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

$Page 1: Crosslinking and stabilization of high fractional free volume polymers for gas separation$
i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 2 ( 2 0 0 8 ) 4 9 2 – 5 0 1

Crosslinking and stabilization of high fractional freevolume polymers for gas separation

Lei Shao a, Jon Samseth b,c, May-Britt Hagg a,*aDepartment of Chemical Engineering, Faculty of Natural Sciences and Technology, Norwegian University

of Science and Technology, Sem Saelandsvei 4, N-7491 Trondheim, Norwayb SINTEF Materials and Chemistry, N-7465 Trondheim, NorwaycAkershus University College, N-2001 Lillestrøm, Norway

a r t i c l e i n f o

Article history:

Received 12 December 2007

Accepted 1 April 2008

Published on line 20 May 2008

Keywords:

Poly(4-methyl-2-pentyne)

Crosslinking

Membranes

Gas separation

a b s t r a c t

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

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

oisopropylidene)diphenyl azide. The composite membranes were transparent and

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

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

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

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

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

pure PMP, the crosslinked membranes containing bis(aryl azide) had lower permeabilities

and higher selectivities, consistent with a reduction in free volume.

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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 poly-

mer. It is one of the most permeable purely hydrocarbon-

based 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

* Corresponding author. Tel.: +47 73594033.E-mail address: [email protected] (M.-B. Hagg).

1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reservedoi:10.1016/j.ijggc.2008.04.005

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 inter-

est in membrane-based separation of gases or vapors. Today

d.

mailto:[email protected]

http://dx.doi.org/10.1016/j.ijggc.2008.04.005

Fig. 1 – Chemical structure of PMP.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 2 ( 2 0 0 8 ) 4 9 2 – 5 0 1 493

crosslinked poly(dimethylsiloxane) (PDMS) is used commer-

cially 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 Yam-

polskii, 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

Fig. 2 – Illustration of crossl

chemical and physical stability. Crosslinked PTMSP mem-

branes 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, char-

acterization and the permeability of crosslinked PMP mem-

branes 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 solution-

diffusion 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 (1)

inking reaction of PMP.


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 ¼PA

PB¼ DA

DB

� �SA

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 selec-

tive 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):

S ¼ S0 exp�DHS

RT

� �(3)

D ¼ D0 exp�Ed

RT

� �(4)

P ¼ P0 exp�Ep

RT

� �(5)

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:

Ep ¼ Ed þ DHS (6)

2.2. 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 meth-

ods (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), small-

angle X-ray scattering (Roe and Curro, 1983), 129Xe NMR

(Golemme et al., 2003), and positron annihilation lifetime

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

The solubility of gases in uncrosslinked PMP and crosslinked

PMPmembranesweremeasuredwith asorption apparatus. The

experimental method and setup is described elsewhere (Lind-

brathen, 2005; Lindbrathen and Hagg, 2005). After the mem-

brane 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 wereperformed at 35 8C with thepressure range of 1–4 bar.

3.3. Polymer synthesis

PMP was synthesized as described in literature (Morisato and

Pinnau, 1996; Khotimsky etal., 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 cyclohex-

ane was added dropwise to the catalyst solution, and the

mixturewas reacted at80 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 cyclohex-

ane and reprecipitated twice from methanol to remove excess

monomer, oligomers and catalysts. The polymer yield was 85%.1H 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),

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

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.


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-Diazidobenzophenone4,40-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. Filtra-

tion and solvent evaporation gave 0.51 g (84%) of pale yellow

4,40-diazidobenzophenone. 1H and 13C NMR and FT-IR ana-

lyses of the product confirmed the chemical structure of BAA

(see Fig. 3).

3.4.2. 4,40-(Hexafluoroisopropylidene)diphenyl azide4,40-(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. Filtra-

tion 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. 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

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 benzophenone-

based 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

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 alumi-

num 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)

Fig. 6 – FT-IR spectra of PMP/3 wt% HFBAA composite membra

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.

4. Results and discussion

The bis(aryl azide)s dissolved easily in PMP to form homo-

geneous 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 per-

formed 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 as-

prepared 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 cm�1 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

nes: (A) as cast, (B) after irradiation and (C) after heating.

http://www.scioncorp.com/

Table 1 – Gas permeabilities (Barrer)a of uncrosslinked and photochemically crosslinked PMP membranes at 35 8C and feedpressure of 2.0 bar

Crosslinking agent Before crosslinking After crosslinking

Azide wt% azide N2 O2 H2 CH4 CO2 N2 O2 H2 CH4 CO2

Noneb 0 950 1780 3970 1790 6700 870 1670 3810 1560 6420

BAA 1.0 810 1570 3680 1530 6230 450 1340 3580 890 5130

2.0 750 1530 3650 1420 6190 340 1250 3480 710 4910

3.0 680 1440 3520 1180 5820 280 1130 3410 630 4450

Nonec 0 950 1780 3970 1790 6700 490 1490 3640 920 4860

HFBAA 1.1 790 1580 3660 1500 6250 380 1190 3420 810 4710

2.0 730 1540 3610 1430 6100 290 1080 3380 630 4320

3.0 690 1420 3490 1070 5710 230 990 3300 510 3950

a Permeability is in unit of Barrer (1 Barrer = 10�10 cm3(STP) cm cm�2 s�1 cmHg�1).b Irradiated at 302 nm for 60 min.c 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 improve-

ment 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

Table 2 – Selectivitiesa of various gas pairs in uncrosslinked a

Crosslinking agent Selectivity (before crosslinking

Azide wt% azide O2/N2 H2/N2 CO2/N2 CO2/CH4

Noneb 0 1.9 4.2 7.1 3.7

BAA 1.0 1.9 4.5 7.7 4.1

2.0 2.0 4.9 8.3 4.4

3.0 2.1 5.2 8.6 4.9

Nonec 0 1.9 4.2 7.1 3.7

HFBAA 1.1 2.0 4.6 7.9 4.2

2.0 2.1 4.9 8.4 4.3

3.0 2.1 5.0 8.3 5.3

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

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 acetylene-

based 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

nd photochemically crosslinked PMP membranes

) Selectivity (after crosslinking)

H2/CH4 O2/N2 H2/N2 CO2/N2 CO2/CH4 H2/CH4

2.2 1.9 4.4 7.4 4.1 2.4

2.4 3.0 7.9 11.4 5.7 4.0

2.6 3.7 10.2 14.5 6.8 4.9

3.0 4.0 12.2 15.9 7.1 5.4

2.2 3.0 7.4 9.9 5.3 4.0

2.4 3.1 9.0 12.3 5.8 4.2

2.5 3.8 11.7 14.9 6.9 5.4

3.3 4.3 14.4 17.2 7.8 6.5

Table 3 – Gas permeabilities (Barrer)a of uncrosslinked and thermally crosslinked PMP membranes at 35 8C and feedpressure of 2.0 bar

Crosslinking agent Before crosslinking After crosslinking

Azide wt% azide N2 O2 H2 CH4 CO2 N2 O2 H2 CH4 CO2

Noneb 0 950 1780 3970 1790 6700 890 1720 3820 1650 6640

BAA 1.0 810 1570 3680 1530 6230 610 1510 3620 980 5780

2.0 750 1530 3650 1420 6190 540 1470 3560 810 5310

3.0 680 1440 3520 1180 5820 460 1390 3510 720 4920

HFBAA 1.1 790 1580 3660 1500 6250 570 1490 3640 950 5690

2.0 730 1540 3610 1430 6100 480 1380 3570 770 5210

3.0 690 1420 3490 1070 5710 390 1260 3420 570 4320

a Permeability is in unit of Barrer (1 Barrer = 10�10 cm3(STP) cm cm�2 s�1 cmHg�1).b Thermal treatment at 180 8C for 90 min.

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

Crosslinking agent Selectivity (before crosslinking) Selectivity (after crosslinking)

Azide wt% azide 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 1.9 4.5 7.7 4.1 2.4 2.5 5.9 9.5 5.9 3.7

2.0 2.0 4.9 8.3 4.4 2.6 2.7 6.5 9.8 6.6 4.4

3.0 2.1 5.2 8.6 4.9 3.0 3.0 7.6 10.7 6.8 4.9

HFBAA 1.1 2.0 4.6 7.9 4.2 2.4 2.6 6.3 10.0 6.0 3.8

2.0 2.1 5.0 8.4 4.3 2.5 2.9 7.4 10.9 6.8 4.6

3.0 2.1 5.1 8.3 5.3 3.3 3.2 8.8 11.1 7.6 6.0

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

Fig. 7 – Nitrogen sorption in uncrosslinked PMP and

crosslinked PMP containing 3.0 wt% HFBAA at 35 8C.


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.

4.2. Thermal crosslinking

After crosslinking, the membranes were insoluble in good

solvents for PMP such as cyclohexane and carbon tetra-

chloride. Permeability data in Table 3 show membranes

with different amounts of crosslinker. For all gases consid-

ered, 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 selectiv-

ities 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.

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.

4.3. Solubility

Fig. 7 presents nitrogen sorption isotherms in uncrosslinked

PMP and crosslinked PMP containing 3.0 wt% HFBAA cross-

linker at 35 8C. Within experimental uncertainty, nitrogen

sorption levels in uncrosslinked PMP and crosslinked PMP are

Fig. 8 – Methane sorption in uncrosslinked PMP and

crosslinked PMP containing 3.0 wt% HFBAA at 35 8C.

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.


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 uncros-

slinked 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. 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.

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 photo-

chemically crosslinked PMP/bis(aryl azides) composite mem-

branes 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 perme-

abilities and selectivities comparable or higher than those of

poly(dimethylsiloxane).

5. 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 mem-

branes.

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 cross-

linking due to the loss in FFV. The selectivities of O2/N2, H2/N2,


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 cross-

linked 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, non-

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