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Structure and Surface Properties of Liquid Crystalline Fluoroalkyl Polyacrylates: Role of the Spacer Jean-Marc Corpart,* Sylvie Girault, and Didier Juhue´ Atofina Company, Centre d’Application de Levallois, 95 rue Danton, 92300 Levallois Perret, France Received February 14, 2001. In Final Form: July 6, 2001 The organization of liquid crystalline fluorinated acrylate polymers has been generally described so far in the literature in terms of phase segregation between hydrogenated and fluorinated moieties within the polymer and preferential orientation induced by the backbone. However, little is known on the effects of the chemical composition on the properties of such fluorine-containing polymers. Our study was then focused on the role played by the spacer group located in the side chain between the backbone and the fluorinated segment and, in particular, on the properties of poly[2-[[[[2-(perfluoroalkyl)ethyl]sulfonyl]methyl]amino]ethyl]acrylates (pASn). Surface and bulk organization of fluorinated side chains of those polymers were investigated by surface tension, X-ray scattering, and differential scanning calorimetry measurements. Results were compared with those obtained with poly[(perfluoroalkyl)ethyl] acrylates (pAFn) of the same perfluoroalkyl chain lengths. A strong correlation between bulk organization and surface properties of our polymers could be established. Surprisingly, in the perfluorohexyl series, pAS6 with a N-methylsulfonamide spacer group was found to be organized in a crystalline lamellar structure whereas pAF6 was found to be amorphous. This indicates that the introduction of a N-methylsulfonamide spacer group in the side-chain allows the system to crystallize with a shorter -C6F13 fluorinated segment, whereas in the presence of methylene groups, an organization is only present with a -C8F17 segment (pAF8). This was mainly attributed to the strong dipole-dipole interaction between N-methylsulfonamide groups that tends to align the fluorinated segments in a lamellar structure that eventually crystallizes as long as the section of the N-alkylsulfonamide group does not induce steric hindrance.
Introduction Fluorinated polymers generally have properties that differ greatly from those of comparable hydrogenated structures: for example, good chemical and thermal stability, antiadhesive properties, low friction coefficients, biocompatibility,1,2 low surface free energies,3 and nonstick behavior.4 Due to their extremely low surface energies, combshaped polymers with fluorocarbon segments in their side chains have received particular attention.3-16 These polymers are indeed used as surface modification agents such as water-and-oil repellents and soil release on different substrates such as textiles,17 paper,18 leather, carpets,19 and nonwoven or building materials. (1) Akemi, H.; Aoyagi, T.; Shinohara, I.; Okano, T.; Kataoka, K.; Sakurai, Y. Makromol. Chem. 1986, 187, 1627-1638. (2) Kambic, H. E.; Murabayashi, S.; Nose, Y. Chem Eng. News 1986, 64 (15), 30-48. (3) Fluoropolymers; Pittman, A. G., Wall, L. A., Eds.; WileyInterscience: New York, 1971; p 419. (4) Schmidt, D. L.; Coburn, C. E.; DeKoven, B. M.; Potter, G. E.; Meyers, G. F.; Fischer, D. A. Nature 1994, 368, 39. (5) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1962, 66, 1207. (6) Fox, H. W.; Zisman, W. A. J. Colloid Sci. 1950, 5, 514. (7) Roitman, J. N.; Pittman, A. G. Polym. Lett. 1972, 10, 499. (8) Pittman, A. G.; Ludwig, B. A. J. Polym. Sci. A1 1969, 7, 3053. (9) Pittman, A. G.; Sharp, D. L.; Ludwig, B. A. J. Polym. Sci. A1 1968, 6, 1729. (10) Ramharack, R.; Nguyen, T. H. J. Polym Sci. C, Polym. Lett. 1987, 25, 93. (11) Yokota, K.; Hirabayashi, T. Polym. J. 1985, 17, 991. (12) Lin, J. W. P.; Dudek, L. P.; Majumdar, D. J. Appl. Polym. Sci. 1987, 33, 657. (13) Park, I. J.; Lee, S. B.; Choi, C. K. J. Appl. Polym. Sci. 1994, 54, 1449. (14) Chapman, T. M.; Benrashid, R.; Marra, K. G.; Keener, J. P. Macromolecules 1995, 28, 331. (15) Guerry-Rubio, C.; Viguier, M.; Commeyras, A. Macromol. Chem. Phys. 1995, 196 (4), 1063. (16) Beyou, E.; Bennetau, B.; Dunogues, J.; Babin, P.; Teyssie, D.; Boileau S.; Corpart, J. M. Polym. Int. 1995, 38, 237.
There are a wide variety of chemical structures, and numerous studies on fluorinated polysiloxanes,16 polyurethanes,14 and polyfluoroalkyl(meth)acrylic polymers3-13,15 have been reported. As far as poly(tetrafluoroethylene) is concerned, the lower critical surface tensions of many polymers with fluorinated side chains reflects a high concentration of CF3 groups on the surface.20 It can therefore be concluded that the conformational arrangements of the fluorinated side chains at the airsolid interface, and the packing that results, must be taken into account. On this basis, Pittman and Ludwig8 were the first to demonstrate that the wetting properties of polymers containing fluorinated side chains can be markedly influenced by packing the side chains resulting from crystallization. Polymers containing crystalline segments involving fluoroalkyl side chains can lead to surface tension values lower than the values obtained for amorphous polymers. More recently, physicochemical and structural studies of acrylate/methacrylate polymers,11,21-23 polyesters24-26 and polystyrene-b-isoprene27 copolymers with perfluoroalkyl side chains have been reported and have shown (17) Corpart, J. M.; Dessaint, A. Meliand Engl. 1997, 9, E 135. (18) Fluorine in Coat., 2nd Int. Conf.; Corpart, J. M., Dessaint, A., Collette, C., Eds.; Paint Research Association: Teddington, U.K., 1994; Paper 21. (19) Bierbrauer, C. J.; Goebel, K. D.; Landucci, D. P. Am. Dyest. Rep. 1979, 68 (6), 19-21, 34. (20) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1954, 58, 236. (21) Volkov, V. V.; Plate, N. A.; Takahara, A.; Kajiya, M. A.; Amaya, N.; Murata, Y. Polymer 1992, 33, 1316. (22) Shimizu, T.; Tanaka, Y.; Kutsumizu, S.; Yano, S. Macromolecules 1993, 26, 6694. (23) Shimizu, T.; Tanaka, Y.; Kutsumizu, S.; Yano, S. Macromol. Symp. 1994, 82, 173. (24) Wilson, L. M.; Griffin, A. C. Macromolecules 1994, 27, 1928. (25) Wilson, L. M.; Griffin, A. C. Macromolecules 1994, 27, 4611. (26) Wilson, L. M. Liq. Cryst. 1995, 18, 347.
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arrangements of the fluorinated units in smectic phases. The structure of the packing of the side chains is governed by their length and chemical structure. The nature and flexibility of the main chain also play a role in the probability of an ordered structure formation.21 According to work by Volkov et al.21 and Shimizu et al.,23 for example, poly(1H,1H,2H,2H-perfluorodecyl)acrylate forms a double layer of packing for the fluoroalkyl side chains at room temperature and poly(1H,1H,2H,2Hperfluorodecyl)methacrylate consists of a blended structure with single- and double-layer packing. Likewise, the fluoroalkyl side chains of acrylate/methacrylate polymers with a fluorinated carbon number of 7 or more can be crystallized or liquid-crystallized.28,29 The present work examines the effects of chemical composition on the properties of such fluorine-containing polymers. We have focused on the properties of poly[2[[[[2-(perfluoroalkyl)ethyl]sulfonyl]methyl]amino]ethyl]acrylates (Figure 1). The aggregation states of fluorinated side chains and the surface tension of the polymers are characterized. The results are compared with those of poly[(perfluoroalkyl)ethyl]acrylates with the same length of perfluoroalkyl chains (Figure 1).
were between 45 000 and 60 000 with a polydispersity between 2 and 3. The polymers are believed to be atactic due to the nature of the free radical polymerization. Measurements. Wetting measurements were used for polymer surface tension evaluation. The different techniques we used to determine the structures of the polymers were optical microscopy, differential scanning calorimetry (DSC), and wideangle (WAXS) and small-angle (SAXS) X-ray scattering. For the surface tension measurements, microscope glass plates were cleaned as previously described.16 They were then immersed in 2 g/L polymer solutions in 1,1,2-trichlorotrifluoroethane for 1 h and withdrawn at a speed of 0.5 mm/min in order to obtain homogeneous films. The residual solvent in the polymer film was removed by drying at 60 °C for 1 h. Confocal microscope observations confirmed that the polymer surfaces obtained were smooth and had a uniform thickness of about 5 µm. Surface tensions were derived from contact angle measurements by the sessile drop method on a Rame-Hart Model A-100 goniometer. About 5 µL of wetting liquid was used for each measurement, at 20 °C. Measurements were made on four drops of each liquid; eight readings of the contact angle (on the front and back of each drop) were taken. The average value was used as the contact angle. The wetting liquids used were water (γLD ) 21.6 mN/m; γLP ) 51.0 mN/m) and n-tetradecane (γL ) γLD ) 26.3 mN/m). Thermal analysis was carried out at a scanning rate of 10 °C/min on a Du Pont 1090 and a Setaram DSC 141 apparatus. After a preliminary heating/cooling cycle, a second heating was performed and used for the measurement. The texture observation under crossed polarizers was made by using a Leitz optical microscope equipped with a programmable Mettler hot stage (Mettler FP82). After being heated to 200 °C, the samples were observed while they cooled slowly at 0.1 °C/min. Wide- and small-angle X-ray diffraction patterns were recorded with Cu KR radiation from an X-ray rotating-anode generator (RU-200, Rigaku Co. Ltd). The samples (white powder or rubbery solid) were contained in 1 mm diameter Lindeman quartz tubes and observed in the transmission mode. The patterns at different temperatures were recorded on photographic film at a distance of 28 mm from the samples. After the samples were heated to a temperature higher than any of the transition temperatures and then cooled at 10 °C/min until room temperature was reached, small-angle X-ray diffraction patterns were recorded on a linearposition-sensitive detector (LPS 50 INEL) and wide-angle X-ray diffraction patterns on a curve-position-sensitive detector (CPS 120 INEL).
Experimental Section
Results and Discussion
Materials: Synthesis and Characterization of the Polymers. The fluorinated acrylic monomers {[2-[[[[2-(perfluoroalkyl)ethyl]sulfonyl]methyl]amino]ethyl]acrylates (AS6 and AS8) and [(perfluoroalkyl)ethyl]acrylates (AF6 and AF8)} were provided by Atofina and used without further purification. The homopolymers pAS6 and pAS8 were prepared by solvent polymerization in acetone under reflux with t-butyl perpivalate as the initiator. Polymers pAF6 and pAF8 were prepared under the same conditions in methanol under reflux. The polymers produced were precipitated in the solvent during polymerization and recovered by means of either filtration (pAS6, pAS8, and pAF8) or solvent evaporation (pAF6). The polymers were purified three times by first dissolving in 1,1,2-trichlorotrifluoroethane and then reprecipitating in methanol. The purified polymers were dried in a vacuum oven at 45 °C for 2 days. Polymer pAF6 was a rubbery solid; the other polymers consisted of white powder. The molecular weights of the fluorinated polyacrylates were determined by size-exclusion chromatography (SEC) in 1,1,2trichlorotrifluoroethane at 30 °C on a Waters apparatus equipped for refractive index detection, by comparison with a poly(dimethylsiloxane) calibration curve. The molecular weights, Mw,
Contact Angles and Surface Free Energies. The validity of Young’s eq 1 between the contact angle θ, the solid surface tension γS, the solid/liquid interfacial tension γSL, and the surface tension of liquid γL is well-known:
Figure 1. Schematic structures of polyacrylates used for this study: (a) poly[2-[[[[2- (perfluoroalkyl)ethyl]sulfonyl]methyl]amino]ethyl]acrylates; (b) poly[(perfluoroalkyl)ethyl]acrylates.
(27) Wang, J.; Mao, G.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906. (28) Okawara, A.; Maekawa, T.; Ishida, Y.; Matsuo, M. Polym. Prepr. Jpn. 1991, 40, 3898. (29) Maekawa, T.; Kamata, S.; Matsuo, M. Paper presented at the 13th Symposium on Fluorine Chemistry, Bochum, 2-6 September 1991.
cos θ ) (γS - γSL)/γL
(1)
According to Fowkes,30 the surface tension can be resolved into a dispersion component and a polar component:
γ ) γ d + γp
(2) d
where γ is the surface tension, γ is the dispersion component, and γp is the polar component. Owens and Wendt31 extended this concept and proposed the following semiempirical equation:
γL(1 + cos θ) ) 2(γSdγLd)1/2 + 2(γSpγLp)1/2
(3)
In this equation, subscript L refers to the wetting test liquid and subscript S to the solid, and θ is the contact angle. (30) Fowkes, F. M. Ind. Eng. Chem. 1964, 56 (12), 40. (31) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
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Table 1. Contact Angles θ and Surface Tension Data for Polymers PAS6, PAS8, PAF6, and PAF8 (T ) 20 °C) polymer
θtetradecane (deg)
θwater (deg)
γS (mN/m)
γSd (mN/m)
γSP (mN/m)
pAS6 pAS8 pAF6 pAF8
74 74 71 76
118 119 113 120
11.0 10.9 12.3 10.4
10.7 10.7 11.6 10.2
0.3 0.2 0.7 0.2
The surface free energy of the solid surface (γS, γSd, and γSp) can be determined with eq 3 and two different liquids whose surface tension (γL), dispersion (γLd), and polar component (γLp) are known. Whereas previous investigators31 used methylene iodide and water to estimate γSd and γSp, we used water and tetradecane as our test liquids to measure the surface energies. These two solvents are of very different polarities and can be used to obtain accurate surface tension data.12,31 The water and tetradecane contact angles measured for polymers pAS6, pAS8, pAF6, and pAF8 and the results obtained for the surface energies are shown in Table 1. It is noticeable that polymers pAS6, pAS8, and pAF8 have the same contact angle for a given liquid within the experimental error ((2°). The contact angles are slightly smaller with pAF6. No significant difference is therefore observed for the polymer surface free energies except in the case of poly[(perfluorohexyl)ethyl]acrylate (pAF6), for which γS is slightly higher. The differences in surface tension are in fact linked to variations in the nature of the chemical groups on the solid surface. In comparison with poly(tetrafluoroethylene), it is a particularly well-known fact that the lower critical surface tensions of the fluorinated polyacrylates reflect the concentration of CF3 groups on the surface.20 It can be concluded that at the polymer-air interface the fluoroalkyl group of pAS6, pAS8, or pAF8 presents a comparable arrangement. Comparison of the γS values for pAF6 and pAF8 indicates that the organization of the perfluoroalkyl groups at the surface improves with the increasing chain length of the perfluoroalkyl segment from C6F13 to C8F17. On the contrary, pAS6 and pAS8 with an N-methylsulfonamide group in the side chain have the same surface tension for C8F17 and C6F13 fluorinated segments. Another interesting point must be stressed. The polar component γSp increases for pAF6 in comparison with the other polymers. A possible explanation for this behavior may be that functional groups such as the ester groups of pAF6 exist at the polymer-air interface, providing a strong hydrogen-bonding interaction with a polar wetting liquid like water. These results may be compared with literature data. When the polymers contain one or two methylene units interspersed between the perfluoroalkyl group and the main chain, it should be noted that as the fluorocarbon chain length is increased, the surface tension at first decreases and then reaches a low and almost constant value. Data by Maekawa et al.29 suggest that there is not much difference in γC calculated from a Zisman plot for polyacrylates with perfluorinated segments containing four or more carbons (γC ∼ 10 mN/m). Using the same method, Hugues32 found similar γC values for poly[(perfluorohexyl)ethyl]methacrylate and poly[(perfluorooctyl)ethyl]methacrylate (γC ∼ 7.5-8.0 mN/m). These results could be seen as inconsistent with those of the present paper. Using dynamic contact angle measurement with Wilhemy’s technique, however, Maekawa et al.29 have also shown that if the water-advancing contact angle has the same value for poly[(perfluorohexyl)ethyl]acrylate and (32) Hugues, T. The`se de Doctorat, Universite´ de Montpellier II, 1991.
Figure 2. DSC thermograms for pAS6 and pAF6. Table 2. Phase Transition Temperatures of the Polymers first transition
pAS6 pAS8 pAF6 pAF8
second transition
Tonset (°C)
Tmelting (°C)
∆H (J/g)
Tonset (°C)
Tmelting (°C)
∆H (J/g)
104 142
107 146
7 13
179 204
182 209
3 3
74
78
12
poly[(perfluorooctyl)ethyl]acrylate, the water-receding contact angle is generally smaller for a polymer with a C6F13 segment than for a polymer with a C8F17 chain. For these authors, in fact, there is a discontinuity in wetting behavior when the chain length is increased from C6F13 to C7F15. Using Owens and Wendt’s equation (eq 3), Hugues32 also found a decrease in γS values from polymethacrylate with a C6F13 segment (γS ∼ 9.9 mN/m) to polymethacrylate with a C8F17 chain (γS ∼ 7.1 mN/m). Park et al.35 also found a high polar component γSp for a polymethacrylate with C6F13 segments (γSp ∼ 0.9 mN/m). In this case, the polar component is about 10% of the surface free energy and this result is well correlated with ESCA experiments, which showed that the outmost layer of the fluorinated coating is composed of mainly perfluoroalkyl groups and some ester groups. This is to be correlated with what Wang et al.27 found on monodisperse polystyrene-semifluorinated alkyl ester side chain block copolymers, where the C6F13 form exhibited surface reconstruction after water exposure whereas the C8F17 form remained crystalline without noticeable surface reconstruction. In the case of polymers containing an N-alkylsulfonamide group in the spacer between the perfluoroalkyl group and the main chain, we have not found literature surface tension data showing the effects of fluorinated segment lengths. Our results for pAS8 are consistent with those of Ramharack and Nguyen10 (γS ∼ 10.7 mN/m). In conclusion, our wetting measurements show that the presence of an N-methylsulfonamide group in the spacer seems to play a positive role on surface tension values. Thus, a slightly lower surface tension value is recorded for pAS6 than for pAF6. Thermal Analysis. For three of the homopolymers, the thermograms obtained show the existence of one or two endothermic transitions. The results are shown in Table 2. We noted the temperatures (in degrees Celsius) at which the transitions began, Tonset, the peak temperatures, Tmelting, and the enthalpy variation corresponding to each transition observed. (33) Me´las, M. The`se de Doctorat, Universite´ de Montpellier II, 1995. (34) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015. (35) Park, I. J.; Lee, S.-B.; Choi, C. K.; Kim, K.-J. J. Colloid Interface Sci. 1996, 181, 284.
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Figure 3. Optical polarizing micrographs displayed for pAS6 during a cooling from 200 °C to room temperature at 0.1 °C/min : (a) at 178 °C ; (b) after 1 h at 177 °C ; (c) at 168 °C ; (d) after crystallization at 100 °C.
No transition was observed in the case of the (1H,1H,2H,2H-perfluorooctyl)acrylate (pAF6). On the other hand, the pAS6 and pAS8 polymers underwent two transitions, and the pAF8, a single transition. The thermograms of polymers pAS6 and pAF6 can be compared in Figure 2. As far as pAF6 was concerned, our experimental conditions did not make it possible to demonstrate glass transition, which concords with previous observations.21 It should be noted that DSC and X-ray diffraction structural characterization has already been reported in the literature for pAF8.21,33 The melting temperature and enthalpy variation measured here were comparable to the values obtained in previous studies. The pAS6 and pAS8 polymers underwent two successive transitions associated with two rather different enthalpy variations: high (respectively 7 and 13 J/g) for the lowtemperature transition and lower (3 J/g) for the higher temperature transition. These results have already been observed24,26 for other polymers and interpreted first as solid/mesophase transitions and then as mesophase/ isotropic transitions. Structural characterization by means of optical microscopy and X-ray diffraction allows us to make this result clearer, later on in this report.
The results obtained here by thermal analysis must be related to previous surface characterizations: pAS6, pAS8, and pAF8 have a structural organization that is shown by a low surface energy, whereas pAF6, which is nonorganized, has degraded surface properties. Optical Microscopy in Polarized Light. Regardless of the temperature, no birefringence was observed for pAF6; it remained isotropic whereas the three other samples showed birefringent textures. The behavior of pAS6 and pAS8 was perfectly comparable, the only difference being the temperatures at which the transitions occurred. Figure 3, for example, shows the evolution of the texture of pAS6 as observed when the temperature was lowered from the isotropic phase. At 178 °C, a number of rods, characteristic of a lamellar phase, appeared (Figure 3a). If the temperature was maintained at a set value of 177 °C, with time, an extension of these lamellar zones was observed (Figure 3b). These zones increased until they occupied the entire space (Figure 3c). When the temperature was then lowered, a transition was observed at 100 °C, where all these zones suddenly changed color (Figure 3d). This rapid evolution of the
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Figure 5. Powder X-ray diffraction diagrams at room temperature for the polymers showing an amorphous structure (pAF6) or smectic phases (pAS6, pAS8, and pAF8). Between spectra, the intensities are not comparable because no normalization was made. The inset shows the lower intensity peaks of pAF8 at 2.89 and 2.50 Å. Table 3. Bragg Spacings and Approximate Intensity from X-ray Experimentsa pAS6 33.0 (s) 16.4 (w) 10.7 (w)
Figure 4. Optical polarizing micrograph displayed for pAF8 at 74 °C after a cooling from 150 °C at 0.1 °C/min.
birefringence can be associated with the crystallization of the lamellar zones. The temperatures observed here concur totally with those presented in Table 2, which enables us to make clear the nature of the transition observed. The first peak, at a low temperature, was associated with a transition of the crystal/mesomorphic phase type, whereas the second, at a high temperature, was attributed to a mesomorphic phase/isotropic transition. The pAF8 polymer did not behave in the same way as pAS6 and pAS8. From the isotropic phase, when the temperature was lowered, the appearance of birefringent patterns was observed: at 74 °C, Maltese crosses could be distinguished (Figure 4). If the cooling was interrupted, these Maltese crosses grew and progressively filled the space.36 No other transition was observed when the cooling was continued. This result confirmed the existence in this case of a single, crystal/isotropic-type transition. X-ray Diffraction. The diffraction spectra performed on the four polymers at room temperature are shown in Figure 5. For three polymers (pAS6, pAS8, and pAF8), the spectra were composed of small-angle peaks (Bragg angles 2θ < 10°) on one hand, corresponding to successive diffraction orders and characteristic of a lamellar order, and on the other, around 2θ ) 15-20°, of an amorphous diffusion bump accompanied by a sharp peak, characteristic of the crystalline order between fluorinated segments. With complementary small-angle diffraction experiments, we checked that there were no supplementary peaks corresponding to greater distances. The first peak observed on the small angles thus corresponds to the lamellar period of the system. The positions of the different peaks are mentioned in Table 3. The pAF6 spectrum did not reveal a sharp peak, merely a diffusion band at 5.25 Å, characteristic of an amorphous state. This result is totally in agreement with the thermogram in Figure 2. In comparison, pAS6, which also has a short C6F13 fluorinated segment, showed a lamellar structure with a (36) de Crevoisier G. The`se de Doctorat, Universite´ de Paris VI, 1999.
4.95
pAS8
pAF6
small-angle region 40.3 (s) 20.4 (w) 13.3 (w) wide-angle region 4.89 5.25b
pAF8 32.6 (s) 16.3 (s) 10.9 (s) 5.0
a
Bragg spacings are given in angstroms. Intensities are classified as sharp (s) or weak (w). bBroad peak.
33 Å period. There was also an order in the lamellae characterized by a peak at 4.95 Å. The order in these lamellae probably did not extend very far because the peak, at 4.95 Å, was relatively wide. pAS8 was organized in the same way as pAS6: lamellar, with a period of 40.3 Å; the crystalline order in the layers extended farther; the peak, situated at a distance of 4.89 Å, was sharper; and as a result, the lamellae were denser. The peaks observed in pAF8 were even sharper. The organization was still lamellar, with a period of 32.6 Å. In this case, the order in the lamellae extended over a great distance: the characteristic peak of this order, situated at 5.0 Å, was very sharp, and furthermore, lowintensity peaks could be observed at 2.89 and 2.50 Å; these three peaks corresponded respectively to the planes (100), (110), and (200) of a hexagonal structure with a parameter a ∼ 5.7 Å. The fluorinated groups were thus organized inside the lamellae in a hexagonal array. The single transition observed in this polymer, at 78 °C, corresponded to a solid/isotropic transition. This type of organization, lamellar and organized along the planes of the layers, is a smectic organization. In accordance with the literature, at room temperature, the polymers pAS6, pAS8, and pAF8 were all of the smectic B type. pAF6 was globally amorphous. Temperature diffraction experiments were carried out on the polymers pAS6 and pAS8. The images obtained for pAS6, shown in Figure 6, show that, at 140 °C (which is between the two phase-transition peaks), the crystalline peak situated around 4.9 Å disappeared whereas the lamellar peaks remained present. Conversely, the lamellar peaks also disappeared above the second phase-transition peak, at 220 °C. This observation is totally coherent with the DSC and optical microscopy in polarized light analyses. It also confirms that the first melting peak can be interpreted as
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Figure 6. Wide-angle X-ray diffraction patterns of pAS6 at (a) room temperature; (b) 140 °C, between the two transitions; and (c) 220 °C, above the second transition.
Figure 8. Schematic arrangement of the polymers in the coating: (a) smectic polymers; (b) amorphous polymers.
Figure 7. Schematic representation of a polymeric chain for pAS6 and pAS8 with the characteristic distances in the extended conformation: L0 ) the extension perpendicular to the main chain with a [-COO-(CH2)2-] motif on each side, LS ) the length of the (N-methylsulfonamido)ethyl spacer, and LF ) the length of the perfluoroalkyl moiety. Lm ) L0 + 2(LS + LF).
a smectic B/smectic A transition and the second as smectic A/isotropic. An organization model will now allow us to carry our analysis of the X-ray diffraction results further. The use of molecular models (CPK) to compare the thickness of the lamellae obtained by X-ray diffraction with the length of the different segments made it possible to see an organizational model at room temperature more clearly. First of all, it could be seen that in order to form lamellae, the steric hindrance of the fluorinated segments (which was significant in comparison with the distance between the two successive side segments along the main chain) prevented them from all taking their position on the same side. This allowed us to design a polymer chain model represented in Figure 7. A comparison of the values obtained from the models (Lm) and from X-ray diffraction data (L) may be made. The characteristic lengths Lm were calculated with L0 ) 12 Å, the diameter of the main chain including the two groups [COO-(CH2)2]; LS ) 4.5 Å, the length of the N-methylsulfonamido ethyl spacer (not
present in the case of pAF8); and LF ) the length of the fluorinated group, calculated on a 1.5 Å per CF2 basis. We obtained the following values for Lm: 39 Å for pAS6, 45 Å for pAS8, and 36 Å for pAF8. In all cases, the ratio Lm/L was close to 1, between 1.1 and 1.18, which means that a lamellar period is composed of the main chain and a fluorinated layer on both sides of this chain; that is, the pendant groups are organized into double layers that are not interpenetrated, as represented in Figures 7 and 8a. It is certainly linked to the rigidity and size of the perfluorinated groups. No hypothesis is made concerning the conformation of the acrylic backbone. The only information we have concerns the arrangement of the side groups. If we consider that the interpenetration of the fluorinated groups is impossible because of their steric hindrance, a possible explanation of the deviation of the Lm/L ratio in comparison to 1 could be found in the existence of a tilt of the fluorinated groups within the lamellae. This tilt would correspond to a 20-30° angle in relation to the lamella perpendicular. This point is rarely discussed in the literature. It could be interesting to orient the polymer lamellae in order to conclude on the existence of this tilt. Discussion The results obtained in our study clearly established a direct relationship between the organization of the fluorinated side chains inside smectic phases and the surface properties of the coatings prepared with the various polymers. It must be pointed out that the presence of polymer crystallinity as measured by X-ray diffraction
Role of the Spacer in Fluoroalkyl Polyacrylates
on the powder does not necessarily involve side-segment crystallinity in the coating. Nevertheless, it should be noted that Maekawa et al.29 also carried out experiments on X-ray diffraction, not on polymers in powder form but directly on the coatings. For pAF8, they obtained a diffraction spectrum highly comparable to ours. This indicates that the organization in the coating is a faithful representation of the organization existing intrinsically in the polymer. The more recent work by Sheiko et al.34 shows that one can admit, at least as a first approximation, that the lamellae become oriented in the coating preferentially in parallel with the surface of the substrate. We then understand that the existence of smectic phases conditions the orientation of the fluorinated segments at the outmost surface of the coating and, as a result, the concentration of the CF3 extremities at the solid-air interface (Figure 8a). Under these conditions, polymers pAS6, pAS8, and pAF8 show organizations of the smectic B type sufficiently extended at room temperature to lead to a conformational arrangement of the fluorinated side groups globally comparable in the coatings. This justifies the fact that their surface properties are quite similar. Conversely, in the case of pAF6, there is no organized phase able to force the orientation of the fluorinated groups at the surface of the coating. For this reason, we can conceive a much less organized surface, and so degradation in the surface properties. Using ESCA and wetting measurements, Park et al.35 studied the outmost surface of a coating made of poly(1H,1H,2H,2H-perfluorooctyl)methacrylate, another amorphous polymer, like its polyacrylate equivalent (pAF6). The results obtained by these authors also show that at the surface of the coating, the fluorinated segments have an irregular conformational arrangement: most of them are not oriented perpendicularly to the coating and directed toward the air (see Figure 8b). In former studies, the effect of crystalline properties on wetting properties has already been reported for polymers containing side fluorinated groups capable of being oriented and organized. Pittman and Ludwig8 thus studied the surface properties of polyacrylates containing a side HCF2(CF2)nCH2- alkyl group with n ) 1, 3, 5, 7, and 9. In this series, polyacrylates with n ) 7 and 9 are semicrystalline polymers at room temperature and have, in parallel, the lowest critical surface tensions, γS, and the highest concentration of -CF2H groups at the solid-air interface. In an interesting manner, these authors show that above the melting point of polymers, the wetting capacity of the fluorinated coatings is increased. Comparable results were reported recently by de Crevoisier et al.,36,37 in a study on copolymers synthesized by solution polymerization of an acrylic monomer with a C8F17C2H4- side chain and of a methacrylic monomer with a long alkyl C17H35- side chain. At room temperature, these copolymers are highly organized in lamellar structures within which the hydrogenated and fluorinated side chains are crystallized at respective characteristic distances of 4.1 and 4.9 Å. DSC measurements show a transition between a mesomorphic phase and an isotropic phase at about 35 °C. At this transition temperature, the wetting hysteresis ∆θ ) θa θr, measured with a low molecular mass polybutadiene telechelic dihydroxy liquid, strongly increases (θr strongly decreases and θa stays more or less constant). This phenomenon can be attributed to a reconstruction of the surface, i.e., to a reorientation of the molecules at the surface, minimizing the interfacial energy with the environment. This minimization should lead, from a (37) de Crevoisier, G.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246.
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thermodynamic point of view, to a fluorinated surface when the copolymer is exposed to the air (when θr is measured) and to a hydrogenated surface when the coating is in contact with the polybutadiene (when θa is measured). But the mobility of the molecules is much lower in the smectic phase, which prevents the reconstruction of this phase. Conversely, the reconstruction kinetics is more favorable in the isotropic phase, which leads to a decrease in θr. The same reasoning makes it possible to understand the wetting difference measured at room temperature between crystalline polymers (pAS6, pAS8, and pAF8) on one hand, and the amorphous polymer (pAF6) on the other. The equilibrium contact angles θc ) (θa + θr)/2 that we measured effectively decreased when passing from a smectic phase (pAS6, pAS8, and pAF8) to an isotropic phase (pAF6). Although, as mentioned in the introduction of this paper, many references in the literature deal with fluorinated polymers,3-16,20-28,32-37 a limited number of studies are aimed at analyzing the structural parameters that make the organization of fluorinated groups possible. This organization, as explained above, is what the surface properties and, ultimately, the end-use properties of the substrates processed (textile, paper, ...) depend on. There are notably few studies that take an interest in the influence of the spacer that links the side perfluorinated group to the acrylic backbone. For the most traditional hydrocarbonated spacers, such as CH2 and (CH2)2, it appears that, globally, the fluorinated side chains of the polyacrylates do not become organized below a critical size, which is situated around 7 or 8 atoms of fluorinated carbon. A more systematic study of the effect of the length of the hydrocarbonated spacer was carried out by Me´las33 in the case of polyacrylates containing a C8F17(CH2)m- alkyl side group with m ) 2, 3, 4, 6, 10, or 11. This author noticed that, regardless of the length of the spacer, all the polymers in the series studied had an organized structure. Nevertheless, although with a short hydrocarbonated spacer (m ) 2, pAF8 of the present study) the fluorinated groups were well organized inside the lamellae, with an intermolecular distance of about 5 Å the organization clearly degraded when the size of the spacer increased from m ) 2 to m ) 6. The organized phases remained of the smectic B type but with more and more limited extension. For the longest spacers (m ) 10 and m ) 11), the structure remained lamellar, in double layers, but the long-distance order between the perfluorinated groups disappeared, translating into a diffuse diffraction at 5 Å. This corresponds to an occurrence of smectic A-type phases. From these results we learn that increasing the length of the hydrocarbonated spacer, and thereby increasing the global flexibility of this segment, has a generally negative effect on the crystallization of the fluorinated segments. Correlatively, Me´las notes that the wetting hysteresis measured with water clearly depends on the structure of the polymers. That is, with m ) 2, ∆θ is low (∆θ ) 19°), whereas with m ) 4, ∆θ is much higher (∆θ ) 60°). This degradation of the wetting properties, with a significant decrease in organization, was expected and refers to the arguments developed earlier in this discussion. In the case of the polymers that we studied, the spacer found between the perfluorinated segment and the acrylic skeleton contained four atoms of hydrogenated carbon in the case of pAS6 and pAS8 but two atoms of hydrogenated carbon for pAF6 and pAF8. In relation to what we have described above, this increase in the number of carbons should be unfavorable in terms of organization and did not justify the improvement in the properties when going
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from pAF6 to pAS6. The presence of the N-methylsulfonamide group in the spacer is thus the key parameter enabling us to explain our results. Whereas it has often been said in the literature that the perfluorinated side segment must attain a critical length of about 7-8 atoms of carbon in order to obtain crystalline polymers, the sulfonamide group improves the organizational capacities of the fluorinated segments and makes it possible to obtain smectic B phases, at room temperature, with -C6F13. An initial justification of a steric nature can be made for the role of this group. In the case of a purely hydrocarbonated spacer, there is effectively a significant section difference between the perfluorinated segment (S ) 28.3 Å2) and the spacer (S ) 18.5 Å2).33 In addition, the size of the fluorine atoms, bigger than the hydrogen atoms, imposes a helixshaped conformation on the perfluorinated chain and a high level of rigidity. The system must then find a molecular arrangement capable of minimizing the gaps, which are not very favorable in thermodynamic terms and which could appear between the side chains, at the level of the hydrocarbonated underlayer. At the same time, it must take into account the high level of rigidity of the fluorinated part of all the pendant groups. With such an image, it can be imagined that an increase in the length of the spacer [for instance, from -(CH2)2- to -(CH2)4-] makes it more difficult on one hand to maintain the organization of the fluorinated segments in the lamellae and on the other hand to minimize the gaps between the hydrocarbonated spacers. The results obtained by Me´las show that the system has no choice but to disorganize the fluorinated segments, at least in part. Conversely, the section of the spacer containing the N-methylsulfonamide group (which is the case for pAS6 and pAS8), measured according to a molecular model, has a size comparable to that of the perfluorinated segment, which is very favorable in steric terms and facilitates the segregation of the perfluorinated segments. This explanation is reinforced by the results obtained by Ramharack and Nguyen.10 These authors say that, in the case of acrylic or methacrylic polymers with pendant chains of the -(CH2)2N(CmH2m+1)SO2C8F17 type with m ) 1, 2, 3, or 4, if the alkyl substituent on the nitrogen has a low volume (ethyl or methyl group), the polymers are organized. On the other hand, polymers with a more voluminous alkyl substituent (propyl or butyl) are amorphous. Me´las33 completed his work with the structural study, by X-ray diffraction, of a certain number of fluorinated molecules of the alcohol, acid, and acrylate types. He noticed that the introduction of a polar function such as a hydroxyl function at the extremity of the hydrocarbonated chain [for instance, C8F17(CH2)10OH] induced the formation of intermolecular hydrogen bonds, which favored an organization of the system into double layers (of the smectic B or smectic G type) and increased the temperatures of smectic/isotropic transitions in comparison to the (perfluoroalkyl)alkane derivatives (for instance, C8F17C10H21). In the case of carboxylic acids [for instance, C8F17(CH2)10COOH], the double layers improved their cohesion thanks to stronger hydrogen bonds between the COOH groups. Conversely, the esterification of the alcohol function in acrylate [for instance, C8F17(CH2)10OOC-CHd CH2] modified the behavior of the molecules. The absence of hydrogen bonds between the extremities of each molecule no longer made the formation of double-layer structures possible. From this, then, we can see that, beyond the purely steric aspects, the inter- or intramolecular attractive interactions can modify and reinforce the organized phases. In the case of pAS6 or pAS8, the presence of an N-methylsulfonamide group makes dipole-
Corpart et al.
dipole-type interactions possible, which strongly reinforce the interactions between the side chains and make it possible to obtain a smectic B order even with a short perfluorinated chain like -C6F13. As a matter of fact, the existence of a strong dipolar moment for the sulfonyl group (-SO2-) has been well documented in the literature.38,39 This dipolar moment, directed in the OdSdO plane, orientates the polymer side chains in parallel and promotes their arrangement in smectic phases. This argument is reinforced by the fact that, for pAS6 and pAS8, the organized phases disappear at higher temperatures than pAF8. In addition, if for pAF8 the disappearance of the crystallinity of the side segments is concomitant with that of the lamellae (a single smectic B-isotropic transition), there is, for pAS6 and pAS8, a smectic A phase over a very wide range of temperatures (smectic B-smectic A-isotropic transition). The presence of the N-methylsulfonamide group makes it possible to stabilize the lamellae up to extremely high temperatures (>200 °C for pAS8). This explanation correlates quite well with that given by Lenk et al.40 Conclusion The organization of liquid crystalline fluorinated acrylate homopolymers has been generally described so far in the literature in terms of phase segregation between hydrogenated and fluorinated moieties within the polymer and preferential orientation induced by the backbone. Our study, focused on the role played by the spacer group located in the side chain between the backbone and the fluorinated segment, brings some new light to address this issue. A strong correlation between bulk organization and surface properties of our polymers has been established. In particular, using X-ray diffraction and DSC, we have shown that pAS6 with an N-methylsulfonamide spacer group exhibits a crystalline lamellar structure whereas pAF6 is amorphous. This indicates that the introduction of a N-methylsulfonamide spacer group in the side chain allows the system to crystallize with a shorter -C6F13 fluorinated segment, whereas in the presence of methylene groups, an organization is only present with a -C8F17 segment. This has mainly been attributed to the strong dipole-dipole interaction between N-methylsulfonamide groups that tends to align the fluorinated segments in a lamellar structure that eventually crystallizes as long as the section of the N-alkylsulfonamide group does not induce steric hindrance. An organization yielding excellent surface properties, i.e., a high water and oil repellency, can therefore be obtained with less fluorine atoms, which is of great industrial interest. This suggests that attention should be paid to the design of new spacer groups that are capable of generating strong interactions, such as dipole-dipole or hydrogen bonding. A close look should also be taken at copolymer systems where up to now the phase segregation approach prevails when their organization is interpreted. Acknowledgment. We thank M. J. Lina and P. Durual for the synthesis of monomers and polymers, A. C. Gayon for the wetting measurements, and A. Ibos, P. Decourval, and B. Lanteri for the structural investigations. We are indebted to B. Pees, M. Sindt, and J. L. Mieloszynski for fruitful discussions. LA010238G (38) Bulgarevich, S. B.; Movshovich, D. Ya.; Ivanova, N. A.; Filippov, S. E.; Finocchiaro, P.; Failla, S. J. Mol. Struct. 1991, 249, 365. (39) Fantoni, A. C.; Corbelli, G. J Mol. Spectrosc. 1994, 164, 319. (40) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F. Langmuir 1994, 10, 4610.
