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Surface Organization, Light-Driven Surface Changes, and Stability ofSemifluorinated Azobenzene Polymers

Marvin Y. Paik,† Sitaraman Krishnan,† Fengxiang You,† Xuefa Li,† Alexander Hexemer,‡

Yushi Ando,† Seok Ho Kang,† Daniel A. Fischer,⊥ Edward J. Kramer,‡,§ andChristopher K. Ober*,†

Department of Materials Science and Engineering, Cornell UniVersity, Ithaca, New York 14853,Department of Materials and Department of Chemical Engineering, UniVersity of California at Santa

Barbara, Santa Barbara, California 93106, National Institute of Standards and Technology,Gaithersburg, Maryland 20899

ReceiVed NoVember 23, 2006. In Final Form: January 15, 2007

A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-inducedchanges in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group,and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorptionfine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganizationin the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differentialscanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formedwell-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. Thedisruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length.Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistantto light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzenephenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenylrings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.

1. Introduction

The reversible photoisomerization of polymers containing theazobenzene group is an active area of research with applicationsin digital optical data storage, optical signal processing, andoptical switching.1 Irradiation of thin films of these polymerswith light causes repeated trans-cis-trans isomerization cycles(cf. Scheme 1), imparting dichroism and birefringence to thefilms.2,3 Photomechanical effects, resulting from reversiblecontraction and expansion, have also been reported.4-6 Geo-metrical changes of the azobenzene group on the molecular scalecan ultimately result in large-scale mass transport and lead to theformation of surface relief structures.7

Recently, we have studied the self-assembly of a variety ofnonphotoactive, fluorinated, diblock copolymers to form thesurfaces of thin films.8-10 The liquid crystalline semifluorinatedalkyl side chains organized into well-ordered surface layers,

resulting in nonreconstructing, low-energy surfaces. It would bedesirable to develop a material in which one could readily modify

* To whom correspondence should be addressed. Tel: 607-255-8417.Fax: 607-255-2365. E-mail: [email protected].

† Department of Materials Science and Engineering, Cornell University.‡ Department of Materials, University of California at Santa Barbara.§ Department of Chemical Engineering, University of California at Santa

Barbara.⊥ National Institute of Standards and Technology.(1) Natansohn, A.; Rochon, P.Chem. ReV. 2002, 102, 4139 and the references

therein.(2) Oliveira, O. N., Jr.; dos Santos, D. S., Jr.; Balogh, D. T.; Zucolotto, V.;

Mendonca, C. R.AdV. Colloid Interface Sci.2005, 116, 179.(3) Cui, L.; Zhao, Y.; Yavrian, A.; Galstian, T.Macromolecules2003, 36,

8246.(4) Ikeda, T.; Nakano, M.; Yu, Y.; Tsutsumi, O.; Kanazawa, A.AdV. Mater.

2003, 15, 201.(5) Strzegowski, L. A.; Martinez, M. B.; Gowda, D. C.; Urry, D. W.; Tirrell,

D. A. J. Am. Chem. Soc.1994, 116, 813.(6) Labarthet, F. L.Phys. Chem. Chem. Phys.2000, 2, 5154.(7) You, F.; Paik, M. Y.; Ha¨ckel, M.; Kador, L.; Kropp, D.; Schmidt, H.-W.;

Ober, C. K.AdV. Funct. Mater.2006, 16, 1577.

(8) (a) Xiang, M.; Li, X.; Ober, C. K.; Char, K.; Genzer, J.; Sivaniah, E.;Kramer, E. J.; Fischer, D.Macromolecules2000, 33, 6106. (b) Wang, J.; Mao,G.; Ober, C. K.; Kramer, E. J.Macromolecules1997, 30, 1906. (c) Mao, G.;Wang, J.; Scott, R. C.; Ober, C. K.; Chen, J. T.; Thomas, E. L.Macromolecules1997, 30, 2556.

(9) (a) Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang, J.; Ko¨rner, H.; Xiang,M.; Yang, S.; Ober, C. K.; Char, K.; Chaudhury, M. K.; DeKoven, B. M.; Bubeck,R. A.; Fischer, D. A.; Sambasivan, S. InApplications of Synchrotron RadiationTechniques to Materials Science IV, Mini, S. M., Perry, D. L., Stock, S. R.,Terminello, L. J.,Eds.;MRSSymposiumProceedings,Vol. 524;MaterialsResearchSociety: Pittsburgh, PA, 1998; p 365. (b) Genzer, J.; Sivaniah, E.; Kramer, E. J.;Wang, J.; Ko¨rner, H.; Char, K.; Ober, C. K.; DeKoven, B. M.; Bubeck, R. A.;Fischer, D. A.; Sambasivan, S.Langmuir2000, 16, 1993. (c) Genzer, J.; Sivaniah,E.; Kramer, E. J.; Wang, J.; Xiang, M.; Char, K.; Ober, C. K.; Bubeck, R. A.;Fischer, D. A.; Graupe, M.; Colorado, R., Jr.; Shmakova, O. E.; Lee, T. R.Macromolecules2000, 33, 6068. (d) Genzer, J.; Sivaniah, E.; Kramer, E. J.;Wang, J.; Ko¨rner, H.; Xiang, M.; Char, K.; Ober, C. K.; DeKoven, B. M.; Bubeck,R. A.; Chaudhury, M. K.; Sambasivan, S.; Fischer, D. A.Macromolecules2000,33, 1882.

(10) Krishnan, S.; Kwark, Y.-J.; Ober, C. K.Chem. Rec.2004, 4, 315 and thereferences therein.

Scheme 1. Chemical Structure of an AzobenzeneChromophore and the Isomerization between Trans and Cis

Configurations

5110 Langmuir2007,23, 5110-5119

10.1021/la0634138 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/31/2007

and control the organization of molecules at the surface in aspatially defined manner. Such a material with controllable surfaceproperties could broaden the array of applications of photore-sponsive polymers, ranging from reversible optical informationstorage media11-14 to regulating the geometry and function ofbiomolecules.15-19

Prior studies of photoinduced switching of surfaces havemainly dealt with self-assembled monolayers of azobenzenemolecules,20-22 or nonfluorinated polymers.23,24There are onlya few reports on fluorinated azobenzene surfaces. Mo¨ller et al.have investigated the photoswitchable wetting behavior of adiblock copolymer consisting of a poly(2-hydroxyethyl meth-acrylate) block and a polymethacrylate block with 4-trifluo-romethoxyazobenzene side groups.25 Monolayer films of theamphiphilic block copolymer were prepared on glass or siliconwafers using the Langmuir-Blodgett (LB) technique. Surfaceswith patterned wettability could be obtained by exposing themonolayers to UV light through a mask; however, they foundthat photoisomerization of the mesogenic side chains could beobserved only if there was some degree of disorder in the sidechain packing. The packing density of the chromophores in thetrans state was higher compared to the cis state. Hence, the sidechains had to be switched to the cis state in solution before theywere transferred onto the substrate. Feng et al. also observed areversible, photoswitchable wetting behavior of LB films of4-trifluoromethylazobenzene-containing molecules, with up to8° difference in water contact angles in the cis and trans states.26

Knobloch et al. were able to control the alignment of liquidcrystals using polymers with azobenzene side chains as sub-strates.27

In this report, we explore several aspects of photoresponsivepolymers with 4-perfluoroalkylazobenzene side chains. Themolecular orientation and surface properties of these polymersare discussed in relation to their mesophase behavior and surfacestability. We present the details of light-controlled reversiblechanges in molecular configuration at the polymer surface,determined using in situ near-edge X-ray absorption fine structure(NEXAFS) spectroscopy and water contact angle measurements.Finally, we assess the differing effects of UV exposure at thesurface and in the interior of the film and show that the stabilityof the surface was strong enough in some cases to inhibit changesin the surface behavior, even in the presence of intense UV light.

2. Experimental Section

2.1. Materials.All chemicals were purchased from Aldrich andused without further purification, unless noted. Tetrahydrofuran(THF) was dried over calcium hydride for 24 h, degassed on avacuum line, refluxed over sodium/benzophenone and distilled intoa reaction flask for anionic polymerization. The styrene and isoprenemonomers were dried over calcium hydride (CaH2); degassed on avacuum line; stirred with neat dibutylmagnesium; and finally, distilledunder vacuum into airtight reservoirs for immediate use.

2.2. Polymer Synthesis.The procedure for the synthesis of theblock copolymers with semifluorinated azobenzene side chains isshown in Schemes 2 and 3. Polyisoprene (PI) and polystyrene-block-polyisoprene (PS-b-PI) polymers with high vinyl contents (3,4-and 1,2- microstructures) were obtained by anionic polymerizationin THF, a polar solvent, at-78°C. Injection of thesec-butyllithiuminitiator was followed by sequential addition of styrene first, andthen isoprene. Isoprene was directly added to the initiator-containingsolvent during homopolymer synthesis. The living chain ends wereterminated with methanol. The PI block was hydroxylated using thehydroboration-oxidation reaction. The hydroxyl groups in the PIblock were finally reacted with the semifluorinated azobenzenemesogen. The syntheses of PI and PS-b-PI precursor polymers andpolymer analogous reactions on these were performed usingpreviously reported procedures,8 but the preparation of the newsemifluorinated azobenzene mesogens will be elaborated in thisreport. As an example, the synthesis of (4-perfluorohexylazobenzen-4′-oxy)hexanoylchloride [F6, cf. (5) in Scheme 2,n ) 6] mesogensis described. The same procedure was employed to synthesize (4-perfluorobutylazobenzen-4′-oxy)hexanoylchloride (F4) and (4-perfluorooctylazobenzen-4′-oxy)hexanoylchloride (F8).

4-Iodo-4′-hydroxyazobenzene (1).A 16.6-g (75.8 mmol) portionof 4-iodoaniline and 250 mL of 2 M aqueous hydrochloric acid weretaken in a 1-L beaker and heated until all the iodoaniline dissolved.The mixture was cooled to 10°C, and a solution of 13.1 g (189mmol) of sodium nitrite in 50 mL of water was added. After 1 hof stirring at 0-10°C, the mixture turned orange. A solution of 17.8g (189 mmol) of phenol in 250 mL of 1 M aqueous sodium hydroxide

(11) Fukuda, T.Opt. ReV. 2005, 12, 126.(12) Ikeda, T.; Kanazawa, A. InMolecular Switches; Feringa, B. L., Ed.;

Wiley-VCH: Weinheim, 2001; p 363.(13) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H.Makromol. Chem.

Rapid Commun.1987, 8, 59.(14) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S.Macromolecules1992,

25, 2268.(15) Willner, I. Acc. Chem. Res.1997, 30, 347.(16) Ulysse, L.; Cubillos, J.; Chmielewski, J.J. Am. Chem. Soc.1995, 117,

8466.(17) Asanuma, H.; Ito, T.; Yoshida, T.; Liang, X.; Komiyama, M.Angew.

Chem. Int. Ed.1999, 38, 2393.(18) Loweneck, M.; Milbradt, A. G.; Root, C.; Satzger, H.; Zinth, W.; Moroder,

L.; Renner, C.Biophys. J.2006, 90, 2099.(19) Khan, A.; Kaiser, C.; Hecht, S.Angew. Chem. Int. Ed.2006, 45, 1878.(20) Ichimura, K.; Oh, S. K.; Nakagawa, M.Science2000, 288, 1624.(21) Feng, C. L.; Zhang, Y. J.; Jin, J.; Song, Y. L.; Xie, L. Y.; Qu, G. R.; Jiang,

L.; Zhu, D. B. Langmuir2001, 17, 4593.(22) Delorme, N.; Bardeau, J. F.; Bulou, A.; Poncin-Epaillard, F.Langmuir

2005, 21, 12278.(23) Wu, L.; Tuo, X.; Cheng, H.; Chen, Z.; Wang, X.Macromolecules2001,

34, 8005.(24) Yuan, W.; Jiang, G.; Wang, J.; Wang, G.; Song, Y.; Jiang, L.

Macromolecules2006, 39, 1300.(25) Moller, G.; Harke, M.; Motschmann, H.; Prescher, D.Langmuir1998,

14, 4955.(26) Feng, C. L.; Jin, J.; Zhang, Y. J.; Song, Y. L.; Xie, L. Y.; Qu, G. R.; Xu,

Y.; Jiang, L.Surf. Interface Anal.2001, 32, 121.(27) Knobloch, H.; Orendi, H.; Bu¨chel, M.; Seki, T.; Ito, S.; Knoll, W.J. Appl.

Phys.1995, 77, 481.

Scheme 2. Synthesis of Semifluorinated AzobenzeneMolecules for Side Chains

PhotoresponsiVe Fluorinated Surfaces Langmuir, Vol. 23, No. 9, 20075111

was then added dropwise to the mixture while keeping the temperaturebelow 5°C, resulting in a dark red coloration. After stirring for 1h, the reaction mixture was neutralized with sodium bicarbonate.The precipitate was filtered, recrystallized from methanol/water anddried to obtain a deep red powder. Yield: 73%.1H NMR (CDCl3,300 MHz,δ): 7.85 (4H,m-protons from-OH,m-protons from-I),7.60 (d, 2H,o-protons from-I), 6.94 (d, 2H,o-protons from-OH).

4-Iodo-4′-(ethoxycarbonylpentanoxy)azobenzene (2).A mixtureof 9.70 g (30 mmol) of 4-iodo-4′-hydroxyazobenzene (1), potassiumcarbonate (8.0 g, 57.8 mmol), potassium iodide (80 mg, 0.482 mmol),and 150 mL of anhydrousN,N-dimethylformamide was heated at120°C for 5 min. Then, ethyl-6-bromohexanoate (6 mL, 33.7 mmol)was added to the mixture, which was heated at 120°C for 30 min.Finally, the reaction mixture was cooled to room temperature andprecipitated in 400 mL of water. The precipitate was filtered off,recrystallized from ethyl acetate and dried under vacuum to obtainan orange powder. Yield: 83%.1H NMR (CDCl3, 300 MHz,δ):7.85 (d, 2H,m-protons from-O-), 7.83 (2H,m-protons from-I),7.60 (d, 2H,o-protons from-I), 6.98 (d, 2H,o-protons from-O-),4.14 (q, 2H,-COOCH2CH3), 4.04 (t, 2H,-CH2OPh-), 2.35 (t,2H,-CH2COO-), 1.4-1.9 (m, 6H,-CH2CH2CH2-), 1.25 (t, 3H,-CH3).

4-Perfluorohexyl-4′-(ethoxycarbonylpentanoxy)azobenzene (3).A mixture of 9.32 g (20.0 mmol) of 4-iodo-4′-(ethoxycarbonyl-pentanoxy)azobenzene (2), copper (3.80 g, 59.8 mmol), perfluo-rohexyl iodide (13.4 g, 30.0 mmol), and 100 mL of anhydrousdimethylsulfoxide was deoxygenated by bubbling nitrogen gas andstirred at 120°C for 12 h. After cooling to room temperature, 100mL of water and 200 mL of diethyl ether were added, and the mixturewas filtered to remove copper. The ether phase was extracted andwashed with water. Finally, the organic phase was dried overanhydrous Na2SO4 and evaporated under reduced pressure. Theresulting orange solid was recrystallized from methanol. Yield: 54%.1H NMR (CDCl3, 300 MHz,δ): 7.95 (m, 4H,m-protons from-O-,m-protons from-CF2-), 7.72 (d, 2H,o-protons from-CF2-),7.01 (d, 2H,o-protons from-O-), 4.14 (q, 2H,-COOCH2CH3),4.06 (t, 2H,-CH2OPh-), 2.36 (t, 2H,-CH2COO-), 1.4-1.9 (m,6H, -CH2CH2CH2-), 1.26 (t, 3H,-CH3).

4-Perfluorohexyl-4′-(hydroxycarbonylpentanoxy)azobenzene (4).A mixture of 8.20 g (12.5 mmol) of 4-perfluorohexyl-4′-(ethoxy-carbonylpentanoxy)azobenzene (3), potassium hydroxide (10 g, 178mmol), and 120 mL of THF/methanol/water blend (1:1:1 volumeratio) was stirred and heated to reflux for 1 h. The reaction mixturewas cooled to room temperature and then precipitated in 1000 mLof water. A yellow product was obtained after neutralization with

hydrochloric acid, which was recrystallized from chloroform anddried under vacuum. Yield: 92%.1H NMR (CDCl3, 300 MHz,δ):7.95 (m, 4H,m-protons from-O-, m-protons from-CF2-), 7.72(d, 2H,o-protons from-CF2-), 7.01 (d, 2H,o-protons from-O-),4.06 (t, 2H,-CH2OPh-), 2.42 (t, 2H,-CH2COOH), 1.4-1.9 (m,6H, -CH2CH2CH2-).

(4-Perfluorohexylazobenzen-4′-oxy)hexanoylchloride (5).A mix-ture of 0.63 g (0.10 mmol) of 4-perfluorohexyl-4′-(hydroxycarbo-nylpentanoxy)azobenzene (4) and 3 mL of thionyl chloride washeated to reflux under nitrogen for 4 h. The excess thionyl chloridewas removed under vacuum, and the resulting solid was diluted with10 mL of anhydrous tetrahydrofuran and used without furtherpurification.

Attachment of Semifluorinated Azobenzene Side Groups toPolystyrene-b-polyisoprene (PS-b-PI) and Polyisoprene (PI) Pre-cursors.Hydroxylated PS-b-PI block copolymer [27 kDa PS and 9kDa PI, polydispersity index (PDI)) 1.10] and PI homopolymer(17 kDa, PDI) 1.09) were used (7 in Scheme 3). In a typical sidechain attachment reaction, 0.24 g (0.84 mmol-OH) of the blockcopolymer and DMAP (6.0 mg, 0.05 mmol) were dissolved in 10mL of anhydrous THF and 0.5 mL of anhydrous pyridine. The mixturewas cooled to 0°C, and 1.0 mmol of a semifluorinated acid chloride(5) in 10 mL of anhydrous THF was slowly added using a cannula.A precipitate formed immediately. The reaction mixture was slowlywarmed and kept at room temperature for 18 h. After further heatingat 40°C for 6 h, 1 mL of anhydrous methanol was injected into theflask to convert the excess acid chloride to ester. The polymer solutionwas then poured into a large amount of a methanol/water (9/1 v/v)blend. The precipitated polymer was filtered, dissolved in THF,reprecipitated in cold and hot methanol (twice and once, respectively),and dried overnight at 60°C in a vacuum oven.

2.3. Characterization.1H NMR spectra were acquired using aVarian Mercury 300 MHz NMR spectrometer using deuteratedchloroform as a solvent. Chemical shifts were referenced to theprotons of tetramethylsilane (TMS). IR spectra were taken with aMattson Galaxy 2020 series Fourier transform infrared (FTIR)spectrometer with 4 cm-1 resolution using 64 scans. Samples werecast on a NaCl crystal plate. Gel permeation chromatography ofTHF solutions of polymers (1 mg/mL) was carried out using fourWaters Styragel HT columns operating at 40°C and Waters 490ultraviolet (254 nm wavelength) and Waters 410 refractive indexdetectors. The molecular weight range of the columns was from 500to 107 g/mol. THF was used as the eluent at a flow rate of 1 mL/min,and toluene was used as marker for flow calibration. Narrow dispersitypolystyrene samples were used as calibration standards.

Scheme 3. Reaction Scheme for the Synthesis of Fluorinated Azobenzene Block Copolymers

5112 Langmuir, Vol. 23, No. 9, 2007 Paik et al.

Thermal transitions were studied by differential scanning calo-rimetry (DSC) measurements using a Perkin-Elmer DCS-7 Seriesinstrument. The samples were first heated under nitrogen to 200°Cat 10°C/min, annealed at 200°C for 1 min, and cooled to-50 °Cat 10°C /min. The DSC data were collected during the cooling andthe second heating steps. Wide-angle X-ray scattering (WAXS)studies were performed using the Scintag Theta-Theta Diffracto-meter with a wavelength of 1.54 Å. Data were collected at 0.02°steps with a 1.2-s collection time. Films cast from 5% (w/v)R,R,R-trifluorotoluene (TFT) solutions were analyzed. The solvent wasallowed to evaporate slowly for 2 days at room temperature. Theas-cast films were dried overnight under vacuum at room temperatureto remove residual solvent and, finally, annealed under vacuum at150°C for 2 days. The UV spectra were recorded on a Lambda 19spectrometer (Perkin-Elmer) at room temperature for thin films.The UV spectrum of the trans state was recorded after annealing thesample overnight at 50°C in the dark. The UV spectrum of thecis-rich state was recorded after exposure to 360-nm light for 5 min.A portable UV source with a mercury vapor short-arc lamp operatingat 100 W (model SCU 110B, UVEXS Inc.) was used for irradiationat 360 nm. The light intensity of the UV source was tunable. Atypical intensity used in this study was 85 mW/cm2, which wasmeasured by a UVEXS PM 600 UV radiometer.

The films for contact angle measurements and NEXAFS studieswere prepared by spin-coating 5% (w/v) solutions of polymer inTFT at a speed of 2000 rpm (Cee model 100CB spin-coater) onsilicon wafers at elevated temperature. The films were annealed at150 °C under vacuum for about 12 h and slowly cooled to roomtemperature. Surfaces of polymers with (4-perfluorooctylazobenzen-4′-oxy)hexanoyl side chains were annealed at 180°C, above theDSC clearing temperature.

Water contact angles were measured using a NRL contact anglegoniometer model 100-00 (Rame´-Hart Inc.), and values wereaveraged over several measurements. The thin-film samples werealso exposed to UV irradiation in a dark room, and contact angleswere determined immediately after exposure.

The NEXAFS experiments were carried out on the U7A NIST/Dow materials characterization end station at the National Syn-chrotron Light Source at Brookhaven National Laboratory. The X-raybeam was elliptically polarized (polarization factor) 0.85), withthe electric field vector dominantly in the plane of the storage ring.The photon flux was∼1 × 1011 photons/s at a typical storage ringcurrent of 500 mA. A spherical grating monochromator was usedto obtain monochromatic soft X-rays at an energy resolution of 0.2eV. C K-shell NEXAFS spectra were acquired for incident photonenergy in the range 270-320 eV. A computer-controlled goniometer,to which the sample holder was attached, was used to vary theorientation of the sample with respect to the X-ray beam. NEXAFSspectra were obtained at X-ray incident angles of 20°, 30°, 40°, 55°,60°, 70°, 80° and 90°. Each measurement was taken on a fresh spotof the sample in order to minimize possible beam damage effects.The partial-electron-yield (PEY) signal was collected using achanneltron electron multiplier with an adjustable entrance grid bias(EGB). All the data reported here are for a grid bias of-150 V. Thechanneltron PEY detector was positioned at an angle of 45° withrespect to the incoming X-ray beam and in the equatorial plane ofthe sample chamber. To eliminate the effect of incident beam intensityfluctuations and monochromator absorption features, the PEY signalswere normalized by the incident beam intensity obtained from thephoto yield of a clean gold grid.28 A linear pre-edge baseline wassubtracted from the normalized spectra, and the edge jump wasarbitrarily set to unity at 320 eV, far above the C K-edge, a procedurethat enabled comparison of different NEXAFS spectra for the samenumber of carbon atoms. Energy calibration was performed usinga highly oriented pyrolytic graphite (HOPG) reference sample. TheHOPG 1s toπ* transition was assigned an energy of 285.5 eVaccording to the literature value.29 The simultaneous measurementof a graphite-coated gold grid allowed the calibration of the photon

energy with respect to the HOPG sample. The error in the energycalibration is expected to be within(0.5 eV. Each measurementwas taken on a fresh spot of the sample in order to minimize possiblebeam damage effects. Charge compensation was carried out bydirecting low-energy electrons from an electron gun onto the samplesurface. To study the effect of UV exposure on the orientation ofthe azobenzene side chains using NEXAFS, an ultrahigh vacuumwindow that was transparent to 360 nm UV light was installed onthe sample chamber. The sample surface was irradiated with UVlight at an intensity of 85 mW/cm2, and partial electron yield spectrawere obtained with simultaneous exposures to X-rays and UV light.The surfaces were UV-irradiated for at least 1 h before the NEXAFSspectra were collected.

3. Results and Discussion

3.1. Polymer Synthesis.The goal of our work was to producea photoresponsive polymer that combined an azobenzenechromophore with a self-assembling fluorinated segment and toinvestigate light-induced changes in both its surface and thebulk. In order to construct such a material, liquid-crystallinemolecules consisting of 4-perfluoroalkylazobenzene units weresynthesized and attached to polymers containing hydroxylatedisoprene groups. PI homopolymer and PS-b-PI diblock copolymerprecursors were investigated. Block copolymers were used toimprove the solubility of these fluorinated materials in commonorganic solvents and to utilize their superior ability to form thinfilms by the spin-coating technique.

Polyisoprene (Mn ) 17 kDa, PDI) 1.09) was prepared byliving anionic polymerization usingsec-butyllithium as initiator.The block copolymer PS-b-PI (Mn, 27 kDa PS and 9 kDa PI, PDI) 1.10) was also synthesized by living anionic polymerizationusing sequential monomer addition. The polymerization condi-tions resulted in predominantly 1,2- and 3,4- additions of isopreneunits (over 1,4- addition) as determined by1H NMR. Thesepolymers were then modified, prior to attachment of fluorinatedside groups, by hydroboration reaction of the pendent doublebonds with 9-BBN. Hydrolysis of the borane intermediate usingH2O2/OH- resulted in polymers containing hydroxyl-terminatedside groups. The synthesis of the 4-perfluoroalkylazobenzenemesogens was achieved in a facile manner and in high yieldusing the synthetic procedure illustrated in Scheme 2. Perfluo-roalkyl segments with 4, 6 and 8-CF2- units were attached tothe para position of the azobenzene chromophore. Finally,attachment of the semifluorinated azobenzene side group wascarried out by an esterification reaction between the correspondingacid chlorides and the hydroxylated PI precursor. About 20%excess of the acid chloride was used to ensure completeattachment. Three solution-precipitation cycles were used toremove the excess side groups. The overall synthetic procedure,outlined in Scheme 3, allowed us to prepare semifluorinatedazobenzene side chain polymers with narrow molar massdistributions.

The resulting homopolymers are designated PI-Fn (n ) 8, 6,and 4), where Fn refers to the number of perfluoromethylenecarbon atoms in the side group. The corresponding blockcopolymers are designated PSPI-F8, PSPI-F6, and PSPI-F4. Inthe preparation of homopolymer PI-F8, the resulting polymerwas not soluble in THF due to a high degree of fluorination. Thepolymer was, therefore, dissolved in hot TFT, precipitated inTHF, and washed with a large amount of THF to remove excessside group. The FT-IR spectra of the hydroxylated polymersshowed that the hydroxyl absorption in the 3400 cm-1 regionhad completely disappeared with a concurrent appearance of a

(28) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992;Chapter 5, p 114.

(29) Rosenberg, R. A.; Love, P. J.; Rehn, V.Phys. ReV. B: Condens. MatterMater. Phys.1986, 33, 4034.


very prominent ester group absorbance at 1716 cm-1 and a C-Fband near 1195 cm-1. To study the effect of packing density onUV-induced reorientation of the side chains, PSPI-F6 blockcopolymers, with 50% and 25% attachment of the side chainsto the hydroxylated block, were also prepared by reacting 0.5and 0.25 equiv of the F6 acid chloride with the PSPI-OH blockcopolymer precursor. These polymers are denoted by PSPI-F60.50

and PSPI-F60.25, respectively.3.2. Liquid Crystalline (LC) Behavior. Differential scanning

calorimetry (DSC) and wide-angle X-ray scattering (WAXS)were employed to examine the liquid crystalline behavior of theazo polymers, and the results are shown in Table 1 with the dataprovided in the Supporting Information. The melting points ofthe carboxylic acid mesogens (4 in Scheme 2) were 132 (n )4), 150 (n) 6), and 165°C (n) 8). No LC-to-isotropic transitionwas observed for PI-F4 and PSPI-F4. No peaks were detectedin the WAXS pattern for the F4 polymers, either. Thus, the sidegroup with the shortest-CF2- chain did not form a liquidcrystalline phase. Side groups with longer perfluoroalkyl chainlengths (n ) 6 and 8), however, formed smectic mesophases.

WAXS patterns of these polymers showed d-spacings of 46 Åfor the F6 side group and 51 Å for the F8 side group. Thesevalues, which are approximately twice the length of the sidechains, can be assigned to smectic A layers.

The clearing temperatures for the F6 homo- and block polymerswere close to 110°C, and those for the F8 polymers were higher.Two transition temperatures (around 140 and 170°C) wereobserved for the PI-F8 and PSPI-F8 polymers. The presence ofan additional transition in the DSC curves of these polymersindicates the existence of an additional mesophase, where170°C is the transition temperature from the smectic A to isotropicphase and 140°C is the transition temperature from a higherorder smectic phase to the smectic A phase. These observationsare consistent with our previous studies of semifluorinatedpolymers;8 however, the semifluorinated azobenzene mesogensformed LC phases with higher transition temperatures ascompared to semifluorinated alkyl mesogens.8The higher clearingtemperature of the azobenzene polymers can be attributed to theincorporation of the azobenzene chromophore, which wouldincrease the intermolecular forces of interaction between theside chains; for example, throughπ-π stacking, thus stabilizingthe mesophase. Similar effects have been observed in othersystems.10 In the case of styrenic polymers with para-substitutedsemifluorinated alkyl groups,30 the presence of a phenyl ring inthe side chain greatly enhanced the thermal stability of the LCmesophase. The PSPI-F60.5 polymer showed a relatively broadmelting peak near 105°C, but the PSPI-F60.25 polymer did notshow a liquid crystalline melting transition.

The PSPI-F4 and PSPI-F8 block copolymers showed a glasstransition (Tg) near 100°C, close to theTg of polystyrene. Thus,the existence of PS microphase separation can be inferred. ThePSTg could not be located in the DSC trace of PSPI-F6 becauseof its proximity to a liquid crystalline melting transition. Theglass transition temperatures of the liquid crystalline blocks couldalso not be clearly seen, except in the case of the PSPI-F4 blockcopolymer, for which a transition was observed near 70°C. TheDSC traces for representative homopolymers and block copoly-mers are shown in the Supporting Information. Because thereare additional-CF2- groups in the longer side chains, theTg’sof the PI-F6 and PI-F8 polymers are expected to be slightlylower than that of PI-F4,31 but enhanced liquid crystallinity maycounteract this effect. Our observations are consistent with theinference that the glass transition temperatures of all theazobenzene-containing liquid crystalline blocks are higher thanroom temperature.

3.3. Contact Angle Measurements.The photoisomerizationof the fluorinated azobenzene side groups at the surface was firststudied using water contact angle measurements. A UV lightsource was set up in conjunction with the contact angle goniometerso that advancing water contact angles could be measuredimmediately after UV irradiation. The entire process was carriedout in the dark room with the lamp of the goniometer coveredby a red filter to avoid reversion to the trans isomer caused by440-nm wavelength light. The results are shown in Figure 1. Forall surfaces, the water contact angle decreased only slightly underUV irradiation. The largest changes in contact angles wereobserved for the F4 polymers, which from DSC and WAXS areknown to lack liquid crystallinity. The water contact angles beganto increase after∼10 min after cessation of UV exposure, andcomplete recovery was achieved within∼12 h. The PSPI-F60.50

surface showed contact angle values similar to the PSPI-F6

(30) Li, X.; Andruzzi, L.; Chiellini, E.; Galli, G.; Ober, C. K.; Hexemer, A.;Kramer, E. J.; Fischer, D. A.Macromolecules2002, 35, 8078.

(31) Tsuwi, J.; Appelhans, D.; Zschoche, S.; Zhuang, R.-C.; Friedel, P.; Ha¨uâler,L.; Voit, B.; Kremer, F.Colloid Polym. Sci.2005, 283, 1321.

Figure 1. Advancing water contact angles of fluorinated azobenzenepolymer surfaces before and during UV irradiation.

Figure 2. UV-vis spectra of PI-F6 before (solid line) and rightafter UV irradiation (dashed line). The surface was exposed to 360-nm UV light for 5, 10, and 15 min. The spectra obtained after 10and 15 min of exposure were indistinguishable from those obtainedafter 5 min of exposure, suggesting that a photostationary state wasreached within 5 min.

Table 1. DSC and WAXS Data for Side Chain LiquidCrystalline Polymers with Fluorinated Azobenzene Mesogens

sample ID transition temp (°C) X-ray d-spacing (Å)

PSPI-F8 145, 175 51PSPI-F6 110 47PSPI-F4 none nonePI-F8 140, 160 50PI-F6 105 46PI-F4 none none


polymer with 100% attachment of side chains. The PSPI-F60.25

block copolymer, with only 25% attachment to the PI-OH block,had lower advancing water contact angles of 108( 1° in theabsence of UV light and 102( 1° when illuminated. The contactangle measurements are in accord with the results of NEXAFSspectroscopy in Section 3.6, where well-packed smectic layersof the fluoroalkyl groups (higher number of-CF2- groups inthe side chain or greater extent of attachment of side chains tothe polymer backbone) did not show a significant decrease inperfluoroalkyl-helix order parameter during UV exposure. Toverify that it was not a change in surface topography that re-sulted in the observed changes in water contact angles, scanningforce microscopy was used to study the surface before andafter UV exposure. No significant difference in surface rough-ness was observed before and after UV irradiation for all thepolymers.

3.4. Microphase Separation in Thin Films of the DiblockCopolymers.The PS mass fractions in PSPI-F4, PSPI-F6, and

PSPI-F8 polymers are 0.26, 0.23, and 0.20, respectively. On thebasis of the estimated densities of the fluorinated blocks, the PSvolume fractions are 0.31, 0.29, and 0.27, respectively. Tappingmode atomic force microscopy of the PSPI-F8 surface (cf.Supporting Information) showed cylindrical polystyrene domains;however, previous NEXAFS studies have shown that irrespectiveof the inner film structure, surfaces of block copolymers withliquid crystalline semifluorinated side chains consist of a smecticlayer of oriented fluoroalkyl helices overlaying the PS block.9

We also noted that the intensities of the C 1sf π*Φ resonanceswere almost the same for both the homopolymers and thecorresponding block copolymers (cf. Section 3.6), which ispossible only in the absence of the PS blocks at the blockcopolymer surfaces. In summary, the PSPI-F4, PSPI-F6, andPSPI-F8 block copolymers have nearly identical surface mor-phologies, which are also similar to those of the homopolymers.The only difference is in the lengths of the fluoroalkyl groupsand, hence, the orientational order of the side chains. Experimental

Figure 3. WAXS spectra of (a) PI-F4, (b) PI-F8, and (c) PI-F60.50 before and immediately after a 5-min UV exposure.

Figure 4. NEXAFS spectra of (a) PI-F4, (b) PI-F6, and (c) PI-F8 at X-ray incidence angles of 20° and 90° with respect to the sample surface;(d) curve fitting of the NEXAFS spectra of PI-F8 surface obtained at an X-ray incidence angle of 55°. The final states of the C1s core shellelectrons corresponding to the observed peaks are also shown.


results provide no indication of any influence of mircophaseseparation in our study.

3.5. Photoisomerization of Semifluorinated AzobenzeneSide Chains in the Bulk.To investigate the photoisomerizationin the film interior rather than the surface, a thin film samplewith a thickness of∼330 nm was allowed to relax overnight at50 °C in the dark so that a saturated trans photostationary statewas achieved. The sample was mounted into a UV spectrometer,and UV spectra were recorded (Figure 2). A strong absorptionpeak near 340-nm wavelength is seen, which corresponds to theπ f π* transition of azobenzene. Subsequently, the thin filmwas irradiated at 360 nm in situ by a UV source for 5 min sothat a new photostationary state could be reached. The UVspectrum was acquired immediately and is also shown in Figure2. A decrease in absorbance near 340 nm and an increase inabsorbance near 440 nm (which corresponds to then f π*transition of azobenzene) can be noted. Even though a photo-stationary state was reached, a maximum of∼22% cis isomercould be obtained in our samples, as can be inferred by significantabsorption near 340 nm, even after the UV irradiation. Thesespectra are consistent with those reported in the literature.32,33

The effects of photoisomerization were also evident in wide-angle X-ray scattering experiments. WAXS scattering patternswere obtained for the polymer thin films before UV irradiationand immediately after 5 min of UV exposure at room temperature.Figure 3a compares the spectra for the PI-F4 surface. As expected,we see no diffraction peak for the F4 sample, which did not alsoshow a liquid crystalline melting peak in DSC. The PI-F8 sample,on the other hand, showed a sharp diffraction peak atq ) 0.125Å-1, which corresponds to a smectic layer d-spacing of 50 Å.There was only a small change in the intensity of this diffractionpeak after UV exposure (Figure 3b). However, in the case of thePSPI-F60.5 surface, a sharp diffraction peak was observed atq) 0.138 Å-1 (45 Å d-spacing) before UV exposure, the intensityof which decreased greatly after UV irradiation (Figure 3c). Thediminished diffraction peak was also shifted to a higher q position,indicating a decrease in the smectic layer thickness due tophotoisomerization. This suggests that the LC order of the F8sample was too strong for any changes to occur as a result ofUV exposure, whereas the less-densely packed side chains in theF60.50sample were able to undergo trans-cis isomerization anddisrupt the smectic layers.

Because the entire film thickness is probed in UV-visspectroscopy and WAXS experiments, changes in UV-vis andWAXS spectra reflect photoisomerization occurring in the bulkof the sample, although the films used for measurements arefairly thin. On the other hand, NEXAFS probes the molecularorientation in a monolayer of side chains at the surface of thefilm, which we discuss in the following section.

3.6. NEXAFS Spectroscopy.Effect of the Length of Per-fluoroalkyl Segment.The self-assembly of the semifluorinatedazobenzene side chains was studied using NEXAFS spectroscopy,which has proven to be an ideal tool for probing the chemicalcomposition of and molecular orientation in the top 1-3 nm ofa surface. Carbon K-shell NEXAFS spectroscopy involvesexcitation of the C 1s electrons toσ* or π* molecular orbitals,using polarized X-ray photons of energy between 270 and 320eV, and studying the intensity of the emitted Auger electrons.By adjusting the bias of the channeltron detector, the escapedepth of Auger electrons can be controlled so that only the partialelectron yield signals from roughly the top 3 nm of the surfaceregion are collected, which is only modestly larger than the lengthof the side chains. The Auger electrons arising from the electronictransitions from C 1s to theσ*C-F andπ*Φ molecular orbitals(whereΦ denotes a phenyl ring) were of particular interest inour study. From the variation of the intensities of these electronswith the angle of incidence of the X-ray beam, the orientationand order of the C-F bonds and the phenyl rings could bedetermined.

Figure 4 shows the NEXAFS spectra of the PI-Fnhomopolymersurfaces. For each sample, spectra at only two angles,θ ) 20°and 90°, are shown, although data were acquired at several otherangles, as stated in the experimental section. The spectra werefitted using Gaussian-type peaks for near-edge resonances andan arctan function for the adsorption edge as shown in Figure4d.34 The C 1sf π*Φ resonance occurred at∼285.5 eV. TheC 1sf σ*C-F peak was found to be near 293 eV, whereas theC 1sf σ*C-C resonance occurred at∼296 eV.

It is evident from Figure 4a-c that the intensity of theσ*C-F

resonance increased with the number of-CF2- groups in theside chain. In addition, the intensities ofσ*C-F, σ*C-C, andπ*Φresonances varied with the angle of incidence of the X-ray beam,indicating that the fluorinated azobenzene side chains had apreferential orientation at the surface. The orientational orderparameter of the perfluoroalkyl segment of the azobenzene sidechains was determined from a plot of the intensity of C 1sfσ*C-F resonance at 293 eV,IC-F, versus sin2 θ, whereθ is theangle between the X-ray beam and the surface. The orientationalorder parameter of C-F bonds is given by35,36

where⟨R⟩ is the average angle made by the C-F bonds with thesurface normal,P is the polarization factor of the X-ray beam(∼0.85),mis the slope ofIC-F versus sin2 θ, andC is the intercept.Assuming that the fluoroalkyl segment forms a rigid helix withthe C-F bonds normal to the axis of the helix, it is evident fromthe spinning chain model35that the helix order parameter,SF-helix,is -2SC-F. An order parameter of 1 indicates a highly ordered

(32) Tamai, N.; Miyasaka, H.Chem. ReV. 2000, 100, 1875.(33) Eisenbach, C. D.Polymer1980, 21, 1175.

(34) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992;Chapter 7, p 211.

(35) Outka, D.; Sto¨hr, J.; Rabe, J.; Swalen, J. D.J. Chem. Phys.1988, 88,4076.

(36) Hayakawa, T.; Wang, J.; Xiang, M.; Li, X.; Ueda, M.; Ober, C. K.;Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fischer, D. A.Macromolecules2000, 33,8012.

Figure 5. Expected change in conformation of the F6 side chainsupon exposure to UV light (λ ) 360 nm).

SC-F )3 cos2 ⟨R⟩ - 1

2) - P-1m

(3 - P-1)m + 3C(1)


surface in which all the perfluoroalkyl groups are oriented normalto the surface. A value of-0.5 denotes an orientation parallelto the surface. An order parameter of 0 indicates either a disorderedsurface or an average tilt angle,⟨τF-helix⟩, of 54.7°, the formerbeing more likely. The order parameter of vectors normal to theplane of the phenyl rings,SΦ, was similarly determined by plottingthe intensities of C 1sf π*Φ resonance versus sin2 θ. Thefollowing sections discuss the effect of UV exposure on theliquid crystalline order of the side chains. The order parametersof the fluoroalkyl and the azobenzene parts of the side chainswere separately calculated and compared.

Photoswitching of Semifluorinated Azobenzene Groups at theSurface.Figure 5 shows the expected trans and cis conformationsof a free (untethered) F6 side chain.37 A large change in theorientations of the fluoroalkyl helices and the azobenzene phenylrings is expected as a result of the photoisomerization process.The normals to the phenyl rings make an angle of 0° with eachother in the trans conformation and 66.8° in the cis conformation.The -NdN- internuclear axis lies in the plane of the phenylrings in the trans state, but it makes angles of 49.7° and 57.4°with the normals toΦ1 andΦ2, respectively, in the cis conforma-tion.38In addition, the magnitude of the dipole moment decreasesfrom 6.42 Debyes in the trans state to 3.94 Debyes in the cisstate. However, NEXAFS analysis presented a different picture.

Figure 6 shows the NEXAFS spectra of the PI-F6 and PSPI-F6 surfaces acquired before and during UV exposure. Thevariation in the intensities of theπ*Φ, σ*C-F, andσ*C-C peakswith the X-ray incidence angle,θ, showed that the side chains

were oriented even under UV light. However, the difference inσC-F/ andσC-C

/ peak intensities atθ ) 20° and 90° decreasedwhen the surfaces were exposed to UV, which indicates a decreasein the orientational order parameters.

Figure 7a shows the perfluoroalkyl helix order parameter,SF-helix, for the PI-Fn (n ) 4, 6, and 8) polymers and thecorresponding block copolymers. It is seen that the helix orderparameter increased with an increase in the number of-CF2-groups in the side chain. For the polymers with 6 and 8-CF2-groups,SF-helix did not show a notable decrease when the surfacewas exposed to UV light. Thus, when the side chains are highlyordered, as in the case of the F6 and F8 polymers, photoisomer-ization of the azobenzene group cannot readily disrupt the liquidcrystalline packing of the perfluoroalkyl helices. It should benoted that the liquid crystalline packing of the perfluoroalkylgroups did not completely prevent the photoisomerization process,which is evident from the UV-vis absorption spectra shown inFigure 2. Figure 8 shows a possible mechanism whereby theazobenzene group can undergo trans-to-cis isomerization, butthe fluoroalkyl groups remain unaffected by UV light. The largerpercentage changes in the orientations of the phenyl ring (peakat 285.5 eV in Figure 6) and C-C bonds (peak at 296 eV) supportthis hypothesis. The phenyl ring order parameters for the F6polymers showed a significant decrease when the surfaces wereilluminated with UV light (Figure 7b).

It is possible, in theory, to calculate the average tilt angles,⟨R⟩, using the orientational order parameters,S(cf. eq 1). However,because the phenyl ring order parameters are averages over thetwo phenyl rings in the azobenzene side groups, either or bothof which can move due to the isomerization, a correlation withthe order parameter of the fluroalkyl helices is not straightforwardand has not been attempted.

(37) The geometry optimization was performed using a restricted Hartree-Fock calculation and the 6-31.G(d) basis set (using Gaussian 98, Revision A.7).

(38) Φ1 andΦ2 are the azobenzene phenyl rings attached to the perfluoroalkyland oxyhexanoyl groups, respectively.

Figure 6. NEXAFS spectra of (a) PI-F6 before UV exposure, (b) PI-F6 during UV exposure, (c) PSPI-F6 before UV exposure, and (d)PSPI-F6 during UV exposure. Spectra obtained at the two X-ray incidence angles of 20° and 90° are shown. Despite the presence of thePS block in the PSPI-F6 polymer, the intensity of theπ* Φ peak is similar to that for the homopolymer PI-F6. Thus, only the smectic layerof azobenzene side chains is probed, and the underlying PS block is not detected. Theπ*Φ peak at 285.5 eV is mainly from the azobenzenephenyl rings of the side chains. The PS phenyl rings must lie below the escape depth of Auger electrons; otherwise, the intensity of theπ* Φpeak would have been higher for the block copolymer surface.


Effect of Density of Attachment of Side Chains to PolymerBackbone.Due to low packing density, spin-coated surfaces ofthe polymer with 25% attachment (PI-F60.25) showed lower orderof the perfluroalkyl helices as compared to the polymers with100 and 50% attachment of side chains. It can be inferred fromFigure 9 that the polymer with a 50% density of attachmentshowed a higher change in the helix and phenyl ring orderparameters (compared to 100% attachment), possibly due to alower steric hindrance for the trans-cis isomerization to occur.100% attachment resulted in a well-packed smectic layer offluoroalkyl groups so that only the phenyl rings showed asignificant change in orientation, whereas 25% degree ofattachment was so low that neither the fluoroalkyl groups northe phenyl rings were highly ordered to begin with. The large

differences observed in the case of the PI-F60.5surfaces are quiteconsistent with the WAXS scattering results discussed in Section3.5 (Figure 3c).

4. Conclusions

Fluoroazobenzene side chains with different perfluorometh-ylene segment lengths were synthesized and attached to PIhomopolymers and the isoprene block of PS-b-PI diblockcopolymers. NEXAFS spectroscopy showed that the longer (n) 6 and 8) semifluorinated azobenzene side chains initiallyorganized into well-ordered surface layers due to a combinationof low surface energy and liquid crystallinity. The orientationalorder parameters of perfluoroalkyl helices increased with thelength of the perfluoroalkyl group, consistent with previous studieson semifluorinated liquid crystalline block copolymers. Theshorter fluorinated segment (n ) 4), which lacked strong orderat the surface, easily underwent light-induced surface reorga-nization, as indicated by its relatively large change in watercontact angle. NEXAFS experiments with in situ UV exposuredemonstrated that with sufficiently long perfluoroalkyl units, itwas possible to stabilize the surface so that photoisomerizationdid not change the surface properties.

Although the perfluorohexyl segments of the F6 side chainsat the surface formed a highly stable smectic mesophase thatcould not be disordered by exposure to UV light, the mesophaseformation did not prevent the trans-to-cis isomerization of theazo benzene groups, as seen in UV-vis spectroscopy. Thus, UVlight caused a change in the orientation of the phenyl rings andthe alkyl spacers, but the perfluoroalkyl segments remained intheir original ordered state at the film surface. By decreasing thegrafting density of mesogens along the polymer backbone, greaterUV-induced changes in the thin film structure and surfaceproperties could be effected.

Water contact angle measurements were also in agreementwith the NEXAFS results. The contact angle measurements

Figure 7. Orientational order parameters of (a) the perfluoroalkylhelix, SF-helix, and (b) the phenyl ring,SΦ, for PI-Fn and PSPI-Fnpolymer surfaces determined, using NEXAFS spectroscopy, in theabsence of UV irradiation and also under UV exposure.

Figure 8. Structural reorganization of F6 side chains at the polymersurface upon exposure to UV light (λ ) 360 nm). The perfluoroalkylpart of the side chain is unable to undergo a change in the orientationdue to liquid crystalline packing. The part of the side chain attachedto the polymer backbone, however, can reorient.

Figure 9. Perfluoroalkyl and phenyl ring order parameters forsurfaces of PI-F6 block copolymers with 100, 50, and 25% attachmentof the azobenzene side chains to the hydroxylated PI block. Theorder parameters were determined using NEXAFS spectroscopybefore and during exposure to 360-nm UV light.


showed that with sufficiently long fluorinated segments, thesurface wettability remained relatively constant upon UVirradiation, which indicated stable surfaces resistant to recon-struction. Such surface stability of azobenzene polymers isimportant in applications such as holographic information storage,for which the ability to suppress the formation of surface structuressuch as surface relief gratings is needed. However, we did observea change in surface wettability in the case of polymers with theshortest fluorinated segments. Surfaces of these polymers couldswitch reversibly from a hydrophobic state to a less hydrophobicstate upon UV irradiation for a short time (∼5 min). Thermalrelaxation of the surface in the dark caused a slow recovery ofthe hydrophobic nature.

The bulky nature and liquid crystallinity of the semifluorinatedazobenzene side chains will increase chain stiffness, whichmanifests as a higher glass transition temperature (Tg ∼ 70 °C)than that of the polyisoprene precursor (Tg∼ -70°C). However,the Tg-dependent cooperative motion of chain segments is animportant consideration, mainly in situations in which large scalemass transport effects predominate; for example, in the formationor suppression of surface relief structures.7 In such cases, it isevident that the effects of higher glass transition temperature andstability caused by the liquid crystalline mesophase will act inharmony to prevent the formation of surface relief structures.Similarly, segmental mobility is also important when theazobenzene groups are part of the polymer backbone, as in thepolymers investigated by Lamarre and Sung.39 In the presentstudy, we have no evidence that the polyisoprene backbone isinsufficiently flexible, at least locally, to freely allow photoi-somerization of thesurfaceazobenzene groups, even at roomtemperature. It is the high grafting density along the polymerbackbone and the formation of a smectic phase that result instable surfaces that are resistant to reconstruction.39

Although glass transition-related temperature effects are notexpected to play a significant role in photoisomerization occurringat surfaces, their role in bulk has been widely demonstrated andwell-understood. Photoisomerization of bulky azobenzene groupsin the interior of the film will depend on the free volume neededfor geometrical rearrangements during the transf cis photoi-somerization. Paik and Morawetz found that the photostationarystate contained a relatively lower number of cis isomers whenthe bulk polymer was irradiated in the glassy state.40 On thecontrary, the kinetics of approach to the photostationary stateand the equilibrium distribution of cis and trans isomers weresimilar in rubbery samples and polymers in dilute solutions.They attributed these observations to differences in free volume(and free volume distribution) in the glassy and rubbery states.They also found that certain polyesters and polyimides containingazobenzene groups in the chain backbone showed stronglyinhibited glassy state photoisomerization. This inhibition wasmorepronounced than in thecaseofpolymersbearingazoaromaticmoieties in side groups, which can be attributed to a lack ofcooperative motion of chain segments below theTg. Such a

cooperative motion would be necessary if several azobenzenegroups along a polymer backbone have to undergo geometricalchanges.

The free volume constraints on geometrical isomerization havebeen detailed in the work of several research groups. For example,Victor and Torkelson used a homologous series of azo chro-mophores and substituted stilbenes to investigate the free volumedistribution in glassy polymers.41 Each of the probes required adifferent volume,υ, to undergo photoinduced trans-cis isomer-ization. Similarly, Mita et al. found that the fraction of theazobenzene groups that are surrounded by a local free volumelarger thanυ increased with temperature.42 Not only will ourpolymers with semifluorinated azobenzene side chains be subjectto the freevolumeconstraint belowTg, butalsophotoisomerizationwill be inhibited by liquid crystallinity of the azobenzenemesogens. This is evident from Figure 2, in which UV-vis spectraof bulk PSPI-F6 samples showed that only∼22% of theazobenzene groups were cis isomers in the photostationary state.

In conclusion, both liquid crystallinity and free volumeconstraint, therefore, contribute toward surface stability ofsemifluorinated side-chain azobenzene polymers. The free volumeconstraint can be somewhat reduced by lowering the graftingdensity along the polymer backbone. Geometrical isomerization,as shown in Figure 8, is expected to be favored by longer alkyltethers than the five methylene groups we have used in ourpolymers. Although liquid crystallinity will still prevent surfacereconstruction, we expect to observe a larger proportion of cisisomers in bulk samples because of greater flexibility impartedto the azobenzene group. The experimental observations of Tianet al. are in accord with this hypothesis.43

Acknowledgment. This research was supported by theNational Science Foundation Division of Materials Research(Grants Nos. DMR-0307233 and DMR-0208825) and the Officeof Naval Research, Grants N00014-02-1-0170, to C.K.O. andE.J.K. All grants are gratefully acknowledged. This researchmade use of the Hudson Mesoscale Processing, PolymerCharacterization and Surface Imaging facilities of the CornellCenter for Materials Research (CCMR) and the Microscopy andMicroanalysis Central Facility of the Materials ResearchLaboratory at UCSB, both with support from the National ScienceFoundation Materials Research Science and Engineering Centers(MRSEC) program (DMR-0079992 and DMR-0520415, re-spectively). The NEXAFS experiments were performed at theNational Synchrotron Light Source at Brookhaven NationalLaboratory, which is supported by the U.S. Department of Energy,Division of Materials Science and Division of Chemical Sciences.

Supporting Information Available: Differential scanningcalorimetry and atomic force microscopy images of the semifluorinatedpolymers. This material is available free of charge via the Internet athttp://pubs.acs.org.

LA0634138

(39) Lamarre, L.; Sung, C. S. P.Macromolecules1983, 16, 1729.(40) Paik, C. S.; Morawetz, H.Macromolecules1972, 5, 171.

(41) Victor, J. G.; Torkelson, J. M.Macromolecules1987, 20, 2241.(42) Mita, I.; Horie, K.; Hirao, K.Macromolecules1989, 22, 558.(43) Tian, Y.; Watanabe, K.; Kong, X.; Abe, J.; Iyode, T.Macromolecules

2002, 35, 3739.
