ARTICLE pubs.acs.org/Langmuir
Coupling of Microphase Separation and Dewetting in Weakly Segregated Diblock Co-polymer Ultrathin Films Derong Yan,† Haiying Huang,† Tianbai He,*,† and Fajun Zhang*,‡ †
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences Changchun 130022, People’s Republic of China ‡ Institut f€ur Angewandte Physik, Universit€at T€ubingen, 72076 T€ubingen, Germany
bS Supporting Information ABSTRACT: We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(Llactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the copolymer forms a lamellar structure in bulk with a long period of L ∼ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ∼ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.
’ INTRODUCTION The behavior of block co-polymers at the surface and interface is of importance for technological applications, such as coatings, corrosion protection, functional surfaces, etc. These applications require an adequate understanding of the morphology and stability of thin block co-polymer films. The presence of a substrate and free surface strongly influence the behavior of block co-polymers in thin films compared to those in bulk because of the specific attraction or repulsion of substrates and the requirement of lowering surface-free energy.13 Generally, the initial spincoated diblock co-polymer thin film is in a non-equilibrium state because of the fast solvent evaporation and imposed centrifugal forces.4 It will be relaxed therefore either through microphase separation and/or dewetting to lower the total free energy when annealing above its glass transition temperature (Tg) or melting temperature (Tm).5 Microphase separation in a block co-polymer can form wellordered nanoscale structures (e.g., spheres, cylinders, gyroids, and lamellae) below the orderdisorder transition temperature (TODT) because of the thermodynamic incompatibility of covalently connected chemically distinct blocks. The driving force is primarily determined by the product of the FloryHuggins interaction parameter (χ) between the blocks and the total degree of polymerization (N). For symmetric diblock co-polymers, microphase separation is expected at χN > 10.5 according r 2011 American Chemical Society
to theoretical prediction.6 The mechanisms of the microphase separation process for symmetric diblock co-polymers have been reported to be either spinodal decomposition or nucleation and growth. In spinodal decomposition, the amplitude of the thickness fluctuations grows exponentially until the whole thin film is in an ordered state.7 For nucleation and growth, microphase separation started through defects or dislocations in thin films.810 For the lamellar-forming diblock co-polymer thin film, if the initial film thickness fulfills the quantization constraints, microphase separation can stabilize the thin films; otherwise, the embryonic patterns will decay into relief structures, i.e., islands, bicontinuous patterns, or holes.11 Each of these relief structures is still period-matched, which means that the terrace height should be equal to nL or (n + 1/2)L. Here, n is an integer, and L is the lamellar period. While if it is relaxed through dewetting, the thin film finally evolved into discrete droplets, and their height variances do not need to match the lamella period. Polymer dewetting has been extensively studied and dedicated to developing a fundamental understanding of this process.1216 Dewetting is driven by the difference of surface energies at polymer/ air, polymer/substrate, and air/substrate, which is characterized Received: June 24, 2011 Revised: August 1, 2011 Published: August 26, 2011 11973
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Langmuir by the formation of holes, their growth, and coalescence, finally leading to a set of droplets on the substrate.12 There are two main rupture mechanisms. (1) Dry spots are nucleated at defect sites (where the energy barrier for hole formation is lower) that can either be pre-existing (that leads to heterogeneous nucleation) or created by thermal agitations (that results in thermal nucleation). (2) Capillary waves are spontaneously amplified, which is known as “spinodal dewetting”, and can take place only if the film is unstable.17 Another intriguing dewetting phenomenon called “autophobicity”,18 which refers to dewetting between molecules with identical chemical composition, is sometimes observed in block co-polymers.1922 In general, it is believed that autophobic dewetting is solely driven by entropy,2326 meaning that the conformation of the first block co-polymer layer contacting the substrate surface is different from that of the other polymer layers. The formation of dewetting and microphase separation patterns each involves complicated nonlinear dynamics, and the simultaneous occurrence of these processes may result in new interesting physical phenomena. By far, the dewetting process has been mainly used either for fabricating hierarchical structures27 or as confined geometry to study the surface topography and the chemical morphology5,28 in thin diblock co-polymer films. The dewetting process is always much faster than the microphase separation in the above studies. Neto et al.29 have observed that the dewetting began to develop when the microphase separation almost totally finished; i.e., the dewetting velocity is much slower than the microphase separation velocity. Less attention has been paid to the coupling behavior of microphase separation and dewetting when their evolution speeds are qualitatively close. Such interplay is very important for understanding the relaxation process in thin films. In this work, we show that microphase separation and dewetting can interplay in a competitive way on the stability of the copolymer thin films. Specifically, we have investigated a weakly segregated lamella-forming diblock co-polymer, poly(ε-caprolactone)-block-poly(L-lactide) (PCL-b-PLLA), spin-coated onto amorphous carbon-coated mica substrates. With the initial film thickness matching the lamellar period, while dewetting leads to the rupture of the thin film, microphase separation stabilizes it. The competition of these two processes leads to a novel relaxation kinetics of co-polymer thin films; i.e., the rupture velocity increases with a decreasing temperature in the current temperature range. Different scenarios because of the competition between microphase separation and dewetting are discussed on the basis of our results and literature reports.
’ EXPERIMENTAL SECTION Materials. The PCL-b-PLLA diblock co-polymer was purchased from Polymer Source, Inc. and used without further purification. The number average molecular weight (Mn) was 60 000 (PCL, 20 000; PLLA, 40 000), with a polydispersity of 1.45 determined by size-exclusion chromatography (SEC). The volume fraction f is calculated to be fPCL = 0.38 and fPLLA = 0.62, using 1.02 and 1.24 g/cm3 as the densities of PCL and PLLA, respectively. The crystallization and melting temperature, Tc and Tm, are determined by standard differential scanning calorimetry (DSC) scanning at a heating and cooling rate of 10 °C/min. For the PCL block, the two values are 31 and 55 °C. For the PLLA block, the crystallization peak is quite broad, ranging from 70 to 110 °C during the cooling scanning. However, a relatively sharp melting peak of 158 °C can be found during the next heating process. The block co-polymer became unstable above 190 °C in the air condition, as indicated by the
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thermogravimetric analysis (TGA) measurement (Figure S1 of the Supporting Information). Thin Film Preparation. The PCL-b-PLLA diblock co-polymer was dissolved in chlorobenzene with 0.5 wt % at 55 °C. Freshly cleaved mica coated with a ca. 4 nm thick amorphous carbon film was used as the substrate. The surface roughness of the carbon film was less than 1 nm from atomic force microscopy (AFM) measurements (Figure S2 of the Supporting Information). The contact angle of the carbon-coated mica is measured less than 10°, which means that the surface is hydrophilic. Thin films were prepared by spin-coating with a speed of 2500 rpm for 30 s. The initial film thickness, h, was ca. 10 nm, as measured by an ellipsometer and the AFM scratching method (Figure S3 of the Supporting Information). The as-cast films were kept in vacuum at room temperature for at least 24 h to remove the residual solvent. For comparison, thin films with h ≈ 16 and 27 nm were prepared using 0.75 and 1 wt % chlorobenzene solution, respectively, on the same substrate. Thin films on freshly cleaved mica without carbon coating were also prepared using 0.5 wt % chlorobenzene solution with h ≈ 11 nm. Small-Angle X-ray Scattering (SAXS). SAXS experiments was performed with a NanoSTAR-U (Bruker AXS) using Cu Kα radiation (λ = 0.154 nm). The generator was operated at 40 kV and 650 μA. The sample was mounted on the hot stage (Linkam THMS 600) in the vacuum, first melted at 180 °C for 5 min, and then decreased to 165 °C isothermally for 2 h. Two-dimensional (2D) SAXS patterns were collected for 2 h at 165 °C using a HI-STAR detector. The distance between the sample and the detector was 1074 mm. The 2D data were converted into the plot as the scattering intensity I(q) versus the scattering vector, q = 4π sin θ/λ (2θ = scattering angle). AFM. Agilent Series 5500 AFM equipped with a high-temperature hot stage controlled by a Lake Shore Model 325 temperature controller (Lake Shore Cryotronics, Inc.) was used to capture images at ambient conditions, without heating, and for in situ observations of microphase separation and dewetting at various temperatures. Silicon cantilevers (OMCL-AC240TS, Olympus) with a resonance frequency of approximately 70 kHz and a spring constant of about 2 N/m were used. The scan rates varied from 1 to 2 Hz. Each scan line contains 512 pixels, and a whole image is composed of 512 scan lines. The interaction of the tip with the specimen was fixed at light tapping mode by adjusting the ratio of the set point amplitude (Asp) to the free oscillation amplitude (Ao) to be 0.85 to track the surface without disturbing nucleation. For in situ hotstage AFM, all of the films were first heated to 180 °C, which was well above the Tm of PCL-b-PLLA for 5 min on a Linkam THMS 600 hot stage to erase any thermal history, and then immediately (less than 1 s) transferred to the nearby AFM hot stage preset at the desired temperature.
’ RESULTS Microphase Separation in the Bulk. The PCL-b-PLLA diblock co-polymers have been reported as homogeneous or microphase-segregated in the melt, depending upon the block length ratio between the blocks.3035 Kim et al.30 synthesized several different Mn PCL-b-PLLA diblock co-polymers, including Mn = 77 000 (fPLLA = 0.63) and Mn = 19 000 (fPLLA = 0.58), which have a similar chemical composition to our sample. They found that the sample of Mn = 77 000 showed microphaseseparated structures at the molten state up to 220 °C, and the sample of Mn = 19 000 became homogeneous at temperatures higher than 175 °C. Hamley et al.33,34 studied a PCL-b-PLLA diblock co-polymer of Mn = 20 900 (fPLLA = 0.55) and showed that the two blocks are miscible in the melt at 190 °C; however, time-resolved SAXS experiments revealed that, upon cooling from the melt, the sample transformed into a transient microphase-separated lamellar structure just before crystallization of 11974
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Langmuir the PLLA block started. According to these literature reports and the Mn of the PCL-b-PLLA used here, we estimate that TODT of our block co-polymer should be in the range of 190220 °C. Figure 1 shows the high-temperature SAXS profile of the PCL-bPLLA diblock co-polymer studied here. In the melt state, the SAXS profile shows two weak peaks q* and 2q*, as indicated by arrows in Figure 1, indicating the formation of the lamellar structure via weakly microphase separation. The long period L can be calculated from the first maximum q* ∼ 0.308 nm1 as L = 2π/q* ≈ 20.4 nm. Microphase Separation and Dewetting in Thin Films. Figure 2a shows the morphology of a 10 nm thick film on a carbon-coated mica substrate. The sample was annealed at 180 °C for 5 min and then quenched to 140 °C. The AFM image was obtained 12.5 min after quenching to 140 °C. The film seems stable in this stage, and no visible evidence of rupture could be seen. Because the initial film thickness (10 nm) is close to a half lamellar period, the microphase separation may not be obvious even it occurred. To prove this, we examined a 16 nm film, whose thickness is mismatching the lamellar period (Figure 2b) on the same substrate and thermal history before quenching to 140 °C. The AFM image was obtained 10.5 min after quenching. The small holes can be clearly seen because of the mismatch of the initial film thickness to the lamellar period. The fluctuation of the hole depth (15.2 ( 4.3 nm) is simply due to the fact that microphase separation is advancing. Thus, we can conclude that the microphase separation is in progress in the case of 10 nm film during the initial annealing process.
Figure 1. High-temperature SAXS profile of bulk PCL-b-PLLA at 165 °C.
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When we prolonged the annealing at 140 °C to a very long time (e.g., about 1 day), dewetting occurs on the 10 nm film. A typical morphology for a sample after 1400 min of annealing was shown in Figure 3a. The film was scratched carefully before annealing; this can help us understand the dewetting mechanism by checking the existence of the residue wetting layer. The AFM cross-section profiles give an average thickness of about 9 nm, including a 4 nm carbon film. Thus, a thickness of ∼5 nm left behind the ruptured film, indicating that the relaxation of the thin films follows an autophobic dewetting mechanism. Typical crosssection profiles of droplets are shown in Figure 3b. The height profiles of the droplets do not have the layered structure, as seen from strongly segregation diblock co-polymer thin films.36 The overall height values do not fit the long period nL or (n + 1/2)L, which further supports the dewetting mechanism of the relaxation of the thin films. Such autophobic dewetting has also been observed for thicker films with h = 27 nm, leaving liquid-like droplets on a wetting layer of ca. 5 nm (Figure S4 of the Supporting Information). Note that, at 140 °C, the crystallization of the PLLA block in ultrathin films is very slow and not observed in the experimental time window. Because of the existence of a 5 nm wetting layer, the rupture in fact occurs only on the top layer of a similar thickness, which does not fit 1/2L. One question that we may ask is whether the microphase separation is still the reason for the stability of the
Figure 3. (a) AFM topography images (20 20 μm) of PCL-b-PLLA ultrathin films isothermally annealed at 140 °C for 1400 min. The sample was first melted at 180 °C for 5 min and then quenched to 140 °C. The cross-section profiles showing the wetting layer thickness are created by the vertical line. (b) Cross-section profiles of the droplets. Their heights are 73 and 68 nm for droplets 1 and 2. The curves are shifted intentionally for clarity.
Figure 2. AFM topography images annealing at 140 °C after quenching from 180 °C. (a) h = 10 nm and 12.5 min; the image size is 20 20 μm. (b) h = 16 nm and 10.5 min; the image size is 10 10 μm. 11975
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Figure 4. AFM topography images show the morphology of the PCL-b-PLLA diblock co-polymer thin film on a pure mica substrate: (a) 20 20 μm, annealed at 180 °C for 5 min and then quenched to room temperature, and (b) 5 5 μm, annealed at 180 °C for 5 min and then quenched to 100 °C isothermally for 380 min.
thin film upon annealing (Figure 2a). Here, we argue that the microphase separation is driven by the thermodynamic incompatibility of the covalently connected chemically distinct blocks. In our system, both blocks, PLLA and PCL, are polar blocks and can almost equally adsorb to the substrate. The formation of the wetting layer should not change the chemical composition through the initial film, although it should change the conformation of molecules within the wetting layer. If we consider the annealing process below TODT, the driving force of microphase separation should still work on the whole film instead of only the top layer because there is no difference in terms of the chemical composition. Therefore, the film with an initial thickness of 10 nm tends to be stabilized by microphase separation (h = 1/2L). To further understand the relaxation process, we prepared a thin film with a similar thickness (h ∼ 11 nm) but on a freshly cleaved mica surface. Without carbon coating, the interactions between the polymer and substrate are much stronger, which leads to a stronger driving force of autophobic dewetting because of the larger reduction of conformational entropy in the wetting layer. The dewetting process under the same annealing condition should be faster, and the effect of the microphase separation becomes relatively less important. Indeed, a complete dewetting occurred already after only 5 min of annealing at 180 °C (Figure 4a). No stabilization effect of the microphase separation could be recognized. The fast autophobic dewetting kinetics on pure mica indicates that the carbon coating could greatly reduce the dewetting velocity. Dry patches were visible in some cases (Figure 4b), indicating an autophobic dewetting mechanism. The thickness of the wetting layer is about 4.5 nm, similar to that on carbon-coated mica substrates. The droplets formed on the residue film are liquid-like without any visible layered structure, and their heights do not fit the long period (data not shown). All of these observations (Figures 24) support that autophobic dewetting is the dominate mechanism of the relaxation process, but the microphase separation can influence the process when the driving force of dewetting is significantly reduced by carbon coating. In the following, we present the novel rupture kinetics caused by this competition effect. Figure 5 shows the in situ AFM observations of typical surface morphologies of 10 nm thick PCL-b-PLLA films on carboncoated mica substrates at different annealing temperatures. All of the samples have first been melt at 180 °C for 5 min and then quenched to preset annealing temperatures. With annealing at
140 °C (panels ac of Figure 5), the rupture process is very slow. After 77 min, small holes with irregular shapes are observed. The cross-section profiles show that the average depth of holes is 67 nm. The holes have a surface fraction of ca. 21%, calculated using the software of the AFM instrument, which is in consistent with the formation of a wetting layer below the considered volume conservation calculation. When the annealing time is increased, the holes start to connect to each other and the heights of the surrounding area increase but do not fit the long period of the microphase-separated structure. After 700 min, droplets with a similar structure in Figure 3 are formed. With annealing at 130 °C (panels df of Figure 5), the rupture process becomes fast. After 15.5 min, many holes are formed and some of them are already connected. When the time is increased, more and more holes are connected and form droplets and ridges. The crosssection profiles indicate that these structures are not periodmatched. The volume conservation calculation supports the existence of a ∼5 nm wetting layer. With annealing at 120 °C (panels gi of Figure 5), the rupture process becomes even faster. After 12.5 min, most of the holes are connected. Droplets evolved quickly as the annealing time increases. The crosssection profiles at each time do not show any layerd structures resulting from the microphase separation. Instead, they look more like liquid droplets. These results show a novel dewetting kinetics; i.e., the dewetting velocity increases with a decreasing annealing temperature. To make this phenomenon more clear, the surface coverage values of the co-polymer (S, %) on the wetting layer as a function of the annealing time (t, min) at each temperature are plotted in Figure 6. It can be seen that the dewetting process is very slow at 140 °C. It takes about 270 min to decrease to 40% surface coverage and form mostly discrete droplets. With annealing at 130 °C, the dewetting velocity increases significantly and the surface coverage is below 40% after only 20 min. The limited data in Figure 6 for lower annealing temperatures (130 and 120 °C) are due to the interference of crystallization of the PLLA block (data not shown). If we continue to lower the annealing temperature to 120 °C, the dewetting velocity is even much faster compared to that at 130 °C, although there are only three data points because of the quick appearance of PLLA crystals. The observed dewetting kinetics is very interesting and uncommon. We further demonstrate it by exploring this phenomenon at two higher and two lower temperatures using the 11976
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Figure 5. In situ AFM topography images (20 20 μm) of PCL-b-PLLA thin films isothermally annealed at different temperatures of 140 °C for (a) 77 min, (b) 230 min, and (c) 502 min, 130 °C for (d) 15.5 min, (e) 24 min, and (f) 92 min, and 120 °C for (g) 12.5 min, (h) 22 min, and (i) 31.5 min.
same thin films, i.e., h ∼ 10 nm on the carbon-coated mica surface. Panels a and b of Figure 7 show the ex situ AFM results of samples annealed at 150 and 160 °C for 360 min, respectively,
and then quenched to room temperature. Samples were first annealed at 180 °C for 5 min, followed by cooling to preset annealing temperatures at a rate of 30 °C/min. There was no 11977
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Langmuir obvious dewetting happening in both cases compared to that at 140 °C. Besides, it seems that the relaxation process at 150 °C is a little faster than at 160 °C because a lot of small holes have already appeared in the former case. Panels c and d of Figure 7 show the hot-stage AFM results of samples isothermally annealed at 110 and 100 °C for 14 and 9.5 min, respectively. Samples were first annealed at 180 °C for 5 min, followed by quenching to preset annealing temperatures. The rupture of the films in both cases happened quickly, which is very similar to the case of 130 and 120 °C. Qualitatively, the dewetting velocities at 110 and 100 °C seem a little slower than that at 120 °C. These results again supported the tendency of the dewetting speed.
Figure 6. Surface coverage of the co-polymer, S (%), on the wetting layer as a function of the annealing time, t (min), at different temperatures, as remarked in the image.
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’ DISCUSSION Autophobic Dewetting. We have shown results above that the autophobic dewetting happened finally in 10 nm thick PCLb-PLLA films on carbon-coated mica substrates in the investigated temperature scope if the annealing time is long enough. It is believed that autophobic dewetting is primarily driven by entropy, which means that the conformation of the first polymer layer contacting the substrate surface is different from that of the other polymer layers. Green et al.19,20 found that the autophobic dewetting happened in polystyrene-b-polymethylmethacrylate (PS-b-PMMA) thin films on the silicon substrates with native SiOx layers because the layer in contact with the substrate formed a dense co-polymer “brush” of highly stretched chains as a result of the preferential attraction of the PMMA block to the substrate, which greatly reduced the conformation entropy compared to the adjacent layer in the rest of the film. Reiter and Sommer37,38 reported autophobic dewetting in poly(ethylene oxide) films annealed on ultraviolet (UV)-treated oxidized silicon surfaces. In this system, strong pinning interactions between the polymer and oxide significantly reduced the conformational entropy of chains next to the surface. Autophobic dewetting can also be realized by intentionally grafting end-functionalized molecules into the surface and forming a “brush-like” layer, which entropically repelled the nongrafted but otherwise identical molecules.3943 In the present case, PCL-b-PLLA was spin-coated on the amorphous carbon-deposited mica substrate. The low-surface-energy carbon coating is supposed to have no preferential interaction with both of them, considering the chemical structure of each block.44 However, contact angle measurements indicate that the carbon-coated
Figure 7. AFM topography images (20 20 μm) obtained (a and b) ex situ and (c and d) in situ at different annealing temperatures and times after cooling or quenching from 180 °C: (a) 150 °C for 6 h, cooling from 180 °C at a rate of 30 °C/min, (b) 160 °C for 6 h, cooling from 180 °C at a rate of 30 °C/min, (c) 110 °C for 14 min, quenching from 180 °C in air, and (d) 100 °C for 9.5 min, quenching from 180 °C in air. 11978
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mica surface is still hydrophilic (static contact angle is less than 10°). Thus, it is not surprising to see the strong interaction between polar PCL/PLLA and the substrate. This can lead to very likely autophobic dewetting, as reported by others.19,20,37,38 In fact, the 4 nm carbon film on mica played an important role in delaying the speed of autophobic dewetting by partially screening the attraction. This is confirmed by the fast autophobic dewetting on the freshly cleaved mica substrate (Figure 4). Seemann et al. proposed a model potential (eq 1) to describe the dewetting mechanism of PS on silicon substrates with various thicknesses of the SiOx layer.45 jðhÞ ¼ c=h8 þ jvdW ðhÞ
ð1Þ
The first term denotes the short-range interaction, and the second term describes the long-range van der Waals potential. The system has been treated as a “three-interface” system, air/ PS/SiOx/Si, when the SiOx layer was very thin. When the thickness of the SiOx layer was increased, the interaction between the polymer and SiOx becomes dominant and can be treated as a “two-interface” system. They found that the global minimum of the potential, which corresponds to the thickness of the wetting layer, increases with the thickness of the SiOx layer. In our case, the wetting layer on pure mica and the carbon-coated mica surface has a similar thickness of about 5 nm, whereas the kinetics of dewetting is significantly reduced with a 4 nm carbon coating. We contribute the effect of carbon coating as reducing the strength of the attractions between the polymer and mica surface, but the position of the global minimum of the overall potential does not change significantly. Thus, we have established a proper system for studying the coupling of microphase separation and dewetting with a tunable dewetting kinetics. Coupling of Microphase Separation and Dewetting. The appearance of autophobic dewetting indicates that the relaxation process in the 10 nm thick film on the carbon-coated mica substrate is not only controlled by the microphase separation but also determined by the dewetting. Generally, the dewetting velocity is monotonously decreasing as the annealing temperature is decreasing. Hamley et al. studied the autophobic dewetting of thin films of low-molecular-weight di- and triblock copolymers of poly(oxyethylene)/poly(oxybutylene) on silicon substrates. They found that the dewetting velocity was an exponential function of the temperature and could be expressed well by the Arrhenius form: v = v0 exp(Ea/RT).22 Okerberg et al. investigated the dewetting of thin films of PCL homopolymer on n-octyldimethylchlorosilane (ODS)-treated substrates, and they observed that the dewetting velocity was approximately a linear function of the temperature.46 Here, in our experiments, although the velocities of autophobic dewetting at different annealing temperatures are not resolved quantitatively, the tendency that the dewetting velocity is increasing with a decreasing annealing temperature could be clearly visible (Figure 6). We consider that this novel dewetting velocity as a function of the annealing temperature is closely related to the weakly segregated microphase separation process in thin films. Here, we propose three cases of the coupling interaction between dewetting and microphase separation according to their relative velocities, as shown in Figure 8. The curve of the dewetting velocity versus the annealing temperature is based on the Arrhenius plot.22 The change of the microphase separation velocity (forming lamellae) with the annealing temperature is considering the factor of viscosity. In case a, the dewetting velocity
Figure 8. Simple models showing the interplay between microphase separation (MS) and dewetting according to their relative velocity.
is always faster than the microphase separation velocity. As a result, the surface relaxation process is entirely dominated by the dewetting. That was the case reported by Hamley et al.22 Our results on freshly cleaved mica without carbon coating also belonged to this situation. In case b, the dewetting velocity is always slower than the microphase separation velocity, except approaching TODT. The surface relaxation process was initially dominated by the microphase separation. However, when the microphase separation was finished, the dewetting may also occur if the annealing time was prolonged, depending upon whether the total free energy is reaching a minimum. Our previous work47 on the PS-b-PCL system showed that, when the film was annealed at 100 °C, only microphase separation occurred. However, when the annealing temperature was increased to 150 °C, which was above TODT, the dewetting happened quickly from microphaseseparated thin films. Recently, Neto et al. observed the autophobic dewetting in a lamellar-forming diblock co-polymer PS-bPEO.29 Although the annealing temperature was well below TODT in their experiments, they found that the autophobic dewetting began to develop after the microphase separation. They also observed that the duration of annealing before the onset of dewetting depended upon film thickness: for films with a thickness of ca. 100 nm, dewetting occurred for annealing times longer than 24 h, and for films with a thickness of ca. 50 nm, dewetting started after ca. 10 h. These results were consistent with the situation b in Figure 8. In case c, the microphase separation velocity is very close to the dewetting velocity in a certain temperature range (T1, T2). Thus, the microphase separation could stabilize the film at an early stage. Once the thickness fluctuation is amplified, the dewetting began to develop. Beyond this temperature scope, the dewetting velocity is gradually faster than the microphase separation velocity. Our experimental results on carbon-coated mica substrates are coincidence with case c. Specifically, T1 ≈ 150 °C; T2 g 180 °C; and TODT g 190 °C. When the annealing temperature Ta was in the scope of 180 g Ta g 150 °C, the microphase separation velocities were very close to the dewetting velocities. Because of the lamellar period match of the initial film thickness, the microphase separation could keep the film stable for a long time. Pitifully, limited by the easy degradation of this diblock co-polymer, we cannot verify the situation of annealing temperatures above 180 °C. When the annealing temperature was decreased to 140 °C or below, the function of stabilization generated by the microphase separation is gradually weakened. Thus, the dewetting velocity is becoming faster and faster first. Then, the dewetting velocity returns to a normal state and slows. The whole process has been unambiguously confirmed by the in situ and ex situ AFM results (Figures 57). 11979
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’ CONCLUSION In summary, we have investigated the coupling behavior between microphase separation and autophobic dewetting of PCL-b-PLLA ultrathin films on carbon-coated mica substrates. The block co-polymer is weakly segregated into a lamellar structure in bulk with a long period of 20.4 nm as determined by SAXS. Polymer ultrathin films with h = 10 nm ∼ 1/2L were annealed at a temperature range between 100 and 160 °C. The relaxation procedure was found to be dominated by an autophobic dewetting with a wetting layer of ca. 5 nm; however, the velocity of the dewetting increases with decreasing temperatures. Autophobic dewetting is due to the attractive interaction between the polar co-polymer and the hydrophilic substrate. In comparison to the autophobic dewetting on a pure mica substrate without carbon coating, we concluded that carbon coating (4 nm) reduces the dewetting velocity by partly screening the attraction. While the dewetting leads to the rupture of the ultrathin films, the microphase separation can stabilize them because of the matching quality (h = 1/2L). This explains our observation of the novel dewetting kinetics; i.e., the dewetting velocity increases with decreasing annealing temperatures. ’ ASSOCIATED CONTENT
bS
Supporting Information. TGA measurement, surface roughness value of the carbon film, thickness measurements of the carbon film, the initial block polymer film and the wetting layer of the 27 nm thick film using the AFM scratching method. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-431-85262123. Fax: +86-431-85262126. E-mail:
[email protected] (T.H.); Telephone: +49-7071-2978670. Fax: +49-7071-29-5110. E-mail: fajun.zhang@uni-tuebingen. de (F.Z.).
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