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Hierarchical Morphology of Polymer Blend Films Induced by Convection-Driven Solvent Evaporation Qinghua Fang, Feng Ye, and Xiaoniu Yang Langmuir, Just Accepted Manuscript • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Hierarchical Morphology of Polymer Blend Films Induced by Convection-Driven Solvent Evaporation Qinghua Fang†‡, Feng Ye*†, and Xiaoniu Yang*† †State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China. ‡ College of Applied Chemistry and Engineering, University of Science and Technology of China, Jinzhai Road No 96, Baohe District, Hefei 230026, P. R. China KEYWORDS: polymer blend film, hierarchical morphology, spin coating, fluid convection, solvent evaporation

ABSTRACT: Homogeneous thin film of polymer blends with desired morphology is necessary for their applications in the fields, e.g. optoelectronics, sensors, biomedicine and so on. The frequently employed approach for thin film preparation, spin coating is only able to achieve homogeneous film at small area due to overwhelming spin-driven solvent evaporation with increased size. Here a convection-guided morphology formation for polystyrene: poly(methyl methacrylate) blend films is reported. In-situ observation shows that the morphology changed from homogeneous deposition with scale less than 10 microns to self-organized cellular pattern with scale of more than 100 microns after the fluid flow is involved. Selective dissolution of the hierarchical films reveals that the cellular morphology is attributed to the flow-field guided

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deposition of sequentially generated precipitates. The coupling of phase separation and fluid convection results in the hierarchical morphology that includes Voronoi cellular division as the primary structure and the detailed heterogeneous inner-cell features as the secondary structure. Isolated modulation of either micro or meso-scale in the hierarchical morphology could be carried out via adjusting phase interaction or the convection disturbance correspondingly, providing a flexible and straightforward strategy to construct designed hierarchical structures for polymer thin films towards desired function or property.

1. INTRODUCTION Phase-separation plays a vital role in the controllable morphology, which dominates the performance of polymer blend films covering a wide range of fields as optoelectronics, sensor, biomedicine and so on.1–4 Spin coating has been the prime choice as a film-forming method due to its easy-to-implement smoothing and accelerated drying of liquid films.5–8 When combined with mixture of solvents, the designed micro-phase separation has been pushed to a far-reaching refinement towards the optimization of device performance at laboratory scale as in the field of optoelectronics.9–11 In the rotating liquid film, however, the stimulated solvent evaporation removes heat from the surface, leaving the inhomogeneous concentration and temperature fluctuations, both of which lead to the fluid flow and disturb the film morphology at mesoscale.12–15 Consequently, it remains challenging to obtain a mesoscale uniform film while keeping desired microphase morphology, suggesting that heterogeneous deposition exists in the film formation under fluid disturbance, which goes beyond the traditional phase separation mechanism and arouses the urgent in-depth research for the coupling effect of fluid disturbance and phase separation.16–19


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Desired phase separation of polymer blend films usually exists at nanoscale. As studied by this group, thermodynamic compatibility of the solution mixtures is proved to be well suitable for the film performance and stability improvement at molecular dimension.20–22 However, it is further found that with the scope extended to mesoscale, maintaining a uniform microphase separation against the ubiquitous fluid disturbance becomes a core issue which is widely associated with device performance in the frontier research.23–25 For example, the topping efficiency by ternary blend system depends highly on the film deposition kinetics of combined solvents volatilization.26 As for the large-area device promotion, coupling of phase separation and fluid disturbance gains weights in such aspects as large scale uniform film deposition, solvent evaporation control for additional thickness requirements, substitution of halogen solvents for environmental reasons, all of which concerns the central problem of uniform expansion of microphase separation under fluid disturbance.27–29 Fluid flow behavior thus plays a leading role for film uniformity and affects the strategy of mixed solutions in the preparation of functional polymer blend films.30,31 Meanwhile, dominant factors of film formation are then changed from the thermodynamic entropy of mixture compatibility to the kinetics of convection-guided deposition. Although initial efforts have been made in experimental trial and error, the coupling kinetics of phase separation and fluid disturbance is still elusive.32–34 Considering the hierarchical morphology at mesoscale is commonly occurred for polymer blend films prepared via spin-coating method, while polymers involved in the blends towards desired functionality are usually thermodynamically incompatible, in this study, a model incompatible blend system, polystyrene:poly(methyl methacrylate) (PS:PMMA) is employed to elucidate the morphology formation and its kinetics.35 In order to in-situ monitor the morphology


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formation details, hereby the fluid convection within the liquid film upon solvent evaporation is introduced by heating instead of rotating the substrate. Apart from the phase separation at nanoscale, the mesoscale cellular morphology is deduced and verified to be induced by fluid convection, which limit the quality of a uniform film. Coupling kinetics of convection disturbance and phase separation is demonstrated and the fluid flow during phase separation is proved to be effective in the precipitate lateral transfer beyond molecular dimension, and is responsible for the heterogeneous deposition at mesoscale. Different parameter ranges to provide an initial investigation of the evolution of hierarchical phase separation in terms of the coupling effect is proposed. As a result, regulation methods combining microscale phase separation and mesoscale fluid flow are proposed to be feasible and the coupling strategy would be employed as a general methodology to construct desired morphology of polymer blend films for functional devices. 2. EXPERIMENTAL SECTION 2.1. Materials Polystyrene (PS, Mw=107 kg mol-1, PDI=1.44) was obtained from Daqing Petro Chemical Company. Polymethyl methacrylate (PMMA, Mw=140 kg mol-1, PDI=1.09) was acquired from J&K Scientific Ltd. P-xylene (≥99.0%) and acetone (≥99.9%) were purchased from SigmaAldrich. All these materials were used as received. 2.2. Sample Preparation Glass and silicon wafers were cleaned in ultrasonic bath with deionized water, acetone and isopropanol alcohol for 20 min each and dried with nitrogen flow. Mixtures of PS and PMMA with various proportions were dissolved in p-xylene while maintaining the total concentration at 30 mg mL-1. Then, all of the solution was heated at 70 oC and stirred overnight before deposited


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on glass and silicon wafers. All film deposition processes are implemented in an open environment with ambient temperature of 20 oC and humidity of 20%. Acetone-soaking treatment is performed with the PS:PMMA solid film on 2020 mm2 glass substrate, by dropping 0.2 mL of acetone on the film surface every 2 min for 5 times in total, while keeping the substrate rotating at a speed of 500 rpm to acquire a gradual selective etching effect of the blend film. 2.3. Characterization Optical microscopy was taken with a Carl Zeiss A1m microscope. Film thicknesses were acquired by a KLA-Tencor D-100 surface profiler. Surface morphology of the films was observed with Agilent 5500 AFM by tapping mode in ambient atmosphere. Micro-zone infrared testing was performed on a Fourier transform infrared spectrometer(IFS66V/S) in the vision of an optical microscope(Hyperion3000). 3. RESULTS AND DISCUSSION 3.1. Hierarchical Morphology and Self-Organized Voronoi Diagram One of the major drawbacks of spin-coated film is the non-uniform deposition which is usually capable to provide satisfactory regional phase separation but accompanied by film-forming defects at the mesoscale. As shown in Figure 1, with the spin-coating speed reaching 500 rpm, the hierarchical morphology with cellular division appears in both the central and edge regions of the deposited PS:PMMA blend film. Apparently, the cellular division does not match in size with phase separation of molecular dimension, and it is therefore reasonable to assume that the cellular division emerges from the dispersed flow disturbances in the mesoscale. However, compared with the orientated cellular structure of the edge region in Figure 1b, isotropic cellular structure has emerged in the central region under less centrifugal force as shown in Figure 1a,


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which indicates that although the intense centrifugal force behaves effective in aligning the cell units from center to edge, the cellular division is not induced by the centrifugal flow, and there is special multi-regional kinetic mechanism responsible for the cellular division morphology. As is well known that accelerated loss of surface solvent would be effective in triggering the convection in the form of cellular patterns, and the spin-coated mesoscale cellular division is thus probably related to a convection-guided deposition process, which is further demonstrated below. In order to exclude the interference from centrifugal flow, the convection-guided deposition effect on the phase separation process is performed in a static PS:PMMA blend solution film with controllable solvent loss rate through modulation of substrate temperature.


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Figure 1. OM pictures of self-organized hierarchical pattern in PS:PMMA blend film (1:6 weight ratio) by spin coating at 500 rpm on silicon wafer. (a) Central area and (b) edge area. As a basic dynamic process for the coupling behavior of phase separation and fluid flow, the deposition of PS:PMMA blend film is first studied in the absence of convection. As shown in Figure 2, when the solvent evaporation temperature is kept as low as 50 oC, the film exhibits a uniform phase separation behavior. According to the real-time in-situ tracking with optical microscopic images, the drying process takes a total time of 205 s. After the liquid film is spread onto the silicon wafer (Figure 2a), dense granulated microdomains appear within the liquid film (Figure 2b), which is corresponding to the start of the precipitate deposition. Immediately, the film is converted from liquid to solid with the progressive advance of the solid-liquid boundary as shown in Figure 2c. Eventually, the solid film is maintained with the surface profile full of uniform granular microdomains as shown in Figure 2d and no heterogeneous deposition on mesoscale is observed.

Figure 2. In situ OM images tracking the drying process of PS:PMMA blend film (1:6 weight ratio) formed through homogeneous deposition on a 50 oC silicon wafer. The time stamp is (a) 10 s, (b) 100 s, (c) 180 s and (d) 210 s.


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It can be further inferred from Figure 2 that with the increasing precipitates, the scale of the dense microdomains is kept within 10 µm. The temperature differences caused by the surface solvent evaporation is not enough to trigger the lateral mesoscale fluid flow against film viscosity. Consequently, the deposition of precipitates is dominated by gravity, resulting in the uniform microphase separation. For polymer blend optoelectronic devices, such homogeneous phase separation is vital to conversion efficiency and device stability. Unfortunately, this is not always the case for various polymer blend systems, especially when the solvent loss rate is sufficient to overcome the solution viscosity, and non-uniform deposition in mesoscale then appears as the result of liquid film instability, which has always been a thorny problem troubling the quality of spin-coating process. In addition to phase separation, fluid disturbance is then introduced by accelerated solvent evaporation. As shown in Figure 3, with the substrate temperature increased to 100 oC, the total time for the drying process is reduced to 20 s. Impressively, not the dense microdomains but the cellular boundaries appear as a sign of the liquid film instability (Figure 3b). Consequently, the liquid-solid transition approaches with the prime cellular division and the drying of inner cells. It is noted that although the composition precipitation in phase separation acts within a scale less than 10 µm, due to the dramatically increased solvent evaporation rate, the temperature fluctuation is sufficient to induce fluid flow against the resistance of viscosity, leading to the cellular division beyond 100 µm in a self-organized manner, thus causing the mesoscale nonuniform deposition.


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Figure 3. (a-d) In situ OM images tracking the formation of Voronoi diagram in PS:PMMA blend film (1:6 weight ratio) on a 100 oC silicon wafer. The time stamp is (a) 5 s, (b) 10 s, (c) 15 s and (d) 20 s, respectively. (e) Visual guide line (black) along the cell boundary and the indicating spots (red) at cell centers corresponding to the optical interference cores. (f) Simulation of the Voronoi cells calculated from the central spots. As shown in Figure 3e, visual guide lines are added according to the cellular boundaries and the interference spots indicating the minimum film thickness marked with red dots are interestingly well isolated and exhibit as the nuclei of the cellular division. Voronoi analysis is therefore achieved to confirm the kinetic association between the cell units.36 Growth kinetics of the cellular boundaries is simulated and Voronoi diagram is calculated from the marked nuclei as shown in Figure 3f (details are given in Table S1 and Figure S1). Impressively, there is excellent agreement between the experiment depiction and the calculated cellular division, which reveals that the mesoscale mess transfer behaves in the form of lateral expansion around the


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nuclei. Correspondingly, the fluid flow sources from the scattered nuclei and spread out as multiregional convection till reaching the intercellular squeezing boundaries, facilitating the deposition enrichment and thus leaving the Voronoi cellular diagram. Consequently, the mesoscale homogeneity of the blend film is destroyed by the convection-guided dissipation and enrichment of precipitate, which inevitably reduces the film quality in optoelectronic devices. 3.2. Mesoscale Non-Uniformity in Morphology and Composition A special focus is laid on the Voronoi cell units for the inner heterogeneous features of disturbed phase separation by fluid convection. As shown in Figure 4a, the hierarchical topography is identified by the cellular boundaries together with the dense microdomains inside. Combined with the profile in Figure 4b, the steep cell boundary outlines the precipitation enrichment areas, which can be attributed to the sinking flow of the regional convection. For the non-uniform distribution of the dense microdomains, selected-area power spectral density (PSD) based on polar coordinates (‫ݎ‬, ߠ) is applied for the scaling analysis.37,38 The PSD for the selected areas is represented as the function of radial spatial frequencies ݂ by averaging it over ߠ as expressed in Equation (1). ܲܵ‫=)݂(ܦ‬

ଵ ଶగ ‫݂(ܦܵܲ ׬‬, ߠ)݀ߠ ଶగ ଴

(1)


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Figure 4. (a) AFM image covering the Voronoi cell boundary in PS:PMMA blend film (1:6 weight ratio). Scan area 100100 µm2. Z-range 500 nm. (b) Profile along the given path of L1. (c) Power spectral density versus radial frequency for the selected areas of A1 and A2. Thereby, selected areas of 2323 µm2 both deep inside the cell (A1) and near the boundary (A2) are calculated (Figure 4a), and the corresponding PSD versus radial spatial frequency is provided in Figure 4c. Apparently, PSD deep inside the cell exhibits a main peak well below 2 µm-1, which indicates that the microdomain size exceeds 500 nm statistically. While, as for the areas near the cell boundaries, the main peak of the PSD is located above 3 µm-1, which reveals the microdomain size generally less than 300 nm. The non-uniform size of microdomains from the cell boundaries to the centers indicates that under fluid convection, the precipitate from phase separation is gradually deposited along a lateral spatial range, leading to the intracellular heterogeneous microdomains which is also distinct from the homogeneous deposition. For the in-depth analysis of the heterogeneous cellular morphology, composition distribution is identified across the cell unit with micro-zone infrared spectrum as shown in Figure 5. The spectrum has been normalized by the ester carbonyl peak at 1730 cm-1 which is positively related to the content of PMMA. The peak at 700 cm-1 corresponds to the out-of-plane hydrogen bending vibration of the monosubstituted benzene ring, which is positively correlated with the PS content. Apparently, the increasing PS-rich domain enrichment evolves from the cell centers to the boundary areas. Therefore, it can be deduced that with the continuous loss of solvent, precipitate of different composition ratio deposits sequentially under the fluid convection with lateral spatial resolution. Thus, the film deposition guided by fluid convection not only enables the self-organized cellular division and intracellular heterogeneous morphology, but also distributes the precipitate of different ratios on the lateral spatial mesoscale. Consequently, the


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influence of fluid convection on blend films goes more than the inhomogeneous morphology, and the non-uniform component distribution under any fluid disturbance would inevitably deteriorates the device performance at a degree. Especially, when the blends are designed to precipitate in desired sequence with the help of mixed solvents of different boiling points, the heterogeneous component distribution at mesoscale is thus more serious in that the instability of the liquid films is technically impossible to avoid in complex solvents.

Figure 5. (a) OM image of the Voronoi cell with the P1-P4 marks from center to boundary. (b) Microregion infrared spectra of the corresponding locations. 3.3. Convection-Guided Deposition and Lateral Cellular Mass Transfer


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In addition to the above analysis of non-uniform composition, selective dissolution of the hierarchical films is achieved through acetone-soaking treatment to obtain the component topography of the cellular division, and the self-organizing kinetics of flow-guided component enrichment is thus demonstrated. For the etching process of PS:PMMA blend film, acetone exhibits effective in the selective dissolution of PMMA component (Figure S2), which is consistent with the miscibility between acetone and the corresponding polymer component.39 As shown in Figure 6, due to the selective dissolution of PMMA, the distribution profile of PS-rich domains is deduced from the remained film skeleton. Consistent with the results of the microregion infrared, the PS-rich domains are concentrated along the cellular boundary. Impressively, the boundaries exhibit ridge-like extension of 30 µm in width, which confirms the non-uniform composition distribution from the cellular boundaries to center areas. Consequently, the film deposition kinetics can be drawn that with the mass transfer of convection fluid, precipitates deposit along the cellular boundaries and gradually spreads towards the cell centers. What is more, as can be deduced from the residual granules shown in Figure 6b, the inhomogeneous morphology at mesoscale is derived from the flow-guided deposition of micro precipitates that self-organize into the cellular division morphology at the scale over hundreds of microns, which implies the interference kinetics of fluid flow on homogeneous phase separation.


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Figure 6. (a) OM picture of the residual morphology after selective dissolution of the PS:PMMA blend film with acetone and (b) the enlarged image of boundary zone outlined by the red rectangle. The effect of fluid flow disturbance on film non-uniform deposition is depicted in detail as shown in Figure 7. The volatilization of the solution occurs on the liquid film surface which causes the continuous component precipitation. Consequently, the surface concentration increases, accompanied by the accumulation of precipitates as depicted in Figure 7a. While then, the subsequent deposition kinetics dominates the final morphology and is determined by the relationship between solvent evaporation rate and liquid film viscosity. As shown in Figure 7b, at a lower substrate temperature, e.g. 50 oC in this work, the solvent loss rate is not high enough to stimulate the fluid disturbance against the resistance of viscosity. As a result, the liquid film


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maintains stable with no lateral flow at mesoscale. Therefore, the sequentially accumulated precipitates on the solution surface deposit directly onto the substrate mainly under the influence of gravity, leading to the mesoscale morphology with homogeneous dense microdomains less than 10 microns. In contrast, however, when the substrate temperature is raised to an appropriate level, e.g. 100 oC in this work, the solvent loss rate is high enough to induce the instability of the liquid film against the resistance of viscosity, and thus the fluid convection is introduced into the film formation process (Figure 7c). As a result, the movement of precipitates is dominated by the dispersed fluid flow, and a self-organized deposition with lateral dissipation and downward enrichment areas is thus evolved in the circulated flow units depicted by the streamlines in Figure 7c, which facilitates the non-uniform mesoscale morphologies, such as the cellular division that exhibits significant boundary and core differences in this study.

Figure 7. Schematic diagram of (a) composition precipitation with the loss of solvent on the surface and the deposition mechanism for (b) homogeneous deposition in static field and for (c) mesoscale self-organized deposition in convection field. Fluid streamlines in the convection movement are depicted. The downward arrows (blue) denote the lateral mass transfer and


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consequent boundary enrichment of precipitates. The upward arrows (red) denote the lateral boundary growth direction with the precipitate accumulation towards the nucleus positions. 3.4. Multi-Dimensional Regulation of the Hierarchical Morphology Although the hierarchical morphology is the combined result of phase separation under flowguided deposition, the scale features of the two processes are both preserved. The phase separation behavior depends on the thermodynamic compatibility of the components at molecular dimension. As for the fluid flow, mesoscale factors such as viscosity, temperature gradient and concentration distribution play a dominant role in the fluid kinetics. Inspired by this, isolated modulation strategies of the hierarchical morphology can then be carried out either from the phase separation behavior or from the disturbance of the fluid flow. From the perspective of a phase separation inducing morphology regulation, the proportion adjustment of the PS:PMMA blend is performed. As shown in Figure 8, with the mass ratio of PS:PMMA increased from 1:6 to 1:4 while maintaining the total concentration of the solution unchanged, the thickness of the cellular boundaries is significantly increased, and the dense microdomains are increased in size as well. It is noteworthy that aside from the coarsening of the dense microdomains, the size of the cellular division remains at around 100 microns, exhibiting the relatively unchanged fluid convection range that is insensitive to the blend proportion. Therefore, the isolated kinetics of phase separation and fluid flow makes the hierarchical scale features independently controllable.


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Figure 8. OM images of the composition-modulated Voronoi diagrams with PS:PMMA weight ratio of (a) 1:6 and (b) 1:4. Aside from the modulation of phase separation behavior, the size of cellular division is adjusted through the mesoscale kinetic factors as comparison. As shown in Figure S3, under a further reduced liquid film thickness (~2 µm), the cellular division scale is significantly limited to about 60 µm from that of exceeding 100 µm shown with film thickness of more than tens of microns (Figure 3). Interestingly, Figure S3d shows that due to the dramatically reduced film thickness, optical interference fringes are recognizable throughout the entire film, which not only exhibits the cellular nuclei, but also maps down the flow-guided lateral mass transfer with interference patterns, confirming the dominant role of the lateral fluid flow in the mesoscale nonuniform deposition.


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Apparently, as for the construction of functional films through thermodynamic behavior of phase separation with incompatible polymer blends, the desired homogeneous deposition would be disturbed by the fluid flow-guided deposition kinetics, thus leading to the heterogeneous morphology at mesoscale. Therefore, achieving the maximum inhibition of fluid instability through well designed mesoscale factors as solution viscosity, spinning speed and solvent volatility would be the straightforward strategy towards the uniform morphology of polymer blend films. The two immediate causes of fluid disturbance are temperature variations and surface tension differences, both of which are inevitably introduced by the solvent evaporation in the spin coating process. Therefore, to meet the strong demand for mesoscale uniform films in fields as optoelectronics and sensors, according to this study, regulating the restriction between solvent evaporation rate and liquid film viscosity is the key towards the solution. Similar solutions have been explored through experimental trial and error in the field of optoelectronics, such as getting the uniform film by increasing solution concentration while raising the rotational speed, which is a good example to maintain the stable liquid film by enhancing the resistance of viscosity. And the fluid flow-guided deposition kinetics in this study would find more use in the more complicated blend systems. Especially, when the blends are designed to precipitate in desired sequence with the mixed solvents of different boiling points, the heterogeneous component distribution at mesoscale is thus more serious in that the instability of the liquid films is technically impossible to avoid in complex solvents. 4. CONCLUSIONS In summary, a model incompatible blend system, PS:PMMA is employed to elucidate troublesome heterogeneous morphology formation and its kinetics. Effect of fluid disturbances on the deposition behavior of thin films during spin-coating is studied, and in-situ monitor of the


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morphology formation under coupling of phase separation and convection disturbance is established by heating instead of rotating the substrate. With the convection introduced into the PS:PMMA liquid film, the self-organized Voronoi diagram is achieved which confirms the convection-guided deposition process as the heterogeneous film formation kinetics. The precipitation deposition achieves a morphology with the scale spanned from less than 10 microns in homogeneous deposition to more than 100 microns in the self-organized deposition after the flow field is intervened. Additionally, intracellular gradient distribution is found to exist both in the topographical scale features and in composition ratio. Thus, coupling of the two processes with features in different scales leads to a hierarchical topography that includes the Voronoi cellular division as the primary structure and the intracellular nonlinear distribution as the secondary structure. The dynamic mechanism for the formation of the self-organized hierarchical pattern is attributed to the flow-field guided selective deposition of sequentially generated precipitates from the phase separation process. As a result, the hierarchical morphology contains two kinds of features from phase separation and convection fields, respectively. Additionally, isolated modulation of the hierarchical morphology features in micro and meso-scale is proved to be feasible through adjusting the effect either from phase separation or from convection disturbance, which provides a flexible and straightforward strategy for the construction of hierarchical structures in polymer blend films with desired function. ASSOCIATED CONTENT Supporting Information Detailed description of Voronoi Analysis, Table S1, Figures S1-S3 (PDF) AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (Feng Ye). *E-mail: [email protected] (Xiaoniu Yang). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21574132 and 21504090), the Jilin Provincial Science & Technology Department (grant no. 20160520118JH), and the Guangdong Provincial Department of Science and Technology (grant no. 2015B020239004). REFERENCES (1)

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Table of contents Hierarchical Morphology under fluid instability is proposed, which denotes the common problem in preparing homogeneous polymer blend films. Mesoscale cellular division is deduced and demonstrated, revealing flow-induced heterogeneous deposition. Morphology regulation combining microscale phase separation and mesoscale fluid flow is proved to be feasible and general methodology for the construction of desired morphology of polymer blend films for functional devices. ToC figure


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Figure 1. OM pictures of self-organized hierarchical pattern in PS:PMMA blend film (1:6 weight ratio) by spin coating at 500 rpm on silicon wafer. (a) Central area and (b) edge area. 85x128mm (300 x 300 DPI)


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Figure 2. In situ OM images tracking the drying process of PS:PMMA blend film (1:6 weight ratio) formed through homogeneous deposition on a 50 oC silicon wafer. The time stamp is (a) 10 s, (b) 100 s, (c) 180 s and (d) 210 s. 85x63mm (300 x 300 DPI)


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Figure 3. (a-d) In situ OM images tracking the formation of Voronoi diagram in PS:PMMA blend film (1:6 weight ratio) on a 100 oC silicon wafer. The time stamp is (a) 5 s, (b) 10 s, (c) 15 s and (d) 20 s, respectively. (e) Visual guide line (black) along the cell boundary and the indicating spots (red) at cell centers corresponding to the optical interference cores. (f) Simulation of the Voronoi cells calculated from the central spots. 85x94mm (300 x 300 DPI)


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Figure 4. (a) AFM image covering the Voronoi cell boundary in PS:PMMA blend film (1:6 weight ratio). Scan area 100×100 µm2. Z-range 500 nm. (b) Profile along the given path of L1. (c) Power spectral density versus radial frequency for the selected areas of A1 and A2. 85x218mm (300 x 300 DPI)


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Figure 5. (a) OM image of the Voronoi cell with the P1-P4 marks from center to boundary. (b) Microregion infrared spectra of the corresponding locations. 85x124mm (300 x 300 DPI)


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Figure 6. (a) OM picture of the residual morphology after selective dissolution of the PS:PMMA blend film with acetone and (b) the enlarged image of boundary zone outlined by the red rectangle. 85x115mm (300 x 300 DPI)


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Figure 7. Schematic diagram of (a) composition precipitation with the loss of solvent on the surface and the deposition mechanism for (b) homogeneous deposition in static field and for (c) mesoscale self-organized deposition in convection field. Fluid streamlines in the convection movement are depicted. The downward arrows (blue) denote the lateral mass transfer and consequent boundary enrichment of precipitates. The upward arrows (red) denote the lateral boundary growth direction with the precipitate accumulation towards the nucleus positions. 85x79mm (300 x 300 DPI)


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Figure 8. OM images of the composition-modulated Voronoi diagrams with PS:PMMA weight ratio of (a) 1:6 and (b) 1:4. 85x115mm (300 x 300 DPI)


Hierarchical Morphology of Polymer Blend Films Induced by

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