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Langmuir 1997, 13, 6360-6362
Dihedral Angle at Solid/Liquid-Polymer Interfaces Determined by Atomic Force Microscopy Tobias Kerle,† Sidney R. Cohen,‡ and Jacob Klein*,† Department of Materials and Interfaces, and Chemical Services Unit, Weizmann Institute of Science, 76100 Rehovot, Israel Received July 14, 1997. In Final Form: September 30, 1997X We describe a novel approach based on atomic force microscopy to determine the dihedral contact angle of a liquid polymer on top of a solid substrate. By cross-linking the polymer in an ion beam, the polymer surface becomes amenable to AFM imaging. This sample preparation enables us to take topography scans of the polymer in a solid state. The cross section of the contact area of a polymer drop and the substrate can then be utilized to determine the contact angle between polymer and substrate. Extremely low contact angles in the range 2-8° are reproducibly measured. Control measurements carried out with optical phase modulated interference microscopy gave contact angles well comparable to the AFM results. We use our method in a study of polymer dewetting on top of a network of itself.
Interfacial phenomena at the solid-liquid interface are quite complex and have been studied extensively during the last few decades. One convenient observable for characterizing the solid-liquid interactions is the dihedral contact angle between a droplet of the liquid and the surface. In a variety of applied disciplines, such as interfacial engineering,1 the measurement of contact angles has proved to be a useful probe for the stability of the system. The contact angle θ is related to the surface tension of the spreading or wetting liquid by Young’s equation, σsv - σsl ) σlv cos θ, where σsv, σsl, and σsv are the surface tensions of the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively. Classical methods of measuring contact angles, e.g. the sessile drop technique, are limited to the measurement of a single macroscopic drop of liquid on the solid substrate. In the system we studied, the droplets were formed spontaneously by dewetting. The characteristic height of the droplets was smaller than 5 µm. Very recently optical microscopy was used to estimate the contact angle in a similar system.2 A micrograph of the rim of a hole obtained by illuminating the sample with a monochromatic light source showed a fringe pattern which results from interference. The fringes were used to calculate the contact angle. However, the uncertainties in determining θ from the fringe spacing can be quite large. We applied atomic force microscopy (AFM) for measurement of the contact angle. AFM has been used previously in similar problems for imaging the dewetting process at the liquid-liquid interface3,4 and for topography scans of copolymer adsorbates on surfaces.5 In the aforementioned cases, the polymers were all scanned at a temperature well below their glass transition temperature. Sheiko and co-workers have measured dewetting of polymeric films above their glass temperature using intermittent contact (tapping mode) AFM. The polymers used were either highly branched interconnected den†
Department of Materials and Interface. Chemical Services Unit. X Abstract published in Advance ACS Abstracts, November 1, 1997. ‡
(1) Fourche, G. Polym. Eng. Sci. 1995, 35, 957. (2) Reiter, G.; Auroy, P.; Auvray, L. Macromolecules 1996, 29, 2150. (3) Lambooy, P.;. Phelan, K. C.; Haugg, O.; Krausch, G.; Phys. Rev. Lett. 1996, 76, 1110. (4) Overney, R. M.; Leta, D. P.; Fetters, L. J.; Liu, Y.; Rafailovich, M. H.; Sokolov, J. J. Vac. Sci. Technol. 1996, B14, 1276. (5) Gesang, T.; Ho¨per, R.; Dieckhoff, S.; Schlett, V.; Possart, W.; Hennemann, O. D. Thin Solid Films 1995, 264, 194.
S0743-7463(97)00782-8 CCC: $14.00
Figure 1. Micrograph of a polymer film on top of a crosslinked network of itself. The sample was annealed under vacuum (p ) 5 × 10-3 Pa) for 1 day at T ) 120 °C. The initially smooth and homogeneous top layer broke up into droplets, forming a polygonal structure on top of the polymer network.
drimers6 or films formed from rigid perfluoralkyl methacrylate.7 These resilient structures exhibit a frequencydependent compliance, enabling successful imaging at the high imaging frequencies (>300 kHz) used. Here we present a general technique which allows imaging of any liquid polymer surface after brief, nonperturbative crosslinking. The method is outlined and demonstrated for the AFM scan of the interfacial area of a thin liquid polymer film dewetting on top of a cross-linked matrix of itself.10 The samples were prepared by depositing a thin film of liquid polymer on top of a cross-linked film of polymers with identical chemistry. The polymers studied were statistical random copolymers of diethylene (-C4H8-) and ethylethylene (-C2H3(C2H5)-) monomers, with mean microstructure ((C4H8)0.14(C2H3(C2H5))0.86)N. The degree (6) Sheiko, S.; Eckert, G.; Ignateva, G.; Muzafarov, A. M.; Spickermann, J.; Rader H. J.; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283. (7) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015. (8) Scheffold, F.; Eiser, E.; Budkowski, A.; Steiner, U.; Klein, J.; Fetters, L. J. J. Chem. Phys. 1996, 104, 8786. (9) Kerle, T.; Yerushalmi-Rozen, R.; Klein, J. Europhys. Lett. 1997, 38, 207. (10) While we focus here on the method itself, a more complete study of the issue of dewetting and the variation of θ with extent of substrate cross-linking can be found in a forthcoming publication: Kerle, T.; Yerushalmi-Rozen, R.; Klein, J. Macromolecules, in press.
© 1997 American Chemical Society
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Figure 2. (a) Typical AFM topography scan of a dewetted polymer film in the noncontact mode. The picture was taken at the rim between dewetted polymer and substrate (cross-linked polymer film) following the ‘fixing’ of the droplets by weak irradiation as described in the text. (b) Cross section through this scan showing the exposed substrate surface on the right. At a distance of 30 µm the rim starts to rise. (inset) Cross section through a topography scan, obtained by optical phase modulated interference microsocopy (OPIM), of the same dewetted polymer/substrate combination. No irradiation was applied to the droplets in this case.
of polymerization was N ) 1520, the molecular weight was MW ) 8.5 × 104, and the polydispersity was well below 1.08.8 The first polymer layer was prepared by spin casting the polymer from a toluene solution onto polished silicon wafers. After thorough drying, these substrate layers were cross-linked by exposing them to a uniform flux of highenergy (1.2 MeV) R particles (4He) from a van de Graaff acceleratorsthe ion dose was in all experiments in the range 1013-1015 ions/cm2. A second layer of polymer was similarly spin cast on a piece of freshly cleaved mica, floated off in water, and then mounted upon the first crosslinked film to cover it and form a bilayer. Figure 1 shows a micrograph of a sample prepared in this way after annealing it under vacuum (p ) 5 × 10-3 Pa) for 1 day at T ) 120 °C. The initially smooth and homogenous top layer broke up into droplets, forming a polygonal structure on top of the polymer network.9 As the dewetted polymer film is liquid even at room temperature, it can easily wet the AFM tip, thus preventing stable feedback and imaging, making any analysis of the contact angle impossible. By exposing the dewetted sample again to high-energy R irradiation, we were able to induce cross-linking. Cross-
linking for very short times (dose < 1014 ions/cm2) makes the polymer an elastic solid. For the AFM measurement we used a Topometrix TMX2010 Discoverer system. The atomic force microscope was operated in noncontact or in intermittent contact mode using microfabricated cantilevers with integrated Si tips (NanoSensors) of radius 15 nm (manufacturer’s specification).11 Topography scans of droplets, holes, and other features were taken at different stages of the dewetting process. It is possible to obtain large scale scans, as shown in Figure 2, both for non-cross-linked and for cross-linked polymer droplets. However, in order to determine precisely the angle at the droplet-substrate dihedral line, a high-resolution scan (8 µm xy, 3 µm z full scale) is necessary. At this resolution stable feedback and imaging could be obtained only on the cross-linked polymer (see (11) In intermediate contact, or even “noncontact” modes, the tip will touch the surface to a degree depending on oscillation amplitude, set point, and the relation between the speed of feedback response and the rate of topography change during the scan. Extremely flat topographies and/or slow scan rates are required to totally avoid tipsurface contact.
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Figure 3. Cross sections through a high-resolution AFM scan (8 µm xy, 3 µm z full scale). The three cross sections taken at different places of the sample show very clearly the good reproducibilty of the contact angle measurements. Table 1. Comparison of the Contact Angle of a Dewetted Polymer Sample Determined by Different AFM and Optical Phase Modulated Interference Microsocopy (OPIM) Scansa drop A (AFM) drop B (AFM) drop C by OPIM drop D by OPIM
5.5° b 6° b 5.5° b 5.3° b
6.2° c 6.2° c
a In the case of the AFM scans the first value was determined by a forward scan and the second by one in the reverse direction on the same sample position. (The dewetted polymer/substrate combination used differs from that shown in Figures 1 and 2, in particular in the extent of substrate cross-linking.10) b AFM scan in forward direction. c AFM scan in backward direction.
Figure 3), while the non-cross-linked sample wetted the tip on initial approach, as noted above. This causes irreversible damping of the cantilever oscillation which cannot be corrected, due to the limited z range. Such difficulties were never experienced when imaging the cross-linked film. We recall that these measurements were taken in the noncontact mode: we were unable to obtain quality images in contact mode operation for either non-cross-linked or cross-linked film. Figure 2b is a crosssection through Figure 2a. One can observe the exposed surface of the substrate (the initially cross-linked polymer layer) on the right side of the line. At a distance of ca. 30 µm the rim starts to rise. By constructing the tangent to the rim at the point of intersection and measuring the angle between this line and the baseline (a linear fit through the data right of the rim), we readily retrieve the contact angle. The contact angle measured in the figure was 7.7° ((0.5°). Characteristic errors of 0.5° and high reproducibility are typical for this method. In Table 1 we give values for two droplets, indicating this reproducibility. Also given in Table 1 as a control are the contact angle values for two different droplets (from the same dewetted polymer/substrate) determined by optical phase interfer-
ence microscopy (OPIM) from profiles such as those shown in the inset to Figure 2b. As the close agreement between the OPIM and AFM results shows, the surface effects (if any) of the irradiation on the intrinsic contact angles are very small and well within any scatter of the data. Contact angles down to 2° or even lower in some cases could be accurately measured. The contact angle could be varied by varying the extent of cross-linking of the substrate polymer prior to spin casting the top layer on it.10 Another feature which makes AFM especially suitable for measuring the topology in this system is the lateral dimensions, which are much bigger than the AFM tip size. Therefore the problem of a convolution of the tip shape with the measured feature, which was observed by Gesang and co-workers when scanning adsorbates (
