Poly(azobenzene acrylate-co-fluorinated acrylate) Spin-Coated Films

Article pubs.acs.org/Langmuir

Poly(azobenzene acrylate-co-fluorinated acrylate) Spin-Coated Films: Influence of the Composition on the Photo-Controlled Wettability Sanae Abrakhi, Sébastien Péralta, Odile Fichet, Dominique Teyssié, and Sophie Cantin* Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI, EA 2528), Institut des Matériaux, Université de Cergy-Pontoise, 5 mail Gay-Lussac Neuville/Oise, 95000 Cergy-Pontoise Cedex, France S Supporting Information *

ABSTRACT: The wetting properties of spin-coated films of copolymers based on azobenzene and fluorinated units have been investigated. The copolymers, denoted as poly(Azo-coAcRf6), have been synthesized by free-radical polymerization of different proportions of acrylate monomers bearing either an azobenzene group or a semifluorinated side chain. The UV−visible spectroscopy analysis of the different spin-coating films through a cycle of UV and visible light irradiation indicates the reversible trans−cis isomerization of azobenzene groups. Simultaneously, atomic force microscopy shows that surface roughness does not exceed 1 nm. Advancing and receding contact angles of water and diiodomethane have been measured before and after UV photoirradiation of the different surfaces. In particular, a decrease in the advancing contact angles has been observed upon trans−cis isomerization of azobenzene groups. Switching variations up to 50° have been evidenced without any introduction of surface nanoroughness. Surface freeenergy evaluations have been deduced from these measurements, including dispersive and polar components. The results show that, through surface composition and UV photoirradiation, a large range of surface free-energies can be obtained, from 7 to 46 mN·m−1.

1. INTRODUCTION Surfaces with switchable wettability achieved through light irradiation have aroused great interest recently in view of potential applications such as, for example, microfluidic devices, self-cleaning surfaces, control of protein adhesion to the surface, or ion transport across a membrane.1,2 Owing to their photoactivity, chromophores like azobenzene, spyropyran, or cinnamate incorporated in thin films have been shown to allow control of surface free-energy.3−5 In particular, the reversible trans−cis isomerization of azobenzene groups through UV/visible irradiation is accompanied by fast and large changes in both geometry and dipole moment. Indeed, the trans isomer has no dipole moment while the cis isomer has a dipole moment of 3.1 D.6 The resulting change in polarity induces wettability variations of azobenzene-modified surfaces. A variety of different methods have been used to generate azobenzene-modified surfaces with tunable wetting properties, such as self-assembly,7 Langmuir−Blodgett,8 layer by layer or spin-coating,9,10 either with or without introduction of surface roughness. For example, UV photoirradiation of azobenzene functionalized alkyltriethoxysilane self-assembled monolayers leads to a few degree decrease in the static water contact angle.11,12 Similar variations have been observed with olive oil droplets.12 Moreover the initial contact angle can be recovered after visible light irradiation of the film. This reversibility can be achieved through several cycles alternating UV and visible © XXXX American Chemical Society

irradiations without altering surface properties. Similar water contact angle variations (∼9−12°) have been observed for selfassembled monolayers of azobenzene functionalized with an alkyl chain.11,13 UV photoirradiation of self-assembled monolayers of azobenzene substituted by a hydrophilic group results in water static contact angle increase, with a wettability contrast varying from 1 to 8° depending on the grafting density.14 This increase is related to the change in surface composition upon azobenzene trans−cis isomerization. The Langmuir−Blodgett (LB) technique has been also used to prepare photoresponsive surfaces containing azobenzene groups. For example, LB films of a poly(methacrylate) based copolymer bearing trifluoromethyl functionalized azobenzene or 2-hydroxyethyl as side chain undergo reversible wettability changes upon UV/visible irradiation.8 In addition, the authors show that the 11° variation in water static contact angle is accompanied by reversible morphological changes and friction force responses attributed to the structural changes associated to photoisomerization. Indeed, the cross-section area of the cis form is larger than that of the trans form. Quite higher static contact angle variations (∼20°) can be obtained using a nematic liquid crystal (5CB) as Received: March 13, 2013 Revised: June 25, 2013

A

dx.doi.org/10.1021/la400938j | Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Chemical structure and synthetic pathway of poly(Azo-co-AcRf6) copolymers; schematic representation of trans and cis states of the azobenzene units of poly(Azo-co-AcRf6) through UV/visible photoirradiation.

layer-by-layer technique. Depending on the pillar spacing, as high as 66° contact angle variation through UV photoirradiation has been thus obtained. Finally, in some cases, depending on the photoinduced variation in advancing and receding contact angles, the resulting surface free energy changes can be used to guide spatially the motion of a liquid droplet through controlled UV/visible photoirradiation of the surface.4,15,13 In order to investigate the possible light-driven motion of a liquid droplet on a surface, dynamic contact angle values are thus required rather than static contact angles. In particular, contact angle hysteresis values are crucial to identify the liquids that can be moved on the surface. Indeed, drop motion requires that the advancing contact angle in the cis-rich state must be lower than the receding contact angle in the trans-rich state. Only a few studies have been reported on the surface freeenergy variation upon photoswitching.13,16 In particular, the estimation of the surface free-energy components can bring new information on the modification of the predominant forces at the surface resulting from trans-cis isomerization. In the present work, the photoswitchable wetting properties of spin-coated films of copolymers synthesized by free-radical polymerization of acrylate monomers bearing either a semifluorinated side chain or an azobenzene group (poly(Azo-coAcRf6)) have been investigated. Such compounds should result in low-energy surfaces with tunable wetting properties. Indeed, well-defined surface properties should be achieved via both azobenzene/fluorinated acrylate proportion and UV/visible light irradiation. The synthesized poly(Azo-co-AcRf6) copolymers of different compositions have been first characterized by 1 H NMR spectroscopy and differential scanning calorimetry (DSC). Then poly(Azo-co-AcRf6) spin-coated films have been studied before and after UV photoirradiation by UV−visible spectroscopy, atomic force microscopy, and dynamic contact

liquid, as shown for azobenzene-terminated calix[4]resorcinarenes monolayers.15 In some cases, specific functionalization of azobenzene moiety can lead to significant contact angle variations. Demirel et al. have prepared self-assembled monolayers of an azobenzene functionalized by a carboxyl group.16 In this case, UV irradiation leads to an increase in advancing contact angle of about 38°. Indeed, the carboxyl group is directed toward the air in the azobenzene trans form, while it is away from the surface in the cis form. However, the difference in contact angle measured after UV light illumination is in general rather small when smooth substrates are employed, even for polymers with semifluorinated azobenzene side chains.9 An enhancement of contact angle variation can be detected by increasing surface roughness. Lim et al. have prepared layer-by-layer films associating a poly(allylamine hydrochloride) and SiO2 nanoparticules as polycation and polyanion, respectively.10 Depending on the surface roughness controlled by the number of bilayers, the decrease in water contact angle after exposure to UV light varies from 5° for a flat substrate to about 150° for a nanostructured multilayer film. Film roughness can be also achieved through substrate roughness. Indeed, Groten et al. have obtained changes in water contact angle as high as 140° on a polymer monolayer bearing fluorinated azobenzene moieties attached on a nanorough surface.17 For comparison, on the corresponding flat surface, a contact angle difference of a few degrees is observed upon photoswitching. Jin et al. have measured a 120° static contact angle variation through UV photoirradiation of trifluoromethyl functionalized azobenzene monolayer grafted on cellulose nanofibre surfaces precovered with an ultrathin titania layer.18 Patterned surfaces can also be employed. Jiang et al. have introduced square pillars on a flat silicon wafer and deposited azobenzene polyelectrolytes by the B


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heating scan recorded at 10 °C.min−1 heating rate between −50 and 180 °C on a Q100 apparatus (TA Instruments). The UV−visible spectra were recorded on a V570 spectrometer (JASCO). Photoirradiation was carried out at a wavelength of 330 ± 10 nm or 430 ± 10 nm using a spectrofluorimeter (Jasco FP-6200) equipped with a Xenon lamp. The atomic force microscopy (AFM) experiments were performed in tapping mode with a Nanoscope IIIA Dimension 3100 microscope from Digital Instruments. Measurements were carried out in air at room temperature. For each sample, AFM images were recorded on at least four samples and in several places in order to check the reproducibility of the displayed images. All contact angle measurements were carried out under air at room temperature. Advancing and receding contact angles of water and diiodomethane were determined using the drop shape analysis-profile device equipped with a tiltable plane (DSA-P, Kruss, Germany). A 40 μL ultrapure water drop or 10 μL diiodomethane drop was first deposited on the substrate using a variable-volume micropipet. With 40 μL as water drop volume, the drops have been observed to slide on both surfaces. This quite large volume was thus fixed for all the measurements. When possible, that is, for surfaces leading to the lowest contact angle hysteresis, it was checked that decreasing the drop volume does not lead to different contact angle values. After drop deposition, the static contact angle was measured by means of a Young−Laplace drop profile fitting. The water surface tension was also deduced in order to check systematically that no water contamination by polymer residues occurs during polymer film exposure. In order to perform dynamic contact angle measurements, the solid surface sustaining the drop was then tilted at a constant speed (1°/s) and the images of the drop simultaneously recorded. The advancing and receding angles were measured, respectively, at the front edge and the rear edge of the drop, just before the triple line starts moving. The angles were obtained using the tangent of the drop profile at the triple line. For each spin-coated film, contact angles were measured on about five samples; six drops per sample were analyzed. The reported contact angle values correspond to the average of all measurements with an error bar corresponding to the standard deviation.

angle measurements of water and diiodomethane. Finally, surface free-energy variations have been assessed.

2. EXPERIMENTAL SECTION 2.1. Materials. 4-Phenyl azophenol, triethylamine (Aldrich), acryloyl chloride (Acros), and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1octanol (ABCR) were used as received. 2,2-Azobisisobutyronitrile (AIBN, Acros) was recrystallized in methanol before use. Chloroform and dichloromethane (puro) were purchased from Carlo Erba, hexane, tetrahydrofurane (THF) and diiodomethane from Acros, and dimethylformamide (DMF) from Fisher Scientific. DMF was distilled before used. 2.2. Synthesis of Poly(Azo) and Poly(AcRf6) Homopolymers. 4-Phenyl azophenyl acrylate (Azo) was synthesized as follows: 10.0 g of 4-phenyl azophenol (5.04 × 10−2 mol) was dissolved in 70 mL of anhydrous dichloromethane, and then 5 mL of triethylamine was added. A volume of 3.5 mL (4.20 × 10−2 mol) acryloyl chloride was added dropwise (Figure 1). The mixture was stirred at 0 °C under argon for 48 h. Then, it was washed several times with water to remove the triethylamine. The organic phase was dried on MgSO4. After filtration, the solvent was eliminated under vacuum. Azobenzene based acrylate (Azo) was purified on a silica column with 50−50 dichloromethane−cyclohexane mixture as the eluent and characterized by 1H NMR in deuterated chloroform. A yield of 90% was obtained. The fluorinated 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate (AcRf6) monomer was prepared by esterification of acryloyl chloride with 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol, as described previously (Figure 1).19 Poly(Azo) and poly(AcRf6) homopolymers were synthesized as follows: 1.0 g of Azo (or AcRf6) was dissolved in 6 mL (or 0.1 mL) of freshly distilled DMF. Then 50 mg (5 wt %, 3.04 × 10−4 mol) of 2,2azobisisobutyronitrile (AIBN) was added, and the mixture was heated at 80 °C for 24 h under argon. After cooling at room temperature, the reaction mixture was washed once with an aqueous 0.5 M HCl solution to remove DMF, and then with water. The polymer was then precipitated three times in 200 mL of cold hexane. It was dried at 70 °C for 48 h. Yields of 60% were obtained. 2.3. Synthesis of Poly(Azo-co-AcRf6) Copolymers. Poly(Azoco-AcRf6) copolymers were synthesized with initial molar fractions of azobenzene acrylate (xiAzo) of 0.94, 0.79, 0.62, and 0.42. The synthesis was performed through the same protocol as homopolymer synthesis (Figure 1). Only the DMF volume was adjusted according to the studied molar proportions: 2.0, 1.0, 0.8, and 0.5 mL of DMF were used to dissolve 1 g of monomers for xiAzo of 0.94, 0.79, 0.62 and 0.42, respectively. The reactivity ratio measurements were carried out on a copolymer series synthesized through the same protocol, with a reaction time fixed to 45 min instead of 24 h. 2.4. Spin-Coating. A 10−2 mol·L−1 chloroform solution of polymer was spin-coated onto glass or quartz substrates at 6000 rpm for 60 s. Prior to use, the substrates were carefully cleaned in a beaker of ultrapure water (Millipore, 18 MΩ·cm) placed in an ultrasonic bath for at least 20 min at room temperature and were then dried with argon stream. Under those conditions, a 60 nm thick film was obtained, as measured with a DEKTAK 150 profilometer. In the case of poly(AcRf6), the glass substrates were covered with a Teflon sheet in order to ensure a better anchorage of the polymer film. 2.5. Analytical Techniques. The polymer molar weights were determined by size exclusion chromatography (Waters) equipped with Stryragel HR columns, a model 486 absorbance detector, and a model 410 differential refractometer. The retention times were recorded at a flow rate of 1.0 mL·min−1 with THF as eluent. The molar weights were determined by fitting of a standard polystyrene calibration curve. 1 H NMR spectra were recorded on a Bruker Avance DPX 250 NMR spectrometer. The compounds were dissolved in deuterated chloroform (Aldrich). Glass transition temperature (Tg) were determined using differential scanning calorimetry (DSC) carried out on a Q100 apparatus (TA Instruments). Tg values were taken as the onset point from the second

3. RESULTS AND DISCUSSION 3.1. Polymer Synthesis and Characterization. The aim of this study was to investigate photosensitive polymer films with adjustable surface free energy. This study was carried out on a copolymer series (poly(Azo-co-AcRf6)) combining units bearing either a semifluorinated side chain (AcRf6 for the low surface energy) or an azobenzene group (Azo for its photoswitchable wetting properties), in different proportions. Poly(Azo-co-AcRf6) copolymers were synthesized by freeradical polymerization initiated with 5 wt % AIBN (Figure 1). Different molar proportions (xiAzo) of azobenzene acrylate were used, and the copolymers were first characterized by size exclusion chromatography, 1H NMR spectroscopy, differential scanning calorimetry (DSC), and UV−visible spectroscopy. The number average molecular weight (Mn) of poly(Azo) is about 6000 g·mol−1. This fairly low value for a free radical polymerization is in agreement with results previously reported in the literature.20 The Mn’s of poly(Azo-co-AcRf6) polymers are 1000, 6000, 10 000, and 14 000 for xiAzo of 0.94, 0.79, 0.62, and 0.42, respectively. These low weights can be due to a solvent effect, since it has been complicated to find a common solvent of both monomers (DMF). However, the corresponding homopolymers are not or few soluble in this solvent. Thus, during polymerization, the copolymers can precipitate, leading to compounds with low molar weight. In addition, the low weights could also be ascribed to the low reactivity of the acrylate in the presence of lateral chains. Finally, the C


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Figure 2. 1H NMR spectra of poly(Azo) (a) and poly(Azo-co-AcRf6) synthesized starting from different molar fractions xiAzo of azobenzene acrylate: (b) 0.94, (c) 0.79, (d) 0.62, and (e) 0.42.

Table 1. Comparison between the Molar Fraction (xiAzo) of Azobenzene Acrylate Used for the Poly(Azo-co-AcRf6) Copolymer Synthesis and the Molar Fraction of Azobenzene Unit (xAzo) in the Copolymer Deduced from 1H NMR Spectraa

copolymers are obtained as nonsticky powders. It is not possible to obtain self-supporting films with good mechanical integrity, which is in agreement with their low molar weight. Figure 2 presents the 1H NMR spectra obtained for deuterated solutions of poly(Azo) and the different poly(Azoco-AcRf6) polymers. The 1H NMR spectrum of poly(AcRf6) cannot be recorded because poly(AcRf6) is not soluble in usual deuterated solvents. The protons of the acrylate function between 6.1 and 6.8 ppm are not detected on the different spectra while the chemical shifts associated to the CH2 groups resulting from the acrylate polymerization are observed at 2.5 and 1.4 ppm (C region). In addition, the detection of both the protons of the CH2 group in α position of the poly(AcRf6) ester function assigned at 4.3 ppm (B region) and the aromatic protons of the azobenzene groups at 7.3, 7.4, and 7.8 ppm (A region) confirms the presence of Azo and AcRf6 units in the copolymers. From these spectra, the effective molar fraction (xAzo) of azobenzene units can be determined from the integrals of the peaks at 7.8 and 4.3 ppm assigned to the azobenzene groups (4H by unit) and to the CH2 group in α position of the poly(AcRf6) ester function (2H by unit), respectively. The xAzo values are presented in Table 1 and compared with the molar fraction xiAzo used for copolymer synthesis. The effective molar fraction xAzo is systematically lower than the initial proportion, indicating that azobenzene acrylate is probably less reactive than AcRf6 fluorinated acrylate. In order to check this hypothesis, monomer reactivity ratios were determined. The instantaneous copolymer composition can be expressed as follows:

xiAzo

xAzo

Tg (°C)

0 0.42 0.62 0.79 0.94 1

0 0.24 0.42 0.71 0.83 1

−12 26 48 57 79 92

a

Glass transition temperatures measured by DSC are also reported for each polymer.

d[Azo] [Azo] rAzo[AcRf6] + [AcRf6] = d[AcRf6] [AcRf6] [Azo] + rF[AcRf6]

(1)

where the reactivity ratios rAzo and rF are defined as: rAzo =

KAzo−Azo KAzo−F

and

rF =

KF−F KF−Azo

where the rate constant nomenclature is illustrated in the following reactions: KAzo−Azo

∼Azo· + Azo ⎯⎯⎯⎯⎯⎯⎯→ ∼∼ Azo· KAzo−F

∼Azo·+AcRf6 ⎯⎯⎯⎯⎯→ ∼∼ AcRf6· KF−Azo

∼AcRf6·+Azo ⎯⎯⎯⎯⎯→ ∼∼ Azo· D


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∼AcRf6·+AcRf6 ⎯⎯⎯⎯→ ∼∼ AcRf6·

The ratio between the initial molar fractions is defined as Ri = (xiAzo/xiF) = ([Azo]/[AcRf6]) while the effective ratio is R=

xAzo d[Azo] = xF d[AcRf6]

Equation 1 can be thus written as follows: R = Ri

rAzoRi + 1 rF + Ri

or

Ri −

Ri Ri2 = rAzo − rF R R

The reactivity ratios can be thus deduced from the slope and the intercept of the linear curve Ri − (Ri/R) as a function of (Ri2/R) (Figure 3). One obtains rAzo = 0.7 and rF =0.9.

Figure 3. Evolution of Ri − (Ri/R) as a function of (Ri2/R) for poly(Azo-co-AcRf6) polymers. Figure 4. (A) DSC thermograms obtained at the second run for poly(Azo-co-AcRf6) with different molar fractions xAzo of azobenzene unit: (a) 0, (b) 0.24, (c) 0.71, (d) 0.83, and (e) 1. (B) Evolution of (1/ Tg) of poly(Azo-co-AcRf6) as a function of the weight fraction of azobenzene group, compared to the dotted line representing Fox law.

r Azo < 1 indicates that azobenzene acrylate reacts preferentially with AcRf6 fluorinated acrylate, while rF ∼1 shows that AcRf6 units react indifferently with both monomers. These results are in agreement with the 1H NMR characterization showing that the measured molar fraction of azobenzene groups is systematically lower than the introduced one. The different polymers were then characterized by differential scanning calorimetry (DSC). The thermograms are shown in Figure 4A, and the glass transition temperatures Tg deduced from these curves are reported in Table 1. First, poly(Azo) and poly(AcRf6) homopolymers show glass transition temperatures at 92 and −12 °C, respectively. These Tg are sufficiently different to inform on the structure of the poly(Azo-co-AcRf6) copolymers. Only one glass transition temperature is detected, meaning that the synthesis of either a homopolymer mixtures or block copolymers can be ruled out. Indeed, in these cases, two Tg should be detected close to 92 and −12 °C. Second, the Tg values were then compared with those deduced from empirical Fox law describing statistical copolymers and expressed as ω ω 1 = 1 + 2 Tg Tg1 Tg2

Figure 4B shows the evolution of the measured (1/Tg) value as a function of the weight fraction of azobenzene group, compared to the dotted line representing Fox law. The measured values are systematically below the calculated ones, in a more pronounced way for both lower fractions of azobenzene group. This deviation can be explained by a rigidity decrease in the bonds in the copolymers compared with that in poly(Azo). Thus the introduction of fluorinated units between azobenzene ones allows decoupling of the rigid bonds and leads to a lower glass transition temperature than that predicted by Fox law. Finally, poly(Azo-co-AcRf6) copolymers can thus be considered as statistical (DSC measurements) with the presence of short AcRf6 blocks (1H NMR analysis). 3.2. Spin-Coated Films. Poly(Azo-co-AcRf6) with different xAzo molar proportions (1, 0.83, 0.71, 0.42, 0.24, 0) were spincoated on glass substrates. 60 nm thick films were obtained. To obtain a homogeneous coating of poly(AcRf6), the glass had to be covered with a Teflon sheet. The azobenzene-containing films were first characterized by UV−visible spectroscopy upon a UV/visible light irradiation cycle. Then, after an atomic force microscopy (AFM) study of the possible change in surface morphology due to photoirradiation, water and diiodomethane wetting behavior was examined. 3.2.1. UV−Visible Spectroscopy Characterization. Figure 5a shows the UV−visible absorption spectra measured for a

where Tg, Tg1, and Tg2 are the copolymer, poly(Azo), and poly(AcRf6) glass transition temperatures respectively, and ω1 and ω2 are the weight fractions of Azo and AcRf6 units in the copolymer. E


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Figure 5. UV−visible spectra of a poly(Azo-co-AcRf6) (xAzo = 0.83) spin-coated film (a) before and right after different times of UV photoirradiation (t = 5, 10, 20, 25 min) at 330 nm. (b) UV−visible spectra of the same film exposed to UV light for 25 min and subsequently irradiated with 430 nm visible light for 10 and 20 min.

Figure 6. AFM images on a 1 μm × 1 μm scale recorded on a poly(Azo-co-AcRf6) (xAzo = 0.83) spin-coated film (a) as prepared, (b) after 20 min UV photoirradiation at 330 nm, and (c) after 20 min UV photoirradiation followed by 20 min visible light photoirradiation at 430 nm. The rootmean-square roughness Rq is also indicated.

F


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poly(Azo-co-AcRf6) (xAzo = 0.83) spin-coated film, as-prepared and submitted to different times of UV photoirradiation at 330 nm. The spectrum of the as-prepared film exhibits the two characteristic bands of azobenzene-modified molecules: a high intensity π→π* band in the UV region (λmax = 317 nm) and a low-intensity n→π* band in the visible region (λmax = 440 nm).21 As expected for a trans-cis isomerization of azobenzene groups, the UV irradiation leads to a gradual decrease in the absorbance at 317 nm and an increase in the intensity of the band at 440 nm. The spectra measured after 20 and 25 min of exposure being undistinguishable, a photostationary state seems to be reached within 20 min. A film preirradiated for 20 min with UV light was subsequently exposed to different times of visible light irradiation at 430 nm. As shown in Figure 5b, this causes the π→π* transition absorption to increase and the n→ π* band intensity to decrease. The spectrum of the as-prepared film is fully recovered when the visible light exposure reaches 20 min. This reversibility is thus attributed to the full cis to trans isomerization. Similar evolutions were monitored for the other copolymers. In addition, the same behavior was observed in chloroform solutions. 3.2.2. Surface Morphology of the Films. Wettability is not only governed by surface chemistry but also by surface roughness. In view of contact angle measurements after UV/ visible light irradiation, the surface morphology of the poly(Azo-co-AcRf6) spin-coated films was analyzed by AFM upon cycles of UV/visible photoirradiation, as a function of the azobenzene molar fraction. Figure 6 shows images recorded on poly(Azo-co-AcRf6) films of molar fraction xAzo = 0.83, asprepared, after 20 min UV exposure, and after subsequent 20 min visible light irradiation. The as-prepared film appears very smooth, with a root-mean-square roughness Rq lower than 1 nm. Then after the different stages of photoirradiation, the surface topography is not modified. The same results were obtained, independently of the molar fraction of azobenzene groups in the copolymer. Indeed, the root-mean-square roughness measured on the films of poly(Azo) and the different copolymers does not exceed 1 nm, both before and after UV photoirradiation. Only the poly(AcRf6) film which was spin-coated on a Teflon sheet appears rougher, with a root-mean-square roughness close to 7 nm. This can be explained by the roughness Rq of 3.0 nm for a glass plate covered by a Teflon sheet compared to 0.2 nm for a glass plate, at this scale, as detected by AFM. These AFM characterizations mean that the significant change in azobenzene group section upon trans−cis isomerization does not manifest itself by surface topography modifications at the considered scale. 3.2.3. Wettability. 3.2.3.1. trans-Rich Azobenzene Surfaces. The advancing contact angles θa of water and diiodomethane measured on the poly(Azo-co-AcRf6) spin-coated films of different compositions are presented in Table 2. Water receding contact angles θr are also indicated as well as contact angle hysteresis θa − θr. Unfortunately, diiodomethane receding contact angles were not obtained due to an anchoring phenomenon of the triple line at the rear edge of the liquid drop. The water advancing contact angle decreases with increasing proportion of azobenzene groups, from 125.3° on the poly(AcRf6) film to 89.8° on the poly(Azo) spin-coated film. The value obtained for poly(AcRf6) is in agreement with those reported for fluorocarbon surfaces.22 Considering the contact angle hysteresis, θa-θr, it is close to 30° on the azobenzene-rich

Table 2. Advancing θa Contact Angles Measured for Water and Diiodomethane on Homopolymers and poly(Azo-coAcRf6) Spin-Coated Films for Different Azobenzene Unit Molar Fractions xAzoa water poly(Azo-coAcRf6) xAzo 0 0.24 0.42 0.71 0.83 1

θa (°) 125.3 124.9 120.5 115.8 100.4 89.8

± ± ± ± ± ±

diiodomethane

θr (°) 0.6 0.6 1.1 1.2 0.7 0.7

80.3 76.0 69.7 66.2 73.8 57.8

± ± ± ± ± ±

0.5 1.2 0.9 1.0 1.3 0.9

θa − θr 45.0 48.9 50.8 49.6 26.6 32.0

± ± ± ± ± ±

1.1 1.8 2.0 2.2 2.0 1.6

θa (°) 105.8 103.7 101.1 95.6 88.6 80.2

± ± ± ± ± ±

0.6 0.5 1.5 0.6 0.6 1.6

Water receding contact angle θr and contact angle hysteresis (θa − θr) are also presented. a

surfaces (poly(Azo) and poly(Azo-co-AcRf6) xAzo = 0.83), while it reaches 50° over the other copolymer films. From AFM characterizations, this hysteresis increase is not related to surface roughness at the probed scales and most probably results from chemical heterogeneities. The contact angle hysteresis measured on the poly(AcRf6) film cannot be compared to those obtained on the other surfaces, since this film was spin-coated on a Teflon sheet rougher than glass substrates. In the same way, diiodomethane advancing contact angles increase with the molar fraction of fluorinated unit, from 80.2° on the poly(Azo) spin-coated film to 105.8° on the poly(AcRf6) surface. The values of water and diiodomethane advancing contact angles measured on the poly(Azo) spin-coated film can be compared to those reported in the literature for azobenzenemodified surfaces. Siewierski et al. and Delorme et al. measured water static contact angles of 75° and 69°, respectively, on selfassembled monolayer of alkyl-siloxane functionalized with azobenzene groups.11,12 These values are significantly lower than the one measured on the poly(Azo) spin-coated film (θa = 89.8°), which could be ascribed to different organizations of the azobenzene moieties in the self-assembled monolayer and in the spin-coated film. Water advancing contact angles were also reported on surfaces functionalized with azobenzene groups bearing a hydrocarbon chain (Azo-(CH2)n-CH3) in the para position. Values of 94° and 101.3° were, respectively, obtained by Oh et al.15 on calix[4]resorcinarenes bearing four Azo(CH2)7-CH3 and by Yang et al.13 on silanized silicon surfaces functionalized with Azo-(CH2)4-CH3. These values are higher than those measured on poly(Azo) spin-coated films and in agreement with the values measured on the films of poly(Azoco-AcRf6) bearing a low proportion of fluorinated chains (xAzo = 0.83). This hydrophobicity increase is in agreement with the presence of the hydrocarbon chains at the surface. The contact angle hysteresis values reported by these authors differ significantly (54° and 25.3°, respectively). The absence of data concerning the roughness of these surfaces makes impossible the comparison with the value obtained on the poly(Azo) spin-coated films (32°). In the case of diiodomethane, the advancing contact angle value reported by Yang et al for Azo-(CH2)4-CH3 surfaces,13 54.1°, is significantly lower than the one measured on poly(Azo) films (θa = 80.2°), contrary to the water advancing contact angle which is higher, as previously mentioned. G


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Figure 7. Plot of cos θa measured for water (left) and diiodomethane (right) on poly(Azo-co-AcRf6) spin-coated films, as a function of the molar fraction xAzo of azobenzene units. The dotted lines represent Cassie’s law.

trans−cis isomerization on their wetting behavior. The advancing and receding contact angles of water and diiodomethane were thus measured before (θa, θr) and after (θa*, θr*) 20 min UV photoirradiation. The variations upon irradiation (θa* − θa) and (θr* − θr) are plotted in Figure 8 as a function of the copolymer molar composition xAzo. The values are reported in the Supporting Information (Table S1). One can first notice that the advancing and receding contact angles measured on the poly(AcRf6) spin-coated films do not vary after UV photoirradiation, in agreement with the absence of azobenzene groups in the polymer. The water and diiodomethane advancing contact angles measured on the different films containing azobenzene groups decrease in agreement with the modification of the azobenzene dipole moment associated to trans-cis isomerization. The amplitude variation is significantly different for the two probe liquids. Indeed, water advancing contact angle decreases relatively moderately, from 3 to 6° depending on the molar fraction. Considering the uncertainties, it is not possible to detect a trend with the molar fraction xAzo of azobenzene groups. In contrast, the amplitude of θa variations measured with diiodomethane increases with xAzo, reaching a significant value of about 50° on poly(Azo) spin-coating film. Since the higher variation is detected on the surface bearing the higher fraction of azobenzene groups, the isomerization does not seem to be prevented by the density of photosensitive moieties. It can be also noticed that the amplitude value of θa* − θa remains quite significant, close to 10°, for the copolymer film of azobenzene fraction xAzo= 0.42, even though the surface fraction xS of azobenzene was previously estimated to be lower (xS ∼ 0.15). This could indicate a possible contribution of the azobenzene groups in the adjacent layer and/or of the copolymer microstructure. The θa variation measured for water on poly(Azo) spincoated films (θa* − θa = −5°) is close but somewhat lower than those reported by Oh et al. for azobenzene-terminated calix[4]resorcinarenes monolayers (θa* − θa = −8°) and by Yang et al. for self-assembled monolayers of azobenzene functionalized with an alkyl chain (θa* − θa = −12°).15,13 Concerning water receding contact angle variations upon photoswitching, quite different behaviors were reported. Indeed, Oh et al. obtained a θr decrease upon UV photoirradiation (θr* − θr = −6°), in the same way as for poly(Azo) spin-coated films, while Yang et al. observed an increase in θr (θr* − θr = 11°). Few studies were undertaken with

In order to analyze more precisely the effect of the copolymer composition on the advancing contact angle values, the evolution of the cosine of θa was plotted as a function of the azobenzene molar fraction xAzo, for both liquids (Figure 7). Indeed, if the fraction of the surface covered by azobenzene units, denoted as x S , corresponds to the copolymer composition xAzo, Cassie’s law describing chemically heterogeneous surfaces should allow reproducing the experimental data. This law can be written as cos θa = xS cos θa,poly(Azo) + (1 − xS)cos θa,poly(AcRf6)

where xS is the surface fraction of poly(Azo), and θa,poly(Azo) and θa,poly(AcRf6) the advancing contact angles measured on poly(Azo) and poly(AcRf6) spin-coated films, respectively. As shown in Figure 7, a negative deviation from the dotted line illustrating Cassie’s law is evidenced for both liquids. This means that the surface fraction of azobenzene units is lower than the corresponding molar fraction within the copolymer. Cassie’s law can thus be used to estimate xS from the advancing contact angles obtained on each of the copolymer spin-coated films. The values of xS deduced from the measurements performed with both water and diiodomethane are presented in Table 3, for the different spin-coated films. Considering the Table 3. Values of the Surface Fraction xS of Azobenzene Units on the Different Poly(Azo-co-AcRf6) Spin-Coated Films Deduced from Cassie’s Law Applied with Both Water and Diiodomethane Advancing Contact Angles poly(Azo-co-AcRf6) 0.83 0.71 0.42 0.24

xS (water) 0.69 0.25 0.12 0.01

± ± ± ±

0.04 0.05 0.04 0.03

xS (diiodomethane) 0.67 0.39 0.18 0.08

± ± ± ±

0.07 0.06 0.09 0.04

experimental uncertainties, the values deduced from the data obtained with both liquids are comparable. The main result is that, for the whole spin-coated films, the surface fraction xS of azobenzene unit is significantly lower than the molar fraction xAzo in the copolymer. Due to their hydrophobic character, the fluorinated groups lie thus preferentially at the film−air interface. 3.2.3.2. Photocontrol of the Wettability. The different poly(Azo-co-AcRf6) spin-coated films were then UV photoirradiated in order to study the influence of the azobenzene H


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Figure 8. Variation, upon 20 min UV photoirradiation, of the advancing (θa* − θa) (left) and receding (θr *−θr) (right) water (up) and diiodomethane (bottom) contact angles measured on homopolymers and poly(Azo-co-AcRf6) spin-coated films as a function of the azobenzene molar fraction xAzo. For comparison, the same scale was used for water and diiodomethane. The horizontal lines at 0 are guides to the eye.

diiodomethane as liquid. Yang et al. reported θa variations of −12° for monolayers of azobenzene groups functionalized with an alkyl chain.13 This variation is significantly lower than the one obtained for poly(Azo) film reaching −50°. These results show that, for these copolymers, surface roughness is not required for amplification of light-induced contact angles switching. Finally, diiodomethane advancing contact angles on poly(Azo-co-AcRf6) spin-coated films can be tuned from 80° to 104° through azobenzene unit proportion while upon photoirradiation, this leads to contact angles between 30° and 102°, according to the copolymer composition. In order to check whether or not the introduction of a small proportion of fluorinated monomers facilitates azobenzene groups isomerization, Cassie’s law was used to calculate the advancing contact angles that should be obtained on the copolymer films if the proportion of isomerized azobenzene groups was the same as in poly(Azo) film. Indeed, it is known that the free volume of the azobenzene-modified film has significant influence on the photoresponse of azobenzene chromophores.23 From Cassie’s law applied to the copolymer spin-coated films, cos θa and cos θa* can be written as

From the first equation, we deduced the surface fraction xS of azobenzene groups, as previously mentioned (Table 3). The second equation then allows calculating θa*. The comparison between the experimental and calculated values obtained in the case of diiodomethane for the different copolymer compositions is shown in Figure 9. For the copolymer films of high azobenzene fraction (xAzo = 0.83 and 0.71), the experimental values of θa* are lower than the calculated ones. The fraction of isomerized azobenzene groups seems thus to be higher in these copolymer films than in the poly(Azo) homopolymer spin-coated film, meaning that isomerization is partly prevented in poly(Azo) film. The decrease in azobenzene group density due to fluorinated unit incorporation has thus a positive effect on the isomerization efficiency. The contact angle variation upon photoirradiation could be thus optimized by synthesizing a copolymer with a molar fraction of azobenzene groups intermediate between 0.83 and 1. 3.2.3.3. Surface Free Energy of Photoswitchable SpinCoated Films. The surface free-energy is an important parameter to predict wetting behavior of a liquid on a surface and estimate the type and magnitude of intermolecular interactions. Nevertheless, there is no direct reliable method for measuring this parameter. As a result, several theoretical approaches have been developed to assess this quantity. The

cos θa = xS cos θa,poly(Azo) + (1 − xS)cos θa,poly(AcRf6) cos θa* = xS cos θa*,poly(Azo) + (1 − xS)cos θa,poly(AcRf6) I


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⎛

cos θ = −1 + 2

⎞ ⎛ γp ⎞ γLd ⎟ L ⎟ p⎜ + 2 γS ⎜ ⎟ ⎟ γ ⎝ γL ⎠ ⎝ L ⎠

γSd ⎜⎜

The surface free energies γS and their dispersive γSd and polar γSp components can thus be estimated from water and diiodomethane advancing contact angle values. These two liquids present significantly different properties, with water being polar (γS =72.8 mN·m−1; γSd = 21.8 mN·m−1) while diiodomethane is apolar (γS =50.8 mN·m−1; γSd = 50.8 mN·m−1). In addition, γS, γSd, and γSp evolution upon UV photoirradiation can be analyzed as a function of the azobenzene molar fraction xAzo in the copolymer. Figure 10, left presents the variation of γS with xAzo before and after UV photoirradiation, that is, for azobenzene groups mainly in the trans and cis form, respectively. Before photoirradiation, γS decreases continuously from 23.2 mN/m for poly(Azo) spincoated film to 7.0 mN/m for poly(AcRf6) surface. This low value is in agreement with those reported for fluorinated surfaces.24 After UV photoirradiation, the surface free energy of poly(Azo) spin-coated film increases up to 46.0 mN/m. Then, as the azobenzene molar fraction decreases, the difference in γS upon UV photoirradiation is less and less important, while no longer variation is detected for poly(AcRf6) surface, in agreement with the absence of a photosensitive group. The γS increase evidenced after UV photoirradiation of the surfaces containing azobenzene groups is related to the conformation change of azobenzene groups upon trans-cis isomerization. This variation is quite significant compared to the values close to 7 mN/m reported by Yang et al. for self-assembled monolayers of azobenzene functionalized with an alkyl chain.13 The percentage of the surface free energy represented by the dispersive component, γSd/γS, is plotted in Figure 10, right as a function of the azobenzene molar fraction xAzo in the copolymer, before and after photoirradiation. Considering the poly(Azo) spin-coated film, the dispersive part represents 75% of γS in the trans-rich state while a value close to 98% is reached after UV photoirradiation. Thus, in spite of the change in dipole moment from 0 to 3D of the azobenzene moiety, these results indicate that the polarization remains weak with respect to dispersion forces. This is confirmed by the strong affinity of nonpolar diiodomethane for the trans-rich poly(Azo) spincoated film. The same trend is observed for the copolymer

Figure 9. Diiodomethane advancing contact angle θa* measured after UV photoirradiation on the different poly(Azo-co-AcRf6) spin-coated films (filled symbols), compared to the values deduced from Cassie’s law (open symbols).

indirect determination of surface free-energy of solid by means of contact angle measurements relies on the modeling of the liquid−solid surface tension γSL or equivalently the work of adhesion for a solid − liquid interface expressed as WSL = γL(1 + cos θa)

where γL is the liquid−vapor surface tension and θa is the liquid drop advancing contact angle. Among the different models developed in the literature, the Owens−Wendt approach is based on the decomposition of the surface tension (or energy) into two components due to dispersion forces (γd) on one hand and dipole−dipole interactions and hydrogen bonding (γp) on the other hand:

γL = γLd + γLp γS = γSd + γSp

where γS is the surface free energy. The work of adhesion is then expressed as WSL = 2 γLdγSd + 2 γLpγSp

One thus obtains the following expression:

Figure 10. Surface free energy γS (left) and percentage of the surface free energy represented by the dispersive component γSd/γS (right) for poly(Azo-co-AcRf6) spin-coated films of different azobenzene molar fraction xAzo, before (trans state) and after 20 min UV photoirradiation (cis state). J


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films, with an increase in the dispersive part which is less and less pronounced as the azobenzene molar fraction decreases. These results show that spin-coating of poly(Azo-co-AcRf6) allows tuning surface free-energy between 7 and 23 mN·m−1 through copolymer composition and between 7 and 46 mN·m−1 upon UV photoirradiation.

(6) Yager, K. G.; Barret, C. J. Novel photo-switching using azobenzene functional materials. J. Photochem. Photobiol., A 2006, 182, 250−261. (7) Jiang, W.; Wang, G.; He, Y.; Wang, X.; Song, Y.; Jiang, L. Photoswitched wettability on an electrostatic self-assembly azobenzene monolayer. Chem Commun 2005, 28, 3550−3552. (8) Feng, C.; Zhang, Y.; Jin, J.; Song, Y.; Xie, L.; Qu, G.; Jiang, L.; Zhu, D. Reversible wettability of photoresponsive fluorine-containing azobenzene polymer in Langmuir-Blodgett films. Langmuir 2001, 17, 4593−4597. (9) Paik, M. Y.; Krishnan, S.; You, F.; Li, X.; Hexemer, A.; Ando, Y.; Kang, S. H.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Surface organization, light-driven surface changes, and stability of semifluorinated azobenzene polymers. Langmuir 2007, 23, 5110−5119. (10) Lim, H.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. Photoreversibly switchable superhydrophobic surface with erasable and rewritable pattern. J. Am. Chem. Soc. 2006, 128, 14458−14459. (11) Siewierki, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Photoresponsive monolayers containing in-chain azobenzene. Langmuir 1996, 21, 5838−5844. (12) Delorme, N.; Bardeau, J.-F.; Bulou, A.; Poncin-Epaillard, F. Azobenzene-containing monolayer with photoswitchable wettability. Langmuir 2005, 21, 12278−12282. (13) Yang, D.; Piech, M.; Bell, N. S.; Gust, D.; Vail, S.; Garcia, A. A.; Schneider, J.; Park, C.-D.; Hayes, M. A.; Picraux, S. T. Photon control of liquid motion on reversibly photoresponsive surfaces. Langmuir 2007, 23, 10864−10872. (14) Pei, X.; Fernandes, A.; Mathy, B.; Laloyaux, X.; Nysten, B.; Riant, O.; Jonas, A. M. Correlation between the structure and wettability of photoswitchable hydrophilic azobenzene monolayers on silicon. Langmuir 2011, 227, 9403−9412. (15) Oh, S.; Nakagawa, M.; Ichimura, K. Photocontrol of liquid motion on an azobenzene monolayer. J. Mater. Chem. 2002, 12, 2262− 2269. (16) Demirel, G. B.; Dilsiz, N.; Ç akmak, M.; Ç aykara, T. Molecular design of photoswitchable surfaces with controllable wettability. J. Mater. Chem. 2011, 21, 3189−3196. (17) Groten, J.; Bunte, C.; Rühe, J. Light-induced switching of surfaces at wetting transitions through photoisomerization of polymer monolayers. Langmuir 2012, 28, 15038−15046. (18) Jin, C.; Yan, R.; Huang, J. Cellulose substance with reversible photo-responsive wettability by surface modification. J. Mater. Chem. 2011, 21, 17519−17525. (19) Darras, V.; Fichet, O.; Perrot, F.; Boileau, S.; Teyssié, D. Polysiloxane-Poly(fluorinated acrylate) Interpenetrating Polymer Networks: Synthesis and Characterization. Polymer 2007, 48, 687−695. (20) Brar, A. S.; Thiyagarajan, M. Microstructure of trans-4acryloyloxyazobenzene/methyl methacrylate copolymers by n.m.r. spectroscopy. Polymer 1998, 39, 5923−5927. (21) Zhao, Y.; Ikeda, T. Azobenzene Chromophores in PhotoReversible Materials. In Smart Light-Responsive Materials: AzobenzeneContaining Polymers and Liquid Crystals; John Wiley: Boca Raton, FL, 2009. Chapter 1, pp 1−46. (22) De Gennes, P. G. ; Brochard-Wyart, F. ; Quéré, D. In Gouttes, bulles, Perles et Ondes; Berlin 2002. (23) Haro, M.; Giner, B.; Gascon, I.; Royo, F. M.; Lopez, M. C. Isomerization behavior of an azopolymer in terms of the LangmuirBlodgett film thickness and the transference surface pressure. Macromolecules 2007, 40, 2058−2069. (24) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. Properties of films of adsorbed fluorinated acids. J. Phys. Chem. 1954, 58, 236−239.

4. CONCLUSIONS Polyacrylate copolymers with different proportions of azobenzene and semifluorinated chain as pendant groups, denoted as poly(Azo-co-AcRf6), were synthesized and characterized by 1H NMR spectroscopy and DSC. It was deduced that azobenzene acrylate is a little less reactive than semifluorinated acrylate, leading to both a molar fraction of azobenzene groups lower than the initial molar fraction. Poly(Azo-co-AcRf6) spin-coated films were prepared and characterized by UV−visible spectroscopy through a cycle of UV−visible light irradiation. Their topography was analyzed by AFM while dynamic contact angles of water and diiodomethane were measured before and after UV photoirradiation. Advancing contact angles were observed to decrease upon UV exposure, in a more pronounced way in the case of diiodomethane and with a switching amplitude increasing with the azobenzene groups fraction. In particular, significant advancing contact angle variations were obtained on poly(Azo) spin-coated film when diiodomethane is used as liquid. Indeed, a contact angle switching of 50° was measured during the process of photoisomerization from trans to cis. This means that large changes can be detected in these systems without any introduction of surface nanoroughness, as frequently required. The surface free energy was deduced from water and diiodomethane advancing contact angles. A significant variation of γS, from 23 mN/m for the trans-rich surface to 46 mN/m for the cis-rich surface, was obtained. The surface free energy is dominated by dispersive interactions in both states, their proportion increasing in the cis state.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 presents the variation, upon 20 min UV photoirradiation, of the advancing (θa* − θa) and receding (θr* − θr) water and diiodomethane contact angles measured on poly(Azo-co-AcRf6) spin-coated films with different azobenzene molar composition xAzo. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

(1) Xin, B.; Hao, J. Reversibly switchable wettability. Chem. Soc. Rev. 2010, 39, 769−782. (2) Wang, S.; Song, Y.; Jiang, L. Photoresponsive surfaces with controllable wettability. J. Photochem. Photobiol,. C 2007, 8, 18−29. (3) Katsonis, N.; Lubomska, M.; Pollard, M. M.; Feringa, B. L.; Rudolf, P. Synthetic light-activated molecular switches and motors on surfaces. Prog. Surf. Sci. 2007, 82, 407−434. (4) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Light-Driven Motion of Liquids on a Photoresponsive Surface. Science 2000, 288, 1624−1626. (5) Abrakhi, S.; Péralta, S.; Cantin, S.; Fichet, O.; Teyssié, D. Synthesis and characterization of photosensitive cinnamate modified cellulose acetate butyrate spin-coated or network derivatives. Colloid Polym. Sci. 2012, 290, 423−434. K


Poly(azobenzene acrylate-co-fluorinated acrylate) Spin-Coated Films

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