Radial Linear Polymer Patterns Driven by the Marangoni Instability

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Radial Linear Polymer Patterns Driven by the Marangoni Instability and Lateral Phase Separation for the Formation of Nanoscale Perforation Lines Bo-Hao Wu, Kai-Chieh Chang, Hsun-Hao Hsu, Yu-Jing Chiu, Tang-Yao Chiu, Hsiao-Fan Tseng, Jia-Wei Li, and Jiun-Tai Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00615 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Nano Materials

Radial Linear Polymer Patterns Driven by the Marangoni Instability and Lateral Phase Separation for the Formation of Nanoscale Perforation Lines Bo-Hao Wu,1 Kai-Chieh Chang,1 Hsun-Hao Hsu,1 Yu-Jing Chiu,1,2 Tang-Yao Chiu,1 Hsiao-Fan Tseng,1 Jia-Wei Li,1 and Jiun-Tai Chen1,2,3* 1Department

of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010

2Sustainable

Chemical Science and Technology, Taiwan International Graduate Program, Academia

Sinica and National Chiao Tung University, Hsinchu, Taiwan 30010 3Center

for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu, Taiwan

30010

*To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +886-3-5731631

ABSTRACT Phase separation under convection, related to the Marangoni instability, is critical in the development of advanced polymer processing technologies. The formation mechanisms of polymer patterns driven by the phase separation under convection, however, still require further investigations. In this work, we study the phase separation of polystyrene (PS)/poly(methyl methacrylate) (PMMA) bilayer films during the spincoating processes. PMMA film-coated glass substrates are dripped by PS solutions in toluene before spinning. The concentration gradients of the PS solutions and the swollen PMMA films cause the surface tension-driven Marangoni instability; lateral phase separations also occur during the spin-coating 1 ACS Paragon Plus Environment

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processes, forming the interesting and uncommon radial linear patterns with nanoscale heights on the PS/PMMA bilayer films. By changing the solution waiting time and spin rates, the surface morphologies of the PS/PMMA bilayer films can be controlled. At longer waiting time, the higher degrees of swelling of the PMMA films by the solvents allow the formation of the polymer patterns; at higher spin-coating rates, the faster solvent evaporation improves the ordering of the polymer patterns. Furthermore, the morphologies of the PS/PMMA bilayer films can be confirmed using the selective removal technique; the PS films with linear-arranged cavities and the PMMA films with bumps can be obtained using acetic acid and cyclohexane, respectively. Finally, a proof of concept on the potentials in applying the cavitiescontaining PS films for nanoscale perforation lines is also demonstrated.

Keywords: Marangoni instability, perforation lines, phase separation under convection, radial linear patterns, spin-coating

INTRODUCTION Phase separation is a common phenomenon for polymer blends and has been widely studied.1-4 Phase separation under convection, which is important in the development of advanced processing technologies for polymer materials, however, has been less studied.5 To study the phase separation under convection, one of the commonly investigated systems is using polymer blend solutions during the spin-coating processes.6-8 For example, Jukes et al. used time-resolved small-angle light scattering to investigate the phase separation of semiconducting polymer blends in the spin-coating processes;9 they concluded that the morphologies of the blends are caused by either the unstable growth of a composition fluctuation or an interfacial instability induced by the loss of solvent. Wu et al. also demonstrated the experiments of forming parallel strip patterns via spin-coating from polystyrene (PS)/ Poly(vinyl pyrrolidone) (PVP)

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blend solutions with chloroform on unpatterned substrates;10 they investigated the impact of film thickness and solution viscosity on the morphologies of the patterns. Kim et al. observed periodic striping patterns with microscale pore sizes on thin films surfaces prepared by spin-coating PS and polyethylene glycol (PEG) blend solutions; they discussed the effect of different solvents on the periodic striping patterns.11 Chiu et al. also reported the formation of aligned droplet patterns by dewetting of PS/P4VP bilayer films.12 Despite these works, the mechanisms of phase separation of polymer blends under convection, especially during the spin-coating processes, are not clear and require further investigation.13-14 In this work, we examine the phase separation of polymer blends during the spin-coating processes by applying a new strategy. Polystyrene (PS) and poly(methyl methacrylate) (PMMA), two commonly used polymers with well-known physical and chemical properties, are chosen here as model materials. Glass substrates coated with dried PMMA films are first prepared. Subsequently, PS solutions in toluene are dripped on the PMMA film-coated glass substrates, followed by spin-coating processes. Toluene is a good solvent for PMMA and can swell the PMMA films before and during the spin-coating processes. As a result, the Marangoni instability, driven by the concentration gradients and therefore surface tension gradients, can be induced. Different from the Marangoni instability triggered by the thermocapillary effect, the Marangoni instability observed in this work is believed to be triggered by the solutiocapillary effect. Lateral phase separation of polymer blends, affected by the Marangoni instability, also involves in the patterns formation processes. Consequently, PS/PMMA bilayer films with radial linear patterns with nanoscale heights can be generated. The waiting time and spin rates are also changed to investigate the morphologies of the linear radial patterns. This work provides further insight on the related studies of the formation of polymer patterns driven by the Marangoni instability15-18. To confirm the morphologies of the linear radial patterns with nanoscale heights of the PS/PMMA bilayer films, a selective removal technique is applied.19-22 The PS films can be



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selectively removed by cyclohexane, leaving PMMA bumps on PMMA films; the PMMA films can be selectively removed by acetic acid, leaving PS films with linearly-arranged cavities. Besides, the cavitiescontaining PS films are used for the tearing experiments, demonstrating a proof of concept in the potential applications of cavities-containing polymer films for nanoscale perforation lines.

RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the experimental processes to fabricate the PS/PMMA bilayer films with radial linear patterns with nanoscale heights.

Figure 1 shows the schematic illustration of the experimental process to fabricate the PS/PMMA bilayer films with radial linear patterns with nanoscale heights. The PMMA film-coated glass substrates are first prepared by dripping and spinning a PMMA solution in toluene on glass substrates. The samples are annealed at 150 °C, higher than the glass transformation temperature (Tg) of PMMA (Tg of PMMA: 110 °C) for 3 h to remove the residual solvents. Subsequently, a PS solution in toluene is dripped on the PMMA film-coated glass substrates, following by the spinning processes. Different waiting time is applied before the spinning processes. After the samples are dried, PS/PMMA bilayer films with radial linear patterns with nanoscale heights can be obtained.


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Figure 2. (a, b) SEM and OM images of the PS/PMMA bilayer films with radial linear patterns. (c) Size distribution of the humps from the OM images. (d) Graphical illustration of the linear patterns. (e, f) 3D and 2D AFM height images of the PS/PMMA bilayer films with radial linear patterns. (g) Height profile from the blue dotted line in (f). (h) Graphical illustration of a hump. The waiting time for the samples are 60 s. The spin-coating rates are 4000 rpm for 90 s and 3000 rpm for 30 s.

After the formation of the PS/PMMA bilayer films with radial linear patterns with nanoscale heights by the spin-coating process, the surfaces of the PS/PMMA bilayer films are examined by scanning electron microscopy (SEM), optical microscopy (OM), and atomic force microscopy (AFM) (Figure 2). From the SEM and OM images (Figure 2a,b), the linear patterns with humps can be observed. To see the sizes of the humps more quantitatively, the distribution of the diameters of the humps from the OM images is plotted in Figure 2c, in which the average diameter is ~3.1 μm. The 3D graphical illustration of the linear patterns with nanoscale heights is drawn in Figure 2d, in which the linear patterns are made of the humps. The surface morphologies and height profiles of the bilayer films can be further studied using AFM. Figure 2e,f shows the 3D and 2D AFM height images of the PS/PMMA bilayer films with radial linear patterns with nanoscale heights, which agree well with the data from SEM and OM (Figure 2a,b). The height



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profile of a hump, indicated by the blue dotted line in Figure 2f, is shown in Figure 2g. In which the height and the diameter are 325 nm and 4.3 μm, respectively. In Figure 2h, a simple model of the hump composed of PS and PMMA is illustrated based on the experimental results by selectively removing the PS or the PMMA parts, which are discussed later. The results can be compared with those from the control experiments. In the first control experiment, the PMMA film-coated glass substrates without dripping PS solutions are examined and the smooth PMMA film without surface patterns can be observed (Figure S1a’). In the second control experiment, the PMMA film-coated glass substrates are dripped by toluene with a waiting time of 60 s and PMMA films without surface patterns are observed (Figure S1b’). In the third control experiment, the PMMA film-coated glass substrates are dripped by PS solutions with a waiting time of 0 s and the surface patterns are not observed (Figure S1c’). The results demonstrate that dripping PS solutions on the PMMA filmscoated glass substrates with enough waiting time is critical in the formation of the surface patterns.


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Figure 3. (a) Graphical illustration of the radial linear patterns. (b-j) OM images of the polymer patterns at different locations of the PS/PMMA bilayer films. The directions of the linear patterns are indicated by red dashed arrows. The waiting time before spinning the PS solution is 60 s. The spin rate for the PS solution is 4000 rpm for 90 s and 3000 rpm for 30 s. Figure 3a shows the graphical illustration of the radial linear patterns with nanoscale heights of the entire PS/PMMA bilayer films, in which a region is enlarged to better demonstrate the directions and distributions of the patterns. The corresponding OM images at different locations of a PS/PMMA bilayer film are displayed in Figure 3b-j. The radiative directions of the linear patterns, indicated by red dash arrows, imply that the formation of the polymer patterns might be related to the centrifugal force in the spin-coating processes. It is noteworthy to see that no patterns is observed near the center of the PS/PMMA bilayer films (Figure 3f), which could be attributed to the balance of the centrifugal forces because of the symmetric geometry. It might also be possible to generate the patterns on selective areas by dropping the PS solutions on specific regions in the processes of spinning the PS solutions on the PMMA films. The formation of the PS/PMMA radial linear polymer patterns with nanoscale heights can be explained by the Marangoni instability, which is induced by the surface tension gradients. It has been reported that the spin-coating defect striation, similar to the radial linear patterns observed here, can be induced by the Marangoni instability.23 Two effects are normally used to explain the Marangoni instability; one is the thermocapillary effect, and the other is the solutiocapillary effect. The thermocapillary effect can be caused by the solvent evaporation, which can lead to the thermal gradients, and can be defined by the following equation.23 𝑴𝒏 = where 𝑴𝒏 is the Marangoni number,

(∂𝑇∂𝜎)𝐻2∇𝑇

∂𝜎 ∂𝑇

𝜂𝛼

(1)

is the change in surface tension with temperature, 𝐻 is the

fluid thickness, ∇𝑇 is the temperature gradient at the surface, 𝜂 is the viscosity, and 𝛼 is the thermal



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diffusivity. The solutiocapillary effect is caused by the concentration gradients and can be defined by the following equation.24 𝑴𝒏 = where 𝑴𝒏 is the Marangoni number,

∂𝜎 ∂𝐶

(∂𝐶∂𝜎)𝐻2𝐶 𝜂𝐷

(2)

is the change in surface tension with concentration, 𝐻 is the

fluid thickness, ∇𝐶 is the concentration gradient at the surface, 𝜂 is the viscosity, and 𝐷 is the diffusion rate of the component. For the thermocapillary effect or the solutiocapillary effect, it has been demonstrated that the Marangoni instability can be triggered when the Marangoni number is larger than 80.25-26 For the thermocapillary effect, the temperature gradient (∇T) in the spin coating processes could not be measured because of the fast solvent evaporation processes; for the solutiocapillary effect, the concentration gradient (∇C) also could not be obtained because of the complex dissolution process. As a result, the values of the Marangoni number can not be determined precisely. Still, the Marangoni instability is believed to be mainly driven by the solutiocapillary effect than the thermocapillary effect, considering the thin fluid thicknesses during the spin-coating processes; at thin fluid thicknesses, it is easier to have enough concentration gradients than to have enough thermal gradients to trigger the Marangoni instability.24 For the experiments by dropping the pure solvent (toluene) on the PMMA films with a waiting time of 60 s and by dropping the PS solutions on the PMMA films without waiting, as shown in Figure S1, the linear patterns are not observed. The results also agree that the dominant effect is the solutiocapillary effect.


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Figure 4. (a) Graphical illustration of the radial linear patterns. (b, c) OM image and corresponding distribution of the distances between the linear patterns from the OM images. (d, e) SEM image and corresponding distances between neighboring humps from the SEM images. (f, g) OM image and corresponding X and Y positions for the humps along the blue dotted line in (f). (h, i) Average coefficients of determination (R2) for the humps verse waiting time and spin rates. For (b-g), the waiting time and the spin rate for the PS solution is 60 s and spin rate 4000 rpm, respectively. For (h), the spin rate is fixed at 4000 rpm. For (i), the waiting time is fixed at 60 s.

We also measure the distances between the linear patterns (blue dotted line in Figure 4a) and distances between neighboring humps (green dotted line in Figure 4a). The measurements are conducted on the linear patterns near the edges of the bilayer films, which are ~1 cm from the center of the films. For the distances between the linear patterns, the data from the OM images and corresponding distributions show that the distances range from 35 to 55 μm with the average distance at 46 μm, as shown in Figure 4b,c; for the distances between the neighboring humps, the data from the SEM images and corresponding distributions show that the distance range from 1.5 to 2 μm with the average distance at 1.8 μm, as shown 9 ACS Paragon Plus Environment


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in Figure 4d,e. The effects of waiting time and spin rates on the morphologies of polymer patterns with nanoscale heights are also investigated. When the PS solutions in toluene are dropped on the PMMA films, toluene can diffuse and swell the underlying PMMA films. Therefore, the lengths of the waiting time should affect the swelling degrees of the PMMA films and the patterns formation behaviors. The surface patterns of the PS/PMMA films at different waiting time (from 30 to 70 s), while the spin rates are fixed at 4000 rpm, are shown in Figure S2; radial linear patterns can be observed at these conditions. When the waiting time is shorter than 30 s, the radial linear patterns are not formed. When the waiting time is too short, the PMMA films are not swollen enough by toluene; the surface tension gradient of the system is not high enough to trigger the Marangoni instability. When the waiting time is longer than 30 s, the PS/PMMA bilayer films with radial patterns can be observed (Figure S2). When the waiting time is too long (longer than 70 s), cracks can start to be observed on the bilayer films, as indicated by the red arrows in Figure S2e,e’. The formation of the cracks may be due to the interdiffusion between the PMMA and the PS. To compare the morphologies of the radial linear patterns at different waiting time, the coefficients of determination (R2), reflecting how close the humps are to the fitted linear lines, are used. Figure 4f,g shows an example OM image and corresponding X and Y positions for the humps along the blue dotted line in Figure 4f, in which a coefficient of determination (R2) with a value of 0.99 is obtained. The average coefficients of determination (R2) for the humps versus the waiting time is plotted in Figure 4h. A maximum value of 0.98 is achieved when the waiting time is 60 s, meaning higher ordered humps are formed on the polymer patterns at suitable waiting time. Another effect on the morphologies of the polymer patterns with nanoscale heights is the spin rates in the spin-coating processes. When the PMMA films are swollen by the PS solutions in toluene and spun, the spin rates can affect the rates of solvent evaporation, influencing the morphologies of the polymer


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patterns. In this work, different first spin rates (from 2000 to 6000 rpm) are used and the sample are spun for 90 s, followed by spinning at second spin rates of 3000 rpm for 30 s. The first spin rates are used to control the morphology of the polymer patterns, and the second spin rates are used to ensure the further evaporation of the solvents. The surface patterns of the PS/PMMA films at different first spin rates (from 2000 to 6000 rpm), while the waiting time is fixed at 60 s, are shown in Figure S3; radial linear patterns can be observed at these conditions. It can be observed that the ordering of the polymer patterns become worse at the spin rates lower than 2000 rpm. At the spin rates of 2000 and 3000 rpm, the ordering of the polymer patterns enhances (Figure S3a,b and S3a’,b’). The optimal ordering of the polymer patterns is achieved at the spin rate of 4000 rpm. As the spin rates increase to 5000 and 6000 rpm, however, the ordering of the polymer patterns become worse again. The coefficients of determination (R2) are also used to compare the morphologies of the radial linear patterns at different spin rates. The average coefficients of determination (R2) for the humps versus the spin rates are plotted in Figure 4i. A maximum value of 0.98 is achieved when the spin rates is 4000 rpm, meaning higher ordered humps are formed on the polymer patterns at suitable spin rates. It can be observed that the data at shorter waiting time (30, 40 and 50 s) have larger error bars (Figure 4h). For the data at different spin rates, the waiting time is fixed at 60 s and the error bars are smaller (Figure 4i). For the effects of spin shear, we observe that the degrees of linearity of the linear polymer patterns are not affected by the spin rates. Therefore, we postulate that the spin shear only has limited effect on the polymer patterns. During the spin-coating processes, the directional evaporation gradient should also be neglected because of the high spin rates and the small sizes of the substrates. The effect of the polymer concentration on the polymer patterns with nanoscale heights is also studied. The OM images of the PS/PMMA bilayer films with different concentrations of the polymer solutions (PS solutions: 10 and 30 wt % ; PMMA solutions: 10 and 30 wt %) are shown in Figure S4. For PMMA



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solutions with higher concentraions, cracks are formed on the polymer films because of the higher thicknesses of the PMMA films, as shown in Figure S4c,d. For PS solution with higher concentrations, the patterns driven by the Marangoni effect are not observed because of the higher viscosities of the PS solutions, as shown in Figure S4b,d. The effect of the polymer molecular weights is also studied. The OM images of the PS/PMMA bilayer films with different molecular weights of the polymer solutions (molecular weights of PS: 35, 192 and 250 kg mol-1; molecular weights of PMMA: 38, 72 and 350 kg mol-1) are shown in Figure S5. The results show that the patterns are only observed for PMMA with the molecular weight of 72 kg mol-1. When the molecular weights are lower, the patterns are not formed because the thicknesses of the PMMA films are too thin, enhancing the effect from the substrate. When the molecular weights are higher, the patterns are not formed because the rates of the PMMA dissolution by the PS solution are too low, decreasing the effective concentration tension gradients. With suitable molecular weights of PMMA (72 kg mol-1), the patterns can all be observed with different molecular weights of PS.

Figure 5. (a) Graphical illustration of the polymer films before and after the selective removal processes. (b-d) OM, tilted-view SEM, and top-view SEM images of the PS films with linearly-arranged cavities after the PMMA films are selectively removed by acetic acid. (e-g) OM, tilted-view SEM, and top-view SEM images of PMMA films with linearly-arranged bumps after the PS films are selectively removed by 12 ACS Paragon Plus Environment

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cyclohexane. The regions with larger bumps are indicated by red arrows. For (b-d), the bilayer film is dipped into acetic acid for 5 days. For (e-g), the bilayer film is dipped into cyclohexane for 5 days.

To further confirm the morphologies of the PS/PMMA bilayer films, the selective removal technique is applied. The graphical illustration of the polymer films before and after the selective removal processes is drawn in Figure 5a, in which the PS and PMMA films can be selectively dissolved using cyclohexane and acetic acid, respectively. When the samples are dipped in acetic acid to selectively remove the PMMA films, the linearly-arranged cavities can be observed on the bottom sides of the PS films, as shown in the OM and SEM images (Figure 5b-d); when the samples are dipped in cyclohexane to selectively remove the PS films, the linearly-arranged bumps can be observed on the top sides of the PMMA films, as shown in the OM and SEM images (Figure 5e-g). The above results confirm our proposed model of the PS/PMMA bilayer films, as illustrated in the cross-section view of the bilayer films (Figure 1). It is interesting to notice that cavities on the bottom sides of the PS films and bumps on the top sides of the PMMA films both have two different ranges of sizes, the larger sizes at ~1–2 μm and the smaller sizes at ~200–300 nm. The cavities and bumps with the larger sizes correspond to the bumps between the PS and PMMA layers illustrated in Figure 2h, which can be explained by the theory of the Marangoni instability; the cavities and bumps with the smaller sizes are not illustrated in Figure 2h, which may be caused by the phase separation of the PS and PMMA during the swelling and spin-coating processes.



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Figure 6. (a) Graphical illustration of the polymer patterns. (b) Graphical illustration of the formation processes of the patterns in the tangential direction (blue dotted line) in (a). (c) Graphical illustration of the phase separation process in the radial direction (yellow dotted line) in (a).

Figure 6a shows the graphical illustrations of the formation mechanisms of the PS/PMMA bilayer films with radial linear patterns with nanoscale heights, which can be divided into two parts. For the radial part of the radial linear patterns (blue dotted line), the formation mechanism is mainly related to the theory of the Marangoni instability. When the PS solution in toluene is dripped on top of the PMMA film, the PMMA film is swollen with a concentration gradient, causing a surface tension gradient. As a result, Marangoni instability is induced and periodic hill-and-valley patterns are formed (Figure 6b). After the solvents are evaporated, the structures can be solidified. For the linear part of the radial linear patterns (yellow dotted line), the formation mechanism is mainly related to the phase separation of PS and PMMA. When the PS solution in toluene is dripped on top of the PMMA film, the phase separation also occurs between the PS solution and the swollen PMMA 14 ACS Paragon Plus Environment

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films. It has been reported that the phase separation affected by the Marangoni instability can become the lateral phase separation.27-31 As a result, the hill-and-valley structures are formed (Figure 6c). After the solvents are evaporated, the structures can be solidified. Therefore, the formation of the radial linear patterns is caused by the combination of the concentration-gradient-driven Marangoni instability and lateral phase separation. The relationship between of the diameter of the humps and the waiting time is also measured, as shown in Figure S6; the results show that the diameters of the humps increase with the waiting time, agreeing with our prediction about the phase separation mechanism. It should be noted that the PMMA films are partially dissolved by depositing the PS/toluene solutions on the PMMA films. In Figure 6, the schematics are illustrated with a sharp interface for easier comprehension on the model.



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Figure 7. (a) Photos of tearing a polymer label film of a soft drink bottle along the perforation lines. (b) Graphical illustrations of the tearing experiment. (c) Photos of the tearing processes. (d, e) Tilted-view SEM images of the torn PS film with lower and higher magnifications.

After selectively removing the PMMA films, the linearly-arranged cavities on the PS films may be used to investigate the mechanical properties of cavity-containing films. The concept is inspired by the perforation lines on polymer label films for soft drink bottles.32 The perforation lines with holes are designed to offer spots with weaker mechanical strengths, which can be easily torn, as shown in Figure 7a. Without such perforation lines, the films are torn along random directions. Following similar concepts, here we investigate the tearing experiments of the cavities containing PS films, as illustrated in Figure 7b; the cavity-containing spots possess locally weaker mechanical strengthens and can be easily torn by external forces. It is interesting to know that, when the polymer films are torn, there are always parallel and perpendicular patterns to the tearing directions because the linear patterns are radially distributed on the films. As expected, the films are torn along the directions of the linearly-arranged cavities (Figure 7c-e), demonstrating the proof of concept to apply the cavities-containing films to nanoscale perforation lines. CONCLUSION In conclusion, we successfully develop a new strategy to investigate the phase separation of polymer blends under convection during the spin-coating processes. PS solutions in toluene are dripped on the PMMA film-coated glass substrates, follow by spin-coating processes. Affected by the concentrationgradient-driven Marangoni instability and lateral phase separation, the PS/PMMA bilayer films with radial linear patterns can be generated. The morphologies of the linear radial patterns with nanoscale heights are observed to be affected by the waiting time and spin rates. To confirm the morphologies of the linear radial patterns of the PS/PMMA bilayer films, a selective removal technique is applied. The PS films with 16 ACS Paragon Plus Environment

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cavities and the PMMA films with bumps can be obtained by using acetic acid and cyclohexane, respectively. The radial linear patterns are discovered to be induced by the combination of the concentration-gradient-driven Marangoni instability and lateral phase separation. The PS films with linearly-arranged cavities, in which the PMMA films are selectively removed by acetic acid, are tested in the tearing experiments. Because of the weaker mechanical strengthens on the regions of the films with the cavities, easier tearing along the linearly-arranged cavities is observed, demonstrating a proof of concept in the potential applications of cavities-containing polymer films to nanoscale perforation lines. Such demonstration has great implication for the control of mechanical properties and failure analysis of polymer thin films.

EXPERIMENTAL SECTION Materials Poly(methyl methacrylate) (PMMA) with molecular weights (MW) of 75 kg mol-1 was obtained from Scientific Polymer Products. Polystyrene (PS) with molecular weights (MW) of 192 kg mol-1 was obtained from Sigma Aldrich. Toluene and cyclohexane were purchased from J. T. Baker. Acetic acid was obtained from Sigma Aldrich.

Preparation of the PMMA Film-Coated Glass Substrates First, a 10 wt % PMMA solution in toluene was prepared. The PMMA solution (volume: ~0.4 mL) was dripped on glass substrates (size: ~600 mm2) and spin-coated at spin rates of 800 rpm for 90 s and 3000 rpm for 30 s. After the spin-coating process, the samples were annealed at 150 °C for 3 h to evaporate the residual solvents.



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Fabrication of the PS/PMMA Bilayer Films with Radial Linear Patterns A 10 wt % PS solution in toluene was first prepared. To fabricate the PS/PMMA bilayer films with radial linear patterns, the PS solution (volume: ~0.4 mL) was dripped on the PMMA film-coated glass substrates. The waiting time were controlled from 0 to 70 s. Subsequently, the samples were spun, following by drying processes. In a typical experiment, the samples were spun at spin rates of 4000 rpm for 90 s and 3000 rpm for 30 s.

Selective Removal Processes and Tearing Experiments For the PS/PMMA bilayer films with radial linear patterns, the PS or the PMMA layers can be selectively removed by cyclohexane or acetic acid, respectively. By soaking the PS/PMMA bilayer films in acetic acid for 5 days to selectively remove the PMMA layers, the PS films were obtained; by soaking the PS/PMMA bilayer films in cyclohexane for 5 days to selectively remove the PS layers, the PMMA films were obtained. The PS films with cavities after the PMMA layers were selectively removed were used for the tearing experiments, in which a tweezer was applied to provide the tearing forces.

Structure Analysis and Characterization The PS/PMMA bilayer films with radial linear patterns were observed by an optical microscope (OM) equipped with a charge-coupled device (CCD). A scanning electron microscope (SEM) (JEOL, JSM7401F) at an acceleration voltage of 5 kV was also used to characterize the samples. Before the SEM measurements, the samples were dried by a vacuum pump and coated with platinum (thickness ~4 nm).

ASSOCIATED CONTENT Supporting Information available 18 ACS Paragon Plus Environment

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.XXXXXXX. OM and SEM images of the PS/PMMA bilayer films with different experimental conditions.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ORCID Jiun-Tai Chen: 0000-0002-0662-782X Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the ﬁnal version of the manuscript. Funding This work was financially supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This work was also supported by the Ministry of Science and Technology of the Republic of China (MOST-106-2221-E-009131-MY3). Notes The authors declare no competing ﬁnancial interest

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Radial Linear Polymer Patterns Driven by the Marangoni Instability and Lateral Phase Separation for the Formation of Nanoscale Perforation Lines



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Bo-Hao Wu, Kai-Chieh Chang, Hsun-Hao Hsu, Yu-Jing Chiu, Tang-Yao Chiu, Hsiao-Fan Tseng, Jia-Wei Li, and Jiun-Tai Chen*


Radial Linear Polymer Patterns Driven by the Marangoni Instability

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