Evaluation of photocrosslinkable fluorinated polydimethylsiloxanes as gas permeation membranes

Journal of Membrane Science 217 (2003) 295–298

Evaluation of photocrosslinkable fluorinated polydimethylsiloxanes as gas permeation membranes

L. Abdellah ^a , B. Boutevin ^b , F. Guida-Pietrasanta ^{b ,∗} , M. Smaihi ^c

aFaculté des Sciences, Université Hassan II Ain Chock, Casablanca, Morocco

bLaboratoire de Chimie Macromoléculaire, UMR 5076 CNRS, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France

cInstitut Européen des Membranes (IEM), UMR 5635 CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France Received 3 October 2002; received in revised form 21 March 2003; accepted 21 March 2003

Abstract

Dense permselective membranes were prepared from two series of polydimethylsiloxanes (PDMS) bearing fluorinated groups and acrylic or vinyl ether type photocrosslinkable groups linked to the polysiloxane chain through urethane bridges.

These dense permselective membranes were evaluated for their application as gas permeation membrane for the purification of natural gas. Permeabilities and selectivities towards the CO

/CH

gas pair were measured.

This study showed that the selectivity increased with the introduction of C

F

fluorinated groups and/or acrylic groups.

© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Polysiloxanes; Membranes; Gas permeation; Photocrosslinking

1. Introduction

During the last two decades, gas separation through membrane technology has found a growing interest [1]. For a long time, polydimethylsiloxane (PDMS) rubber –[Si(Me)

O]

– was considered as the more gas permeable polymer, due to its free volume characteris- tics and to the great flexibility of polysiloxane chains.

The relation between gas solubility in silicon poly- mers and the chemical structure of these polymers has been studied by Koros et al. [2] and Stern et al. [3].

Thus, it was shown that the introduction of a fluo- rinated group (CF

CH

CH

)– lateral to the siloxane chain, induces an increase of the CO

solubility co-

∗Corresponding author. Tel.:+33-4-6714-4307;

fax:+33-4-6714-7220.

E-mail address: [email protected] (F. Guida-Pietrasanta).

efficient, due to the specific interaction between the electronegative fluorinated group and the polar CO

molecule [4]. Similarly, the grafting of ester groups on the side of the polydimethylsiloxane chain gives rise to a CO

selectivity increase because of the interaction with carbonyl groups present in esters and ketones [5].

Recently, crosslinked fluorinated PDMS polymers as well as fluoroalkyl grafted PDMS chains have been reported to be highly effective for the separation of volatile organic compounds (VOCs) from air [6,7].

This paper will present gas transport properties of two kinds of photocrosslinkable fluorinated poly- dimethylsiloxanes: either with acrylate or with vinyl ether grafted groups linked to the main chain through a urethane bridge and with fluorinated chains including C

F

or C

F

moieties. Gas permeation measure- ments and selectivities of the membranes obtained after crosslinking of these polymers were evaluated

0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0376-7388(03)00166-2

296 L. Abdellah et al. / Journal of Membrane Science 217 (2003) 295–298

for carbon dioxide (CO

) and methane (CH

). The influence of the polymer and the grafted groups have been highlighted.

2. Materials and methods

The polymers were synthesized by polycon- densation of the commercial ␣ , ␻ -dihydroxy poly- dimethylsiloxane with the suitable dichlorosilane and subsequent reactions [8,9]. The four polymers pre- pared present different photocrosslinkable groups and different proportions of fluorine. Their formulae are given in Table 1.

For vinyl urethanes PDMS, the formulations prepared contain 98% weight of polymer, 4%

dichloromethane or chloroform and 2% of cationic

Table 1

Permeability and selectivity characteristics of the CO2/CH4gas pair

Polysiloxane I/S (%) Fexp(%) Permeability^a Selectivity

␣(CO2/CH4) CO2 CH4

[(CH3)2SiO]_x^b 4553 1353 3.1

[(CH3)(C2H5)SiO]_x^b 1506 474 2.9

[(CF3CH2CH2)(CH3)SiO]_x^b 1388 201 5.6

[(C6H5)(CH3)SiO]_x^b 239 36 6.4

5.1 14.4 175 13.5 12.8

5.1 14 577 27 21

5.1 14.4 1071 245 4.4

7.3 17.6 197 15.6 12.6

I/S: unsaturations number/(Si–O) number.

aPermeability in 10⁻¹⁰cm³cm/cm²s cmHg.

bResults obtained by Stern et al.[3].

photoinitiator (Ph

S

, SbF

). For acrylate ure- thanes PDMS, the formulations prepared contain 92%

weight of polymers, 15–20% dichloro methane or chloroform, 4% darocur 1173, 2% isopropyl thioxan- thone (ITX) and 2% ethyl-4-dimethyl aminobenzoate (EDB).

Formulations containing the polymers and a pho- toinitiator were laid onto a porous support and pho- tocrosslinked under UV to give the polymer network constituting the thin and selective layer. Polyacryloni- trile (PAN) has been chosen as support (surface pore diameter = 0 . 2 ␮ m), because it exhibits a good resis- tance and allows the obtention of layers.

The mixtures were laid onto PAN supports using a

“hand-coater” threaded stick in order to get a 1 ␮ m

thick layer. The deposits were dried at room tem-

perature to let the solvent evaporate and were then

L. Abdellah et al. / Journal of Membrane Science 217 (2003) 295–298 297

crosslinked under UV with a PHILIPS 80 W/cm

lamp, during 3–6 s. The membranes obtained were well dried in order not to stain or stick and to exhibit a good adherence of the layer onto the support.

The morphological characteristics of the films were determined by scanning electron microscopy (SEM) with a HITACHI S-4500 apparatus.

The gas flux and selectivity of the polymers were determined from measurements of pure gas fluxes.

Experiments were performed at room temperature (25

C). The gas permeation area was 10.7 cm

. Two gases (99.9% purity) having various kinetic diam- eters were used during this study: carbon dioxide (3 . 23 × 10

m (3.23 Å)) and methane (3 . 8 × 10

m (3.8 Å)).

3. Results and discussion

The synthesis and properties of the polymers, in particular, thermal and release properties have been already presented elsewhere [10].

Surface and cross-section of the coatings as well as the PAN substrate surface and cross-section have been observed using scanning electron microscopy.

All samples present an homogeneous dense coating at the nanometer scale with no infiltration in the sub- strate. The coating thickness is of a few micrometers.

No cracks or defects have been observed either on the surface nor on the substrate/coating interface.

PAN substrate permeance measurements performed at room temperature give values equal to 1 . 5 × 10

cm

/cm

s cmHg for CO

and CH

. As substrate per- meances are 100–1000 times higher than those mea- sured on the coated samples, the PAN substrate gas transport is negligible compared to the membrane ma- terial and the coated substrate permeance can be con- sidered to be equal to the film permeance.

Table 1 presents permeability and ideal selectivity values obtained at room temperature on the polymers bearing different grafted groups.

These data show that the introduction of a long C

fluorinated chain in both series of vinyl urethane and acrylate membranes results in an increase of the ideal selectivity ␣ (CO

/CH

) respectively from 4.4 to 12.6 and from 12.8 to 21.

Moreover, acrylate urethane membranes are much more selective than vinyl ether urethane ones. Mem-

brane selectivities, when the fluorinated chains con- tain C

F

groups, are respectively equal to 12.8 and 4.4 whereas, when the fluorinated chains con- tain C

F

groups, they are respectively equal to 21 and 12.6. The same tendency is observed for other polymers. Organopolysiloxane membranes contain- ing a fluorinated group showed a selectivity of 5.6 [3], whereas polysiloxanes bearing acrylic functions present ␣ (CO

/CH

) selectivities values ranging be- tween 2.6 and 3.9 [5].

The best combination in terms of permeability and selectivity coefficients is then the acrylate urethane membrane bearing a long C

fluorinated chain.

The diffusion coefficient being inversely propor- tional to the size of the gas molecules, the selectiv- ity of mobility (D

/D

) must be approximately close to 1. Thus, the values presented in Table 1 indi- cate that the differences observed for the permeability values can be much more attributed to differences in solubility coefficients.

4. Conclusion

PDMS membranes containing perfluorinated groups and vinyl ether or acrylate urethane functional groups, lateral or at the chain ends, were prepared and their gas permeation properties were evaluated.

Permeability measurements were carried out on the CO

/CH

gas pair and ideal selectivities ␣ (CO

/CH

) have been determined. The results show that PDMS membranes including acrylate urethane groups exhibit a better ideal selectivity ␣ (CO

/CH

) than vinyl ure- thane PDMS membranes.

On the other hand, the introduction of C

perfluo- rinated groups results in an increase of the selectivity

␣ (CO

/CH

) compared to C

perfluorinated groups.

Acrylate urethane PDMS membranes including a C

fluorinated group exhibit the best gas transport properties with selectivity values [ ␣( CO

/ CH

) = 21]

that are much higher than those reported previously for fluorinated polymers.

References

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