wettability and surface tension of amphiphilic polymer films: time-dependent measurements of the...

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Wettability and Surface Tension of Amphiphilic Polymer Films: Time-Dependent Measurements of the Most Stable Contact Angle

Elisa Martinelli, Giancarlo Galli,* Dory Cwikel, Abraham Marmur*

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The wettability of new amphiphilic block copolymer fi lms is studied by measuring different types of water contact angles (static, receding, advancing, and most stable). The most stable contact angles are used to calculate the surface tensions of the fi lms following the recently developed Marmur–Valal correlation. Most stable contact angles of water in an air-in-water system are also measured to study the changes in surface ten-sion over relatively long periods of immersion time. The new approach yields surface tension values that are higher than those calculated by previous methods. The results suggest that reconstruction of the surface takes place at short time scale and continues to occur at long time scale. The responsive character of the polymer fi lms can have implications for their use as biofouling release coatings.

1. Introduction

The wettability of a solid surface is a characteristic that is important for many practical applications. Two of the main factors that affect wettability are the chemical composition and the geometric micro- and nanostructures of the sur-face. Wettability can be decreased by either incorporating a low surface tension component in the material [ 1 , 2 ] or, for hydrophobic surfaces, creating a local geometry with a large area relative to the projected area. [ 3 , 4 ] Concerning the latter possibility, a unique example coming from nature is the lotus leaf, for which a combination of surface hydro-phobic chemistry and proper roughness imparts a super-hydrophobic behavior to the system that is responsible for the natural self-cleaning process. [ 5 ]

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E. Martinelli , G. Galli Dipartimento di Chimica e Chimica Industriale and UdR Pisa INSTM, Università di Pisa, 56126 Pisa, Italy E-mail: [email protected] D. Cwikel , A. Marmur Department of Chemical Engineering, Technion-Israel Institute of Technology, 32000 Haifa, IsraelE-mail: [email protected]

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Polymers carrying perfl uoroalkyl side chains [ 6–8 ] are typically hydrophobic/lipophobic, low surface tension materials, which have been widely explored for low adhe-sion and solvent repellency applications. [ 9 ] While this type of polymers can promote the release of adhering materials in both air and water, they do not necessarily prevent the adhesion of foreign substances and organ-isms. [ 10 ] However, in the case of underwater applica-tions, such as anti-biofouling uses, the polymeric mate-rial should also exhibit low values of interfacial tension with water, so that the driving forces for the adsorption and adhesion of biological entities are minimized. [ 11–14 ] Hydrophilic surfaces based on polyethylene glycol (PEG) have shown signifi cant promise in this area, [ 15–17 ] since they present excellent resistance to protein adsorption and cell adhesion.

According to the above concepts, amphiphilic polymers containing hydrophilic and hydrophobic chemical con-stituents may be used as surface-active polymers being capable of both reducing adhesion and facilitating the release of soil and biological matter. Such compounds are able to modify spontaneously their surface proper-ties according to the nature of the contacting medium to minimize the interfacial tension with both air and water.

elibrary.com DOI: 10.1002/macp.201200163

Wettability and Surface Tension of Amphiphilic Polymer Films . . .

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MacromolecularChemistry and Physics

Among the different strategies to create polymers with switchable surface wettability, [ 10 , 18–23 ] one is represented by the combination of PEGylated and fl uorinated moieties in the same macromolecular structure. [ 12 , 13 , 24 , 25 ] The syn-thesis and the physico-chemical properties of a new class of amphiphilic block copolymers composed of a polysty-rene block and a PEGylated–fl uoroalkyl polystyrene block have recently been reported [ 26 ] together with their bio-fouling release properties when incorporated into elasto-meric SEBS based coatings. [ 27 ] Their surface consisting of both hydrophilic and hydrophobic domains turned out to be ambiguous to test marine biofouling organisms. This property was enhanced by immersion in water owing to a water-induced reorganization of the amphiphilic, respon-sive surface.

The confi guration of a polymer fi lm can change quickly when in contact with water. [ 12 , 26 , 28–30 ] Generally, the mod-ifi cations involved in the surface reorganization process of environmentally responsive materials can be detected by the variations of the water contact angle (CA) value as a function of the contact time. Indeed, the CA measure-ments are much more sensitive to the quick transforma-tion of surface confi guration than many other surface analytical techniques, although they do not provide details of the spatial rearrangements at the molecular level. However, the measurement of CAs is not as straight-forward as it deceptively appears to be. [ 5 ]

When a droplet of a wetting liquid makes contact with an ideal solid surface (smooth, chemically homogeneous, rigid, insoluble, and non-reactive) in air, the contact angle with the solid is defi ned by the Young equation:

cosθY =

σs + σl

σsl (1)

where θ Y is the Young CA, σ s and σ l are the surface ten-sions of the solid and liquid, respectively, and σ sl is the solid–liquid interfacial tension. Therefore, to be able to assess the surface tension of a solid, one needs to calcu-late the Young CA; this CA, in turn, can be estimated from the apparent (experimentally measurable) CA only if the most stable CA is known. [ 31 , 32 ] The most stable CA is the CA associated with the global minimum in the Gibbs energy of the wetting system, and is the only CA that can currently be theoretically linked with the Young CA. It can be estimated either from CA measurements following vibration of the system, [ 33 − 35 ] or from measurements of the advancing and receding CAs. [ 31 , 32 ]

However, even when the values of σ l and Young CA are known, a correlation between σ sl , σ s , and σ l is still required to solve Equation (1) for σ s . Following the pioneering con-tributions of Girifalco and Good [ 36 ] and Fowkes, [ 37 ] two main methodologies have been developed. One is the equation-of-state approach, [ 38 ] where the solid–liquid

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interfacial tension, σ sl , is assumed to be a function of the individual surface tensions only:

σsl = σl + σs − 2

√σlσs

(1 − β σl − σs

)2) (2)

where the recommended value for β is 0.0001057 (mN m − 1 ) − 2 . [ 38 ]

The other approaches consider the surface tension to be the sum of various contributions in addition to that of the van der Waals interactions, such as hydrogen bonding, polar interactions, or acid − base interactions. [ 39 , 40 ] The Owens − Wendt approach, for example, is based on the assumption that the surface tension of the solid consists of a dispersive contribution, σ s d , and a polar contribution, σ s p , such that

σs = σds + σps (3)

Since there are two unknowns ( σ s d , σ s p ), it is necessary to use at least two probing liquids.

The latter approach yields more information than the former, in terms of polar and non-polar components of the surface tension; however, this approach introduces more unknowns, and therefore experiments need to be done with several test liquids. The choice of these liq-uids may be critical to the accuracy of the results. [ 41 , 42 ] These two approaches, though very different, have a common starting point: the square-root dependence on the individual surface tensions, which derives from the approximate expression for the van der Waals interaction between two phases. [ 32 ] This approximation, while some-what reasonable for non-polar materials, has very lim-ited theoretical basis for the polar interactions. Therefore, although there is no doubt that polar components affect surface and interfacial tensions, their quantitative evalu-ation is still uncertain.

Recently, a new interfacial tension correlation has been developed by Marmur and Valal (MV) on the basis of the Gibbs dividing surface concept. [ 32 ] The main advantages of the new correlation are that (a) it derives from a fun-damental theoretical basis; (b) it is a rather universal correlation that was directly tested with many data for liquid − liquid systems and was found to yield the best fi t to date; and (c) it can be applied to results of CA measure-ments done with only a single liquid. The correlation is given by

cosh(σe f /σo)(σe f )−mσAB = cosh(σA/ σo)(σA)1−m

− cosh(σB /σo)(σB)1−m (4)

where σ AB is the interfacial tension between phases A and B, σ A and σ B are the surface tensions of A and B (by

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defi nition σ A is the phase with the higher surface tension), and σ o and m are constants. The effective surface tension is given by

σe f ≡ σA + σB −ησn

Aσ1−nB (5)

where η and n are constants. The best fi t of this correla-tion to over 120 experimental data for liquid–liquid sys-tems yielded the following values for the constants: σ o = 42.121, m = 0.93884, η = 0.83755, and n = 0.94965. [ 32 ]

The objective of the present work was to use the approach based on measurements of the most stable CA and their interpretation by the MV correlation to study the hydrophobic-to-hydrophilic transformations of amphiphilic block copolymer fi lms as a function of immersion time in water. Special attention will be given to the ability of characterization within short time scales (CA of a sessile water drop on the fi lm surface) and long time scales (captive bubble underneath the fi lm surface that is immersed in water).

2. Experimental Section

2.1. Materials

1-Phenylethyl bromide (1-(PE)Br), 2,2 ′ -bipyridine (Bipy), and CuBr were purchased from Aldrich and used without further purifi cation.

The monomer Sz was synthesized according to the procedure previously reported [ 26 ] from 4-vinylbenzoic acid (from Aldrich) and the PEGylated–fl uoroalkyl alcohol Zonyl FSO-100 (registered trademark of E. I. du Pont de Nemours & Co) (from Aldrich). The Zonyl FSO-100, F(CF 2 CF 2 ) y (CH 2 CH 2 O) x CH 2 CH 2 OH, has a distribution of molecular weights ( M

— w / M

— n ≈ 1.2), with x ≈ 5 and y ≈ 4. According

to SEC and NMR measurements, the monomer Sz synthesized also presented a relatively broad distribution of molecular weights ( M—

w / M—

n ≈ 1.3) and average degrees of polymerization x ≈ 5 and y ≈ 4. Styrene (S) (from Fluka) was washed with 5% NaOH and water, dried over Na 2 SO 4 , and distilled under vacuum prior to use. Poly(styrene- b- (ethylene- co- butylene)- b- styrene) (SEBS) triblock thermoplastic elastomer (Kraton G1652M) and SEBS grafted with 1.4–2.0 wt% maleic anhydride (SEBS-MA) (Kraton FG1901X) were kindly provided from Kraton Polymers.

For static CA measurements, the wetting liquids were water (HPLC grade; J. T. Baker) and n -hexadecane (99%; Aldrich). For most stable CA measurements, high-purity water was obtained by treatment of de-ionized water (reverse osmosis treatment) with a UHQ Elgar apparatus and had a resistivity larger than 18 M Ω cm. A.R. grade acetone, ethanol, and methanol used for cleaning were obtained from Biolab or Frutarom. Air was used without further purifi cation.

2.2. Polystyrene Macroinitiators

In a typical preparation, 27.270 g (262.21 mmol) of S, 0.771 g (4.94 mmol) of Bipy, and 223 μ L (1.63 mmol) of 1-(PE)Br were

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introduced into a dry Schlenk fl ask under nitrogen. The solution was purged with nitrogen for 15 min and then 0.237 g (1.65 mmol) of CuBr was added. After four freeze-thaw-pump cycles, the polymerization was let to proceed under nitrogen for 90 min at 110 ° C. When the reaction was stopped the polymer mixture was dissolved in THF and then eluted on neutral alumina to remove the catalyst. The solvent was removed under vacuum and the polymer was purifi ed by repeated precipitations from THF solutions into methanol (59% yield). The number-average degree of polymerization ( n ) of the polymer sample was 81 ( M

— n =

8400, M—

w / M—

n = 1.27), and the macroinitiator is named here S81. 1 H NMR (CDCl 3 ): δ (ppm) = 1.0 − 2.2 (CH 2 CH), 6.1 − 7.4 (H aromatic). FTIR (fi lm): v̄ (cm − 1 ) = 3082 − 3026 ( ν C — H aromatic), 2924 ( ν C — H aliphatic), 1601 ( ν C = C aromatic), 1493 and 1452 ( δ C — H aliphatic), 756 and 698 ( δ C — H aromatic).

2.3. Diblock Copolymers

In a typical preparation, 0.800 g (0.09 mmol) of S81 and 0.045 g (0.29 mmol) of Bipy were introduced into a dry Schlenk fl ask, which was then evacuated and fl ushed with nitrogen three times. A solution of 3.946 g (4.85 mmol) of Sz in 20 mL of dig-lyme was then added under nitrogen. The mixture was purged with nitrogen for 30 min and then 0.014 g (0.097 mmol) of CuBr was added. After four freeze–thaw–pump cycles, the polymeri-zation was let to proceed for 66 h at 115 ° C. When the reaction was stopped, the polymer mixture was dissolved in chloroform and then washed with water. The solvent was removed under vacuum and the polymer was purifi ed by repeated precipita-tions from THF solutions into methanol (44% yield). The number average degrees of polymerization of the S and Sz blocks of the sample were 81 and 19, respectively ( M

— n = 23 900, M

— w / M

— n =

1.50), and the block copolymer is named here S81Sz19. 1 H NMR (CDCl 3 ): δ (ppm): 0.9 − 2.1 (CH 2 CH), 2.4 (CH 2 CF 2 ), 3.2 − 4.2 (CH 2 O), 4.4 (COOCH 2 ), 6.2 − 8.0 (H aromatic). 19 F NMR (CDCl 3 /CF 3 COOH): δ (ppm): − 6 (3F, CF 3 ), − 38 (2F, CF 2 CH 2 ), − 46 to − 49 (10F, CF 2 ), − 51 (2F, CF 2 CF 3 ). FTIR (fi lm): v̄ (cm − 1 ) = 3080 − 3000 ( ν C — H aromatic), 1722 ( ν C = O), 1400 − 1000 ( ν C — O and ν C — F), 759 and 699 ( δ C — H aromatic), 654 ( ω CF 2 ).

2.4. Preparation of Two-Layer Films

The detailed procedure followed for the preparation of two-layer fi lms has previously been described. [ 27 ] In brief, (3-glycidyloxypropyl)-trimethoxysilane functionalized glass slides were coated by casting on a 12% (w/v) toluene solution of SEBS-MA and SEBS (56/44, w/w). The fi lms were allowed to dry slowly in a closed chamber for two days and then annealed in an oven overnight at 120 ° C to form the bottom layer. A 1.5% (w/v) toluene solution of either a block copolymer alone or a blend of a block copolymer with SEBS was spray-coated on the bottom layer using a Badger model 250 airbrush (50 psi air pressure). These two-layer fi lms were vacuum-dried in an oven at 60 ° C for 8 h and then annealed at 120 ° C overnight to form the top layer.

The bottom layer was the same for all the fi lms, while the top layer differed in terms of either the chemical composition of the block copolymer or the content of the block copolymer in the blend with SEBS (Table 1 ). Accordingly, the fi lms are denoted as

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Table 1. Composition of the top layer of the two-layer fi lms SnSzm_p.

Film Top layer composition [wt%]

n a) m b)

S26Sz23_100 S26Sz23 (100%)

26 23

S26Sz23_90 S26Sz23/SEBS (90%/10%)

26 23

S26Sz23_70 S26Sz23/SEBS (70%/30%)

26 23

S81Sz19_100 S81Sz19 (100%)

81 19

S81Sz19_90 S81Sz19/SEBS (90%/10%)

81 19

a) Degree of polymerization of the polystyrene block; b) Degree of polymerization of the PEGylated–fl uorinated polystyrene block.

Figure 1 . General structure of block copolymers SnSzm ( x ≈ 5, y ≈ 4).

SnSzm_p where n and m indicate the degrees of polymerization of the blocks of polystyrene and PEGylated–fl uorinated polystyrene respectively, and p is the weight percentage of copolymer in the blend with SEBS in the top layer (Table 1 ).

2.5. Characterization

Static CAs were measured on fi lms using the sessile drop tech-nique with a FTA200 Camtel goniometer. The receding and advancing CAs of water in air were measured using a Kruss DSA 100 drop analysis system.

For the determination of the most stable CA of water in air, the sample was vibrated, using a horizontal drop shaking apparatus. Several sessile water drops (50 to 100 μ L) were dispensed on the same sample with the help of a precision microsyringe. The drops were shaken till the three-phase contact lines started to move. The amplitude and frequency of the vibrating plate were usually around 0.2 mm and 13 Hz, respectively. Digital pictures of the drops were taken from above, using a A 034451 Tamron or a Computar MLH-X10 magnifying lens, attached to a high resolution U-eye digital camera that was aligned perpendicularly above the surface. For calibration of the pixel size, a small cylinder of precisely known diameter and of the same thickness as that of the sample was used. The pictures were processed with the Image-Pro Plus program, version 4.0 (Media Cybernetics). All drops that were not axisymmetric or almost axisymmetric (ratio of longest to shortest radius > 1.2) were excluded. The contact angles were then calculated by fi tting to solutions of the Young–Laplace equation, using the maximum diameter, volume of the drop, and the surface tension of water as input values. All contact angles were measured at 23 ° C.

For the determination of the most stable CA of air in water, the fi lm samples were tightly attached under a Tefl on platform. The samples (duplicates) were immersed in a glass tank fi lled with water that was placed on a transparent glass table. A digital camera equipped with a magnifying lens and an illumination lamp were placed and aligned under the table. An air bubble of known volume was dispensed on the immersed surface with the

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help of a microsyringe equipped with a bent needle. The water was agitated with the help of a magnetic stirrer. These vibrations are transferred via the water to the air bubble trapped under the surface. The axisymmetry of the bubble was ascertained. The most stable CA was calculated from the radius of the bubble as for most stable CA of water in air (note that the contact angle always refers to the water side of the interface). The samples were kept immersed in water for 15 d and measurements were performed after ≈ 15 min ( t = 0) and at days 1, 3, 5, and 15.

Atomic force microscopy (AFM) experiments under ambient conditions (dry state) were carried out in tapping mode with a multimode system equipped with a Nanoscope IIIa controller (Veeco Instruments) using silicon cantilevers with a nominal force constant of 42 N m − 1 from Olympus type OMCL-AC160TS at a resonance frequency of about 320 kHz. To investigate structural evolution of the surfaces, the fi lms were stored in water for 7 d, and then transferred into the experimental chamber of the AFM fi lled with water, thereby avoiding drying-out. All in situ experiments were performed in tapping mode on a MFP-3D system (Asylum Research) using silicon cantilevers with an aluminum refl ex coating and a nominal force constant of 2.8 N m − 1 from Nanosensors type PPPFMR at a resonance frequency of about 24 kHz. In all experiments, the scan rate was kept at 1 Hz, while the tip-sample forces were carefully minimized to avoid artifacts. Tip radius of less than 7 nm (manufacturer's information) was used. Root-mean-square of surface roughness ( R rms ) was determined over regions of 20 × 20 μ m 2 :

Rrms =

√√√√ 1

mn

n∑j=1

m∑i=1

Z2(xi, yj ) (6)

with Z the height and x , y the in-plane coordinates stored by the AFM software.

3. Results and Discussion

3.1. Polymer Films

The polystyrene–amphiphilic polystyrene block copoly-mers (Figure 1 ) were prepared by atom transfer radical polymerization (ATRP) of the PEGylated − fl uoroalkyl sty-rene monomer Sz, starting from a bromo-terminated polystyrene macroinitiator. Their synthesis and detailed characterization have recently been described. [ 26 , 27 ]

By taking advantage of the ATRP controlled character, the contents of the hydrophobic polystyrene block (S)

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Table 2. Measured water and hexadecane contact angles ( ° ).

Film Static CA water

Static CA hexadecane

Most stable CA water

Advancing CA water

Receding CA water

Average CA a) water

S26Sz23_100 113 ± 2 69 ± 2 70.3 ± 1.4 102.9 ± 0.9 65.9 ± 2.3 89.2

S26Sz23_90 112 ± 2 66 ± 1 72.5 ± 1.1 117.5 ± 0.3 61.4 ± 1.1 89.5

S26Sz23_70 111 ± 2 67 ± 1 71.1 ± 0.7 121.6 ± 1.3 62.8 ± 2.4 91.9

S81Sz19_100 110 ± 1 64 ± 1 71.4 ± 0.7 109.2 ± 0.2 57.0 ± 2.2 83.8

S81Sz19_90 107 ± 1 65 ± 1 70.1 ± 0.3 109.0 ± 0.7 49.3 ± 1.1 80.6

a) Calculated from the averages of the cosines of the advancing and receding contact angles.

and the amphiphilic PEGylated − fl uoroalkyl styrene block (Sz) were tuned by adjusting the adopted polymerization conditions. The block copolymers are denoted as SnSzm, where n and m indicate the degrees of polymerization of the respective polymer blocks (Table 1 ).

The two-layer fi lms consisted of a thick elastomeric bottom layer (thickness 150 − 250 μ m) with a spray-coated thin top layer (thickness ≈ 500 nm). In any case, the bottom layer was a blend SEBS/SEBS-MA, while the top layer con-sisted of the surface-active block copolymer either alone or blended with SEBS (Table 1 ). The fi lms were annealed at 120 ° C for 12 − 15 h to promote the segregation of the fl uorinated component to the polymer − air interface thus favoring the formation of an equilibrium structure. The fi lms are denoted as SnSzm_p, where p is the wt% of SnSzm surface-active block copolymer in the blend with SEBS in the top layer of the fi lm.

According to this strategy, the fi lms comprised a low surface tension top layer deposited on a low elastic mod-ulus bottom layer. Young elastic modulus at 100% strain and elongation at break of the fi lms were found to be in the ranges of 2.8 − 4.2 MPa and 450 − 500%, respectively, typical of thermoplastic elastomers, such as the pristine SEBS. Therefore, an advantage of this strategy would con-sist in the use of small amounts of a surface-active block copolymer notably in the top layer of a two-layer fi lm to control independently the bulk modulus and the surface chemistry of suffi ciently thick polymer fi lms to be applied as fouling release coatings.

3.2. Contact Angle Measurements

The polymer fi lms were used for measurements of the static, receding, advancing, and most stable contact angles. Two different types of experiments were performed: (a) measuring the static, receding, advancing, and most stable CAs of water in air and the static CA of n -hexadecane in air; (b) measuring the most stable CA of water in an air-in-water system. The former was used to assess the surface tensions of the fi lms at shorter times (order of magnitude


of seconds-minutes) from initial contact, while the latter was used for studying the changes in surface tension over a longer period of time (weeks).

The values of CAs measured in the former set of experi-ments are shown in Table 2 . The most stable CA was always in-between the receding and advancing CAs, as it should be. [ 35 ] The static CAs also were within the hyster-esis range (except for the S26Sz23_100), however higher than the most stable CAs and closer to the advancing CAs. The static CAs with water and n -hexadecane for the two-layer fi lms were both larger than those measured for the respective pristine block copolymers, ≈ 106 ° and ≈ 66 ° , respectively, and signifi cantly larger than those for SEBS, 102 ° and 26 ° , respectively. Therefore, the wettability of the two-layer fi lms was dictated by the amphiphilic polymer that was preferentially segregated at the poly-mer–air interface. No infl uence was attributable to the SEBS underlayer that would obviously depress the n -hex-adecane CA. Nevertheless, it should be noted that there is an element of randomness in the positioning of the drop in a static CA measurement, therefore there is some doubt regarding its meaning and interpretation. [ 31 ]

An appreciable hysteresis was recorded for all the fi lms (advancing − receding CAs ≈ 40 − 60 ° ). The fi lms presented very smooth surfaces, the root-mean-square roughness (20 × 20 μ m 2 area) being 3–6 nm for the as-received fi lms and 15–40 nm for the fi lms after underwater immersion. Therefore, it is unlikely that roughness is the source of hysteresis. On the other hand, chemical heterogenei-ties, even in the nanoscale, may cause CA hysteresis. [ 34 ] The fi lms displayed surface morphologies with spher-ical or cylindrical nanodomains of the S discrete phase embedded in the Sz continuous matrix (20–30 nm diam-eter) (Figure 2 ). Reorganization of surface chemistry and structure may also lead to CA hysteresis, especially for amphiphilic polymer fi lms where a quick rearrangement can occur at the interface immediately following contact with water, owing to their responsive hydrophilic–hydro-phobic nature. [ 26 ] Our underwater AFM investigations revealed a transformation from a well-ordered surface

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Figure 2 . AFM phase images (0.5 × 0.5 μ m 2 area) of fi lms of SEBS (left), S23Sz26_90 (center) and S23Sz26_100 (right).

Figure 3 . Underwater AFM phase images (2 × 2 μ m 2 area) of fi lms of S23Sz26_90 (left) and S23Sz26_100 (right) after 7 d of immer-sion in water.

morphology to a mixed surface structure, in which the nanoscale heterogeneity was increased (Figure 3 ). There-fore, hysteresis, in the present case, most probably arises from the surface chemical heterogeneity and surface reorganization. The most stable CA represents the average Young CA for these samples.

The most stable CA may be roughly estimated by aver-aging the cosines of the advancing and receding CAs (Table 2 ). Table 2 also shows that the average CAs may be quite different from the measured most stable CAs. This may be expected, [ 35 ] since the concept underlying this average has been empirically hypothesized, without any fundamental indication.

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Table 3. Calculated surface tensions a) (in mN m − 1 ) using various corre

Film OW from static CA

EoS from most stable CA

S26Sz23_100 13.5 40.7

S26Sz23_90 14.2 38.8

S26Sz23_70 14.1 39.7

S81Sz19_100 14.9 39.5

S81Sz19_90 15.3 40.3

a) Calculated using the Owens–Wendt (OW), Equation-of-State (EoS) an


As explained in the introduction, in order to interpret CA measurements in terms of surface tension of the solid surfaces, empirical equations need to be used. As can be seen in Table 3 , the calculated values of the surface ten-sion may be quite different from each other, depending on the method of assessing the CA and the equation used to calculate the surface tension. Regarding the contact angle, the most stable CA is the preferable one, as indi-cated by theory, since it is the only CA that may be linked by theory with the Young CA. [ 31 ] The average of the cosine of the advancing and receding CAs is only an approxima-tion for the most stable CA and it should be used only in the absence of direct measurement of the most stable CA. As shown in Table 2 and as explained above, this average CA may be quite different from the most stable CA.

Table 3 shows that the surface tension values calcu-lated according to Equation (4) are higher than the values calculated by Equation (2) . Since the surface tension of the solid cannot, in most cases, be directly measured, these equations can be fully tested only for liquid − liquid − air systems. For such systems, Equation (4) fi ts experimental data better than Equation (2) . [ 32 ] Table 3 also presents the surface tension values calculated according to the Owens − Wendt method ( Equation 3 ), using water and n -hexadecane static CAs. Equation (3 ) predicts low surface tension values for the fi lms ( < 16 mN m − 1 ), much lower than those calculated according to Equations (2) and (4) . These values are consistent with those evaluated for other

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lations.

MV from most stable CA

EoS from average CA

MV from average CA

52.6 28.1 40.9

51.0 27.9 40.7

51.7 26.4 39.1

51.5 31.5 44.4

52.2 33.6 46.3

d Marmur–Valal (MV) correlations.


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Figure 4 . Time-dependent most stable contact angles of water in an air-in-water system.

fl uorinated fi lms for which water and n -hexadecane were used as contact liquids. [ 10 ] The calculated values of sur-face tension of the fi lm may be higher in some cases than in others because of two possible reasons: (a) the CA used for the calculations is lower, and (b) the empirical corre-lation used yields, in general, higher values. Indeed, the most stable CAs are the lowest among all CAs, except for the receding CAs (that are not used for the surface tension calculations). In contrast, the static CAs are the highest, except for the advancing CAs. Thus, the difference in cal-culated surface tension of the fi lm is not surprising. In addition, the MV correlation seems to lead to somewhat higher values of the surface tension than the EoS correla-tion, when the same CA is used. Thus, it is understandable why the MV correlation yields values that, on a fi rst sight, seem to be high.

In order to assess the ability of these amphiphilic fi lms to respond to external water environment over long time periods, the most stable CAs of water in an air-in-water system were measured at different immersion times. As evident from Figure 4 , the most stable CA decreased as a function of immersion time for all the tested surfaces. In fact, taking S26Sz23_70 as one example, CA decreases from 49.9 ° to 34.9 ° (Table 4 ). Moreover, one can also

Table 4. Most stable contact angles (in degrees) of water in an air-in

Sample t = 0 t = 1

S26Sz23_100 56.8 ± 3.0 54.6 ± 3.9

S26Sz23_90 50.9 ± 3.1 49.9 ± 2.1

S26Sz23_70 49.9 ± 5.2 48.4 ± 3.8

S81Sz19_100 57.9 ± 5.1 55.4 ± 2.0

S81sSz19_90 54.8 ± 3.4 54.1 ± 3.6


note that all the fi lms substantially exhibit the same descending trend with a difference between the initial and the fi nal values of CA of 14–20 ° . All the fi lm surfaces undergo a reconstruction and become progressively more hydrophilic. On the other hand, in any case, the surface tension values calculated with both Equations (2) and (4) increased with the immersion time (Figure 5 and Table 5 ). This suggests that the overall kinetics of reconstruction is similar for all the fi lms and basically independent of the chemical composition of the amphiphilic block copolymer and the amount of SEBS in the top layer within the ranges investigated.

A plausible explanation of such surface reorganization assumes fl ipping of the amphiphilic side chains. When the annealed fi lms are in contact with air, the fl uoroalkyl segments are selectively segregated to the polymer–air interface, being the lowest surface tension components in the system. On the other hand, the PEG segments are confi ned to stay in the bulk of the fi lm because of their higher surface tension. However, since the PEG segment is covalently bonded to the lower surface tension fl uori-nated segment, it is also dragged close to the surface. Therefore, when the fi lm is in contact with water the side chains reorganize themselves such that the PEG segments are preferentially exposed at the surface, thus lowering the interfacial tension with water. Such fl ipping of the pendent chain was reported to occur over a short time scale (seconds according to dynamic contact angle meas-urements). [ 26 ] This fast reconstruction process results in the above reported relatively low, most stable CAs of water in an air-in-water system detected upon contact with water ( ≈ 15 min, t = 0). Recently, some of us have proven by NEXAFS analysis of the fi lm surface that the amphiphilic block copolymers undergo an additional sur-face reconstruction process, which involves the migration of the hydrophobic polystyrene block away from the outer surface into the surface inner layers. [ 43 ] Such reconstruc-tion at the macromolecular level occurs simultaneously with the fl ipping, but requires a longer time scale. The two effects would add to each other and render the fi lm surface less hydrophobic. This explains the descending trend of the most stable CAs of water in an air-in-water


-water system at different immersion times t (in days).

t = 3 t = 5 t = 15

52.1 ± 1.1 n.d. 38.3 ± 0.8

50.1 ± 3.1 45.8 ± 2.1 36.9 ± 3.7

45.1 ± 3.5 40.7 ± 4.0 34.9 ± 2.2

50.8 ± 1.2 46.2 ± 1.1 38.1 ± 2.7

48.4 ± 2.1 42.4 ± 0.9 37.7 ± 1.9



www.mcp-journal.de


Figure 5 . Surface tension values at different immersion times in water calculated according to the Equation-of-State (left) and Marmur–Valal (right) approaches.

system (Figure 4 ) detected after prolonged periods of con-tact with water.

The responsive character of the amphiphilic polymer fi lms to the external environment can have implications for their use as coatings, namely as coatings for the release of marine biofouling. [ 44 ] In fact, it has been shown that several fouling organisms differently attach to substrata of different wettability. For example, sporelings (young plants) of the macroalga Ulva linza adhere more strongly to hydrophilic surfaces compared to hydrophobic surfaces, whereas cells of the diatom Navicula perminuta adhere more weakly to hydrophilic surfaces. [ 7 , 12–14 ] Accordingly, amphiphilic reconstructing coatings could better release such fouling organisms that have contrasting tendencies for adhesion.

4. Conclusions

The wettability of a set of amphiphilic polymer fi lms of regularly varied composition was tested by measuring the most stable water CA and evaluating the surface tension by the new Marmur–Valal approach. This approach was used here for the fi rst time to assess the dependences of


Table 5. Calculated surface tensions a) (in mN m − 1 ) using Equation-oat different immersion times t (in days).

Sample t = 0 t = 1

EoS MV EoS MV

S26Sz23_100 48.6 62.9 49.9 63.6

S26Sz23_90 52.2 64.7 52.8 65.0

S26Sz23_70 52.9 65.0 53.9 65.5

S81Sz19_100 48.0 62.5 49.5 63.3

S81sSz19_90 49.9 63.5 50.3 63.7

a) Calculated using the Equation-of-State (EoS) and Marmur–Valal (M

Macromol. Chem. Phys© 2012 WILEY-VCH Verlag G

surface tension on the chemical composition and nature of the polymer fi lm. The relatively low values of the most stable CAs of water in air as well as those of water in an air-in-water system at t = 0 point to a quick, initial surface reorganization that occurs after seconds–minutes after immersion in water. By measuring the most stable CA of water in an air-in-water system over a period of 15 d, it was shown that for all the tested fi lms the CA decreases, and by contrast the surface tension increases, as a consequence of the amphiphilic character of the fi lm surface.

The value of most stable CA, which at present is the only one that can be theoretically linked with the Young CA, was compared with those of other three types of CAs (static, advancing, and receding). The measured CAs were used to calculate the surface tension according to the three fundamentally different methods of Owens–Wendt, the Equation-of-State and Marmur–Valal. The approach based on the most stable CA with its interpretation by the MV correlation provided here values at variance with the previous two methods, which are widely used to char-acterize the surface tension of polymer fi lms. The above hypothesis regarding the two timescales of reorganization attempts to explain this variance. Future work with other polymer fi lms, including highly fl uorinated polymers that

1455

f-State and Marmur–Valal correlations from most stable contact angles

t = 3 t = 5 t = 15

EoS MV EoS MV EoS MV

51.5 64.4 n.d. b) n.d. b) 59.5 68.0

52.8 65.0 55.3 66.1 60.3 68.3

55.7 66.3 58.2 67.5 61.4 68.7

52.3 64.8 55.0 66.0 59.7 68.0

53.7 65.9 57.3 67.0 59.8 68.1

V) correlations; b) Not determined.

. 2012, 213, 1448−1456mbH & Co. KGaA, Weinheim

1456


www.mcp-journal.de


are common low surface tension, non-reconstructing materials, may assist in understanding this question of differences in the assessment of the surface tension of the fi lm.

Acknowledgements : We thank Dr. F. Bartels and Ms. K. Graf for assistance with AFM measurements. The work was funded by the EC Framework 6 Integrated Project “AMBIO” (Advanced Nanostructured Surfaces for the Control of Biofouling). This article refl ects only the authors’ views and the European Commission is not liable for any use that may be made of information contained therein.

Received: March 29, 2012 ; Published online: June 5, 2012; DOI: 10.1002/macp.201200163

Keywords: amphiphilic polymer; contact angle; fi lms; fl uoropolymers; surface tension

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