Article pubs.acs.org/Langmuir
Highly Water Repellent but Highly Adhesive Surface with Segregation of Poly(ethylene oxide) Side Chains Kaya Tokuda, Motoko Kawasaki, Masaru Kotera, and Takashi Nishino* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan S Supporting Information *
ABSTRACT: Polymer surfaces were modified using methacrylate terpolymers containing both perfluoroalkyl (Rf) groups and poly(ethylene oxide) (PEO) as side chains in the same molecule. The structure and properties of the modified surfaces were evaluated using X-ray photoelectron spectroscopy and by measuring the dynamic contact angles and 90° peel strength. It was found that not only Rf groups but also PEO side chains were segregated on the surface being against the order of the surface free energy. The terpolymer modified surface is hydrophobic in air because Rf groups are predominant, but it becomes hydrophilic in water because the surface is covered with PEO side chains. This response to the environment is rapid and reversible. The modified surface showed high water repellency because of the surface Rf groups and high adhesive strength because of the side chains. strength simultaneously, has been predicted theoretically10−12 and observed experimentally.8,9,12−15 Jannasch investigated the surface compositions of poly(styrene(S)-block-ethylene oxide (EO)) and poly(S-graf t-EO) films. He reported that S segments were segregated on the film surface of the copolymer having hydroxy groups at the EO chain ends, while EO segments predominated on the surface of films having fluorine at the EO chain ends. These findings show hydrophilic EO unit could be surface localized against the request of surface enrichment of hydrophobic segments under some conditions.16 In this study, we synthesized methyl methacrylate (MMA) terpolymers containing Rf groups and PEO side chains of various lengths. We investigated their surface structures and properties and found the composition where EO units were predominantly segregated on the surface. We then investigated how the surface segregation of PEO side chains affects the surface structure and properties.
1. INTRODUCTION Surface properties determine the performance of adhesives, paints, coatings, lubricants, and biomaterials. A lot has been reported on hydrophilic surface modification of polymers by chemical treatments using acids and alkalis1−3 and physical treatments such as plasma, corona, ultraviolet, and γ-ray irradiations.4−7 A hydrophobic surface, or low-free-energy surface, is useful for a number of applications, such as water/oil-repellent coatings and coatings that prevent the adhesion of soil, ice, and other unwanted contaminations. The use of fluorine is wellknown to be effective in the production of low-energy surfaces, and there have been a great deal of developments aimed at optimizing the use of fluoropolymers for lowering the surface free energy. A typical and first example is polytetrafluoroethylene (PTFE), which is widely used in coatings, biomaterials, and soil-resistant and breathable textiles. Fully fluorinated polymers, however, have disadvantages from the viewpoints of mechanical properties, processing, and cost.8−10 In industrial applications, a surface modifier is commonly used to change just surface properties without affecting bulk properties. The disadvantages of using fully fluorinated polymers as surface modifiers can potentially be avoided by instead using copolymers with smaller amounts of fluorinecontaining comonomers. This is because the fluorinecontaining moieties tend to localize at the air/polymer interface in order to lower the surface free energy, so polymers with perfluoroalkyl (Rf) side chains and their copolymers were reported to show very low surface free energy. Enrichment of the low-surface-free-energy component at the surface, which brought the surface high water repellency and low adhesive © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Methyl methacrylate (MMA, Nacalai Tesque, Inc., Kyoto, Japan) was distilled under reduced pressure before use. 2,2′Azobisisobutyronitrile (AIBN, Nacalai Tesque, Inc., Kyoto, Japan) was recrystallized from methanol. The fluorine-containing monomer 2(perfluorooctyl)ethyl acrylate (PFEA-8, Clariant Corp., Tokyo, Japan) and several macromonomers with EO side chains of different length (PEO-OMe: BLEMMER PME-100, 400, 1000, 2000, and 4000, NOF Co., Tokyo, Japan) were used as received. The PEO side chain lengths (n) in these PEO-OMe macromonomers were 2, 9, 22, 45, and 90, Received: September 22, 2014 Revised: December 3, 2014
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DOI: 10.1021/la503781b Langmuir XXXX, XXX, XXX−XXX
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The films were irradiated with Al Kα radiation generated at 15 kV and 10 mA, and the XPS spectra were collected at a 15° takeoff angle between the sample and the analyzer. No radiation damage was observed during the data collection. The dynamic contact angles of distilled water in air were measured at room temperature.19−21 The advancing contact angle (θa) and the receding contact angle (θr) were measured when the droplet enlarged (< 2 mm diameter) and reduced in size, respectively. The dynamic contact angle of an air bubble in distilled water was measured via the sessile bubble method. To evaluate the adhesive property, a 90° peel test against epoxy resin was performed. The sample film, 0.5 mm thick, was first glued to an Al plate (the thickness was 0.5 mm) using the epoxy resin Araldite (AR-R30, NICHIBAN Co., Ltd., Tokyo, Japan) allowed to harden under constant pressure (270 Pa) for 24 h at room temperature. After that, the sample film was peeled from the Al plate at the angle of 90°, and the 90° peel strength was measured by using an Autograph AGS1kND tensile tester (Shimadzu Co., Kyoto, Japan); the peel rate was 50 mm/min. For each film, the number of tested specimens was more than five. Macromonomer density was measured at 65 °C using a picnometer calibrated with distilled water (0.996 g/cm3). The density of the surface modifier was determined by a floatation method using mixtures of methanol and sodium bromide aqueous solution. The surface morphology was observed with an atomic force microscope (AFM, NanoNavi Station/E-sweep, Seiko Instruments Inc., Chiba, Japan) operated in the dynamic force mode (DFM). The force constant of the Si cantilever was 15 N/m. The scan area was 1 μm square. The surface roughness was evaluated in terms of the rootmean-square (RMS) value determined from the AFM image of the modified surface.
respectively. The terminus of each PEO side chain was a methoxy group. Poly(methyl methacrylate) (PMMA) (Acrypet VH, Mitsubishi Rayon Co. Ltd., Mn = 41 100, Tokyo, Japan) was purified by reprecipitation from methyl ethyl ketone (MEK) solution into methanol. Vinylidene fluoride (VdF)-tetrafluoroethylene (TFE) copolymer (VdF/TFE = 8/2 (mol ratio)) (P(2F-4F), Kynar SL, Atfina Chemicals Inc., Mn = 69 300) was used as received. 2.2. Sample Preparation. The chemical formula of P(MMA/ PFEA-8/PEO-OMe), the surface modifier used throughout this study, is as shown below.
P(MMA/PFEA-8/PEO-OMe)s were synthesized by free-radical polymerization in ethyl acetate (Wako Pure Chemical Industries, Ltd., Osaka, Japan) at 75 °C for 16 h using AIBN as an initiator (0.5% w/w vs monomers). The monomer concentration was totally 30% w/w. After 16 h, an excess of a mixture of hexane (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and n-butanol (Nacalai Tesque, Inc., Kyoto, Japan) (hexane/n-butanol = 4/1 v/v) was poured into the reactant solution at 55 °C and the polymers were obtained as precipitate. The resultant terpolymer was used as the surface modifier after being dried at 60 °C in vacuum. In order to evaluate the PEO side chain content of the surface modifier, we used nuclear magnetic resonance (1H NMR, DPX-250, Bruker BioSpin K. K., Kanagawa, Japan, 250. 63 MHz) analysis at 60 °C using deuterated chloroform (EURISO-TOP Co.) as a solvent. The 1H NMR spectrum of P(MMA/PFEA-8/PEO-OMe) (n = 9) is shown in the Supporting Information. The multiplet signals assigned as PEO side chain were observed at around δ = 3.60 ppm. The fluorine content was evaluated by using the lanthanum-alizarin complexone method followed by the oxygen flask combustion method.17,18 The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were measured using a gel permeation chromatography (GPC) column (Waters Co., Ltd., Model 410) equipped with a reflective index detector. Tetrahydrofuran (Wako Pure Chemical Industries, Ltd., Osaka, Japan) used as the eluent, and polystyrene was used for the calibration. The compositions and molecular weights of the terpolymers are listed in Table 1. We used a blend of PMMA and P(2F-4F) as a matrix resin of the modifier. The blend mixture of PMMA, P(2F-4F), and each surface modifier was codissolved into methyl ethyl ketone/methyl isobutyl ketone = 7/3 w/w as a 10% w/w clear solution. After sitting overnight, the solution was dip coated on poly(ethylene telephthalate) (PET) film and then annealed at 140 °C for 1 h. The coated surface is hereafter referred to as the modified surface. 2.2. Measurements. The surface composition was investigated using X-ray photoelectron spectroscopy (XPS) measurements carried out with a Kratos AXIS-HS. The radiation source was a monochromated X-ray gun (LMX-30, Shimadzu Co., Kyoto, Japan).
3. RESULTS AND DISCUSSION 3.1. Effect of PEO Side Chain Length on Surface Composition. Figure 1 shows the (a) wide and (b) C1s narrow XPS spectra of the surface modified with MMA terpolymer, P(MMA/PFEA-8/PEO-OMe) with n = 9. The depth from the surface evaluated is around 1.55 nm (see the Supporting Information), less than the conventional depth because of the low incident angle of the X-ray beam.22 The results of the angular dependence are shown in the Supporting Information. On the modified surface, together with C1s and O1s peaks, a F1s peak originating from the Rf groups was clearly seen around 680.0 eV. The C1s spectra for the modified surfaces could be curve resolved into eight peaks: at 294.1 eV (−CF3), 291.7 eV (CF2−(CF2)), 291.0 eV (CF2−(CH2)), 288.8 eV (CO), 286.5 eV (C−O−(CO)), 286.4 eV (C−O−(CH2)), 285.5 eV (CH2−(CF2)), and 285.0 eV (C−(CH2)). These peak assignments agreed well with the previously reported ones.23,24 The curve resolving was performed under following conditions: the area of the C−O−(CO) peak was equal to that of the CO peak, and the area of the CH2−(CF2) peak was equal to that of the CF2−(CH2) peak.
Table 1. Effect of PEO Side Chain Length on the Polymer Composition and Molecular Weight n 2 actual composition (mol ratio)
MMA PFEA-8 PEO-OMe
average molecular weight
Mn (103) Mw (103) Mw/Mn
9
1000 166 110
1000 151 107
21.8 58.2 2.7
22
45
90
1000 128 339
1000 139 390
1000 140 401
23.7 65.8 2.8 B
19.9 61.3 3.1
12.7 68.9 5.4
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when the PEO side chain length n was 9. If the modified surface were totally covered with PEO side chains (−(CH2CH2O)9− OMe), the segregation factor would be about 340. In other words, for the modified surface, about one-fifth based on molar ratio of the surface was covered with the surface segregated PEO side chains. Both the PEO side chains and terpolymer main chain existed near the surface because it was random copolymer, but the surface-segregated PEO side chains rather than terpolymer main chain were predominantly detected because of the low XPS detection depth. To explain these phenomena, we next investigated the free volume effect of the PEO side chains. Generally, polymer chain ends are well-known to possess high free volume, which brings about the surface segregation of the chain ends because of the entropy effect.25 Bates et al. reported that a chain with a large free volume tends to localize on the surface because of the entropy effect.26 In the case of comb-shaped molecules like the macromonomers used in this study, it is expected that there is also a very large free volume due to the side chains. Figure 3 shows the effect of the PEO side chain length (n) of macromonomer PEO-OMe on the densities of the surface
Figure 1. XPS (a) wide and (b) C1s narrow spectra of MMA terpolymer, P(MMA/PFEA-8/PEO-OMe) (n = 9). Incident angle of the X-rays is 15°.
Figure 2 shows the relationships between the PEO side chain length (n) of the surface modifier and the segregations factors
Figure 2. Relationships between the PEO side chain length of P(MMA/PFEA-8/PEO-OMe) and the segregation factors for C−O and F/C. The matrix resin is composed of PMMA/P(2F-4F) = 40/50.
Figure 3. Effect of the PEO side chain length in macromonomer PEOOMe on the densities of the surface modifier P(MMA/PFEA-8/PEOOMe) and the macromonomer.
of the ether (C−O) bond and the fluorine atom. The weight composition of each terpolymer and the matrix resin is terpolymer (n=2, 9, 22, 45 and 90): MMA: P(2F-4F) = 10:50:40. The segregation factors are here defined as the ratio between the observed (obs.) and calculated (cal.) values. Each of the C−O (obs.) and C−O (cal.) was evaluated by the result of the curve fitting of C1s spectra, and the bulk value of surface modifier (calculated from the elemental analyses), respectively. In order to evaluate the C−O atom % assigned to the ether bond of PEO side chains, those from the ester groups were eliminated based on the curve fitting process. The F/C atom % (obs.) was also evaluated from the C1s spectra. As mentioned above, in general, the component with the lower γ is segregated on the surface. From this viewpoint, the surface should be covered with Rf groups because the γ value for the homopolymer of PFEA-8 is 8.5 mJ/m2 and that for the macromonomer is 44.0 mJ/m2. However, the segregation factor of fluorine was rather less than the unity. This indicates that the surface fluorine concentration is less than that of the bulk. On the contrary, hydrophilic PEO side chains were predominantly segregated on the surface. The segregation factor of PEO side chains was as much as 70 times compared with that of the bulk
modifier and the macromonomer itself. Though the density of the macromonomer increased with the PEO side chain length, the density of the surface modifier showed minimum values when n = 9 and 22. The increase of macromonomer density is mainly due to their crystallization (see the Supporting Information). A macromonomer with a long PEO side chain is thought to show low mobility, so the longer PEO side chains could easily align with one another and show high density when crystallized. For surface modifier, the random sequence with which the macromonomers were fixed into the polymer chain of the surface modifier prevented their crystallization, which brought lower density to the terpolymer compared with the macromonomer. The low density suggests that the terpolymers with short PEO side chains (n = 9, 22) have a large free volume. As a result, the large free volume of the PEO side chains stimulates the surface segregation of the hydrophilic segments due to its entropy effect, which is consistent with the results plotted in Figure 2. In the work reported hereafter, we used P(MMA/PFEA-8/ PEO-OMe) with the PEO side chain length of 22 and investigated the effect of the P(2F-4F) contents of the matrix resin on the surface composition. C
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Langmuir 3.2. Effects of P(2F-4F) Content on the Surface Composition. Figure 4 shows the relationships between the
Figure 5. Mechanism explaining the segregation of the hydrophilic EO units on the modified surface.
Figure 4. Relationship between the P(2F-4F) content in matrix resin and the segregation factors for C−O and F/C on modified surfaces.
P(2F-4F) content in the matrix resin and the segregation factors for C−O and F/C. The composition of surface modifier, PMMA and P(2F-4F) was shown in Supporting Information Table 1. Whether P(2F-4F) is present, no phase separation was observed (see Supporting Information Figure 4). Compared with the bulk composition, fluorine concentrated and PEO side chains appeared on the surface for just blended the surface modifier with PMMA matrix. This is due to the conventional surface segregation effect of fluorine based on the lowering of surface free energy, and free volume effect of PEO side chains. Almost the same results were observed when dip coating just the terpolymer without matrix resin. On the other hand, instead of fluorine, PEO side chains were preferentially segregated on the surface modified by the PMMA/P(2F-4F) blended matrix. Especially, in case of 40% w/w of P(2F-4F) blended with PMMA, PEO side chains were largely segregated on the surface. This reveals that the incorporation of P(2F-4F) to the matrix resin promotes the surface segregation of PEO side chains. The interaction between fluorine segments of the modifier and P(2F-4F) of the matrix is also presumed to promote surface segregation of the PEO side chains. When 50% or more of P(2F-4F) polymer is added in to the matrix, change of the balances between three driving forces is considered to bring the changes in surface composition. The mechanism of the segregation of the hydrophilic PEO side chains on the modified surface suggested by the above results is shown schematically in Figure 5. Not only Rf groups but also PEO side chains are segregated on the surface by balances between three driving forces: the surface free energy of the Rf groups, the entropy effect due to the free volume of the PEO side chains, and the interaction between Rf and P(2F-4F) in the matrix resin. These factors act competitively, so the surface composition can be controlled by adjusting the PEO side chain length and the compounding ratio of the matrix resin. 3.3. Dynamic Contact Angle and Environmental Response. Figure 6 shows the hysteresis of dynamic contact angle of (a) water in air and (b) air in water on the matrix, P(MMA/PFEA-8/PEO-OMe), and P(MMA/PEO-OMe) surface when PEO-OMe with n = 22 was used. Generally, polymer surface shows the hysteresis between θa and θr. This phenomenon, called the pinning effect,27 can be separated into two types.28,29 One is based on a physical effect, known as the Wenzel effect or Cassie−Baxter effect, of surface roughness.
Figure 6. Hysteresis of dynamic contact angle of (a) a water droplet on the modified surface in air and (b) an air bubble on the modified surface in water. (Solid blue circle) Modified with P(MMA/PFEA-8/ PEO-OMe), (open circle) modified with P(MMA/PEO-OMe), and (solid black up triangle) matrix resin. The matrix resin is composed of PMMA/P(2F-4F) = 40/50.
The AFM topographical image of the modified surface with P(MMA/PFEA-8/PEO-OMe) is shown in Figure 7 along with the scale for and the RMS value of surface roughness. The modified surface very smooth and flat, with an RMS roughness of 0.5 nm. This indicates that the effect of physical roughness on the dynamic contact angle can be regarded as negligible. The other type of pinning effect is the chemical pinning effect due to chemical heterogeneity/environmental change of the surface during the measurement. A huge variety of studies of this pinning effect on the heterogenetic surface have been performed.30−35 One can see in Figure 6 that the modified surface showed high water repellency due to the “chemical pinning effect” of Rf D
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Figure 9. Relationship between the 90° peel strength and the advancing contact angle θa of water on various polymers.
has a low θa and a high peel strength. Figure 9, however, shows that the modified surfaces had both a high θa and a high peel strength. All modified surfaces with different PEO side chain length showed higher θa and the 90° peel strength than conventional polymer surfaces. The amount of surface segregated PEO side chains, the 90° peel strength, and θa were summarized in Supporting Information Table 2. The Rf groups made the modified surface highly water repellent, and the PEO side chains made it readily wettable by the adhesive.
Figure 7. AFM Topographical image of the modified surface with P(MMA/PFEA-8/PEO-OMe) (n = 9) and the root-mean-square roughness (RMS) obtained by DFM mode operating in air.
groups on the surface during the enlargement of a water droplet. During the θr measurement, in contrast, the surface became hydrophilic because the extended PEO side chains predominated the surface property when the water droplet wetted the surface. These features are shown schematically in Figure 8. On the modified surface without matrix resin, θa and
4. CONCLUSIONS Surface modifiers composed of methacrylate terpolymers containing both PEO side chains and Rf groups in the same molecule were prepared and mixed with a PMMA/P(2F-4F) matrix. It was found that not only Rf groups but also PEO side chains were segregated on the surface. This is considered to be brought by the balances among the three driving forces: the surface free energy of the Rf groups, the entropy effect due to free volume of the PEO side chains, and the interaction between Rf and P(2F-4F) in the matrix resin. In air, the surface showed high water repellency due to the chemical pinning effect of the Rf groups. In water, the surface was very hydrophilic because it was covered with extended PEO side chains. These environmental responses were reversible and rapid, taking place within 5 s. The modified surface showed high water repellency due to Rf groups and high peel strength due to PEO side chains.
Figure 8. Mechanism of high water repellency on modified surface.
θr were 106° and 61°, respectively. In the case of there being no matrix resin, it was presumed that hydrophilicity derived from PEO side chains did not occur because of the lack of surface segregated PEO side chains. Conversely, in water, the modified surface is hydrophilic because PEO side chains cover the surface and an air bubble therefore shows a high θa, while the θr of an bubble air is low due to spreading of Rf groups on the surface by contacting with air, that is, the modified surface showed reverse pinning effect on the air bubble in water. Moreover, though the dynamic contact angle measurements (of θa and θr) were alternatively performed successively at the same point up to ten times, these environmental responses were reversible and rapid: the change from water repellent to hydrophilic one or from hydrophilic water repellent one took place within 5 s. 3.4. Adhesion Property. Figure 9 shows the relationship between the θa value of water on the modified surface and the 90° peel strength against epoxy resin. Generally, a surface with a high θai.e., a highly water repellent surface such as PTFE, polyethylene, or isotactic polypropylenehas a low peel strength because it is poorly wettable by adhesives. A hydrophilic surface such as poly(vinyl alcohol), in contrast,
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ASSOCIATED CONTENT
S Supporting Information *
Figures showing a schematic representation of the depth profiling using the XPS measurements, relationship between the C−O atom % and the depth from the air surface by XPS measurements, effect of the PEO side chain length in macromonomer PEO-OMe on the crystallinity, AFM topographical image of the modified surface with P(MMA/PFEA-8/ PEO-OMe) (n = 9) and PMMA without P(2F-4F) obtained by DFM mode operating in air, and 1H NMR spectrum of P(MMA/PFEA-8/PEO-OMe). Tables showing composition of surface modifier and matrix resin, and C−O atom %, 90° peel strength, and θa of surfaces modified by P(MMA/PFEA-8/ PEO-OMe) with different PEO side chain length. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +81-78-803-6164. Fax: 81-78-803-6198. E-mail:
[email protected]. E
DOI: 10.1021/la503781b Langmuir XXXX, XXX, XXX−XXX
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Langmuir Notes
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The authors declare no competing financial interest.
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DOI: 10.1021/la503781b Langmuir XXXX, XXX, XXX−XXX
