Dewetting Process of Deuterated Polystyrene and Poly(vinyl methyl

May 21, 2013 - Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, ... Chinese Journal of Polymer Science 2018 36 (9), 1036-104...
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Dewetting Process of Deuterated Polystyrene and Poly(vinyl methyl ether) Blend Thin Films via Phase Separation Tian Xia,†,‡ Hiroki Ogawa,*,§ Rintaro Inoue,‡ Koji Nishida,‡ Norifumi L. Yamada,⊥ Guangxian Li,† and Toshiji Kanaya*,‡ †

College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan § Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo-ken 679-5198, Japan ⊥ High Energy Accelerator Research Organization, Tokai, Ibaraki-ken 319-1106, Japan ‡

ABSTRACT: We studied the structure development in the dewetting process of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) thin films, which were prepared by a spin-coating method on a quartz substrate, after the temperature jump into the two-phase region, using timeresolved specular and off-specular neutron reflectivity (NR), specular X-ray reflectivity (XR), light scattering (LS), and optical microscope (OM) measurements. The OM and specular XR measurements showed that there clearly existed an incubation period before dewetting. In the incubation period, composition fluctuations between dPS and PVME were developed due to phase separation. The specular NR results revealed that composition fluctuations began to occur just after the temperature jump along the depth direction over the film, giving rise to the composition gradient heterogeneously over the film. Composition fluctuations were also observed in the in-plane direction by the off-specular NR and LS measurements: the offspecular NR measurements revealed that composition fluctuations with micrometer scale in the in-plane direction occurred in the incubation period, and they also grew up with the annealing time before dewetting, implying composition fluctuations induced dewetting, while the LS measurements detected only droplet formation of dewetting. On the basis of the off-specular NR results, the inside structure of the dewetting was additionally discussed, showing tilted structure along the depth direction. separation, studies on fluctuations in the in-plane as well as the out-of-plane directions are indispensable. In a previous work, we investigated the time evolution of the morphology in deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) thin films with various film thicknesses from 65 μm to 42 nm in the two-phase region.15 When the film thickness was below 200 nm, we found that there was a long incubation period before dewetting although the spinodal decomposition (SD) type of dewetting was observed. Generally speaking, the SD type dewetting occurs in an unstable region, meaning that it occurs immediately without the incubation period. The observed long incubation period suggests that composition fluctuations occur inside the film before dewetting, and we found in the specular NR measurements on the dPS/ PVME thin films below 100 nm composition fluctuations occurred in the depth direction (or in the out-of-plane direction).16 However, we have not discussed composition fluctuations in the in-plane direction before dewetting. In the present work, hence, we focused our attention on composition

1. INTRODUCTION Dewetting of polymer thin film is a key issue in the instability for many fields, including coatings, adhesives, and lubrications. The dewetting mechanism of the one-component polymer thin films on the substrate (or another component polymer) including capillary wave and density fluctuation has received considerable attention to understand the stability of the films.1−4 Since two component films such as polymer blend are often utilized to achieve high performance of thin films, much attention is also paid to phase separation because of the important roles to affect dewetting. The interplay between phase separation and dewetting has been investigated experimentally and theoretically in recent years, and two possible mechanisms have been mainly proposed.5−8 One possibility is that dewetting occurs due to capillary fluctuations after phase separation (capillary wave mechanism).9,10 Another possibility is that composition fluctuations of phase separation directly induce dewetting (composition fluctuation mechanism).11−14 However, it is still obscure how fluctuations affect dewetting in the in-plane directions as well as the out-of-plane directions. In addition, it is not clear if only the fluctuations in the out-of-plane or the in-plane direction can induce dewetting. Therefore, for the understanding of dewetting via phase © XXXX American Chemical Society

Received: March 9, 2013 Revised: April 21, 2013

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fluctuations in the in-plane direction and the relation between the in-plane and out-of-plane composition fluctuations before dewetting. To observe the structure and/or composition fluctuations in the in-plane direction, we performed optical microscope (OM), light scattering (LS), and off-specular neutron reflectivity (NR) measurements for dPS/PVME thin films on a quartz substrate. Besides these measurements, the quartz substrate was utilized for the measurements of specular X-ray reflectivity (XR) and specular NR measurements, which provide information on the out-of-plane structure in the film. The usage of quartz substrate also enables us to measure the sample in the in-plane directions. On the basis of the results we discuss the structure development of dewetting via composition fluctuations caused by phase separation, especially focusing on the structure development during the incubation period before dewetting. Additionally, we discuss the in-plane structure dependence on the depth direction in the dewetting process, which is measured by off-specular NR measurements.

The specular and off-specular NR measurements were performed on a beamline BL16 installed at the Materials and Life Science Experimental Facility (MLF) in Japan Proton Accelerator Research Complex (J-PARC), Tokai, which is a time-of-flight (TOF) or energy dispersive reflectometer.17,18 The specular NR measurements were performed in a Qz range from 0.1−0.9 nm−1 with an incident angle of 1.0° and a two-dimensional scintillation counter. The acquisition time for one measurement was 2−5 min. XR measurements were performed on a beamline BL03XU at the Synchrotron Radiation Facility, SPring-8, Nishiharima.19 A wavelength of an incident X-ray was 0.1 nm and a zero-dimensional scintillation counter was used as a detector. The data acquisition time per point was 500 ms and the total acquisition time was 2 min in a limited Qz range from 0.5 to 1.2 nm−1 for time-resolved measurements. LS and OM measurements were done using a homemade apparatus20 and Olympus BX50 equipped with a CCD camera, respectively. All the measurements were performed at 110 °C after the temperature jump from room temperature (=25 °C). The temperature of 110 °C is far above the critical temperature of 102.0 °C at the critical composition (WPS = 10%). In thin polymer films it was often reported that phase diagram changed due to the effects of surface and interface interactions.21,22 The temperature of 110 °C guarantees that the measurements were done in the two-phase region of the phase diagram. Note that we used quartz substrates 1 mm thick (not Si substrate) for all of the NR, XR, LS, and OM measurements, which guaranteed that interactions between polymers and substrate are the same in every measurement.

2. EXPERIMENTAL SECTION dPS and PVME used in the experiments have the weight-average molecular weights of Mw = 315 000 and 90 000, and the molecular weight distributions of Mw/Mn = 1.09 and 1.88, respectively. dPS and PVME in toluene solution were purchased from Polymer Source, Inc., and Aldrich, respectively. dPS was used as received. PVME was diluted in toluene and then pored into excess n-heptane after filtering the toluene solution for precipitation, and the purified PVME was placed in a vacuum at room temperature for 72 h to remove the solvent. It is well-known that PS/PVME blend has a phase diagram of lower critical solution temperature (LCST) type. For the dPS/PVME blend used in the experiment we have estimated the cloud points using LS intensity under the heating process with a rate of 0.2 °C/min from room temperature. On the basis of the LS measurements we have constructed the phase diagram for the dPS/PVME blend which is shown in Figure 1. As seen in the diagram, the critical composition is of ∼10% of dPS by weight (WPS = 10%) and the critical temperature is Tc = 102.0 °C.

3. RESULTS AND DISCUSSION We first performed the OM measurements to see the time evolution of morphology of the dPS/PVME thin film 36 nm thick in the in-plane direction after the temperature jump into the two-phase region. The time evolution of the OM images is shown in Figure 2. In the initial stage of annealing no changes

Figure 2. Time evolution of the optical microscope (OM) images for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

were observed in the surface morphology, and then some holes were formed in the surface (in-plane direction) at ∼27 min after the temperature jump. These holes grew up with the annealing time and gradually changed to continuous networklike structure as seen at 30, 35, and 40 min of Figure 2. The continuous network-like structure is probably unstable due to excess surface energy, and finally the droplets are formed through the coalescence to reduce the surface energy. The result clearly shows that dewetting occurs after a certain incubation time. The incubation time for dewetting was determined to be ∼25 min based on the results of the fast Fourier transform of the OM images.

Figure 1. Phase diagram of dPS/PVME blend in bulk determined by light scattering with a heating rate of 0.2 °C/min.

The dPS/PVME blend thin films with WPS = 10% were prepared as follows. Given amounts of dPS and PVME were dissolved in toluene to obtain the 1.0 wt % solution, which was spun-coated onto the quartz substrate 1 mm thick to prepare the thin films. Then, the thin films were annealed at 60 °C for 24 h under vacuum to remove residual solvent. The thickness of as prepared thin film was confirmed by ellipsometer measurement to be 36 nm. B

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To examine the surface roughness in the incubation period, time-resolved XR measurements were carried out under the same experimental conditions as the OM measurements. The time evolution of XR profiles is shown in Figure 3 at various annealing times.

Figure 4. Time evolution of the top interface roughness and bottom interface roughness evaluated from the NR profiles, and the surface roughness evaluated from the XR profiles for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

shown in the figure by a down arrow, is almost identical with that estimated from the XR measurement. From the OM and XR results, it is concluded that the surface roughness of the film does not change during the incubation period (∼25 min), and the increase in the surface roughness is clearly caused by the onset of dewetting. A question we have to ask is what happened inside the film during the incubation period both in the in-plane and out-ofplane directions. To confirm it, we performed the TOF-NR measurements on the dPS/PVME thin film 36 nm thick after the temperature jump into the two-phase region. In Figure 5, we plotted the specular NR profiles as a function of Qz at the various annealing times after the temperature jump. In the specular NR data we can evaluate composition fluctuations in the out-of-plane direction. The observed fringed pattern was not so clear in the profile since the low dPS composition in the blend, leading to the low scattering contrast. However it is safely mentioned from the NR data that the fringed pattern was gradually smeared in the incubation period and it was completely obscured at around 26 min. In a previous work, we found that the dPS/PVME thin films with WdPS = 30% had a trilayer structure, consisting of surface PVME layer, middle dPS/PVME blend layer and bottom PVME layer, due to the preferential interactions of PVME to the surface and the substrate even in the one-phase region.14 We therefore applied a trilayer model for fitting the specular NR data. The results of fitting are shown by solid curves in Figure 5. As seen in the figure, we succeeded to fit the trilayer model to the data due to the fringed pattern although it is not so clear. After 26 min, however, we could not evaluate the thickness and roughness due to the disappearance of the fringes. It is noted that the surface roughness of the top PVME layer could not be characterized by fitting due to the very low scattering contrast of PVME. In the present study, we focused on the time evolution of the top interface roughness between the surface PVME layer and the middle blend layer, and the bottom interface roughness between the middle blend layer and the bottom PVME layer. In Figure 4 the time evolution of the top interface roughness and the bottom interface roughness are shown. The top interface roughness began to increase just after the temperature jump, while the bottom interface roughness started to increase at ∼10 min after the temperature jump. On the other hand, the surface

Figure 3. Time evolution of the specular XR profiles (a) and depth profile of density (b) for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

The well-defined fringed patterns were observed in the profile just after the temperature jump into the two-phase region. As the annealing proceeded the fringed pattern was abruptly smeared at around 23 min, suggesting that the surface roughness increased before the dewetting observed by OM. At 27 min, the fringed pattern was completely smeared. The electron densities of dPS and PVME are 9.1 × 1010 and 8.8 × 1010/cm−2 at the wavelength of 0.1 nm, respectively, and the small difference between these values suggests that we cannot distinguish between dPS and PVME in the XR measurement. Hence, the fringed patterns were analyzed using a single layer model on the quartz substrate. From the results of the fitting to the measured XR profile, which are shown by solid curves in Figure 3a, the surface roughness of the dPS/PVME film as a function of annealing time was evaluated and plotted in Figure 4. The surface roughness did not change until 23 min and drastically increased at 27 min. Therefore, it can be said that the incubation time of dewetting exists between 23 and 27 min from the XR measurement. The incubation time of dewetting estimated from the OM measurement (25 min), which is also C

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type phase separation would be observed, but it is not observed in the dPS/PVME thin film. As shown above, it is clear from the specular XR and NR data, composition fluctuations occur along the out-of-plane direction during the incubation period of dewetting, implying phase separation plays an important role in dewetting. However, composition fluctuations over the in-plane direction were not clarified during the incubation period. As shown in Figure 2, the OM measurements told us that nothing happens inside of the film over the in-plane direction due to a dark contrast. It is, however, very natural to speculate about composition fluctuations in the in-plane direction because the dPS/PVME thin film was in the two-phase region. We therefore examined composition fluctuations over the in-plane direction using the LS and off-specular NR measurements. We carried out the time-resolved LS measurements just after temperature jump into the two-phase region to examine the time evolution of morphology in the in-plane direction. Figure 6 shows the time evolutions of the one-dimensional scattering

Figure 6. Time evolution of the LS profiles for the dPS/PVME thin film 36 nm thick after atemperature jump to 110 °C. Figure 5. Time evolution of the specular NR profiles (a) and depth profiles of scattering length density (b) for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

profiles, which were obtained by circular averaging the twodimensional scattering profiles. The scattering intensity was not observed until 20 min but slightly began to increase at ∼20 min in the incubation period. After the incubation period (∼25 min) estimated by the OM and XR measurements the drastic increase in the intensity was observed, which must be due to dewetting. The very small increase in the LS intensity before 25 min suggests composition fluctuations in the in-plane direction. However, it is not enough to say something about this. After the incubation period, the scattering intensity increased and the scattering peak appeared at around 40 min. The peak position is at ∼0.5 μm−1 and independent of the annealing time. The position was very different from the characteristic length of phase separation of dPS/PVME bulk, which was appeared at 3.0 μm−1. We considered that the scattering peak at 40 min was not attributed to phase separation (or composition fluctuation). The characteristic size derived from the scattering peak was about 11.9 μm. This value corresponds to the correlation between the droplets of the dewetted film, which was estimated from the fast Fourier transform of the OM results. Thus, the LS results mainly show only the formation process of the droplets of dewetting. It is not surprising because contrasts in LS and OM measurements are almost identical because both of them arise from the difference of refractive indices.

roughness evaluated from the specular XR data did not change and kept constant during the incubation period, which is also shown in Figure 4. What we have to emphasize here is an increase in both the top and bottom interface roughness occur even in the incubation period, showing that composition fluctuations (or phase separation) occurred in the out-of-plane direction during the incubation period. The results also show that composition fluctuations (or phase separation) first occur at the top interface along the out-of-plane direction in the film and subsequently occur at the bottom interface. The delay of the onset of composition fluctuations in the bottom interface may be due to the hard wall effect of the substrate, which leads to motional slowing down of polymers.23,24 In any case, the specular NR data clearly show that phase separation created composition fluctuations in the out-of-plane direction. It should be noted that composition fluctuations observed here does not coherently occurred in the out-of-plane direction, which is a socalled surface directed spinodal decomposition (SDSD).25,26 If SDSD occurs in this system the Bragg scattering corresponding to the characteristic length scale in the spinodal decomposition D

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Figure 7. Time evolution of the 2D reflectivity intensities in the Qx−Qz geometries for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

In order to confirm if composition fluctuations occur during the incubation period more rigorously, we also analyzed the offspecular NR data which have different scattering contrast comparing with LS. In the out-of-plane off-specular configuration (see Figure 8 in ref15.), scattering vector Q includes the small component of Qx vector along the surface direction, meaning that structure information in the in-plane direction (or the surface direction) can be investigated based on the offspecular NR data. Qx and Qz components in Q are given by eqs 1 and 2 Qx =

2π (cos θf − cos θi) λ

(1)

Qz =

2π (sin θf − sin θi) λ

(2)

Figure 8. Time evolution of the integrated intensity profiles in Qz =0.12−0.17 nm−1 for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

where θi, θf, and λ are the incident angle, the reflected angle, and the wavelength of the neutron. Using eqs 1 and 2, one can translate the raw 2D off-specular NR data into the Qx − Qz data. Figure 7 shows the Qx − Qz plots of the reflectivity intensities in various periods of annealing time in the two-phase region. The acquisition time for the one frame was 5 min in the measurement. In the beginning of 5 min, only the specular intensity was observed at Qx = 0. However, the off-specular intensities gradually increased after 10 min even in the incubation period, and the profile is tilted along the Qx vector. To analyze these profiles, we calculated the off-specular intensity Iinteg(Qx) integrated in the Qz region from 0.12 to 0.17 nm−1 as a function of Qx and plotted in Figure 8 in various periods of annealing time. The off-specular intensity Iinteg(Qx) reflects structure or composition fluctuations along the surface direction (or the inplane direction). At 10 min after the temperature jump Iinteg(Qx) began to increase with a clear peak and grew up with the annealing time, suggesting some structure or composition fluctuations occurred in the in-plane direction even in the incubation period. As shown above, no changes in the OM and XR measurements were observed on the surface during the incubation period and hence the increase of Iinteg(Qx) must reflect composition fluctuations in the thin film blend in the inplane direction. The off-specular peak position Qinteg,max and the peak intensity Iinteg,max are plotted in Figure 9, parts a and b,

Figure 9. Time evolution of the peak position Qinteg,max (a) and peak intensity Iinteg,max (b) estimated from the LS and Off-specular profiles for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C.

respectively, to see the time evolution of composition fluctuations in the in-plane direction. The peak intensity Iinteg,max appears at ∼10 min after the temperature jump and grows up with the annealing time until ∼25 min during the incubation period of dewetting. This observation suggests that the in-plane composition fluctuations grow before dewetting. During this period, the peak position shifts to the lower Qx region (Figure 9a), suggesting that the in-plane composition fluctuations grow inside the films before dewetting. Coinciding E

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related to the in-plane fluctuations although the details are not clear. The off-specular results also suggested that composition fluctuations caused by phase separation have a characteristic wavelength of ∼8.4 μm in the beginning, which is much larger that that of bulk phase separation, and grow with annealing time. These in-plane composition fluctuations give rise to height fluctuations from the inside of the film and finally induce dewetting, resulting in the droplets of the dewetted film, as illustrated in Figure 10, parts d and e, respectively. We give some comments on the inner structure of droplets (Figure 10e). As shown by the specular NR measurements, PVME is segregated in the surface and bottom layers before dewetting, and the top and bottom interface roughness increase due to composition fluctuations (or phase separation) with annealing time, meaning that the PVME chains segregate and gather more in the surface and bottom layer, and finally the film is broken up into droplets, which may include dPS rich core encapsulated by PVME. This is illustrated in Figure 10e. X-ray photoelectron spectroscopy (XPS) measurements on PS/PVME droplets by El-Mabrouk et al.20 support our picture. In the next part, we discuss the 2D off-specular NR data to consider the in-plane composition fluctuations before the dewetting and the droplet structure after the dewetting in more details. As seen in Figure 7 the Qx − Qz off-specular NR profile slightly but clearly directed toward the diagonal direction (see the difference of scales in Qx and Qz). This result indicates that the characteristic wavelength of the in-plane composition fluctuations depends on Qz. In Figure 8 the off-specular NR intensity Iinteg,max integrated in the Qz range of 0.12 to 0.17 nm−1 was shown to see the average structure in the Qz range. To see the detailed structure depending on Qz we have calculated Ioff(Qx) at various Qz positions and plotted Ioff(Qx) at Qz = 0.12 and 0.17 nm−1 in Figure 11, parts a and b, respectively. As seen in Figure 11, the peak positions at Qz = 0.12 nm−1 are smaller than those at Qz = 0.17 nm, showing that

with the onset of dewetting, the growth of both the peak intensity and peak position stop and remain constant. As seen in Figure 9a, the peak position agrees with that of the peak position of LS, meaning the distance between the droplets of the dewetted film is detected by the off-specular NR measurements after dewetting. The scattering peak position Qinteg,max at 10 min in the initial stage is ∼0.75 μm−1, corresponding to a characteristic wavelength of ∼8.4 μm, which is larger than the characteristic wavelength of phase separation in dPS/PVME bulk (∼2.1 μm),14 and there is an incubation period of ∼10 min before the onset of the in-plane composition fluctuations (see Figure 9b). These observations imply that the in-plane composition fluctuations observed here in the dPS/PVME thin film 36 nm thick are different from composition fluctuations of bulk phase separation. The effects of out-of-plane composition fluctuations and/or the confinement may affect the in-plane composition fluctuations although the details are still unclear. On the basis of the measurements of the OM, XR, NR, and LS, a schematic illustration of the cross-sectional view in the dewetting process via phase separation (or composition fluctuations) is drawn in Figure 10. From the results of the specular NR measurements, the out-of-plane composition fluctuations occur randomly over the in-plane direction in the beginning of the incubation period, as shown in Figure 10b. Consequently, the in-plane composition fluctuations arise after the out-of-plane fluctuations (Figure 10c) as seen in the offspecular NR data. The out-of-plane fluctuations are somehow

Figure 10. Schematic representation of the structure formation in the dewetting process. (a) Thin film formed trilayer structure in the onephase region. (b) Composition fluctuations along the out-of-plane direction to produce the composition gradient in the in-plane direction occurred. (c) Composition fluctuations in the in-plane direction occurred. (d) Composition fluctuations induced height fluctuations. (e) Droplets are formed with the inside structure of dPS rich segregation.

Figure 11. Time evolution of the off-specular intensity profiles at Qz = 0.12 (a) and 0.17 nm−1 (b), and the time evolution of peak position Qx,max (c) and peak intensity Ioff,max (d) evaluated from the LS and Offspecular NR profiles for the dPS/PVME thin film 36 nm thick after the temperature jump to 110 °C. F

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the in-plane composition fluctuations are in larger scale at lower Qz. The time evolutions of the off-specular peak position Qoff,max and peak intensity Ioff,max at Qz = 0.12, 0.15, and 0.17 nm−1 are also shown in Figure 11, parts c and d, respectively. The scattering peak position Qoff,max clearly depends on the Qz value even before and after dewetting. It is clear that the inplane composition fluctuations have the different characteristic wavelength along the depth direction. The peak positions in the LS profiles exist between 0.12 and 0.15 nm−1 of the off-specular NR peaks, confirming that we observed the average distance between the droplets after dewetting in the LS measurements. On the other hand, the time evolutions Qoff,max and Ioff,max in Figure 11, parts c and d, are almost independent of Qz, suggesting the similar growth manners of the in-plane fluctuations at each Qz. Finally, we discuss the reason why composition fluctuations have the dependence on the depth direction. Even after the onset of dewetting, the correlation length depends on the Qz position. If the inside of the droplets is symmetry along the depth direction, the correlation length corresponding to a distance between the centers of the droplets is independent of the Qz position. In other words, the scattering peaks must accord with each other after the onset of dewetting. Hence, the observed results suggest that the dPS rich phase in the droplets is asymmetry along the depth direction (see Figure 10e). In order to minimize the surface free energy, the dPS rich phase may segregate inside the droplets after phase separation. From atomic force microscope observation,16 the droplet diameter and height are approximately 200 and 2000 nm, respectively, and the cross sections of the droplets have an aspect ratio of 1:10. Consequently, the droplets of the dPS rich layer may form a disc-shaped structure and diagonally tilt along the depth direction. The tilt angle of each droplet may have a distribution over the out-of-plane direction, which is indicated from the wide distribution of the peak position in the off-specular profiles.

phase separation occurs during the incubation period in the inplane and out-of-plane directions before dewetting, supporting an idea that composition fluctuation induced dewetting in the dPS/PVME thin films. Finally it should be emphasized that in the off-specular NR measurements we could measure the correlation length over the in-plane direction until the onset time of dewetting and discuss the inside structure of dewetting segregate, showing a powerful potential of off-specular NR measurements for the in-plane structure studies of thin films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.O.), [email protected]. jp (T.K.). Tel.: +81-75-58-0833 (H.O.), +81-74-38-3142 (T.K.). Fax: +81-75-58-1873 (H.O.), +81-74-38-3146 (T.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Collaborative Research Program of Institute for Chemical Research, Kyoto University (Grant No. 2012-16). The neutron reflectivity measurements were performed in a S-type research project, High Energy Accelerator Organization (KEK) (Proposal No. 2009S08). The synchrotron radiation X-ray reflectivity measurements were carried out at the first hutch of the Consortium of Advanced Softmaterial Beamline (FSBL) (Proposal No. 2012B1950).

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REFERENCES

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4. CONCLUSION In this article, we have investigated the structure development of the dPS/PVME thin film 36 nm thick on the quartz substrate over the in-plane and along the out-of-plane directions in the dewetting process of the two-phase region using the OM, LS, specular XR, specular and off-specular NR measurements. It was found from the OM and specular XR measurements that no changes were observed on the surface of the thin film after the temperature jump into the two-phase region, showing that the incubation period existed before dewetting. To see the change of the inner structure in the out-of-plane direction during the incubation period, the time-resolved specular NR measurements were performed. It was found that the dPS/ PVME thin film has a trilayer structure even in the one−phase region, consisting of top PVME layer, middle dPS/PVME blend layer and bottom PVME layer. The top interface roughness between the surface PVME layer and the middle dPS/PVME blend layer, and the bottom interface layer between the middle dPS/PVME blend layer and the bottom PVME layer increased during the incubation period, suggesting that the out-of-plane composition fluctuations occurred before dewetting. The off-specular NR measurements, on the other hand, revealed that the in-plane composition fluctuations also began to increase during the incubation period and grew up to induce the height fluctuations, giving rise to dewetting. These observations clearly shows that composition fluctuations due to G

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dx.doi.org/10.1021/ma400506f | Macromolecules XXXX, XXX, XXX−XXX