Solvent On-Film Annealing (SOFA): Morphological Evolution of

Jun 23, 2017 - In this work, we develop a solvent on-film annealing (SOFA) method to fabricate anisotropic polymer particles by studying the morpholog...
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Solvent On-Film Annealing (SOFA): Morphological Evolution of Polymer Particles on Polymer Films via Solvent Vapor Annealing Hsiao-Fan Tseng, Ming-Hsiang Cheng, Jia-Wei Li, and Jiun-Tai Chen* Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010 S Supporting Information *

ABSTRACT: Over the past few decades, anisotropic polymer particles are of significant interest because of their unique properties which can be applied in various areas, such as drug delivery, biotechnology, and electronics. Most approaches to synthesize anisotropic polymer particles, however, are complicated, and the three-dimensional shapes of the anisotropic particles are usually difficult to be controlled. In this work, we develop a solvent on-film annealing (SOFA) method to fabricate anisotropic polymer particles by studying the morphological evolution of polystyrene (PS) microspheres on poly(methyl methacrylate) (PMMA) films annealed in toluene vapor. At different annealing stages, the isotropic PS microspheres gradually transform to anisotropic PS particles with different morphologies, such as UFO-, cymbal-, peanut-, and bowl-shaped particles. The morphology evolution is driven by the surface and interfacial tensions during the annealing processes and can be confirmed by a selective removal technique. Acetic acid, a selective solvent for PMMA, and cyclohexane, a selective solvent for PS, are also used as the annealing solvents to further investigate the effect of the annealing solvent.

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INTRODUCTION Anisotropic polymer particles have attracted considerable attention in recent years because of their distinctive properties and can be utilized in different areas, such as interface stabilizers, drug delivery, and sensors.1−3 For example, the asymmetry provided by Janus particles imparts them unique chemical or physical properties within a particle, causing unusual self-assembly behaviors.4,5 There exist many approaches to fabricate anisotropic polymer particles. Among these approaches, however, most synthetic procedures are complicated, and additives or surfactants are usually added, limiting the applications of anisotropic polymer particles.6,7,8 Previously, we demonstrated an effective strategy to fabricate anisotropic polymer particles by thermally annealing polymer microspheres on polymer films.9,10 In one case, by annealing polystyrene (PS) microspheres on poly(methyl methacrylate) (PMMA) films at 240 °C, the PS microspheres can gradually sink into the PMMA films and transform to PS hemispheres embedded in the PMMA films.9 In another case, by annealing PS microspheres on poly(vinyl alcohol) (PVA) films at 240 °C, the PS microspheres transform to PS particles which contain two hemispheres with different curvatures.10 Although anisotropic polymer particles can be fabricated, the properties of the polymer particles may be deteriorated by the high annealing temperatures; therefore, possible applications of these anisotropic polymer particles are limited. To overcome the limitations, here we develop a versatile solvent on-film annealing (SOFA) method to fabricate © XXXX American Chemical Society

anisotropic polymer particles with different morphologies by annealing the microspheres deposited on polymer films using solvent vapors. The usage of the solvent vapors can effectively avoid the problem of thermal degradation of the polymer structures under high temperatures.11,12 In this work, PS microspheres deposited on PMMA films are used as a model system and annealed in toluene vapor, a good solvent for both PS and PMMA. During the toluene vapor annealing processes, the PS microspheres gradually sink into the PMMA films and transform into anisotropic PS particles, driven mainly by the surface and the interfacial tensions of the polymers.13−15 The morphological evolution of the PS/PMMA composites can be observed at different annealing times using optical microscopy (OM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The morphologies are also confirmed by using a selective removal technique, a powerful tool to understand the morphology evolution.16−18 The PS particles can be selectively removed by using cyclohexane, and porous PMMA films can be obtained; the PMMA films can be selectively removed by using acetic acid, and PS particles can be obtained. In addition to toluene, acetic acid, a selective solvent for PMMA, and cyclohexane, a selective solvent for PS, are also used as the annealing solvents to further investigate the effect of Received: April 1, 2017 Revised: June 1, 2017

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DOI: 10.1021/acs.macromol.7b00670 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the experimental process to fabricate PS/PMMA composites using solvent (acetic acid, cyclohexane, and toluene) vapor annealing at 30 °C. PMMA films and anisotropic PS particles can be obtained after the selective removal processes.

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RESULTS AND DISCUSSION The schematic illustration of the experimental process to fabricate PS/PMMA composites is shown in Figure 1. PMMA films are first prepared by the blade-coating method on glass substrates using a 25 wt % PMMA solution in toluene, followed by an annealing process at 150 °C for 90 min to reduce the roughness of the films. Subsequently, PS microspheres with an average diameter of 10 μm are suspended in ethanol and spincoated on the PMMA films at 1000 rpm for 60 s. The samples are then placed into a sealed chamber containing saturated solvent vapor (acetic acid, cyclohexane, or toluene) that has been equilibrated at 30 °C for 12 h. The solvent annealing processes are conducted for different lengths of time at 30 °C. The morphological evolution of the PS/PMMA composites during the solvent annealing process is driven not only by the surface tensions of the polymers but also by the interfacial tensions between the polymers. For example, the surface tensions of PS (44K) and PMMA (3K) at 20 °C are 40.7 and 41.1 mJ/m2, respectively. The interfacial tension between PS and PMMA at 20 °C is 3.2 mJ/m2.19 Because of the similar values of the surface tensions, the surface morphologies of PS/ PMMA composites are also determined by the interfacial tensions.20 Before the solvent annealing process, the surface energies are higher because of the exposed PS microspheres and PMMA film surfaces. When the samples are annealed in acetic acid vapor, selective for PMMA films, the PS microspheres gradually sink into the PMMA films while maintaining the spherical shapes, reducing the total surface energies. When the samples are annealed in cyclohexane vapor, selective for PS microspheres, the PS microspheres transform into anisotropic

the annealing solvent. For the acetic acid vapor annealing process, the spherical PS particles are maintained and gradually covered by the PMMA films. For the cyclohexane vapor annealing process, the PS microspheres wet the PMMA films and form disk-like structures. This approach can effectively produce anisotropic polymer particles, and the morphologies of the anisotropic particles can be controlled by changing the annealing solvents or the annealing times. Compared with the thermal annealing method, the solvent annealing method has many unique advantages. First, the thermal degradation problems at high temperatures which occurred during the thermal annealing process can be effectively avoided in the solvent annealing process; for functional polymers, especially for conjugated polymers, the optoelectronic properties are seriously deteriorated because of thermal degradation, affecting their potential applications. Second and more importantly, selective annealing of one polymer component in the polymer samples can be easily achieved using selective solvents in the solvent annealing process. Such selective annealing controllability is usually difficult to be achieved in the thermal annealing process because many common polymers have similar thermal transition temperatures, such as the glass transition temperatures (Tg) or the melting temperatures (Tm). In addition, the selective annealing feature in the solvent annealing process has the versatility in controlling the morphologies by using annealing solvents that have different degrees of swelling ability on individual polymer components. B

DOI: 10.1021/acs.macromol.7b00670 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. Illustration and SEM images of samples annealed in solvent (acetic acid) vapor at 30 °C: (a−d) single PS microspheres on PMMA films; (e−h) PMMA films, in which single PS particles are removed; (i−l) pairs of PS microspheres on PMMA films; (m−p) PMMA films, in which pairs of PS particles are removed.

particles on the PMMA films. The total surface energies decrease mainly because some areas of PMMA films are covered by the melted PS particles. When the samples are annealed in toluene vapor, a good solvent for both PS microspheres and PMMA films, the PS microspheres gradually transform and sink into the PMMA films, during which the surface energies of the PS particles decrease but the interfacial energies between PS and PMMA increase. Eventually, the PS microspheres transform to anisotropic particles embedded in the PMMA films, whose morphologies depend on the balance of the surface and the interfacial energies of PS/PMMA. To isolate the anisotropic PS particles from the PMMA films, we apply the selective removal technique by dissolving the PMMA films with acetic acid. First, we study the morphology transformation of PS microspheres annealed on PMMA films in acetic acid vapor. Acetic acid is a nonsolvent for PS but a good solvent for PMMA; only the PMMA chains in the films can adjust the

conformation while the PS microspheres maintain the spherical shapes. Figure 2 shows the illustration and SEM images of samples annealed in solvent (acetic acid) vapor at 30 °C. For single PS microspheres on PMMA films, the PMMA chains gradually cover the outer surfaces of the PS microspheres, resulting in the sinking of PS microspheres into the PMMA films (Figure 2a−d). The morphology transformation can be further confirmed by selectively removing the PS particles, as shown in Figure 2e−h, where volcano-like structures can be observed. For pairs of PS microspheres on PMMA films, the PMMA chains gradually cover the neighboring PS microspheres at different annealing times (Figure 2i−l). After the PS microspheres are selectively removed, PMMA films with twin holes (pig-nose-like structures) can be observed. For more microspheres aggregating together, interesting volcano-like structures with multiple openings can also be observed after the PS microspheres are selectively removed (Figure S1). The annealed samples before and after the selective removal C

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Figure 3. Illustration and SEM images of samples annealed in solvent (cyclohexane) vapor at 30 °C: (a−d) single PS microspheres on PMMA films; (e−h) single PS particles, under which PMMA films are removed; (i−l) pairs of PS microspheres on PMMA films; (m−p) pairs of PS particles, under which PMMA films are removed.

together from the connected regions while the PS chains also wet the PMMA films, as shown in Figure 3i−l. At longer annealing times, the two microspheres can merge and transform into disk-like structures. The transformation process of the pairs of PS microspheres can be confirmed again by selectively removing the PMMA films with acetic acid, as shown in Figure 3m−p. It is reasonable to see that the structures before and after the selective removal processes are similar because the PMMA films do not transform during the cyclohexane vapor annealing process; the transformed bottom parts of the PS microspheres at the early stage of the cyclohexane annealing process, however, can be observed after the selective removal processes. The generality of the morphology transformation for the samples annealed in cyclohexane vapor can be demonstrated from the SEM images with lower magnifications, in which PS particles with similar morphologies can be observed, as shown in Figure S3.

processes are also examined by SEM at lower magnifications, as shown in Figure S2, in which the generality of the morphology transformation is demonstrated. We then study the morphology transformation of PS microspheres annealed on PMMA films in cyclohexane vapor. Only the PS microspheres can transform because cyclohexane is a good solvent for PS but a nonsolvent for PMMA. Figure 3 shows the illustration and SEM images of samples annealed in solvent (cyclohexane) vapor at 30 °C. For single PS microspheres on PMMA films, the PS chains on the microspheres start to wet the PMMA films, changing shapes from spheres to disk-like structures (Figure 3a−d). The shape transformation of the PS microspheres can be confirmed by removing the PMMA films with acetic acid, as shown in Figure 3e−h. From the results, we can see that the PS chains near the bottom parts of the microspheres in contact with the PMMA films wet first during the solvent annealing process. For pairs of PS microspheres on PMMA films, the two spheres can merge D

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Figure 4. (a−d) OM images of single and multiple PS microspheres on a PMMA film annealed in solvent (toluene) vapor at 30 °C for different lengths of time: (a) 0, (b) 3, (c) 6, and (d) 12 h. (e−g) AFM data of a PS microsphere annealed on a PMMA film in solvent (toluene) vapor at 30 °C for 12 h: (e) height image, (f) height profile along the green dashed line in (e), and (g) 3D height image.

Figure 5. SEM images of single and pairs of PS microspheres on PMMA films annealed in solvent (toluene) vapor at 30 °C for different lengths of time: (a, g) 0, (b, h) 3, (c, i) 4, (d, j) 6, (e, k) 9, and (f, l) 12 h.

profile of the PS/PMMA composite (Figure 4f) is measured from the AFM height image (Figure 4e) by drawing a line across the particle. The height profile of the PS/PMMA composite indicates that the surface is slightly sunken at the location of the annealed PS particle. The size and the depth of the sunken dimple are estimated to be ∼14.5 μm and ∼708 nm, respectively. When the polymer composite films are swollen by the toluene vapor, a flat surface is expected; after the solvent evaporation, higher portions of toluene may be evaporated from the PS particles than those from the PMMA films, resulting in the formation of sunken surfaces at the locations of the annealed PS particles. The formation of the sunken surfaces due to the higher degrees of volume contraction of the PS particles than those of the PMMA films during the solvent evaporation process is illustrated in Figure S4. Although the size changes of the PS particles during the toluene vapor annealing process can be observed by OM and AFM measurements, it is still unclear about the threedimensional morphologies of the PS/PMMA composites at different annealing stages. Therefore, we examine the three-

After studying the morphology transformation by annealing the samples using selective solvents, we then investigate the morphology transformation of the PS microspheres on PMMA films by annealing the samples using toluene vapor, a good solvent for both PS and PMMA. Figure 4a−d shows the OM images of single and multiple PS microspheres on PMMA films annealed in toluene vapor at 30 °C for different times. For single PS microspheres, as indicated by arrows in the OM images, the diameters increase at longer annealing times, demonstrating the shape transformation of the PS microspheres during the toluene vapor annealing process. For multiple PS microspheres, the microspheres merge together and form structures with special shapes; for example, two microspheres and three microspheres can merge and form peanut- or triangle-shaped structures, respectively. For the annealed samples, AFM is also used to study the surface morphologies of the PS/PMMA composites and to obtain more quantitative information about the sizes of the annealed particles. Figure 4e−g shows the AFM images of a PS microsphere annealed on a PMMA film for 12 h. The height E

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Figure 6. (a, d) Schematic illustrations of the experimental processes to remove the PMMA films or PS particles selectively using acetic acid or cyclohexane, respectively. (b, e) SEM images of anisotropic PS particles. (c, f) SEM images of PMMA films, in which the PS particles are removed. The samples are annealed in toluene vapor at 30 °C for 6 h (a−c) and 12 h (d−f).

Figure 7. (a) Graphical illustrations (cross-sectional view) of the PS/PMMA composites at different annealing times. (b) SEM images of PS particles annealed on PMMA films for different lengths of time: 0, 3, 4, 6, 9, and 12 h. The PMMA films have been removed using acetic acid after the annealing processes.

removal technique. For example, Figure 6a illustrates the experimental process to dissolve the PMMA films or the PS particles selectively for the samples annealed for 6 h. Acetic acid is used as the selective solvent to remove the PMMA films, and cymbal-shaped PS particles can be obtained, as shown in Figure 6b; cyclohexane is used as the selective solvent to remove the PS particles, and PMMA films with dimples can be observed, as shown in Figure 6c. For the samples annealed for other times (3 and 12 h), similar removal processes are also conducted, as shown in Figure S6a−c and Figure 6d−f. With a longer annealing time (12 h), bowl-shaped PS particles or PMMA films with cavities can be obtained. To further understand the morphology evolution of the PS/ PMMA composites, we compare the PS particles at different annealing times by removing the PMMA films using acetic acid. The graphical illustrations (cross-sectional view) and SEM images at different annealing times are shown in Figure 7. At first, the PS microspheres with an average diameter of ∼10 μm are deposited on the PMMA films. When the samples are annealed in toluene vapor, the morphologies of the PS/PMMA composites gradually change to achieve a lower energy state, driven mainly by the surface and interfacial tensions. After annealing for 3 h, the surface of the PS microspheres starts to wet the PMMA films and transform to bell-shaped structures. At longer annealing times (4 h), more polymer chains on the particles wet the PMMA films and transform into UFO-shaped structures. It has to be noted that the PS particles also slightly sink into the PMMA films. As the annealing time increases to 6

dimensional morphologies transformations of the PS particles in more details by SEM measurements, as shown in Figure 5. From the SEM images, the PS microspheres are observed to sink into the PMMA films during the toluene vapor annealing process. At different annealing times, PS microspheres transform into different anisotropic PS particles embedded in the PMMA films. For shorter annealing times (∼3 h), the PS microspheres first transform to bell-shaped particles, as shown in Figure 5b. When the annealing times increase to ∼4 h, UFO-shaped particles can be observed, as shown in Figure 5c. As the annealing times further increase to ∼6−12 h, the particles merge into the films and form dimples on the films, as shown in Figure 5d−f. The diameter of the dimple shown in Figure 5f is ∼15.7 μm, close to that measured by AFM (∼14.5 μm) shown in Figure 4g. In the case of pairs of PS microspheres, the two particles in each pairs can also merge together and sink into the PMMA films during the toluene vapor annealing process, as shown in Figure 5g−l. The peanut-shaped particles, previously observed in the OM images (Figure 4c,d), can also be observed in the SEM images (Figure 5j−l). The generality of the morphology transformation for the samples annealed in toluene vapor can be demonstrated from the SEM images with lower magnifications, in which PS particles with similar morphologies can be observed, as shown in Figure S5. To further confirm the morphologies of the PS/PMMA composites at different annealing stages, we remove either PS or PMMA domains selectively by applying the selective F

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h, the PS particles fall deeper into the PMMA films and form cymbal-shaped structures. As the annealing time further increases to 9 h, frisbee-shaped PS particles can be obtained. Later, bowl-shaped particles are formed when the annealing time increases to 12 h. For the PS/PMMA composites annealed at longer times, it is interesting to consider whether there are thin PMMA films coated on the top of the PS particles. For the samples annealed at longer times (9 and 12 h), we observe that it is more difficult to remove the PS particles of the PS/PMMA composites selectively by using cyclohexane. Therefore, we speculate that there are thin PMMA films covering the PS particles, preventing the dissolution of PS particles in cyclohexane. The morphology differences between the samples annealed for 0 and 12 h are shown in Figure S7, in which the graphical illustrations (cross-sectional view), SEM images, and mathematical illustrations of the PS/PMMA composites are included. In the mathematical illustrations, O is the center of the sphere, R0 is the radius of the sphere, R1 is the long radius of the ellipsoid, h is the short radius of the ellipsoid, and r is the radius of the top circle. The values of R0, R1, and r can be estimated from the SEM images to be 5, 7.4, and 6.6 μm, respectively. The value of h, however, is difficult to be obtained from the experimental data. For the morphologies shown in Figure 7, the model is illustrated according to the SEM images of the residual PS particles after the selective removal process. It has to be noted that the proposed model is a simplified one because the sinking and shape changing of the PS particles may occur simultaneously during the solvent annealing process. In our model, for example, we assume that the particles change shapes first, without sinking into the PMMA films during the initial stages, as shown in the first three cartoons in Figure 7a. But the actual experimental data indicate that the PS particles also slightly sink into the PMMA films during the initial annealing stages.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00670. Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +886-3-5731631 (J.T.C.). ORCID

Jiun-Tai Chen: 0000-0002-0662-782X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of the Republic of China. REFERENCES

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CONCLUSION

In conclusion, we study the morphology evolution of PS/ PMMA composites under solvent vapor annealing, driven by the surface and interfacial tensions of the polymers. The morphologies of the three-dimensional PS/PMMA composites are characterized by OM, AFM, and SEM. For the acetic acid vapor annealing process, the spherical PS particles are maintained, and the PMMA films with holes can be obtained after removing the PS particles. For the cyclohexane vapor annealing process, the PS microspheres wet the PMMA films. For the toluene vapor annealing process, both PS microspheres and PMMA films can transform, resulting in the formation of anisotropic PS particles embedded in PMMA films. After the PMMA films are removed, anisotropic PS particles with unique morphologies can be obtained, such as bell-, UFO-, cymbal-, frisbee-, and bowl-shaped structures. PMMA films with cavities can also be obtained after selectively removing the anisotropic PS particles. In the future, we will study the self-assembly behaviors of these anisotropic particles by using chemical modification techniques on specific regions of the particles. These asymmetric polymer particles can also be applied in areas such as sensors and surfactants. Other polymer systems treated by environmentally benign solvents will also be investigated. G

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DOI: 10.1021/acs.macromol.7b00670 Macromolecules XXXX, XXX, XXX−XXX