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Water Induced Dewetting of Ultrathin Polystyrene Films on Hydrophilic Surfaces E. Bonaccurso, H.-J. Butt,* V. Franz, K. Graf, M. Kappl, S. Loi, and B. Niesenhaus Physikalische Chemie, Universita¨ t Siegen, 57068 Siegen, Germany
S. Chemnitz and M. Bo¨hm Elektrotechnik, Universita¨ t Siegen, 57068 Siegen, Germany
B. Petrova University of Chemical Technology and Metallurgy, Kliment Ohridski Blv. 8, 1756 Sofia, Bulgaria
U. Jonas and H. W. Spiess Max-Planck-Institut fu¨ r Polymerforschung, 55099 Mainz, Germany Received May 7, 2002. In Final Form: July 29, 2002 The wetting of ultrathin films of polystyrene on the hydrophilic surfaces of mica and silicon oxide was studied by atomic force microscopy. After annealing, the surfaces were covered with a homogeneous, continuous polystyrene film of roughly 1 nm thickness. On top of this film, polystyrene droplets with microscopic contact angles of 7°-16° were observed. After exposure to an oversaturated water vapor, the continuous polystyrene film disintegrates and dewets the surfaces. Polystyrene structures on silicon oxide indicate a homogeneous dewetting process starting from few nucleation sites. On mica the density of nucleation sites for water is much higher and the polystyrene dewets the surface in an inhomogeneous process. The structural changes observed imply that ultrathin polystyrene films are highly mobile in the presence of water.
Introduction The wetting of thin polymer films on solid substrates plays an important role in many practical applications such as coating, lubrication, and adhesion.1 Scientifically, the investigation of wetting or dewetting structures provides information about the dominating interaction forces at the molecular level.2-5 In this study we focus on polystyrene films with an average thickness (total volume of deposited polystyrene divided by the surface area) of 0.2-2 nm. Polystyrene films with a thickness between 5 nm and several tens of nanometers have been investigated extensively on nonwettable silicon surfaces.6-13 Polymer films are formed by spin casting from solution. After * Corresponding author. E-mail:
[email protected]. Phone: +49-6131-379 111. Fax: +49-6131-379 330. (1) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827-863. (2) Daillant, J.; Benattar, J. J.; Leger, L. Phys. Rev. A 1990, 41, 19631977. (3) Heslot, F.; Cazabat, A. M.; Levinson, P. Phys. Rev. Lett. 1989, 62, 1286-1289. (4) Brochard-Wyart, F. Langmuir 1991, 7, 335-338. (5) Redon, C.; Brzoska, J. B.; Brochard-Wyart, F. Macromolecules 1994, 27, 468-471. (6) Reiter, G. Phys. Rev. Lett. 1992, 68, 75-78. (7) Stange, T. G.; Mathews, R.; Evans, D. F.; Hendrickson, W. A. Langmuir 1992, 8, 920-926. (8) Orts, W. J.; van Zanten, J. H.; Satija, S. K. Phys. Rev. Lett. 1993, 71, 867-870. (9) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251-1254. (10) Jacobs, K.; Herminghaus, S.; Mecke, K. R. Langmuir 1998, 14, 965-969. (11) Scha¨fer, E.; Thurn-Albrecht, T.; Russel, T. P.; Steiner, U. Nature 2000, 403, 874-877.
annealing above the glass transition temperature, the films spontaneously dewet the silicon surface, leaving only an ultrathin continuous film on the silicon oxide. The dewetting process is driven by a competition of van der Waals interactions within the polymer and between the polymer and the substrate surface.14-16 Both substrates used in this study, mica and silicon oxide, are hydrophilic; water spreads readily, and the contact angle is 0°. Hence, one could expect that water has an influence on the wetting of polystyrene because the less polar polystyrene would have to replace the strongly adsorbing water. The aim of this study was to analyze how the presence of water changes the structure of ultrathin polystyrene films on hydrophilic surfaces. Therefore, the films were exposed to an oversaturated water vapor in an enclosed chamber. The resulting film structures were imaged with an atomic force microscope (AFM). Materials and Methods If not explicitly mentioned otherwise, experiments were done with polystyrene synthesized by anionic polymerization. The molecular weight, characterized by gel permeation chromatog(12) Karapanagiotis, I.; Evans, D. F.; Gerberich, W. W. Langmuir 2001, 17, 3266-3272. (13) Wang, C.; Krausch, G.; Geoghegan, M. Langmuir 2001, 17, 62696274. (14) Reiter, G.; Sharma, A.; Casolie, A.; David, M. O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551-2558. (15) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. Rev. Lett. 2001, 86, 5534-5537. (16) Mu¨ller, M.; MacDowell, L. G.; Mu¨ller-Buschbaum, P.; Wunnike, O.; Stamm, M. J. Chem. Phys. 2001, 115, 9960-9969.
10.1021/la020429z CCC: $22.00 © 2002 American Chemical Society Published on Web 09/21/2002
Dewetting of Ultrathin Polystyrene Films
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Figure 1. Chamber to moisturize polymer films on solid surfaces. raphy and calibrated with polystyrene standards, was Mw ) 10.660 g/mol, and the polydispersity was Mw/Mn ) 1.043 (Mn is the number average of the molar mass). The glass transition temperature measured by differential scanning calorimetry was Tg ) 94.5 °C. The density was 1040-1090 kg/m3. In addition, we used commercial polystyrene (Fluka) of low (Mw ) 1.000 g/mol) and high molecular weight (Mw ) 500.000 g/mol). As substrates we used mica (Plano GmbH, Wetzlar, Germany) and silicon wafers (Wacker Siltronic AG, Burghausen, Germany). Silicon wafers were used as received with the native oxide film, or they were oxidized. Oxidation of silicon wafers was done in an oven containing an atmosphere of 97% oxygen (quality 4.5) and 3% chlorine. The temperature at the beginning was 800 °C. It was increased at the rate 10 °C/min to 1000 °C. After 9 min at 1000 °C the gas was exchanged to nitrogen (quality 5.0). After 15 min, the sample was cooled at the rate 5 °C/min down to 800 °C. Then it was taken out of the oven. The thickness of the oxide films was 30 nm, as measured with a spectrophotometer (Leitz MPV SV, Wetzlar, Germany). Before the polymer was spin coated, each wafer was cleaned by sonicating it for 15 min in aqueous detergent solution (5% Tickopur R33, Stamm GmbH, Germany), 5 min in aqua dest, 5 min in ethanol, and 5 min in aqua dest three times. Then the surfaces were plasma cleaned for 8 min. Polystyrene was dissolved in toluene (spectroscopy grade, Merck, Darmstadt, Germany) at a concentration of 0.01 mg/mL if not mentioned otherwise. Samples were prepared by spin casting a drop of the solution on freshly cleaved mica or on silicon oxide. Immediately after putting the drop on the samples, they were rotated for 10 s at 810 rpm on a home-built spin coater. Afterward, the films were annealed at 130-150 °C for 30 min in air. Samples were imaged at room temperature with a commercial AFM (Multimode, Nanoscope III, Veeco Instruments, Santa Barbara, CA) in tapping mode. We used rectangular silicon cantilevers (125 µm long, 30 µm wide, 4 µm thick, Nanosensors, Wetzlar, Germany) with an integrated tip of nominal tip radius 10 nm, a nominal spring constant of 42 N/m, and a resonance frequency of 280-330 kHz. Some images were filtered by flattening. Usually, sample preparation and imaging were performed in a glovebox under controlled relative humidity (RH). The humidity was adjusted with an open beaker filled with drying agent (leading to 10-20% RH), saturated aqueous salt solutions of MgCl2 (33% RH) or NH4NO3 (62% RH), or pure water (8189% RH). The temperature inside the glovebox was 25-28 °C. Temperature and humidity were constantly measured. To study the effect of moisture, ultrathin annealed films of polystyrene were cooled on a Peltier element in a chamber of constant humidity (Figure 1). Within the chamber the temperature was 22 °C. The humidity inside the chamber was adjusted with a saturated solution of KCl. At 22 °C the RH was 85%, leading to a dew point of 19 °C. The temperature of the sample was adjusted to either 10 or 15 °C. At 15 and 10 °C, the oversaturation (vapor pressure divided by the vapor pressure of water at the respective temperature) directly at the sample was 1.31 and 1.82, respectively. After the samples were exposed to moisture, they were taken out of the chamber and imaged at 30-35% RH. The condensation of water onto a cooled substrate in 87% RH was also imaged by video microscopy. Therefore, we used a Zeiss Axiotech equipped with a Zeiss Epiplan (×100) objective and a CCD camera (WAT-202 D, Watec Co. Ltd., Japan). Images were directly recorded in a personal computer.
Figure 2. (A) Polystyrene on mica after annealing at 130 °C for 30 min at 65% humidity formed from a 0.03 mg/mL concentrated toluene solution. The height scale is 3 nm (black to white). (B) A square hole scraped into a film of polystyrene on mica formed from a 0.01 mg/mL concentrated solution in dichloromethane after annealing at 140 °C for 2 h and imaged at roughly 50% humidity. The height scale is 4 nm. All errors reported are the random errors of a single measurement, as determined from many independent experiments. They indicate the variation of a certain value from experiment to experiment.
Results and Discussion Polystyrene on Mica. Directly after spin casting polystyrene onto mica at humidities below 50%, we observed drops (5 ( 2 per µm2) with a typical height of H ) (4.4 ( 1.3) nm and a diameter of w ) (136 ( 24) nm. The shape of drops was that of a spherical cap. Microscopic contact angles were 7.4° ( 1.8°. At a humidity of 62% or above, often patches with an irregular rim of roughly 4 nm thickness were observed instead of drops. After annealing at 150 °C, polystyrene forms a continuous film of defined thickness. On top of the continuous polystyrene film, small droplets of roughly 2 nm height and 30-50 nm diameter were observed (Figure 2a). In reality, the diameter of the droplets is probably smaller because the image is a “convolution” between the real structure and the tip shape. For a detailed discussion of the tip effect, see ref 17. With increasing polystyrene concentration (0.1 instead of 0.01 mg/mL), the size of the droplets increased to typically 200 nm diameter and a height of 10 nm. The droplets had a spherical shape. Microscopic contact angles varied from experiment to experiment from 8° to 14°. To verify the existence of the continuous film, we scratched holes into it (Figure 2b). Therefore, we switched to contact mode scanning. Directly after switching to contact mode and before being able to adjust the force, the AFM tip penetrates the continuous film and rectangular holes are formed. From the depth of these holes, the film thickness was estimated to be (1.4 (17) Butt, H.-J.; Gerharz, B. Langmuir 1995, 11, 4735-4741.
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Figure 3. Polystyrene film on naturally oxidized silicon after annealing at 125 °C for 30 min imaged at room temperature at roughly 50% humidity. The film was formed from a 0.1 mg/ mL concentrated toluene solution. The height scale is 6 nm.
( 0.3) nm. After the scanning parameters were adjusted to forces below ≈40 nN, the film remained intact and only the droplets were laterally displaced. Qualitatively, the same structure was observed using dichloromethane as solvent instead of toluene. Polystyrene on Silicon Oxide. Directly after spin casting polystyrene onto silicon oxide at a humidity below 50%, we observed drops (3 ( 2 per µm2) with a typical height of H ) (5.0 ( 1.6) nm and a diameter of w ) (147 ( 53) nm. The shape of drops was that of a spherical cap. Microscopic contact angles were 7.9° ( 1.0°, which agrees with results of Seemann et al.,18 who report a contact angle for polystyrene nanodroplets of 6.9°. No significant differences were observed at high humidities (3 ( 2 drops per µm2, H ) (4.7 ( 1.2) nm, w ) (185 ( 38) nm, Θ ) 5.9° ( 0.8°). Usually, a homogeneous film of defined thickness of typically (1.0 ( 0.4) nm was extending from the drop. Its rim was not circular but usually had an irregular shape. For a given sample, the drops and films were quite homogeneous in size and height. Between different samples we observed a higher variation. On some samples only films of defined height were observed. On others the rim of a homogeneous film around a drop was barely visible. When the samples were annealed at 125 °C for 30 min, the homogeneous film extended from the drops but did not yet form a continuous layer while the droplets shrunk in size (Figure 3). After annealing at 150 °C, the films extended further and formed a continuous layer of ≈1 nm thickness. These observations agree with those of Seemann et al., who found a homogeneous layer of 1.3 nm thickness with polymer drops on top after annealing.15 Influence of Humidity. Does a high humidity change the structure of polystyrene films on hydrophilic surfaces? At high humidity (63-90%) annealed polystyrene films showed roughly the same structure as that at moderate humidity (
