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Deformation of a Glassy Polymer Film by Long-Range Intermolecular Forces M. O. David,* G. Reiter, T. Sitthaı¨, and J. Schultz Institut de Chimie des Surfaces et InterfacessCNRS, 15 rue Jean Starcky, B.P 2488, 68057 Mulhouse Cedex, France Received April 27, 1998. In Final Form: July 24, 1998 Long-range van der Waals forces may govern the physical behavior of thin films with thicknesses lying in the nanometer range scale. Here, we show that these forces can also be highly efficient across multilayer systems. The observed destabilization of a thin fluid layer, confined between a rigid substrate and a thin solid polymeric film, leading to the deformation of the solid covering film, revealed the strength of these interactions. Furthermore, simply by modifying the thickness of the layers or by changing the bounding medium, the effect of these interactions could be controlled and even reversed.
Introduction The stability of thin layers is an essential criterion in the coating industry. Sometimes, multilayered structures have to be prepared to obtain the desired properties. The behavior of such complex structures becomes difficult to predict because it depends on the stability of each layer and the interaction between these layers. The experimental and theoretical behavior of a thin fluid film, in contact with at least one elastic or fluid layer has been well studied in the literature.1-11 Depending on film thickness instabilities can be induced by intrinsic factors (capillary intermolecular forces) and/or external factors (defects).1-11 If the film thickness gets much thinner than 1 µm or approaches molecular dimensions, intermolecular interactions of short and long range across the film become relevant and govern the thermodynamic behavior.8-16 Short-range interactions, corresponding to forces acting mainly on contact between molecules, determine to a large extent interfacial tensions, wettability, or adhesion. Short-range and long-range forces may vary independently and even may have different signs as they originate from different types of interactions.16 However, it has been (1) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. Rev. Lett. 1996, 76 (7), 1110-13. (2) Pan, Q.; Winey, K. I.; Hu, H. H.; Composto, R. J. Langmuir 1997, 13, 175-80. (3) Shull, K. R.; Karis, T. E. Langmuir 1994, 10, 334-339. (4) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11, 48554861. (5) Brochard-Wyart, F.; de Gennes, P. G. J. Phys.: Condens. Matter 1994, 6, A9-A12. (6) Brochard-Wyart, F. J. Phys. II 1994, 4, 1727-1735. (7) Brochard-Wyart, F.; Debregeas, G.; Fondecave, R.; Martin, P. Macromolecules 1997, 30, 1211-1213. (8) Martin, P.; Brochard-Wyart, F. Phys. Rev. Lett. 1998, 80 (15), 3296-3299. (9) Reiter, G. Langmuir 1993, 9, 1344-1351. (10) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682-3690. (11) Brochard-Wyart, F.; Redon, C.; Sykes, C. C. R Aad. Sci. Ser. II 1992, 314, 19-24. (12) Ivanov, I. B. Thin Liquid Films: Fundamentals and Applications; Marcel Dekker: New York, 1988. (13) Israelachvili, J. N. Intermolecular and Surfaces Forces; Academic Press: London, 1992. (14) Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927-941. Van Oss, C. J. Colloids Surf., A 1993, 78, 1-49. Van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994. (15) Sharma, A. Langmuir 1993, 9, 861-869 and 3580-3586. (16) Brochard-Wyart, F.; di Meglio, J. M.; Que´re´, D.; de Gennes, P. G. Langmuir 1991, 7, 335-338.
Figure 1. Schematic representation of the system considered in this study. The excess free energy of a thin layer (2) inside a multilayer is given in the text.
shown that long-range interactions, even if these forces are weak at short distances, can overcome the short-range interactions and govern the film behavior.17 Such longrange interactions may be due to apolar Lifshitz-van der Waals (LW) forces and can have an effect on multilayer structures1-4 up to relatively large distances (about 100 nm and more). For a thin film sandwiched in a multilayer structure, all the interactions between all the surrounding materials across the thin layer12 will contribute to the total excess free energy ∆G2 of this layer.12,18,19 Considering the configuration depicted in Figure 1 which corresponds to the system studied here: layer 2 (thickness h) sandwiched between substrate 1 and layer 3 (thickness δ) with bounding medium 4, the total thickness of the two layers (PDMS and PS) lies in the nanometer range. A sum of two terms describes then the nonretarded interactions between the layer 2 and the different materials:12
∆G2 )
(
)
A1234 -1 A123 + 12π h2 (h + δ)2
(1)
(17) Reiter, G.; Sharma, A.; Casoli, A.; David, M. O.; Khanna, R.; Auroy, P. Submitted to Nature. (18) Hamaker, H. C. Physica 1937, 4, 1058-1072. (19) Visser, J. Adv. Colloid Interface Sci. 1972, 3, 331-363.
S0743-7463(98)00478-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/04/1998
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The first term accounts for the interactions between the substrate 1 and the layer 3 across the layer 2. The second term accounts for the interactions between the substrate 1 and the bounding medium 4 across the layers 2 and 3. The effective Hamaker constants A123 and A1234 can be expressed as a function of the respective Hamaker constant of each medium12,18,19
A123 ) (xA11 - xA22)(xA33 - xA22)
(2)
A1234 ) (xA11 - xA22)(xA44 - xA33)
(3)
In this study, the thicknesses exceed hundred nanometers, which lie in the limit of validity of the expression for nonretarded LW interactions. However, the transition between the nonretarded (eq 1) and retarded expression is not abrupt. As we are comparing the behavior of different systems only qualitatively, the absolute value of the forces is less important. The thermodynamic stability of layer 2 is directly related to the sign of the second derivative of ∆G2 with respect to h.15,20-22 Then, in the case depicted here, ∂2∆G2/∂h2 will depend on the different values of the two effective Hamaker constants and also on the thickness h and δ of the layers 2 and 3. When ∂2∆G2/∂h2 is negative, film 2 is unstable. Taking into account mass conservation, thinning the film locally reduces the free energy more than it “costs” to increase the film thickness some place else. Small undulations of the surface thus should grow spontaneously. This phenomena is called spinodal decomposition15,21,22 because it is a spontaneous decomposition of a thin layer induced by thermodynamic factors and leading to droplets lying on the substrate and surrounded by the environmental medium. It should be noted that eq 1, which describes the situation for a stratified system, is used as a criterion to discriminate between thermodynamically stable or unstable systems. However, whether an instability can appear depends on the deformability of the thin cover layer 3 to allow for flow in the fluid layer 2. The kinetics of the instability is governed by several factors: driving force due to intermolecular interactions, Laplace pressure due to curvature of the film surface and accounting for interfacial tensions, and also dissipation due to viscoelastic forces inside the layers (related to the rheology of the different materials).1-11,15 The experimental dewetting behavior of multilayered structures with total thicknesses lying in the nanometer range has already been studied in some cases, but it concerned only thin layers bounded by at least one fluid medium.1-4 In that case, dissipation could be reduced to viscous dissipation and acted solely on the growth kinetics of instabilities. However, in these studies the origin of the driving force of the instability was never considered as such. It is one purpose of this study to determine to what extent the LW interactions can govern the behavior of such multilayer systems. The theory of LW interactions across stratified media was well-known for quite some time, but no systematic experimental study of their effects on multilayers has yet been described in the literature. We are now considering the system depicted in Figure 1, composed of a thin liquid film (2) of poly(dimethylsiloxane) (PDMS) confined between a nondeformable substrate (silicon wafer) (1) and a rigid thin layer (polystyrene (20) Vrij, A. Discuss. Faraday Soc. 1966, 42, 23-33. (21) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391-464. (22) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 10841088.
Figure 2. Characteristic final pattern, as observed by optical microscopy, for a 50 nm PS film on top of a 25 nm PDMS layer. The bounding medium was air: the evolution of instability began at ambient temperature but was completed rapidly by heating the sample at 60 °C for 1 min. Identical patterns were obtained at ambient temperature with water as bounding medium.
(PS)) (3) (Figure 1). The appearance of instabilities is mainly controlled by a balance between the LW forces across the fluid layer and the mechanical resistance of the rigid layer. As we will show in the following, even in this confined system, instabilities could be observed and led to deformation of the rigid layer. Furthermore, the effect of the long-range interactions could be controlled and even switched simply by changing the bounding medium and/or the thicknesses of the different layers constituting the sandwich. Experimental Section The polystyrene (PS) used (Mn ) 186 000 g mol-1, I ) Mw/Mn < 2) was kindly supplied by Atochem Chemie Lacqrene. The glass transition temperature was found equal to 99 °C (as determined by differential scanning calorimetry at a rate of 10 °C/min). The poly(dimethylsiloxane) (PDMS) (η ) 106 cSt; Mw ) 139 000 g mol-1; I < 1.1) was purchased from ABCR (France), and the silicium wafers were obtained from Mat. Technology. The silicium wafers with a natural oxide layer of about 2 nm were exposed for 4 h to UV-ozone (for cleaning purposes and to generate hydroxyl groups at the surface). PS (trichlorosilaneterminated Mw ) 18 000 g mol-1, I ) 1.03) was end-grafted onto the wafers to avoid adsorption phenomena of PDMS chains which may drastically slow flow in the PDMS layer. It should be noted that the same experiments conducted without the PS brush gave qualitatively the same results. The presence of the brush solely effected the kinetics of the phenomenon, which was not the major goal of this study. In the following, we thus will not further discuss the influence of this brush layer. The PDMS layer was deposited by spin-coating toluene solutions to obtain thicknesses of 25 and 100 nm. The PS layer was deposited onto the PDMS surface by putting down a freely suspended PS film. This suspended film resulted from a PS film previously prepared on a glass slide, floated onto a water surface, and picked up with a wire frame. The PS layer thicknesses varied between 25 and 120 nm. The thicknesses of the different layers were measured by means of a spectroscopic ellipsometer (Sopra ES4G). All subsequent observations were made at ambient temperature or temperatures below the glass transition temperature of PS, ensuring that the PS layer remained glassy. Two different
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Figure 3. AFM measurements (tapping mode) for the same bilayer sample (25 nm PDMS and 50 nm PS) as in Figure 2: (a) topography; (b) phase; (c) 3D representation; (d) profile analysis of the scan line indicated in (a) and (c); (e) schematic illustration of the final pattern.
bounding media, air or water, were used. The behavior of the bilayers was followed with an optical microscope and recorded with a video camera. Atomic force microscopy (AFM) measurements were made with a Nanoscope IIIa (Digital Instrument), operated in the tapping mode.
Results With water as the environmental medium, the PDMS film was always unstable. If the sandwich was bounded
by air, stability depended on the thicknesses of the PS and PDMS films. A characteristic pattern was obtained as the instability developed. This is shown in Figure 2 for a 50 nm PS film on top of a 25 nm PDMS layer. In that case, whatever bounding medium was used, air or water, the final patterns were identical. All the optical microscopy photographs presented in this paper are reproduced in gray level.
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Figure 4. Characteristic evolutions observed at ambient temperature by optical microscopy for a bilayer composed of 25 nm PS on top of 100 nm PDMS. Water as bounding medium: (a) t ) 0; (b) t ) 15 min; (c) t ) 30 min. After exchanging water by air: (d) t ) 0; (e) t ) 3 min; (f) t ) 12 h.
However, due to the interference of reflected light from the (bounding medium/PS) and (PDMS/Si wafer) interfaces, all samples showed colors that could be directly related to the local thickness of the bilayer. During the evolution of the instability we could follow in situ the variations of the colors with optical microscopy. Two ways were used to establish the correspondence between the
colors and the thicknesses: ellipsometry measurements on flat layers and AFM measurements presented hereafter. In the case considered here (50 nm PS and 25 nm PDMS), the thickness appears then to vary between roughly two limits: from about 50 nm (regions of dark gray color (brown color in reality)) to about 120 nm (regions of pale gray color (pale blue color in reality)). This
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variation in thickness is rather important and leads to the conclusion that the amplitude of the deformation has to be considered in an interpretation of these results. Furthermore, the smallest thickness (50 nm) is roughly equal to the thickness of the PS cover layer. It thus seems that the PS film, in the thinnest regions, has touched the substrate. To confirm that these patterns were really due to the deformation of the total bilayer and not just of the underlying PDMS film, we performed AFM measurements in the tapping mode on this sample. As shown by the topography view (Figure 3a) and the three-dimensional representation (Figure 3c), the upper film was undoubtedly deformed but not fractured. In addition, a profile analysis (Figure 3d) confirmed the large amplitude of the deformation already revealed by optical observations. Furthermore, the topography (Figure 3a) is almost perfectly reproduced by the AFM phase mode measurements (Figure 3b). The mechanical properties of the thinnest regions and that of the bumps appear to be rather different. In fact, a plausible interpretation of this final pattern is the following: liquid PDMS “droplets” covered by a deformed glassy PS film adhering partially to the substrate (Figure 3e). As the AFM measurements were done several months after the optical observations without any visible differences, the pattern described here can be considered as an equilibrium pattern. An increase of the PDMS thickness to 100 nm and a decrease of the PS thickness to 25 nm led to a different behavior. A characteristic evolution is presented in Figure 4. Under water, the film was still unstable (Figure 4ac). The upper PS film was deformed by undulations of the underlying PDMS film. However, the deformation amplitude never became sufficiently large to allow the PS film to reach the substrate. Furthermore, when water was exchanged by air, the pattern formed under water disappeared (Figure 4d-f). Discussion The system studied was composed of noncharged components, and the thickness of the two polymeric layers was in the nanometer range. LW interactions between the different component thus became relevant. From the observed behavior of the first bilayer (50 nm rigid PS on top of 25 nm fluid PDMS), a first conclusion can be drawn: LW interactions across the bilayer can be sufficiently strong to destabilize the system and to deform the rigid PS film. The absence of fracture in the PS layer in spite its large deformation (Figures 2 and 3) is not yet well understood: one plausible reason could be a reduced Tg value as compared to the bulk. Experiments described in Figure 2 indicate that the same morphology has been obtained at different temperatures and 60 K below Tg. This means that a Tg reduction of almost 60 K should have occurred. As PS and PDMS are known to be incompatible, some diffusion of PDMS into PS is highly improbable. Nevertheless, this point is now under investigation. In the second case (only 25 nm rigid PS on top of 100 nm PDMS), the destabilizing effect of the LW interactions seems to be less important: under water, the instabilities never led to a sufficient deformation amplitude of the PS film to allow it to reach the substrate. Furthermore, exchanging water by air led to the disappearance of the pattern resulting in a smooth film again. This is surprising at first glance as, considering the PS film thickness which
is thinner in this system, the rigidity of the PS film should be weaker. Accordingly this effect cannot be attributed to the PS film and its glassiness. A modification of the LW interactions has to be considered. Varying the PDMS and PS thicknesses alters the strength of the interactions forces across the bilayer, as can be seen from eq 1: a decrease of the interaction energy is indeed expected when replacing the first system (50 nm PS on 25 nm PDMS) by the second one (25 nm PS on 100 nm PDMS) due to the decrease of both terms of eq 1 (This would be also the case with a retarded LW expression). The inverse behavior observed for the two systems when the bounding medium is varied (water exchanged by air) is more complex. In the first system, the instabilities occurred in both cases. In the second one, the deformation observed under water disappeared completely when water was exchanged by air. In fact, simply exchanging the bounding medium affects only the second effective Hamaker constant A1234 (eq 3). Here the Hamaker constants can be estimated from the dispersion component of the surface energy of the different materials.13,16 A positive value of the Hamaker constant refers to attractive interaction while a positive one represents repulsive. Thus, the interaction between PS (3) and SiO2 (1) across PDMS (2) is always attractive (A123 ) 1.53 × 10-21 J) while it is always repulsive between the environmental media (water or air) and SiO2 (1) across PS (3) and PDMS (2) (A123air ) -5.4 × 10-21 J; A123water ) -1.4 × 10-21 J). The net interaction (attractive or repulsive) depends thus on the thicknesses of the different layers (eq 1). From these values of the Hamaker constants it can be estimated for the two systems considered here and compared to their experimental behavior. In the first case (50 nm PS on 25 nm PDMS) the net interaction is always attractive, whatever the environmental medium: the system should be always unstable. In the second case, (25 nm PS on 100 nm PDMS) the estimated net interaction is attractive under water and repulsive in air. These predictions are in complete agreement with the experimental observations: the first system is always unstable whatever the environmental medium (air or water) while the second one is unstable under water and stable in air. These results clearly show that, for the second system, the stability can be switched simply by exchanging the bounding medium: unstable under water and stable in air. This means that one can switch the effective LW interactions across the bilayer from attractive to repulsive by a change of the bounding medium; this modification being efficient even across an intermediate layer. However, the rigidity of the PS layer, which appears here to exert a minor influence on the behavior of the bilayers, should not be neglected in both cases. A more complete study of the influence of this parameter is now under way. Conclusion We clearly showed that LW interactions can be efficient across multilayer systems over distances up to at least 125 nm. The effective range of LW interactions seems to be even larger. The bounding medium has an influence on the stability of the liquid film even if it is separated from this film by a glassy polymer layer. This undoubtedly shows that the substrate and the bounding medium can experience mutually their presence across the PDMS and PS layers mediated by LW interactions. Furthermore, the strength of the destabilizing forces due to these LW interactions can be sufficiently important to even overcome the rigidity of a solid cover layer and to lead to its
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deformation. For these reasons, LW interactions cannot be neglected when considering the behavior of multilayered structures. Finally, the attractive or repulsive nature of the LW interactions can be controlled simply by modifying the bounding media and/or the thicknesses of the different layers constituting the system. However, the relations between these modifications and the different behaviors observed are complex, and a more systematic study of the influence of these parameters has not yet been carried
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out. Nevertheless, this last point appears to be extremely important because it can open new ways of pattern formation and control of film stability. Acknowledgment. This work was supported by the Indo-French Centre for the Promotion of Advanced Research/Centre Franco-Indien pour la Promotion de la Recherche. LA9804785
