Two-Step Solvent On-Film Annealing (2-SOFA) Method: Fabrication of

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Two-Step Solvent On-Film Annealing (2-SOFA) Method: Fabrication of Anisotropic Polymer Particles and Implications for Colloidal Self-Assembly Hsiao-Fan Tseng, Yu-Jing Chiu, Bo-Hao Wu, Jia-Wei Li, and Jiun-Tai Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00866 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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Two-Step

ACS Applied Nano Materials

Solvent

On-Film

Annealing

(2-SOFA)

Method: Fabrication of Anisotropic Polymer Particles and Implications for Colloidal Self-Assembly Hsiao-Fan Tseng,1 Yu-Jing Chiu,1,2 Bo-Hao Wu,1 Jia-Wei Li,1 and Jiun-Tai Chen1,2,3* 1

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

2

Sustainable Chemical Science and Technology, Taiwan International Graduate Program, Academia

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

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

30010 *To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +886-35731631

ABSTRACT In recent years, anisotropic polymer particles have gained increased interest owing to their special properties and broader applications, such as drug delivery, optical traps, and e-paper display. Most strategies to produce anisotropic polymer particles, however, require sophisticated instruments or additional surfactants. Here, we develop a simple and versatile method, the two-step solvent on-film annealing (2-SOFA) technique, to make anisotropic polymer particles with different shapes. Polystyrene (PS) microspheres spin-coated on poly(methyl methacrylate) (PMMA) films are chosen as model materials. By sequentially annealing the PS/PMMA composites in different solvent vapors, anisotropic

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polymer particles with distinctive and diverse shapes can be produced, such as half-eaten peach-shaped, snowman-shaped, and bowler hat-shaped morphologies. An exquisite selective removal strategy is applied to check the morphologies of the PS/PMMA composite films and to comprehend the transformation mechanism at different annealing steps and times. For particles merged from multiple microspheres, quantitative studies are also performed to figure out the relationships between the sizes of the merged particles and the numbers of the original microspheres. These results have implications for colloidal self-assembly.

Keywords: anisotropic, polymer films, selective removal, solvent vapor annealing, surface tension

INTRODUCTION In recent years, anisotropic polymer particles have received growing attention because of their unique properties and morphologies.1-5 Anisotropic polymers particles can be applied to a variety of areas, such as drug delivery, optical traps, and e-paper display.6-9 For example, patchy polymer particles have various compositional patches in the corona and can be applied in electronics.10-13 Until now, few ways to produce anisotropic polymer particles have been reported; most proposed techniques, however, are complicated or require the addition of surfactants, restraining the possible applications of anisotropic polymer particles.14-16 A simple and facile method to make anisotropic polymer particles, the thermal annealing strategy, has been previously proposed, in which polymer microspheres are annealed on polymer films at elevated temperatures.17,

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Although anisotropic polymer particles have been

successfully made, the functions of the prepared particles could be altered because of the thermal degradation problems at elevated annealing temperatures.

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To conquer the challenges, we recently reported a solvent annealing strategy to make anisotropic polymer particles by annealing polymer microspheres coated on polymer films under solvent vapors.19 UFO-shaped, cymbal-shaped, peanut-shaped, and bowl-shaped polymer particles were obtained. Although anisotropic PS particles with interesting shapes can be produced, there are still challenges need to be addressed. First, the complete mechanisms for the polymer microspheres changing to anisotropic polymer particles remain unclear; therefore, more complicated structures such as snowmanshaped particles can not be obtained using the reported method. Second, the reported method only allows whole microspheres to transform the morphologies, limiting the possible control on the shapes of the polymer particles; the unique strengths of the solvent annealing processes are not used. Third, quantitative investigations on the relationships between the sizes of anisotropic particles and the experimental parameters such as the annealing time and the number of the original microspheres are less discussed. To address these challenges, in this study we propose an adaptable two-step solvent on-film annealing (2-SOFA) approach, which enables the fabrication of anisotropic polymer particles with complicated morphologies. This method relies on the sequential annealing processes of polymer composite samples in different solvent vapors, by which certain parts of the composite samples can be selectively annealed at different steps. To demonstrate the possibilities of this powerful method, we choose polystyrene (PS) microspheres spin-coated on poly(methyl methacrylate) (PMMA) films as model materials. Acetic acid is used as a preferable solvent for the PMMA films in the first solvent annealing step; cyclohexane is used as a preferable solvent for the PS microspheres in the second solvent annealing step. During the first step solvent annealing process in acetic acid vapors, the PMMA films wet and coat the surfaces of the PS microspheres while the microspheres maintain the spherical shapes; during the second step solvent annealing process in cyclohexane vapors, the PS particles wet the

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transformed PMMA films while the PMMA films maintain the morphologies. To characterize the morphological evolution of the annealed PS/PMMA composite films at different annealing steps, driven primarily by the interfacial and the surface tensions of the polymers, optical microscopy (OM) and scanning electron microscopy (SEM) are conducted. This method can successfully prepare anisotropic polymer particles with unique and complicated shapes, such as half-eaten peach-shaped, snowman-shaped, and bowler hat-shaped morphologies. The strategy to use the solvent vapors not only can solve the problem of thermal degradation of polymers at high temperatures but also can have the advantage to selectively anneal different components of the samples sequentially, which is inaccessible by other approaches.

EXPERIMENTAL SECTION Materials Poly(methyl methacrylate) (PMMA) (weight-average molecular weight (Mw):

75 kg/mol) was

purchased from Sigma-Aldrich. Polystyrene (PS) microspheres with mean diameters of ~10 µm (variation: 5 %) were obtained from Polysciences. Ethanol was obtained from Echo Chemical. Acetic acid, cyclohexane, and toluene were purchased from Tedia. The glass substrates were purchased from Matsunami Glass Ind., Ltd. with the sizes of 1.8 cm × 1.8 cm. The glass chambers used for the solvent annealing experiments were obtained from Tung Kuang Glassware company with the sizes of 960 mL.

Fabrication of PS Microspheres on PMMA Film-Coated Glass Substrates To fabricate the PMMA film-coated glass substrates, a 20 wt % PMMA (Mw: 75 kg/mol) solution in toluene was prepared and spin-coated at 1000 rpm for 60 s. The PMMA film-coated glass substrates were then annealed at 150 °C for 1.5 h to decrease the roughness of the PMMA films. The thicknesses of

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the PMMA films were measured to be ~8 µm. Subsequently, an aqueous suspension of PS microspheres (2.6 wt %) was diluted with ethanol and spin-coated on the PMMA film-coated glass substrates at 1000 rpm for 1 min. Finally, the PS/PMMA composites were dried and stored.

Preparation of PS/PMMA Composites by the Two-Step Solvent On-Film Annealing (2-SOFA) Method Open bottles of solvents (acetic acid) were first placed in sealed glass chambers for 12 h. The samples containing the PS microspheres deposited on the PMMA film-coated glass substrates were then placed in the glass chambers. The solvent vapor annealing processes were conducted at 30 °C for various periods of time. After the samples were removed from the annealing chambers, they were dried at room temperature to remove any residual solvents. The samples were again placed in glass chambers that contained open bottles of solvents (cyclohexane), and the solvent vapor annealing processes were carried out at 30 °C for various periods of time. Finally, the samples were taken out from the annealing chambers and dried at room temperature to remove any residual solvents. To confirm the structures of the annealed PS/PMMA composites, the selective removal technique was applied. The samples were dipped in acetic acid or cyclohexane for 48 h to remove the PMMA or the PS domains, respectively. When the samples are dipped in acetic acid, anisotropic PS particles can be obtained; when the samples are dipped in cyclohexane, cavity-containing PMMA films can be obtained.

Structure Characterization and Analysis A Zeiss optical microscope (OM) was employed to examine the morphologies of the PS microspheres deposited on the PMMA films under different solvent vapor annealing conditions. A JEOL scanning

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electron microscope (SEM, JSM-7401F) with the accelerating voltage of 5 kV was also employed to characterize the morphologies of the PS/PMMA composites and anisotropic PS particles annealed under different solvent vapor annealing conditions. Before the SEM measurements, the polymer samples were dried using a vacuum pump and coated with ~4 nm of platinum.

RESULTS AND DISCUSSION The schematic illustration of the experimental steps to prepare PS/PMMA composites utilizing the twostep solvent on-film annealing (2-SOFA) method is demonstrated in Figure 1. PMMA film-coated glass substrates are first fabricated by spin-coating a PMMA solution (20 wt % in toluene) on glass substrates, followed by annealing the samples at 150 °C for 1.5 h to decrease the roughness of the PMMA films. The thicknesses of the PMMA films are ~8 µm, as measured by SEM (Figure S1). Then, an aqueous suspension of PS microspheres (diameter: ~10 µm) is spin-coated on the PMMA film-coated glass substrates at 1000 rpm for 1 min. The polymer samples are subsequently moved into a sealed chamber filled with solvent vapors (acetic acid), which has been equilibrated for 12 h at 30 °C. After the first step solvent annealing processes, the samples are dried to remove any residual solvents. The samples are subsequently moved into a sealed chamber filled with solvent vapors (cyclohexane) for the second step solvent annealing processes, followed by the drying processes. To confirm the morphologies of the annealed PS/PMMA composites, the samples are dipped in acetic acid to selectively remove the PMMA films and anisotropic PS particles can be obtained.20, 21

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Figure 1. Graphical illustration of the experimental steps to prepare PS/PMMA composites utilizing the two-step solvent on-film annealing (2-SOFA) method. PMMA films can be selectively dissolved using acetic acid, and anisotropic PS particles can be released and collected.

The morphology changes of the PS/PMMA composites during the two-step solvent annealing processes are caused by not only the surface tensions but also the interfacial tensions of the solvent vapor-swollen polymers.22-24 It has been reported that the surface tensions of PS (molecular weight: 44 kg/mol) and PMMA (molecular weight: 3 kg/mol) are 40.7 and 41.1 mJ/m2 at 20 °C, respectively.25 When the samples are annealed in solvent vapors, the atmospheres are filled with solvent vapors and the polymers are also swollen by solvent vapors, resulting in new interfaces and different wetting behaviors. When PS microspheres and PMMA films are annealed in acetic acid vapors, the degrees of swelling for the PS microspheres are much less than those for the PMMA films. Therefore, the surface tensions of the PS microspheres with solvent annealing should be similar to those without solvent annealing; by comparison, the surface tensions of the PMMA films with solvent annealing should be affected by the presence of acetic acid. The values of the surface tensions of the PS microspheres and the PMMA films 7



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annealed in acetic acid vapors, however, are difficult to be obtained, concerning that the measurements should be conducted in solvent annealing chambers. In the first annealing step in this work, the samples are annealed in the vapors of acetic acid, a good solvent for the PMMA films and a non-solvent for the PS microspheres. The PMMA films are selectively swollen by acetic acid and the PMMA chains are mobile and gradually climb up and coat the surfaces of the PS microspheres, depending on the first step annealing time; the spherical shapes of the PS particles are maintained while the total surface and interfacial energies are decreased, mainly because parts of the surfaces of the PS particles are coated by the PMMA films. In the second annealing step, the samples are annealed in the vapors of cyclohexane, a good solvent for the PS particles and a non-solvent for the PMMA films. Only the PS chains in the particles are mobile and can cover back the PMMA structures; the morphologies of the PMMA structures are maintained while the total surface and interfacial energies are decreased, mainly because parts of the surfaces of the PMMA structures are covered by the PS particles. First, the morphology evolution of PS microspheres annealed on PMMA films in acetic acid vapors for 12 h and then in cyclohexane vapors for 12 h are investigated. Bowler hat-shaped PS particles with crown and visor parts can be generated, as illustrated in Figure 2a. Figure 2b−e presents the OM images of the PS microspheres annealed at different stages with lower and higher magnifications, in which the green and blue arrows indicate single and double PS particles, respectively. Before the annealing processes, only circles are observed in the OM image (Figure 2b) because of the optical contrast between the PS microspheres and the PMMA films. After the first step annealing process in acetic acid vapors for 12 h, two concentric circles can be observed in the OM image (Figure 2c) because of the additional contrast from the climbed PMMA films around the PS spheres. After the second step annealing processes in cyclohexane vapors for 12 h, circles with lighter corona parts can be observed while cracks are also formed in the OM image (Figure 2d), mainly due to the stress induced by the

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swollen PS particles confined in the PMMA structures. The anisotropic PS particles can be released after the PMMA films are selectively removed by acetic acid, as shown in Figure 2e. From Figure 2d,e, it can be seen that bowler hat-shaped particles with larger visor parts are formed after the two-step annealing processes. Figure 2f−q shows the SEM images of the samples at different annealing stages, which include single and double PS particles at lower and higher magnifications. It can be seen that the PS microspheres are unchanged before the annealing processes and the diameters of the PS microspheres are ~10 µm (Figure 2f,j,n). After the samples are first annealed by acetic acid, the wetted volcanoshaped PMMA films can be seen around the PS microspheres (Figure 2g,k,o). When the polymer samples are annealed in cyclohexane vapors for the second annealing processes, the polymer chains of the PS particles can cover back the PMMA films, revealing holes of the particles (Figure 2h,l,p). After the PMMA films are selectively removed by acetic acid, the bowler hat-shaped PS particles with crown and visor parts can be released (Figure 2i,m,q). The generality of the morphology transformation of the PS/PMMA composites at different stages can be exhibited in the SEM data with lower magnifications (Figure 2f−i), in which many PS particles on the PMMA films can be observed simultaneously. It can also be seen that two or more PS microspheres may aggregate together and form interesting structures. For the aggregated PS particles (Figure 2n), the PMMA chains start to wet the neighboring PS microspheres after the samples are first annealed by acetic acid, forming double volcano-shaped PMMA structures (Figure 2o). After the samples are treated by the second step solvent annealing process in cyclohexane, the aggregated PS particles can cover back the PMMA structures, generating PS particles with twin holes (Figure 2p). After the PMMA films are selectively removed by acetic acid, bowler hat-shaped PS particles from single, double, or even triple PS microspheres can be observed (Figure 2q). It can be seen that larger visor parts are observed by merging more PS particles. The sizes and morphologies of the samples

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observed from the SEM images agree well with those obtained from the OM images. Moreover, the three-dimensional structures of the anisotropic PS particles can be observed in more detail from the SEM images.

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Figure 2. (a) Graphical illustration to prepare PS/PMMA composites by annealing the samples using the two-step solvent on-film annealing (2-SOFA) method (acetic acid vapors for 12 h and cyclohexane vapors for 12 h at 30 °C). (b,f,j,n) PS microspheres deposited on PMMA films before the solvent annealing processes: (b) OM images with lower and higher magnifications and (f,j,n) SEM images. (c,g,k,o) PS/PMMA composites after the samples are annealed in acetic acid vapors for 12 h: (c) OM images with lower and higher magnifications and (g,k,o) SEM images. (d,h,l,p) PS/PMMA composites after the samples are annealed first in acetic acid vapors for 12 h and then in cyclohexane vapors for 12 h: (d) OM images with lower and higher magnifications and (h,l,p) SEM images. (e,i,m,q) Anisotropic PS particles after the PMMA films are selectively removed by acetic acid: (e) OM image and (i,m,q) SEM images. In (b,c,d), the scale bars of the insets are 5 µm.

For the two-step solvent on-film annealing process, the most interesting phenomenon is the morphology transformation of the PS/PMMA composites during the second solvent annealing step using cyclohexane, for which the PS microspheres cover back the climbed PMMA structures. To understand the morphology evolution processes from the PS spheres to the bowler hat-shaped PS particles, we examine the morphologies of the samples at different times in the second annealing processes. Here we fix the annealing time for the first step annealing process at 12 h, for which the PMMA films cover most regions of the PS microspheres. The samples are then annealed in cyclohexane vapors for different times, as shown in the graphical illustration with a cross-sectional view (Figure 3a), in which the PS particles wet the PMMA films and transform into different morphologies. The front and back sides of the bowler hat-shaped PS particles are also indicated in the illustration. The morphologies of the PS/PMMA composites by annealing the samples in acetic acid vapors for 12 h and cyclohexane vapors for various periods of time (0, 3, 6, 12, 15, and 30 h) at 30 °C are shown in Figure 3b−g. It can be seen that the top uncovered parts of the PS particles start to bud from the opening and wet the outer PMMA films. At a later stage, holes are formed inside the PS particles as the PMMA films are wetted by more PS chains. The sizes of the holes become larger at longer annealing times. After the PMMA films are selectively 11



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removed by acetic acid, the anisotropic PS particles at different times for the second step solvent annealing process can be seen, revealing the morphology transformation at different stages, as shown in Figure 3b’–g’. For Figure 3c’, the small protrusion on the PS particles, indicated by a red arrow, corresponds to the bump structure on the top of the PMMA film shown in Figure 3c, indicated by a green arrow. During the second step solvent annealing procedure, only the PS chains can move and wet the PMMA films to reduce the surface and interfacial energies of the PS/PMMA composites. When the annealing times are longer, more PS chains from the PS particles can crawl outside to cover more surfaces of the PMMA films, forming the bowler hat-shaped PS particles. After the PMMA films are selectively removed by acetic acid, the front or back sides of the bowler hat-shaped PS particles can be seen, as demonstrated in Figure 3e’−g’.

Figure 3. (a) Graphical illustration (cross-sectional view) of the PS/PMMA composites by annealing the samples in acetic acid vapors for 12 h and cyclohexane vapors for various periods of time at 30 °C. (b−g) SEM images of the PS/PMMA composites by annealing the samples in acetic acid vapors for 12 h and cyclohexane vapors for various periods of time at 30 °C: (b) 0, (c) 3, (d) 6, (e) 12, (f) 15, and (g) 30 h. 12


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(b’−g’) SEM images of anisotropic PS particles by annealing the samples in acetic acid vapors for 12 h and cyclohexane vapors for various periods of time at 30 °C, followed by selectively removing the PMMA films by acetic acid: (b’) 0, (c’) 3, (d’) 6, (e’) 12, (f’) 15, and (g’) 30 h. In (e−g) and (e’−g’), the front or back sides are also indicated.

In addition to the partially covered PS microspheres by the PMMA films, completely covered PS microspheres are also prepared. The PS microspheres are first annealed in acetic acid vapors for 12 h and PMMA films are placed on top of the samples, followed by annealing the samples again by acetic acid vapors for 12 h. Subsequently, the samples, in which the PS microspheres are completely covered by the PMMA films, are annealed by cyclohexane vapors for 12h. After selectively removing the PMMA films using acetic acid, it can be observed that the spherical shapes of the PS particles are maintained, as shown in Figure S2, indicating that the PS particles are not able to transform when they are covered completely by the PMMA films. For the snowman-shaped PS particles (Figure 3d’), which are prepared by annealing the samples in cyclohexane vapors for 6 h followed by selectively removing the PMMA films, it is questionable whether or not holes are really present in the particles as illustrated in Figure 3a, even though it might be reasonable considering the volume conservation of the particles. The snowman-shaped particles can be divided into the head and the body parts (Figure 4a−e). To find out if there are empty spaces in the particles, we develop an ingenious way to separate the head and body parts of the particles by taking advantage of the formation of the cracks during the solvent annealing processes. Figure 4a’ shows the SEM image of the bump structure on a PMMA film, in which cracks passing across the particle can be observed, as indicated by two red arrows. As mentioned before, such cracks are formed during the second annealing process due to the stress induced by the swollen PS particles confined in the PMMA structures. Using the cracks, which lead to weaker mechanical strengths at surrounding regions, we have

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combined breaking up the PS/PMMA composite films and applying the selective removal technique, which can cause the release of the particles and the separation of the head and the body parts, as shown in Figure 4b’−e’. It can be noted that holes are observed on both the separated head and body parts, confirming the hypothesis that there are empty spaces present inside the snowman-shaped particles after the two-step solvent on-film annealing processes. The formation of the holes in the snowman-shaped PS particles is probably related to the solvent evaporation after the solvent annealing process. In the solvent annealing process, the polymer chains are swollen by the solvent vapor; the PS chains in the PS particles are swollen by cyclohexane, causing the volume expansion of the particles. When the solvent is evaporated, the PS chains adhere to the PMMA walls, leaving holes or empty spaces in the snowmanshaped PS particles.

Figure 4. (a−e) Graphical illustrations of the snowman-shaped PS particles encapsulated in the PMMA films. (a’–e’) Corresponding SEM images of the snowman-shaped PS particles encapsulated in the PMMA films.

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For the above results, the times for the first step annealing processes are fixed at 12 h, at which the PS microspheres are almost covered by the PMMA films. Here we also study the effect of the times for the first step annealing processes on the morphology changes of the polymer composites, while the times for the second step annealing processes are fixed at 12 h. Figure 5 presents the graphical illustration and corresponding SEM data of the samples annealed for 3 and 6 h in the first step annealing processes. When the samples are annealed for 3 h in the first step annealing processes, only the bottom parts of the PS microspheres are covered by the PMMA films, as shown in Figure 5b. The shapes of the annealed PMMA films can be checked by selectively removing the PS microspheres using cyclohexane, as presented in Figure 5c, in which the volcano-shaped PMMA films with shallow cavities can be observed. When the PS/PMMA composite films are then annealed in cyclohexane for 12 h, the polymer chains of the bottom-covered PS microspheres are able to wet the PMMA films, as presented in Figure 5d. After removing the PMMA films selectively, half-eaten peach-shaped PS particles can be observed, as shown in Figure 5e, in which the pit parts are from the PMMA-covered bottom regions of the PS microspheres in the initial annealing step. For the samples annealed by the first step annealing processes in acetic acid vapors for 6 h, around two-thirds of the surfaces of the PS microspheres are covered by the PMMA films, as shown in Figure 5f. The shapes of the climbed PMMA films can be observed more clearly by selectively removing the PS microspheres using cyclohexane, as shown in Figure 5g, in which the volcano-shaped PMMA films are observed. When the PS/PMMA composites are then annealed in cyclohexane for 12 h, the polymer chains of the largely covered PS particles wet the volcano-shaped PMMA films, as presented in Figure 5h. After removing the PMMA films selectively, snowman-shaped PS particles can be observed (Figure 5i), similar to the shapes of the samples annealed in acetic acid vapors for 12 h and then in cyclohexane vapors for 6 h (Figure 3d’). Compared with the samples annealed for 12 h in the first step annealing

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process, the samples annealed for shorter periods of times (3 and 6 h) allow the PS chains to coat the PMMA films from larger openings, producing PS particles with different shapes. It should be noted that the shapes of the samples shown in Figure 5f and 5h look similar because both the PMMA films and the PS particles are present. After the selective removal process (Figure 5g and 5i), it can be seen that the morphologies of the samples shown in Figure 5f and 5h are different; Figure 5f and 5h contain the spherical PS particle and the snowman-shape PS particle, respectively. The samples are also annealed in the first annealing step for 24 h, followed by being annealed in cyclohexane vapors for the second annealing step for 12 h. The results demonstrate that snowman-shaped PS particles are also formed, as shown in Figure S3.

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Figure 5. (a) Graphical illustration of the PS/PMMA composites by annealing the samples in acetic acid vapors for 3 or 6 h and then in cyclohexane vapors for 12 h at 30 °C. (b,f) SEM images of the PS/PMMA composites by annealing the samples in acetic acid vapors for 3 and 6 h at 30 °C. (c,g) SEM images of the PMMA films with cavities after the samples are annealed in acetic acid vapors for 3 and 6 h at 30 °C, followed by selectively removing the PS particles by cyclohexane. (d,h) SEM images of the PS/PMMA composites by annealing the samples in acetic acid vapors for 3 and 6 h and then in cyclohexane vapors for 12 h at 30 °C. (e,i) SEM images of anisotropic PS particles after the samples are 17



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annealed in acetic acid vapors for 3 and 6 h and then in cyclohexane vapors for 12 h at 30 °C, followed by selectively removing the PMMA films by acetic acid.

For the PS/PMMA composites first annealed in acetic acid vapors for 12 h and then annealed in cyclohexane vapors for 12 h, interesting bowler hat-shaped PS particles can be generated (Figure 2q), where single or more PS particles are merged into single anisotropic particles. To further characterize the morphologies of the bowler hat-shaped PS particles and to understand the relationships between the sizes of the merged particles and the numbers of the original PS microspheres, we have measured the sizes of different parts (holes, visors, and crowns) of the particles. Figure 6a−d shows the 3D illustrations and corresponding SEM images of bowler hat-shaped PS particles by merging single, double, triple, and quadruple PS particles from the back sides of the particles, in which the hole parts can be observed. It is reasonable that the numbers of the holes on the bowler hat-shaped PS particles correspond to the numbers of the original PS microspheres. Figure 6a’−d’ shows the 3D illustrations and corresponding SEM images of single, double, triple, and quadruple PS particles from the front sides of the particles, in which the crown parts can be observed. Similar to the numbers of the holes, the numbers of the crowns correspond to the numbers of the original PS microspheres. To understand the formation of the bowler hat-shaped PS particles more quantitatively, the sizes of the holes, visors, and crowns are measured from the SEM images (Figure 6a−d and 6a’−d’) and the relationships between the sizes of the merged particles and the numbers of the original PS microspheres are plotted (Figure 6e−h). It can be observed that the diameters of the holes decrease with the numbers of the original PS particles, mainly due to the refilling effect of the polymer chains back into the holes from neighboring particles, as plotted in Figure 6f. It is also reasonable to see that the diameters of the visors increase with the numbers of the original PS microspheres because more polymer chains are involved in the wetting processes when there are more PS particles merging together, as plotted in Figure 6g; for the case of many spherical particles 18


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merging into a circular disk with a defined thickness, the diameter of the disks should increase with the square root of the number of the spherical particles. For the diameters of the crowns of the bowler hatshaped PS particles, the values are smaller than those of the original PS microspheres but remain nearly constant at different numbers of the original PS microspheres, as plotted in Figure 6h, implying that the reduction of the diameters of the crowns is dependent on the annealing time in the second annealing process instead of the numbers of the original PS particles.

Figure 6. (a−d and a’−d’) SEM images of bowler hat-shaped PS particles from the back and front sides. (a,a’) single, (b,b’), double, (c,c’) triple, and (d,d’) quadruple. (e) Graphical illustration of bowler hat-

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shaped PS particles, in which the hole, visor, and crown parts are indicated. (f−h) Plots of the diameters of the holes, visors, and crowns versus the numbers of the original PS microspheres.

In addition to the effect of the number of the original PS particles, we also study the effect of the time in the second annealing process on the morphologies of the bowler hat-shaped PS particles. For particles transformed from single PS microspheres, the diameters of the holes, visors, and crowns are measured and plotted versus the time in the second annealing process, as shown in Figure 7a−d. First, we can see that the sizes of the holes are ~3 µm and increase slightly with the annealing times because more polymer chains of the PS particles come out and coat the PMMA films, as presented in Figure 7b. Second, the sizes of the visors are measured to increase with the annealing times because the PS particles wet larger surfaces of the PMMA films, as shown in Figure 7c. We expect that the sizes of the visors should reach a plateau value at very long annealing times and wetted films with equilibrium thicknesses can be obtained. Finally, the sizes of the crowns are measured to decrease with the annealing times, as shown in Figure 7d; the surprising and interesting results indicate that larger gaps between the crowns and the PMMA walls are formed at longer annealing times.

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Figure 7. (a) Graphical illustrations and corresponding SEM images of bowler hat-shaped PS particles by annealing the samples in acetic acid vapors for 12 h and then in cyclohexane vapors for various periods of time at 30 °C, followed by selectively removing the PMMA films by acetic acid. (b−d) Plots of the diameters of the hole, visor, and crown parts versus the second solvent (cyclohexane) vapor annealing times.

CONCLUSION We investigate the morphology transformation of PS/PMMA composite films using a versatile two-step solvent on-film annealing (2-SOFA) approach. The PMMA films and PS microspheres can be selectively swollen using acetic acid and cyclohexane vapors sequentially during the first and the second annealing steps. Anisotropic polymer particles with special and complicated shapes can be prepared, such as half-eaten peach-shaped, snowman-shaped, and bowler hat-shaped morphologies. The results show that the wetting behaviors of the polymer composites, driven by the surface and interfacial tensions, are strongly affected by the annealing environments. For the morphology evolution from the PS spheres to the bowler hat-shaped PS particles, we examine the morphologies of the samples at different times in the second annealing processes. When the annealing times are longer, more PS chains from the PS particles are observed to crawl outside to cover the PMMA films, generating the bowler hatshaped PS particles. For the snowman-shaped particles, the empty spaces in the particles are confirmed by breaking up the PS/PMMA composite films caused by the cracks during the annealing process. For the bowler hat-shaped particles, quantitative studies are conducted to understand the relationships between the diameters of the holes, visors, and crowns and the times in the second annealing process. In the future, we will optimize the 2-SOFA method using other environmentally benign solvents; we will also investigate the applications of these anisotropic particles.

ASSOCIATED CONTENT 21



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Supporting Information available The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.XXXXXXX. SEM images of the PS/PMMA composite films treated by the two-step solvent annealing method.

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 Pro-gram 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-103-2221-E009-217-MY3). Notes The authors declare no competing ﬁnancial interest

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for Table of Contents use only

Two-Step

Solvent

On-Film

Annealing

(2-SOFA)

Method: Fabrication of Anisotropic Polymer Particles and Implications for Colloidal Self-Assembly Hsiao-Fan Tseng, 1 Yu-Jing Chiu, 1,2 Bo-Hao Wu, 1 Jia-Wei Li, 1 and Jiun-Tai Chen 1,2,3*

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Two-Step Solvent On-Film Annealing (2-SOFA) Method: Fabrication of

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