Molecular Design for Preparation of Hexagonal-Ordered Porous Films

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Molecular Design for Preparation of Hexagonal-Ordered Porous Films Based on Side-chain Type Liquid-Crystalline Star Polymer Yumiko Naka, Hiromu Takayama, Teruhisa Koyama, Khoa V. Le, and Takeo Sasaki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01104 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Molecular Design for Preparation of HexagonalOrdered Porous Films Based on Side-chain Type Liquid-Crystalline Star Polymer Yumiko Naka,* Hiromu Takayama, Teruhisa Koyama, Khoa Van Le, Takeo Sasaki Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

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ABSTRACT Fabrication of regularly porous films by the breath-figure method has attracted much attention. The simple, low-cost technique uses the condensation of water droplets to produce these structures, but the phenomenon itself is complex, requiring control over many interacting parameters that change throughout the process. Developing a unified understanding for the molecular design of polymers to prepare ordered porous films is challenging, but required for further advancements. In this article, the effects of the chemical structure of polymers in the breath-figure technique were systematically explored using side-chain type liquid-crystalline (LC) star polymers. The formation of porous films was affected by the structure of the polymers. Although the entire film surface of poly(11-[4-(4-cyanobiphenyl)oxy]undecyl methacrylate) (P11CB) had a hexagonal ordered porous structure over a certain Mn value, regularly arranged holes did not easily form in poly(methyl methacrylate) (PMMA), even though the main chain of PMMA is similar to that of P11CB. Comparing P11CB and poly(11-[(1,1'-biphenyl)-4yloxy]undecyl methacrylate) (P11B) (P11CB without cyano groups) showed that the local polar groups in hydrophobic polymers promoted the formation of ordered porous films. No holes formed in poly(4-cyanobiphenyl methacrylate) (P0CB) (P11CB without alkyl spacers) films due to its hydrophilicity. The introduction of alkyl chains in P0CB allowed the preparation of honeycomb-structured films by increasing the internal tension. However, alkyl chains in the side chain alone did not result in a porous structure, as in the case of poly(undecyl methacrylate) (P11). Aromatic rings are also required to increase the Tg and improve film formability. In the present study, suitable molecular designs of polymers were found, specifically hydrophobic polymers with local polar groups, to form a regularly porous structure. Development of clear


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guidelines for the molecular design of polymers is the subject of our current research, which will enable the fabrication of porous films using various functional polymers. KEYWORDS. Liquid-crystalline polymer, well-defined star polymer, breath-figure method, porous film


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INTRODUCTION Highly regular porous films produced by a solution casting process—the so-called breathfigure method—has fascinated many researchers because of their applications in a wide variety of fields such as functional materials with superhydrophobic surfaces,1 optical materials,2 lithography templates,3 and cell scaffolds.4-5 The technique is simple and low-cost, using the condensation of water droplets to easily fabricate an ordered porous structure. However, the mechanism is unexpectedly complicated because the conditions during fabrication are constantly changing. A simple mechanism can be derived based on previous studies.6-9 In this method, water droplets are formed at the surface of a polymer solution that has been cooled under high humidity, are aligned by the Marangoni effect and/or capillary forces between water droplets, and are immobilized by the condensation of the polymer solution. Finally, evaporation of water results in the formation of a porous structure at the film surface. To obtain a regularly porous structure by this method, stabilized water droplets of equal size must be formed on the surface of the polymer solution.8, 10 The precipitation of polymers around the water droplets prevents them from combining with one another, leading to hexagonal porous films. Therefore, the molecular design of polymers is crucial to the formation of highly ordered porous films. Previous work has involved the fabrication of regularly porous films from various polymers, such as rod-coil block copolymers,6, 11 branching polymers12-13, amphiphilic block copolymers1416

and amphiphilic random polymers.5 However, not all polymers form an ordered structure

when using the breath-figure method. In terms of polymer architecture, it has been previously reported that branching polymers easily form regularly porous films compared with linear polymers.6, 12 This is due to the rapid deposition of branching polymers at the interface between water and the polymer solution.10 It has also been reported that the pore size could be controlled


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by the molecular weight of well-defined star polymers with multi-arm chains.13 In terms of the chemical structures of polymers, polymers containing styrene moieties have worked well for the fabrication of porous films.2, 4, 6, 8, 12-19 Control of the porous morphology has also been reported by changing the chemical structures at the end of dendrimers.19 As mentioned above, the design of polymers plays a vital role in the formation of porous films using the breath-figure method. However, it is difficult to develop a unified set of guidelines for designing polymers to form regular porous films, despite many reports. The conditions of film preparation, such as the type of solvent and substrate used,17 polymer concentrations, velocity of airflow across the surface,18 and humidity, directly affect the regular pattern of the porous structure, and are not unified between reports. Therefore, the guidelines for molecular design are not sufficient and new guidelines should be developed. More knowledge with respect to molecular design would enable production of regularly porous films using many different kinds of polymer, including polymers for which porous structure formation is typically challenging, polymers with functional moieties, etc. If regularly porous films can be fabricated using any kind of polymer, the application field will expand more than ever. In our search of molecular designs for the preparation of hexagonal-ordered porous films, we put particular emphasis on liquid-crystalline (LC) polymers with mesogens in the side chain because side-chain LC polymers often exhibit self-organization, similar to low-molecular weight LC compounds.20 The branching number and molecular weight of LC polymers can also be controlled by living radical polymerization, which can minimize the effects of polymer architecture on the preparation of porous films by the breath-figure method. In the present study, the ability to form porous films using well-defined LC polymers with mesogens in the side chain was investigated by systematically changing the chemical structure of the polymers. The


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research objective of this paper is to offer effective and versatile molecular designs of polymers for the preparation of ordered porous films.

EXPERIMENTAL Synthesis of polymers 11-[4-(4-Cyanobiphenyl)oxy]undecyl methacrylate (11CB), 11-[(1,1'-biphenyl)-4-yloxy]undecyl methacrylate (11B), 4-cyanobiphenyl methacrylate (0CB), and undecyl methacrylate (11) were first synthesized as monomers using methods reported elsewhere.21 Commercially available methyl methacrylate (MMA, ≥99.0%, Kanto Chemical Co., Inc.) was washed with an aqueous solution of sodium sulfite (pH 4), sodium hydroxide (5%), and sodium chloride (5%) to remove the stabilizer (hydroquinone). The resulting MMA was then stirred over anhydrous magnesium sulfate overnight and distilled under vacuum. Compounds with 3- or 6-initiation sites for atomtransfer radical polymerization (ATRP) were purchased from Aldrich and used without purification. Linear polymers, 3-arm, and 6-arm star polymers were prepared by ATRP. The synthesized homopolymers, PMMA, P11CB, P11B, P0CB, and P11, were composed of MMA, 11CB, 11B, 0CB, and 11, respectively. 1:1 random copolymers (P(0CB-11)) comprising 0CB and 11 were prepared to investigate effects of alkyl chains in the polymers. In ATRP, the initiator, the monomer, Cu(I)Br, 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), and solvent were mixed in an ampule equipped with a reflux condenser, which was degassed and filled with nitrogen. Dried tetrahydrofuran (THF) was used as a solvent in all cases except for the polymerization of MMA. The mixture was stirred while in an oil bath preheated to 80 °C. The solution was passed through a column (silica gel 60N, spherical neutral, particle size 63-210 µm,


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Kanto Chemical Co., Inc.) using chloroform as an eluent to remove the catalyst after polymerization. The obtained polymers in chloroform were precipitated in a large excess of methanol and finally dried under vacuum. A white solid product was obtained. The numbers in the prefix, 3-, 4-, or 6-, indicate the branching number of star polymers, while the word, linear-, indicates that the polymer is linear. Film preparation by breath-figure method Polymer films were prepared on washed glass plates by dropping the polymers dissolved in chloroform (50 mg/0.5 mL) in a pre-humidified glove bag at >90 %RH and room temperature. The concentration of the polymer solution was set to 100 g/L and a volume of 20–50 µL of polymer solution was used. A static field was maintained by detaching the humidifier from the glove bag when the films were being prepared. The size of the glove bag was about 110 cm3. Characterization The monomers were characterized by 1H-NMR spectroscopy and elemental analysis (Perkin Elmer 2400 Series II CHNS/O). NMR spectra were recorded in CDCl3 using the JNM-AL500 spectrometer (JEOL Ltd.). The molecular weight of the polymer was determined by gel permeation chromatography (GPC; TOSOH HLC-8220GPC; column, Super Multipore HZ-M; eluent, THF) calibrated with standard polystyrenes. The thermal properties of the polymers were determined with a differential scanning calorimeter (DSC; Perkin Elmer DSC4000) at a scanning rate of 10 °C/min. At least three scans were performed for each sample to check the reproducibility. Liquid crystalline (LC) properties were investigated using a polarizing optical microscope (Olympus, BX53LED) with a hot stage (Mettler FP82, FP80HT). The surface of the obtained film was observed by scanning electron microscopy (SEM; Hitachi Miniscope®


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TM3030). Surface and interface tensions were measured by the plate method using a surface tensiometer K100 (KRUSS). RESULTS AND DISCUSSION To investigate the effects of polymer structures for the preparation of porous films by the breath-figure method, several linear and star polymers with different molecular weights and branching numbers, as well as star polymers with different chemical structures, were synthesized by ATRP (Figure 1). Well-defined polymers showing a unimodal peak with narrow Mw/Mn by GPC were obtained. To diminish the effects of the film-preparation conditions, the films were fabricated in controlled conditions using the glove bag. The initial polymer concentration was set to 100 g/L, which was decided based on the results of pore formation at different concentrations, as shown in Figure SI-1. Using the obtained star and linear homopolymers made of 11CB, the effects of molecular weight and branching number on pore formation were investigated. A point where many holes were formed in the films was selected and observed by SEM. As shown in Figure 2, holes of various sizes were formed, and no regularly porous structure was observed over the entire surface in low-molecular-weight linear-P11CB, 3-P11CB, and 6-P11CB (Mn < 15,000). In contrast, high-molecular-weight linear-P11CB, 3-P11CB, and 6-P11CB (Mn ≥ 25,000) formed hexagonally ordered arrays of spherical holes at the film surface. The holes, of course, were almost of the same size. In particular, hexagonally ordered holes formed over the entire surface of the 6-P11CB films with Mn of 26,000, 28,000, and 38,000 (Figure 3). The honeycomb structure was also observed in some areas of 3-P11CB films (Mn = 17,000, 22,000, 24,000) and linear-P11CB (Mn = 36,000). The increase in both the molecular weight and branching number significantly influenced the formation of regularly porous films using P11CB.


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Figure 1


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Figure 2


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Figure 3


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The influence of the chemical structures on the preparation of porous films was also explored. Linear-PMMA, 3-PMMA, 4-PMMA, and 6-PMMA with Mn = 18,000 ∼ 52,000 were synthesized (Table 1). The chemical structure of PMMA is similar to that of the main chain of P11CB. The SEM images of three parts of the film were obtained: central part of the film (③), edge of the film (①), and intermediate position between the central and the edge part (②). Large holes were formed over the entire film made using linear-PMMA, while the numbers of the large holes tended to decrease with increasing branching number, as observed for 3-arm, 4-arm, and 6-arm star PMMA films (Figure 4). In 6-PMMA films, holes of the same size were closely arranged. These results implied that the combining of water droplets was inhibited in the process of pore formation in films made using PMMA with a large branching number. 6-PMMA was more able to precipitate around the water droplets in comparison with linear-PMMA, 3-PMMA, and 4-PMMA. However, 6-PMMA films did not have a honeycomb structure, in contrast to the 6-P11CB films. This is likely due to deformation of water droplets by the Marangoni effect and capillary forces between the water droplets during film formation when 6-PMMA is used. The condensed PMMA layer that precipitates around the water droplets may be softer than that formed by P11CB.


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Figure 4


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It can be surmised from the differences in the porous structures between 6-PMMA and 6P11CB that the side chain of P11CB played an important role in the formation of honeycomb structures using the breath-figure method. First, the formation of the porous structure using biphenyl-containing polymers without cyano groups (6-P11B) was investigated. Spherical holes formed at the film surface, but were not regularly ordered (Figure 5(b)). The size of the holes also varied widely. Compared with 6-P11CB with Mn of 26,000, 6-P11B with Mn of 24,000 had a poor ability to form pores. The surface tensions of the 6-P11B and 6-P11CB solutions (γair/6P11Bsol,

γair/6-P11CBsol) and the internal tension between water and the dilute polymer solution (1

g/L) of 6-P11B and 6-P11CB with Mn ≥ 20,000 (γw/6-P11Bsol, γw/6-P11CBsol) were measured by the plate method. The surface tensions, γair/6-P11Bsol and γair/6-P11CBsol, were almost the same (about 26 mN/cm2). The internal tensions, γw/6-P11Bsol and γw/6-P11CBsol, were about 3–5 mN/cm2 and 10–20 mN/cm2, respectively. Although γw/6-P11CBsol was higher than the internal tension between water and pure chloroform (γw/solv), γw/6-P11Bsol. was only slightly larger than γw/solv. These results suggest that polar groups, such as cyano groups, promoted the aggregation of polymers around water droplets, and the interaction between polymer-stabilized water droplets encouraged the formation of honeycomb structures. Next, the ability to form pores was investigated in cyanobiphenyl-containing polymers without alkyl spacers (P0CB). The 3-arm and 6-arm star polymers, 3-P0CB and 6-P0CB, were synthesized by ATRP, and porous films were fabricated using the same conditions as mentioned above (concentration of polymer solution, 100 g/L; drop volume, 20 µL; 90 %RH, room temperature). The SEM images of 6-P0CB with Mn of 20,000 are shown in Figure 5(c). The surface of the 6-P0CB films did not have entirely spherical holes. To understand the poreformation behavior, 6-P0CB and 3-P0CB films were prepared under different conditions; the


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concentration of the polymer solution was 1, 5, 10, or 100 g/L, and the drop volume was 10, 20, or 50 µL per film. Spherical holes did not form at all on the surface of the films (Figure SI-2). When water was added to the 6-P0CB solution and the solution was stirred, the solution became cloudy without any phase separation. The internal tension between water and the 6-P0CB solution (γw/6-P0CBsol) was drastically lower than γw/6-P11CBsol. The value of γw/6-P0CBsol was almost equal to the value of γw/solv. The 6-P0CB did not precipitate around water droplets, as predicted from these results and the polar 6-P0CB film could partially dissolve in water. Since no water droplets were present at the surface of the 6-P0CB solution, they could not become large through the vapor-liquid equilibrium, so spherical holes using water droplets as templates were not formed. The thermotropic properties of 6-P11CB and 6-P0CB in bulk also differed: 6-P11CB showed an LC phase, but 6-P0CB did not (Figure SI-3). It was found that the ability to form pores depends on the solution properties at the early stage of film preparation rather than on the self-organization nature of molecules in bulk. The structural difference between 6-P0CB and 6P11CB is the presence of an alkyl spacer. It is not clear whether the alkyl spacer, located between the cyanobiphenyl moieties and the polymer chain, is involved in stabilizing the water droplets. To confirm the effect of the alkyl chain on the formation of porous films, 6-arm startype random copolymers of 0CB and 11 at a 1:1 ratio (6-P(0CB-11)) were designed and synthesized. 6-P(0CB-11) formed a regularly porous structure on a relatively broad area of the films, as shown in Figure 5(d). The internal tension of 6-P(0CB-11) (γw/6-P(0CB-11)sol) in dilute solution was the same as γw/6-P11CBsol. These results show that the introduction of an alkyl chain in P0CB increases the internal tension between water and the solution regardless of the position of the alkyl chains in the polymers, resulting in stable water droplets in the solution. However, pores were not present for films of 6-P11, despite the polymer possessing alkyl chains (Figure


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5(e)). This is because the glass transition temperature of 6-P11 was below room temperature, so a flat surface forms while drying, even if stable water droplets as templates are induced in the polymer solution. These results show that the desired chemical structure for the preparation of regularly porous films is not satisfied only by the introduction of long alkyl chains.

Figure 5


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Figure 6 sums up the relationship between the chemical structure of the polymers and porous structures. These results demonstrated how each part of P11CB plays a role in the formation of porous structures using the breath-figure method. The polar cyano groups encouraged the condensation of the hydrophobic polymer around the water droplets, the biphenyl moieties improved the formability of films at room temperature, and the alkyl chains increased the interface tension between water and the polymer solution and changed hydrophilic polymers to hydrophobic polymers. The introduction of alkyl chains using monomer 11 is undesirable for pore formation since the Tg decreases.

Figure 6

CONCLUSIONS In this article, we demonstrated the influence of both polymer architecture and chemical structure on porous-film preparation by the breath-figure method. With respect to polymer architecture, it was found that the greater both the molecular weight and branching number of


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star polymers, the easier the preparation of regularly porous films by promoting the deposition of polymers around water droplets. The chemical structure of the polymers substantially impacted the ability to form porous films. Hydrophobic polymers without polar groups, like 6-P11B, formed pores of various sizes due to a limited ability to deposit around water droplets. Polymers without an aromatic ring had lower Tgs, which prevented a porous structure from forming due to the spontaneous collapse of holes. Hydrophilic polymers with a large number of polar cyano groups, like 6-P0CB, did not form porous films by utilizing water droplets as a template because the polymer slightly dissolved in water, preventing stabilization of the interface between water and chloroform by the condensation of the polymer. The introduction of alkyl chains to the hydrophilic polymer could lead to the stabilization of water droplets in the polymer solution. The above results have revealed that hydrophobic polymers with local polar groups are capable of forming regularly ordered porous arrays by the breath-figure method. Specifically, 6-P11CBs, the hydrophobic polymers with local polar groups, exhibited a honeycomb structure over the entire film surface. The present study demonstrates some suitable molecular designs of polymers that can form a regularly porous structure. Development of clear guidelines for the molecular design of polymers is the subject of our current research, which will enable the fabrication of porous films using various functional polymers.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of films prepared 6-P11CB, 6-P0CB, and 3-P0CB solutions at different concentrations, and thermal properties of 6-P11CB, 6-P11B, 6-P0CB, and 6-P11. (PDF)

AUTHOR INFORMATION Corresponding Author * Yumiko NAKA Department of Chemistry, Faculty of Science, Tokyo University of Science 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan E-mail: [email protected] Author Contributions Y. Naka conceived this research, supervised, and coordinated the project. H. Takayama, and T. Koyama carried out synthesis of monomers and polymers, film preparation, polymer characterization and SEM measurement. Khoa V. Le. and T. Sasaki provided beneficial advice regarding the research.


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REFERENCES (1) Yabu, H.; Shimomura, M. Single-Step Fabrication of Transparent Superhydrophobic Porous Polymer Films. Chem. Mater. 2005, 17, 5231-5234. (2) Yabu, H.; Shimomura, M. Simple Fabrication of Micro Lens Arrays. Langmuir 2005, 21, 1709-1711. (3) Connal, L. A.; Qiao, G. G. Preparation of Porous Poly(Dimethylsiloxane)-Based Honeycomb Materials with Hierarchal Surface Features and Their Use as SoftLithography Templates. Adv. Mater. 2006, 18, 3024-3028. (4) Martínez-Campos, E.; Elzein, T.; Bejjani, A.; García-Granda, M. J.; Santos-Coquillat, A.; Ramos, V.; Muñoz-Bonilla, A.; Rodríguez-Hernández, J. Toward Cell Selective Surfaces: Cell Adhesion and Proliferation on Breath Figures with Antifouling Surface Chemistry. ACS Applied Materials & Interfaces 2016, 8, 6344-6353. (5) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.-I.; Wada, S.; Karino, T.; Shimomura, M. Honeycomb-Patterned Thin Films of Amphiphilic Polymers as Cell Culture Substrates. Materials Science and Engineering: C 1999, 8-9, 495-500. (6) Widawski, G.; Rawiso, M.; Francois, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystyrene Films. Nature 1994, 369, 387-389. (7) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Mesoscopic Patterns of Molecular Aggregates on Solid Substrates. Thin Solid Films 1998, 327–329, 854-856. (8) Pitois, O.; Francois, B. Formation of Ordered Micro-Porous Membranes. Eur. Phys. J. B 1999, 8, 225-231. (9) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. Formation of Honeycomb-Structured, Porous Films Via Breath Figures with Different Polymer Architectures. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2363-2375. (10) Pitois, O.; Francois, B. Crystallization of Condensation Droplets on a Liquid Surface. Colloid Polym. Sci. 1999, 277, 574-578. (11) Jenekhe, S. A.; Chen, X. L. Self-Assembly of Ordered Microporous Materials from Rod-Coil Block Copolymers. Science 1999, 283, 372-375. (12) François, B.; Ederlé, Y.; Mathis, C. Honeycomb Membranes Made from C60(Ps)6. Synth. Met. 1999, 103, 2362-2363. (13) Stenzel-Rosenbaum, M. H.; Davis, T. P.; Fane, A. G.; Chen, V. Porous Polymer Films and Honeycomb Structures Made by the Self-Organization of Well-Defined Macromolecular Structures Created by Living Radical Polymerization Techniques. Angew. Chem., Int. Ed. 2001, 40, 3428-3432. (14) de León, A. S.; del Campo, A.; Fernández-García, M.; Rodríguez-Hernández, J.; Muñoz-Bonilla, A. Hierarchically Structured Multifunctional Porous Interfaces through Water Templated Self-Assembly of Ternary Systems. Langmuir 2012, 28, 9778-9787. (15) Hayakawa, T.; Horiuchi, S. From Angstroms to Micrometers: Self-Organized Hierarchical Structure within a Polymer Film. Angew. Chem., Int. Ed. 2003, 42, 22852289. (16) Escale, P.; Save, M.; Lapp, A.; Rubatat, L.; Billon, L. Hierarchical Structures Based on Self-Assembled Diblock Copolymers within Honeycomb Micro-Structured Porous Films. Soft Matter 2010, 6, 3202-3210.


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(17) Ferrari, E.; Fabbri, P.; Pilati, F. Solvent and Substrate Contributions to the Formation of Breath Figure Patterns in Polystyrene Films. Langmuir 2011, 27, 1874-1881. (18) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Three-Dimensionally Ordered Array of Air Bubbles in a Polymer Film. Science 2001, 292, 79-82. (19) Connal, L. A.; Vestberg, R.; Hawker, C. J.; Qiao, G. G. Cover Picture: Dramatic Morphology Control in the Fabrication of Porous Polymer Films. Adv. Funct. Mater. 2008, 18, 3706-3714. (20) Ikeda, T. Photomodulation of Liquid Crystal Orientations for Photonic Applications. J. Mater. Chem. 2003, 13, 2037-2057. (21) Naka, Y.; Kawamura, H.; Sasaki, T. Synthesis of Liquid-Crystalline Star Polymers with Sulfonyl Groups in the Central Core and Selective Degradation of Their Cores by Base. Mol. Cryst. Liq. Cryst. 2014, 593, 141-150.


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Figure 1. Chemical structures and abbreviations of polymers synthesized in this study Figure 2. SEM images of films prepared from cyanobiphenyl-containing polymers (P11CB) in chloroform at a relative humidity of 90%. Concentration of the polymer solutions set to 100 g/L. Drop volume: 20 µL. Figure 3. Polarizing optical micrograph of porous films prepared from 6-P11CB (Mn = 26,000) in chloroform at a relative humidity of 90%. Concentration of the polymer solutions set to 100 g/L. Figure 4. SEM images of PMMA films prepared from the chloroform solution at a relative humidity of 90%. Concentration of the polymer solutions set to 100 g/L. Drop volume: 20 µL. (a) Linear-PMMA (Mn = 25,000), (b) 3-PMMA (Mn = 22,000), (c) 4-PMMA (Mn = 23,000), and (d) 6-PMMA (Mn = 22,000). Each number (①-③) in SEM images indicates different observation points on the sample films. Figure 5. SEM images of porous films prepared from (a) 6-P11CB (Mn = 26,000), (b) 6-P11B (Mn = 24,000), (c) 6-P0CB (Mn = 20,000), (d) 6-P(0CB-11) (Mn = 22,000), and (e) 6-P11 (Mn = 20,000) in chloroform (100 g/L) at a relative humidity of 90%. Each number (①-③) in SEM images indicates different observation points on sample films. Figure 6. Influence of the chemical structure of polymers on pore structure


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Table 1. Molecular weights of linear and star polymers synthesized in this study, and their poreformation behavior Conv. (a) (%)

Porous structure (b)

Sample

Mn(a)

Mw/Mn(a)

Linear-PMMA

25,000

1.29

×

3-PMMA

22,000

1.11

×

3-PMMA

52,000

1.56

×

4-PMMA

18,000

1.17

×

4-PMMA

36,000

1.15

×

4-PMMA

23,000

1.09

×

6-PMMA

22,000

1.24

△

Linear-P11CB

9,000

1.11

11

×

Linear-P11CB

13,000

1.08

38

×

Linear-P11CB

36,000

1.21

86

〇

3-P11CB

11,000

1.08

×

3-P11CB

17,000

1.10

〇

3-P11CB

22,000

1.14

53

〇

3-P11CB

24,000

1.12

90

〇

6-P11CB

15,000

1.06

47

×

6-P11CB

19,000

1.10

50

×

6-P11CB

26,000

1.20

90

◎

6-P11CB

28,000

1.14

96

◎

6-P11CB

38,000

1.15

83

◎

6-P11B

20,000

1.10

61

×

6-P11B

24,000

1.10

48

×

3-P0CB

12,000

1.17

6-P0CB

20,000

1.14

37

–

6-P(0CB-11)

22,000

1.19

(54)

◎

6-P(0CB-11)

31,000

1.05

◎

6-P11

13,000

1.26

–

6-P11

20,000

1.04

–

–

(a) Number average of molecular weight (Mn) determined by GPC.


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(b) –, Non-porous films; ×, scattered holes of different sizes; △, closely arranged holes of similar sizes; 〇, hexagonally ordered arrays of spherical holes in part of film; ◎, honeycomb structure over whole surface of film. (c) Concentration of dissolved polymer in chloroform is 1 g/L. Measured at room temperature.


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SYNOPSIS


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Molecular Design for Preparation of Hexagonal-Ordered Porous Films

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