From Block Copolymer Nanotubes to Nanospheres: Nonsolvent

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From Block Copolymer Nanotubes to Nanospheres: NonsolventInduced Morphology Transformation Using Porous Templates Chun-Wei Chang, Yi-Hsuan Tu, Ke-Hsuan Luo, and Jiun-Tai Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03250 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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From Block Copolymer Nanotubes to Nanospheres: Nonsolvent-Induced Morphology Transformation Using Porous Templates Chun-Wei Chang,1 Yi-Hsuan Tu,1 Ke-Hsuan Luo,1 and Jiun-Tai Chen1,2* 1

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

2

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

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

ABSTRACT

Block copolymer nanostructures have attracted great attention because of the wide range of applications such as sensors and drug delivery. The fabrication of block copolymer nanostructures with controlled morphologies and sizes, however, is still challenging. Here, we study the fabrication of nanotubes and nanospheres of polystyrene-block-polybutadiene (PS-b-PBD) using anodic aluminum oxide (AAO) templates. When PS-b-PBD solutions in N-methyl-2-pyrrolidone (NMP) are introduced into the nanopores of the AAO templates applying the traditional solution wetting method, PS-b-PBD nanotubes can be obtained. When PS-b-PBD solutions in the nanopores are in contact with nonsolvent, acetic acid, PS-bPBD nanospheres are formed. Two possible mechanisms are proposed to discuss the formation of the nonsolvent-driven morphology transformation, including the Rayleigh-instability-type transformation mechanism and the nucleation and growth mechanism. The effect of the polymer concentrations on the 1 ACS Paragon Plus Environment

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internal morphologies of the PS-b-PBD nanostructures is discussed; at higher concentrations, PS-b-PBD nanocapsules can also be prepared. Furthermore, core-shell PS-b-PBD/polymethylmethacrylate (PMMA) nanospheres can be fabricated using this strategy with polymer blend solutions. This work not only demonstrates a simple strategy to control the morphologies of block copolymer nanostructures but also deepens the understanding of the interactions between polymer solutions and solvents.

KEYWORDS: anodic aluminum oxide, block copolymers, core-shell nanospheres, solution wetting, templates

INTRODUCTION Block copolymer nanostructures have received significant attention because of their self-assembly behaviors, which are applied to a variety of applications, such as sensors, drug delivery, and photovoltaics.1-3 For block copolymer colloids, there have been efforts to fabricate particles with controlled shapes and internal morphologies. Previously, polystyrene-block-poly(2-vinyl pyridine) and other copolymer particles have been prepared by a solvent exchange procedure; for example, nonsolvent is added gradually into block polymer/THF solutions and block copolymer particles are formed.4-7 The internal morphologies of these particles can be further manipulated by thermal or solvent annealing. Also, in the perspective article reported by Hayward et al., several techniques such as epitaxial crystallization of micelles, rapid nanoprecipitation, emulsion processing, and membrane extrusion have been introduced to control the self-assembled behavior of block copolymers in solutions.8 To make polymer nanostructures with controlled internal and surface morphologies and sizes, the template wetting method is considered to be one of the most frequently used approach, which had been pioneered by Martin et al.9, 10 For the template wetting technique, polymers can be infiltrated into the

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channels of the porous templates by wetting the surface of the nanopores through the capillary force. After infiltrated polymers are solidified, the porous templates can be selectively etched to reveal the polymer nanostructures. Porous template materials, such as the anodic aluminum oxide (AAO) membranes, are commonly used.11 Generally, there are four ways to introduce polymer chains into the nanopores of porous templates, including the solution wetting method, the microwave annealing method, the melt wetting method, and the solvent vapor annealing method.12-16 To make polymer nanomaterials with different morphologies, the solution wetting method is more suitable compared to other three methods because of the numerous experimental parameters that can be utilized. For the solution-based template wetting method, one of the recently developed approach is called the double solution wetting method, in which templates are dipped in two different solutions in sequential order.17 In contrast to the traditional solution wetting method, the double solution wetting method offers higher possibility in manipulating the morphologies of the prepared nanostructures. Parameters such as the type of polymers and the type of solvents can be used to change the interactions between the polymers and the solvents. Different nanostructures with a variety of morphologies, such as nanospheres, core-shell nanospheres, nanopeapods, and mesoporous structures, can be prepared.17-21 Furthermore, functional polymer nanoparticles can be fabricated using thiol-ended PS to encapsulate gold nanoparticles.22 Wu et al. also utilized this procedure to generate multifunctional polymer nanoparticles, which provides three distinct functionalities.23 Although the double solution wetting method has been exploited to make polymer nanomaterials with different morphologies, most of the studies focus on homopolymer or polymer blend systems. In addition, the mechanisms involved in the double solution wetting method are still unclear. Here, we investigate the morphologies of block copolymer nanostructures by applying the traditional solution wetting method and the double solution wetting method. AAO membranes are used as the template


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materials. Polystyrene-block-polybutadiene (PS-b-PBD) is chosen as a model block copolymer, in which the PBD domain can be selectively stained for TEM measurements. Using the traditional solution wetting method by placing the AAO templates in PS-b-PBD solutions in N-methyl-2-pyrrolidone (NMP), nanotubes of PS-b-PBD with the cylindrical PBD domains perpendicular to the surface can be obtained. Using the double solution wetting method by placing AAO templates in PS-b-PBD solutions and in acetic acid sequentially, nanospheres of PS-b-PBD with the cylindrical PBD domain perpendicular to the surface can be obtained. For the fabrication of polymer nanospheres using the double solution wetting method, most of the previous works focus on homopolymers, polymer blends, and block copolymer micellar nanostructures. Different from the previous studies, this work is the first demonstration of the precipitation of block copolymer nanoparticles through solvent exchange process in the nanopores of AAO templates. The double solution wetting method provides the possibility to study the self-assembled behaviors of block copolymers in spherical confinement. The confined geometries of the particles can be altered by polymer concentrations, and capsule-like nanoparticles can be prepared. Furthermore, we also demonstrate that core-shell PS-b-PBD/PMMA nanoparticles can be fabricated by blending homopolymers and block copolymers. The results show the feasibility to investigate the self-assembled behaviors of homopolymer/block copolymer blends in spherical confined environment.

EXPERIMENTAL SECTION Materials Poly(styrene-block-butadiene) (PS-b-PBD) copolymer with the number-average molecular weight (Mn) of (35-b-11) kg/mol (PDI= 1.09) and poly(methyl methacrylate) (PMMA) homopolymer with the numberaverage molecular weight (Mn) of 24 kg/mol (PDI= 1.25) were obtained from Polymer Source. Glacial


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acetic acid was purchased from Sigma Aldrich. Toluene (reagent grade) was obtained from Tedia. Deionized (DI) water was obtained from Milli-Q system. N-Methyl-2-pyrrolidone (NMP) (HPLC grade) and sodium hydroxide (NaOH) were purchased from J.T. Baker. Anodic aluminum oxide (AAO) membranes with a pore diameter of ~150-400 nm and a thickness of ~60 μm were obtained from Whatman. Osmium tetroxide (OsO4) solution with a concentration of 4% was purchased from Alfa Aesar. Polycarbonate membrane filters with pore size of ~0.1 μm were obtained from Millipore.

Fabrication of PS-b-PBD Nanotubes and Nanospheres To fabricate PS-b-PBD nanotubes, a small piece of the AAO template (~0.2 cm×0.2 cm) was first placed in a 2.5 wt % solution of PS-b-PBD (Mn: 35-b-11 kg/mol) in NMP (3 mL) for 10 s. After the sample was removed from the polymer solution, the residual solutions on the surface of the AAO template was cleaned using Kimwipes. Subsequently, the obtained sample was further dried by a vacuum pump at room temperature for 12 h. Finally, the nanotubes were released by immersing the template in 5 wt % NaOH(aq) at room temperature for 2 h to etch the AAO membrane selectively. To fabricate PS-b-PBD nanospheres, a small piece of the AAO template (~0.2 cm×0.2 cm) was placed in a 2.5 wt % solution of PS-b-PBD (Mn: 35-b-11 kg/mol) in NMP (3 mL) for 10 s. After the sample was removed from the polymer solution, the residual polymer solutions on the surface of the AAO template was cleaned using Kimwipes. The AAO template was subsequently immersed in acetic acid for 10 s. Finally, the sample was again removed from the solution and the residual solvents on the surface of the AAO template was cleaned using Kimwipes. After the obtained sample was further dried by a vacuum pump at room temperature for 12 h, the AAO template was selectively etched by placing the template in 5 wt % NaOH(aq) at room temperature for 2 h to release the PS-b-PBD nanospheres.


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Fabrication of PS-b-PBD/PMMA Core-Shell Nanospheres To fabricate PS-b-PBD/PMMA core-shell nanospheres, an AAO template was first put in a 5 wt % mixed solution of PS-b-PBD (Mn: 35-b-11 kg/mol)/PMMA (Mn: 24 kg/mol) (weight ratio= 1:1) in NMP for 10 s. After the sample was removed from the polymer solution by a tweezer, the sample was subsequently placed in water for 10 s. After the sample was again removed from the solution and the residual solutions on top of the AAO template was cleaned using Kimwipes. The obtained sample was further dried by a vacuum pump for 12 h and the AAO template was selectively etched using 5 wt % NaOH (aq) to reveal the PS-b-PBD/PMMA core-shell nanospheres. The nanospheres were filtered by a polycarbonate membrane filter and washed by water. To confirm the spatial distribution of the PS-b-PBD/PMMA nanospheres, a selective removal process was conducted. The PS-b-PBD/PMMA nanospheres were immersed in acetic acid, a good solvent for PMMA and nonsolvent for PS-b-PBD, for 24 h to selectively etch the PMMA domain, leaving the PS-b-PBD nanospheres.

Structure Analysis and Characterization A scanning electron microscope (SEM) (JEOL JSM-7401F) at an acceleration voltage of 5 kV was utilized to characterize the surface morphologies of the polymer nanostructures. Before SEM measurements, the polymer samples were dried using a vacuum pump and coated with 4 nm of Pt. A bright-field transmission electron microscope (TEM) (JEOL JEM-2100) at an acceleration voltage of 200 kV was utilized to characterize the internal morphologies of the polymer samples. For TEM measurements, the samples were deposited onto copper grids coated with a thin layer of Formvar. Before TEM measurements, osmium tetroxide (OsO4) was used to selectively stain the PBD domain in the PS-b-PBD nanostructures. For the characterization of the average diameters of the PS-b-PBD nanostructures, the SEM images of the samples were measured by image J. The selective removal process of the PS-b-PBD/PMMA core-shell


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nanospheres before and after PMMA removal was examined by 1H nuclear magnetic resonance (NMR) spectra using a NMR spectrometer (Varian 400 MHz). The chemical shifts (δ) were recorded in parts per million. Deuterated chloroform (CDCl3) with TMS was used as the internal reference.

RESULTS AND DISCUSSION Here, porous AAO membranes are used as template materials to make the block copolymer nanotubes and nanospheres. AAO templates have been widely used because of the regular pore arrangement, the tunable pore diameter, and the easy removal process.24, 25 The top view and cross-sectional view SEM images of the porous AAO membranes are displayed in Figure S1.

Figure 1. (a) Schematic illustration of the fabrication process of the PS-b-PBD (Mn: 35-b-11 kg/mol) nanotubes using the solution wetting method. The blue parts and red parts in the nanotubes represent the PS and PBD domains, respectively. (b and c) SEM images of the PS-b-PBD nanotubes at lower and higher magnifications. (d) TEM image of a PS-b-PBD nanotube.


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We first characterize the morphologies of the cylinder-forming PS-b-PBD (Mn: 35-b-11 kg/mol) nanotubes by applying the solution wetting method. The fabrication process of the PS-b-PBD nanotubes is illustrated in Figure 1a. First, an AAO template is placed in a PS-b-PBD solution in NMP with a concentration of 2.5 wt % for 10 s. After the template is taken out and the solvents are evaporated using a vacuum pump, the polymer chains then deposit on the pore wall surfaces of the template, resulting in the formation of nanotubes. In the last step, the template is selectively etched using 5 wt % NaOH(aq) to reveal the nanostructures. The hollow nature of the PS-b-PBD nanostructures can be confirmed from the broken nanotubes, as shown in the SEM image (Figure 1b), in which the broken part is indicated by a red arrow. From the magnified SEM image (Figure 1c), the surface of the nanotubes is smooth and no selfassembled structure from the block copolymers is observed. In order to check the internal morphologies of the PS-b-PBD nanotubes, TEM measurements are conducted. Before the TEM measurements, the PBD domains in the PS-b-PBD nanotubes are selectively stained by OsO4 vapor for 15 min to increase the electron-density contrast. The TEM image of a PS-b-PBD nanotube is shown in Figure 1d, in which the PBD domains appear darker than the PS domains do. From the TEM image, the cylindrical PBD domains are arranged perpendicularly to the AAO wall (the perpendicular tubular morphology). Such morphology indicates that the surface tensions and the corresponding interfacial tensions of the PS and PBD domains to the AAO walls are mediated during the solution wetting and solvent evaporating processes.


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Figure 2. (a) Schematic illustration of the fabrication process of the PS-b-PBD (Mn: 35-b-11 kg/mol) nanospheres using the double solution wetting method. The blue parts and red parts in the nanospheres represent the PS and PBD domains, respectively. (b and c) SEM images of the PS-b-PBD nanospheres at lower and higher magnifications. (d) TEM image of a PS-b-PBD nanosphere.

We then investigate the formation of PS-b-PBD nanospheres by adding nonsolvent after the polymer solutions are infiltrated into the nanopores of the AAO templates. Figure 2a shows the fabrication process of the PS-b-PBD nanospheres. First, an AAO template is placed in a polymer solution of PS-b-PBD in NMP with a concentration of 2.5 wt % for 10 s. After the template is removed from the solutions, the solutions on the surface of the AAO template is cleaned using Kimwipes. This wiping step is critical in the fabrication process, which prevents the formation a thick polymer solution layer outside the nanopores and allows the infiltration of other solutions. Second, the template is again placed in a nonsolvent (acetic acid) of both PS and PBD for 10 s, resulting in the formation of nanospheres. In the last step, the nanostructures are revealed by selectively etching the AAO template using 5 wt % NaOH(aq). It has to be noted that the choice of solvent and nonsolvent is of crucial importance in the formation of PS-b-PBD nanospheres. We choose NMP as the solvent for PS-b-PBD because of the relatively high boiling point 9 ACS Paragon Plus Environment

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(∼203 °C), which prevents the polymer solution from evaporation. If the polymer solution evaporates completely before the nonsolvent infiltration, nanotubes instead of nanospheres are formed. Acetic acid is selected as the nonsolvent because it does not dissolve PS-b-PBD and is miscible with NMP. The interactions between acetic acid and the pore walls are also stronger than those between NMP and the pore walls. Therefore, the nonsolvent can be readily infiltrated into the channels of the AAO template when the sample is immersed in the nonsolvent for the precipitation of the block copolymer nanospheres. The SEM images of the PS-b-PBD nanoparticles are shown in Figure 2c and d, in which spherical structures with smooth surfaces can be observed. The average diameter of the nanospheres is ～146 nm. The internal morphology of the PS-b-PBD nanospheres can also be measured by TEM. Before the TEM measurements, the PBD domains are stained selectively by OsO4 vapor; therefore, the PBD domains appear darker than the PS domains do. As shown in Figure 2d, the darker PBD domains are arranged perpendicularly to the surface of the spheres (the perpendicular spherical morphology), forming golf ball-like structures. Similar to that of the PS-b-PBD nanotubes, the perpendicular morphology of the PS-b-PBD nanospheres indicates that the interfacial tensions of the PS and PBD domains to the surrounding mediums are mediated during the solution wetting and solvent evaporating processes, which avoids the formation of surface preferential layers (the parallel spherical morphology). In addition to acetic acid, water is also used as the nonsolvent. The results show that PS-b-PBD nanospheres can also be obtained (Figure S3), indicating that both water and acetic acid can be utilized as the nonsolvent. When the PS-b-PBD nanostructures are prepared, the confinement effect does play a critical role in the morphologies of the block copolymers, which are different from those in the bulk state. When PS-bPBD nanospheres are prepared, the higher curvatures and smaller confined sizes (three-dimensional confinement) causes higher degrees of chain compression or stretching, resulting in the formation of the golf ball-like structures. For the confined environment (AAO template) used here, the average pore


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diameter is ~230 nm. If the diameters of the AAO template decrease, the confinement effect would be stronger and highly distorted self-assembled structures can be expected. The nanospheres with the golf ball-like morphology are similar to those studied by Hirai et al.26 In that work, sphere-forming polystyrene-block-poly(t-butyl acrylate) (PS-b-PtBA) nanoparticles are fabricated using a solvent evaporation process. Block copolymer particles with virus-like structures are prepared by blending homopolymers (PS) with different amounts and molecular weights. To clarify the confinement effect of the AAO template, two additional sets of experiments are conducted. The first additional set of experiment is by dropping 25 μL of 1 wt % solution of PS-b-PBD in NMP into a large volume of nonsolvents (water or acetic acid) under sonication. The use of sonication is to ensure homogeneous mixing of the polymer solution and the nonsolvent and prevent the formation of large polymer aggregates. As displayed in Figure S4, without the confinements of the AAO templates, irregular polymer aggregates are formed when water or acetic acid are used as the nonsolvents. The second set of experiment is by slowly adding 100 μL of nonsolvent (water and acetic acid) into same amount of PS-b-PBD solution in NMP at a concentration of 1 wt % with stirring, followed by solvent evaporation. As shown in Figure S5, when nonsolvents (water and acetic acid) are gradually added into PS-bPBD/NMP solutions, spherical particles with average diameters ~3.4±1.4 and 3.8±1.1 μm can be prepared, respectively. The sizes of the PS-b-PBD particles are much larger than those using the AAO templates. Also, the size distributions of the particles fabricated using the AAO templates are narrower, showing that the AAO templates provide better controllability on the sizes of the block copolymer particles. From the two additional sets of experiments, we may infer that the nanopores of the AAO templates provide a space for mixing the polymer solutions and the nonsolvents, which allow the diffusion of the solvent molecules in a relatively small scale, resulting in the formation of spherical particles.


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Figure 3. Graphical illustrations of two possible formation mechanisms of the PS-b-PBD nanospheres using the double solution wetting method: (A) the Rayleigh-instability-type transformation mechanism and (B) the nucleation and growth mechanism.

We then discuss the formation mechanism of the PS-b-PBD nanospheres; two possible mechanisms are discussed. The first proposed mechanism is the Rayleigh-instability-type transformation mechanism (Figure 3A), which was proposed previously.17 When the nonsolvent is wetted the polymer solutioncontaining channels of the AAO template, a wetting layer of nonsolvent is formed on the surface of the nanopores because of the stronger interactions between the nonsolvent and the AAO pore walls than those between the polymer solution and the channel walls. As a result, cylindrical polymer solution domains are isolated in the middle parts of the nanopores. To decrease the total interfacial energies, the cylindrical solution domains form wave-like structures and change into spherical solution domains, driven by the Rayleigh instability-type transformation.27-29 The second proposed mechanism for the generation of the PS-b-PBD nanospheres is the nucleation and growth mechanism (Figure 3B).30,

31

When the nonsolvent (acetic acid) is infiltrated into the 12

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cylindrical pores of the AAO templates, the phase separated state is more energetically favorable. The rapid blending between the nonsolvent and the polymer solutions creates a local supersaturation of the polymer solutions, resulting in the spontaneous coalescing of polymer molecules to form nuclei.32 In this mechanism, therefore, the mixing rates and the molecular diffusion rates should be much faster than the nucleation rates.33 After the formation of the nuclei, single polymer chains diffuse from the supersaturated solution to the surface of the nuclei, resulting in the growth of the precipitant sizes. The small precipitants further aggregate together to form particles with larger sizes.34 If the Rayleigh-instability-type transformation mechanism is dominant, the particle sizes should be independent of the mixing times of the polymer solutions and the nonsolvents; if the nucleation and growth mechanism is dominant, the particle sizes should increase with the mixing times. From the experimental results, the dependence of the particle sizes on the mixing times are not observed, indicating that the Rayleigh-instability-type transformation mechanism is more favorable.

Figure 4. (a-d) SEM images of the PS-b-PBD nanostructures prepared by the double solution wetting method with different polymer concentrations: (a) 0.5, (b) 1, (c) 5, and (d) 8 wt %. (e) Graphical illustration


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of the PS-b-PBD nanostructures with increasing polymer concentrations. (f) Plot of the average diameters and the aspect ratios of the PS-b-PBD nanostructures verses the polymer concentration, presented as black curve and blue curve, respectively.

We further study the morphologies of the PS-b-PBD nanospheres using polymer solutions with different initial concentrations. At the lowest polymer concentration (0.5 wt %), PS-b-PBD nanospheres with a smaller average diameter (~106 nm) are formed (Figure 4a). As the concentrations of the polymer solution increase to 1 and 5 wt %, the average diameter increases to ~123 and ~183 nm, respectively (Figure 4b,c). As the concentration further increases to 8 wt %, both spherical particles with larger diameters and capsule-like nanostructures can be observed (Figure 4d). The graphical illustration of the PS-b-PBD nanostructures with increasing polymer concentrations is shown in Figure 4e, in which the diameters of the nanospheres increase with the concentration and capsule-like nanostructures are also formed at higher concentrations because of the cylindrical confinement of the channels of the AAO template. Figure 4f shows the plot of the average diameters and the aspect ratios of the PS-b-PBD nanostructures verses the concentration of the polymer solution. The aspect ratio of the nanostructures is defined by the length divided by the diameter. When the concentration of the polymer solution increases, both the average diameters and aspect ratios of the nanostructures increase, confirming that more polymer chains are in the confined cylindrical channels of the AAO templates at higher concentrations. For the nanostructures fabricated at concentrations lower than 2.5 wt %, the values of the aspect ratio are close to 1, indicating that the formation of nanospheres; for the nanostructures fabricated at higher concentrations (5 and 8 wt %), the increased values of the aspect ratio suggest the formation of nanocapsules.


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Figure 5. (a) Schematic graphics of the making processes of PS-b-PBD/PMMA core-shell nanospheres. The red parts and the yellow parts indicate the PS-b-PBD cores and the PMMA shells, respectively. (b) SEM image of PS-b-PBD (Mn: 35-b-11 kg/mol)/PMMA (Mn: 24 kg/mol) core-shell nanospheres. (c) Diameter histogram of the PS-b-PBD/PMMA core-shell nanospheres. (d) SEM image of PS-b-PBD (Mn: 35-b-11 kg/mol) nanospheres. The nanospheres are made by placing the PS-b-PBD/PMMA core-shell nanospheres in acetic acid to etch the PMMA domains. (e) Diameter histogram of the PS-b-PBD nanospheres.

In addition to block copolymer nanospheres, the double solution wetting method can also be applied to make homopolymer/block copolymer core-shell nanospheres. Blend solutions of PMMA and PS-bPBD are used to make core-shell nanospheres. The schematic illustration of the fabrication process of the core-shell nanospheres is displayed in Figure 5a. First, the AAO template is placed in the blend solution of PS-b-PBD/PMMA in NMP. Water, a nonsolvent for both PMMA and PS-b-PBD, is then introduced in the channels of the AAO templates. After the solvent evaporation and template removal, core-shell polymer nanospheres can be obtained. It has to be noted that the nonsolvent used here is water instead of acetic acid. This is because acetic acid is considered as a good solvent for PMMA; it cannot cause the precipitation of PMMA chains after the PS-b-PBD/PMMA solution is brought into contact with acetic 15 ACS Paragon Plus Environment

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acid. To confirm the morphologies of the PS-b-PBD/PMMA nanospheres, a selective etching process of the PMMA domains using acetic acid is conducted. The sample is placed in acetic acid for 24 h to completely dissolve the PMMA shells, revealing PS-b-PBD cores. We choose acetic acid as a selective solvent to etch the PMMA domains and keep the PS-b-PBD domains unaffected. The SEM image of the PS-b-PBD/PMMA nanospheres is shown in Figure 5b, and the average diameter of the nanospheres is ~198 nm (Figure 5c). After the PMMA outer layers are selectively etched, the PS-b-PBD nanospheres can be observed (Figure 5d) and the average diameter of the PS-b-PBD cores is ~146 nm (Figure 5e), which is smaller than the diameter before the selective removal process. The decreased diameter of the nanospheres confirms the core-shell morphology. The removal of the PMMA shells is also confirmed by NMR measurements. Before the removal process, the presence of the PMMA can be identified from the peak associated with the −CH3 group in ester linkage, indicated as peak c in the NMR spectrum (Figure S6). After the removal process, the much weaker peak associated with the −CH3 group in ester linkage of the PMMA confirms that the PMMA parts are almost removed, as shown in the NMR spectrum (Figure S7). In Figure 5, the concentration of the blend solution of PS-b-PBD/PMMA is 5 wt %. Two other concentrations (1 and 2.5 wt %) are also utilized to prepare the core-shell nanostructures (Figure S8). It can be observed that nanospheres with smaller sizes can be obtained at lower concentrations. Compared with the results using PS-b-PBD alone, similar trends are observed; spherical nanoparticles with smaller sizes are obtained at lower concentrations. Previously, Ko et al investigated the fabrication of PS/PMMA core-shell nanospheres.19 In that work, the morphologies of the nanospheres were determined by the interactions between solvents (DMF), nonsolvents (water), polymers (PS and PMMA), and alumina walls. Because the interactions between PMMA and water is higher than those between PS and water. Therefore, the PMMA chains aggregate at


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the solution/water interface during solvents evaporation, causing the precipitation of PS/PMMA core-shell nanoparticles. In the case here using PS-b-PBD/PMMA blend solutions, the core-shell nanospheres comprise PMMA shells and PS-b-PBD cores, which is reasonable concerning the stronger interactions between PMMA and nonsolvent (water) than those between PS-b-PBD and water. To further demonstrate the formation of the core-shell nanostructures, additional experiments using PMMA with a higher molecular weight of 398 kg/mol are conducted. When a 2.5 wt % PMMA (Mn: 398 kg/mol)/NMP solution is used to make the polymer nanostructures using the double solution wetting method, nanorods with undulated surfaces instead of nanospheres are observed (Figure S9a). When a 2.5 wt % of PS-b-PBD/PMMA mixed solution is used to prepare the polymer nanostructures, bead-on-string structures are observed (Figure S9b). After the PMMA domains are selectively removed by acetic acid, PS-b-PBD nanospheres are observed (Figure S9c). These results also indicate that the PMMA chains tend to form the outer layers of the nanostructures, which are similar to the case using homopolymers blends of PS/PMMA.

CONCLUSION We study the fabrication of nanotubes and nanospheres of PS-b-PBD using porous AAO membranes. When PS-b-PBD solutions are infiltrated into the channels of the AAO templates followed by solvent evaporation, PS-b-PBD nanotubes can be prepared, in which perpendicular cylinder morphology is observed. When nonsolvent is introduced into the polymer solution-containing AAO templates, PS-b-PBD nanospheres with golf ball-like structures can be obtained. Two possible mechanisms are discussed to elucidate the formation of the block copolymer nanospheres. The sizes and aspect ratios of the PS-b-PBD nanoparticles induced by addition of nonsolvents can be controlled by the polymer concentrations.


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Furthermore, PMMA/PS-b-PBD core-shell nanospheres are prepared using PMMA/PS-b-PBD blend solutions. This work provides a simple way to control the morphologies of block copolymer nanostructures confined in cylindrical and spherical geometries, which can be applied to multicomponent functional nanostructures.

ACKNOWLEDGEMENT This study was financially supported by the Ministry of Science and Technology of the Republic of China. This research was also supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

SUPPORTING INFORMATION SEM images of the AAO membranes and the PS-b-PBD nanostructures, histograms of the diameters of the PS-b-PBD nanostructures fabricated with different concentrations and NMR spectrum of the PS-bPBD nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

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for Table of Contents use only

From Block Copolymer Nanotubes to Nanospheres: Nonsolvent-Induced Morphology Transformation Using Porous Templates Chun-Wei Chang,1 Yi-Hsuan Tu,1 Ke-Hsuan Luo,1 and Jiun-Tai Chen1,2* 1

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

2

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

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


From Block Copolymer Nanotubes to Nanospheres: Nonsolvent

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