Swelling kinetic of polymeric membranes in ionic liquids P. Izák, Š. Hovorka, T. Bartovský, L. Bartovská, J. G. Crespo

The aim Possibility to obtain new experimental data, which will contribute to better understanding of the swelling process. Development of an optical technique allowing follow small and even very slow swelling kinetics. Cation - exchange membrane Nafion®, hydrophobic membranes, polyurethane-polybutadiene elastomer (PU/PBDO) and polydimethylsiloxane (PDMS).

Introduction During the transport into the membrane, two ways of mutual affecting of components may be considered: The free volume effect, generally increasing the diffusivity of components (i.e. plasticizing effect). The coupling effect, due to remaining interaction among molecules in the polymer, which can increase or decrease diffusivity of molecules in the membrane (i.e. the interaction effect).

Swelling - balance between the forces where Fmix - force of thermodynamic mixing between the net polymer and the solvent, Fion – forces between immobilized and free ions, Fel – elastic force of the polymer, Fint – inter-chain attractive force The expansion of the polymer is due to the entropic diffusion of its constituent chains and their counterions. On the other hand, swelling is countered by elastic forces in the chain and inter-chain attractive forces.

Experimental Ethyl acetate, ethyl hexanoate, and hexyl acetate were of analytical grade (Fluka, Germany). Room temperature ionic liquids (RTILs): [C4mim] [BF4], [C4mim] [PF6], [C8mim] [PF6] PU/PBDO, PDMS membrane, Nafion® 122

Experimental We looked for ways to promote a micro-turbulence to the binary mixture and thus minimize the concentration polarization at the membrane surface during pervaporation separation process Fine-tuned surface of the dense membrane by UV radiation and application of shear stress The fine-tuned surface of the membrane prepared from liquid crystalline polyurethane shows primary (stripes) and secondary (bands) set of periodic structures, which are perpendicular to each other

Structure of surface modified PU/PBDO blend Controlled shear rate (F = 85 N m-2) was then periodically (at least 30 stretching cycles) applied reaching maximum elongation of 1.2 UV radiated (λ = 254 nm) for 24 hours Solvent + ≡ X Transverse direction Casting direction (Toluene) (PBDO) (PU) Y where Shear ( ≈ 47 Å) PU: PBDO: ≡ ( ≈ 263 Å)

FCT - Universidade Nova de Lisboa Portugal 3D topography image (50 × 50 μm2 scan with a image surface area of 2598 μm2) of the PU/PBDO-UV dense membrane in region 2 after pervaporation experiment. FCT - Universidade Nova de Lisboa Portugal

Apparatus for swelling kinetics of membranes MG SH TV TCS RCT digital camera magnifying glass circle Teflon cell thermostated vessel infra-red remote control and timer stand with holders

Results and discussion Table I. Membrane swelling equilibrium and water saturation of “pure” RTILs at 25°C Table II. Membrane swelling equilibrium of binary mixture [C4mim] [BF4] and hexyl acetate at 25°C

Isotropic swelling kinetics of PU/PBDO membrane in hexyl acetate at 60°C

Results and discussion Table III. Membrane swelling equilibrium of binary mixture [C4mim] [BF4] and hexyl acetate at 60°C Table IV. Membrane swelling equilibrium of binary mixture [C4mim] [BF4] and hexyl acetate at 25°C

FCT - Universidade Nova de Lisboa Portugal Membrane swelling equilibrium with binary mixture [C4mim] [BF4] and hexyl acetate at 25°C FCT - Universidade Nova de Lisboa Portugal

Results and discussion Table V. Membrane swelling equilibrium of binary mixture [C4mim] [BF4] and ethyl hexanoate at 25°C Table VI. Membrane swelling equilibrium of binary mixture [C4mim] [BF4] and ethyl acetate at 25°C

FCT - Universidade Nova de Lisboa Portugal Swelling equilibrium of Nafion in binary mixture H2O and [C4mim] [BF4] at 25°C FCT - Universidade Nova de Lisboa Portugal

Yeager and Steck 3-phase model of Nafion clusters A - fluorocarbon backbone material B - large fractional void volume, which contains side-chain material, a small amount of water, and some free sulfonate exchange sites C - ion clusters, counterions and sorbed water [C4mim]+ [C4mim]+

Stylized view of polar/nonpolar microphase separation in a hydrated ionomer

FCT - Universidade Nova de Lisboa Portugal Anisotropic swelling and deswelling kinetics of Nafion membrane in H2O at 25°C FCT - Universidade Nova de Lisboa Portugal

FCT - Universidade Nova de Lisboa Portugal Anisotropic swelling and deswelling kinetics of Nafion membrane in binary mixture 50% w/w H2O and 50% w/w [C4mim] [BF4] at 25°C FCT - Universidade Nova de Lisboa Portugal

Conclusions Nafion - anisotropic swelling and isotropic swelling of PU/PBDO, and PDMS membranes in all measured mixtures Swelling of Nafion, PU/PBDO, and PDMS membranes is faster in casting than in transverse direction Membrane swelling of all polymers increased with the content of ester in all mixtures (RTIL + ester) Nafion: δhexyl acetate > δethyl hexanoate > δethyl acetate

Conclusions, cont. PU/PBDO, PDMS: δhexyl acetate >δethyl hexanoate >δethyl acetate Swelling equilibrium of Nafion membrane in liquid binary mixture ([C4mim] [BF4] + H2O) clearly shows the maximum in the middle of the concentration range at 25°C This is caused by forming of the larger clusters contains [C4mim]+ and H2O between hydrophobic and ionic parts of Nafion network It was observed only little increase of polymer swelling with temperature

Acknowledgement The financial support of the FEDER through the project grant (POCTI/EQU/35437/1999) is gratefully acknowledged. P. Izák would like to acknowledge the post-doc grant (SFRH/BPD/9470/2002) from Fundação para a Ciência e a Tecnologia, Portugal. The co-authors from Czech Republic acknowledge the financial support from the MSM 2234 00008 grant from Ministry of Education of the Czech Republic.

FCT - Universidade Nova de Lisboa Portugal