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Dewetting Instabilities in Thin Block Copolymer Films: Nucleation and Growth R. Limary and P. F. Green* Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712 Received December 8, 1998. In Final Form: March 26, 1999 We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order-disorder transition temperature, TODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t2/3. The shape of the rim was highly asymmetrical and its width L increased as t1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer “brush” of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.
Introduction The stability of thin liquid polymeric films on substrates is important in numerous applications, including coatings, adhesives, lubricants, and dielectric layers and is a topic of appreciable scientific interest.1-12 Perturbations of the free surface of a liquid film on a substrate due to external disturbances, for example, from thermal fluctuations or mechanical vibrations, can induce pressure gradients into the film. These pressure gradients result from a competition between the Laplace pressure, the ratio of the surface tension to the local radius of curvature, and the intermolecular disjoining pressure. Flow induced by the Laplace pressure tends to flatten the film. Thin films can become unstable and deform spontaneously whenever the intermolecular disjoining pressure increases with an increase in film thickness. Formally, a thin nonwetting film becomes unstable and ruptures when the second derivative of the excess free energy/area with respect to the local film thickness is negative (d2∆G/dh2 < 0). Generally two types of morphologies are documented in thin films destabilized by intermolecular forces. The most well-documented case involves the appearance of discrete holes that nucleate randomly throughout the surface of the film. Hole growth is influenced by interfacial and hydrodynamic forces. The other morphology is a bicontinuous spinodal pattern reflecting fluctuations in the local thickness of the destabilized, nonwetting, film. (1) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682. (2) Brochard-Wyart, F.; Debregeas, G.; Fondecave, R.; Martin, P. Macromolecules 1997, 30, 1211. (3) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (4) Reiter, G. Langmuir 1993, 9, 1344. (5) Segalman, R. A.; Green, P. F. Macromolecules 1999, 32, 801. (6) Shull, K. R.; Karis, T. E. Langmuir 1994, 10, 334. (7) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11, 4855. (8) Qu, S.; Clarke, C. J.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Phelan, K. C.; Krausch, G. Macromolecules 1997, 30, 3640. (9) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. Rev. Lett. 1996, 76, 1110. (10) Brochard-Wyart, F.; Martin, P.; Redon, C. Macromolecules 1997, 30, 1211. (11) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251. (12) Sharma, A.; Khanna, R. Phys. Rev. Lett. 1998, 81, 3463.
This occurs in sufficiently thin films where the destabilization is due largely to the long-range dispersive forces.12 The existence of such patterns was recently reported for thin liquid films that dewet solid5,11 and liquid substrates.5 It is also possible to observe patterns composed of discrete holes and spinodal patterns, as shown by simulations12 and experiment.13 The morphology will depend on the nature of the intermolecular potential, as discussed by Sharma.12 Research on this topic has, for the most part, been exclusively devoted to the dewetting of simple polymeric fluids from solid and from liquid substrates.1-12 Consequently, little is known about the dewetting of structured fluids such as block copolymers. Bulk symmetric A-B diblock copolymers form ordered alternating lamellae of A-rich and B-rich regions below a characteristic orderdisorder transition temperature TODT, that is, an isotropic to lamellar transition.14 Generally, in the case of thin films, the molecules self-assemble on the substrate with the lamellae parallel to the substrate. Even at temperatures above the bulk TODT, where bulk copolymer is disordered, the substrate can induce the system into the ordered state. Hence for T > TODT the copolymer can become ordered in the vicinity of the substrate while the bulk remains disordered.15,16 For sufficiently thin films, the film thickness is smaller than the correlation length and ordering will be induced throughout the entire film. We examined the dewetting of thin film symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers of degree of polymerization N ) 201 and dispersion index Mw/Mn < 1.14 above the TODT. Thin films (19 nm < thickness < 35 nm) of this copolymer are shown to undergo dewetting via nucleation and growth. We show that when the hole initially forms, the rim is absent and the hole grows at a rate consistent with an exponential dependence on time. This, to our knowledge, (13) Masson, J.-L.; Green, P. F. In preparation. (14) See for example: Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (15) See for example: Orso, K. A.; Green, P. F. Macromolecules 1999, 32, 1087. (16) Milner, S. T.; Morse, D. C. Phys. Rev. E. 1996, 54, 3793. Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989, 22, 5677.
10.1021/la981693o CCC: $18.00 © 1999 American Chemical Society Published on Web 06/16/1999
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Figure 1. AFM image of the topology of an isolated hole after 30 min of annealing at 170 °C in vacuum. An accompanying line scan of the hole is also shown. This was typical of all holes during the early stage of development. The total thickness of this film was H ) 32 nm.
appears to be the first experimental evidence of the nature of the early stage of dewetting. A rim subsequently forms at the edge of the hole, and the growth rate of the radius of the hole undergoes a transition from an exponential to a power law dependence on time when the rim is “mature”. At the end of the process, droplets of the destabilized melt remain on a dense copolymer brush of thickness 7 nm “anchored” to the substrate. This is a clear indication of autophobic behavior whereby a liquid does not spread on itself. This behavior was first documented in small molecule liquids, and its origins in these systems are believed to be due to steric, orientation, or dipolar effects.17 We show later that, for our system, the origins of autophobicity are primarily entropic. Experimental Section The dewetting of the thin films of pure PS-b-PMMA diblock copolymers was studied using an Autoprobe CP atomic force microscope (AFM) from Park Scientific. Images of the surface morphologies of the films were made with the AFM operated in the contact mode. To maintain the integrity of the film, the set point for the force exerted by the cantilever tip for each scan was minimized to values just large enough to maintain contact with the surface. Intermittent contact experiments yielded the same results. Thin films with thicknesses ranging from 19 to 35 nm were prepared by spin coating a toluene solution of the PS-bPMMA diblock copolymer (N ) NPS + NPMMA ) 201; f ≈ 0.5, Mw/Mn ) 1.14) from Polysciences, Inc., onto silicon substrates. The silicon substrates had native SiOx layers of approximately 2 nm on the surface, as determined by spectroscopic ellipsometry. The films were subsequently annealed at 170 °C in a vacuum oven for various times. The glass transition temperature of PS is 100 °C, and that of PMMA is 115 °C. The samples were removed periodically, quenched to room temperature, and probed with the AFM. The same area of the samples was probed after each annealing, and the growth of selected holes was tracked throughout the study.
Results Different stages of hole growth were identified during the dewetting of the diblock copolymer. Figure 1 shows (17) Hare, E.; Zisman, W. A. J. Phys. Chem. 1955, 59, 335; 1955, 59, 1097.
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Figure 2. Topography of the same hole shown in Figure 1 after 430 min of annealing. A rim at the edge of the hole is evident from the line scan and from the image. This hole is identified as hole no. 2 later in Figure 4.
Figure 3. Schematic diagram illustrating relevant hole dimensions.
an AFM image accompanied by a line profile of a typical hole during the early stage of dewetting. When the hole first forms, the characteristic outer rim at the periphery, invariably observed in films which dewet via a nucleation and growth mechanism, is absent, as evident from the accompanying line scan. Figure 2 shows a hole at a “mature” stage of development. The characteristic rim at the periphery, formed from the accumulation of material excavated from the hole, is now prominent. Note that the initial thickness of the film was H ) 32 nm. A film of thickness h ) 25 nm dewets a layer of thickness 7 nm. The hole depth is 25 nm. Figure 3 shows a schematic of characteristics of the hole, the radius R, the height of the rim q, and the width of the rim L. During the initial stage of hole growth, the radius of the hole grew exponentially with time, as shown by the filled squares in Figure 4A. In this regime, the growth of the radius of the hole is best described by
R ) 0.079 exp(0.0038t)
(1)
During the time interval 250 min < t < 550 min there is an intermediate stage involving the development of the rim where the increase in R is linear with time. The line drawn through the data in this regime has a slope of 1. An examination of the rims in this regime (Figure 5) indicates that they are in an evolutionary stage and not fully developed. For the film thickness shown, h ) 25 nm, the critical radius denoting rim formation was Rc ≈ 2.2 µm. We were unable to get more data on this particular hole because it impinged with others in its vicinity. We
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Figure 4. Time dependence of the growth of two holes. The rim width L and height q for both holes are shown here as a function of time.
Figure 5. Development of the rim for hole no. 1.
subsequently examined another hole (hole no. 2) at a later stage in rim formation. The radius of this hole, represented by the circles, increased as t2/3, particularly for the case where R > 4 µm. In addition to the radii of the holes, we also examined the width of the rim L and the height of the rim q. The data shown in Figure 4b indicate that the width of the rim L increased as t1/2 for both holes. It is also clear that the rim heights q are considerably smaller than the rim widths L. This strongly indicates that the rim is not symmetrical. The fact that L and q have approximately the same scaling at later times, where R > 4 µm, indicates that the rim shape does not change appreciably before the holes impinge. A sharp depression of lateral dimensions 550 min, the growth law appears to change; however, we were unable to examine the growth
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Figure 8. Schematic representation of a layer of ordered diblock copolymers in contact with a disordered layer.
of the hole further because the rims impinged. We then proceeded to examine another hole in order to get more information. Hole no. 2 (Figure 4) was subsequently examined, and the data are consistent with R ∼ t2/3, indicating the influence of slippage on the dynamics of the dewetting melt. The observation of the 2/3 power law dependence occurred when L was comparable to Rc. The ratio of q to L (Figure 5) indicates that the rim is highly asymmetrical. Segalman and Green5 reported similar behavior in the dewetting of a high-viscosity film of polystyrene from a lower viscosity polystyrene-co-acrylonitrile (SAN) liquid substrate. This asymmetry in the shape of the rim is expected when slippage is significant.22 Let us now examine the final stages of dewetting. Reiter4,5 observed two distinct late stages of dewetting of PS from the substrate. When the rims impinge, a network of polygonal-like patterns is formed. These patterns break up to form droplets. The series of AFM images shown in Figure 6 illustrates qualitatively the stages of the PS-bPMMA diblock copolymer dewetting process in a film of thickness h ) 18 nm. Figure 6a depicts a region of the film of early hole growth. The same region is shown in Figure 6b after the film was annealed for 8 h. The rims are evident from the micrograph. After 12 h, the rims impinged, as seen in Figure 6c. After 47.5 h, the honeycomb structure had broken up due to a Rayleigh instability and formed droplets. The morphology shown in Figure 6d remained relatively unchanged even after 100 h at 170 °C. It should be mentioned that the equilibrium contact angle that the droplets make with the underlying layer is approximately 1° with a typical droplet base diameter of 7 µm and a height ∼60 nm, as determined from AFM line scans. The small equilibrium contact angle is expected for polymers that dewet from an absorbed polymer layer of the same composition. It is also important to note that the droplets are randomly dispersed throughout the surface of the film. The configuration observed here is similar in appearance to the liquid-liquid dewetting case examined by Segalman and Green.5 To gain some insights into the behavior of this diblock copolymer when it dewets, a region at the edge of the film was scored prior to annealing and subsequently examined using the AFM. As seen in Figure 7, the copolymer dewets, leaving a uniform layer of approximately 7 nm on the
silicon substrate. It is noteworthy to mention that the surface of this layer is rather smooth, having a root-meansquared roughness less than 0.3 nm, as determined from topographic line profiles. Below we examine the significance of this 7 nm layer. When a symmetric diblock copolymer orders on a surface, the lamellae align parallel to the substrate. The arrangement of layers results from the preferential attraction of one component to the substrate while the lower surface energy component migrates to the air surface. For thin films of a symmetric diblock copolymer of PS-b-PMMA on silicon, the PMMA component is more polar and is strongly attracted to the SiOx/Si substrate. When χN . 10.5, the system is in the strong segregation regime and the interlamellar spacing D ∼ χ1/6N2/3. The Flory interaction parameter χ for this copolymer is 0.0375,23 indicating that χN ) 7.5. This is less than the value χN ) 10.5, above which the copolymer is expected to undergo a transition from an isotropic to a lamellar phase, characterized by alternating layers of PS- and PMMA-rich domains.15,16 Consequently, a bulk sample of our copolymer of N ) 201 used in this study should be disordered. Because of the substrate interaction, it is possible for the substrate to induce ordering into a copolymer at temperatures T > TODT(bulk), the bulk order-disorder transition temperature. We can determine the proportionality constant for an ordered block copolymer film made from a symmetric PS-b-PMMA copolymer of N ) 642 to be 6.7 at 170 °C.15 If this copolymer is ordered because of a substrate-induced effect, then a value of D/2 ) 6.8 nm would be expected. The thickness of the layer of polymer remaining on the substrate is 7 nm. In essence, the PS-b-PMMA orders in the vicinity of the silicon substrate and forms half of an interlamellar layer while the excess material remains disordered, χN , 1 (see Figure 8). When the thickness of the layer residing on top of the lower 7 nm layer, x0, is greater than 28 nm, dewetting does not occur and the film remains stable and flat beyond days of annealing. The above discussion indicates that the layer which remains on the substrate is of thickness D/2, where D is the interlamellar spacing of our copolymer. Clearly, this is a densely packed “brush” in which the chains are highly stretched, as is expected in an ordered copolymer. On the basis of entropic considerations, the adjacent layer cannot
(22) Brochard-Wyart, F.; de Gennes, P. G.; Hervert, H.; Redon, R. Langmuir 1994, 10, 1566.
(23) Russell, T. P.; Hjelm, R. P.; Seeger, P. A. Macromolecules 1990, 23, 890.
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interpenetrate, leading to this partial wetting condition.24 The conditions are therefore ideal for slippage of the destabilized melt19,25 on the “brush”. Conclusion The basic physical picture regarding the dewetting of this thin liquid diblock copolymer film above its bulk order-disorder transition temperature is as follows. The substrate induces ordering into the film, thereby creating a dense “brush”. An interface forms between the ordered phase (“brush”) and the disordered phase (Figure 8). The disordered phase (autophobicity) subsequently dewets the ordered phase by the nucleation of a hole, which penetrates to the substrate. At the initial stage, the outer rim is absent and the radius of the hole grows exponentially with time. This behavior is reminiscent of the rupturing of a freely (24) Shull, K. R. Faraday Discuss. 1994, 98, 203. (25) Durliat, E.; Hervet, H.; Leger, L. Europhys. Lett. 1997, 38, 383.
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supported film. There appears to be a relatively short intermediate stage during which the rim develops and R increases linearly with t. At later times when the rim is mature, the growth undergoes a transition to a power law dependence on time, t2/3. This power law dependence is consistent with the hole dynamics in which polymer slippage occurs. This behavior is to be expected, since the underlying ordered layer forms a smooth dense copolymer “brush”. Moreover, the asymmetry of the shape of the rim is consistent with slippage of the melt. The final stages of dewetting correspond well to that found for homopolymer films dewetting solid or liquid substrates, with the formation of honeycomb structures which eventually break up to form droplets. Acknowledgment. This work was supported by The National Science Foundation (Grant DMR-9705101) and a DuPont Aid to Education Grant. LA981693O
