Surface modification of pervaporation membrane by UV-radiation and application of shear stress

1. Institute of Chemical Process Fundamentals, Czech Republic Universidade Nova de Lisboa, Portugal Surface modification of pervaporation membrane by UV-radiation and application of shear stress P. Izák,M. H. Godinho, P. Brogueira, J.L. Figueirinhas, J. G. Crespo

2. Outline • Surface modified PU/PBDO membrane • Characterization of a modified PU/PBDO-UV • Swelling of polymeric membranes • Pervaporation experiment • Impact of results and conclusions

3. The aim • We looked for ways to promote a microtur- bulence to minimizethe concentration polarization at the membrane surface during pervaporation separation process

4. The aim • fine-tuned surface of the dense membrane UV radiation and application of shear stress • different feed flow rate in the pervaporation module (0.34ml/min; e.g. Reynoldsnumber, Re = 2.06×10−4and 2.49 ml/min; e.g. Re = 1.51×10−3); long channel module 0.25m laminar flow

5. Introduction • the major problem associated with the solute recovery by pervaporationa restrictive compromise between selectivity and flux • the urethane–urea elastomers - unique elastic properties of the microphase separation of hard (isocyanate) and soft (polyol) segments into domain structures during material preparation and processing

6. Introduction • the fine-tuned surface of the membrane, prepared from liquid crystalline polyurethaneprimary (stripes) and secondary (bands) set of periodic structures, which are perpendicular to each other • the solute recovery fromionic liquids by pervaporation the solutes permeate the membrane and arerecovered in the permeate, while the solvent (RTIL) does notpermeate due to its extremely low vapour pressure

7. N N N R N R R Cl Reflux 80ºC RT – 24h + Cl - X - 24 h (X _ = PF6_, BF4_ ) N + N + + HX Room Temperature Ionic Liquids (R= methyl group, R= butyl group, R= decyl group), X _ = PF6_, X _ = BF4 _ ) • Non-measurable vapour pressure  Green solvent • High ionic conductivity and thermal stability • Ability to solubilize a large range of organic molecules and transition metal complexes

8. Room Temperature Ionic Liquids • Do not permeate through either organophilic or hydrophilic dense membranes • Possibly an environmentally benign alternative to classical organic solvents • High viscosity and low heat transfer (m25°C= 0.12 Pa.s) • Purification of ionic liquids

9. Transverse direction X Casting direction ≡ 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 + (Toluene) (PBDO) (PU) Y where Shear ( ≈ 47 Å) PU: ≡ PBDO: ( ≈ 263 Å)

10. POM images of a surface modified PU/PBDO-UV dry membrane (a) and of PU/PBDO-UV membranes after 7 days of immersion in binary mixtures of 1% w/w hexyl acetate and [C4mim] [BF4] at T=25ºC (b),T=35ºC (c) and T=45ºC (d).

11. Characterization of a modified PU/PBDO-UV membrane by AFM before (a) and after (b) a pervaporation experiment.Thecross-section = 9 – 10mmThe peak-to-valley height h = 350 - 500 nmThe mean roughness Ra = 118.85 nmThe roughness root mean square(rms) Rq = 170.59 nm.

12. Characterization of a modified PU/PBDO-UVmembrane by POM before (a) and after (b) a pervaporation experiment. Characterization of a modified PU/PBDO-UVmembrane by SEM after a pervaporation experiment (c).

13. Small Angle Light Scattering patterns (SALS) obtained before the pervaporation experiments. SALStechnique = detection of periodic patterns in the membrane interior that cannotbe accessed by AFM.

14. Swelling of polymeric membranes • Transport into the membranetwo ways of mutual affecting of components may be considered: • The free volume effectgenerally increasing the diffusivity of components (i.e. plasticizing effect). • The coupling effect due to remaining interaction among molecules in the polymerincreased or decreased diffusivity of molecules in the membrane (i.e. the interaction effect).

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

16. The accuracy of this optical method better than that of usually used gravimetric methods • To exclude the influence of the liquid meniscus destorsion in Teflon cell, blank experiments with Teflon square (which does not swell) were done for each mixture and pure liquids. • Optical correction was then subtracted from the length and the width of the wet membrane (typically 2 pixels, where 1 pixel is 13 μm).

17. The evaluation of the measurement commercial software Zoner Media Explorer 6 from Olympus, Czech republic. • Dimensions of the membrane in dry and wet state in pixels, the extension was evaluated as a relative change • The absolute error in determining the membrane extension 1 pixel i.e. ± 0.3 % or ± 13 mm for used membrane dimension

18. Membrane swelling equilibrium in hexyl acetate at 25°C

19. Pervaporation experiment • Pervaporation experiments - different feed flow rate in the pervaporation module (0.34 and 2.49 ml/min) and both flat and fine-tuned surfaces of the dense membrane • The whole separation process - monitored by gas chromatography in a classical pervaporation arrangement (FFAP polar capillary column) • Extraction - tridecanwas added as an internal calibration standard

20. Retentate Feed Permeate Reaction vessel Vacuum pump Cold trap Permeate Pervaporation set-up Pervaporation experiment – standard laboratory pervaporation set-up with effective membrane area of 5 cm2 ; downstream pressure p = 60 Pa Permeate samples were analysed by 1H NMR - only hexyl acetate passed through the membrane Thermostat

21. 3D topography image (50 × 50 μm2 scan with an image surface area of 2598 μm2) of the PU/PBDO-UV dense membrane after pervaporation experiment.

22. Effect of surface modification and feed flow rate on the flux of hexyl acetate versus weight fraction of hexyl acetate in the feed

23. Effect of surface modification and feed flow rate on the enrichment factor of hexyl acetate versus time

24. Conclusions • The increase of the feed flow rate in pervaporation module from 0.35 to 2.50 ml/min improved the enrichment factor of hexyl acetate by 15%. • The surface modification of the PU/PBDO dense membrane obtained by UV radiation and application of shear stress increased the enrichment factor by 14%.

25. Conclusions, cont. • The active membrane area increased due to the modification by 4% only we can state that the surface modification of PU/PBDO-UV helps to promote micro-turbulence in the feed boundary layer at the membrane surface this effect also contributes to increase the enrichment factor. • This work demonstrates the potential of using surface modified dense membranes to enhance the pervaporation separation process.

26. Conclusions, cont. • The approach presented may be used in other membrane processes mass transport represents a significant limitation. • These nano-structured surfaces use in nanobio-applications, such as tissue culture and biosensors,where orientation at a molecular/nano-dimension becomes relevant.

27. Conclusions, cont. • Applications for improved external mass transfer extremely interesting both at a nano- and at a micrometer scale mini- misation of concentration polarisation effects – better fluxes and improved selectivity • The use of room temperature ionic liquids is particularly interesting in combination with pervaporation in this case the solvent does not permeate through the dense membrane and therefore it is not lost to the environment high enrichment factors are attained.

28. Acknowledgement • P. Izák would like to acknowledge the post-doc grant (SFRH/BPD/9470/2002) from Fundação para a Ciência e a Tecnologia, Portugal. • This research was supported by the Czech Science Foundation grant No. 104/08/0600. • Thank you for your attention