Langmuir 2001, 17, 6269-6274
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Dewetting at a Polymer-Polymer Interface: Film Thickness Dependence Chun Wang, Georg Krausch, and Mark Geoghegan* Lehrstuhl fu¨ r Physikalische Chemie II and Bayreuther Zentrum fu¨ r Kolloide und Grenzfla¨ chen (BZKG), Universita¨ t Bayreuth, D-95440 Bayreuth, Germany Received April 23, 2001. In Final Form: June 22, 2001
We have used optical microscopy and scanning force microscopy to study the dewetting of polystyrene from poly(methyl methacrylate) on silicon substrates as a function of film thickness. We have performed measurements for bilayers with a lower layer more viscous than the upper layer, as well as for the opposite situation. For a solidlike (highly viscous) lower layer, the dewetting speed is constant and independent of the thickness of the polystyrene film. However, for a liquidlike lower layer, the radius of the dewetted holes grows as t2/3, where t is the annealing time, and depends on the thickness of both layers. The absolute values of the dewetting speed are in reasonable agreement with theoretical predictions.
Introduction The dewetting of polymer films has been a much-studied problem in recent years1,2 due to the technological importance of dewetting and also due to the usefulness of polymer films as model systems. The former arises from the need, for example, to create stable coatings (paints, biocompatible implants, packaging materials, and insulating layers among others), and the latter, from the fact that the high molecular weights involved lead to time scales that are much greater than those in simple small molecule fluids. Furthermore, one can study both liquidliquid dewetting and liquid-solid dewetting with the same materials2 simply by varying the molecular weights to alter the respective viscosities. An extremely good model system for such studies is that of polystyrene (PS) and poly(methyl methacrylate) (PMMA). These polymers are immiscible, and when a film of one is placed on top of a film of the other, they are either metastable3,4 or unstable.5 Such bilayers may dewet by either spinodal dewetting (the spontaneous amplification of capillary waves) or nucleation and growth. The former is caused by dispersive forces and depends on the magnitude and sign of the Hamaker constants for the components of the system. Spinodal dewetting is realized when the PMMA layer is placed on top of PS. We do not consider spinodal dewetting in this work and consider only situations whereby the dewetting is triggered by the nucleation and growth of holes. Holes may be either thermally nucleated or nucleated by the presence of defects such as dust particles or other imperfections in the film. A complete discussion of the factors governing such nucleation, as well as the conditions under which spinodal dewetting occurs, is given elsewhere.6 Holes will only grow * Corresponding author. Present address: Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, United Kingdom. E-mail:
[email protected]. (1) For a review, see the following: de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (2) For a review, see the following: Krausch, G. J. Phys.: Condens. Matter 1997, 9, 7741. (3) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. Rev. Lett. 1996, 76, 1110. (4) Qu, S.; Clarke, C. J.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Phelan K. C.; Krausch, G. Macromolecules 1997, 30, 3640. (5) Sferrazza, M.; Heppenstall-Butler, M.; Cubitt, R.; Bucknall, D.; Webster, J.; Jones, R. A. L. Phys. Rev. Lett. 1998, 81, 5173.
if the spreading coefficient, S, is negative. For a film A, on a substrate B, S is given by Young’s equation,7
S ) γB - (γAB + γA)
(1)
where γA, γB, and γAB are the energies of the A and B surfaces and the AB interface, respectively. Interfacial and surface energies also play a key role, however, in the spinodal dewetting of polymer films.8 A final factor that can also be relevant is gravity. This, however, is only important when considering films with thicknesses into the micrometer range. Thus far, the dewetting of (liquid) polymer films at a solid interface has been relatively well studied, beginning with films of polystyrene on silicon substrates.9 Later work has been extended to consider dewetting at the liquidliquid interface,3,4,10-12 with theoretical work concentrating on the growth of the holes and rims as well as the shape of the profile.13-16 The previous experimental work on the PS on PMMA system considered the effect of viscosity (by altering the molecular weight, Mw) on the growth laws4 and the shape of the profile.3 In this paper we continue this work by altering the thicknesses of the two layers to better understand and test the role that a solid or liquid substrate plays in the dewetting process. We shall then be able to compare our work with the relevant theory,14 as well as previous experimental work.11 Experimental Section Thin films of PMMA were created by spin coating from toluene solution onto silicon substrates (covered by the native oxide layer). (6) Seemann, R.; Herminghaus, S.; Jacobs, K. J. Phys.: Condens. Matter 2001, 13, 4925. (7) Young, T. Philos. Trans. R.. Soc. London 1805, 95, 65. (8) Reiter, G.; Khanna, R.; Sharma, A. Phys. Rev. Lett. 2000, 85, 1432. (9) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (10) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11, 4855. (11) Pan, Q.; Winey, K. I.; Hu, H. H.; Composto, R. J. Langmuir 1997, 13, 1758. (12) Segalman, R.; Green, P. F. Macromolecules 1999, 32, 801. (13) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (14) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682. (15) Brochard-Wyart, F.; Debregeas, G.; Fondecave, R.; Martin, P. Macromolecules 1997, 30, 1211. (16) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715.
10.1021/la010585q CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001
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Table 1. Molecular Weights, Polydispersities, and Viscosities17,18 of the Polymers Used in This Study polymer
Mw (kDa)
polydispersity (Mw/Mn)
viscosity (Pa s) (at 162 °C)
PS PS PMMA
32 520 31
1.02 1.03 1.03
960 1.3 × 107 2.4 × 106
PS films were spun cast from toluene solution onto glass and subsequently floated off onto deionized water. They were then picked up onto the PMMA film on the silicon substrate. This method of creating bilayers is preferable to spin coating a PS layer on PMMA from a selective solvent for PS (e.g. cyclohexane), because casting from toluene tends to produce films of better quality than those produced with most other solvents. The thicknesses of the individual layers were measured either by scanning force microscopy (SFM) or by ellipsometry. For SFM film thickness measurements, a scratch was applied to the polymer film and the height was determined relative to the underlying substrate. After preparation, the films were allowed to dry before being annealed under flowing nitrogen at 162 °C. Monodisperse batches of the polymers (Mw/Mn < 1.10) were purchased from Polymer Standards Service (Mainz, Germany). The molecular parameters for the polymers are listed in Table 1. The viscosities for the polystyrenes are calculated from earlier results.17 The viscosity of PMMA depends on the tacticity of the polymer. The material used in the present study is about 70% syndiotactic and 30% atactic, corresponding well to the data of Fuchs et al.18 After each annealing step, the samples were removed from the oven thus quenching the polymer films below their glass transitions of 100 °C (PS) and 110 °C (PMMA) and freezing in the dewetted structure. Optical microscopy was used to follow the growth of the holes. Nucleated holes were observed under the optical microscope, and when they formed, their diameter was measured as a function of annealing time, t. After each measurement the bilayers were annealed further. It is not expected that the continuous heating and cooling will affect the growth of the holes, because once formed their continuation is expected to be independent of sample treatment. Equally, we saw no evidence that this treatment gave rise to more holes. To study the morphology of the growing holes in more detail, we used scanning probe microscopy. The measurements were made using a Digital Instruments Nanoscope III controller with a Dimension 3100 microscope. All measurements were performed in tapping mode. To image the structure of the PS/PMMA interface in the vicinity of the growing holes, the PS layer was dissolved in cyclohexane, leaving a bare PMMA layer.3 In some circumstances, optical microscopy images show the dewetted holes to consist of a series of concentric rings (we shall see later a good example of this in Figure 5). We therefore used SFM to ensure that the hole diameters measured using optical microscopy corresponded with the inner diameter of the hole.
Results Solid PMMA Layer. In a first series of experiments, we consider the dewetting speed as a function of the upper layer thickness for a lower layer thickness of 200 ( 30 nm. The molecular weights were chosen such that the viscosity of the PMMA lower layer (Mw,PMMA ) 31 kDa) is about 2500 times greater than that of the PS upper layer (Mw,PS ) 32 kDa). We therefore should expect the dewetting to proceed linearly with time, provided that the width of the rim was smaller than the radius of the hole.14 In our experiments this was the case, and the growth velocity of the holes remained constant as a function of time. For comparison, we also fitted the data to 2R ) 2m(t - t0)2/3, (17) Fox, T. G.; Flory, P. J. J. Phys. Colloid Chem. 1951, 55, 221. (18) Fuchs, K.; Friedrich C.; Weese, J. Macromolecules 1996, 29, 5893. Fitting their data to Mw3.4 gives for the viscosity (in Pa s) ηPMMA ) 2.1 × 10-(11+9.4((T-190)/(T-15))Mw3.4, with T the temperature in °C.
Figure 1. Hole diameter plotted as a function of time for five holes in a film of PS (32 kDa) (160 nm thick) on PMMA (230 nm) after annealing at 162 °C. The lines are linear fits to the data. The mean dewetting speed is 4.3 ( 0.1 nm s-1 (the speed is half the gradient of the fits).
Figure 2. Dewetting speed as a function of PS thickness for the Mw ) 32 kDa PS at 162 °C. The PMMA thicknesses for the different bilayers lie between 170 and 230 nm. The error bars are obtained form a statistical analysis of the data for the growth of the individual holes.
where t0 and m are fitting parameters. For all PS thicknesses, the linear fit proved overall to be the better. In Figure 1, for example, the linear fit better described four of the five holes. In these experiments the viscosity of the PS was so low that the dewetting proceeded very rapidly, meaning that some samples could only be annealed for 30 min or less. (We decided to measure at 162 °C rather than a lower temperature to facilitate comparison with the earlier measurements of this system.4) To overcome the inaccuracies inherent in measuring the dewetting speeds over such short times, we measured several holes in each case. An example is shown in Figure 1. For different PS film thicknesses, the dewetting speed is constant, at v ) 3.9 ( 0.1 nm s-1 (see Figure 2). We were also able to look at the morphology of the holes using SFM. We show in Figure 3 an example of a section through a hole, before and after washing the bilayer with cyclohexane. Note that there is very little deformation of the PMMA layer in the vicinity of the growing holes, as would be expected with a high-viscosity substrate. Liquid PMMA Layer. In a second series of experiments we use a high molecular weight PS (Mw,PS ) 520 kDa) to ensure that the bulk viscosity of the PS is greater than that of the PMMA layer. At 162 °C the PS viscosity is about 5.3 times that of the PMMA (Mw,PMMA ) 31 kDa). In this case we should not expect the dewetting speed to remain constant in time, but rather the diameter of the holes is expected to grow proportionally to t2/3. Indeed this is the case. In Figure 4 we show typical optical microscopy
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(a)
(b)
(c)
Figure 3. (a) SFM scan of a 140 nm PS (Mw ) 32 kDa) layer dewetting from a 170-nm-thick PMMA film after 30 min annealing at 162 °C. (b) As in (a) but with the PS layer removed by washing the bilayer in cyclohexane. (c) Two different layers in cross section, as measured along the solid lines shown in the images in (a) and (b).
data from which we measured the growth of the holes (of diameter 2R). In Figure 5, we show the growth of the holes for different films as a function of time. The data are fitted to 2R ) 2m(t - t0)2/3. We note that this fit, rather than 2R ) 2(R0 + mt2/3), is appropriate, because the growth should depend on the size of the hole rather than how long it has been annealed. We note that when the PMMA layer is thinner than 50 nm, holes are formed in the PMMA lower layer within the growing holes in the PS upper layer. This can be seen clearly in the SFM images shown in Figure 6. The image indicates that the retreating PS rim pulls the PMMA back with it. In the following, we do not discuss the time
Figure 4. Optical micrographs of a 100 nm PS (Mw ) 520 kDa) film on a 495 nm PMMA lower layer after annealing for (a) 80, (b) 150, and (c) 220 min at 162 °C. The scale bar is 50 µm.
dependence of the growth of the holes in the upper PS layer after the PMMA film has begun to break up. Discussion The dynamics of hole growth in liquid/liquid dewetting has been analyzed in a seminal paper by BrochardWyart et al.14 If the viscosity of the lower layer is significantly higher than that of the upper layer (“solid
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(a)
(b)
Figure 5. Hole growth as a function of time at 162 °C for nine different bilayers with a Mw ) 520 kDa PS upper layer. In (a) the upper PS layer is approximately 60 nm thick, while in (b) the lower PMMA layer is approximately 490 nm thick. The time scale for the 100 nm PS on 495 nm PMMA bilayer has been shifted by subtracting 250 min from the annealing time.
substrate regime”), the dewetting speed is predicted to be
γPSΘe3
v)
(2)
12ηPSκx2
where γPS and ηPS are the surface tension and the viscosity of the polystyrene, respectively. Θe is the equilibrium contact angle, and κ is a numerical dissipation factor13 that is expected to be of order 10. We see that our data support this general behavior. If we use a surface tension γPS ) 30 mJ m-2 and contact angle Θe ) 0.2 (taken from the earlier work4), then, with our value v ) 3.9 nm s-1, we obtain κ ≈ 4, which is not unreasonable. For thin PS upper layers (dPS < 30 nm), we were unable to measure the dewetting speed because the extremely large number of holes meant that the dewetting occurred far too quickly to be able to obtain reliable results. However, in some experiments with dPS ≈ 10 nm no dewetting at all was observed. This result was not regularly reproducible and so suggests that preparation conditions are very important in these situations. However, this does point to the possibility that, in such bilayers, long-range (van der Waals) forces are stabilizing the films against dewetting. We now turn to the growth of the dewetting for the case of a liquid substrate. In the model of Brochard-Wyart and co-workers14 the radius of the holes is not expected to increase linearly with time but rather as
R)t
(
2/3
)
γ2dPMMA2Θe ηPMMA2dPS
1/3
(3)
where dPMMA and dPS are the thickness of the PMMA and
(c)
Figure 6. (a) SFM scan of a 60 nm PS (Mw ) 520 kDa) layer dewetting from a 50-nm-thick PMMA film after 175 min annealing at 162 °C. (b) As in (a) but with the PS layer removed by washing the bilayer in cyclohexane. (c) Two different layers in cross section, as measured along the solid lines shown in the images in (a) and (b).
the PS layers, respectively, and γ is an effective surface tension given by γ-1 ) γPS -1 + γPMMA-1. In Figure 7, we plot m as a function of dPMMA2/3/dPS1/3. The agreement with theory is surprisingly good. The fit to the data is bounded by two broken lines, which show the effect of a 1 °C temperature difference in the viscosity (according to eq 3 and the previously measured viscosities18). The temperature variation is clearly the largest source of error. (This is not the case for the experiments with a solid substrate, because the PS viscosity has a much smaller temperature dependence than that of PMMA.) We also have very good absolute agreement with theory. If we use γPS ) 30 mJ m-2 and γPMMA ) 35 mJ m-2, we find that
Dewetting at a Polymer-Polymer Interface
Figure 7. Plot of m against dPMMA2/3/dPS1/3. The solid line is a linear fit to the data, and the broken lines represent the effect of a 1 °C variation in temperature on the viscosity assuming eq 3. Error bars are obscured by the data, and the fit is to lower values of m than would be obtained from the raw results, because errors have been included in the analysis (error bars are obscured by the data points).
(γ2Θe/ηPMMA2)1/3 ) 2.1 nm2/3 s-1. The gradient of our plot is 0.8 nm2/3 s-1, which is a difference of less than a factor of 3. When considering the inaccuracies present in measurements of viscosity and its large temperature dependence, these results present a convincing confirmation of the theory of Brochard-Wyart and co-workers.14 Our results present a quantitative confirmation of the theory of Brochard-Wyart and co-workers. This is in contrast with the results obtained on a bilayer consisting of a polycarbonate (PC) film on a layer of a random copolymer of styrene and acrylonitrile (SAN), where discrepancies were observed.11 In these latter experiments the upper PC layer had a viscosity some 10 times greater than the lower SAN layer. One would therefore expect, a priori, similar results to our experiments with the upper layer of PS with Mw ) 520 kDa. In a comparison between their results and ours, we note some similarities (i) The dewetting proceeded more rapidly for thicker substrate layers (ii) One usually expects a lifting of the contact line during dewetting. In both the experiments on the PC/SAN system and the present work, the bottom of the dewetting hole is observed to be deeper than the height of the lower liquid layer (iii) In the PC/SAN system the dewetting speed (for 50 nm SAN films) was observed to decrease with time before recovering its original speed. This was attributed to the interaction of the upper layer with the silicon substrate. Although we did not test the time dependence in the present study, we did observe an interaction of the PS layer with the substrate, when the PMMA layer was 50 nm thick. The differences between our liquid lower layer results and the results on the PC/SAN system are also noteworthy (i) The holes in the PC/SAN system have a slight depression in the center. In the PS/PMMA system with the liquid substrate, this is not observed. (For the experiments with a solid substrate, we do observe a slight depression in SFM images (ii) The growth of the holes in some PC/SAN bilayers was faster than a linear dependence (i.e. the dewetting speed increased with time (iii) A cross-sectional transmission electron microscopy (TEM) image of PC dewetting SAN showed the SAN being pulled up the rim, giving rise to a concave shape. We have no evidence for such a phenomenon in our data. Pan et al.11 noted that the approach of dissolving the upper layer could not allow us to image such a structure, but our SFM
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Figure 8. SFM scan (solid line) of the rim of a growing hole of 270 nm PS (Mw ) 520 kDa) on 500 nm PMMA after annealing 990 min at 162 °C (measured from the line of contact between the air, PMMA, and PS). The dashed line is a third-order polynomial fit to the data, corresponding to a rim, which would satisfy the theory of Brochard-Wyart and co-workers (eq 4).14
Figure 9. Sections of differently sized holes from a 495-nmthick PMMA film, after the 100 nm upper PS (Mw ) 520 kDa) film was dissolved in cyclohexane after annealing for 220 min at 162 °C. Note that the receding PS layer penetrates further into the PMMA film for the smaller two holes than it does for the largest hole. This implies that, even after the hole has formed, there is a time-dependent behavior in the shape of the PS-PMMA interface (since we assume a one-to-one correspondence between the size of a hole and the effective annealing time).
images after washing show no evidence for a PMMA contribution to the rim being broken away during solvent washing. To explain the difference between their results and the theory of Brochard-Wyart and co-workers,14 Pan et al.11 pointed out that an essential simplification made in the theory is to assume a homogeneous flow field in the rim. They suggested that if h is the height of the rim, then the speed of the retreating polymer in the rim is given by
v(r) )
∂ 1 ∂ ∂h r ∂r r ∂r ∂r
( ( ))
(4)
where r is the lateral position. Since our data are in good agreement with the theory, one would expect the speed of the dewetting PS layer to satisfy eq 4. This requires a third-order polynomial for the shape of the rim because we are assuming a homogeneous flow field. A sample rim and corresponding fit are shown in Figure 8. Clearly the agreement is not perfect, but neither is it dreadful. How poor the agreement must be before the theory of BrochardWyart and colleagues14 is no longer valid remains an open question. An interesting phenomenon that was not expected concerns the shape of the hole during its growth. In Figure 9 we present sections through holes taken from the same bilayer, after dissolution of PS in cyclohexane. In the case
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of the smaller two holes (the top two in Figure 9), there is a ∼90 nm indentation in the PMMA layer due to the receding PS layer. In the largest hole, this indentation is less than 50 nm deep. Even though the sample was annealed for 220 min, the holes have different diameters, which we take to imply that they nucleated at different times. It was expected that only the diameters of the rim and hole increase with time, and that the precise shape of the PS-PMMA contact area would remain largely unchanged. This implies that the flow of PS in the rim is not as uniform as predicted by Brochard-Wyart et al.14 On the contrary, however, in the PC/SAN system, SAN is pulled strongly into the rim as dewetting progresses, as illustrated by cross-sectional TEM. Another issue that may play a role here, and has not so far been discussed, is the interface between the two films. If there is more mixing at the PC/SAN interface than at our PS/PMMA interface, the greater interfacial width of PC and SAN may well explain the discrepancy in the shape of the respective holes.
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For bilayers with a viscous PS layer (Mw ) 520 000 Da), the dewetting is retarded as the holes grow. The radius of the holes has a growth law dependent on t2/3. In this case, the dewetting speed can also be predicted from theory.14 We also noted that thin PMMA layers (
