Anisotropic Dewetting Holes with Instability Fronts in Ultrathin Films of

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Anisotropic Dewetting Holes with Instability Fronts in Ultrathin Films of Polystyrene/Poly(ε-caprolactone) Blend Meng Ma, Feng Chen,* Ke Wang, Qin Zhang, Hua Deng, Zhongming Li, and Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *

crystal structure in the blend films leads to the initial film structure in the film plane to be anisotropic, which in turn has a great impact on the following annealing-induced dewetting process and mechanism. Figure 1a,b displays the crystal morphology of PS3.7K/PCL (50/50) blend films. Dendritic crystal analogue of the diffusioncontrolled33 finger-like crystal of the pure PCL film32 with a flat-on lamellae thickness about 11 nm is obtained, and it shows that most parts of that film is occupied with dendritic crystals. The XPS results show that PS3.7K not only located at the interval of those crystals as the PS-rich domains but also with a little part at the surface or in the PCL crystals domains (see the XPS results in Figure S1a,b). The crystallization of PCL leads to the dendritic PCL crystals formation in the blend film. PCL crystal lamellae with thickness about 11 nm and a thinner one about 4 nm are obtained in the blend films, as can be seen from the cross-section profile in Figure 1d and the phase image in Figure 1c. The crystallization of PCL rejects most of the amorphous PS3.7K out of the crystals, resulting in the PS-rich domains formation located at the intervals of the dendritic PCL crystal, as marked by a circle and arrows in Figure 1a, and with a little part of PS3.7K located at the surface or in the PCL crystals domains. This initial of PCL crystals embedded in PS3.7K layer structure leads to the composition in the film plane and the film thickness varying at different spots, which may trigger the film wetting ability and stability to be anisotropic. The normal circular holes nucleate at the PS-rich domains between the dendritic PCL crystals, as shown in Figure 2, when the PS3.7K/PCL (50/50) blend films is annealed at 70 °C for 2 min. This indicates that holes nucleation occurs selectively in the region of the PS3.7K segregation domains at the initial stage of annealing. While noncircular and faceted holes are obtained in the blend films when annealed longer time for 10 and 60 min, as shown in Figure 3a,b and Figure 3 c,d, respectively. This unusual anisotropic dewetting structure has been reported in the liquid−liquid dewetting of a bilayer system when viscous dissipation in the two layers and slippage of the dewetting layer on the lower layer are dominated.17,18 For our system, what induces the holes growth to be anisotropic? After carefully examining the dewetting holes, we find that high rims and low rims are formed at the fronts of the holes faceted sides and the front end of the arrows (Figure 3a), respectively. Most of PCL lamellae crystals aggregate at faceted sides of the holes as

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n the past several decades, the stability and dewetting of thin polymer films on solid supports have drawn much attention for its fundamental scientific interests and its numerous technological applications in coatings, paints, adhesives, lubrication layers, dielectric layers, and multilayer adsorption. A lot of work has been done on the dewetting process by experiments, simulations, and theories.1−7 Holes nucleation and growth is the common dewetting mechanism except for the spinodal dewetting. For the nucleation and growth mechanism, holes formation is initiated by an impurity or a defect in the film,8 and the usual scenario at the initial stages of dewetting of a thin film on a solid substrate with no slip involves the formation and growth of nearly circular holes.9−11 With the growth of the holes, the dewetted polymer mass aggregates as a narrow elevated rim at the perimeter of the holes, which in some cases may show periodic undulations owing to the crosssectional curvature. At later stages, faceted sides form when the rims of the neighboring holes merger, resulting in a polygonal structure. The edges of polygons are instable and decay into droplets eventually. The structure and mechanism of the formation and growth of holes are now reasonably well understood.2,12,13 However, recently, a fractal holes pattern has been reported for the thin film of poly(styrene)-blockpoly(methyl methacrylate) (PS-b-PMMA)14,15 and polystyrene-block-poly(ethylene oxide) (PS-b-PEO)16 diblock copolymer after being annealed in a solvent vapor. Some recent simulations17 and experiments18 also found that irregular and faceted shapes in the dewetting of the upper polystyrene layer for a thin polymer bilayer (silica−PMMA−PS−air) could be constructed when viscous dissipation in the two layers and slippage of the dewetting layer on the lower layer are dominated. To the best of our knowledge, although the dewetting, phase separation, and their coupling behaviors in thin polymer blend films have been studied extensively for the blend thin films of poly(styrene-ran-acrylonitrile) (SAN) and poly(methyl methacrylate) (PMMA),19−25 polystyrene (PS) and poly(vinyl methyl ether) (PVME),26−29 and some crystalline polymer blends film,30−32 an anisotropic dewetting of a blend films has never been discovered up to now. In this Note, a crystalline polymer poly(ε-caprolactone) (PCL) with molecular weight Mw = 50 000 g/mol is introduced into PS film with different molecular weight (PS3.7K and PS54K represent PS with Mw = 3700 and 54 000 g/mol, respectively) to explore the morphology of the growing holes in dewetting of the blend films. The Mw/Mn of the used monodiseperse PS3.7K and PS54K is 1.04 and 1.06, respectively. (The experimental details are presented in the Supporting Information.) The © 2012 American Chemical Society

Received: January 11, 2012 Revised: May 5, 2012 Published: May 14, 2012 4932

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Figure 1. (a) AFM height images of PS3.7K/PCL (50/50) blend films crystallized at room temperature for 24 h with film thickness about 15 nm. (b) Zoom-in height image of the square box in (a). (c) Corresponding AFM phase image of (b). (d) Corresponding cross-sectional profiles along the black line in (b).

Figure 2. AFM height images of PS3.7K/PCL (50/50) blend films with thickness about 15 nm annealed at 70 °C for 2 min.

has polar interaction with the mica substrate, it is stable for molten PCL layer on the mica. On the other hand, the glass transition temperature (Tg) of PS3.7K is about 70 °C, and it has a molecular weight clearly below critical molecular weight (Mc)34 with low viscoelasticity and part compatibility with PCL, resulting in the PS3.7K segregation domains containing part of amorphous PCL. It is enough for polymer chains in PS3.7K segregation domains to move when annealed at 70 °C. So we speculate that holes nucleation and growth occurs selectively at the PS3.7K segregation domains with less stability and resistance at the interval of the molten and stable PCL domain layer, as

encircled in Figure 3b. It is well-known that the stability of thin film is determined by the combination of short- and long-range intermolecular forces. It is embodied in the excess free energy per unit area of a film of thickness h and the effective interface potential φ(h). When the curvature, Π(h) = dφ(h)/dh, of the interface potential is negative, the film is unstable and breaks up via spinodal dewetting mechanism; otherwise, it would be stable or metastable, wherein the destabilization occurs via a nucleation and growth process. For thin PS film on mica, it is metastable and dewet through holes nucleation and growth when the chains are movable, as shown in Figure 4. While PCL 4933

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Figure 3. AFM height images of PS3.7K/PCL (50/50) blend films with thickness about 15 nm annealed at 70 °C for (a, b) 10 min and (c−e) 60 min. (b) Zoom-in height image of (a), (c) AFM height image of the dewetting fingers on the substrate, and (d) zoom-in height image of the square box in (c), (e) zoom-in height image of the square box in (d), and (f) corresponding cross-sectional profiles along the black line in (e).

the composition of the blend film and study the growing kinetics of the holes. These are under investigation in our group. This abnormal dewetting process of the PS3.7K/PCL blend films is quite different from that of thin PS films and PS54K/ PCL blend films with the same thickness. For thin PS3.7K films with thickness about 15 nm, it is the norm nucleation and growth dewetting process that results in the formation of circular holes, as shown in Figure 4. The Tg of PS3.7K is about 70 °C, so it is stable after annealed at 70 °C even for 1 h (Figure 4a). However, circular holes initiated by an impurity with undulation rims are obtained after being annealed at 120 °C for 10 min (Figure 4b,c). The cross-section profiles display the typical 15 nm depth dewetting holes with a rim and trough surrounding (the arrow in Figure 4d).35 The aggregation of PS at the perimeter of the holes forms the rims, which are instability, as observed by Masson et al.36 It is showed that for the film thickness, h < hc (hc is the critical film thickness, hc ∝ M3/7), the fluctuations in the rim are amplified owing to the small rim and the large hole growth velocity resulting in fingers at dewetting fronts. Further annealing, dewetting rims with ruptured fingers at fronts are formed due to Rayleigh instability,37−39 as shown in Figure 4e. Eventually, stable PS drops on mica are obtained after annealed for 30 min (Figure 4f). For PS54K/PCL blend films, its compatibility is worse than that of PS3.7K/PCL. As shown in our previous reports,31,32 an enriched two-layer structure with the dendritic PCL-rich crystal layer covered by layer of PS54K, is formed during spin-coating (see the XPS results in Figure S1c). This unique initial structure leads to the holes nucleation and growth of liquid−solid

marked by red circular and arrows in Figure 1a. Further annealing, the irregular dewetting holes with selective growth directions are remarkable, as illustrated by the marked red and blue arrows from the holes center in Figure 3c. The aggregation of polymer at the perimeter of the holes leads to the formation of periodic undulations rims owing to the cross-sectional curvature, forming the fingers at the dewetting fronts are obtained (Figure 3c,d). After the crystallization of PCL in the blend films at room temperature, we can observe the composition distribution of the dewetting blend films by examining the location of the PCL crystal. No crystal is observed in the fingers, indicating that they are mostly composed of PS3.7K, and distinct PCL lamellae crystal is only observed at the faceted sides of the noncircular holes encircled in Figure 3d. This indicates that most of PCL aggregates at the dewetting sides with better stability, which makes holes to grow slowly at this direction resulting in the formation of the noncircular holes. The aggregation of the dewetting parts of the blend films makes the unbroken regions to thicken, and a compact PCL crystal (∼12 nm in thickness) with many wide branches is obtained, as shown in Figure 3e and the crosssection profile in Figure 3f. The anisotropic dewetting process indicates the wetting ability and stability of the blend films on the substrate are anisotropic. In order to thoroughly and deeply acknowledge this anisotropic wetting behaviors and for better characterization of the morphology and the composition of the blend film during the dewetting process, other powerful techniques, such as in-situ AFM or optical microscopy (OM) with a temperature control stage and scanning transmission Xray microscopy (STXM), should be carried out to characterize 4934

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Figure 4. AFM height images of the dewetting morphology of thin PS3.7K films with thickness about 15 nm annealed at (a) 70 °C for 60 min, (b, c) 120 °C for 10 min, (e) 120 °C for 20 min, (f) 120 °C for 30 min, and (d) corresponding cross-sectional profiles along the black line in (c).

films annealed at 120 °C, which rupture into drops before the merge of the dewetting holes (Figure 4e). The phase image of Figure 5a illustrates the fact that there are selective directions for the holes to expand during dewetting again, as marked by arrows in Figure 5b. Figure 5c,d displays the morphology of the merged dewetting fronts; four dewetting frontiers are merged as marked by arrows and camber lines in Figure 5c. The rims instability at the dewetting fronts leads to the fingers formation with a fixed distance about 2 μm corresponding to the undulations of a certain wavelength (λ), which has been observed in the systems of solvent annealed PS40,41 or PMMA42,43 films with a constant wavelength (λ) for the corresponding film thickness. The frontier lines shorten with the merge of the dewetting fronts, resulting in less space to contain so many fingers with a fixed distance because of the constant wavelength for the blend film. Therefore, the number of fingers decreases with the merge of the dewetting fronts resulting in the combined of two fingers, as the branched fingers at the dewetting fronts shown in Figure 5c,d. Unique and row by row of small drops along the dewetting fingers, looked like the dewetting trails, are found at the front of the fingers, as displayed in Figure 6. The arrow and camber lines in Figure 6a represent the selective direction of the holes growth and the drawing back route of the fingers, respectively, which fully illustrates the unique anisotropic dewetting process with instability fronts and the dewetting trails left. Considering the polar interaction between PCL and mica, we speculate that the small drops of the trails with diameter ∼200 nm (Figure 6b) may mostly be comprised of PCL, which phase separates from

dewetting of PS from the mica, liquid−liquid dewetting of PS54K from the melt PCL layer, and then the PCL layer wetting the mica substrate process during annealing, resulting in the formation of a stable pattern of PS54K drops embedded in voronoi dendritic PCL crystal finally.32 For PS3.7K/PCL blend films, though, there are a little parts of PS3.7K at the surface or in the PCL crystals domains; the PS3.7K/PCL blend films does not form the two-layered structure with clear interface between PS3.7K and PCL just as that of PS54K/PCL (see the XPS results in Figure S1a,b). On the other hand, the stability of the liquid PS on liquid PCL varies with the PS molecular weight. The layer of PS54K on PCL is unstable, and a liquid−liquid dewetting process occurs during the later stages of annealing for PS54K/PCL blend films, whereas it is stable for liquid PS3.7K on liquid PCL. The changes of the PS molecular weight causes the different distribution of PS within the blend films, which combines with the different stability of liquid PS on liquid PCL leading to the different dewetting process for the blend films. To further investigate the pattern formation process and mechanism of the anisotropic dewetting holes with instability fronts, the PS3.7K/PCL blend films are annealed for even more time for 6 h at 70 °C. Long fingers with gap about 2 and 5 μm in length are exhibited at the dewetting fronts and wide ribbons and big islands with long fingers pattern that looked like a centipede and beetle with many legs is also obtained in the blend films (Figure 5a). This indicates that the fingers are stable during the dewetting process owing to the fact that the fingers are mostly composed of PS3.7K, and the chains are barely movable at 70 °C. This is different from the fingers of PS3.7K 4935

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films with different initial state, thin PMMA9K/PEO6K (50/50) blend films was selected to investigate its morphology evolution process. (PMMA9K and PEO6K represent PMMA with Mw = 9000 and PEO with Mw = 6000 g/mol, respectively.) The same dendritic PEO crystal as that of PCL is obtained in the blend films (see Figure S2a), as been reported previously.44,45 By controlling the spin-coating humidity, dendritic PEO crystal on the top of the underlying PMMA layer structure can be constructed in the PEO/PMMA blend films.46 Since the blend film are prepared at humidity about 80%, the dendritic PEO crystal are located on the surface of the PMMA layer. After annealed at 70 °C, PEO chains are mobile while PMMA not, causing the dewetting process to be quite different from that of PS3.7K/PCL blend films. It is the upper PEO in combination with a thin mobile PMMA layer that autophobic dewets from the adsorption PMMA layer (see Figure S2B,b).43 Further annealing, PEO ribbons or islands lamellae with dendritic PEO crystal surrounding are constructed on the surface of the PMMA layer, as shown in Figures S2C,c and S2D,d, respectively. For the PS3.7K/PCL blend, its compatibility is better than that of PS54K/PCL leading to the formation of the dendritic PCL crystal embedded in thin PS3.7k films structure. As a result, the dewetting morphology and process is quite different and unique, as described schematically in Figure 7.

Figure 5. AFM height and phase images of PS3.7K/PCL (50/50) blend films with thickness about 15 nm annealed at 70 °C for 360 min. (a) AFM height image, (c) zoom-in height image of the square box in (a), and (b, d) corresponding AFM phase images.

Figure 7. Schematic illustration interpreting the anisotropic dewetting holes with fingers at fronts pattern formation for ultrathin PS3.7K/PCL blend films. (a, a′) Spin-coated PS3.7K/PCL blend film with PCL dendritic crystal morphology. (b, b′) Holes nucleation at the PS segregation domains. (c) Holes growth occurs selectively in the directions of PS3.7K segregation domains forming irregular and faceted shapes. (d) Fingers pattern formation at the dewetting fronts.

Figure 6. AFM height images of PS3.7K/PCL (50/50) blend films with thickness about 15 nm annealed at 70 °C for 360 min. (a) AFM height image of the dewetting fingers trials on the substrate and (b) zoom-in height image of (a).

The introduction of a crystallization phase into the blend film leads to the formation of PCL crystal aggregation domains (light blue part in Figure 7a′) embedded in the segregation PS3.7K domains (gray part in Figure 7a′), inducing the wettability and film stability of the blend film to be anisotropic. During annealing, the holes nucleation occurs selectively in the region of the less stability PS3.7K segregation domains (Figure 7b,b′), as observed in Figure 2. Over time, the holes grow selectively along the directions of the arrows at the interval of dendritic PCL crystal of the PS3.7K domains with less resistance (Figure 7c), resulting in noncircular and faceted shapes. Further annealing, however, rims at the perimeter of the holes are formed, which are instability for the ultrathin polymer films,36 leading to the fingers pattern formation (Figure 7d). Row by

the dewetting fingers and leaves on mica as small drops along the drawing back line of the fingers forming the unique morphology (Figure 6a,b). On the other hand, the small drops of the trails may be the result of the autophobic dewetting of the films leaving an unstable mesolayer and leading to the formation of the nanodroplets due to entropic effects. The precise component and reasons of the nanodroplets formation needed further investigation. In order to illustrate the determined effect of dendritic PCL crystal embedded in thin PS films structure on the film dewetting mechanism and process, and compare this unique dewetting process to that of other crystalline polymer blend 4936

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row of small PCL drops, which indicates the rails of the dewetting fingers, at front of the fingers are not marked out in the schematic. In summary, the introduction of PCL into PS3.7K film leads to the wetting ability and stability of the blend films to be anisotropic, resulting in the dewetting process to be quite different from the formation of normal circular holes in the spontaneous dewetting of thin PS films, and the morphology evolution process of PS54K/PCL and PMMA9K/PEO6K (50/50) blend films. The holes nucleation and growth occurs selectively in the regions and directions of PS3.7K segregation domains, forming the noncircular and faceted holes. Over time, however, noncircular holes with fingers are obtained due to the rim instability at the perimeter of the holes. This unique dewetting noncircular hole with fingers at fronts, on one hand, enriches the dewetting morphology of polymer blend films but also, on the other hand, offers a new perspective to study dewetting mechanism of polymer blend films. To further study the effect of substrate wettability on tuning the dewetting process and morphology, the investigation of the morphology evolution of PS/PCL and PMMA/PEO blend films on silanized (trimethylchlorosilane) modified substrate is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, XPS results of the PS/PCL blend films, and further information on the dewetting morphology evolution of PMMA9K/PEO6K blend films during annealing. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax 086-28-85405401; e-mail [email protected] (F.C.), [email protected] (Q.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (grants 50973068 and 51121001) and Cultivation Found of the Key Scientific and Technical Innovation Project (grant 708076), Ministry of Education of China.

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