Article pubs.acs.org/Macromolecules
Control of Morphology in Pattern Directed Dewetting of a Thin Polymer Bilayer Sudeshna Roy, Debarati Biswas, Namrata Salunke, Ajit Das, Pavanaphani Vutukuri, Ravdeep Singh, and Rabibrata Mukherjee* Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, West Bengal, 721 302, India S Supporting Information *
ABSTRACT: We report the dewetting of a thin polymer bilayer on a low surface energy topographically patterned substrate with grating geometry. The bilayer, comprising of a polystyrene (PS) top and poly(methyl methacrylate) (PMMA) bottom layer was prepared by direct sequential spin coating on the patterned substrate, using mutually exclusive solvents. Depending on the coating conditions, three distinct initial morphologies of the as coated bilayer is possible: type 1, a discontinuous bottom layer under a discontinuous top layer, resulting in polymer threads confined within the substrate grooves; type 2, discontinuous threads of bottom layer polymer (PMMA) confined within the substrate grooves under a continuous top layer; type 3, continuous bottom and top layers. Our experiments reveal that the initial morphology of the film, particularly, that of the bottom layer significantly influences the final dewetted patterns. For example, in a type 1 or type 2 bilayer the morphology depends significantly on the relative widths of the PMMA threads (LT−PMMA) and that of the substrate grooves (LP). In case LT−PMMA < LP, the bottom PMMA layer disintegrates into isolated droplets aligned along substrate grooves, irrespective of the thickness or morphology of the top PS layer. On the other hand, the overall morphology of the dewetted film is rather strongly influenced by the thickness of the PS layer and the configuration of the bilayer. In case the PMMA threads span the entire width of the substrate grooves (LT−PMMA = LP), the droplet formation is suppressed in favor of an intact PMMA thread, with periodic undulations, submerged under either an undulating thread or an intact layer of PS. In case of a type 3 bilayer, the continuous PMMA bottom layer in most cases ruptures over the substrate stripes, where it is thinnest. This result in the top PS layer coming in direct contact with the substrate and subsequently rupture over the same locations, resulting in core shell threads localized over the substrate grooves. In case of a type 3 bilayer with an ultrathin top film, the two layers rupture simultaneously at different locations and subsequent dewetting results in an exotic structure comprising alternating array of PS droplets and undulating PMMA threads. For a thicker bottom layer, the PMMA film is seen to remain intact, over which the PS film dewets, forming undulating threads. We also construct a morphology phase diagram that depicts the influence of the individual layers on the final dewetted morphology.
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INTRODUCTION Ultrathin polymer films tend to become unstable, spontaneously rupture, and dewet on a solid substrate following variety of mechanisms such as spinodal dewetting engendered by attractive interfacial van der Waal’s interaction,1−6 sudden release of residual stresses accumulated due to entanglement of the long chain molecules during film preparation,7 density variation, etc.8 Slightly thicker films tend to rupture by nucleation.4,5,9 A ruptured film subsequently evolves and selforganizes into variety of meso scale structures such as array of droplets, holes, bicontinuous patterns etc. due to dewetting.1−9 Any form of film instability is undesirable in applications such as coatings, adhesives, lubricants, paints, membranes, etc. as film integrity is essential for these applications.10 However, the morphological self-organization during dewetting of an unstable polymer thin film has the potential to become a nonlitho© 2013 American Chemical Society
graphic technique for engineering meso and nanoscale patterns, where the feature size and the periodicity of the structures can be controlled by varying the initial film properties such as the film thickness (h0) and surface/interfacial tensions.11−36 Typically instability in a homopolymer film is manifested only when it is heated above the glass transition temperature (TG) of the polymer or is exposed to its solvent vapor. This allows additional morphology control of the structures, as the evolution sequence can be arrested at any intermediate stage by simply quenching the film to a lower temperature,16 or by removing it from the solvent chamber. Thus, instability mediated patterning offers greater flexibility in terms of feature Received: September 5, 2012 Revised: December 11, 2012 Published: February 4, 2013 935
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layer, reducing the dewetting velocity significantly.36 In the second experimental configuration, both layers are thin and therefore become unstable, either simultaneously or sequentially, resulting in complex structures.44−59 In this case, the final dewetted morphology depends largely on the initial instability mode. de Silva et al. have experimentally shown a distinct stability transition between the two layers depending on the ratio of individual film thickness in an unstable PS (bottom)/ PMMA (top) bilayer coated on a silicon (substrate).52 They argued that a thin PMMA film on a thick PS film is destabilized due to dispersion forces, in contrast to a thin PS film below a thick PMMA film which undergoes spinodal dewetting due to long-range forces.52 Xu et al. have recently showed that in a PMMA/PS bilayer on an oxide coated silicon substrate, the evolution starts with a strong deformation at the film/air interface over a nondeforming film/film interface when the bottom PMMA layer is very thin. In contrast, a rapid deformation at the film/film interface is observed when the PMMA layer is thick and has lower viscosity.58 Also irregular and faceted structures form due to anisotropic growth of the holes, due to strong viscous dissipation in a high viscosity, low thickness PS top layer, which is associated with significant interfacial slippage between the two layers.59 Interestingly, dewetting of a thin bilayer on a heterogeneous substrate has received far less attention,68−72 in comparison to both pattern directed dewetting of a single layer film,11−31 and dewetting of a thin polymer bilayer on a flat substrate,33−65 probably due to complex experimental protocol involving large number of parameters. We must highlight that a variety of ordered structures with complex morphologies such as core shell columns, hierarchical and enclosed cage like structures etc. have been successfully created by applying an externally applied electric filed across a thin polymer bilayer.66−71 Regarding dewetting of a bilayer on a patterned substrate, based on long wave nonlinear simulations, Sharma et al. have predicted the morphology of a dewetted thin bilayer on topographically and chemically patterned substrates, and have identified conditions under which ordered structures can be obtained.73,74 Their prediction show the formation of parallel stripes of the constituent polymers of the bottom and top layers along the grooves and ridges of the substrate from the dewetting of a bilayer on grating substrate with high γS. In contrast, on a low γS grating, the top layer ruptures sequentially after the bottom layer has ruptured, forming ordered encapsulated patterns.73,74 Very recently, Ding and co-workers have experimentally studied the dewetting of a thin PS film coated on a topographically patterned thick PMMA substrate.75−77 The morphological evolution proceeds with the disintegration of the PS layer into stripes and threads aligned along the PMMA ridges and grooves respectively, that subsequently rupture into aligned small and large droplets.75 The kinetics of film rupture depends on the viscosity of the two layers and is governed by the more viscous component.72 For a thick and continuous PS top layer, the dynamics is also strongly influenced by the molecular weight of the layer.77 They have also reported the vertical pattern decay and lateral in-phase capillary breakup of a nano imprinted polymer bilayer, where a complex ‘‘in-phase’’ capillary breakup of the PS stripes on the PMMA substrate ridge eventually results in strings of PS ellipsoids aligned orthogonal to the imprinting direction. At later stages, these ellipsoids anisotropically coalesce into thick threads before disintegrating into droplets due to Rayleigh instability.76,86
size and morphology control, in comparison to the standard top down lithography methods, where in most cases only a particular pattern can be created using a specific stamp or a mask.32 The only limitation of instability mediated patterning approaches lies in the inherently random nature of the structures, which hinders their practical utility.1−8 Strategies have therefore been worked out to align the structures for their potential use in in fabrication of devices and functional surfaces. In this regard, dewetting a thin polymer film on a chemically,11−21 or a topographically patterned substrate12,22−31 is a promising route, which has been successfully used to align the instability patterns. Theoretical studies based on nonlinear simulations,11−14 as well as experiments15−30 have shown that a perfectly ordered structure on a patterned substrate results only within a narrow parameter range, which depends on the commensuration between the natural instability length scale of the film (λ) and the periodicity of the substrate patterns (λP).11−16,28 Additionally, on a topographically patterned substrate, the positioning of the dewetted features can be tailored by adjusting the nature of the initial adhesion of the film with the substrate.27,28 A recent work outlines the influence of substrate feature height on the dewetting dynamics and pattern morphology.30 Interestingly, most papers on pattern directed dewetting report the instability of a single layer film.11−31 though instability and dewetting of thin bilayers comprising of two immiscible polymers is a fascinating and important problem.33−65 Bilayers are important in variety of settings including resist layers in lithography, layered composites, pressure sensitive adhesive,66 attachment of biological membranes to a solid surface under liquid etc.67 Instability of a bilayer is complex as it involves the coupled deformation of multiple, confined interfaces. This leads to possible creation of exotic patterns such as submerged, embedded, hollow and core shell type meso scale structures, which are beyond the fabrication capability of the standard lithography methods.63−65 In a thin bilayer, the interaction between the two deformable interfaces can lead to two distinct short time modes of instability.60−62 When instability is engendered due to a preferential attraction between the film/film and film/substrate interfaces, an in phase relative deformation of the two layers result, which is known as the “bending” mode of deformation. In contrast, when the instability is triggered due to a preferential attraction between the film/film and the film/air interfaces, the deformations in the two layers is out of phase, which is termed as the “squeezing” mode.60,62 The final dewetted morphology is strongly influenced by the initial mode as well as the relative growth rates of instability at the two interfaces.62 Various aspects related to dewetting of a bilayer, such as the effects of thickness and viscosity of the individual layers have been investigated, both based on nonlinear simulations 60−65 and experiments.33−59 Experimentally, dewetting of a polymer bilayer has been investigated in two distinct configurations. In the first configuration, dewetting of a thin top layer is studies on a thick bottom layer which is stable and does not dewet. In this setting, the problem essentially transforms into dewetting of a single layer film on a viscous substrate.33−43 where the final droplet morphology is defined by the Newman configuration, instead of the customary Young’s configuration which specifies it on a solid substrate.33,34 The rims formed around ruptured holes in the top layer often penetrate into the softened bottom 936
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The bilayer samples were prepared by sequential spin coating (Apex Instruments, India) of PMMA (MW: 305 K, Sigma, U.K.) and PS (MW: 280 K, Sigma, U.K.) from their dilute solutions in HPLC grade Toluene and 1-chloropentane (synthesis grade, Merck India) respectively, on the patterned substrates. The drop volume, rpm, and spinning durations were 200 μL, 2500, and 1 min respectively, for both the layers. After coating each layer, the films were air-dried for 3 h followed by annealing in an air oven at 60 °C for 6 h to remove any remnant solvent. The morphology of the as cast PMMA bottom layer and that of the bilayers coated on the patterned substrates were investigated using an AFM in intermittent contact mode with a silicon cantilever (PPP-NCL, Nanosensors Inc. USA). Dewetting of the bilayer films was triggered by thermally annealing the films in a vacuum oven at 130 °C, a temperature that is higher than the TG of both the polymers. Samples were withdrawn from the oven after every 30 min to observe the progressive morphological evolution using an AFM. To investigate the deformation at the polymer/polymer interface, it was necessary to preferentially remove the top PS layer, which was accomplished by washing the samples with 1-chloropentane for 2 h, followed by drying under vacuum for 6 h at 60 °C. It is worth pointing out that the UVO exposed patterned cross-linked Sylgard 184 substrate does not swell during the selective washing step with chloropentane, which is discussed in details in section S2 of the Supporting Information. For removing the PMMA layer, which was necessary only in certain cases, the samples were washed in acetic acid for 10 min followed by washing in deionized water and drying in a vacuum oven for 6 h. Further, it is well-known that a film directly spin coated on a topographically patterned substrate is either discontinuous or has an undulating top surface and therefore there is no simple way to define its exact thickness (h).78,83 Consequently, for such a film we define an equivalent film thickness (hE), which corresponds to the thickness of a film coated under identical conditions (same cn and rpm) on a flat substrate of the same material.78 The thickness of the flat films are measured using a variable angle imaging ellipsometer (Accuron GmbH, Model: EP3SW, 532 nm laser source). The effective thickness of the different bottom (hE‑PMMA) and the top (hE‑PS) layer films used in the present study are listed in Table 1.
It becomes evident from the preceding discussion that dewetting of a thin polymer bilayer on a topographically patterned substrate appears to be a fascinating problem which has never been experimentally investigated. In this article, we report the dewetting of a thin polymer bilayer comprising of a PS top and a PMMA bottom layer, on a grating substrate made of low surface energy cross-linked PDMS. The bilayers were prepared by direct sequential spin coating on the topographically patterned substrate using mutually exclusive solvents. It is well-known that depending on the coating condition a continuous or a discontinuous film may result on a topographically patterned substrate.78 We identify three distinct possible initial morphologies of the bilayer film on the grating substrate, which are as follows: type 1, a discontinuous bottom layer under a discontinuous top layer, resulting in bilayer threads confined within the substrate grooves; type 2, a discontinuous thread of the bottom layer polymer within the substrate grooves, buried under a continuous top layer; type 3, both layers continuous. By varying the thickness of the two layers independently, we show that it becomes possible to create complex ordered structures, such as aligned array of core shell droplets, undulating multilayer ribbons and laterally coexisting ordered structures of both PS and PMMA. We also construct a morphology phase diagram that shows the influence of each layer independently on the final dewetted morphology. Our experiments reveal that both morphology and periodicity of the dewetted structures are strongly influenced by the dewetted PMMA bottom layer.
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EXPERIMENTAL DETAILS
The grating substrates were soft lithographically fabricated on ∼5 μm thick Sylagrad 184 (a two part cross-linkable poly(dimethylsiloxane) (PDMS); Dow Corning, USA) films. The concentration ratio of oligomer (part A) to cross-linker (part B) was maintained at 10:1 (v/ v) in the PDMS composition. Dilute Sylgard 184 solution in n-hexane (SRL, India) was spin coated on cleaned double side polished quartz pieces (15 mm ×15 mm, Applied optics, India). The Sylgard 184 films were cured for 12 h at 120 °C for complete cross-linking. The crosslinked films were subsequently kept immersed in n-hexane for 15 h for removal of the un-cross-linked oligomer molecules.79 Subsequent to solvent wash, the films were dried at 60 °C for 6 h in a vacuum oven. The dried samples were subsequently patterned by a pressure-less room temperature imprinting technique, using flexible patterned metallic foils peeled from commercially available CD discs as stamps.80,81 The imprinted patterns were made permanent by exposing the film while in contact with the stamp to UV−ozone exposure, which oxidizes the cross-linked PDMS film surface with the formation of a stiff oxide layer.80,82 The foils are porous,81 which allows the atomic oxygen and ozone to reach the PDMS film surface and oxidize. The oxide layer also acts as a diffusion barrier during spin coating as well as during selective solvent washing, and does not allow the solvent to penetrate the into PDMS matrix. Details about the patterning methodology can be found elsewhere.80,81 The periodicity (λP), substrate groove width (LP) and the stripe height (HS) of the patterned substrates were measured using an AFM (Agilent Technologies, USA Model 5100) and are 1.5 μm, 750 and 120 nm, respectively. Patterned PDMS substrates were preferred owing to their low cost and ease of fabrication. Similar substrates have also been used in several recent dewetting related studies.26−28,30 It is important to highlight that though Toluene is a good solvent for PDMS, it does not swell the PDMS substrate during spin coating, primarily due to the presence of the thin impermeable silicon oxide crust on the PDMS substrate.78 That there is no swelling of a cross-linked PDMS substrate during spin coating of a polymer on it has been highlighted in several recent publications.66,78,82
Table 1. Details of the As Cast Films
The equilibrium contact angles of toluene and 1 − chloropentane on the patterned cross-linked Sylgard 184 substrates ware measured using a contact angle goniometer (make: Ramé-Hart, USA, Model 290) and were found to be ∼35° and 32° respectively. This implies that the cross-linked Sylgard 184 substrate is partially wetted by the solvent used for coating both the layers. It was also observed that a PMMA film was fully wetted by 1-chloropentane. Further, we calculated the surface energy of the substrate, γS‑sub = 24.2 mJ/m2 from Owen’s equation, using water, ethylene glycol, and toluene as probing liquids.84 937
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Figure 1. (A) Morphology of an as cast bilayer comprising hE−PMMA = 11.1 nm and hE−PS = 12.8 nm. Inset A1 shows cross sectional line scan of the bilayer threads as well as the PMMA under layer thread. Inset A2 shows the existence of gap between the bilayer thread and the confining substrate groove walls. (B) Undulating threads observed after 2 h of annealing. (C) Disintegration of the undulating threads into droplets after 4 h of annealing. (D) Final dewetted morphology comprising array of aligned droplets after 5.5 h of annealing. (E) Array of PMMA droplets after preferential removal of the PS layer. AFM line scans show the core shell nature of each droplet.
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RESULTS AND DISCUSSION For cn−PMMA ≤ 1.0%, a discontinuous bottom layer comprising isolated polymer threads or strips confined within the substrate grooves results due to insitu dewetting of the polymer solution during spin coating.78 At even lower concentrations the polymer threads do not span the entire width of the substrate grooves. For example, when cn‑PMMA ≈ 0.25% with corresponding hE−PMMA = 11.1 nm, the width of the PMMA threads LT−PMMA is ≈340 nm, which is narrower than the width of the substrate groove (LP = 750 nm). In this specific case, the height of the PMMA thread is hT−PMMA ≈ 28 nm. At higher cn−PMMA, the isolated threads span the entire width of the substrate groove (LT−PMMA = LP), which is observed in films with hE−PMMA =19.1 and 38.3 nm. For cn‑PMMA ≥ 1.5%, a continuous PMMA film results, with an undulating top surface. The amplitude of the surface undulations (aS‑PMMA) progressively reduces with increase in cn‑PMMA, vis−a−vis hE−PMMA. As the substrate is partially wetted by toluene, the undulations on a continuous PMMA film surface are 180° out of phase with respect to the substrate patterns.78 In cases where a continuous PS film is coated on a discontinuous PMMA film (type 2 bilayer), the surface undulations on the PS film are also 180° out of phase with respect to the substrate pattern, due to partial wettability of cross-linked PDMS substrate by the solvent, 1− Chloropentane. A continuous film is therefore thinnest over the
substrate stripes, where the local thickness is denoted by hL. From measured values of hE and as, we estimate hL following a simple calculation procedure discussed in the Supporting Information. On the other hand, as 1-chloropentane fully wets PMMA, the surface undulations of the two layers are in phase with each other, when the top PS film is coated on a continuous PMMA bottom layer. The values of spreading coefficients (SPS−Sub = −24.2 mJ/m2, SPMMA−Sub = −24.8 mJ/m2 and SPS−PMMA = 2.7 mJ/m2) calculated based on the individual surface energies (γS) of the two constituent polymers (γPMMA = 41.8 mJ/m2, γPS = 38.3 mJ/ m2),85 that of the substrate (γS‑sub = 24.2 mJ/m2), and the interfacial energies between them (γPMMA−PS = 1.2 mJ/m2, γPMMA−Sub = 3.8 mJ/m2 and γPS−Sub = 6.1 mJ/m2) shows that the PMMA bottom layer is unstable on the substrate (negative SPMMA−Sub). A PS layer is stable on a continuous PMMA (positive SPS−PMMA) layer but becomes unstable when it comes in direct contact with the PDMS substrate (negative SPS−Sub). With this background, we now discuss the dewetting and morphological evolution of PS−PMMA bilayers with 8 different thickness combinations on a grating substrate. Additional experiments were also performed, with different thickness combination of the two layers, which have been used to formulate a morphology phase diagram shown in Figure 9. 938
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Figure 2. (A) Morphology of an as cast bilayer comprising hE−PMMA = 11.1 nm and hE−PS = 41.8 nm. Inset A1 shows cross sectional line scan of the bilayer threads as well as the PMMA under layer thread. (B) Undulating threads observed after 2.5 h of annealing. The overall dewetted morphology remains nearly identical with further annealing. (C and D) Morphology of the PMMA under layer after 2.5 and 5.5 h of annealing, respectively, showing a transition from undulating threads to isolated PMMA droplet array.
Figure 3. (A) Partially dewetted morphology of a bilayer comprising of hE−PMMA = 11.1 nm bottom layer and hE−PS = 41.8 nm top layer, after 30 min of annealing. The continuous PS film has started to rupture over the substrate stripes. Inset A1 schematically shows the cross section of the bilayer, which comprises of a discontinuous bottom layer (LT−PMMA < LP), buried under a continuous top layer. Inset A2 shows the as cast morphology of the bilayer. (B) Final dewetted morphology of the film comprising undulating threads observed after 5.5 h of annealing. (C) Array of dewetted PMMA droplets aligned along the substrate grooves, formed under the undulating threads, observed after removing the top PS layer, from the structure shown in part B.
involving hE−PMMA =73.3 and 138.9 nm films (Figures 6-8). For every thickness combination; experiments were performed with at least four identical samples prepared in different batches, and AFM scans were performed at five locations for each sample, to check the reproducibility of the results and to strengthen the error bars in Figures 9 and 10. Dewetting of Bilayers with Discontinuous Bottom Layer. First, we discuss the instability in a type 1 bilayer, comprising of a hE‑PS = 12.8 nm top and a hE‑PMMA = 11.1 nm bottom layer. The cross-sectional profiles of the PMMA and the PS threads and the contours of the confining substrate groove walls is shown in the form of a superimposed AFM line
The discussion is organized in terms of progressively increasing bottom layer thickness (hE−PMMA). The first five case studies involve a discontinuous bottom layer. Out of them, in the first four cases (Figures 1−4) hE−PMMA = 11.1 nm, which implies that LT−PMMA < LP. The next example (Figure 5) involves a discontinuous bottom layer (hE−PMMA = 38.3 nm), but the threads span the entire width of the substrate (LT−PMMA = LP). It is worth pointing out that though only one example of this specific case is shown, the final dewetted morphology is rather generic and is observed in bilayers with different thickness ratios, which can be seen in Figure 9. The last three examples involve bilayers with a continuous bottom layer film, 939
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A2 of Figure 1A. The first signature of instability in the bilayer threads appears after ∼2 h of thermal annealing, which is in the form of alternating narrower and wider domains along each thread (Figure 1B). The relatively long time required for the onset of instability is attributed to the high molecular weight of both the polymers, which makes the dynamics sluggish. The undulations appear due to the well-known Rayleigh−plateau instability,86 engendered by the cross-sectional curvature of the long polymer threads. The amplitude of the undulations grow with time, as Laplace pressure drives flow of liquid from the narrower zones of the threads to wider, bulgy zones. The periodicity of the undulations along the axial direction of the thread is measured to be λA ≈ 1.21 ± 0.081 μm. On the basis of linear stability analysis (LSA), the characteristic wavelength (λA) of capillary instability in an infinitely long liquid thread of width LT is λA = 4LT.70,85,86 On the basis of LT−B ≈ 530 nm, the calculated value of λA ≈ 2.2 μm, which is much higher than the experimentally obtained value of λA ≈1.21 μm. However, if the same calculation is performed based on the width of the inner PMMA thread (LT‑PMMA ≈ 340 nm), then λA turns out to be ≈1.36 μm, which is close to the experimentally observed value of λA. On the basis of the fact that λA exhibits the scaling with LT−PMMA rather than LT−B, we argue that the undulations appear in the inner PMMA threads first and the outer PS thread simply deforms along the contours of the under layer. It can also be argued that since the inner PMMA thread has a lower radius compared to the bilayer threads, the magnitude of the Laplace pressure in the inner PMMA thread is higher, leading to its preferential rupture.
Figure 4. (A) Array of dewetted PMMA droplets under an intact PS film observed after 24 h of annealing in a bilayer comprising of hE−PMMA = 11.1 nm bottom layer and hE−PS = 123.7 nm top layer. The morphology of the dewetted film attains this form after 6 h of annealing and does not change subsequently. Inset A1 schematically shows the cross section of the type 1 bilayer, where (LT−PMMA < LP). Inset A2 shows the as cast morphology of the bilayer film. Inset A3 shows the phase contrast image of the sample after 24 h of annealing. (B) Array of aligned PMMA droplets formed under the intact PS film, observed after removing the top layer.
scan (inset A1 of Figure 1A), which shows that the as cast bilayer is discontinuous, comprising of isolated bilayer threads with thread width LT−B ≈ 530 nm and thread height hTB ≈ 37 nm. In this specific case not only LT−PMMA < LP, but even the width of the bilayer threads are narrower than that of the substrate groove (LT−B < LP). Consequently there is a clear gap between the as cast bilayer threads and the confining substrate walls on both sides of each thread, which can be seen in inset
Figure 5. (A) Morphology of a type 2 bilayer comprising of hE−PMMA = 38.3 nm bottom layer and hE−PS = 59.8 nm top layer, after 30 min of annealing. The continuous PS film is seen to rupture over the substrate stripes. Inset A1 schematically shows the cross section of the bilayer, which comprises of a discontinuous bottom layer that covers the entire width of the substrate groove (LT−PMMA=LP), buried under a continuous top layer. Inset A2 shows the as cast morphology of the bilayer. (B) Polymer thread localized over the substrate grooves after 1.5 h of annealing. (C) Final dewetted morphology comprising undulating threads observed after 5.5 h of annealing. The morphology did not change any further with longer annealing. (D) PMMA threads with periodic undulations aligned along the substrate grooves, observed after removing the PS layer, from the structure shown in part C. 940
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Figure 6. (A) Morphology of a type 3 bilayer comprising of hE−PMMA =73.3 nm bottom layer and hE−PS =19.4 nm top layer, after 1 h of annealing. The image shows rupture of the PMMA film as well as that in the PS film (at some locations) over the substrate stripes. (B) Mildly undulating bilayer threads localized over the substrate grooves after 5.5 h of annealing. (C) Undulating PMMA threads, after preferential removal of the PS from the structure shown in part B. (D) Schematic representation of the dewetting mechanism.
Figure 7. (A) Dewetted morphology comprising undulating PMMA threads over substrate grooves and array of PS droplets over the substrate stripes, in a type 3 bilayer comprising of hE−PMMA =73.3 nm bottom layer and hE−PS = 12.8 nm top layer, after 5 h of annealing. (B) and (C): Fully dewetted morphology after Preferential removal of PS and PMMA, which shows undulating PMMA threads along substrate grooves and aligned PS droplets over substrate stripes, respectively. (D) Schematic representation of the mechanism of dewetting.
With time the inner PMMA threads disintegrate into aligned isolated droplets, buried under undulating, still intact PS threads. As the PMMA threads break down into droplets, the PS threads now come in direct contact with the base of the substrate grooves at the locations where the PMMA threads have snapped off. As a PS film is also unstable on a low γS PDMS substrate, the PS threads also start to break up at the same locations with progressive annealing for approximately 4 h, resulting in isolated droplets (Figure 1C). Initially the droplets are slightly elongated, which eventually take a more
spherical shape, leading to minimization of surface energy. The final dewetted morphology, which appears after 5.5 h of annealing and does not change subsequently, comprises of a highly ordered and periodic array of nearly equal sized droplets aligned along the substrate grooves (Figure 1D). The periodicity of the droplets along the direction of the stripes is λD‑B ≈1.184 ± 0.112 μm, which is close to the value of λA of the undulating threads before their disintegration into droplets. The droplet diameter (dD‑B) is ≈750 nm, which seems to be governed by the width of the substrate grooves (LP). As the 941
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(dD−PMMA ≈ 543 ± 7 nm) as compared to the bilayer droplets, though their periodicity (λD‑PMMA ≈ 1.221 ± 0.162 μm) is nearly the same as λD‑B. Thus, it can be argued that each composite droplet resulting from the dewetting of the bilayer threads has a core shell structure, which can be seen in the superimposed AFM line scan shown as inset E1 of Figure 1E. In the next three cases (Figures 2−4), we investigate how the thickness of the top layer (hE−PS) influences the overall morphology of the dewetted structures, when the bottom layer configuration remains unaltered (hE‑PMMA = 11.1 nm). When hE‑PS = 41.8 nm, the as cast morphology of the bilayer is again type 1 (Figure 2A). However, it can be seen from the superimposed AFM line scans (inset A1 to Figure 2A), that unlike the previous case, where LT−B < LP, here the isolated bilayer threads span the entire width of the substrate grooves (LT−B = LP). As LT−PMMA is ≈340 nm, the gap between the PMMA thread and the substrate groove walls are entirely filled up by PS. For this bilayer, the first sign of instability is observed after 2.5 h of annealing, which is in the form of periodic undulations along the threads (Figure 2B). Interestingly, in this case with progressive annealing the stripes do not decay into droplets and the overall morphology remains nearly unaltered as that seen in Figure 2B, even after annealing the samples for 36 h. For an undulating thread to decay into droplets, liquid from the thinner zones of the thread has to physically flow to the wider areas. However, as the entire substrate groove is filled with polymer, therefore the space required for the growth of the bulges along the substrate stripe is not available, which in turn hinders the decay of the threads into droplets. In fact, we will see that in many subsequent cases a similar mechanism is responsible for suppression of droplet formation. The periodicity of the undulations is λA ≈ 1.39 ± 0.211 μm, which is close to the value of λA observed in the previous case as well as the theoretically predicted value of λA based on LSA,86,87 which implies that similar to the previous case, the Rayleigh instability first sets in along the inner PMMA thread.
Figure 8. (A) Nucleated holes in a hE−PS = 19.4 nm thick top PS layer over an bottom layer of hE−PMMA =138.9 nm, after 1 h of annealing. (B) Dewetting of the PS film over the PMMA humps, with growth of holes after 3 h of annealing. (C) Final dewetted morphology comprising undulating PS threads confined within the valleys on the surface of an intact PMMA bottom layer, after 6 h of annealing. (D) Schematic representation of the dewetting mechanism.
droplets form due to disintegration of a bilayer thread, it is obvious that they possess a composite structure comprising both PS and PMMA. To understand the precise nature of each droplet, the PS layer was preferentially removed. This reveals the structure of the dewetted PMMA threads, which also comprises of an array of aligned droplets along substrate grooves. However, the PMMA droplets are smaller in diameter
Figure 9. Morphology phase diagram as a function of the individual layer thicknesses. The solid symbols represent the overall morphology of the dewetted bilayer and the open symbols represent the same for the dewetted bottom layer only. Inset A and B shows the morphologies of the dewetted bilayer and the bottom layers separately. Symbols have following meaning: (■, □): droplet array; (●, ○): undulating threads; (▼, ▽): Intact layers; (▲): undulating or dewetted PS layer. The demarcating lines act as guides only. 942
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Macromolecules
Article
Figure 10. (A) Variation of structure periodicity λA (droplet/undulating threads) along the stripes as a function of bottom layer thickness (hE−PMMA). Inset A1 shows variation of the feature periodicity of the bottom layer λB as a function of hE−PMMA. The demarcation boundaries of the three zones act as guides only. A11, B21, B22, C21, C22 are the schematic morphologies of the bilayers in the respective zones. (B) Variation of feature periodicity λA as a function of total effective thickness of the bilayer (hE−T). Different symbols represent different bottom layer thickness: 11.1 nm (■), 19.1 nm (●), 28.7 nm (▲), 38.3 nm (▼), 48.7 nm (◊), 59.4 nm (solid triangle pointing left) and 73.3 nm (solid triangle pointing right).
prevent any overflow of liquid over the top of the substrate stripes when the threads become undulating, which is seen in Figure 2B. In contrast, in the latter case, as the bilayer threads form due to initial rupture and dewetting of the top layer, a continuous contact line is formed over the top of the substrate stripes, close to the edges. This allows the liquid to overflow over the top of the stripes, when the thread become undulating, as seen in Figure 3B. The phenomena of thread undulations impinging over the substrate stripes are rather generic and observed in many type 2 bilayers with different thickness combinations. The threads attain their final undulating morphology after about 5.5 h of annealing (Figure 3B). The periodicity of the undulations is λA ≈ 1.273 ± 0.029 μm, which is close to λA observed in the previous two cases. Preferential removal of the PS top layer reveals an aligned array of PMMA droplets (Figure 3C), which is morphologically similar to that observed in the previous two cases, with near identical values of λD‑PMMA (≈ 1.186 ± 0.135 μm) and dD−PMMA (≈ 564.6 ± 6 nm). We argue that in this case also the submerged PMMA threads disintegrating into isolated droplets due to Rayleigh instability. Also, similar to the previous case, lack of space within the substrate grooves prevents the decay of the undulating PS threads into droplets. Thus, dewetting in this case results in complex morphology comprising an array of PMMA droplets below well-ordered undulating PS threads, aligned over each substrate groove. The two sequential instability modes associated with the dewetting process, that is rupture of the continuous PS films over substrate stripes and disintegration of the PMMA threads into droplets, have distinctly different time scales, with the latter being significantly slower than the former. This is evident from Figure 3A, which shows isolated threads without any undulation in the early stages of dewetting. This implies that the instability in the PMMA threads set in later after the PS film has ruptured and the isolated bilayer threads have formed. Figure 4 shows the dewetted morphology of a type 2 bilayer comprising hE‑PMMA = 11.1 nm under a thick continuous PS layer having hE‑PS =123.7 nm, with aS = 6.8 nm and hL−PS ≈ 65 nm (inset A2, Figure 4A). The indicative morphology of the system is schematically shown as inset A1 of Figure 4A. Unlike in the previous case, where a low hL−PS (
