Formation of High-Density Brush of Liquid Crystalline Polymer Block

Jul 18, 2019 - In this paper, we will report on the morphology evolution of a layer of LC ..... ordering even if the brush layer does not entirely cov...
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Formation of High-Density Brush of Liquid Crystalline Polymer Block Associated with Dewetting Process on Amorphous Polymer Film Koji Mukai,†,§ Mitsuo Hara,† Shusaku Nagano,*,‡ and Takahiro Seki*,† †

Department of Molecular Design and Engineering, Graduate School of Engineering and ‡Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

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S Supporting Information *

ABSTRACT: The understanding of polymer dewetting on solid surfaces is significant in both fundamental polymer physics and practical film technologies. When liquid crystalline (LC) polymers are dewetted, LC ordering is involved in the dewetting process. Here, we report on the characteristic dewetting processes of a diblock copolymer composed of a cyanobiphenyl side chain liquid crystalline polymer (SCLCP) block connected with polystyrene (PS) taking place on a PS base film. Thin films of the block copolymer were prepared by the water-floating method onto the PS film, and the dewetting process is observed in a softened state above the glass transition temperature of the PS. At the smectic A phase temperature of the SCLCP block, the dewetted surface layer generated a flat unique fingering pattern leading to a monolayered (two-dimensional) highdensity LC polymer brush through the LC ordering. The important role of the anchoring PS block on the base PS film surface is suggested for the formation of highly stretched LC polymer brush. Above the isotropization temperature, in contrast, ordinary three-dimensional droplet morphologies with smooth round edges were observed. By photo-cross-linking the base PS film, the lateral diffusion rate was significantly reduced. This can be applied to an entropy-driven morphology patterning via dewetting. The polymer brush formation and its spatial controls are expected to provide new opportunities for the modification strategies of polymer surfaces.

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conformation) directing vertical to the surface.8 In terms of structural features and photoalignment property, the LC polymer brush obtained this way is similar to those obtained by the synthetic procedure through surface-initiated living polymerization.4,5 Although the resultant structure of the LC polymer brush is well characterized,7 how such high-density polymer brush is formed on the base polymer surface and the role of the anchoring block is not well understood. For this purpose, we have designed another type of experimental approach. In this paper, we will report on the morphology evolution of a layer of LC block copolymer placed on a PS film during the dewetting process. Here, the layer of polystyrene-blockpoly[12-(4′-cyanobiphenyl)-4-oxydodecyl acrylate] (PS-bPCB; Scheme 1) floating on water is transferred onto a PS film to obtain a double-layered film (water-floating method; Scheme 2). For such double-layered films, the PS-b-PCB thin film on the PS is found to exhibit characteristic dewetting morphologies on the PS film upon annealing above Tg of PS. The resulting morphology is strongly coupled with the LC ordering of the SCLCP block. Dewetting of a polymer film is a process in which a polymer thin film spontaneously withdraws from the surface.9−14 This is

INTRODUCTION High-density polymer brush surfaces on a solid substrate are mostly prepared by synthetic procedures by grafting from living polymerization starting from an initiator layer on the substrate surface.1−5 Alternatively, recent investigations have revealed that spontaneous self-assembly processes of block copolymers via surface segregation on softened amorphous polymer film can also provide high-density polymer brushes. For example, a hydrophilic poly(ethylene oxide) block chain on polystyrene (PS) or poly(dimethyl siloxane) network upon contact with water6,7 and a side chain liquid crystalline polymer (SCLCP) block8 on polystyrene film towards the air are expected to provide a self-healing surface, where if part of the surface brush layer is damaged, it will be restored again by following surface segregation as long as the buried block copolymer components are still reserved.6−8 In the latter case of SCLCPs, the process involves (i) mixing small amount of diblock copolymer having an amorphous polymer film (e.g., PS) and (ii) annealing above the glass transition temperature (Tg) of the base amorphous film.8 During the annealing step, the diblock copolymer segregated to the free surface onto an amorphous polymer film and simultaneously the surface segregated a liquid crystalline (LC) block laterally aggregate to form a high-density polymer brush through LC ordering. This method is simple but provides a highly stretched main chain conformation (approximately 80% of the fully stretched all-trans-zigzag © XXXX American Chemical Society

Received: June 4, 2019 Revised: July 11, 2019 Published: July 18, 2019 A

DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX

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The average unit numbers (N) of the block copolymer were 106 and 165 for the PS and PCB blocks, respectively (Mn = 8.3 × 104, Mw/Mn = 1.36). N of the PCB homopolymer was 134 (Mn = 5.8 × 104, Mw/ Mn = 1.19). Polystyrene (PS), which was used as the base polymer of the films, was purchased from Aldrich (Mw = 2.8 × 105) and used without further purification. The thermophysical properties of the polymers were evaluated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray scattering (XRS) measurements. The phase transition behavior of PS-b-PCB was from glass (27 °C) to smectic A (135 °C) to isotropic, which was essentially identical to that of the PCB homopolymer (Figures S2 and S3), which indicates the microphase separation of the two blocks. Tg of PS was 105 °C. Preparation of Double-Layered Films. The double-layered polymer films in this study were prepared by the following procedures (Scheme 2). First, a PS film was prepared onto a silicon wafer by spin coating from a toluene solution. The PS film was annealed at 120 °C (above Tg) for 1 h (the film thickness: approximately 90 nm). A glass substrate was washed with saturated potassium hydroxide ethanol solution, followed by washing with pure water under sonication. PS-bPCB film was prepared onto a glass substrate by spin coating with chloroform solution and dried at 25 °C. The film thickness was controlled by changing the concentration of the polymer solutions and the spinning rate. Then, the films were floated off on water and transferred onto the PS base film (water-floating method32,33). The resulting doubled-layered film was dried under vacuum at 25 °C for 12 h. The thickness of the LC top layer was evaluated by the height profile measurement of the pretransfer film after scratching using a white light interferometric microscope (BW-S501, Nikon Instruments). The thickness of the base PS film was estimated from the surface profile after scratching the film on a silicon wafer using a Nanopics 2100 (Seiko Instruments Inc.). The cross-linked PS film was fabricated by UV light irradiation treatment under vacuum.34,35 UV light (254 nm) irradiation was performed by using a Hg−Xe lamp (UVF-204S, San-ei Electronic) through an illumination guide unit (5Φ-1B-1000L, San-ei Electronic) at a light dose of 64 J cm−2 in a vacuum chamber at 3 × 102 Pa. For the cross-linked pattern, photomasks with the line and space pitch of 1, 10, and 20 μm were used. Measurements. In situ observation of the surface morphology on the bilayer films was performed by a reflecting optical microscope (BX51-P, Olympus) with a CCD camera (DP26-B, Olympus). The double-layered film was heated from room temperature and maintained at a target temperature during microscopic observations on a thermostated hot stage (FP90, Mettler-Toledo). Atomic force microscopic (AFM) observations were made in the tapping mode to obtain the surface topography. The height and phase images were recorded simultaneously by using a MFP-30 system (Asylum Research, Oxford Instruments) in ambient conditions (in the air at 25 °C). The image scan was performed in the repulsive region by using an Al-coated cantilever (OMCL-AC240TS, Olympus). Transmission electron microscopy (TEM) observation was performed by JEM2100Plus (JEOL) equipped with an acceleration voltage of 200 kV. For the cross-sectional TEM observation, the polymer bilayer films were prepared on Kapton (polyimide) substrate, which was hydrophilically treated by a Xe2* excimer light emission unit (UER 20-172VB, USHIO) under 7 × 102 Pa. The films

Scheme 1. Chemical Structure of Polymers

a serious nuisance in the polymer thin film technology such as microelectronics and painting, and usually much effort has been made to avoid dewetting to produce defect-free polymer thin films. However, dewetting itself can be utilized as an attractive microfabrication when it is controlled on a patterned substrate surface.15−19 Dewetting behavior is associated with enthalpic and entropic factors, chain entanglements, chain mobility, and chain anchoring onto the substrate surface. Therefore, in addition to ordinary linear polymers, endfunctionalized polymer,20 block copolymers,21−23 star polymers,24 and ring polymers25 are also interesting class of polymer materials for dewetting processes. Additionally, the dewetting behavior of LC polymers has aroused another interest, i.e., a hierarchical LC-ordered structure involved during the dewetting process.26−29 Based on these backgrounds, we will discuss the features of the dewetting and self-assembly (LC ordering) processes of PS-b-PCB on the PS film leading to the high-density LC polymer brush formation. In this work, cyanobiphenyl (CB) mesogen is selected instead of azobenzene mesogen examined in the previous work 8 because CB affords a higher isotropization temperature (Tiso = 135 °C). This enables comparisons of dewetting behavior above and below Tiso in the softened state of the base PS film above Tg (105 °C). This work also proposes a new type of entropy-driven patterning via dewetting based on the difference in segmental mobility of the PS surface attained by patterned photo-cross-linking. This attempt is in contrast to the examples of the chemically patterned surfaces.15−19

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EXPERIMENTAL SECTION

Materials. LC diblock copolymer (PS-b-PCB; Scheme 1) and the LC homopolymer (PCB; Scheme 1) were synthesized by activators regenerated by electron-transfer atom-transfer radical polymerization (ARGET ATRP) method30 (see the Supporting Information for details). The synthesis of the CB monomer and the characterization methods of the polymers have been described in the previous paper.31

Scheme 2. Schematic of the Preparation Procedure for Double-Layered Polymer Films by the Water-Floating Method

B

DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX

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Langmuir embedded in epoxy resin (Quetol-812, MNA, NSA and DMP-30, Nisshin EM Co.) were sliced to a thin film with approximately 100 nm thickness using a ultramicrotome (Laica EMUC7RT, LAICA Microsystems). The ultrathin slice was transferred to the copper grid and stained with RuO4 vapor for 20 min. Grazing angle incidence small-angle X-ray scattering (GI-SAXS) measurements were performed with a FR-E X-ray diffractometer (Rigaku) with Cu Kα radiation (λ = 0.154 nm). The scattering images were captured with a two-dimensional imaging plate detector R-AXIS IV (Rigaku). The camera length was set as 300 mm. Young’s modulus of the thin film was evaluated by the force− indentation curve in the MFP-30 AFM system with a heater stage (MFP-3D CoolerHeater Stage, Asylum Research). In one measurement, 256 force curves were captured in a 50 μm × 50 μm area (16 × 16 points) with an indentation depth of 2.5−13 nm and an indentation speed of 400 nm s−1. The spring constant of an Alcoated cantilever (SD-Sphere-NCH-S-10, Nanosensors, tip radius: 400 nm) was calculated to be 39−47 N m−1 in the thermal spring constant calibration. The inverse optical laser sensitivity (InvOLS) of the photodiode was calibrated from the force−indentation curve on a clean mica surface. The force−indentation curves in the retraction process were processed through a Johnson−Kendall−Roberts model fitting to obtain Young’s modulus of the sample film.36 Static contact angle was measured with a CA-XP (Kyowa Interface) at room temperature. Three microliters of Milli-Q water was dropped onto polymer films, and the contact angle was measured at 10 s after dropping. The average value was calculated from 5 measurements at different points.

Figure 2. Time evolution of the optical microscopy images for PS-bPCB (20 nm)/PS bilayer films observed during heating at 120 °C for 5 min (a), 20 min (b), 2 h (c), and 4 h (d).

further annealing, distinct fingering patterns as dendritic structures developed (c and d). No smooth edges were observed in the film surface in this case. Further, the film thickness of PS-b-PCB was kept constant seeing from the absence of the interference color even for 4 h (d) annealing (cf. Figure 1d). PS-b-PCB in the bulk state exhibited a cylinder-type microphase separation (PS: cylinder, cylinder-tocylinder distance of 49 nm) as revealed by the small-angle Xray scattering measurement. We assume that this morphological feature in the bulk did not directly reflect on the surface dewetting process under study. Figure 3 depicts the topographical and phase-mode AFM images of the dewetted films upon annealing at 140 °C (a−c) and 120 °C (d−f). Dewetting motions almost ceased in 20 h. In both images, the areas of PS-b-PCB (higher regions) and the exposed PS film surface (lower flat regions) in the topographical images are clearly visualized with sharp contrast. In the phase mode, the lower and higher modulus regions are imaged as bright and dark areas, respectively. The former and latter correspond to the surfaces of the PS-b-PCB layer and the exposed PS film, respectively. Upon annealing above Tiso (140 °C), the height of the island domains reached 195 ± 19 nm from the underlying PS layer (a and c). The thickness of the initial layer of PS-b-PCB was approximately 20 nm; in this case, therefore, the increase in thickness reaches approximately ten times that of initial layer. On the other hand, annealing at 120 °C (smectic A phase of PS-b-PCB) provided films with almost uniform thickness of 34 ± 2 nm (d and f). This thickness is comparable to the length of the fully extended PCB block of PS-b-PCB (see below). The film thickness is essentially unchanged regardless of the extent of dewetting, strongly suggesting the formation of a monolayered polymer brush. The fingering morphology was also characterized by the formation of rims at the edges (f). It is known that the formation of the fingering patterns (Figure 2) and rims is observed in a highly viscous state of the dewetting material.10,11,37−39 Therefore, the fingering patterns observed at 120 °C indicate that the dewetting occurs under limited polymer diffusion conditions in a more viscous smectic A phase. Changes in height of the film during dewetting with time were evaluated by AFM (Figure 4). The height of the film reached over 100 nm within 20 min at 140 °C and increased to a large extent along with the annealing time. At 120 °C, in contrast, the height of the film increased only slightly (20 → 34 nm) within the initial 60 min and remained unchanged with a

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RESULTS AND DISCUSSION Dewetting Behavior of PS-b-PCB Layer on PS Substrate. Figure 1 displays optical microscopy images of a

Figure 1. Time evolution of the optical microscopy images for PS-bPCB (20 nm)/PS bilayer films observed during heating at 140 °C for 1 min (a), 20 min (b), 2 h (c), and 4 h (d).

PS-b-PCB (20 nm)/PS double-layered film upon annealing at 140 °C (above Tiso of PS-b-PCB block). Upon annealing, holes originating from the instability of the PS-b-PCB layer appeared. The holes expanded in the initial stage (a and b) and adjacent holes fused (c and d) later. The gradual interference coloration in the PS-b-PCB film (see d, for example) suggests that dewetting above Tiso progressed as a polymer liquid. Such a dropletlike dewetting behavior of PS-b-PCB on the PS film is typical for liquefied polymer thin films on a solid substrate.9−11 In contrast, when the annealing temperature was lowered to 120 °C (corresponding to the smectic A phase of PCB), completely different morphologies appeared (Figure 2). The PS-b-PCB thin layer started to break up around 5 min (a), and the dewetted regions grew after some minutes (b). Upon C

DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. AFM topographic (a, d) and phase (b, e) images of PS-b-PCB(20 nm)/PS bilayer film after annealing. The dark and bright regions in the phase images in (b) and (e) correspond to the lower- and higher-modulus regions, respectively. Cross-sectional height profiles along the red lines in (a) and (d) are shown in (c) and (f), respectively.

Figure 4. Height growth of the dewetted domains as a function of annealing time at 140 °C (squares) and at 120 °C (diamonds) evaluated from the AFM topographical images.

smooth surface even at longer annealing time as stated above. The characteristic dewetting morphology induction should be a two-dimensional (2D) fractal phenomenon, similar to the one observed in diffusion-limited aggregation,40 crystallization on roughened surfaces,41 and fluid flow in the Hele−Shaw cells.42 Dynamic viscous fingering has also been studied in the Hele−Shaw cells containing thermotropic LCs.43,44 We assume that the morphology induction observed here for the SCLCP brush should be closely related to the processes in such Hele− Shaw cells of LCs. Structural Evaluations of Dewetted PS-b-PCB Layer. Figure 5 shows 2D GI-SAXS images of the PS-b-PCB (20 nm)/PS double layered film before and after annealing at 120 °C (smectic A phase) for 20 h. Before annealing, the PS-bPCB/PS film exhibited no scattering signals (a), suggesting that the PCB block possesses no regular structure. After annealing, X-ray scattering spots were observed in the in-plane direction at the scattering vector (q) values of 1.48 and 2.93 nm−1 (b), giving the spacing (d) of 4.2 and 2.1 nm, respectively, which agree well with the X-ray data in the bulk (Figure S4). These scatterings are attributed to the first- and second-order scatterings of the smectic lamellar of PCB with a bilayer formation.45 The in-plane scattering shows that the

Figure 5. GI-SAXS 2D pattern image of the PS-b-PCB(20 nm)/PS bilayer film before (a) and after (b) annealing at 120 °C for 20 h.

smectic lamella is formed vertically to the surface plane8 in the dewetted bilayer film. The cross-sectional TEM was performed on the PS-b-PCB (20 nm)/PS double-layered film supported on a polyimide substrate after annealing at 120 °C (Figure 6). The film was

Figure 6. Cross-sectional TEM images of the PS-b-PCB (20 nm)/PS bilayer film after annealing at 120 °C for 20 h. D

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regarded as the SAM formation process at the macromolecular level. Considering the above results, initial thickness of the transferred PS-b-PCB film from the water surface was changed from 10 to 90 nm. The relationship between the initial (ho) and the resulting thickness (h) of the PS-b-PCB layer after dewetting at 120 °C was compared (Figure S7). When the initial thickness was in the range of 9−30 nm, the dewetted film thickness was almost the same (approximately 30 nm). As discussed, this thickness corresponds to the length of the vertically oriented PCB brush layer, which agrees with the thickness after dewetting in Figures 3f and 6. When ho was above 34 nm, the h value increased linearly with increase in ho (Figure S7). These results can be interpreted as follows. In the thinner region below 30 nm, the dewetting simply induces a high-density polymer brush formation through the lateral migration to form a vertically oriented smectic A structure. When ho becomes thicker than the critical level (34 nm), the excess amount of PS-b-PCB chain that cannot be accommodated in the monolayered polymer brush is extruded without anchoring on top of the polymer brush monolayer. Brush Formation on Patterned Photo-Cross-Linked PS. As indicated above, the lateral migrating process is the key to the high-density brushes when ho ≤ 30 nm. Under this condition, the lateral migration behavior is expected to be tuned by cross-linking the base PS. For this purpose, UV irradiation under vacuum was performed. The UV (254 nm) irradiation with a reduced pressure avoids the oxidation of PS, leading to the cross-linking between the PS chains.34,35 Figure 8 shows optical microscopy images of the dewetting structure of PS-b-PCB (ho = 23 nm) on a locally photo-cross-

stained by RuO4 vapor, which preferentially stains the aromatic benzene rings. The less stained (brighter) top layer corresponds to the PCB block region with a lower phenyl group density compared with the PS base film. The layer thickness of the PCB block was estimated to be 34 ± 4 nm in the TEM image, in good agreement with the AFM data (Figure 3f). By dividing the layer thickness of 34 nm by the degree of polymerization (N) for the PCB block, the monomer unit length of the PCB block was calculated to be 0.206 [nm]. For the ideal all-trans-zigzag conformation of poly(methacrylate), the main chain should provide this value as 0.254 [nm].46 Thus, the PCB block of PS-b-PCB is extended to 80% of the ideal all-trans-zigzag conformation in the thickness direction. This degree of chain extension is exactly the same as previously observed for the azobenzene-containing SCLCP via surface segregation.8 It is stressed here that the high-density brush is attained by the lateral self-assembly via LC ordering even if the brush layer does not entirely cover the surface. This aspect is different from the one widely studied for the high-density polymer brush formation of amorphous polymer chains attained by the 2D exclusion effect.1−3,6,7 All of the above structural characterization data indicate that the PCB block forms a high-density polymer brush on the base PS substrate. PS-b-PCB extends the PCB block chain in the vertical direction with the anchored PS block having a high affinity to the base PS surface. We can assume that the highdensity polymer brush formation is attained by the LC ordering of smectic lamellar layer formation of the PCB block through the dewetting process. Role of the PS Block in the Brush Formation. In the case of a PCB homopolymer/PS double-layered film, dewetting was also observed at 120 °C (Figures S5 and S6). However, the morphology was completely different from that of PS-b-PCB. Although the dewetting of the top PCB layer proceed under the smectic phase, and dropletlike dome structure with the height of 200−300 nm was formed within 30 s, similar to the case of PS-b-PCB at 140 °C (>Tiso, cf. Figure 1). This fact suggests that the monolayered brush structure of the PS-b-PCB layer was stabilized by the anchoring effect of the PS block on the PS base film (Figure 7). Thus, the

Figure 8. Optical microscopy images of the PS-b-PCB (23 nm) films on the cross-linked patterned PS film after annealing at 120 °C for 10 h. UV irradiation is performed using photomasks with line and space (equal width) patterns at 20 μm (a), 10 μm (b), and 1 μm (c) and with a figure of “T” (the figure part is exposed to UV light) (d).

linked PS film using a line and space photomask with pitches of 20 (a), 10 (b), and 1 μm (c) after annealing at 120 °C. By optimizing the line and space pitch, the brush layer of PS-bPCB could be guided into lined patterns. The lined dewetting pattern can be recognized for 20 and 10 μm line pitch patterns (a and b); however, when the pattern was far below such a distance (1 μm), the directed dewetting pattern was not distinct due to the collective domain formation (c).16,19,48 The dewetting feature could be also visualized with a characteristic letter pattern, for example, a figure of “T” (d). The evolution rate of the dewetting of the PS-b-PCB film on PS was compared to those of the photo-cross-linked and pristine

Figure 7. Schematics of the dewetting processes of PS-b-PCB (upper) and PCB (lower) upon annealing.

lateral migration of PS-b-PCB with the retention of the anchored PS block on the PS base film is essential for LC ordering, resulting in a vertically aligned smectic lamellar structure. The important role of lateral migration in the highly oriented ordered structure formation resembles the process of a self-assembled monolayer (SAM) formation of alkyl thiol compound on the gold surface.47 The present example can be E

DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX

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Langmuir (shaded) PS film surfaces (Figure S8). The dewetting rate of PS-b-PCB on the photo-cross-linked PS surface was considerably lowered than that on the pristine PS surface. Additionally, the percentage of the resultant withdrawn area of the PS-b-PCB film on the photo-cross-linked PS was significantly smaller (25%) than that of the pristine PS surface (70%) (Figure S8). Therefore, on the photopatterned PS film, the dewetting of PS-b-PCB preferentially takes place on the pristine surface to form a line patterned PS-b-PCB brush surface. The PS chain dynamics should be reflected to the rheological properties. Therefore, Young’s modulus was evaluated by the force−indentation curves on the PS films with an AFM cantilever tip (Figure S9). In these measurements, the cantilever tip was indented into the PS film at a depth of 2.5−13 nm from the surface for a film 90 nm thick. Therefore, Young’s modulus will reflect the characteristics of the PS film surface, avoiding the influence of the Si wafer substrate. Figure 9 shows Young’s moduli on the pristine (blue

properties of the PS-b-PCB layer. The cross-linked PS network is less favored for the molecular mobility (lowered entropy), resulting in the slower migration rate of PS-b-PCB on the PS film (Figure S8). The surface free energy of the cross-linked PS will become higher than that of the pristine PS, which consequently generates the PS-b-PCB brush patterns preferentially placed on the photo-cross-linked PS surface to minimize the surface free energy.

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CONCLUSIONS In this study, we have investigated the aggregation processes of PS-b-PCB on the PS film surface at temperatures above and below Tiso of the LC polymer block using double-layered films prepared by water-floating method. The dewetting of the PS-bPCB layer below Tiso and above Tg of PS provides the characteristic fingering patterns. Under these conditions, the vertically oriented monolayered (2D) state of PS-b-PCB is maintained with the aid of surface anchoring of the PS block. At higher temperatures above Tiso, in contrast, dewetting at larger scales progresses as is usually observed for amorphous polymer liquids on solid substrates. It is stressed that, for the formation of high-density polymer LC brush, the lateral migration accompanied by the self-assembling LC ordering with the retention of the anchoring chain of the PS block is essential. In our previous approach,7 both the surface segregation and lateral migration occur in the process of high-density polymer brush formation. The present approach has clearly unveiled the lateral migration behavior. In addition, we have proposed the entropy-driven self-assembly of the polymer brush by using the patterned photo-cross-linked PS surface. The facile polymer brush preparation procedure has potential to expand the possibilities of the surface polymer brush applications.

Figure 9. Young’s modulus of the pristine PS substrate (●: 25 °C, ○: 120 °C) and photo-cross-linked PS substrate (⧫: 25 °C, ◇: 120 °C) at different indentation depths.

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ASSOCIATED CONTENT

S Supporting Information *

symbols) and photo-cross-linked (red symbols) PS films at 25 and 120 °C. At 25 °C (glass state of PS), Young’s modulus values (closed symbols) did not show appreciable differences between the pristine PS (3.4−4.1 GPa) and the photo-crosslinked PS (3.6−4.3 GPa). In contrast, clear differences in Young’s modulus were observed by heating to 120 °C (above Tg of PS). For the pristine PS film, Young’s modulus at 120 °C was 40−740 MPa, depending on the indentation depth, which is 1−18% of the values at 25 °C. On the other hand, the decrease in Young’s modulus by heating was considerably suppressed on the photo-cross-linked PS film surface (1.0−2.6 GPa). Young’s moduli of the photo-cross-linked PS were 4−25 times larger than those of the pristine PS. The dependence of Young’s modulus on the indentation depth at 120 °C can be ascribed to the hysteresis character in the force−indentation curves49 (Figure S9c,d), namely, the strong adhesive nature of the PS film. The suppression of the decrease of Young’s modulus for the cross-linked PS reflects the hardening resulting from more restricted polymer chain motions. Static contact angle measurements of the water droplet (θw) indicated that the contact angles of the two substrates were essentially identical. The pristine and photo-cross-linked PS films gave θw = 95.3 ± 0.3 and 94 ± 0.6°, respectively. This implies that photo-cross-linking does not alter the enthalpy factor in the surface free energy. The entropic factor stemming from the PS chain mobility should affect the dewetting

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01689. Synthesis of polymers, DSC data, POM images, XRS profiles, POM images of PCB/PS bilayer film, AFM images of PCB/PS bilayer film, relationship between the initial resulting thickness of PS-b-PCB by annealing, dewetting rate data, force−indentation curves in the AFM measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.N.). *E-mail: [email protected] (T.S.). ORCID

Shusaku Nagano: 0000-0002-3929-6377 Takahiro Seki: 0000-0003-3010-2641 Present Address §

Toray Industries Incorporation, Nagoya 455-0024, Japan (K.M.). Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We thank Ryota Higashi for experimental assistance. This work was supported by JSPS KAKENHI Grant Number 16H01084 (to T.S.), 19H02774 (to S.N.) for Basic Scientific Research, 18K14283 (to M.H.) for Grant-in-Aid for Young Scientists, and JSPS Research Fellowship for Young Scientists (to K.M.).

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DOI: 10.1021/acs.langmuir.9b01689 Langmuir XXXX, XXX, XXX−XXX