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Hierarchical Polymer Structures Using Templates and the Modified Breath Figure Method Lin-Ruei Lee, Chih-Ting Liu, Hsiao-Fan Tseng, Kuan-Ting Lin, Chien-Wei Chu, and Jiun-Tai Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01381 • Publication Date (Web): 27 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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Hierarchical Polymer Structures Using Templates and the Modified Breath Figure Method Lin-Ruei Lee, 1 Chih-Ting Liu, 1 Hsiao-Fan Tseng, 1 Kuan-Ting Lin, 1 Chien-Wei Chu, 1and 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]. Tel.: +886-3-5731631

ABSTRACT Hierarchical structures are commonly observed in nature and possess unique properties. The fabrication of hierarchical structures with well-controlled sizes in different length scales, however, is still a great challenge. To further understand the morphologies and properties of hierarchical structures, here we present a novel strategy to prepare hierarchical polymer structures by combining the modified breath figure method and the template method. Poly(methyl methacrylate) (PMMA) honeycomb films with regular micropores are first prepared using the modified breath figure method by dipping PMMA films into mixtures of chloroform and methanol. The polymer chains on the honeycomb films are then annealed and wetted into the nanopores of anodic aluminum oxide (AAO)

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templates via capillary forces, resulting in the formation of hierarchical polymer structures. The morphologies of the polymer structures, which can be controlled by the molecular weights of the polymers and the concentrations of the polymer solutions, are characterized by scanning electron microscopy (SEM). The surface wettabilities of the polymer structures are also examined by water contact angle measurements, and the hierarchical structures are observed to be more hydrophobic than the flat films and honeycomb films. This work not only provides a feasible approach to fabricate hierarchical polymer structures with controlled sizes but also gives a better understanding in the relationship between surface morphologies and properties.

Keywords: anodic aluminum oxide, breath figure method, hierarchical structures, templates, wettabilities

INTRODUCTION Hierarchical structures with multiple length scales have received tremendous attention because of their large surface areas and unique surface properties. In nature, there are numerous hierarchical surface structures, such as geckos’ feet and lotus leaves.1 Hierarchical structures have be applied in different fields, such as sensing, catalysis, filtration, and self-cleaning coating.2-5 For example, hierarchical structures have been used as the substrates for cell growth and or for catalytic activities.3, 6

Although many studies have been reported on the fabrications, the characterizations, and the


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applications of hierarchical structures, it is still a great challenge to prepare hierarchical structures with well-controlled sizes in different length scales. Here, we present a novel strategy to fabricate hierarchical polymer structures by combing the modified breath figure method and the template method. For the breath figure method, it has been widely investigated in making honeycomb polymer films with regular pores.7 In 1895, the formation of breath figures was preliminarily examined by John Aitken.8-9 Later, the patterns of water droplets on cold surfaces were unveiled by Lord Rayleigh in 1911.10 In 1994, Francois et al. used polystyrene-b-polyparaphenylene (PS-b-PPP) dissolved in carbon disulfide via a flow of moist air to prepare porous films with honeycomb patterns.11 Compared to the other methods to prepare ordered porous films, this simple process, which has been named the breath figure method, has many advantages, such as low cost, time saving, and easy operation. Over the past few decades, the breath figure method has been used to produce porous films with hexagonally packed pores from different organic and inorganic materials.12-14 In the conventional breath figure method, polymer solutions are dropped on solid surfaces in humid environment. After the successive evaporation of the solvent and the condensed water droplets, porous honeycomb films can be obtained. The processes of the conventional breath figure method can be divided into dynamic and static processes, depending on how the humid environment is obtained.11, 15-16 The sizes of the pores on the honeycomb films can be controlled by different factors, such as the temperature, the type of substrates, the humidity, the rate of air flow, the molecular weight of polymers, or the concentration of polymer solutions.17-18 In the


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past, many polymer materials, such as polystyrene, poly(methyl methacrylate), poly(vinyl alcohol), and polysulfone, have been used to generate honeycomb films by using the breath figure method.17-22 Polymer materials with specific structures, such as amphiphilic polymers, comb-like polymers, or block copolymers, are suitable for the breath figure method because they can easily aggregate and cause the phase separation at the rims of water droplets by self-assembly, preventing the water droplets from collapsing. Recently, Choi et al. developed a modified breath figure method by dip-coating polymer films in the mixtures of chloroform and methanol, by which honeycomb polymer films with highly ordered pores can be obtained.17, 21-22 The morphologies and surface properties of honeycomb films prepared by the breath figure method have been widely studied.23-29 Zhang et al. have also used the breath figure method to generate hierarchical porous structures.30 For the template method, it has been widely used to fabricate one-dimensional polymer nanomaterials. After polymer chains are introduced into the nanopores of porous templates, the templates can be selectively removed to release the polymer nanomaterials. Using porous templates, such as anodic aluminum oxide (AAO) membranes, the polymer chains can be introduced into the nanopores of the templates by different methods, such as the solution wetting method,31 the melt wetting method,32-33s the solvent vapor annealing method,34 and the microwave annealing method.35 In this work, we combine the modified breath figure method, developed by Choi et al.,17 and the template method to fabricate hierarchical polymer structures. Poly(methyl methacrylate) (PMMA), a commonly used polymer with well-known properties, is used as a model material to demonstrate this


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concept. Using the modified breath figure method, the honeycomb PMMA films are first prepared. Nanorods are then formed on the honeycomb PMMA films by the template method, resulting in hierarchical polymer structures. The sizes of the micropores on the honeycomb films can be controlled by the molecular weights of PMMA and the concentrations of the PMMA solutions; the sizes of the nanorods can be controlled by the pore sizes of the templates. Using water contact angle measurements, it is also found that more hydrophobic surfaces can be obtained from the hierarchical polymer structures than the flat films and honeycomb films.

RESULTS AND DISCUSSION


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Figure 1. Schematic illustration of the experimental processes to fabricate hierarchical polymer structures.

The schematic illustration of the experimental processes to prepare the hierarchical polymer nanostructures is shown in Figure 1. First, the PMMA films coated on glass substrates are prepared by spin-coating, followed by a drying process at 150 °C for 2 h using an oven to remove the residual solvents and to reduce the roughness of the films.17 The PMMA films are then dipped into a mixture of 85 vol. % chloroform and 15 vol. % methanol for 5 s. After the samples are taken out and the solvents are evaporated, honeycomb PMMA films with regular micropores can be obtained.17 Here, chloroform is a volatile solvent that can remove the heat from the surface of the films rapidly, causing the decrease of the temperatures and the condensation of the water vapor in the air on the


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films. Moreover, methanol can form gel-like protective layers between water and the polymer phases, which improve the stability of the water droplets and prevent the collapsing of the water droplets, enhancing the regularity of the micropores on the honeycomb films.17 The AAO templates are then placed on the honeycomb films, and the samples are annealed at 150 °C for 30 min using an oven. In this step, a weight of 5 g weight is placed on the top of the sample to ensure a good contact between the AAO templates and the polymer films. During the thermal annealing process at temperatures above the glass transition temperatures of the polymers (Tg of the PMMA: 105 °C), the polymer chains wet the nanopores of the AAO templates via capillary force33, 36. As a result, polymer nanorods in the nanopores of AAO templates are formed on the honeycomb films. After the AAO templates are removed selectively using 5 wt % NaOH(aq), the hierarchical polymer structures can be obtained.


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Figure 2. Graphical illustrations (a) and SEM images of AAO templates: (b) top-view and (c) side-view images.

The AAO templates are used as scaffolds for the infiltration of polymer chains, which can be selectively removed by NaOH(aq) after generation of the polymer nanostructures. The AAO templates used in this work are prepared by the two-step anodization method.37-39 The pore sizes of the AAO templates can be controlled by changing the anodization conditions, such as the oxidation voltage, the type of the electrolytes, or the concentration of the electrolytes. The pore sizes of the AAO templates can also be further controlled via a pore-widening process by using phosphoric acid at 30 °C. The surface morphologies of the prepared AAO templates are shown in Figure 2. Hexagonally arranged pores with an average diameter of 40~50 nm are shown on the surface of the templates. From the side-view image (Figure 2b), well-ordered channels can be observed. It should be noted that the top and the bottom sides of the AAO templates are different. The top side of AAO templates possess open nanopores, which is used for the fabrication process of the PMMA nanorods; the bottom side of AAO templates possess closed nanopores. The side-view SEM images of the AAO templates showing the top side (open nanopores) and the bottom side (closed nanopores) are shown in Figure S1.


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Figure 3. (a–d) SEM images of honeycomb PMMA films using the breath figure method from different molecular weights and concentrations: (a) Mw = 350 kg mol-1, concentration = 10 wt %, (b) Mw = 350 kg mol-1, concentration = 15 wt %, (c) Mw = 996 kg mol-1, concentration = 10 wt % and (d) Mw = 996 kg mol-1, concentration = 15 wt %. (e–h) Corresponding size distributions of the pores of the porous films shown in (a–d).

The polymer honeycomb films, as shown in Figure 3, are prepared by the modified breath figure method. PMMA with two different molecular weights, Mw: 350 and 996 kg mol-1, are used. From the SEM images, we can see that honeycomb films with hexagonally packed pores can be obtained, of which the sizes are controlled by the molecular weights of the polymers and the concentrations of the polymer solutions. At larger molecular weights of PMMA and lower concentrations of the PMMA solutions, honeycomb films with the larger pore sizes can be obtained. This observation can be explained by considering the vapor pressure and the rate of evaporation of the polymer solutions. The relationship between the vapor pressure and the mole fraction of a polymer solution follows the Henry’s law:18


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(1)

where P is the vapor pressure of the polymer solution, P0 is the vapor pressure of the pure solvent, and XB is the mole fraction of the solute in the solution. When the concentration of the PMMA solution is higher, the higher mole fraction of PMMA (solute) causes the lower vapor pressure and the lower rate of evaporation of the PMMA solution. Therefore, less thermal energies are lost, resulting in less amounts of water droplets condensing on the PMMA films and the smaller pores sizes. On the other hand, as the molecular weight of PMMA decreases, the mole fraction of PMMA solutions increases, also causing PMMA films with smaller pore sizes.18

Figure 4. Graphical illustration of the formation of the induced gel-like layers by the aggregation of methanol.

In the modified breath figure method to prepare the honeycomb films, the methanol in the mixture of the chloroform and methanol plays a critical role.17 During the evaporation of the solvent (chloroform), the water droplets condense on the polymer films, during which the methanol molecules aggregate at the interfacial regions between the polymer materials and water droplets,


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forming the induced gel-like layers,17 as shown in Figure 4. Such gel-like layers can prevent the collapsing of the water droplets, enhancing the regularity of the micropores. Because the polymer materials do not dissolve in methanol and water droplets, there is no mass transfer from solvent (chloroform) to water droplets or methanol. Besides, the added methanol can also increase the lateral capillary force of adjacent water droplets, which also enhance the regularity of the nanopores.17

Figure 5. (a) Graphical illustration of hierarchical polymer structures. (b–e) SEM images of hierarchical polymer structures from different molecular weights and concentrations: (b) Mw = 350 kg mol-1, concentration = 10 wt %, (c) Mw = 350 kg mol-1, concentration = 15 wt %, (d) Mw = 996 kg mol-1, concentration = 10 wt %, and (e) Mw = 996 kg mol-1, concentration = 15 wt %.

After the fabrication of the honeycomb PMMA films, the PMMA nanorods on the films are formed by using the melt wetting method.33 In the melt wetting method, the polymer samples are heated above the glass transition temperatures or the melting temperatures. The surface energy of the


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polymer melts is much lower than that of the AAO templates, so the polymer melts wet the nanopores of the porous templates to reduce the surface energy of the templates via capillary force. Zhang et al. investigated the morphologies of polystyrene (PS) melts in cylindrical nanopores at different annealing temperatures.33 A transition from partial to complete wetting was reported when the polymer melts are heated above a critical temperature (Tw). Partial wetting of the polymer melts occurs at temperatures lower than the Tw and polymer nanorods can be obtained. Complete wetting of the polymer melts occurs at temperatures higher than the Tw and polymer nanotubes can be obtained.33 Here, the PMMA melts are heated at 150 °C and PMMA nanorods are obtained, indicating that the PMMA melts wet the nanopores in the partial wetting regime. By removing the AAO templates selectively using aqueous NaOH, hierarchical PMMA structures can be obtained. Figure 5 shows the SEM images of the hierarchical PMMA structures using PMMA with different molecular weights (Mw: 350 and 996 kg mol-1) with two different concentrations (15 and 10 wt %). From the SEM images, hierarchical PMMA structures with hexagonally arranged nanorods on the inter-pore regions can be observed. The sizes of the micropores of the hierarchical structures are controlled by the breath figure method; the sizes and the packing of the nanorods are controlled by the nanopores of the AAO templates. The samples shown in Figure 5 are from samples annealed for 30 min, in which standing nanorods are formed. For longer annealing times, nanorods with greater lengths can be generated and collapsed nanorods may be observed after the selective removal process of the AAO templates.


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Figure 6. Water contact angles on different samples: (a) glass substrate, (b) PMMA film, (c) honeycomb PMMA film, and (d) hierarchical PMMA structures. A 10 wt % PMMA (Mw = 350 kg mol-1) is used to prepare the honeycomb PMMA films and the hierarchical structures.

To further understand the surface properties of the hierarchical structures,40 we also conduct the water contact angle measurements of the samples. Figure 6 shows the data of water contact angle measurements on the glass substrate, PMMA film, honeycomb PMMA film, and hierarchical PMMA structures. The water contact angle of the glass substrate is 46.65°, showing the more hydrophilic nature of the glass substrate. The water contact angle of the PMMA flat film increases to 72.27°, attributed to the ester group on the polymer structures. For the honeycomb PMMA films, the water contact angle increases to 127.25° because of the reduction of contact areas between the water droplets and the films. The water contact angle of the hierarchical structures, which contain PMMA nanorods on the honeycomb PMMA films, further goes up to 132.90°. The higher water contact angle of the hierarchical structures indicates that introducing the nanorods onto the microporous PMMA can further enhance the hydrophobicity of the structures. The enhanced hydrophobicity can


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be attributed to the air between the nanorods on the inter-pore regions, which supports the water droplets, and is related to the Cassie-Baxter state.41-42

CONCLUSIONS In this work, we develop a novel strategy to fabricate hierarchical polymer structures by combing the modified breath figure method and the template method. The micropores on the hierarchical polymer structures are controlled by the conditions in the breath figure method, such the molecular weight of polymers or the concentration of the polymer solutions. The sizes of the nanorods on the hierarchical structures are controlled by the pore sizes of the templates. The morphologies and the surface wettabilities of the honeycomb polymer films and hierarchical structures are examined by SEM and water contact angle measurements, respectively. It is observed that the water contact angles of the hierarchical PMMA structures are higher than those of the flat PMMA films and the honeycomb PMMA films. The hierarchical structures prepared using this simple and feasible approach may have potential applications in different fields, such as membranes, liquid reprography, or anti-fouling surfaces.

EXPERIMENTAL SECTION Materials


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Poly(methyl methacrylate) (PMMA, Mw: 350 and 996 kg mol-1) was obtained from Sigma Aldrich. Chloroform (anhydrous, containing amylenes as stabilizer, 99%(w/w)) was purchased from Sigma Aldrich. Methanol (HPLC grade, 99.9%) was purchased from Aencore. Microscope glass slides were purchased form DGS. Aluminum sheet (thickness: 0.2 mm, 99.99%), oxalic acid (98%), and potassium dichromate (99%) were obtained from Alfa Aesar. Ethanol (99.5%) and acetone were purchased from Echo. Isopropyl alcohol (IPA, 99%) was obtained from Mallinckrodt Chemistry. Sodium hydroxide was brought from Macron Fine Chemicals. Perchloric acid (60-62%) was purchased from J.T. Baker.

Preparation of the Homemade AAO Templates The homemade AAO templates were fabricated using the two-step anodization method pioneered by Masuda and co-workers.37-38 First, aluminum sheets were cleaned with isopropyl alcohol, acetone, and DI water for 10 min, respectively. The aluminum sheets were then dipped into 5 wt % NaOH to remove the impurities on the surfaces of the aluminum sheets. Subsequently, the aluminum sheets were electropolished in the mixed solution of 80 vol % perchloric acid and 20 vol % ethanol at 0 °C and 20 V for 90 s. The aluminum sheets were then anodized at 40 V in 0.3 M oxalic acid at 0 °C for 6 h, followed by a chemical etching process by placing the anodized aluminum sheets in a mixed solution of 6 wt % phosphoric acid and 1.8 wt % chromic acid at 60 °C for 3 h. Later, the etched aluminum sheets were anodized again using the same condition as in the first anodization process.


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Next, the pore sizes were enlarged using 5 wt % phosphoric acid at 30 °C for 30 min. The anodized aluminum sheets were then placed on the surface of a 5 wt % NaOH solution for 10 min to remove one side of the aluminum. Afterwards, the unoxidized aluminum was removed by 98 wt % copper chloride, leaving the transparent AAO templates.

Preparation of the Spin-Coated and Honeycomb PMMA Films A PMMA (Mw: 350 or 996 kg mol-1) solution (10 or 15 wt %) in chloroform was first prepared. The PMMA solution was spin-coated on glass substrates at spinning rates of 600 rpm for 60 s and 2000 rpm for 30 s. The spin-coated PMMA films were further annealed at 150 °C for 2 h in an oven to remove the residual solvents and to decrease the roughness of the films. The honeycomb PMMA films were prepared using the breath figure method. The spin-coated PMMA films were dipped into a mixture of 85 vol. % chloroform and 15 vol. % methanol, which had been stirred for 24 h, for 5 s, followed by a drying process, as shown in Figure S2.

Fabrication of Hierarchical Polymer Nanostructures The homemade AAO templates were placed on the honeycomb PMMA films. A weight of 5 g was also placed on top of the samples. The samples were then annealed at 150 °C for 30 min using an oven. The AAO templates were later removed by using a 5 wt % NaOH solution, releasing the hierarchical polymer structures.


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Structure Analysis and Contact Angle Measurements A scanning electron microscope (SEM) (JEOL, JSM-7401F) at an acceleration voltage of 5 kV was used to observe the surface morphologies of the samples. Before the SEM measurements, the samples were dried by a vacuum pump and then coated with platinum. Additionally, a contact angle meter (First Ten Angstroms, FTA125) equipped with a CCD camera was used to measure the water contact angles of the samples.

ASSOCIATED COTNET Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX SEM images of the AAO templates. Graphical illustration of the dipping process of the PMMA films.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. ORCID


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Jiun-Tai Chen: 0000-0002-0662-782X Notes The authors declare no competing financial interest.

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

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for Table of Contents use only Hierarchical Polymer Structures Using Templates and the Modified Breath Figure Method Lin-Ruei Lee, Chih-Ting Liu, Hsiao-Fan Tseng, Kuan-Ting Lin, Chien-Wei Chu, and Jiun-Tai Chen*


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Hierarchical Polymer Structures Using Templates ... - ACS Publications

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