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Langmuir 1993,9, 1184-1186
Perfluoropolyethers Tethered to the Air-Water Interface Werner A. Goedel, Chengbai Xu, and Curtis W. Frank* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received November 9,1992. In Final Form: March 19,1993 The surface pressuretarea isotherms of the perfluoropolyethers (PFPEs) 1 (F-(-(CFZ)~~-)~-(CFZ)Z-X) and 2 (X-CFZ-O-(-((CF~)~O)O.~-(CFZO)~.~-)~-CFZ-X), with the end groups X = -CF3, -CHzOH,or -COOH, have been determined on water. 1-CF3and 2-CF3are not surface active. The collapse pressure is lower for the hydroxy- than for the carboxy-terminated polymers. Below collapse, the isotherm shapes are independent of the nature of the hydrophilic head groups. The isotherms of the polymers with two head groups are more expanded than those of polymers with only one head. Filmsof 1-COOHcan be transferred to glass substrates by the Langmuir-Blodgett technique. Introduction The surface active compounds used in the LangmuirBlodgett (LB) technique may be low molecular weight amphiphilesor polymers built from amphiphilic repeating The thermodynamic descriptions of both cases have been given by several authors..es In contrast to polymers with amphiphilic repeat units, the concept of tethered or grafted chains deals with polymers that adsorb to an interface with the chain ends but not with their repeating units. Most theoretical papers7-12 deal with polymers tethered to an interface in contact with a good solvent or polymer melt. In this case the tethered polymer chains assume a stretched conformation in a surface layer that is swollen with solvent or free homopolymer. Experimentally these systems have been studied in the form of diblock copolymers dissolved in a selective solvent, in which one block adsorbs from the solution onto a solid substrate,l3or diblock copolymers spread onto the surface of a solvent that dissolves only one of the polymeric bl0cke.l4J5 In these etudies the less-soluble or insoluble block has a collapsed conformation and has only minor influence compared to the swollen part. The case of dense, grafted polymer layers, in which polymers are tethered to an interface but not swollen with a solvent or homopolymer, has been treated in the framework of phase-separated block copolymers,16-18but no analogous investigations of polymer layers grafted to the liquidlair interface of a nonsolvent have as yet been published. The most suitable solvent for the investigation of f h adsorbed to the liquid/air interface is water,' which, however, limits the available temperature range. In order (1) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interrcience: New York, 1966. ( 2 ) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (4) Gaines, G. L., Jr. J . Chem. Phys. 1978, 69, 924. (5) Huggins, M. L. Makromol. Chem. 1965,87, 119. (6) Huggins, M . L. Kolloid Z . Polym. 1979, 251, 449. (7) Alexander, S. J . Phys. 1977,38,983. (8)deGennes, P. G. Macromolecules 1980,13,1069. (9) Milner, 5.T.; Witten, T. A.; Cates, M. E. Macromolecules 1988,21, 2610. (10) Venema, Paul; Odijik, The0 J . Phys. Chem. 1992,96, 3922. (11) Wijmans, C. M.; Scheutjens, J. M. H. M.; Zhulina, E. B. Macromolecules 1992,25, 2657. (12) Review article: Halperin, A.; Tirrell, M.; Lodge, T. P. Adu. Polym. Sci. 1992, 100, 31. (13) Milner, S . T. Europhys. Lett. 1988, 7, 695. (14) Granick, S.; Herz, J. Macromolecules 1986, 18, 460. (15) Kent, M. 5.;Lee, L.-T.; Farnoux, B.; Rondelez, F. Macromolecules 1992,25,6231. (16) Helfand, E. Macromolecules 1975,8, 552. (17) Semenov, A. N. Sou. Phys. JETP 1985,61, 733. (18) Ohta, T., Kawasaki, K. Macromolecules 1986,19, 2621.
to achieve equilibrium properties, the experiments should be done at temperatures above the glass transition (T,) of the polymer. For glassy systems,nonequilibriumbehavior is expected. Polystyrene (T,> 100 "C), for example, that is spread from solution onto the water surface forms hard disklike particles, which do not change their shape upon compression of the s u r f a ~ e .As~ ~ ~~~ a consequence, the properties of polystyrene change only slightly when the polymer chains are tethered to the water via a hydrophilic bloCk.21*22 In order to observe the effects of chain stretching with minimal contributions from other surface related phenomena, the repeat unit of the polymer under study should be non-surface active, and the anchoring moiety should be as small as possible. However, most water-insoluble polymers with a low glass transition are surface active themselves (e.g. polyacrylates, poly~iloxanea~~). To our knowledge the only non-surface-active polymers with a low glass transition temperature are polydienes. It has been shown, for example, that polybutadiene (T,< 0 OC) is not surface active unless derivatized with hydrophilic side groupsz4and that polybutadieneZ5and polyisoprene26 with hydrophilic head groups are surface active. However, none of these previous publications includes a variation of the nature and the number of hydrophilic head groups per chain. In the present study we investigate a new class of water-insoluble polymers, telechelic perfluorinated polyethers, including avariation of the number and nature of the head groups. Hydrophobic perfluorinated polyethers (PFPEk) with a glass transition below 0 "C are available with hydrophobic (-CF3) as well as hydrophilic (-OH, -COOH) end groups. Because they can be displaced from substrates by washing with waterz7 and non-fluorinated analogs like poly(oxyethylene) are surface active,%PFPEk might be surface active as well, but no investigationsof their surface activity have as yet been reported. It has been reported, however, (19) Kumaki, J. Macromolecules 1986, 19, 2258. (20) Kumaki, J. Macromolecules 1988,21, 749. (21) Niwa, M.; Hayaahi, T.; Higaahi, N. Langmuir 1990,6,263. (22) Yoshikawa, M.; Worsfold, D. J.; Matauura, T.; Kimura, A.; Shimidzu, T. Polym. Common. 1990, 31, 414. (23) Mann, E. K.; Langevin, D. Langmuir 1991, 7, 1112. (24) Kim, M. W.; Chung, T. C. J . Colloid Interface Sci. 1988,124,365. (25) Fujimura, Y.; Sakai, I.; Nakamae, K.; Hatada, M.; Okada, T.; Matsumoto, T. Kobunshi Ronbunshu 1980,37,29. (26) Christy, P.; Petty, M. C.; Roberts, G. G.; Richards, D. H.; Service, D.; Steward, M. J. Thin Solid Films 1986,134, 76. (27) Ruhe, J.; Kuan, S.;Blackman, G.; Novotny, V.; Clarke,T.; Street,
G. B. In Surface Science Inuestigations in Tribology; Chung, YipWah, Homola, A. M., Street, G. B., Eds.;ACS SymposiumSeries 485; American Chemical Society: Washington, DC, 1992; p 156. (28) Shuler, R. L.; Zisman, W. A. J . Phys. Chem. 1970, 74, 1523.
0 1993 American Chemical Society
Letters
Langmuir, Vol. 9,No.5, 1993 1185 0.06 1 -
14- 1::
0.02
v1
0.01 0 0
a)
0
0
100
200
300
400
[Aa]
Area per Molecule Figure 1. Isotherms of the polymers in Table I on water at 25 "C recorded with a compressionspeed of 10-l A2s-1(repeatunit)-'. Table I. Polymers Used in This Study abbreviated name
mol-'
M,l M,,
F-((CZ)~-~)~-((CFZ)Z)-X -CF3 2700 DemnumSA -CH20H 4000 Demnum SH -COOH 3800
1.1 1.4
trade name
headgroup -X
MJg
50
100 150 50 100 b)o c) 0 50 Area per Molecule
100
n
40
150
150
[A2] Figure 2. Effects of compression speed and hysteresis experiments with 1-COOH on water at 25 O C . (a) Variation of compression speed (i) 1 A* 8-1 (repeat unit)-', (ii) 10-1 A2 8-1 (repeatunit)-', (iii)10-2A2s-1(repeatunit)-'. (b)Two consecutive cycles of compression and expansion up to a maximum pressure of 0.04 N m-l. (c) Compression down to 30 A2 and subsequent re-expansion.
perfluoropoly(oxypropy1ene) 1-CF3
DemmunS20
1-OH 1-COOH Perfluoropoly(oxyethy1ene-co-oxymethylene)
XCFZ-O-(-((CFZ)~O)O.~-(CFZO)O.~)~-CF~X 203 -CF3 3600 2 2-OH ZDOL -CH20H 2200 1.9
2-CF3
2-COOH
ZDIAC
-COOH
2200
that low molecular weight perfluoroethers with carboxylic end groups are surface active as well as water soluble.29 The scope of this study is (i) to investigate whether the main chains of PFPEs are surface active and, if this is not the case, (ii) to determine whether PFPEs can be made surface active by attaching hydrophilic end groups, thus being suitable as model compounds for a dense grafted polymer layer. Experimental Section Water (minimum resistivity 17 X 106 s2 cm-l) was purified with an ion exchange/filtersystem (Millipore). The properties of the investigated perfluoropoly(oxypropy1ene) (obtained from Daikin Ltd.)and perfluoropoly(oxyethy1ene-co-oxymethylene) (Montedison)samples are given in Table I. 1-OHand 1-COOH contained approximately 30% of 1-CF3. The polymers were spread on the surface of water using solutions (3 g L-l) in 1,1,2chloro-l,Z,Z-fluoroethane(Aldrich)or perfluoroalkane mixture FC72 (3 M). The isothermswere recorded at 298 K, 15min after spreading,using a 11cm X 70 cm rectangular Langmuir trough made of poly(tetrafluoroethy1ene) (LB5000,KSV, Helsinki).The number average molecular weight was used to calculate the mean molecular area. Hydrophilic glass substrates were cleaned by ultrasonicating for 15 min first in butyl acetate and then in a solution of potassium hydroxide in 2-propanol (100 g L-l). They were finally washed with copious amounts of purified water.
Results When the polymers without hydrophilic head groups (1-CF3 and 2-CF3) were spread, small droplets formed on the water surface after evaporation of the solvent. No significant pressure rise was detected during compression (Figure 1). By contrast, the polymers with hydrophilic head groups (1-OH, 2-OH, 1-COOH,2-COOH) formed no droplets, and isotherms with detectable pressure rise were recorded. (29) Caporiccio,G.;Burzio, F.; Carniselli, G.;Biancardi, V. J. Colloid Interface Sci. 1984,98,202.
-50
'
-IO
in
I 20
30
50
Lcm2]
Substrate Area Dipped into the Subphase
Figure3. Transfer of a film of 1-COOHkept on the water surface at 25 O C at a pressure of 0.04 N m-1 onto a glass substrate.
The isotherms of the polymers with two hydrophilic head groups (2-OH, 2-COOH) are more expanded than those of polymers with only one hydrophilic group (1-OH, 1-COOH)(Figure 1). The collapse pressurea (see also next paragraph) of the hydroxy-terminated polymers (1-OH, 2-OH) are lower than the collapse pressures of the carboxyterminated polymers (1-COOH, 2-COOH). Up to the collapse of the hydroxy-terminated polymers, the isotherms of the carboxy-terminated and hydroxy-terminated polymers of a given main chain (1-OH, 1-COOH; 2-OH, 2-COOH) are identical. The point of collapse and the reversibility of the isotherms were investigated in detail using polymer 1-COOH (Figure 2). Up to the point of abrupt change in the slope (at 40 A2, 0.045 N/m), the isotherm was independent of the compression speed (Figure 2a). Figure 2b shows two consecutive cycles of compression to a maximum pressure of 0.04 N/m and subsequent reexpansion. No significant hysteresis was detected. When 1-COOHwas compressed to a mean molecular area of less than 40A2(see Figure 2c),the isotherm in the re-expansion of the film was shifted toward lower mean molecular area. The observations combined in Figure 2 thus show that at pressures below the abrupt change in slope, the isotherm is reversible, whereas above that pressure, it is not. Therefore, this point was interpreted as the onset of collapse of the PFPE films. Polymer 1-COOHwas spread on the water surface and kept a t a constant pressure of 0.04 N/m while a hydrophilic glass substrate was dipped into the surface (Figure 3). During the first down stroke of the dipping process, the
1186 Langmuir, Vol. 9, No. 5, 1993 film area on the trough remained unchanged. During the upstroke, the f i b area decreased proportional to the area of the substrate being lifted out of the trough; the ratio of both areas was 1.02. In the following down stroke, the film area increased proportional to the area of the substrate dipped into the trough. Therefore, transfer takes only place in the upstroke, and the transferred films are displaced by subsequent dipping into water. Discussion Because perfluoropolyethers with hydrophobic end groups do not form a uniform layer and no pressure rise can be detected upon compression, we consider the main chain non-surface active. Nonfluorinated polyethers like poly(oxyethy1ene) (POE), on the other hand, are surface active as well as water soluble.28 The higher hydrophobicity of perfluoroalkanes (compared to hydrocarbons) alone can account for the difference in water solubility between POE and PFPEs. The complete absence of surface activity in PFPEs, however, shows that the ether group in PFPEs must be significantly less hydrophilic than in POE. Though hydrophilic properties are a complex function of several parameters, it is instructive to compare the dipole momenta of simple alkyl and perfluoroalkyl ethers. While the dipole moment of dimethyl ether ( p = 1.30 D3O~39is comparable to the dipole moment of water ( p = 1.85 D 3 2 9 ,the high electronegativity of the fluorine atoms causes a significantly lower dipole moment of perfluorodimethyl ether ( p = 0.29 D34), which is comparable to the dipole moment of alkenes (e.g. propene p = 0.36 D30*35). It is therefore reasonable to assume, in accordance with our results, that the ether bridge in perfluorinated ethers is not hydrophilic. The following observations lead us to the conclusion that the hydrophilic head groups of 1-OH,2-OH, 1-COOH, (30) Hellwege, K. H., Ed. Landolt-Boernstein, Zahlenwerte und Funktionen aus Naturwissenschaft und Technik,Neue Serie; Springer Verlag: Berlin, 1967; part II/4. (31) Blukis, U.; Kasai, P. H.; Myers, J. J. Chem. Phys. 1963,38,2753. (32) Hellwege, K. H., Ed. Landolt-Boernstein, Zahlenwerte und Funktionen au8 Naturwissenschaft und Technik,Neue Serie; Springer Verlag: Berlin, 1982; part 11/14. (33) Dvke. T. R.: Muenter. J. S. J. Chem. Phvs. 1973.59. 3125. (34) Piesnicar, B'.; Kocjan,'D.; Murovec, S.; Aiman, A:J. Am. Chem. SOC.1976,98, 3143. (35) Linde, D. R.; Mann, D. E. J. Chem. Phys. 1957,27,868.
Letters and 2-COOH are adsorbed to the water surface: (i) only PFPEs with hydrophilic head groups are surface active; (ii) the shape of the isotherm depends on the number of head groups per chain; (iii) the collapse pressure depends mainly on the chemical nature of the head group. At pressures below collapse, the shapes of the isotherms are nearly independent of the nature of the hydrophilic head group. Therefore, the packing of the polymer chain might contribute significantly to the surface pressure. We therefore propose that all hydrophilic head groups are attached to the water surface, while the polymer chains form a dense layer above the water and are free to arrange in constrained, random coils. If we treat the polymer as an incompressible fluid,16the thickness of the layer will increase during compression, thus forcing the polymer chains into a less favorable,stretched conformation. These conformational changes increase the free energy of the system17and might cause a significant contribution to the surface pressure. The area per molecule of 1-COOHnear collapse (40 A2) is larger than either the cross-sectional area of a perfluorinated alkyl chain (27.7 A2 36) or the molecular area of perfluorinated fatty acids (33.2 A2 37). Moreover, the thickness of the polymer layer (85 A, calculated from surface coverage and bulk density 1.8 g ~ m - is ~ smaller ) than the contour length of the polymer (120 A39. Thus, we conclude that the polymer chain is significantly but not completely stretched near collapse. Finally we note that since transfer occurs only during the upstroke of the dipping process, it is likely that the head groups in the transferred films are attached to the hydrophilic substrate and that the conformation of the polymer chains is preserved during transfer. Acknowledgment. We thank the Claussen-Stiftung for fellowship support for W.A.G. and J. Lin, G. B. Street, J. Burns, and G. Vurens for providing the polymers along with the characterization data. This work was supported in part by the NSF Polymers Proaram. (36) Estimated from crystallographicdata of polytetrafluoroethylene: Clark, E. S.;Muus, L. T. 2 . Krist. 1962, 117, 119. Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1973; Vol. 1, p 97. (37) Nakahama, H.; Miyata, S.; Wang, T. T.; Tasaka, S. Thin Solid Films 1986, 141, 165. (38) Estimated from data given in ref 36 under the assumption that the oxygen bridge gives the same contribution to the chain length as a CF2 unit.
