Self-Dewetting of Perfluoroalkyl Methacrylate Films on Glass

Langmuir 1996, 12, 4015-4024

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Self-Dewetting of Perfluoroalkyl Methacrylate Films on Glass Sergei Sheiko, Eva Lermann, and Martin Mo¨ller* Organische Chemie III/Makromolekulare Chemie, Universita¨ t Ulm, D-89069, Ulm, Germany Received March 11, 1996. In Final Form: May 20, 1996X Side chain crystallization in spin cast films of polymethacrylates with perfluoroalkyl substituents (i.e., F(CF2)n(CH2)m-, n ) 8, m ) 2 and n ) 10, m ) 6) is shown to yield double layers of the hydrocarbon backbone and fluorocarbon side chains. Disordering to a smectic mesophase has been observed by differential scanning calorimetry and X-ray scattering. Regular packing of the side chains in the top layer at the polymer-air interface resulted in a very low critical surface tension of ca. 6 mN/m. Above the bulk isotropization temperature of the material, the multilayered structure collapsed resulting in a peculiar self-dewetting which could be observed directly by scanning force microscopy. On the basis of the dewetting behavior, three states have been distinguished depending on the film thickness: (i) At distances at least more than seven bilayers away from the flat substrate, the film melted according to the bulk isotropization temperature and demonstrated macroscopic dewetting of an ordered sublayer. (ii) Also this sublayer was not stable above the isotropization transition, but collapsed in a peculiar stepwise manner. A bilayer started to dewet the next bilayer only after the top layer had collapsed into microscopic droplets. This repetitive process proceeded until (iii) a thin film with the thickness of only one and a half bilayers remained to cover the substrate. This layer was stable even when the temperature was raised 50 °C above the isotropization temperature. Apparently, the geometric constraint by the “hard wall” and the interactions with the substrate caused this stabilization. The self-dewetting is discussed in terms of a progressive disordering of the side chains with increasing distance from the wall.

Introduction Wetting and dewetting of thin liquid polymer films have been the subject of increasing scientific interest. General and distinctive features were elaborated theoretically1-3 and tested experimentally in comparison with those of low molecular weight liquids.4-7 Four factors promote molecular ordering in thin films:8,9 (i) geometric constraints caused by the “hard wall”; (ii) lowering of the surface energy; (iii) interactions with the substrate; (iv) microphase separation. This has raised the question of whether a film confined between two interfaces may be considered as truly liquid. Peculiar phase transformations were predicted at an interface10-13 and demonstrated experimentally.14-19 * To whom correspondence should be addressed: e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) de Gennes, P.-G. Rev. Mod. Phys. 1985, 57, 827. (2) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (3) Srolovitz, D. J.; Safran, S. A. J. Appl. Phys. 1986, 60, 247. (4) Ausserre, D.; Picard, A. M.; Leger, L. Phys. Rev. Lett. 1986, 57, 2671. Silberzan, P.; Leger, L. Macromolecules 1992, 25, 1267. (5) Heslot, F.; Cazabat, A. M.; Levinson, P. Phys. Rev. Lett. 1989, 62, 1286. (6) Reiter, G. Phys. Rev. Lett. 1992, 68, 75; Langmuir 1993, 9, 1344. (7) Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Fetters, L. J.; Plano, R.; Sanyal, M. K.; Sinha, S. K.; Sauer, B. B. Phys. Rev. Lett. 1993, 70, 1453. (8) Horn, R. G.; Israelechvili, J. N. Macromolecules 1988, 21, 2836. (9) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1992; Vol. 2. (10) Binder, K. In PhaseTransitions and Critical Phenomena; Domb, C., Lebowitz, J. L., Eds.; Academic: New York, 1983; Vol. 8. (11) Schoen, M.; Diestler, D. J.; Cushman, J. H. J. Chem. Phys. 1987, 87, 5464. (12) Lipowsky, R. Phys. Rev. Lett. 1982, 49, 1575. (13) Hansen, F. Y.; Newton, J. C.; Taub, H. J. Chem. Phys. 1993, 98, 4128. (14) Jackson, C. L.; McKenna, G. B. J. Chem. Phys. 1990, 93, 9002. (15) Herwig, K. W.; Trouw, F. R. Phys. Rev. Lett. 1992, 69, 89. (16) Zhang, J.; Liu, G.; Jonas, J. J. Chem. Phys. 1992, 96, 3478. (17) Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N.; Homola, A. M. J. Chem. Phys. 1990, 93, 1895. (18) Klein, J.; Kumacheva, E. Science 1995, 269, 816. (19) Pershan, P. S.; Braslau, A.; Weiss, A. H.; Als-Nielsen, J. Phys. Rev. A 1987, 35, 4800.

S0743-7463(96)00229-6 CCC: $12.00

The most important differences between low and high molecular weight compounds originate from the elastic free energy contribution upon deformation of the polymer coils in the vicinity of an interface, the slow relaxation times of macromolecules, and long range steric effects.20,21 In the case of multicomponent polymer liquids or polymer blends, an alternating composition profile can be caused by the affinity of one component to concentrate at the interface.22 Typically such a profile damps exponentially into the bulk and has a periodicity which can reach the scale hundreds of nanometers.23 This is also observed in the case of block and graft copolymers, if one of the constituent components goes preferentially toward the surface. Microphase separation24 results in an ordered layer morphology even at a temperature where the bulk structure is disordered.25 Amphiphilic polymers can yield relatively thick films possessing a layered structure. No distinguishable disordering transition was observed in thin films of symmetrical block copolymers with a thickness of 100-1000 nm.26 Because of the interfacial ordering, wetting properties of amphiphilc polymers may be changed considerably. In the present paper, we report on experimental results on the self-dewetting of poly(perfluoroalkyl methacrylate) films. Perfluoroalkyl-substituted methacrylates are widely used to produce coatings imparting soil resistance and water- and oil-repellent properies.27 Yet little is known about the structure and thermal behavior of such coatings. In contrast to block copolymers the chain does not consist of two antagonistic polymer units, but each monomer is (20) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189. (21) van Alsten, J.; Granik, K. S. Macromolecules 1990, 23, 4856. (22) Jones, R. A. L.; Norton, E. J.; Kramer, E. J.; Bates, F. S.; Wiltzius, P. Phys. Rev. Lett. 1991, 66, 1326. (23) Krausch, G.; Kramer, E. J.; Bates, F. S. Phys. Rev. Lett. 1993, 73, 3669. (24) Leibler, L. Macromolecules 1980, 13, 1602. (25) Fredrickson, G. H. Macromolecules 1987, 20, 2535. (26) Menelle, A.; Russel, T. P.; Anastasiadis, S. H.; Satija S. K.; Majkrzak, C. F. Phys. Rev. Lett. 1992, 68, 67. (27) Kissa, E. Fluorinated Surfactants, Synthesis-Properties-Applications; Marcel Dekker: New York, 1984.

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Table 1. Transition Temperatures Determined by DSC, Heating Rate 10 K/mina sample name

Tg, °C

Td, °C

Ti, °C

P(F8H2-MA) P(F10H6-MA-co-MA)

8 99

79 85

90 109

a T , glass transition; T , disordering; T , isotropization temg d i peratures.

Figure 3. SFM micrograph of a 20 nm thick film of the P(F8H2-MA) homopolymer on a glass substrate as prepared by spin casting from 0.1 wt % in trichlorotrifluoroethane.

Figure 1. The X-ray diffraction pattern of the P(F8H2-MA) homopolymer displays a perfect layered structure and crystallization of the side chain at 30 °C. Arrows indicate three orders of layer reflections (L) corresponding to the layer spacing of 3.2 nm and (S) corresponding to the spacing of 0.5 nm and packing of the side chains.

Figure 2. Two-dimensional scheme illustrating principal arrangements of alternating layers of the fluorocarbon side chains and PMA backbone: (a) double comb organization and (b) alternating orientation of the side chains.

an amphiphilic subunit by itself linked by the flexible polymethacrylate backbone. In particular, in thin films or surfaces some competition might be expected between the ordering of the side chains and coiling of the polymer backbone.

Experimental Section The synthesis of perfluoroalkyl methacrylate, F(CF2)n(CH2)mOOC(CH3)CdCH2 with n ) 8, 10, and 12 and m ) 2 and 6, has been described before.28 Homopolymers were prepared by radical polymerization with a small amount of solvent added in order to avoid incomplete conversion due to vitrification during polymerization. Statistical copolymers containing 10% of perfluorodecyl-substituted methyl methacrylate units were prepared by radical polymerization in the bulk. Due to the limited solubility in suitable solvents, it has not been possible to determine the molecular weights accurately. Size exclusion chromotography in toluene yielded values around 2 × 104 g/mol for the copolymer according to polystyrene calibration. Because of the interaction of the perfluoroalkyl groups, a reduced hydrodynamic radius can be expected, and the actual molecular weight might be higher. The homopolymer could not be characterized the same way. SEC experiments in trichlorotrifluoroethane, which was the only solvent we found, has not been possible. From solution viscosities we estimated the molecular weight to be similar to the case of the copolymer, thus exceeding 2 × 104 g/mol. A Perkin-Elmer DSC-7 equipped with a PE-7700 computer and TAS-7 software was used to monitor thermal transitions at scan rates of 10 K/min. Sample weights were typically around 10 mg. Cyclohexane and indium were used for calibration. The onset of the endotherm recorded upon heating was taken as the transition temperature. X-ray diffraction patterns of polymer powders were recorded using Ni-filtered Cu KR radiation and a flat camera with a sample to film distance of 90 mm. Exposure times were approximately 3 h. Diffractograms were collected from the photographically obtained patterns using a linear microdensitometer LS20 (Delft Instruments) controlled by SCANPI software. Polymer films were prepared by spin casting at 2000 rpm. Solutions of two different concentrations, i.e., 0.1 and 0.5 wt %, in trichlorotrifluoroethane were cast at ambient conditions on glass substrates. In the first case, films with a thickness of about 20 nm were produced. The higher concentration yielded films with a thickness of about 100 nm. Specimens were kept for 1 h at ambient temperature under vacuum to remove the solvent. The procedure resulted in smooth films with an average roughness below 4 Å. The surface roughness and the film thickness were measured by scanning force microscopy (SFM). SFM micrographs were measured by means of a Nanoscope III (Digital Instruments, St. Barbara) operated in the tapping mode29 at a resonance frequency of ∼360 (28) Ho¨pken, J.; Faulstich, S.; Mo¨ller, M. Mol. Cryst. Liq. Cryst. 1992, 210, 59.

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kHz. The measurements were performed at ambient conditions using Si probes with a spring constant of ∼50 N/m. Tips with an apex radius below 10 nm were selected by means of a welldefined stepped surface structure of a SrTiO3 single crystal wafer constituted by alternating (103) and (101) planes.30 Dynamic contact angles θ of the polymer coatings were measured against different liquid n-alkanes at 25 °C by the Wilhelmy method31 using a Kru¨ss K12 tensiometer. Double sided coated glass plates with a perimeter of 36 mm were used for the measurements. The surface tension was derived from the force which affected the plates upon immersing and withdrawing from the liquid. The average advancing and receding contact angles were calculated by regressive extrapolation of the force to zero immersion depth. A platinum plate with a perimeter of 59.5 mm was used for the surface tension measurements of the liquids. Temperature dependence of the contact angle against nhexadecane was investigated by means of the sessile drop technique (Kru¨ss G40). The reported angles are the average values of five measurements from different droplets.

Results and Discussion A. The Structure of Perfluoroalkyl Methacrylate Films. Poly(1,1,2,2-tetrahydroperfluorodecyl methacrylate), P(F8H2-MA), and a copolymer of perfluorodecylhexyl methacrylate with 90 mol % of methyl methacrylate, P(F10H6MA-co-MMA), were chosen for the investigation. Degrees of polymerization have been estimated to exceed DP ) 4028 based on SEC data and solution viscosity.

The polymers formed partially ordered solids as the side chains crystallized. Table 1 summarizes the thermal transitions determined by DSC. Besides the glass transition, both the homopolymer and the copolymer show a disordering transition Td at 79 and 85 °C, respectively, before they transform into an isotropic state at Ti ) 90 and 109 °C. The large difference between the glass transition temperatures is consistent with generally observed lowering of Tg by flexible side chains.32 Because of the high fraction of methyl methacrylate in the copolymer, a high Tg is observed. The intermediate state before isotropization can be identified as a smectic liquid crystalline phase according to the microscopic birefringence texture33 and X-ray results.34 The X-ray diffraction pattern of a bulk sample of F8H2PMA below Td is depicted in Figure 1. The small angle reflections (L) can be assigned to the stacking of bilayers. The observation of 3 orders corresponds to a well-developed layered structure with a spacing of 3.2 nm. Wide angle reflections (S) are consistent with a regular arrangement of the perfluoroalkyl chains within the layers with a (29) Zhong, Q.; Inniss, D.; Elings, V. B. Surf. Sci. 1993, 290, L688. (30) Sheiko, S. S.; Mo¨ller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Phys. Rev. B 1993, 48, 5675. (31) Smith, L.; Doule, C.; Gregoris, D. E.; Andrade, J. D. J. Appl. Polym. Sci. 1982, 26, 1269. (32) Plate, N. A., Shibaev V. P., J. Polym. Sci., Macromol. Rev. 1974, 8, 117. (33) Ho¨pken, J. PhD Thesis, University of Twente, Enschede, The Netherlands, 1991. (34) Sheiko, S.; Turetskii, A.; Ho¨pken, J., Mo¨ller, M. In Macromolecular Engineering; Mishra, M. K., et al.; Plenum Press: New York, 1995; p 219.

Figure 4. SFM micrograph of the film shown in Figure 3 after annealing for 20 h at 85 °C. Two-dimensional coarsening of the holes in the top layer is observed. The cross sectional profile runs along the line depicted in the micrograph and yields a layer thickness of 3.2 nm.

spacing of d ) 0.5 nm. For the copolymer a long period of 3.7 nm was determined in agreement with the longer side chains. X-ray studies on solution cast film revealed the same layered structure.34 The obtained values suggest a bilayered organization in which the side chains are in a rather extended conformation and oriented perpendicular to the surface of the bilayers. Formation of a bilayer structure is explained by the amphiphilic nature of the semifluorinated side chains. The critical surface tension according to Zisman35 was determined by dynamic contact angle measurements to be as low as 6 mN/m for a P(F8H2-MA) film.36 This can be attributed to a surface layer of CF3 end groups which is formed as the side chains crystallize with the CF3 groups directed toward the surface. Such a surface arrangement of the side chains was also supported by SFM observations with spin cast films showing an ordered square pattern with a periodicity of 5.6 Å.34,36 Figure 2 gives a schematic representation of the film structure as deduced from the experimental data discussed so far. The figure represents two principally different models for the formation of bilayers. In the first one (Figure 2a) all perfluoroalkyl side chains of one macromolecule are directed to one side and two macromolecules form one bilayer. In the second model (Figure 2b), the side chains are directed up and (35) Zisman, W. A. Ind. Eng. Chem. 1963, 55, 18. (36) Ho¨pken, J.; Sheiko, S.; Czech, J.; Mo¨ller, M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem) 1992, 33 (1), 937.

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Figure 5. SFM micrographs of the thin film from Figure 4 after annealing for 10 min above the bulk isotropization temperature at 100 °C. (left) The topology image of a top layer, a sublayer, and formation of droplets. (right) The friction micrograph does not show significant contrast between the top layer, sublayers, and the droplets. From the cross sectional profiles along the lines drawn in (left image) the film thickness is determined to be 3.2 nm and the “contact angle” of the droplets to be about 18°.

down as well and each macromolecule contributes to two layers of perfluoroalkyl segments. So far, the X-ray data do not allow one to distinguish which of the models is more appropriate. Regarding the dense substitution of the molecular backbone by the rather bulky perfluoroalkyl substituents, the second model appears to be more favorable.37,38 However, for energetical reasons, the interaction of the rather polar polymer backbone to the substrate is favored compared to an arrangement of the interface where the side chains are in contact with the substrate. In this case, at least the first layer must correspond to the model in Figure 2a. Common to both models is that the arrangement in layers is only possible at the expense of a positive contribution of the elastic free energy due to the unfavorable configuration of the polymer backbone. Such elastic effects have been shown before to be responsible for the dewetting of polymethacrylate coatings by ultrathin polystyrene films.39 B. Dewetting of Perfluoroalkyl Methacrylate Films As Observed by SFM. In the following, we describe tapping SFM observations on a 20 ( 1.5 nm thick homopolymer film consisting of about 6.5 bilayers. While the surface of the homopolymer film was highly regular on the molecular scale,34,36 it appeared to be rough or (37) Schwickert, H.; Strobl, G.; Kimmig, M. J. Chem. Phys. 1991, 95, 2800. (38) Russell, T. P.; Rabolt, J. F.; Twieg, R. J.; Siemens, R. L.; Farmer, B. L. Macromolecules 1986, 19, 1135 (39) Krausch, G.; Hipp, M.; Bo¨ltau, M.; Marti, O.; Mlynek, J. Macromolecules 1995, 28, 260.

Table 2. Dynamic Contact Angles of the Homopolymer Coatings against n-Hexadecane (γ ) 27.6 mN/m) samples

Θadva (deg)

Θreca (deg)

∆Θ (deg)

corresponding SFM images

spin-cast 81.4 54.9 26.5 Figure 4 85 °C, 20 h 81.3 71.4 9.9 Figure 5 110 °C, 10 min 81.5 70.8 10.7 Figure 6 a The advancing and receding contact angles were measured by Wilhelmy plate technique.

uneven on the mesoscopic scale. This is demonstrated by the topography image in Figure 3. The surface contained small holes with a diameter of about 10 nm (black spots) and a depth of 3.2 nm. The depth of the holes corresponds to the bilayer spacing determined by X-ray analysis. We believe that these holes formed upon lateral shrinkage of the surface layer as the side chains improved their packing after evaporation of the solvent. When the sample was annealed before reexamination by SFM, the picture changed depending on the annealing temperature. Annealing in the mesophase state resulted in two-dimensional coarsening of the top layer as it is shown in Figure 4. The coarsening was apparently strictly confined to the first bilayer. The small holes fused to larger areas, but the overall area of the holes remained constant. Thus, the effect may be considered as an agglomeration of the holes driven by the line tension.40 (40) Bassereau, P.; Brodbreck, D.; Russel, T. P.; Brown, H. R.; Shull, K. R. Phys. Rev. Lett. 1993, 11, 1716.

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Figure 6. Development of the self-dewetting of a thin film of P(F8H2-MA) homopolymer film. Scanning force micrographs were measured at ambient temperature after annealing the film at 105°C for different times: (a) 15 min; (b) 45 min; (c) 3 h; (d) 20 h. The number of bilayers which did coagulate to form the droplets was determined from the volume of the droplets and is indicated in the top-right corner of the corresponding micrograph. Cross sectional profiles along the lines indicated in the micrographs demonstrate the layered structure and a contact angle of 15-23° for the growing droplets.

Annealing above the isotropization temperature at 100 °C changed the surface structure in a different way (Figure 5a). First, the top layer coagulated and microscopic

droplets were formed in the dewetted area. Also in this case, the thickness of the collapsed layer was measured to be equal to the bilayer spacing L ) 3.2 nm. The volume

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of the droplets corresponded to the amount of material which had been coagulated. Typically, the diameter of one droplet was around 500 nm and the height was 50 nm. The contact angle to the sublayer was 18°. Images based on lateral forces (friction SFM) were taken to get an indication as to whether the surface of the droplets and the underlying layer differed in their chemical composition. Figure 5b depicts the lateral force image of the same area from which the topography image in Figure 5a was taken. The featureless image did not reveal a friction contrast between the top layer, the sublayer, and the droplets. The lack of any contrast indicates a uniform surface composition. This is in agreement with macroscopic contact angle measurements summarized in Table 2. The advancing contact angle against n-hexadecane did not differ significantly regardless of whether the films were annealed or not. The value of about 81° is characteristic for surfaces composed of densely packed CF3 groups.41-42 Figure 6 demonstrates how the process proceeded after the collapsing of the first layer. The sequence of SFM images shows the effect of isothermal annealing at 105 °C after 15 min and up to 20 h. As soon as the second layer became exposed to air, it started to dewet the third layer (Figure 6a). Subsequently, layer after layer coagulated and the size of the droplets increased by taking up the material which was contained in the collapsed layers. Within each layer, dewetting started by formation and two-dimensional growth of small holes with a depth of just one bilayer. This is more clearly demonstrated by the series of the cross sectional profiles in Figure 6. Calculation of the volume of the droplets allowed the estimation of the number of layers which had collapsed. Collapsing of the multilayer coating did not result in complete dewetting of the substrate. In repeated experiments, a minimum thickness was reached. The remaining film turned out to be stable and did not change further even when it was annealed at 135 °C for 48 h. In order to measure this apparently critical thickness, a small area of the film was removed by scanning the sample with a high force. Afterward, a micrograph was taken with minimum force and the film thickness was determined from the cross sectional profile (Figure 7) to be 4.7 nm, which is equal to one and a half bilayer spacings. So far, two different states have been observed, differing in their melting and concurrent dewetting behavior. Firstly, the one and a half bilayers attached directly to the substrate which turned out to be remarkably stable upon heating and, secondly, the adherent five bilayers which showed the peculiar layer by layer dewetting. Both phenomena can be assigned to the proximity of the flat substrate, and even different behavior might be expected for thicker films, when the melting of the surface layer occurs at a greater distance from the substrate. This was indeed observed when we studied annealing of thicker films. Figure 8 shows tapping scanning force micrographs of a film of the homopolymer P(F8H2-MA) with a thickness of 100 nm. The surface of the as cast film shows small holes similar to those observed for the 20 nm thick film in Figure 3 and in addition relatively big holes which looked like small craters. The depth of the holes was determined to be 3.2 and 70 nm, respectively. Because the 3.2 nm is again consistent with the thickness of one bilayer, the small holes can be regarded as indicative of the bilayer structure. The larger craters might be (41) Ho¨pken, J.; Mo¨ller, M. Macromolecules 1992, 25, 2482. (42) Jonson, R. E., Jr.; Dettre, R. H. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem) 1987, 28, 48.

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Figure 7. SFM micrograph of the P(F8H2-MA) film after it was annealed for 48 h at 135 °C. A small area of the substrate was cleaned from the polymer by scanning with a relatively high force (200 nN). The film thickness was determined from the cross sectional profile to be 4.7 nm.

explained by the eruption of gas bubbles from solvent residues in the later stage of film formation. Upon annealing above the bulk isotropization temperature, the small holes disappeared completely, in contrast to what was observed in the case of the thin films. The larger crater-like holes apparently induced macroscopic dewetting of the whole film (Figure 8b-d). However, close inspection of the structure of the dewetted areas by SFM demonstrated that the substrate was still covered by a layer of the homopolymer (indicated in the figure by the arrows). The surface structure at the bottom of the large holes is the same as was observed for the thin films which demonstrated layer by layer self-dewetting; i.e., the growth of small holes with a thickness of the bilayer spacing L ) 3.2 nm. In this case droplets were not observed, which might be explained by the coalescence of the material of the collapsing layers with the molten polymer surrounding the dewetted areas. In principle, the SFM images in Figure 8 show evidence at a third state at distances roughly beyond the first seven double layers. This state is evident by self-healing of the small holes and macroscopic dewetting, instead of the 2D coarsening of the top layer and layer-by-layer dewetting observed for the thin films. For this part of the film, melting and dewetting may correlate to the isotropization transition of the bulk material. Different results were observed for the copolymer films. Figure 9 depicts a series of SFM micrographs of a 25 nm thick film of the copolymer. Also in this case, the as cast films exhibited small holes, whose depth of 3.7 nm corresponded to the bilayer spacing of the copolymer

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Figure 8. The series of 13 × 13 µm2 scanning force micrographs displays the dewetting of a thick film. The spin cast film (a) was annealed at 105 °C for 10 min (b), 30 min (c) and 20 h (d). The cross sectional profile corresponds to the line indicated in (c). The depth and diameter of the large holes in (c) and (d) are shown to be about 90 and 2000 nm, respectively. The bottom of the large holes is formed by a flat thin film of the polymer. Small holes in this film have a depth of 3.2 nm, which is consistent with the bilayer spacing schematically shown in Figure 2.

determined by X-ray. Annealing above the bulk isotropization temperature resulted in the coarsening of the two-dimmensional structure as explained by the line tension, i.e., decreasing borderlines in favor of a minimization of the surface free energy. In contrast to the case of the homopolymer, there was practically no further dewetting to yield the droplets shown in Figure 5, even after extended annealing.

Also, thick films of the copolymer with a thickness of about 100 nm were stable against dewetting irrespective of whether their structure contained defects. SFM micrographs in Figure 10 demonstrate that both small and large holes healed upon extended annealing at 130 °C resulting in a smooth surface. The question of whether the peculiar self-dewetting or autophobic behavior of the homopolymer can be correlated

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Figure 10. Thick films of the copolymer P(F10H6MA-co-MA) demonstrated self-healing of the small and the large holes upon annealing at 130 °C: (a) as prepared; (b) after annealing for 20 h.

Figure 9. Spin cast film of the copolymer P(F10H6MA-coMA) after annealing for different times at 125 °C: (a) asprepared; (b) 30 min; (c) 20 h.

to variations in the surface energy caused by structural changes43-45 was addressed by temperature dependent contact angle measurements with thick films against n-hexadecane by the sessile drop technique (Figure 11). The contact angle remained constant upon raising the temperature up to the disordering transition temperature at 79 °C. Raising the temperature further resulted in a decrease of the contact angle, which indicates an increase (43) Sauer, B. B.; Dee, G. T. J. Colloid Interface Sci. 1994, 144, 527. (44) Poser, C. I.; Sanchez, I. C. J. Colloid Interface Sci. 1979, 69, 539. (45) Newman, A. W. Adv. Colloid Interface Sci. 1974, 4, 105.

of the surface energy upon disordering of the CF3 surface as the interaction between the perfluoroalkyl groups and n-hexadecane is mainly due to dispersion forces. Above the melting transition, the contact angle decreased apparently further. However, the sessile drop experiments became time dependent and less reliable as the molten polymer started to wet the hexadecane droplets. Conclusions Perfluoroalkyl-substituted polymethacrylates have been shown to form ordered structures based on the crystallization of the perfluoroalkyl side chains and the incompatibility of the fluorinated and the hydrocarbon segments. In thin films as well as in the bulk, a regular bilayer morphology is developed. Disordering into an isotropic melt occurs in two steps. A smectic mesophase is formed before the layered structure breaks up finally at elevated

Self-Dewetting of Films

Figure 11. Sessile drop contact angle of the F8H2-PMA film against n-hexadecane as a function of temperature. The doted lines indicate disordering (79 °C) and isotropization (90 °C) transitions of the polymer.

Figure 12. Schematic representation of the disordering/ dewetting phenomena as a function of the film thickness: (a) thick films demonstrate macroscopic dewetting of the ordered sublayers; (b) the sublayers collapse via stepwise coagulation of the bilayers; (c) a thin film of one and a half bilayer thickness remains stable.

temperatures. This transition is characteristically effected by the interfaces in thin films on a flat substrate. As a consequence a peculiar self-dewetting was observed. Figure 12 gives a schematic representation of the observed disordering/dewetting as a function of the film thickness: 1. Thick films disordered above the isotropization temperature and appeared to be metastable against dewetting. The films remained fairly stable as far they did not contain large holes. In this case, macroscopic dewetting of an ordered sublayer was observed.

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2. Thin films showed a peculiar stepwise dewetting where one by one bilayer collapsed. The layers broke up irreversibly into droplets with an apparent contact angle of 15-23°. 3. The bilayer by bilayer dewetting proceeded until a thin film with a thickness of one and a half bilayers was left. The later remained remarkably stable even at temperatures far above the bulk disordering transitions. 4. The stepwise dewetting of thin films was not observed when instead of a perfluoroalkyl methacrylate homopolymer, a copolymer with methacrylate units was used. Although films up to a thickness of at least seven bilayers showed the characteristic coarsening of the top layer, they were stable against dewetting far above the bulk isotropization temperature. Three direct conclusions may be drawn. (i) The proximity of a flat hard wall can stabilize the bilayer structures of the poly(perfluoroalkyl methacrylates) significantly. This is in agreement with the expectations as pointed out in the introduction. (ii) Copolymers can form more stable thin films than the more regular homopolymer. This effect may correlate to the competition between the layered packing of the side chains and the coiling of the polymer backbone. (iii) The peculiar self-dewetting, or autophobic behavior, of the homopolymer must be related to different structures in the dewetting layer and the dewetted layer caused by heating the samples above the isotropization temperature. As the molecular structure is uniform, we do not have any evidence to the contrary. Such an explanation is consistent with the increase of the surface energy of the homopolymer upon disordering as was indicated by the temperature contact angle measurements. We propose the following qualitative model for the observed dewetting. Spin casting or film formation by solvent evaporation resulted in a layered structure as shown in Figure 2. Due to perfect orientation of the side chains and regular arrangement of the -CF3 groups at the surface, the films exhibited a uniquely low surface energy. Increasing temperature favors coiling of the polymer backbone and thus disordering of the side chains. Apparently disordering caused perhaps by the exposure of groups other than CF3, e.g., CF2 and MMA, which exhibit a higher surface energy, results in a small but significant increase in the surface energy. At the same time the substrate stabilizes the layered structure. Progressive disordering of the layers at increased distances from the liquid-solid interface might explain an increase of the surface energy for thick films compared to thin films and thus describe why a thick film dewets a flat sublayer. Thus, contrary to the film thickening controlled by long range forces,2,46 the formation of the droplets with finite contact angle is supposed to correspond to different surface energies caused by different stages of disordering of the perfluoroalkyl groups. However, for the peculiar layer by layer dewetting it is not sufficient to discuss the different stability of each of the layers. Kinetic effects have to be considered as well. The dynamics of the self-dewetting will be a subject of further work. Here, we would only like to make a few suggestions. Apparently, the selfdewetting process starts by the formation of the small holes. Hole formation might be rationalized by the scheme in Figure 13. The “caterpillar” type relaxation of polymer backbones within one bilayer represents a relatively easy process as it does not require large translations of the macromolecules. However, hole formation should face an energetic barrier caused by the increase of the free surface (46) Riegler, H.; Asmussen, A.; Merkl, C.; Schabert, F.; Rabe, J.; Davydov, A. Poster contribution at International Workshop on Wetting and Self-Organization in Thin Liquid Films; 18-21 September, Konstanz, Germany.

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size will become stable and form a nucleus for further dewetting. The dewetting process as a whole is controlled by the rate of nucleation and the growth rate of the dewetted areas involving surface transport of the molecules segregating into the droplets. A difference between the rates can explain why collapsing of the bilayers was resolved in time.

Figure 13. Hole nucleation via “caterpillar” like retraction of the top bilayer.

area.3 The height of the barrier and corresponding radius of the hole depend on the hole depth and surface energies of the top layer and sublayer. Thus, only holes of a certain

Acknowledgment. This work was financially supported by SFB-239 and the Fond der Chemischen Industrie. The authors are grateful to Professor L. Leibler for a very helpful discussion. LA960229L