Effect of Film Thickness on Morphological Evolution in Dewetting and

Sep 21, 2011 - In this Article, the morphological evolution in the blend thin film of polystyrene (PS)/poly(ε-caprolactone) (PCL) was investigated vi...
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Effect of Film Thickness on Morphological Evolution in Dewetting and Crystallization of Polystyrene/Poly(ε-caprolactone) Blend Films Meng Ma, Zhoukun He, Jinghui Yang, Feng Chen,* Ke Wang, Qin Zhang, Hua Deng, and Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China

bS Supporting Information ABSTRACT: In this Article, the morphological evolution in the blend thin film of polystyrene (PS)/poly(ε-caprolactone) (PCL) was investigated via mainly AFM. It was found that an enriched two-layer structure with PS at the upper layer and PCL at the bottom layer was formed during spinning coating. By changing the solution concentration, different kinds of crystal morphologies, such as finger-like, dendritic, and spherulitic-like, could be obtained at the bottom PCL layer. These different initial states led to the morphological evolution processes to be quite different from each other, so the phase separation, dewetting, and crystalline morphology of PS/PCL blend films as a function of time were studied. It was interesting to find that the morphological evolution of PS at the upper layer was largely dependent on the film thickness. For the ultrathin (15 nm) blend film, a liquid solid/ liquid liquid dewetting wetting process was observed, forming ribbons that rupture into discrete circular PS islands on voronoi finger-like PCL crystal. For the thick (30 nm) blend film, the liquid liquid dewetting of the upper PS layer from the underlying adsorbed PCL layer was found, forming interconnected rim structures that rupture into discrete circular PS islands embedded in the single lamellar PCL dendritic crystal due to Rayleigh instability. For the thicker (60 nm) blend film, a two-step liquid liquid dewetting process with regular holes decorated with dendritic PCL crystal at early annealing stage and small holes decorated with spherulite-like PCL crystal among the early dewetting holes at later annealing stage was observed. The mechanism of this unusual morphological evolution process was discussed on the basis of the entropy effect and annealing-induced phase separation.

’ INTRODUCTION Polymer thin films on solid substrate are used for a number of technological applications that range from the fabrication of microelectronics and sensors1,2 to novel drug delivery systems3 and biocompatible coatings.4 The film instability is an important factor for its technological applications. The unstable process is called the dewetting process. In the past two decades, a lot of work has been done on the dewetting process by experiments, simulations, and theories.5 11 Two main mechanisms are commonly proposed for dewetting: (1) nucleation and growth, and (2) spinodal dewetting. In the nucleation and growth mechanism, holes formation is initiated by an impurity or a defect in the film.12 In contrast, the breakup of films by spinodal dewetting is initiated by capillary waves on the surface that arise from density fluctuations in the polymer film.13 Another unique dewetting phenomenon between molecules with identical chemical composition is called autophobic dewetting.14 It is believed that autophobic dewetting is solely driven by entropy15 17 due to the fact that the conformation entropy of the layer contacting the substrate surface is greatly reduced as compared to the adjacent layer in the rest of the film. Recently, the coupling of dewetting and phase separation in polymer blend thin films is also of great interest because of technological applications as well as fundamental r 2011 American Chemical Society

importance.18 For example, Stamm et al.19 21 have extensively investigated thin films of weakly incompatible blends of poly (p-methylstyrene) (PpMS) and deuterated polystyrene (dPS), and a bilayer structure was found to be formed via phase separation, and the upper layer of PpMS dewetted on the bottom layer of deuterated polystyrene. In a study of a thin film of fourarm poly(4-vinylbenzyl chloride) (pVBC) and linear polystyrene (PS) blends, a bilayer structure was also formed during spincoating, carried out by An et al.22 Both spinodal and heterogeneous nucleation dewetting processes have been observed. The blend film shows a richer dynamics than a one-layer system as a result of the interplay of phase separation and dewetting processes during annealing. Recently, the influence of the composition and thickness on the phase separation and dewetting behaviors has been studied extensively for the blend thin films of poly(styrene-ran-acrylonitrile) (SAN) and poly(methyl methacrylate) (PMMA), 23 32 polystyrene (PS) and poly(vinyl methyl ether) (PVME).33 38 Different physical processes, such as dewetting phase separation/wetting, dewetting/wetting phase Received: September 15, 2011 Revised: September 19, 2011 Published: September 21, 2011 13072

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Langmuir separation, and phase separation/wetting pseudodewetting, as a result of the coupling of phase separation and dewetting, have been observed by changing the film thickness and composition, which determine the relative rates of phase separation and dewetting.29 32 A dewetting process induced by composition fluctuation was also observed for the ultrathin blend film.28,30,32,34 36 Although a lot of papers on the phase separation and dewetting process of blend film have been reported, to the best of our knowledge, there are few reports on the morphology evolution of crystalline polymer blend film.39 42 For the crystalline blend film, the coupling of the phase separation, dewetting, and crystallization processes may cause the morphological evolution processes to be very complicated. Particularly, the film thickness and crystal structure may lead to quite different initial states in the blend films, which in turn have a great effect on the later annealing-induced morphology evolution process. In this Article, we add a crystalline polymer poly(ε-caprolactone) (PCL) into the amorphous polystyrene (PS) film to study the morphology evolution of their blend films on the mica wafer. It is found that the addition of PCL leads to the formation of a bilayer with PCL crystalline layer at the bottom during spin-coating, due to its favorable interaction with mica wafer.43 By changing the blend film thickness, different dewetting processes and morphologies were observed, especially for the ultrathin and thick blend films. For an ultrathin (15 nm) blend film, liquid solid/liquid liquid dewetting wetting process was observed, forming ribbons that rupture into discrete circular PS islands on voronoi finger-like PCL layer. For relatively thicker (60 nm) blend films, a two-step liquid liquid dewetting process with different PCL crystal decorating the dewetting holes was found during annealing. The understanding of the complex morphology evolution process provides the effective method to control the blend film surface composition and morphology, which are very important for application.

’ EXPERIMENTAL SECTION Materials. Polystyrene (Mw = 54 000 g/mol, Mw/Mn = 1.04) used in this work was purchased from Polymer Source, Inc. Poly(ε-caprolactone) (Capa6800, Mw = 50 000 g/mol) was obtained from Perstorp Co., Ltd., Sweden. Glass transition temperatures of the PS and PCL, measured by differential scanning calorimetry, were 100 and 60 °C, respectively. All of the materials were used as received. Film Preparation and Characterizations. The blend composition was fixed to PS/PCL = 50/50. After being dissolved in toluene, the thin blend films were prepared by spin-coating the toluene solution onto the mica wafers at 3000 rpm for 30 s, using a commercial spin-coater KW-4A (XingYouYan, China). Prior to spin-coating, the blend solution was put at room temperature for 30 min to ensure that the polymer is completely dissolved. The blend film thickness was controlled by tuning the concentration of the solution as well as the spinning speed, and was measured with AFM by scratch method. After spin-coating, the residual solvent was removed by putting the films in a vacuum oven for 24 h at room temperature, and the surface morphology of the films was observed by atomic force microscopy (AFM) operating in a tapping mode using a instrument with a SPI4000 Probe Station controller (SIINT Instruments Co., Japan) at room temperature. The height as well as phase contrast images were collected at the same time. Olympus tapping mode cantilevers with the spring constants ranging from 51.2 to 87.8 N/m (as specified by the manufacturer) were used with a scan rate in the range 1.0 2.0 Hz. The samples were then annealed at 120 or 140 °C for different time in a vacuum oven and then quenched to room temperature to be observed by AFM. The surface chemical composition of the films was determined on a XSAM800 multifunctional surface analyze

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equipment using monochromatized Al Ka rays (1486.6 eV) under the circumstance of 12 kV  15 mA, Kratos, Inc., at room temperature and at 2  10 7 Pa. Binding energies were referenced to the hydrocarbon peak at 284.8 eV. The takeoff angle was 20° with sampling depths of approximately 6 10 nm. High-resolution C1s spectra were decomposed into various contributions from carbon atoms that experience different chemical shifts due to their different chemical environments.

’ RESULTS AND DISCUSSION Crystal Morphology of PCL and Its Blend Films with Different Thicknesses. Figure 1a c shows the crystal morphol-

ogy of PCL films after being crystallized at room temperature for 24 h with thicknesses of about 7 ( 1, 15 ( 3, and 30 ( 3 nm, respectively. From these AFM height images, it is seen that the 30 nm thickness PCL film (Figure 1c) exhibits spherulitic crystals with a radial centrosymetric organization. No Maltese cross extinction images are observed from polarized light microscopy (PLM), which indicates that they are not intact spherulites. This is consistent with Mareau’s studies.44 It is reported that the number of overgrowth lamellae with different orientation decreases and the PCL chains are oriented normal to the substrate surface resulting in flat-on lamellae with the decreasing of the film thickness. Figure 1b shows that a single layer PCL lamella with a dendritic pattern is formed when the film thickness decreases to 15 nm, and the roughness decreases greatly as compared to that of the 30 nm thickness PCL crystal. The diffusion-controlled45 flat-on finger-like PCL crystal with lamellae thickness of about 11 nm is obtained, as shown in Figure 1a, when the polymer concentration decreases to 0.25 wt %. Figure 1d f shows the AFM height images of the different thickness blend films. Only finger-like, dendritic, and spherulitelike crystal morphologies just like the PCL crystal in Figure 1a c are observed. The XPS analysis of the blend film shows that there is not the ester peak of PCL in the XPS spectra on the film surface and the shakeup peak of π π* is clear at 291.5 eV (Figure S1). This indicates that PS enriches on the surface in 6 10 nm of XPS analyzed thickness for the blend film. Thus, an enriched twolayer structure, in which the amorphous PS-rich layer located on the PCL-rich layer, is formed during spin-coating, and the upper PS-rich layer has little influence on the PCL crystallized process.43 So the crystal morphology can also be observed from the blend films surface, and the low PCL crystal morphology is only determined by the PCL concentration in the solution, as shown in Figure 1d f. The typical diffusion-controlled crystal morphology in the blend films is shown in Figure 1d. The upper PS has a confine effect on the diffusion of PCL chains especially for the ultrathin blend films. So as compared to the pure PCL film with the same concentration (Figure 1a), the width of the crystal becomes larger and the edge becomes rough for the thin blend film (Figure 1d). For the blend film of PS/PCL, three different unique morphologies of an enriched two-layer structure with finger-like, dendritic, and spherulite-like PCL crystal underneath were constructed during spin-coating by adjusting the blend film thickness(Figure 1d f). These different initial states may cause the morphology evolution processes to be quite different from each other during annealing, which was studied as follows. Morphology Evolution of the Ultrathin (15 ( 2 nm) Blend Films. The PS domains and the PCL domains covered by a thin PS layer are separate in contact with the mica substrate in the film plane; as shown in Figure 1d, the PS domains are located at the interval of the finger-like PCL crystal forming the “two interface” 13073

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Figure 1. AFM height images of PCL (a c) and PCL/PS (50/50) (d f) blend films crystallized at room temperature for 24 h after spin-coating at 3000 rmp with different PCL concentrations: (a,d) 0.25 wt %, (b,e) 0.5 wt %, and (c,f) 1 wt %.

system, air/PS+PCL/mica. The slash represents the interface, and “PS+PCL” is defined as the blend film layer with PS domains and PCL domains separately contacting the substrate. Figure 2 shows the dewetting and crystal morphologies after being annealed at 120 °C for different times. Holes with irregular shape rupturing into the substrate and worm-like holes decorated with PCL crystal are obtained on the surface of the blend films after annealing for 2 min (Figure 2a,b and Figure S2a). The layer of PS on the mica is unstable, so the holes extending into the substrate first nucleate at the interval of the finger-like PCL crystal because there is no PCL layer under the PS layer at this place (Figure 1d). Prolonging the annealing time, PS dewets from the substrate to aggregates on the top of the low PCL layer with parts of it exposed, and thus the irregular and worm-like holes increase in size (Figure 2c,d and Figure S2b). The polar interaction between PCL and mica causes the aggregated PCL layer to wet the substrate, making the dewetted PS holes from the mica to be wetted by PCL; as shown in Figure 2e,f and Figure S2c, smaller holes with regular shape and aggregation of PS on top of the low PCL crystal layer are obtained for the samples annealed 135 min. Figure 2g,h and Figure S2d show the unique PS ribbons embedded in the low comb-like PCL crystal with smaller and roundness holes structure. This indicates that most of the PCL is aggregated around the PS domains, crystallization nucleates at this place, and other PCL chains diffuse to the crystal fronts, forming the low comb-like PCL crystal. The PS ribbons on the top of PCL are not stable from the point of thermodynamics due to Rayleigh instability.46 49 Figure 3 shows the eventually thermodynamics stable morphology of the sample. PS islands with diameter 2 3 μm (Figure 3d) embedded in the low voronoi finger-like PCL crystal structure are obtained (Figure 3a c). The underlying single PCL lamellae crystal has a thickness of ∼10 nm, which is in accord with the sample in Figure 1a. This unique structure varies little with the annealing times, as shown in Figure S3. For this sample, it is the unique initial state (Figure 1d) that determines the

unique liquid solid dewetting of PS from the mica, liquid liquid dewetting of PS from the PCL layer, and then the PCL wetting the mica substrate physical processes. Morphology Evolution of the Thin (30 ( 3 nm) Blend Films. A “three interface” system, air/PS/PCL/mica, with an enriched two-layer structure and a single dendritic PCL crystal layer at the bottom is constructed in the blend film prepared by spin-coating the 1 wt % PS/PCL (50/50) solution onto the mica wafers (Figure 1e). Figure 4 gives the evolution of the blend film root-mean-roughness (rms) and dewetting area with the annealing time at 140 °C. For short annealing times less than 150 min, the rms and the dewetting area of the blend films increase quickly to 30 nm and 50%, respectively. It is observed that scattered and irregular holes with quite different size form immediately upon annealing the uniform blend film (Figure S4a). There is low phase contrast between the holes and the intact regions (Figure S5), which most likely arises from the dewetting of only the upper PS layer from the underlying PCL layer without exposing the mica substrate. The lateral dimensions of the holes increase with increasing annealing times (Figure S4b,c). For long annealing times of more than 150 min, the rms and dewetting area of the blend film change little with annealing time. Furthermore, ribbons of PS on the surface of the PCL crystal morphology (Figure S4d) are observed for the blend films that are annealed for 480 min. Upon annealing the blend films at 140 °C for substantially longer times, >720 min, the PS ribbons rupture due to Rayleigh instability,46 49 resulting in discrete circular PS islands on the film surface (Figure S4e), which is stable in thermodynamic. So the rms and dewetting area become constant with the annealing time. Closely observing, there are some isolated dewetting holes in the low layer (Figure S4e), which is the result of the defectinduced dewetting of the PCL layer.12 An enlargement of the AFM images provides an in-depth understanding of the morphological evolution in the blend film. Figure 5 shows AFM images of the blend film annealed at 140 °C 13074

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Figure 2. AFM height images of PS/PCL (50/50) blend films with thickness of about 15 nm annealed at 120 °C for (a) 2, (c) 15, (e) 135, and (g) 360 min. Corresponding AFM phase images with annealing time of (b) 2, (d) 15, (f) 135, and (h) 360 min.

for 20 min. The blend film forms isolate irregular holes with low phase contrast surrounded by rims immediately upon annealing (Figure 5a,b,d). The rim of the hole is clearly visible in Figure 5c, which shows a cross section of an AFM image. Close inspection of the rim profile reveals that the rim decreases to the thickness of

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the unperturbed film via a trough. The presence of this undulatory rim profile, which in very thin films may lead to the formation of so-called satellite holes,50 was investigated in previous studies and believed to be present only in films of polystyrene with very low molecular weight.51 Our experiments show that when the substrate is liquid, the characteristic trough outside the rim is maintained even for higher molecular weight polymers. The trough may be the result of the interface deforming out the plane between the upper PS layer and the low PCL layer, forming a trench of PS extended into the low PCL layer at the advancing dewetting rims due to the fact that there are very different viscosities of the two components for the liquid liquid dewetting systems.52 In the dewetting holes, the single dendritic PCL crystal layer of the same as that of the pure PCL (Figure 1b) is clearly observed (Figure 5a,b and Figure S5b). This provides direct evidence for the upper PS layer dewetting from the low PCL layer during annealing above the Tg of polystyrene. Figure 6 shows the AFM images of the blend films after annealing at 140 °C for 720 min. Discrete circular PS islands with diameter of about 5 μm embedded in the single lamellar PCL dendritic crystal are formed (Figure 6a and Figure S5e). The dendritic PCL crystal with thickness of ∼11 nm is clearly found around the PS domains (Figure 6b,c). These results also illustrated the liquid liquid dewetting of upper PS layer from the low PCL layer. During spin-coating, for the blend film with thickness about 30 nm, a “three interface” system, air/PS/PCL/mica, with clear interface between the PS and PCL is formed due to the preferential interaction between PCL and the substrate, and the low surface energy PS wetting the polymer air interface. The morphology is not stable from the point of thermodynamics because the PS layer is unstable on the top of PCL layer due to the fact that the spreading coefficient S of the PS on PCL is negative.53 So it dewets from the PCL layer with no effect on the low PCL crystal morphology (Figure S5e). The PCL has great interaction with the polar mica substrate, so it is stable on the substrate, and there is no effect of annealing on the PCL crystal morphology except some defects-induced dewetting points.12 Two-Step Liquid Liquid Dewetting Process of the Thicker (60 ( 5 nm) Blend Films. A “four interface” system, air/PS/PSPCL/PCL/mica, is formed for the thick blend film (Figure 1f). The PS-PCL is defined as the blend layer with a composition gradient along the film thickness. The effect of polymer air and polymer substrate interaction on the phase separation into the “three interface” system, air/PS/PCL/mica, is limited to the contacting surface. So an interface layer of PS-PCL between PS and PCL layer forms and thickens with the increasing of the film thickness, making the dewetting and phase separation process complicated for the relative thicker blend films. Figure 7 shows a series AFM image of the blend film with thickness of about 60 ( 5 nm annealed at 140 °C as a function of time. After 20 min annealing, no features are observed (Figure 7a). As compared to Figure 1c, the surface becomes smooth, and the boundary of the spherulite is too obscure to find. This indicates that the blend film becomes stable with the increase of the film thickness. The typical nucleation and growth dewetting process leading to scattered holes with different size and low rims surrounding is observed (Figure 7b) when the films are annealed at 140 °C for 60 min. By closely inspecting the holes, a single dendritic PCL lamellae crystal is found in the dewetting holes, as the enlargement image is shown in Figure 7b. This indicates that it is the upper wetting PS layer combined with the PS-PCL layer that dewets from the PCL layer at the early annealing. From the cross-section profile 13075

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Figure 3. (a) AFM height image, (b) zoom-in height image of (a), (c) AFM phase image of PS/PCL (50/50) blend films with thickness about 15 nm annealed at 140 °C for 5 min, and (d) the corresponding cross-sectional profiles along the black line in (b).

Figure 4. The evolution of rms and dewetting area with annealing time at 140 °C for the PS/PCL (50/50) blend films with thickness of about 30 nm.

(Figure S6), the intact upper layer is ∼47 nm higher than the dewetting holes plane; this also illustrates that there is only a single PCL lamellae with thickness of about 11 13 nm leaving in the holes. Prolonging the annealing times led to the size of the holes to increase and the coalescence of the rims resulting in the formation of interconnected rim structures (Figure 7c). Yet by carefully inspecting the dewetting morphology, we found except for the big holes with interconnected rim around them, there are many smaller nucleation holes in the rims. Dendritic and spherulitelike crystal, which corresponded to different PCL thicknesses, is obtained in the big and small holes, respectively (Figure 7e,f). It is clear that the smaller holes on the rims are formed during the later annealing stages. Our results above clearly demonstrate a two-step dewetting process for the 60 nm thick PS/PCL blend films on mica

substrates: for short annealing times, it is the autophobic dewetting of the upper wetting PS layer combined with the PS-PCL layer that dewets from the PCL layer (region A in Figure 7d,e). It is widely accepted that autophobic dewetting is primarily driven by the entropy effect due to the fact that the conformation of the polymer layer contacting the substrate surface is quite different from that of the polymer layer at the top of the film. Han et al.54 reported autophobic dewetting in solvent annealed polymethylmethacrylate (PMMA) thin films on the silicon substrates with native SiOx layers because the layer in contact with the substrate was formed by highly stretched chains due to the preferential attraction of PMMA to the substrate, which greatly reduced the conformation entropy as compared to the solvent swelling adjacent layer in the rest of the film. Reiter and Sommer55 found that the strong pinning interactions between the oxide of the UV-treated oxidized silicon and poly(ethylene oxide) significantly reduced the conformational entropy of chains next to the surface, leading to the autophobic dewetting in poly(ethylene oxide) films during annealing. In our system, PS/PCL blend film was spin-coated on the mica substrate. The polar PCL has a preferential interactive with mica substrate, so a “four interface” system, air/PS/PSPCL/PCL/mica, is formed for the system through vertical phase separation during spin coating. The strong pinning interactions between the PCL and mica greatly reduced the conformational entropy of the chains of PCL layer next to the surface. Moreover, the PS chains in the interface layer of PS-PCL have a swelling effect just like that of a solvent, which makes the PS-PCL layer incompatible with the PCL layer. All of these reasons can lead to autophobic dewetting very likely as reported by others.54 58 For long annealing time, the broad blend interface layer of PS-PCL phase separated along the direction of the film thickness, thoroughly forming a clear interface between PS and PCL, causing the “four interface” system, air/PS/PS-PCL/PCL/mica, to transform into the “three interface” system, air/PS/PCL/mica. At later annealing time, the interface instability between the new formed PS 13076

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Figure 5. (a) AFM height image, (b) zoom-in height image of the square box in (a), and (d) AFM phase image of PS/PCL (50/50) blend films with thickness of about 30 nm annealed at 140 °C for 20 min. (c) Representative line profile along the black line in (b).

Figure 6. AFM height image (a) dewetting and (b) PCL crystal morphology of the PS/PCL (50/50) blend films with thickness about 30 nm annealed at 140 °C for 12 h. (c) Representative line profile along the black line in (b). 13077

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Figure 7. AFM height images of the PS/PCL (50/50) blend films with thickness of about 60 nm annealed at 140 °C for (a) 20, (b) 60, (c) 180, and (d) 720 min. (e) Zoom-in height image of the square box in (d), and (f) zoom-in height image of the square box in (e).

Figure 8. AFM height images of the PS/PCL (50/50) blend films with thickness of about 80 nm annealed at 140 °C for (a) 180 min, (b) zoom-in height image of the square box in (a), and (c) zoom-in height image of the square box in (b).

layer and PCL layer due to the fact that the spreading coefficient S of the PS on PCL is negative53 leads to the second liquid liquid dewetting process of PS from PCL among the first dewetting holes (region B in Figure 7d). The dewetting morphology is also studied for thicker blend film with thickness of about 80 ( 8 nm, as shown in Figure 8. Several holes decorated with a thickness of 10 12 nm dendritic PCL crystal, the same as the sample with 60 nm thickness at early annealing time, are obtained in the blend film after being annealed at 140 °C for 180 min. Apparently, it is the first autophobic liquid liquid dewetting process at this annealing stage, and the thickness difference (Figure S7) also illustrates this mechanism. The thickness of the adsorbed PCL layer contacting the mica is invariant with the increase of the blend film thickness, which only leads to the thickening of the PSPCL layer and does not change the initial structure of the “four interface” system, air/PS/PS-PCL/PCL/mica. Thus, the same dewetting mechanism is followed even for blend film with thickness

more than 60 nm. Further annealing induces the blend film to phase separate along the direction of the film thickness into two layers with clear interface. So the second step liquid liquid dewetting of PS layer from PCL layer happens in the later annealing process among the first dewetting rims, leaving spherulitelike PCL crystal in the small holes (Figure 7e). Schematic Representation of the Evolution of Dewetting and Crystal Morphologies for the PS/PCL Blend Films with Different Thicknesses. The introduction of a crystallization phase into the blend film may cause the film initial state to be quite different by changing the film thickness, which in turn will have a great effect on the later annealing-induced morphology evolution process. For our system of the PS/PCL blends, during annealing, the dewetting morphology evolution for the blend films with different thickness is described schematically as shown in Figure 9. The dewetting and crystal morphology of blend films varies with the increase of film thickness (15, 30, and 60 nm), 13078

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Figure 9. Schematic representation of the evolution of dewetting and crystal morphologies during annealing and crystallization at room temperature for PS/PCL (50/50) blend films with different thicknesses: (a) ultrathin, 15 ( 2 nm; (b) thin, 30 ( 3 nm; and (c) thick, 60 ( 5 nm.

including three characteristic morphology evolution processes, that is, liquid solid/liquid liquid dewetting wetting, liquid liquid dewetting, and two-step liquid liquid dewetting, respectively. For the ultrathin (15 nm) blend film, the PS layer at the interval of the finger-like PCL crystals first ruptures into the substrate and then dewets to aggregate on the top PCL layer forming discrete circular PS islands. At the same time, the PCL layer wets the substrate making the holes to smaller and rounder, forming voronoi finger-like PCL crystal morphology underneath (Figure 9a). For the thin (30 nm) blend film, by prolonging the annealing time, the number and lateral dimensions of the irregular holes in the upper PS layer increase, forming interconnected rim structures, which rupture into discrete circular PS islands embedded in the single lamellar PCL dendritic crystal due to Rayleigh instability finally (Figure 9b). This is the result of the liquid liquid dewetting of the upper PS layer from the underlying adsorbed PCL layer process. For the thick (60 nm) blend film in short annealing times, the entropy effect causes the first-step autophobic liquid liquid dewetting of the upper PS wetting layer combined with the PS-PCL interface layer leaving a ∼10 nm single dendritic PCL crystal in the regular holes. At later annealing times, small holes decorated with spherulite-like PCL crystal are constructed among the interconnected rims. Annealing induces the phase separation of the interface layer along the film thickness direction, leading to the “four interface” system turning into the “three interface” system, and then the capillary fluctuations at the interface between PS and PCL dominate the film instability process, resulting in the second-step liquid liquid dewetting process of the upper PS layer from the low continuous PCL layer among the first-step dewetting holes (Figure 9c). The blend film

composition in the film plane and along the film thickness direction varies greatly with the increasing film thickness. Three different unique morphologies of an enriched two-layer structure with finger-like, dendritic, and spherulite-like PCL crystal underneath are formed in the blend film due to the coupling of phase separation and crystallization. These different initial states lead to the film instability driving forces to be quite different from each other in various conditions and at various stages during annealing, resulting in the dewetting mechanism transformations. The dewetting mechanism transforms from the liquid solid/ liquid liquid dewetting wetting into the liquid liquid dewetting process when the mica substrate is covered by a single lamellae of PCL crystal completely (Figure 1e). With the increasing of the blend film thickness, the interface layer of PS-PCL is generated, and its thickness thickens when the blend film thickness is more than 30 nm, leading to the dewetting mechanism transformation into the two-step liquid liquid dewetting process.

’ CONCLUSIONS During spin-coating, for the blend film PS/PCL (50/50), an enriched two-layer structure with different kinds of crystal morphologies, such as finger-like, dendritic, and spherulitic-like, was constructed in the low PCL layer via vertical phase separation and crystallization by changing the solution concentration. The different initial states lead to the different morphology evolution processes and the transformation of dewetting mechanism with the thickening of the blend film thickness. For the ultrathin blend film (15 nm) annealed at 120 °C, 2 3 μm discrete circular 13079

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Langmuir PS islands decorated with voronoi finger-like PCL crystal structure are formed in the blend films through the liquid solid/ liquid liquid dewetting wetting physical process. Annealed at 140 °C, for the thin (30 nm) blend film, only the liquid liquid dewetting process of the upper PS layer from the low PCL layer is observed, and discrete circular PS islands embedded in a single lamellar PCL dendritic crystal structure are formed in the blend films eventually. For the thicker (60 nm) blend films, the unique composition gradient along the thickness direction of the blend film brings about two nonwettable interfaces in this system at different stages of the annealing process: one is between the broad interface layer and the adsorbed PCL monolayer at the early stage, and the other is between the low PCL layer and the upper PS layer at later annealing process. As a result, a two-step dewetting process was observed: the first step included autophobic liquid liquid dewetting of the upper PS layer combined with the PS-PCL layer from the adsorbed PCL layer at the early stage, and the second step included liquid liquid dewetting of the vertical phase separation to give a new clear interface between PS and PCL among the first dewetting rims, resulting in different PCL crystal morphology corresponding to different thicknesses of the low PCL layer.

’ ASSOCIATED CONTENT

bS

Supporting Information. XPS data of the PS/PCL (50/50) blend film with thickness of 30 nm, data on the larger area AFM height images of PS/PCL blend films, morphology evolution, and thickness of the upper wetting parts. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: 086-28-85405401. E-mail: [email protected] (F.C.); [email protected] (Q.F.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant no. 50973068) and the Cultivation Fund of the Key Scientific and Technical Innovation Project (grant no. 708076), Ministry of Education of China. ’ REFERENCES (1) Chabinyc, M. L.; Wong, W. S.; Salleo, A.; Paul, K. E.; Street, R. A. Appl. Phys. Lett. 2002, 81, 4260. (2) Thomas, S. W.; Amara, J. P.; Bjork, R. E.; Swager, T. M. Chem. Commun. 2005, 36, 4572. (3) Grayson, A. C. R.; Voskerician, G.; Lynn, A.; Anderson, J. M.; Cima, M. J.; Langer, R. J. Biomater. Sci., Polym. Ed. 2004, 15, 1281. (4) Ratner, B. J. Biomed. Mater. Res. 1993, 27, 837. (5) Sharma, A.; Reiter, G. J. Colloid Interface Sci. 1996, 178, 383. (6) Brochard-wyart, F.; Debregeas, G.; Fondecave, R.; Martin, P. Macromolecules 1997, 30, 1211. (7) Jacobs, K.; Seemann, R.; Schatz, G.; Herminghaus, S. Langmuir 1998, 14, 4961. (8) Becker, J.; Grun, G.; Seemann, R.; Mantz, H.; Jacobs, K.; Mecke, K.; Blossey, A. Nat. Mater. 2003, 2, 59. (9) Luo, H.; Gersappe, D. Macromolecules 2004, 37, 5792. (10) Gabriele, S.; Sclavons, S.; Reiter, G.; Damman, P. Phys. Rev. Lett. 2006, 96, 156105.

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