryl azide)s to PMP decreased the permeability slightly before thermal crosslinking, with hardly any improvement of selectivity (Table 4). After thermal treatment, the crosslinked membranes show decreased permeabilities. The permeabilities decreased as crosslinker content increased for the crosslinked membranes, hence the selectivities of O2/N2, H2/N2, CO2/N2, CO2/CH4 and H2/CH4 increased with increasing crosslinker. Thermally crosslinked membranes show higher permeabilities and lower selectivities compare to photochemically crosslinked membranes. It may be assumed there is a more open network structure for the thermally cured membranes compared to membranes crosslinked using UV irradiation. After crosslinking, the free volume fraction in the membranes decreased, with the photochemically crosslinked membranes having lower free volumes than thermally crosslinked membranes.

Solubility

Fig. 7 presents nitrogen sorption isotherms in uncrosslinked PMP and crosslinked PMP containing 3.0 wt% HFBAA crosslinker at 35 8C. Within experimental uncertainty, nitrogen sorption levels in uncrosslinked PMP and crosslinked PMP are

Fig. 7 – Nitrogen sorption in uncrosslinked PMP and crosslinked PMP containing 3.0 wt% HFBAA at 35 8C.

international journal of greenhouse gas control 2 (2008) 492–501

Fig. 8 – Methane sorption in uncrosslinked PMP and crosslinked PMP containing 3.0 wt% HFBAA at 35 8C.

almost identical. The permeability (P) is the product of the gas diffusivity (D) and solubility (S) (Eq. (1)), it is most likely that the decrease in permeability in crosslinked PMP is due to a decrease in the diffusivity caused by a reduced FFV. This trend would also be expected for an ideal gas like nitrogen with very low sorption level. The same trend is documented for methane (Fig. 8), a gas which is less ideal but has about the same molecular size as nitrogen. Hence it seems to be a general trend applying to gas transport in crosslinked PMP; at least in this low pressure region (!4 bar). Fig. 8 presents methane sorption isotherms in uncrosslinked PMP and crosslinked PMP containing 3.0 wt% HFBAA crosslinker at 35 8C. The isotherms are slightly concave to the pressure axis, which is typical behavior for gas sorption of non-ideal gases in glassy polymers (Ghosal and Freeman, 1994). Consistent with the nitrogen data provided in Fig. 7, methane sorption data in PMP are independent of HFBAA crosslinker content, indicating that methane solubility in PMP

Fig. 10 – Temporal stability of PMP, photochemically crosslinked PMP/bis(aryl azide) composite membranes measured as oxygen permeability. Temperature: 35 8C; feed pressure: 2.0 bar. 1 Barrer = 10S10 cm3(STP) cm cmS2 sS1 cmHgS1.

containing varying amounts of HFBAA is virtually identical to that in the pure polymer—again in this low pressure region.

4.4.

Membrane stability

The stability of the uncrosslinked PMP and photochemically crosslinked PMP/bis(aryl azides) composite membranes stored in air were checked over time. The results are shown in Figs. 9 and 10. The nitrogen and oxygen permeabilities of photochemically crosslinked PMP/bis(aryl azides) composite membranes were almost constant over a fairly long time. The uncrosslinked PMP membranes showed a large decrease in the nitrogen and oxygen permeabilities during the same time. The permeability stability of crosslinked PMP for the gases is clearly improved. After crosslinking, the PMP can be easily converted to mechanically stable membranes with permeabilities and selectivities comparable or higher than those of poly(dimethylsiloxane).

5.

Fig. 9 – Temporal stability of uncrosslinked PMP, photochemically crosslinked PMP/bis(aryl azide) composite membranes measured as nitrogen permeability. Temperature: 35 8C; feed pressure: 2.0 bar. 1 Barrer = 10S10 cm3(STP) cm cmS2 sS1 cmHgS1.

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Conclusions

Crosslinkable poly(4-methyl-2-pentyne) membranes were cast from carbon tetrachloride solutions containing PMP and either BAA or HFBAA crosslinker. The composite membranes were transparent and homogeneous and were crosslinked by UV irradiation at room temperature or thermal annealing at 180 8C. After crosslinking, the membranes were insoluble in solvents that typically dissolve PMP. Thus, the effect is a significant increase in the chemical stability due to crosslinking. The process is simple and effective, and thus PMP can be easily converted to mechanically stable membranes. For all gases considered permeability decreased as amount of crosslinking agent increased. The permeability decrease can be correlated with the fractional free volume decrease. The permeability of PMP decreased with increasing crosslinking due to the loss in FFV. The selectivities of O2/N2, H2/N2,

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CO2/N2, CO2/CH4 and H2/CH4 increased as the FFV decreased, showing that crosslinked PMP is more size selective to gases than uncrosslinked PMP. The permeability stability of crosslinked PMP for the gases is clearly improved. The increased stability may be caused by crosslink constraining the PMP chains and not allowing relaxation of the excess, nonequilibrium FFV that is inherent in PMP. The sorption levels of N2 and CH4 were measured and found to be independent of crosslinker content (i.e. the solubility remains relatively constant), within experimental uncertainty. Therefore gas solubility in PMP does not seem to be affected by the FFV decrease accompanying the increase in crosslinker content. The permeability (P) is described as the product of the gas diffusivity (D) and solubility (S), hence the decrease in permeability in crosslinked PMP is most likely due to a decrease in the diffusivity.

Acknowledgements The authors want to thank the Norwegian Research Council for the financial support to the work. We also gratefully acknowledge Dr. Keith Redford and Siren M. Neset from SINTEF Oslo for valuable help with synthesis and analysis work.

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