10456
Langmuir 2000, 16, 10456-10462
Crossover between Dewetting and Stabilization of Ultrathin Liquid Crystalline Polymer Films Armelle B. E. Vix,*,†,‡ Peter Mu¨ller-Buschbaum,§,| Wolfgang Stocker,†,⊥ Manfred Stamm,§,# and Ju¨rgen P. Rabe† Institut fu¨ r Physik, Humboldt Universita¨ t zu Berlin, Invalidenstrasse 110, 10115 Berlin, Germany, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received June 15, 2000. In Final Form: August 21, 2000 Thin spin-coated films of a smectic liquid crystalline main chain polymer with an azoxybenzene as mesogen were investigated as a function of temperature and thickness. Both lateral and vertical order of the polymer film in the various mesophases were determined by scanning force microscopy in tapping mode and by specular and off-specular X-ray scattering. For films thicker than 10 nm, we observed a stabilizing effect of the smectic organization within the film. No dewetting was detected. The bulk structure of the film consists of smectic layers with a thickness of 3.5 nm, oriented parallel to the substrate. The excess material formed smectic islands on top of the film, corresponding to a noncomplete smectic layer. For films thinner than 10 nm, corresponding to one or two smectic layers only, we observed dewetting with the formation of holes. The holes were dry without any remaining monolayer on the substrate. However, the withdrawing polymer did not accumulate in rounded rims such as for amorphous polymers but in smectic towers, due to the packing of the polymer chain.
Introduction The dewetting of thin polymer films is of great importance for applications as well as fundamental investigations. In contrast to simple amorphous homopolymers, liquid crystalline polymers exhibit several mesophases in the bulk and in thin films. This results from the chain architecture and is widely used in applications. With respect to the stability of a once created film, it opens new possibilities comparable to the use of block copolymers. To improve the knowledge about stabilization mechanisms, the fundamental investigations of the dewetting process itself are important. Thin polymer films are metastable if they are spincoated on nonwettable substrates. Annealing above the glass transition temperature (Tg) of the polymer enables a relaxation of the polymer chains within the films toward their thermodynamical equilibrium. During this relaxation, dewetting can take place,1,2 which means rupture of the homogeneous film, creation of holes, and accumulation of the withdrawing polymer in rims.3-7 For thicknesses * To whom correspondence should be addressed. † Humboldt Universita ¨ t zu Berlin. ‡ Current address: Sektion Physik and CeNS, Ludwig-Maximilians-Universita¨t, Geschwister-Scholl Platz 1, 80539 Mu¨nchen, Germany. § Max-Planck-Institut fu ¨ r Polymerforschung. | Current address: TU Mu ¨ nchen, Lehrstuhl E13, James-FranckStr. 1, 85747 Garching, Germany. ⊥ Current address: Westfa ¨ lische Wilhelms Universita¨t, Institut fu¨r Medizinische Physik und Biophysik, Robert Koch Str. 31, 48129 Mu¨nster, Germany. # Current address: Institut fu ¨ r Polymerforschung e.V., Hohe Str. 6, 01069 Dresden, Germany. (1) Ruckenstein, E.; Jain, R. K. Faraday Trans 2 1974, 70. (2) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (3) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (4) Brochard, F.; di Me´glio, J. M.; Que´re´, D. C. R. Acad. Sci., Ser. II 1987, 304, 553. (5) Brochard, F.; de Gennes, P. G. Langmuir 1992, 8, 3033. (6) Brochard-Wyart, F.; Redon, C.; Sykes, C. C. R. Acad. Sci., Ser. II 1992, 314, 10.
below 1 µm, the dewetting of homopolymer films such as polystyrene annealed above their Tg values (Tg ) 120 °C) was observed.8-9 The aim of this work was to realize polymer films on silicon wafers or glass substrates with thicknesses below 20 nm which are stable, that is, which do not dewet. In earlier publications on thin films with various kinds of side chain liquid crystalline (LC) polymers,10-14 it could be shown that the polymer chains form smectic layers oriented parallel to the substrate in a defined temperature range depending on the polymer, more precisely in the smectic phase of the polymer. The thickness of these smectic layers was only about 3-5 nm. In this paper we present a study of the stabilizing influence of such an organization in smectic layers on the stability of the polymer film. We studied a smectic LC main chain polymer (containing the mesogens within the polymer backbone) with an azoxybenzene as mesogen. Scanning force microscopy (SFM) in tapping mode and X-ray specular and off-specular scattering were employed as a function of temperature and thickness of the films. Experimental Section LC Polymer. The liquid crystalline polymer used for the investigations is a main chain polyester (Figure 1). Its backbone contains an azoxybenzene group as stiff, rodlike mesogen and flexible alkyl chains as spacers. It was obtained by polycondensation from 4,4′-bis(6-hydroxyhexyloxy)azoxybenzene and di(7) Brochard-Wyart, F.; de Gennes, P. G.; Hervet, H.; Redon, C. Langmuir 1994, 10, 1566. (8) Reiter, G. Macromolecules 1994, 27, 3046. (9) Reiter, G. Langmuir 1993, 9, 1344. (10) Van der Wielen, M. W. J.; Cohen-Stuart, M. A.; Fleer, G. J. Langmuir 1998, 14, 7065. (11) Van der Wielen, M. W. J.; Cohen-Stuart, M. A.; Fleer, G. J.; de Boer, D. K. G.; Leenaers, A. J. G.; Nieuwhof, R. P.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 1997, 13, 4762. (12) Henn, G.; Stamm, M.; Poth, H.; Ru¨cker, M.; Rabe, J. P. Physica B 1996, 221. (13) Elben, H.; Strobl, G. Macromolecules 1993, 26, 1013. (14) Mensinger, H.; Stamm, M.; Boeffel, C. J. Chem. Phys. 1992, 96, 3183.
10.1021/la000824u CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000
Ultrathin Liquid Crystalline Polymer Films
Figure 1. Chemical structure of the polymer. ethylphenylmalonate.15 Its molecular weight (MW) is equal to 15.6 × 103 g/mol, as determined by GPC against a polystyrene standard. The formation of several mesophases could be established by DSC, polarization microscopy and X-ray diffraction.16 Above 123 °C, the polymer is isotropic. Between 123 and 23 °C, it organizes in a smectic SA phase, and below 23 °C, it is glassy. The thickness of the smectic layers is 3.5 ( 0.05 nm.16 The azoxybenzene mesogens are oriented perpendicular to the smectic layers and the flexible spacers between them are very often folded, forming hairpins.16 Thin Film Preparation. Thin films of the polymer were spincoated from a solution in a (50/50) (v/v) mixture of dichloroethane and chloroform. A drop of 2-3 mL of the solution was deposited on the substrate, which was then rotated at 2000 rps during 30 s. Silicon wafers and glass slides were used as substrates. They were cleaned by organic treatment before use.17 After preparation, the samples were conserved at temperatures below the Tg (23 °C) of the polymer. The thickness of each film was determined by X-ray reflectivity right after preparation. This information was used to scale the results of the SFM experiments. SFM. SFM measurements were carried out with a commercially available microscope (Nanoscope III from Digital Intruments, Santa Barbara, CA) in tapping mode. In this mode, the tip is oscillating near its resonance frequency and is only touching periodically the sample. This enables the minimization of lateral destructive forces at smooth polymeric surfaces. Silicon cantilevers (length 125 µm, width 30 µm, thickness 3-5 µm) with a spring constant between 17 and 64 N/m and a resonance frequency in the range of 240-400 kHz were used. For some investigation (ultrathin films) height and phase images were recorded simultaneously. The tapping mode phase shift was always zeroed before scanning. X-ray Specular Scattering. X-ray reflectivity measurements were performed at a laboratory X-ray source18 as well as at the A2 Polymer Beamline of the DORIS storage ring at HASYLAB/ DESY in Hamburg.19 The selected wavelength in both cases was λ ) 1.54 Å. X-ray reflectivity enables a nondestructive analysis of the thin films. The specular reflection of X-rays is accurately described by Fresnel’s equations in conjunction with the proper refractive index, which is given by n ) 1 - δ for hard X-rays, with δ ) r0/2πλ2F, where r0 (r0 ) 2.818 × 10-15 m) denotes the classical electron radius, λ the X-ray wavelength, and F the electron density of the material. As n is less than unity, total reflection occurs for grazing angles of incidence below the critical angle Rc ) x2δ, which is typically a few tenths of a degree. Specular reflectivity scans are sensitive to variations of the electron density perpendicular to the surface and therefore give information about the film thickness (interferences of the reflected beams at the airpolymer and polymer-substrate interfaces lead to thickness oscillations), the existence of a lamellar stacking within the film, oriented parallel to the substrate, and the roughness of the interfaces. X-ray Off-Specular Scattering. Diffuse X-ray scattering was observed at the A2 Polymer Beamline as well. A one-dimensional detector was placed behind the sample. For the measurements (15) Wilbert, G.; Traud, S.; Zentel, R. Macromol. Chem. Phys. 1997, 198, 3769. (16) Vix, A.; Stocker, W.; Stamm, M.; Wilbert, G.; Zentel, R.; Rabe, J. P. Macromolecules 1998, 31, 9154. (17) Organic cleaning: the wafers are washed up two times for 5 min in several baths at 60 °C with three different organic solvents: first with trichloroethylene, then, with acetone, and finally, with ethanol. After that, they are rinsed with deionized water and dried with an N2 stream. (18) Foster, M.; Stamm, M.; Reiter, G.; Hu¨ttenbach, S. Vacuum 1990, 41, 1441. (19) Elsner, G.; Riekel, C.; Zachmann, H. G. Adv. Polym. Sci. 1985, 67, 1.
Langmuir, Vol. 16, No. 26, 2000 10457 of detector scans, the sample is held fixed at one angle of incidence Ri and a complete diffuse scattering slice with exit angles Rf > 0 is recorded. Detector scans are mainly sensitive to changes in qz, that is, in the direction perpendicular to the surface, and only to very small changes in the qx direction, parallel to the surface. In a detector scan the specular peak at Ri ) Rf is visible as well as the Yoneda peak at Ri ) Rc. The Yoneda peak20 is a typical dynamical feature which arises from the enhancement of the diffusely scattered intensity due to a standing wave field. In addition to these two peaks, further peaks are visible in the case of an imperfect internal ordering of equally spaced layers, such as in smectic phases. In contrast to perfectly ordered layers which yield a sharp Bragg peak, the lateral imperfection leads to a broadened Bragg peak21 with tails which are cut in a detector scan. Thus, the presence of peaks at typical Bragg positions in a detector scan can be taken as a sign for smectic layers which are disturbed in their in-plane correlation.
Results and Discussion To investigate the film stability and the stabilizing influence of the smectic order within the film, films were investigated as a function of their thicknesses. More precisely, the behaviors of films whose thicknesses enable the formation of three, two, and one complete smectic layer were analyzed. Three Complete Smectic Layers. At first, measurements on a 12.5 nm thick film shall be described, for which the formation of three complete smectic layers is possible. The SFM investigations at the air-polymer interface were carried out at room temperature as a function of time, just after preparation of the film, after 1 day, and finally after one month. In addition, we performed X-ray measurements at 30, 105, and 130 °C. In the case of the SFM investigation, the film was stored at room temperature (often at 25 °C) and therefore above Tg between each measurement. Figure 2 illustrates the evolution of the surface topography during the above-mentioned time. Right after preparation (Figure 2a), the substrate coverage by the film is uniform and homogeneous, with a roughness of about 25 Å as determined from the height histogram, which can be described by a simple Gaussian distribution (Figure 3a). After 1 day, no significant change can be noticed in the topography image, but the statistical analysis on the heights present on the film shows a broadening of the previous Gaussian distribution (Figure 3b). Thus, a time dependent structuring of the film has started. After 1 month the height histogram reveals two Gaussian distributions corresponding to two dominating heights in the film (Figure 3c). This means that the film is composed of steps at the polymer-air interface, as can be seen on the topography image (Figure 2b). The thickness of the steps is given by the distance between both maxima of the Gaussians, and amounts to 3.5 ( 0.4 nm. Since this value agrees well with the thickness of the smectic layers formed by the bulk polymer in the smectic SA phase,16 these steps are attributed to islands of smectic layers. The inner structure of thin films with three complete smectic layers or more was analyzed by X-ray reflectivity. The reflectivity data for the two films presented in Figure 4 were measured at room temperature (25 °C) after the formation of the smectic islands on top of the films. The two films exhibit different thicknesses, but for both, the Kiessig fringes due to the film thickness are modulated by a Bragg peak at θ ) 1.2°. This is the signature of a smectic layering within the film, where the layers are oriented parallel to the substrate. For the thicker film of both (thickness: 40 nm) the modulation by the Bragg peak (20) Yoneda, Y. Phys. Rev. 1963, 131, 2010. (21) Baumbach, T. Ecole franc¸ aise de re´ flectivite´ ; Marseille, France, 1997.
10458
Langmuir, Vol. 16, No. 26, 2000
Vix et al.
Figure 2. Time dependent evolution of the surface topography of a 12.5 ( 0.1 nm thick film, as determined by SFM images measured in tapping mode. Similar scan and z-scale ranges are presented to show the evolving morphology: (a) topography image after preparation showing a homogeneous film; (b) topography image after 1 month storage at 25 °C showing smectic islands (bright domains) on top of the film, corresponding to an incomplete smectic layer.
Figure 3. Time dependent evolution of the height histograms of the 12.5 ( 0.1 nm thick film: (a) height histogram after preparation; (b) height histogram after 1 day of storage at room temperature (up to 25 °C); (c) height histogram after 1 month of storage at room temperature (up to 25 °C).
is enhanced in comparison to the thinner one, for which the modulation is weaker due to a smaller scattering volume. Finally, a time dependent structuring of the film consisting of a smectic layering accompanied by the formation of smectic islands on the top could be observed. These smectic islands correspond to an incomplete smectic layer. In fact, as the polymer film is spin-coated from the solution, it is practically not possible to adjust perfectly the solution concentration so that the resulting film thickness corresponds to an integer number of a smectic layer thickness. But no sign of dewetting with formation
of holes as deep as the film thickness could be detected during the observations. Therefore, it can be concluded that this smectic layering has a stabilizing effect on the film. In contrast, above the melting temperature the polymer simply behaves like a common amorphous polymer. Following Figure 5b, after 2 h at 130 °C no sign of dewetting was detectable. But, after sufficient annealing time, the film dewets. In-situ X-ray reflectivity measurements at temperatures well above room temperature were performed with synchrotron radiation (Beamline A2, Hasylab, DESY,
Ultrathin Liquid Crystalline Polymer Films
Langmuir, Vol. 16, No. 26, 2000 10459
Figure 4. X-ray reflectograms measured at 25 °C on two polymer films with various thicknesses: (a) thick film with a well distinguishable Bragg peak modulating (black arrow) the Kiessig fringes and being the evidence for the presence of a smectic layering parallel to the substrate; (b) thin film showing only a smooth modulation from the Kiessig fringes by the Bragg peak.
Germany) in order to study the reversibility of the smectic layering through annealing of the confined polymer in the isotropic phase. From the specular reflectivity scans (Figure 5a) it can be deduced that the smectic layering exists at room temperature and at 100 °C. At 130 °C the film is thicker and the smectic order tends to vanish. In fact, in this temperature range, the polymer is in the isotropic phase and the thin film is liquid. By cooling, the layer structure is formed again. The detector scans (Figure 5b) were measured for a fixed incident angle Ri ) 0.8°. In the scan measured at 30 °C, the presence of the Bragg peak at Φ ) Ri + Rf ) 2.6° can be observed, which indicates the existence of a smectic layering parallel to the substrate. Upon annealing, the smectic order vanishes around 123 °C, and thus at 130 °C, no sign of order is present. During cooling the order is reinstalled again at 123 °C, and of course it is still present at 105 °C. This implies that the smectic order is reversible. One Complete Smectic Layer. Before discussing the case of two smectic layers, let us consider ultrathin films in the range 3-5 nm, so that, at the most, only one complete smectic layer could be formed on the substrate. The thickness of the film described in the following is 4.2 nm, as determined by X-ray reflectivity. Like for thicker films, the morphology evolution was observed as a function of time. Figure 6 illustrates the time dependent variation of its surface topography. Right after preparation (Figure
Figure 5. Temperature dependent X-ray reflectograms measured with synchrotron X-ray radiation: (a) Specular reflectivity data measured at the 12.5 ( 0.1 nm thick LC film during in situ annealing. Data are presented for T ) 35, 100, 130, and 30 °C, corresponding to an annealing cycle below and above TM (melting temperature). The data are shifted for clarity against each other. (b) Off-specular data taken for the same temperature as above at Ri ) 0.8°.
6a), the substrate coverage by the film is rather uniform, exhibiting shallow valleys with smooth edges. The latter are responsible for the flanked form of the Gaussian distribution of heights (Figure 7a). This means that, at
10460
Langmuir, Vol. 16, No. 26, 2000
Vix et al.
Figure 6. Time dependent evolution of the surface topography of a 4.2 ( 0.1 nm thin film measured with SFM in tapping mode: (a) topography image after preparation; (b) topography image after annealing of the film at 50 °C for 15 min on a heat plate sheltered by a glass cover; (c) topography image after annealing the same film for 15 min more under the same conditions. The z-scale is equal for all three images and covers a range of 15 nm. Dark regions are deeper compared to brighter ones.
room temperature, structuring of this ultrathin film is already starting very quickly after preparation. Thus, immediately after preparation the film starts to dewet at room temperature. This can be attributed to its small thickness, causing the Tg of the confined polymer to be lowered.22 To follow the film structuring on a day scale, the film was annealed ex situ (i.e. outside the SFM apparatus on a heat plate) to 50 °C for 15 min. After annealing, the image (Figure 6b) of the film topography shows that the shallow valleys lead to holes with sharp edges. These give rise to a second well-separated peak on the height histogram (Figure 7b). From the distance between both peaks, amounting to 4.2 nm, it can be deduced that these holes are penetrating the complete film; that is, the film is dewetting from the substrate. If this process goes on as a function of time, which means that the holes get larger, the withdrawing polymer has to accumulate somewhere. For amorphous polymers, it is well-known that it accumulates in rims surrounding the holes. In the present case, however, the polymer is able to organize itself in layers, similar to a synthetic diblock copolymer.23 Former studies were done on comparable stratified systems by Ausse´re´ et al.24-26 These authors investigated thin films of diblock A/B copolymers with regard to their structure and stability through annealing. These polymer systems are also able to organize themselves in layers constituted of alternatively the A and the B part with a thickness in the range 40-50 nm at temperatures between 140 and 170 °C. They observed the formation of wells and towers by dewetting of the film and established a theoretical model for the dewetting of stratified liquids.27 In a system constituted of one state or layer, dry regions can be nucleated. They grow through dewetting, and regions with two layers, named towers, (22) Reiter, G. Europhys. Lett. 1993, 23 (8), 579. (23) Anastasiadis, S. H.; Russel, T. P.; Satija, S. K.; Majkrzak, J. Phys. Rev. Lett. 1989, 62, 1852. (24) Coulon, C.; Collin, B.; Ausse´rre´, D.; Chatenay, D.; Russel, T. P. J. Phys. Fr. 1990, 51, 2801. (25) Maaloum, M.; Ausse´rre´, D.; Chatenay, D.; Coulon, G.; Gallot, Y. Phys. Rev. Lett. 1992, 68, 1575. (26) Maaloum, M.; Ausse´rre´, D.; Chatenay, D.; Gallot, Y. Phys. Rev. Lett. 1993, 70, 2577. (27) Ausse´rre´, D.; Brochard-Wyart, F.; de Gennes, P. G. C. R. Acad. Sci., Ser. IIb 1995, 320, 131.
Scheme 1. Model for the Dewetting of Stratified Liquids through a Flow Perpendicular to the Surface with Formation of Deep Wells Accompanied by Building of Towers Corresponding to a Stack of Smectic Layers
are formed thanks to a polymer flow perpendicular to the layers (Scheme 1). First the towers are rounded, and then they become dissymmetric by growing faster in the direction of the nearest well. The final observation of our system was made after 30 min of annealing at 50 °C. On the topography image (Figure 6c), we notice three levels corresponding respectively to the presence of holes with larger diameter than before (B zone in Figure 6c), polymer domains with a thickness equal to the initial film thickness (C zone in Figure 6c), and domains with a higher thickness (A zone in Figure 6c). Of course, these three levels appear also on the height histogram, giving rise to three distinct peaks (Figure 7c). The left one corresponds to the bottom of the holes (B), the middle one to the remaining polymer film (C), and the right one to the higher domains (A). The height difference between the peak for C and A is 3.3 ( 0.3 nm. From this value, which is comparable to the thickness of a smectic layer, it is concluded that the higher domains are due to an accumulation of the withdrawn polymer in smectic towers. Within these towers, the polymer is organized in smectic layers oriented parallel to the substrate. Thus, these results are well comparable to
Ultrathin Liquid Crystalline Polymer Films
Figure 7. Time dependent evolution of the height histograms of the 4.2 ( 0.1 nm thick film: (a) height histogram after preparation; (b) height histogram after annealing of the film at 50 °C for 15 min, on a heat plate, sheltered by a glass cover; (c) height histogram after annealing the same film for 15 min more at 50 °C, also sheltered by a glass cover.
those of Ausse´re´ et al. Like for the dewetting scenario described above for the diblock copolymers, it seems that a polymer flow perpendicular to the substrate takes place, leading to the formation of smectic towers. Another interesting point concerns the remaining material in the holes. Therefore, phase contrast SFM images of the various regions in the dewetted film were recorded. The relatively hard silicon wafer and the softer polymer layer differ strongly in their Young modulus, so that both these regions should be distinguished by an SFM phase shift image. Recently a model to interpret phase shifts on the basis of stiffness and contact radii has been proposed.28 On the phase image (Figure 8b) recorded simultaneously to the topography image (Figure 8a), indeed a contrast between the polymer domains (remaining film and towers) and the holes is observed. On the (28) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, 385.
Langmuir, Vol. 16, No. 26, 2000 10461
Figure 8. Dry dewetting of the 4.2 ( 0.1 nm thin film measured with SFM in tapping mode: (a) Topography image of the surface of the dewetted film after 30 min of annealing at 50 °C. Dark domains are deeper and correspond to holes in the film. The brighter domains correspond to smectic towers, that is, stacking of smectic layers formed by the withdrawing polymer. (b) Corresponding phase shift image of the surface showing only a contrast between the holes (bright domains) and the polymer but no contrast between the remaining film and the polymer towers.
other hand, no contrast between the remaining film and the towers can be noticed, as expected, since they have very similar Young moduli. This is an evidence for a dry dewetting without any remaining polymer layer covering the substrate within the holes. Two Complete Smectic Layers. Finally, thin films with an intermediate thickness were investigated, that is films which are able to form, at most, two complete smectic layers. After 2 h at 30 °C, the film stays stable, exhibiting a smectic order, but after 2 h at 100 °C, the film has dewetted. At higher temperatures, dewetting occurs even
10462
Langmuir, Vol. 16, No. 26, 2000
Vix et al.
by X-ray reflectivity before dewetting was about 8 nm. After annealing, the height histogram (Figure 9b) evidences four dominating heights. In Figure 9b from the right to the left, the first peak corresponds to the top of smectic islands (thickness 3.3 nm), the second one to top of the remaining polymer film (thickness 8.2 nm), the third one to the top of the smectic towers (two smectic steps), and the last one to the bottom of the holes going throughout the film on its whole thickness. A line scan is shown in Figure 9c displaying the individual heights as a function of the sample position. Note that the x- and y-scales in Figure 9c differ significantly. Therefore, as for the thinner film, the competition between the dewetting process and the structuring of the film is dominated by dewetting: first holes are formed, and then, the withdrawing polymer accumulates, forming smectic islands and towers. Conclusion The investigations on the dewetting and the morphology of ultrathin films with various defined thicknesses from one up to three smectic layers revealed the following facts: First, it is shown that a lateral organization of the film in at least three smectic layers has a stabilizing effect on the film. Through annealing, the polymer film organizes itself in smectic layers oriented parallel to the substrate. On top of the film, smectic islands form, corresponding to incomplete smectic layers, due to an excess amount of polymeric material. The film does not dewet. The molecular organization within the film vanishes upon annealing the film in the isotropic phase of the polymer but is reversible upon cooling. In contrast, thinner films, which enable the formation of only one or two smectic layers, are not stable, despite their lateral organization, and dewet. Holes or wells are formed as well as smectic towers, due to the accumulation of the withdrawing polymer. Within the holes, no remaining polymer material was observed, which suggests a dry dewetting. A similar behavior, the instability of very thin films and the stability at larger film thicknesses, was reported for nematic films.29 Theoretically, this crossover in the stability was attributed to the influence of attractive fluctuation-induced structural forces.30 Thus, in systems with long-range correlations such as liquid crystals, the long-range structural force dominates the stability.
Figure 9. Investigation of a 8.0 ( 0.1 nm thick film after annealing for 1 h at 125 °C: (a) topography of the surface as measured by SFM showing holes, smectic islands, and smectic towers formed due to dewetting of the film; (b) height histogram evidencing the four dominating layers described before; (c) line scan also evidencing the four dominating levels existing on the substrate.
earlier. Figure 9a exhibits the film topography after annealing for 1 h at 125 °C. The film thickness determined
Acknowledgment. The authors thank Dr. G. Wilbert and Prof. Dr. R. Zentel for providing the LC main chain polymers and A. Meyer and R. Do¨hrmann for helpful assistance during the X-ray experiments at the A2 beamline at Hasylab/DESY. A.B.E.V. thanks Pr. F. Brochard-Wyart and Dr. D. Ausse´re´ for fruitful discussions. This work was supported by the European HCM Network under Contract No. CHRX-CT 94 0448. LA000824U (29) Vandenbrouck, F.; Valignat, M. P.; Cazabat, A. M. Phys. Rev. Lett. 1999, 82, 2693. (30) Ziherl, P.; Podgornik, R.; Zumer, S. Phys. Rev. Lett. 2000, 84, 1228.
