DOI: 10.1021/la101809r 14821Langmuir 2010, 26(18), 14821–14829 Published on Web 08/27/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Light-Induced Switching of the Wettability of Novel Asymmetrical

Poly(vinyl alcohol)-co-ethylene Membranes Blended with

Azobenzene Polymers

Bartosz Tylkowski,† Sergio Peris,‡ Marta Giamberini,*,† Ricard Garcia-Valls,†

Jos�e A. Reina,‡ and Joan C. Ronda‡

†Departament d’Enginyeria Quı́mica, Universitat Rovira i Virgili, Av. Paı̈sos Catalans, 26, E-43007 Tarragona,Spain, and ‡Departament de Quı́mica Analı́tica i Quı́mica Org�anica, Universitat Rovira i Virgili, CarrerMarcel.

lı́ Domingo s/n, E-43007 Tarragona, Spain

Received May 6, 2010. Revised Manuscript Received July 1, 2010

Novel composite asymmetrical membranes based on poly(vinyl alcohol)-co-ethylene (EVAL) as the hostmaterial andnew polyethers that contain azobenzene moieties in the side chain were prepared by dry-cast phase inversion afterdissolving the azo polymers in tetrahydrofuran and EVAL in dimethylsulfoxide and subsequently mixing the resultingsolutions. By taking advantage of the proper temperature variation in the oven used for solvent evaporation,asymmetrical membranes that exhibited a dense, crystalline layer on the bottom and a porous, mainly amorphouslayer on the top were obtained. Remarkable changes in the surface morphology and the contact angle with water wereobserved on the top surfaces of the composite membranes. This was ascribed not only to the enhanced concentration ofazo polymer on the top surface but mostly to a conformational change in EVAL induced by the photoisomerization ofthe guest azo groups, as shown by HRMAS 1H NMR. The morphological and structural changes in EVAL could bereversed on exposing the membrane to visible light for 24 h.

1. Introduction

“Smart surfaces” with reversibly switchable wettability haveattracted great interest because of their myriad applications: bio-sensors, biomedical applications, as microfluidic devices, intelligentmembranes, data storage, light-powered molecular machines, andso on.1-4

This reversible switching can be achieved through an externallyapplied stimulus such as light irradiation,5,6 electrical potential,7-9

temperature,10 solvent,11 andpH.12Among these external stimuli-responsivematerials, azobenzene and its derivatives are known toexhibit large changes in both geometry and dipole moment as aresult of UV/visible irradiation because of reversible photoisome-rization between the Z and E configurations, which means

that the wettability of azobenzene-modified surfaces can betriggered by means of UV/visible irradiation.2,13

Although the wettability of azobenzene-based materials hasattracted a great amount of attention, the reported change in thewater contact angle (CA) of membranes containing azobenzenegroups is limited to about 10� in the case of smooth, flat surfaces.6,14

However, much larger reversible CA changes were obtained onirradiation when a photoresponsive azobenzene monolayer wasprepared on a rough substrate14 or on an inverse opal.15 In thesecases, the enhanced hydrophilicity results from the combined effectof changes in surface chemistry and surface geometry. Recently,Lim et al. have presented a method for the fabrication of a wettingsurface that can be photoswitched from superhydrophobicto superhydrophilic, which combines layer-by-layer assemblyand the introduction of photoresponsive moieties onto the topsurface.2

In industrial applications, symmetrical membranes have beenalmost completely displaced by asymmetrical membranes, whichhave much higher fluxes; asymmetrical membranes have a thin,finely microporous or dense permselective layer supported on amore open porous structure. The transport rate of a membrane isinversely proportional to its thickness; therefore, the membraneshould be as thin as possible in order to achieve a high transportrate. In the case of asymmetrical membranes, the surface layerexclusively determines the separation properties and permeationrate of the membrane and the substructure acts as a mechanicalsupport for the very thin upper layer. Therefore, all commercial

*Towhom correspondence should be addressed. E-mail: marta.giamberini@urv.net. Tel: þ34 977558174. Fax: þ34 977559621.(1) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samor�i, P.;

Mayor, M.; Rampi, M. A. Angew. Chem. 2008, 47, 3407–3409.(2) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. J. Am. Chem. Soc. 2006,

128, 14458–14459.(3) Weidner, T.; Bretthauer, F.; Ballav, N.; Motschmann, H.; Orendi, H.;

Bruhn, C.; Siemeling, U.; Zharnikov, M. Langmuir 2008, 24, 11691–11700.(4) Usov, D.; Nitschke, M.; Chitry, V.; Ulbrich, K.; Minko, S.; Stamm, M.

PMSE Prepr. 2004, 90, 622–623.(5) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624–1626.(6) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996,

12, 5838–5844.(7) Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L.

J. Am. Chem. Soc. 2006, 128, 14446.(8) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.;

Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371–374.(9) Mativetsky, J. M.; Pace, G.; Elbing, M.; Rampi, M. A.; Mayor, M.; Samor�i,

P. J. Am. Chem. Soc. 2008, 130, 9192–9193.(10) Koc-er, A.; Walko,M.;Meijberg, W.; Feringa, B. L. Science 2005, 309, 755–

758.(11) Minko, S.;Muller,M.;Motornov,M.; Nitschke,M.; Grundke, K.; Stamm,

M. J. Am. Chem. Soc. 2003, 125, 3896.(12) Matthews, J. R.; Tuncel, D.; Jacobs, R. M. J.; Bain, C. D.; Anderson, H. L.

J. Am. Chem. Soc. 2003, 125, 6428–6433.

(13) Ding, L.; Russel, T. P. Macromolecules 2007, 40, 2267–2270.(14) Jiang, W.; Wang, G.; He, Y.; Wang, X.; An, Y.; Song, Y.; Jiang, L. Chem.

Commun. 2005, 3550–3552.(15) Ge, H.; Wang, G.; He, Y.; Wang, X.; Song, Y.; Jiang, L.; Zhu, D.

ChemPhysChem 2006, 7, 575–578.

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Article Tylkowski et al.

processes use asymmetricalmembranes because of the advantagesof the higher fluxes provided.16,17

In this article, we report a new method of preparing poly(vinylalcohol)-co-ethylene (EVAL) asymmetrical composite mem-branes blended with azobenzene polymers using a dry-castprocess in which the introduction of rough structures onto thesubstrate surface and the presence of a photosensitive moietyresult in a large change in wettability after UV irradiation. In ourexperiment, we used two types of polymers (amorphous azo-P1 orazo-P2 polymers synthesized by us18 (Figure 1) and semicrystal-line EVAL19) and two different solvents (dimethylsulfoxide(DMSO) and tetrahydrofuran (THF)). EVAL was chosen as ahost material because the structure of the membrane depends onthe applied temperature used in the preparation (as will beexplained later), and this allowed us to obtain different morpho-logies in the final film. Azo-P1 and azo-P2, which possess apolyether main chain, were chosen after considering their ex-pected compatibility with EVAL: they are polar polymers bearingside azo groups with different polarities. Our aim was to test howthe different polarities of the side group influenced themembranecharacteristics after photoirradiation. The azo polymers weredissolved in THF solvent, and EVAL was dissolved in DMSO.In the typical theoretical, dry phase-inversion process, which isnot covered very much in the literature,20 the polymer wasdissolved in a mixture of a volatile solvent and a less-volatilenonsolvent. In the present work, the blended (guest) azo polymerswere dissolved in a highly volatile solvent (THF), which in turn isa nonsolvent forEVAL (solution 1). The hostEVALpolymerwasdissolved in a less-volatile solvent (DMSO), which in turn is anonsolvent for the azo polymers (solution 2). The casting solutionwas finally obtained by mixing solutions 1 and 2.

The dry-cast process is characterized by the evaporation ofsolvent from a homogeneous polymer solution. Generally, thecomplete evaporation of a polymer solution consisting only of apolymer and a single solvent will result in a dense or nearly densemembrane. However, the dry phase-inversion technique offersseveral advantages compared to the traditional wet immersion

precipitation process because it is a simple method and there is noneed to use a coagulation bath; in fact, the polymer is dissolved ina mixture of a volatile solvent and a less-volatile nonsolvent. Inthis case, membranes with a porous substructure and a dense skinwill be formed.20-22

Young et al.23 and Luo et al.24 prepared EVALmembranes bya dry-casting process, and they proved that the membranemorphology was strongly dependent on the evaporation tem-perature. To explain the membrane-formation mechanism, thestarting point23 is a polymeric homogeneous solution inwhich thepolymer molecules relax and behave like expanded coils. As thesolvent is evaporated from the solution, the polymer concentra-tion is increased; finally, polymer coils collapse and give rise to agelled casting solution. In the case of crystalline polymers, whenthe solution becomes supersaturated with respect to crystalliza-tion, polymermolecules can crystallize from the solution.When asolvent is evaporated, the gelation of a crystalline polymer cantherefore occur either by crystallization or by vitrification. Crys-tallization is an equilibrium state only; the transition from arandomly coiled configuration in the liquid state to a 3D orderedconfiguration in the solid state is a kinetically hindered processbecause of the time needed for the orientation of the polymermolecule both for nucleus formation and growth. It is possiblethat the induction time is not sufficient for crystal nuclei to formor to grow when the composition change during membraneformation is too fast. Consequently, upon rapid elimination ofsolvent, polymer molecules may directly vitrify. In this situation,the membrane structure from crystallizable polymers is the sameas that from amorphous ones.

Luo et al.24 obtained dense, porous EVALmembrane by usingan EVAL/isopropanol/water system and different membranecasting temperatures. The crystalline EVAL polymer melts ataround 165 �C, the equilibrium crystallization temperature of thepolymer that precipitated from the 15%polymer solution is about35 �C, and the glass-transition temperature of amorphous EVALis 55 �C. When the membrane preparation temperature lies nearthe glass-transition temperature of pure EVAL, vitrification canoccur for the amorphous portion in the concentrated polymersolution system. Luo and co-workers proved that by decreasingthe temperature of solvent evaporation from 60 to 45 �C themembrane crystallinity decreased.

2. Experimental Part

2.1. Materials. Azo-P1 and azo-P2 polymers were synthe-sized by a reported procedure18 and had molecular weights Mn

about 4000; their structures are reported inFigure 1. In the case ofazo-P1, a glass-transition temperature of 58 �Cwas found; azo-P2exhibited a glass-transition temperature of 88 �C and a nematicphase up to 182 �C.

Poly(vinyl alcohol)-co-ethylene copolymer (ethylene content44 mol %, mp 165 �C) was supplied by Sigma and used asreceived. Tetrahydrofuran (synthesis grade, THF) and dimethylsulfoxide (reagent grade, DMSO) were supplied by Scharlau andused without further purification.

2.2. Membrane Preparation. All azo-containing solutionsandazo-containingmembraneswere preparedunder red light andwere kept in dark bottles or a dark box to ensure that the azomoieties were in E configurations.

Figure 1. Molecular formula of azo-P1 and azo-P2.

(16) Prasad, K. Downstream Process Technology: A New Horizon in Biotech-nology; PHI: New Delhi, 2010; p 249.(17) Baker, R. W. Membrane Separation Systems: Recent Developments and

Future Directions; Noyes Data Corporation: Park Ridge, NJ, 1991; p 109.(18) Peris, S.; Tylkowski, B.; Ronda, J. C.; Garcia-Valls, R.; Reina, J. A.;

Giamberini, M. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5426–5436.(19) Bakr, N. A.; Ishra, M. Polym. Test. 2002, 21, 571–576.(20) Jansen, J. C.; Macchione, M.; Drioli, E. J. Membr. Sci. 2005, 255, 167–180.

(21) Shojaie, S. S.; Krantzand, W. B.; Greeberg, A. R. J. Membr. Sci. 1994, 94,255–280.

(22) Shojaie, S. S.; Krantzand, W. B.; Greeberg, A. R. J. Membr. Sci. 1994, 94,281–298.

(23) Young, T.-H.; Huang, J.-H.; Chuang,W.-Y.Eur. Polym. J. 2002, 38, 63–72.(24) Luo, R.-L.; Young, T.-H.; Sun, Y.-M. Polymer 2003, 44, 157–166.

DOI: 10.1021/la101809r 14823Langmuir 2010, 26(18), 14821–14829

Tylkowski et al. Article

In our experiment, the hostmaterial was EVAL and the weightratio between the azo-P1 or azo-P2 solution and the EVALsolution was 1:1. The final compositions of the membranes aregiven in Table 1:

- membrane A: 100 wt % EVAL;- membrane B: 99.94 wt % EVAL þ 0.06 wt % azo-P1;- membrane C: 99.94 wt % EVAL þ 0.06 wt % azo-P2

Membranes were prepared as follows. EVAL was dissolved inDMSOat 70 �C to forma 20wt%polymer solution. The solutionwas stirred at room temperature for 24 h. In the case of B and Ccompositemembranes, eachazopolymerwas separatelydissolvedin THF at room temperature to form a 0.0112 wt % polymersolution. Then, azo-P1 and azo-P2 solutions were mixed sepa-rately with a 20 wt % solution of EVAL, stirred for about 2 h atroom temperature, and then degassed under vacuum for 30 min.Afterwards, the solutions were cast on Petri dishes with a uniformthickness to prepare films. The solvents were evaporated in afurnace (Figure 2) for 48 h. After evaporation, the dry films wereremoved from the Petri dishes and kept in a dark box. In the caseofmembraneA, a 20wt%solutionofEVALwasmixedwith pureTHF in a 1:1 ratio and the film was prepared as previouslydescribed.

To obtain asymmetrical cross sections of the membranes, weused a generally known principle of an oven: the temperature ofthe panel heater in the furnace adjusted to temperature x is a fewdegrees higher with respect to the temperature prevalent at itscenter. Temperature measurements carried out at differentheights from the panel heater, in the furnace, confirmed ourassumptions: when the furnace temperature was set to 40 �C, itsvalue on the surface of the panel heater of the furnace turned outto be 46 �C and was 8 �C higher than in the center of the furnace(Figure 2).

The effect of temperature on crystallinity and the consequentmorphology of EVAL membranes were verified by preparing anadditional membrane (D) according to the same procedure asused for membrane A but by evaporating the solvent at a settemperature of 65 �C.2.3. Characterization. Sample irradiation was carried out

with a Vl-4.LC UV Vilber Lourmat lamp (France;-230 V, 8 W)at 365 nm. During the experiment, the membrane temperaturewas monitored at three different points by means of a type-Kthermocouple. In previous investigations,18 we established that

light intensity was sufficient to achieve the photostationary statein 15 min. Back-isomerization of samples was achieved byexposure to visible light at λ > 420 nm (The Lamp Company,18 W, model F18DBX/840) for 24 h.

The cross sections and the surface morphologies of the EVALor EVAL-azo containing membranes were characterized by en-vironmental scanning electron microscopy (ESEM, Quanta 600,FEI)). Cross sections were prepared by fracturing in liquidnitrogen the membranes that were previously wet in ethanol.

The pore sizewas calculated bymeans of IFME software25 andthe ESEMmicrographs of the membrane cross sections.

The changes in surface morphology of the membranes beforeand after UV irradiation and after subsequent exposure to visiblelight were detected by atomic force microscopy.

Thermal transitions were detected with a Mettler-Toledo dif-ferential scanning calorimeter (DSC, model 822) in dynamicmode at a heating or cooling rate of 10 �C/min. Nitrogen wasused as the purge gas. The calorimeter was calibrated with anindium standard (heat flow calibration) and an indium-lead-zinc standard (temperature calibration).

X-ray diffraction experiments were performed with a SiemensD5000 diffractometer in the θ-θ configuration. CuKR radiationwas used, and graphite was the second monochromator. TheBragg angle step was 0.05�, and the time per step was 3 s. TheX-ray diffractograms were analyzed using the fundamental para-meters approach convolution algorithm implemented in theprogram TOPAS 3.0. The crystallite size was calculated fromthe net integral breadthof the peaks,βi, according to the followingequation derived from the Scherrer expression: βi =

λ/ε cos ϑ,where λ is theX-raywavelength, ε is the crystallite size, andθ is theBragg angle.

Contact angles of water drops on a membrane surface weremeasured with Kruss D10 apparatus. The 1.5-2.0-mm-diameterdrops of water, dripped from a microsyringe, were placed on themembrane at room temperature. The contact angle wasmeasuredimmediately after putting the water drop on the membrane sur-face. Measurements were repeated using different areas of thematerial: for each test reported, at least five drops of water wereused. Themembraneswere immersed in acetone for 10min beforeuse, washed in an ultrasonic bath for 10 min, and then dried for24 h at 25 �C.

HRMAS spectra were recorded on a Bruker Avance 500spectrometer operating at a proton frequency of 500.13 MHz.The instrument was equipped with a 4 mm triple-resonance (1H,13C, and 31P) gradient HRMAS probe. A Bruker cooling unit(BCU) was used to keep the sample temperature at 30 �C. For allNMR experiments, samples were spun at 6 kHz to keep therotation sidebands out of the spectral region of interest. One-dimensional (1D) 1H spectra were acquired in 16 scans with arecycle delay of 3 s. The amount of sample used was approxi-mately 16mg. The sample was introduced into a 4mmZrO2 rotorfittedwith a 50μL cylindrical insert. A total of 20μLofDMSO-d6was then added to the rotor to provide a lock frequency for thenuclear magnetic resonance (NMR) spectrometer. To measurespin-lattice relaxation times (T1H), the standard saturationinverse-recovery pulse sequence was used. Thirty-two variabledelays were used in the range of 10 ms-15 s; 16 scans/spectrum

Table 1. Chemical Composition, Thickness, and Contact Angles of Membranes A-C

composition contact angle (CA) (deg)

membrane

EVAL

(wt %)

guest

(wt %)amembrane

thickness (μm)

CA of the top porous

surfaces before irradiation

CA of the top porous

surfaces after irradiation

CA of the bottom dense

surfaces before irradiation

CA of the bottom dense

surfaces after irradiation

A 100 274 ( 5 48.1 ( 1.2 48.1 ( 0.7 44.4 ( 0.9 44.1 ( 1.0

B 99.94 0.06 254 ( 5 62.0 ( 0.7 36.1 ( 0.8 56.9 ( 0.5 54.4 ( 0.6

C 99.94 0.06 256 ( 5 62.1 ( 0.9 36.1 ( 0.4 56.2 ( 1.0 54.6 ( 0.3aMembrane B: azo-P1; membrane C: azo-P2.

Figure 2. Thermal device used for asymmetrical membrane pre-paration. In the inset, the asymmetrical morphology of the result-ing membrane is shown.

(25) Torras, C.; Garcia-Valls, R. J. Membr. Sci. 2004, 233, 119–127.

14824 DOI: 10.1021/la101809r Langmuir 2010, 26(18), 14821–14829

Article Tylkowski et al.

with a d1 of 20 s and an acquisition time of 2.15 s were performed.The following equation was used to curve fit the magnetizationrecovery26

lnðM0 -MðτÞÞ ¼ ln 2þ lnM0 -τ

T1

whereτ is thedecay timeof theexperimentandM(τ)=-M0 atτ=0.If relaxation was due to a single component, then the experi-

mental data resulted in a straight line; if this was not the case, thenmulticomponent analysis by a computer-aided nonlinear least-squares method had to be performed.

The thickness of the membranes was measured using a micro-meter with a sensitivity of 2 μm. The measurements were carriedout at various points on the membrane and put into evidence aconstant membrane thickness.

3. Results and Discussion

3.1. Membrane Morphology. One of the aims of our workwas to obtain an asymmetrical membrane based on a commercialpolymer blended with polyethers containing azobenzene moietiesthat we synthesized in our previous research.18 Because of goodcompatibility with the azo polymers in our synthesis, the highsolubility in DMSO at 60 �C, and the relatively similar values ofcrystallization and vitrification temperatures (45 and about 40 �C,respectively), the EVAL polymer was chosen as the matrixpolymer in our research.19,27-29

The first indication of the successful blending of azo polymers(azo-P1 and azo-P2) into the EVAL structure was the change inthe color of the membranes from white (membrane A) to light-yellow homogeneously colored membranes B and C. From thethickness data reported inTable 1, it can be clearly concluded thatthe membrane thickness is not affected by the very small addi-tional number of azobenzene polymers blended into the EVALstructure.

Figures 3 and 4 show the results of the ESEM observationof the morphologies of the cross sections of membranes A(Figure 3a), D (Figure 3b), B (Figure 4a), and C (Figure 4b),respectively. From the comparison of membranes A, B, C, and D(Figures 3 and 4), itwas observed that all of themembranes cast at40 �C exhibited a typical skinned asymmetrical structure; on thecontrary, membrane D, prepared at 65 �C, showed the expecteddense structure. Moreover, by comparing the asymmetricalmorphologies of membranes A, B, and C, it can be concludedthat the cross section of the membranes is not significantly

dependent on the additional number of azobenzene polymersblended into EVAL. Further inspection of the ESEM imagesreveals that the left side (i.e, the membrane surfaces which was inthe contact with air, Figure 5) and the central part of the crosssection show a structure formed by a very uniform array of smallcavities and the right side of the cross section (i.e, the area near thesurface of themembrane that was in contact with the Petri dish) ischaracterized by a structure with a dense layer beneath the outersurface layer. The pore mean size of the membranes was calcu-lated using IFME25 software and yielded values of 0.984( 0.003μm for membrane A, 1.104 ( 0.005 μm for membrane B, and0.600 ( 0.001 μm for membrane C. With the same program, themembrane asymmetrywas calculated tobe 30% formembraneA,10% for membrane B, and 47% for membrane C. The higherasymmetry of membrane C can be attributed to the more regular,less porous top surface of this membrane (Figure 5c); this in turncould be due to the higher polarity of polymer azo-P2with respectto that of azo-P1, which determines a different value of theFlory-Huggins interaction parameter with DMSO.30

Figure 6 displays the X-ray diffraction patterns of the differentmembranes: dense membrane D prepared at 65 �C (O); mem-branes A (a), B (b), and C (c); neat azo-P1 (d); and azo-P2 (e)prepared at a set temperature of 40 �C. The first pattern of densefilmD (O) prepared at 65 �C presents a sharp peak at 2θ=21.4�and another peak at 2θ=23.7� that corresponds to the crystallinestructure of the EVAL membrane;19 as far as membrane A (a) isconcerned, XRD analysis showed two broad peaks with very lowintensity at 2θ= 20.2 and 22.0�, which indicate a lower extent ofcrystallinity. As a matter of fact, the range of order calculatedfrom the main peak, which is at 2θ = 21.4� in the case ofmembrane D and at 2θ= 20.2� in the case of membrane A, gaveextents of order of 13.02 and 4.28 nm, respectively. As far asmembranes B andC are concerned (Figure 5b,c, respectively), thepresence of such small amounts of amorphous polymers azo-P1and azo-P2 did not seem to affect the behavior of EVALconsiderably. Actually, when membranes A-C were compared,no significant variations could be found in their X-ray patterns,which exhibited the same peaks at 2θ ≈ 20 and 22�.

This evidence was also confirmed by DSC analysis: as can benoticed from the data reported in Table 2, membrane A ischaracterized by a higher change in heat capacity (ΔCp) at theglass-transition temperature, a lower melting enthalpy, and ahigher melting temperature with respect to the same parametersfor membrane D. This suggests that porous membrane A ismainly amorphous;31,32 however, the crystallites that are formed

Figure 3. ESEMmicrographs of the cross sections of membranesA (a) and D (b).

Figure 4. ESEMmicrographs of the cross sections of membranesB (a) and C (b).

(26) Bovey, F. A. Mirau, P. A. NMR of Polymers; Academic Press: San Diego,1996; p 79.(27) Chang, K. Y.; Chen, L.W.; Young, T. H.; Hsieh, K.H. J. Polym. Res. 2007,

14, 229–243.(28) Lai, P. S.; Shieh,M. J.; Pai, C. L.;Wang, C. Y.; Young, T.H. J.Membr. Sci.

2006, 275, 89–96.(29) Lima, J. A. d.; Felisverti, M. I. Eur. Polym. J. 2008, 44, 1140–1148.

(30) Pitol-Filho, L. Thermodynamic Studies and Applications of PolymericMembranes to Fuel Cells and Microcapsules. Ph.D. Thesis, Universitat Rovira iVirgili, Tarragona, Spain, 2007

(31) Menczel, J. D.; Jaffe, M. J. Therm. Anal. Calorim. 2007, 89, 357–362.(32) Hong, P.-D.; Chuang, W.-T.; Yeh, W.-J.; Lin, T.-L. Polymer 2002, 43,

6879–6886.

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Tylkowski et al. Article

in the case ofmembraneD, obtained at 65 �C, are smaller becauseof the faster evaporation of DMSO at this temperature. As far asmembranes B and C are concerned, they exhibited very similarcalorimetric features to membrane A, thus confirming that thepresence of a small number of azo polymers did not alter theEVAL structure significantly (Table 2).

Considering the results reported by Young and Luo,23,24 wepropose the following mechanisms for membrane A-C formation:

(a) During the casting of membrane A in the furnacefrom the THF/DMSO/EVAL system, two indepen-dent processes occurred:

• crystallization of the solution that was in contactwith a Petri dish on a panel heater (at 46 �C),giving rise to a dense, crystalline membranebottom surface and

• vitrification of the solution that was in contactwith the air in the furnace (at 38 �C),which at firstgives rise to a plasticized structure with bonded

THF and DMSO molecules and subsequentlyturns into a vitrified porous amorphous structureafter solvent evaporation.

(b) During the casting of blended membranes B and Cin the furnace from the THF/DMSO/EVAL/azopolymer system, the crystallization and vitrificationprocesses occurred together as in mechanism a;however, because of the faster evaporation of THF(which is a solvent for azo polymers) from the systemand the consequent increase in the concentration ofDMSO (which in turn is a nonsolvent for azopolymers), azo-P1 and azo-P2 begin to precipitateinto the DMSO-rich phase, thus forming compositemembranes B and C.

(c) In the case of membrane D, which was prepared at65 �C, which is higher than the EVAL vitrificationtemperature, only a dense, crystalline structure couldbe obtained, in agreement with the literature.23,24

3.2. Membrane Interaction with UV Light. Morphologi-cal changes in azo composite membranes B and C by photoi-somerization were monitored using atomic force microscopy(AFM). Figures 7-9 show the AFM images of the top surfacesof EVALmembraneA and compositemembranesB andCbefore(image a) and after (image b) irradiation with a UV lamp at 364nm for 15min. The temperature of themembraneswasmonitoredduring the photoirradiation experiment, and it was found to varyas much as 1( 0.1 �C. Before irradiation, the film surface of eachmembrane was quite smooth. As expected, the morphology ofneat EVAL membrane A (Figure 7) was not changed afterirradiation. However, when composite films B and C wereirradiated with UV light, the morphology of the top surfaceswas changed drastically, as can be seen in Figures 8 and 9. Anumber of hills on the surfaces can be put into evidence, whichsuggest the formation of ordered structures as a consequence of

Figure 5. ESEMmicrographs of the top surfaces of membranes A (a), B (b), and C (c).

Figure 6. X-ray diffraction pattern at room temperature formem-brane D (O), membrane A (a), membrane B (b), membrane C (c),azo-P1 (d), and azo-P2 (e).

Table 2. Glass-Transition Temperatures (Tg), Heat Capacity

Changes (ΔCp) at Tg, Melting Temperatures (Tm), and Melting

Enthalpies (ΔHm) of Membranes A-D

membrane Tg (�C) ΔCp (J/g �C) Tm (�C) ΔHm (J/g)

A 52 1.5 163.5 57.3D 54 0.20 111.0 81.6B 47 1.0 165.0 57.9C 49 1.6 163.0 62.1Ba 38 0.98 163.6 66.0Ca 37.5 0.69 166.0 58.7

aAfter photoirradiation with UV light.

Figure 7. AFM images of EVAL membrane A before irradiation(a) and after irradiation (b) with UV light. The scanned areas are5 μm � 5 μm.

14826 DOI: 10.1021/la101809r Langmuir 2010, 26(18), 14821–14829

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irradiationwithUV light. The rms roughness changed from4.186( 0.012 nmbefore irradiation to 41.98( 0.12 nmafter irradiationin the case of membrane B and from 3.588 ( 0.010 to 23.592 (0.067 nm in the case of membrane C. Similar morphologicalchanges were reported for the Langmuir-Blodgett films ofazobenzene tethered to a polymer33 and cyanine dyes.34However,no sharply outlinedmodifications on the dense bottomsurfaces ofthe composite membranes after direct irradiation were observed.This suggests that the concentration of blended azo polymersdecreases from the top to the bottom surface of the compositemembranes.

The wettability of EVAL membrane A and azo compositemembranes B and C was studied by measuring the contact angle(CA), and the results are collected in Table 1.

As shown in Table 1, the values of the contact angle ofmembrane A, measured both on the top and the bottom surfaces,did not change after exposure toUV light (364 nm) for 15min andwere found to be about 48�. As far as composite membranes Band C are concerned, the results of the CA on the bottom and topsurfaces were different: for membrane B, the CA was 62� on thetop surface and 56� on the bottom surface before irradiation withUV light, and it changed to 36� on the top surface and 54� on thebottom surface, respectively, after irradiation; in the case ofmembrane C, the CA was 62� on the top surface and 56� on thebottom surface before irradiation and was converted to 32 and54�, respectively, after irradiation. Therefore, in contrast tomembrane A, the hydrophilic wettability of the composite mem-branes obtained by blending EVAL with polymers azo-P1 andazo-P2 was considerably enhanced as a result of UV irradiation,but this phenomenon was not observed on the dense bottomsurfaces: these results suggest that the CA decreases as themembrane roughness increases and possibly that the blended

azo polymers aremainly concentrated on the top porous surfaces.The enhanced wettability is strongly dependent not only on theazo polymer concentration but also on the surface roughness. Itwas shown35 that the effect of a roughened surface is to magnifythe wetting properties of a solid: amaterial with a positive wettingtendency, such as the hydrophilic polymer under investigation,EVAL, will wet more readily the rougher its surface. When thesamples are irradiated with UV light, E-to-Z photoisomerizationof azobenzene moieties is induced (Scheme 1). In the E state, theazobenzene molecules have a smaller dipole moment, a lowsurface free energy, and a higher CA with water. The E-to-Zisomerization of azobenzene induced by UV light irradiationleads to a large increase in the dipole moment of this molecule.36

After photoirradiation, a remarkable change in surface geometryis induced not only by the enhanced concentration of azo polymeron the top surface but also by a conformational change in EVAL,as discussed later. Therefore, the wettability enhancement of thecomposite membranes can be reasonably attributed to azoben-zene photoisomerization and to the consequent surface geometrychange.After irradiationwithUV light for 15min, composite filmBwas subsequently exposed to visible light (λ>420 nm) for 24 h:after this treatment, the morphology of the top surface wasrestored (Figure 8c) and the value of the contact angle with waterreturned to 62�. However, no changes in CA or in surfacegeometry could be detected when the irradiated membranes werestored in the dark even for severalmonths or held at 60 �C for 96 hand at 80 �C for 48 h, as described later. We performed severalcycles of UV-visible light irradiation and measured the CAchanges (Figure 10): the results suggested that irradiation withUVor visible light can be used to switch themembranewettabilityreversibly after blending the photoresponsive azobenzene poly-mers into the EVAL membrane structure.

Figure 11 shows the HRMAS 1H spectrum of membrane Bbefore (a) and after (b) UV irradiation and after exposure to

Figure 8. AFM images of composite membrane B before irradiation (a), after irradiation with UV light (b), and after UV irradiationfollowed by 24 h of exposure to visible light (c). The scanned areas are 5 μm � 5 μm.

Figure 9. AFM images of composite membrane C before irradia-tion (a) and after irradiation (b) with UV light. The scanned areasare 5 μm � 5 μm.

Scheme 1. E-Z Isomerization of Azobenzene

(33) Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401–6403.(34) Matsumoto, M.; Terrettaz, S.; Tachibana, H. Adv. Colloid Interface Sci.

2000, 87, 147–164.(35) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994.(36) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915–1925.

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Tylkowski et al. Article

visible light for 24 h (c). No signals due to the presence of azo-P1could be put into evidence, aswas expected as a consequence of itsvery low concentration. The attribution of the different signalswas performed on the basis of literature data on poly(vinylalcohol)37-39 and on the quantitative 1H NMR spectrum ofEVAL in DMSO-d6 solution. We assumed random copolymeriza-tion and calculated the probability of existence of the different regio-and stereosequences in theEVALcopolymerunder investigationand

compared themwith the integrations from the quantitative 1HNMRspectrum inDMSO-d6; we also took into account the shielding effectof the ethylene moiety (E) with respect to the vinyl alcohol moiety(VOH), which is expected to induce the resonance of hydroxyl andmethine protons at progressively higher fields on increasing Econtents in triads and dyads. After these considerations, the peakassignments in themembraneBspectrumbeforeUV irradiationwerethe following:

- Peak 1, 4.74 ppm: hydroxyl protons fromVOH-VOH-VOH with the isotactic (mm) triad configuration.

- Peak 2, 4.55 ppm: hydroxyl protons from VOH-VOH-VOH with the heterotactic (rm and mr) triad

Figure 10. Reversible wettability transitions of membrane B after several cycles of irradiation: 15 min at λ = 354 nm followed by 24 hat λ>420 nm.

Figure 11. HRMAS 1H spectrum of membrane B before (a) and after (b) UV irradiation and after UV irradiation followed by 24 hof exposure to visible light (c).

(37) Masuda, K.; Kaji, H.; Horii, F. Polym. J. 2001, 33, 190–198.(38) Brewer, S. A.; Apperley, D. C.; Stone, C. A. Chem. Mater. 2008, 20, 287–293.(39) Moritani, T.; Kuruma, I.; Shibatani, K.; Fujiwara, Y. Macromolecules

1972, 5, 577–580.

14828 DOI: 10.1021/la101809r Langmuir 2010, 26(18), 14821–14829

Article Tylkowski et al.

configuration; VOH-VOH-E and E-VOH-VOHwith the m dyad configuration.

- Peak 3, 4.30 ppm: hydroxyl protons from VOH-VOH-VOH with the syndiotactic (rr) triad config-uration; VOH-VOH-E and E-VOH-VOH withthe r dyad configuration; the E-VOH-E triad.

- Peak 4, 3.83-3.9 ppm: methine protons fromVOH-VOH-VOH triads.

- Peak 5, 3.59 ppm: methine protons from E-VOH-VOH and VOH-VOH-E triads.

- Peak 6, 3.35 ppm: methine protons from E-VOH-Etriads.

- Peak 7, 1-1.6 ppm: methylenic protons.

Irradiation of membrane B with UV light induced a change inthe conformation of host copolymer EVAL, as indicated by thechanges in the 1H NMR spectrum (Figure 11b). The mostremarkable evidence was the inversion between the relativeintensities of peaks 4 and 5; furthermore, a downfield shift wasobserved for peaks 1-3. This indicates a deshielding effect on thehydroxyl protons that can be due to an increase in the number orstrength of the hydrogen bonds that they formed. As far as T1H

relaxation times are concerned (Table 3), in most cases UVirradiation induced the appearance of a second T1H componentand slowed down the relaxation of both hydroxyl and methineprotons. The effect was especially relevant in the case of peaks 1(hydroxyl protons from VOH-VOH-VOH with the isotactic(mm) triad configuration) and 4 (methine protons fromVOH-VOH-VOH triads). If we assume a planar zigzag con-formation for the copolymer backbone,40 then the hydroxylgroups of isotactic VOH-VOH-VOH triads can form one ortwo intramolecular hydrogen bonds (Scheme 2) because of theshorter O-O separation. In the case of syndiotactic triads, theO-O separation is longer, and this precludes intramolecular bondformation between neighboring hydroxyls, as is also the case forVOH-VOH-E and E-VOH-VOH moieties with the r dyadconfiguration and E-VOH-E moieties. One intramolecularhydrogen bond can be formed in the case of hydroxyl protonsfrom VOH-VOH-VOH moieties with the heterotactic triadconfiguration andVOH-VOH-E andE-VOH-VOHmoieitieswith the m dyad configuration. Therefore, the NMR resultssuggested that, after UV irradiation, in the host EVAL copolymerthe number of hydrogen bonds is increased; this is probably due to

an increase in the number of intermolecular hydrogen bonds thatpartially occur at the expense of intramolecular hydrogen bondsand drives the copolymer toward a conformation with a lowerchain mobility.

The observed change in surface morphology is thereforeinduced not only by the enhanced concentration of azo polymeron the top surface but mostly by the conformational change inEVAL caused by photoirradiation, as shown by NMR experi-mental evidence. Because this effect was not observed in the neatEVAL copolymer after photoirradiation performed in the ab-sence of azo-P1 and azo-P2, it must be a consequence of thepresence of the azo polymers, even at such a low concentration.When the side azo groups are converted into Z isomers, thenitrogen atoms are more easily accessible to the hydroxylichydrogens (Scheme 1): this could facilitate the formation ofhydrogen bonds between the lateral groups of the azo polymersand the EVAL copolymer chains. This in turn could increase thenumber of intermolecular hydrogen bonds betweenEVAL chainsbecause of a domino effect. As observed byAFM (Figures 8b and9b), a much more uniform needle structure was obtained in thecase of membrane C after photoirradiation: this can be explainedby taking into account the presence of cyano groups in the sidechain of azo-P2; they can establish hydrogen bonds with EVALand therefore enhance the magnitude of this domino effect.

The Z form of azobenzene can be converted back to the Eisomer by thermally driven back-isomerization or by irradiatingthe solution with visible light.41 For this reason, after UVirradiation, membrane B was held at 60 �C for 96 h and at 80 �Cfor 48 h in order to verify whether under these conditions thethermal Z-E isomerization of azo-P1 and a subsequent relaxa-tion of EVAL toward the original conformation could be in-duced: in both cases, no change in EVAL structure could bedetected by NMR. Actually, previous investigations showed thatthe half-life for the thermal Z-E isomerization process of azo-P1lies between 2.6 � 103 and 0.4 � 103 s when the experiment isperformed in DMSO solution between 60 and 80 �C.18 Further-more, when membrane B was stored in the dark at roomtemperature even for several months, no change in EVALstructure occurred.However, the original conformation ofEVALwas restored when the membrane was exposed to visible light(λ > 420 nm) for 24 h, as shown by NMR (Figure 11c): thisconfirmed that the changes in the morphology and in the contactangle are directly related to a conformational change in EVAL.

4. Conclusions

In this article, we prepared novel composite asymmetricalmembranes based on EVAL as a host material and concentra-tions as low as 0.06 wt % new polyethers that containedazobenzene moieties in the side chain. The membranes wereprepared by dry-cast phase inversion after dissolving the azopolymers in THF and EVAL in DMSO and mixing the resultingsolutions. The two solvents were then evaporated in an oven,where a proper temperature variation allowedus toobtain a densecrystalline layer at the bottom of the membrane and a porous,mainly amorphous layer at the top of the membrane. Atomicforce microscopy experiments put into evidence that when thecomposite membranes were irradiated with UV light the mor-phology of the top surfaces was changed drastically; contact anglemeasurements showed that upon UV-irradiation the contactangle with water decreased about 30� on the top membranesurfaces. The observed changes in the surface morphology and

Table 3. Proton Spin-Lattice Relaxation Times of Membrane B

before and after UV Irradiation

peak T1H (s) before UVa T1H (s) after UVb

1 0.72 1.25; 3.12 0.70 1.14; 1.63 0.70 1.13; 1.74 0.73 1.12; 5.95 0.72 0.97; 2.76 0.69 0.83a(1%. bFaster component, ( 1%; slower component, (10%.

Scheme 2. Intramolecular Hydrogen Bond Formation in

VOH-VOH-VOH Triads of the EVAL Copolymer

(40) Ha, M.-A.; Vietor, R. J.; Jardine, G. D.; Apperley, D. C.; Jarvis, M. C.Phytochemistry 2005, 66, 1817–1824.

(41) Rau, H. Photochromism: Molecules and Systems; Elsevier: New York, 1990;pp 172-173.

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Tylkowski et al. Article

the contact angle are induced not only by the enhanced concen-tration of azo polymer on the top surface but mostly by aconformational change in EVAL caused by photoirradiation, asshown by HRMAS 1H NMR: this was ascribed to the E-Zphotoisomerization of the azo polymers, which in turn couldincrease the number of intermolecular hydrogen bonds betweenEVAL chains. The EVAL conformation achieved after UVirradiation was stable in the dark and also after heating underdifferent conditions, whereas the original conformation, morphol-ogy, and wettability could be restored by the back-isomerizationof the azo polymer on exposing the membrane to visible light for24 h, as shown byNMR, AFM, and contact angle measurements.The observed effect is intriguing and deserves further effort with

the aimof better understanding the phenomenon on themolecularlevel. Furthermore, the possibility of triggering the wettability ofthese membranes by the use of a very small amount of a properadditive opens new insights into thedesignof “smart”membranes.

Acknowledgment. Financial support from the Ministerio deCiencia e Innovaci�on (MAT2008-00456) and the OPEN TokMarie Curie Project (MTKD-CT-2005-030040) “Process andEngineering of Nanoporous Materials” is gratefully acknowl-edged. We are also grateful to Dr. Carles Torras for his help withthe IFME data, Dr. Giulio Malucelli for contact angle measure-ments, and Dr. Miguel �Angel Rodrı́guez and Mr. Ram�onGuerrero for NMR experiments.