Direct Observation of the Relief Structure Formation in the Nearly

Nov 14, 2012 - Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Ningbo 315201, People's Republic o...
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Direct Observation of the Relief Structure Formation in the Nearly Symmetric Poly(styrene)-block-poly(ε-caprolactone) Diblock Copolymer Thin Film Peng Zhang,† Zongbao Wang,‡ Haiying Huang,† and Tianbai He*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Ningbo 315201, People’s Republic of China S Supporting Information *

ABSTRACT: The relief structure formation associated with microphase separation and dewetting of the nearly symmetric poly(styrene)-block-poly(ε-caprolactone) diblock copolymer thin film was studied in this work, for which a suite of complementary methods, namely atomic force microscopy, optical microscopy, and X-ray photoelectron spectroscopy combined with hot stages, were applied. Through control of the microphase separation strength, indicated by the χN (where χ and N indicate the Flory−Huggins interaction parameter and the total degree of polymerization, respectively), varied relief structures were observed. On one side, when there was no microphase separation (χN = 3.9), typical droplets resulting from autophobic dewetting were revealed. On the other side, when there was intermediate microphase separation (χN ≥ 14.0), superimposed lamellae except for droplets was discerned. Moreover, with continual heating, the formation of the superimposed lamellae and its dynamic transition to ordered droplets were first revealed with the in situ AFM scanning. On the basis of these findings, we conclude that the superimposed lamella is a metastable structure, resulting from the coupling of dewetting and microphase separation, and it finally reaches the equilibrium droplets. The formation of superimposed lamellae was attributed to that the microphase separation strength was forced to yield to the minimization tendency of surface tension.

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value.1,3,23 It is believed that terracing, referring to the nucleation and subsequent growth of the surface relief structure, is a dynamic coarsening process. The driving force for terracing is the tendency to minimize the total length of the terrace edges.2,23 Meanwhile, autophobic dewetting is another frequently mentioned phenomenon in the block copolymer thin film. The occurrence of autophobic dewetting was due to the selective interactions between molecules and substrate lead to entropy difference between the chain conformations of the adsorbed and free molecules.9,11,15,24,25 For instance, Limary et al.25,26 studied the autophobic dewetting of PS-b-PMMA thin film, in which free molecules collapsed into droplets while the adsorbed molecules formed the wetting layer. Thus, the relief structure formation in block copolymer thin film is closely associated with terracing and dewetting. Moreover, in a recent work, Croll et al.8 reported that the internal order resulting from microphase separation had significant influence on the rupture of the thin film, for example, dominant decreased rate of film rupture was found in the system with lamellae order.

INTRODUCTION Chasing after equilibrium state due to free energy minimization promotes the structure reorganization in polymer thin films which attracts the interest of scientists and engineers in recent years. Among which, the dynamics of microphase separation1−3 and the relation between dewetting and film stability4−12 were investigated extensively as mentioned in the previous publications. An interesting finding was the formation of superimposed or terraced lamellae, which was generally mentioned in block copolymer thin films, such as poly(butadiene)-block-poly(ethylene oxide),13 poly(butadiene)block-poly(ε-caprolactone),14 poly(styrene)-block-poly(2-vinylpyridine),15 poly(styrene)-block-poly(4-vinylpyridine),16,17 and poly(styrene)-block-poly(methyl methacrylate) (PS-bPMMA).18−20 Because of the complex coupling and competition between dewetting and microphase separation, the formation of superimposed lamellae and its transition mechanism are yet unclear.17,21,22 Therefore, an intensive study is needed to disclose the relief structure formation process. It is generally recognized from numerous studies of the block copolymer thin film that island−hole structure results from the mismatching of the film thickness and bulk periodicity © 2012 American Chemical Society

Received: July 23, 2012 Revised: October 14, 2012 Published: November 14, 2012 9139

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solution, a mixture of concentrated sulfuric acid and hydrogen peroxide (70/30, v/v), for about 30 min at 120 °C to generate a clean, hydrophilic oxide surface. The substrate was then rinsed with a large volume of distilled water and then purged with dry nitrogen flow. Polymer thin films were obtained by spin-coating a block copolymer solution in toluene (5 and 10 mg/mL) onto the substrate at 2000 rpm for 30 s. The as-cast thin films were set in vacuum environment at room temperature for 24 h to remove the remaining solvent. The initial film thickness casting from 5 and 10 mg/mL toluene solution were 24 and 43 nm, respectively. In addition, for the thermal annealed sample (at 120 °C for 24 h), one observed a wetting layer with thickness about 10.5 nm lying close to the substrate, determined by scratching the thin film but not the underlying SiOx/Si substrates using a sharp blade. Atomic Force Microscopy (AFM). The AFM data were acquired in air with an Agilent 5500 AFM (Agilent Technologies, Inc., Santa Clara, CA) operated in taping mode. The silicon cantilevers (OMCLAC160TS, Olympus Co.) with a resonance frequency of about 300 Hz and a spring constant of about 42 N/m were used. For hot-stage AFM scanning, the sample was set on a hot plate driven by the Lakeshore 332 temperature controller. Temperature calibration of the hot plate was recalibrated by measuring the melting point of benzophenone laid on silicon wafer. Samples were first imaged at ambient conditions to set the scanning zone. The AFM probe was disengaged before heating. For tracking the transition from the superimposed lamellae to the droplets, the sample was annealed at 90 °C for 414 min before the in situ AFM scanning at 120 °C. This was designed to eliminate the influence of melting of PCL blocks. The free oscillation amplitude was set to 2.0 V, and the amplitude set point was 85%. The room temperature and relative humidity were between 19.8 and 21.1 °C and 19.6 and 23.9%, respectively. Optical Microscopy. Optical images were obtained using a LeicaD2500P microscope, equipped with a Linkam TM600 hot stage and heating rate was 30 °C/min. The surface morphology was observed in reflection using a white light source to obtain interference colors. The levels of pink indicate the local thickness of the thin film, and dark regions indicate thicker portions. The images were captured by the charge-coupled device (CCD) camera. X-ray Photoelectron Spectroscopy (XPS). The XPS measurement was performed in a Thermo ESCALAB 250 spectrometer by means of monochromatic Al Kα radiation, without additional charge compensation. The surface composition was obtained from measurement of the areas of the C 1s and O 1s peaks, obtained at 20.0 eV pass energy in constant analyzer energy mode, at a photoemission angle of 0° (i.e., normal to the substrate surface). The binding energies of the photoelectrons were correlated by the aliphatic hydrocarbon C 1s peak at 284.6 eV. The temperature heating and cooling rate was 5 °C/min. To assess the temperature-dependent transition of the surface composition, the following thermal annealing process was applied. First, the sample was heated to 120 °C and annealed for 30 min; then, it was cooled to 25 °C and annealed for 2 months.

However, there is still no clear illustration on the relation of the late stage developments of terracing and dewetting, although it was theoretically predicted that the late stage development of terrace resembled the formation of the dewetting controlled droplets.2,3,7,24,27 According to the microphase separation strength (χN, where χ and N indicate the Flory−Huggins interaction parameter and the total degree of polymerization, respectively), the microphase separation in symmetric block copolymers is generally categorized into three regimes, namely, weak-separation limit (10.5 < χN < 12.5), intermediate-separation limit (15 < χN < 100), and strong-separation limit (100 < χN).1 χ and N indicate the enthalpic and entropic contributions, respectively, and the stability of the microphase structure is closely related to the annealing temperature because the χ is inversely proportional to the annealing temperature.1,28 In the meantime, thermal means was illustrated to be a powerful tool to explore the dewetting of polymer thin films.5−7,10,11,29 Thus, regulating the temperature seems to be a simple and promising method to study the dynamic process of relief structure formation. In this study, we explored in situ the relief structure formation by a suite of complementary techniques, i.e., atomic force microscopy, optical microscopy, and X-ray photoelectron spectroscopy combined with hot stages. To make the complex competition between dewetting and microphase separation readily to be tracked, the nearly symmetric poly(styrene)-blockpoly(ε-caprolactone) (PS-b-PCL) thin film was chosen as the model because of the following reasons. First, PCL is a crystallizing block, and it was proven that the solution-cast PSb-PCL thin film had crystallized lamellar structure when the relative volume fraction of PCL lay between 34% and 76%.30,31 Second, the melting temperature of PCL is much lower than the order−disorder transition temperature, TODT, of PS-b-PCL, which offered us a wide temperature range to study the coupling and competition between dewetting and microphase separation.9

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EXPERIMENTAL SECTION

Materials. A nearly symmetric PS-b-PCL sample was obtained from Polymer Source, Inc., Canada. The molecular weights and molecular-weight distributions of the diblock copolymer were characterized by gel permeation chromatography. Detailed molecular characteristics are listed in Table 1. The melting point (Tm) of PCL

Table 1. Characteristics of the PS-b-PCL Diblock Copolymer Mn (×103 g/mol) no. of monomers, N volume fraction, f a polydispersity

PS

PCL

9.5 86 0.54 1.13

9 79 0.46 1.13

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RESULTS AND DISCUSSION Coupling of Microphase Separation and Dewetting. Optical Microscopy. The structure transition of the PS-bPCL thin film annealed at 167 °C was real-time observed with optical microscopy, where χN = 3.9 (according to eq 1 of the Supporting Information), and no microphase separation was proposed in the bulk sample of this diblock copolymer. As shown in Figure 1a, isolated holes are observed, indicating they formed during heating. Moreover, we also observed the isolated holes (see Figure S2a of the Supporting Information) in the thin film thermal-annealed at 60 °C. As we all know, 60 °C was above the Tm of PCL but below the Tg of PS. Thus, we inferred that hole nucleation was closely related to melting of the PCL blocks. Kressler et al.32 and Okerberg et al.33 illustrated separately that the coupling of dewetting and melting/ crystallization of PCL would result in hole nucleation and

Calculated from f PS = (wPS/ρPS)/[wPS/ρPS + (1 − wPS)/ρPCL], where f PS, wPS, ρPS, and ρPCL indicate the volume content of PS, weight content of PS, density of PS, and density of PCL, respectively. Here, ρPCL = 1.146 g/cm3 and ρPS = 1.047 g/cm3 were applied. a

block and glass transition temperature (Tg) of PS block were determined using a PerkinElmer DSC-7 at a heating rate of 5 °C/ min. We detected that Tm,PCL and Tg,PS were 58.92 and 97.75 °C, respectively (see Figure S1 of the Supporting Information). Sample Preparation. The single-crystal silicon wafers were supplied by the Shanghai Institute of Ceramics, China. They were cut into strips of about 12 × 12 mm and then treated with “piranha” 9140

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Figure 1. In situ optical micrographs of the thermal annealed 43 nm PS-b-PCL thin film. At 167 °C at different periods, i.e., (a) 0, (b) 13, and (c) 25 min. The scanning area is 82 × 62 μm. At 140 °C at different periods, i.e., (d) at 0, (e) 64, (f) 73, (g) 85, and (h) 95 min. The scanning area is 205 × 154 μm. From panels f to h, the superimposed lamellae are indicated with arrows.

growth on the basis of their works. When the annealing time was extended to 13 min, the dewetting resulted characteristic rim structure was observed in the central big hole (Figure 1b). When the annealing time was further increased, as shown in Figure 1c, the impingement of the adjacent holes was observed. To detect the coupling of microphase separation and dewetting, we set the annealing temperature at 140 °C, where χN = 14.0 and intermediate microphase separation was proposed in the bulk sample. The time-related morphology transition is shown in Figure 1d−h. At 0 min (Figure 1d), isolated holes are observed; this agrees with that as shown in Figure 1a. Both of them reveal that the dewetting of thermal annealed PS-b-PCL thin film belongs to the nucleation and growth mode.5,7,8,10 With the increase of the annealing time, the preformed holes grew up. Among which, one observed a dominant hole formed in the center of the focused zone (Figure 1e). Intriguingly, as indicated by the arrow in Figure 1f, superimposed lamellae formed in the brim of the central hole, and as shown in Figure 1g, it extended circling around the hole. Furthermore, as shown in Figure 1h, the superimposed lamellae collapsed into droplets, which could be explained by the Plateau−Rayleigh instability.26,27,34 Dewetting Dynamics. To study the dewetting dynamics, the central holes of Figures 1b and 1e were chosen, and the change of hole radius (R) with annealing time (t) is shown in Figures 2a and 2b, respectively. By comparison, we observed that the hole growth rate at 140 °C was much lower than that at 167 °C. This phenomenon can be rationalized by the fact that the hole growth rate is proportional to the annealing temperature.10,11,33 Besides, considering there was a residual layer with thickness ca. 10.5 nm (corresponding to half periodicity) lying near the substrate, autophobic dewetting was expected in the thermal annealed PS-b-PCL thin film. Figure 2a shows that the hole growth at 167 °C was fitted with linear function (y ∼ t, the coefficient of determination, R2 = 0.999), while Figure 2b shows that the hole growth at 140 °C was well fitted with power (y ∼ t2/3, R2 = 0.999) and linear (y ∼ t, R2 = 0.998) functions. Clearly, the fitting results in Figure 2b implied that the hole growth experienced two different stages. For the liquid slipped over the substrate, Brochard-Wyart et al.35 proposed that the change from R ∼ t2/3 to R ∼ t indicated the hole growth shifted from strong slippage regime to the

Figure 2. (a) Radius of the central hole in Figure 1b as a function of annealing time. (b) Radius of the central hole in Figure 1e as a function of annealing time.

nonslip or viscous flow regime. The hole growth with R ∼ t2/3 was also observed in the autophobic dewetting of PS thin films.36 Therefore, we can ascertain that the hole growth at 140 °C shifted from strong slippage regime to the viscous flow regime, while that at 167 °C only experienced the viscous flow regime. By connecting the inflection point (ca. 90 min) in Figure 2b with the morphology development in Figure 1h (rim structure is observed at 95 min), the different time-dependent hole growths as reflected in Figures 2a and 2b could be attributed to that the superimposed lamellae formation process was only observed at 140 °C. In other words, the formation of superimposed lamellae occurred in the strong slippage regime. 9141

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Figure 3. Typical AFM height images of the 43 nm PS-b-PCL thin films: (a) as-prepared; (b) at 120 °C at 80 min; (c) at 120 °C at 86 min. The height profiles in panels b and c, showing the height fluctuations along the white lines. The scanning area is 80 × 80 μm.

In the meantime, Green et al.3,26 illustrated that R increased exponentially with t in the early stage of dewetting for PS-bPMMA thin film. However, here it is difficult for us to study the hole nucleation because it occurred during heating. Reviewing the height profile of the 43 nm PS-b-PCL thin film thermal annealed at 60 °C (see Figure S2b of the Supporting Information), we find that the hole depth (20.4 nm) corresponds to the periodicity of PS-b-PCL. Thus, it is confirmed that the microphase separation occurred in the asprepared PS-b-PCL thin film. Furthermore, on the basis of the regularity of the color difference (see the brim of the central hole in Figure 1f), we inferred that the relief structure formation was achieved through lamellar sliding. In the next section, the morphology development was tracked in situ with AFM. Atomic Force Microscopy. AFM combined with hot stage is well recognized as a powerful tool to detect the nanostructure change at elevated temperatures.21,37,38 Figure 3 shows the typical AFM experimental results of the PS-b-PCL thin film. In the as-prepared sample (Figure 3a), crystals cover the surface. For the sample thermal-annealed at 120 °C (Figure 3b), hole with depth ca. 44.6 nm is observed. Moreover, in the lower right of the hole, the hole-in-hole structure with step depth ca. 22.6 nm is discerned. The value of step depth corresponds to the bulk spacing of PS-b-PCL, indicating that microphase separation occurred before the dewetting controlled film rupture. This finding echoes the demonstration offered in the last section. As shown in Figure 3c, a step with depth of 44.6 nm replaces the substeps in Figure 3b. By comparing Figure 3c with Figure 3b, one can find a growing dewetted hole, layer-by-layer from the top to the bottom. In this regime, the enlargement of the hole was achieved through lamellar sliding. This finding agrees well with the optical microscopy experimental results. We also notice that analogous two-step dewetting process was found in poly(styrene)-block-poly(ethylene-co-butylene)-block-poly(styrene) thin film, and the cause was ascribed to the surfaceinduced kinetic difference.39 Besides, the height profiles (insets of Figures 3b and 3c) exhibit that the surface outside the hole shows no distinguishable fluctuation, indicating that it may be comprised of one component. The hole growth was further studied when we sequentially lowered the annealing temperatures to 90, 60, and 50 °C, respectively (see Figure S4 of the Supporting Information),

among which hole nucleation and growth were detected. Moreover, as reflected in the height profiles in Figure S4, the hole depths are about 20 nm, indicating the lamellae maintained the unit periodicity. However, considering PS and PCL blocks gradually turned into glass or supercooled state with the decrease of annealing temperature, these phenomena could not be simply attributed to the effect of microphase separation and dewetting. To give a qualitative study on the transition of the relief structure resulting from the coupling of microphase separation and dewetting, ex situ AFM results are offered in Figure 4. In Figure 4a, one observes that the thermal annealed 43 nm PS-bPCL thin film shows complex features, including holes,

Figure 4. AFM experimental results of the thermal annealed 43 nm PS-b-PCL thin film at 120 °C at 186 min: (a) height image, the scanning area is 25 × 25 μm; (b) height profiles, showing the height fluctuations along the white lines in panel a. 9142

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Figure 5. Typical AFM height images of the thermal annealed 24 nm PS-b-PCL thin film, showing the transition from superimposed lamellae to droplet with continual heating: (a) at 120 °C at 9 min; (b) at 120 °C at 42 min; (c) at 120 °C at 76 min; (d) at 120 °C at 153 min. The inserted height profile shows the height fluctuation along the white lines. Scanning area is 80 × 80 μm. Panels e to f, showing the corresponding 3D patterns of the dashed-line-marked zones in panels a to d. The location of the dirt is marked with arrows in panels e to g.

droplets, and superimposed lamellae. Figure 4b reveals that the thicknesses of the superimposed lamellae have an integral number of the bulk spacing. Therefore, microphase separation always played a role when χN ≥ 14.0. Transition from Superimposed Lamellae to Droplets. In the last section, the superimposed lamellae with each lamella retaining the bulk spacing were revealed in the thermal annealed PS-b-PCL thin film. With continual heating, the equilibrium structure, i.e., droplet, was expected under the function of minimization of surface free energy.22,40 For

example, the droplets are discerned, as indicated by arrows in Figure S5 of the Supporting Information, in the thermalannealed 43 nm PS-b-PCL thin film at 120 °C at 24 h. To detect the transition from the superimposed lamellae to droplets, we examined a 24 nm PS-b-PCL thin film and tracked in situ the morphology development at 120 °C; typical results are shown in Figure 5. The choice of the 24 nm rather than the 43 nm PS-b-PCL thin films was due to the film thickness consideration because we found the superimposed 9143

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minimization tendency of the surface tension is bigger than the microphase separation strength, molecules might be forced to dissociate from the microphase structure and diffuse upward around the dirt. The reorganization could be treated as the relaxation of the surface tension toward its equilibrium state during dewetting.41 Analogous material transfer during terrace formation was mentioned by Heier et al.,42 where they stressed that it was the defects in the thin film that made convenience for chain flow. We whereby do not want to imply that dirt is necessary for the formation of superimposed lamellae. In fact, transmission electron microscope results exhibit that there is no distinguishable dirt observed for some isolated superimposed lamellae (see Figure S7 of the Supporting Information). Thus, it is the pinning effect of the defects that played a role in the formation of superimposed lamellae. Furthermore, the present scenario is consistent with the previous illustrations; namely, the higher the drop, the larger is the capillary pressure at the drop basement.43 Surface Composition Transition. As mentioned above, the transition of superimposed lamellae was achieved through lamellar sliding. To further explore this course, the top 0−10 nm surface composition transition was characterized in situ with XPS. Figure 7a reveals that the C 1s region envelops at 284.6

lamellae and its collapse to the droplets early in the 24 nm sample. Figures 5a−d show the lamella/droplet structures grow with the enlargement of the hole. Figures 5e−h exhibit the 3D patterns of the zones outlined with dashed lines in Figures 5a− d, respectively. By observing the structure transition from Figure 5e to Figure 5h, we can find that the height of the superimposed lamellae increased with the enlargement of the hole. Especially, as reflected in the height profiles (insets of Figures 5a−d), triple (Figure 5a) and quadruple (Figure 5b) lamellae are visible, while in Figures 5c,d droplets are discerned. To check if the droplet in Figure 5h was in ordered state, the height profile of the droplet was fit to the generic equation Ax2 + Bxy + Cy2 + Dx + Ey + F = 0. As mentioned by Croll et al.,20 if the droplet was in ordered state, the discriminant Δ = B2 − 4AC > 0, and the height profile fit to a hyperbolic curve. In contrast, if Δ < 0, the droplet was in disordered state and the height profile fit to a spherical curve. Figure 6 reveals that the

Figure 6. AFM height profile of the droplet as shown in Figure 5h and its fitting curve (indicated by the solid line, the coefficient of determination, R2 = 0.999).

height profile fits to a hyperbolic curve (indicated by the solid line) and Δ > 0, indicating that the droplet in Figure 5h was in ordered state. Thus, it is proved that the droplets were consisted of multilayers of PS-b-PCL lamellae. In fact, the phenomenon that microphase structure retained in the dewetted droplets was reported in some other publications.12,23 However, they did not mention the dynamic formation process. Furthermore, the existence of dirt is marked with arrows in Figures 5e−g, which was expected to be related to the formation of superimposed lamellae. In this section, to explore the inherent mechanism for the transition of superimposed lamellae, we made a qualitative phenomenological analysis on the basis of the surface tension changed with the growth of hole. It is generally accepted that the relation between the pressure difference and the shape of the wall could be quantitatively analyzed with Young−Laplace equation. Here, we treated the dewetted holes as cylinders full of air. It is natural to understand the spiral growth of the top layer because the increase of radii will lead to increased inner pressure (Supporting Information). In the meantime, the existence of defect (for example, dirt) can stabilize the retracted material because it has much lower surface tension than the polymer. Moreover, as shown in Figures 5e,f, the dirt could inverse the free surface from concave (ΔP, pressure difference between Pi and outer pressure, P0, < 0) to convex (ΔP > 0) because of the pinning effect. Accordingly, once the

Figure 7. (a) XPS spectra of the 43 nm PS-b-PCL thin film before and after thermal annealing. (b) Selected C 1s XPS spectra, showing the temperature-dependent change of the ester C of PCL (dashed-cut line) and the aromatic C of PS (dotted-cut line).

eV, and the O 1s region shows twin peaks enveloped at 532 and 533 eV, respectively. A further distinction was based on the C 1s XPS spectra. As shown in Figure 7b, the bands at 289.1 eV (indicated by the dashed-cut line of Figure 7b) and 291.1 eV (indicated by the dotted-cut line of Figure 7b) are assigned to the ester C of PCL and the aromatic π−π* shakeup peak of PS, respectively.44 A similar and clear surface composition was observed in the ex situ XPS experimental results (see Figure S8 of the Supporting Information). Furthermore, to give a 9144

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formation is dominated by the coupling and competition of dewetting and microphase separation. Consequently, coexistence of superimposed lamellae and ordered droplets are observed. Moreover, in the thermal-annealed samples, a wetting layer with thickness corresponding to half-periodicity was detected. Furthermore, it should be noted that, in the present work, we cannot give a description of the relief structure transition in the strong microphase separation regime, i.e., χN > 100. This is because the PS block and PCL block turned into glass and supercooled states, respectively, when the temperature was cooled down to the theoretically predicted strong microphase separation regime.

quantitative analysis of the surface composition change, the results represented by area ratio of O 1s to C 1s are collected in Table 2. As shown in Table 2, the as-prepared/original form Table 2. Quantitative Characteristics (Indicated by the Area Ratio of O 1s to C 1s), Showing the Surface Composition Change with the Thermal Annealing Treatment as-prepared

at 120 °C at 30 min

at 25 °C at 2 months

0.155

0.179

0.201

has the lowest value, 0.155; the amorphous form (at 120 °C) has intermediate value, 0.179; the recrystallized form has the highest value, 0.201. These results indicate that thermal annealing and recrystallization promoted the aggregation of PCL at the polymer−air interface. As mentioned in the previous publications, surface tensions of the amorphous PS and crystalline PCL were 40.7 mJ/m2 (at 20 °C)45 and 34.1 mJ/m2 (at 25 °C),46 respectively. Considering that surface free energy minimization will promote the component with lower surface energy to aggregate at polymer−air interface, it is easy to understand that PCL blocks tend to aggregate at the polymer−air interface. Although XPS cannot offer optimal information on the depth profile of polymer layers,24,44 we supposed that lamellar sliding should be attributed to the free diffusion of PCL molecule rather than that of PS molecule. This is because the lamellar sliding could take place at temperatures lower than the Tg of PS, as indicated by the experimental results in Figure S4. Thus, the material contacted the wetting layer should be PCL. In fact, this inference could be rationalized by the XPS experimental results. As illustrated above, the free surface was occupied by the PCL in the thermal-annealed PS-b-PCL. Moreover, the PSb-PCL formed superimposed lamellae, and each lamella retained the bulking spacing. Thus, we can recognize the PCL occupied the layer just above the wetting layer. Furthermore, we concluded that the lamellae inside the droplets were paralleling to the substrate because PCL wet both the free surface and the wetting layer. The schematic drawing of the temperature-dependent relief structure formation is offered in Figure 8. The molecular

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CONCLUSION The temperature-dependent relief structure formation in the nearly symmetric PS-b-PCL diblock copolymer thin film was studied, among which the coupling and competition of dewetting and microphase separation were highlighted by regulating the microphase separation strength (indicated by χN). When there was no microphase separation (χN = 3.9), typical droplets resulting from autophobic dewetting were observed. When there was intermediate microphase separation (χN ≥ 14.0), superimposed lamellae except for the droplets was observed. As proved by the AFM results, microphase separation occurred during the preparation and the lamellar structure retained in temperature regime below the TODT. Dynamic analysis of the hole growth at 140 °C revealed that the formation of superimposed lamellae was achieved through lamellar sliding. Moreover, with continual heating, the formation of the superimposed lamellae and following transition to ordered droplets were first revealed with the in situ AFM scanning. We attributed the transition of superimposed lamellae to that the microphase separation strength was forced to yield to the minimization tendency of surface tension, and the corresponding molecular diffusion was concerned with the existence of defects (for example, dirt). Furthermore, PCL preferentially aggregated at the polymer−air interface, as indicated by XPS experimental results. This molecular arrangement rationalized the occurrence of lamellar sliding at lower temperatures than Tg of PS. On the basis of these findings, we can ascertain that the generally mentioned superimposed lamellae is a metastable structure, resulting from the coupling of microphase separation and dewetting.

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ASSOCIATED CONTENT

S Supporting Information *

DSC thermograms (Figure S1); AFM height image and profile (Figure S2); calculation of the TODT (S3); AFM height images (Figure S4); optical micrograph (Figure S5); Young−Laplace equation (S6); TEM image (Figure S7); XPS spectra (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Schematic drawings for the temperature dependent relief structure formation: (a) as-prepared thin film; (b) droplets; (c) coexistence of superimposed lamellae and ordered droplets.

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AUTHOR INFORMATION

Corresponding Author

structure of the as-prepared PS-b-PCL thin film is shown in Figure 8a, where the microphase-separated lamellar structure is visible. As shown in Figure 8b, the sample is in disordered melt (for example, χN ≈ 3.9) when it is annealed at high temperature. There autophobic dewetting occurs, and typical droplets are observed. In Figure 8c, when the microphase separation strength is large enough to compete with the dewetting tendency, for example, χN ≥ 14.0, the relief structure

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Fajun Zhang for valuable discussions and Dr. Yuming Yang for the kind help in optical microscopy. This 9145

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work is supported by the National Nature Science Foundation of China (No. 21074135).

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dx.doi.org/10.1021/ma301531a | Macromolecules 2012, 45, 9139−9146