Fabrication of Well-Ordered Mesoporous Polyimide Films by a Soft

Apr 22, 2019 - ku, Tokyo, 152. -. 8552, Japan. b) Nanomaterials Research Institute, National Institute of Advanced Industrial Science and. Technology...
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Article Cite This: ACS Appl. Polym. Mater. 2019, 1, 1209−1219

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Fabrication of Well-Ordered Mesoporous Polyimide Films by a SoftTemplate Method Takahiro Komamura,† Kenta Okuhara,† Shin Horiuchi,‡ Yuta Nabae,† and Teruaki Hayakawa*,†,§

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†

Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-S8-36, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0031, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: A well-ordered mesoporous polyimide film was fabricated by a soft-template method using a microphaseseparated structure of an amphiphilic block copolymer (BCP) as a template. Poly(amic acid)s (PAAs) as polyimide precursors and resol as a cross-linker for the stabilization of the nanostructure were found to be selectively miscible with the hydrophilic domain of the template BCP. Their mixture coassembled and formed the desired ordered nanostructures. The composite films tended to form well-ordered nanostructures when PAAs with a flexible domain such as ether or methylene were used. Mesoporous polyimide films were obtained on thermal treatment of the composite films formed via the sequential processes of cross-linking of the resol, imidization of the PAAs, and selective decomposition of the template BCPs. A polyimide film with well-ordered hexagonal-packed cylindrical mesopores was successfully fabricated after the thermal treatment of the composite film based on PAA/polystyrene-block-poly(methacrylic acid)/resol. The domain spacing based on small-angle X-ray scattering was 29.4 nm, and the pore diameter was determined by scanning electron microscopy to be ∼19 nm. KEYWORDS: polyimide, self-assembly, block copolymer, mesoporous polymer, soft-template method

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INTRODUCTION Polyimides are well-known for their excellent heat resistance, mechanical strength, and chemical stability and are prepared on an industrial scale for use as aerospace and electronic materials.1−6 In terms of expanding their applicability, controlling the morphology of polyimides is an important research topic.7−10 Especially, introducing a mesoporous structure into polyimides is an attractive option because ordered mesoporous polymers (OMPs) have high surface areas and narrow pore size distributions. Indeed, OMPs have attracted considerable attention in recent years because they have numerous applications as catalysts, separation membranes, electronic materials, and photonic crystals and in biotechnology and other fields.11−17 However, the fabrication of ordered mesoporous polyimides is still a challenging task. The difficulty of obtaining ordered mesoporous polyimides arises from the mismatch between the fabrication method of OMPs and the synthetic chemistry of polyimides. A typical approach to obtaining OMPs involves using a highly ordered microphase-separated structure by block copolymer (BCP) self-assembly.18−20 However, the self-assembly of polyimide© 2019 American Chemical Society

containing BCP cannot be used to obtain well-defined microphase-separated structures because polyimides are typically synthesized through the polycondensation of two monomers, namely, tetracarboxylic dianhydrides and diamines, to obtain poly(amic acid) (PAA), followed by thermal or chemical imidization. Because PAA synthesis is essentially stepgrowth polymerization, it is difficult to control the molecular weight with a fairly narrow polydispersity. Consequently, only limited examples have been reported for ordered porous polyimides, which were obtained by other methods such as the breath figure method21−24 and a strategy involving silica particles as templates.25−31 As the breath figure method typically provides only macropores and the silica template method is associated with chemical hazard risks such as in the use of hydrofluoric acid in the etching process, a much simpler method for the fabrication of ordered mesoporous polyimide is strongly desired. Received: March 5, 2019 Accepted: April 19, 2019 Published: April 22, 2019 1209

DOI: 10.1021/acsapm.9b00211 ACS Appl. Polym. Mater. 2019, 1, 1209−1219

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Figure 1. Schematic illustration of the screening of PAAs synthesized from various monomers for PAA/F127 composite films.

scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM).

In this context, our research group is interested in developing a soft-template method to easily obtain wellordered mesoporous imide-containing materials. The softtemplate method has been previously used to fabricate OMPs by self-assembly of a BCP that was employed as a template.32 In the reported method, the polymer or polymer precursor was mixed with the template BCP, and because of selective affinity, they coassembled into microphase-separated structures. Subsequently, the composite was heat-treated to decompose the template BCP selectively and formed a well-ordered mesoporous structure. This attractive soft-template method has not been applied to polyimides to date, although examples of some low molecular weight imide compounds33−39 and conventional phenolic resins40−42 have been reported. If such a soft-template method could be combined with polyimides having relatively high molecular weights, more various attractive mesoporous films could be developed because of their excellent thermal and mechanical properties. This study was conducted in an effort to expand the concept of the soft-template method to high-performance polymers such as polyimides. Generally, the high performance of such aromatic polymers relies on the rigidity of the main chains, and their miscibility with other polymers can be quite poor. However, we envisioned that some PAAs would exhibit fairly good miscibility with templating BCPs if suitable flexible main chains were selected for the PAAs. Therefore, various PAAs were investigated as a precursor of well-ordered polyimide films by blending them with various amphiphilic template BCPs. Subsequently, the microphase-separated structure was characterized by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to evaluate the miscibility of the PAAs with the templating BCPs. Furthermore, promising composite films were heat-treated in the presence of a resol cross-linker to obtain mesoporous polyimide films. The successful fabrication of well-ordered mesoporous structures was demonstrated by field emission

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EXPERIMENTAL SECTION

Materials. The triblock copolymer Pluronic F127 (F127) (poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide), PEO106-b-PPO70-b-PEO106, Mn = 12700) was purchased from Sigma-Aldrich (St. Louis, MO) and used as received. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP, Mn = 80500, PS389-P2VP380, Mw/Mn = 1.10) and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP, Mn = 44000, PS211-P4VP209, Mw/Mn = 1.15) were purchased from Polymer Source (Quebec, Canada) and used as received. Polystyrene-block-poly(methacrylic acid) (PS-b-PMA, Mn = 34300, PS199-PMA158, Mw/Mn = 1.05) was synthesized via living anionic polymerization as described in the Supporting Information (Figure S1).43 Tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were purchased from Kanto Chemical (Tokyo, Japan) and used as received. N,N-Dimethylacetamide (DMAc) was also purchased from Kanto Chemical and stirred with calcium hydride for 24 h and then distilled under pressure before use. Four and six different tetracarboxylic dianhydrides and diamines, respectively, were used as monomers of PAA. Pyromellitic dianhydride (PMDA), 4,4′biphthalic anhydride (BPDA), p-terphenyltetracarboxylic dianhydride (TPDA), 4,4′-oxidiphthalic anhydride (ODPA), 1,4-phenylenediamine (PPDA), 4,4′-oxydianiline (ODA), 4,4″-diamino-p-terphenyl (DAPT), 1,4-bis(4-aminophenoxy)benzene (PAPB), 1,3-bis(3aminophenoxy)benzene (MAPB), and 4,4′-diaminodiphenylmethane (DDM) were purchased from commercial sources and purified as summarized in Table S1. The chemical structures of these are shown in Figure 1. Phenolic resol was synthesized as described elsewhere.35,44 Synthesis of PAAs. A series of PAAs were synthesized via condensation polymerization of the diamines and tetracarboxylic dianhydrides. The typical synthetic procedure of PAAs, given here for PAA:PMDA-ODA as a representative example, was as follows. In a 50 mL three-necked flask purged with nitrogen, 1.000 g of ODA (5.000 mmol) was dissolved in 10.0 mL of DMAc at room temperature by a mechanical stirrer. Next, 1.089 g of PMDA (5.000 mmol) was added to the solution, and the mixture was left to stir for 24 h. Subsequently, the solution was diluted with 10.0 mL of DMAc, and the solution was 1210

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ACS Applied Polymer Materials Table 1. Properties of Synthesized PAA and Nanostructures of the PAA/F127 Composite Films

a

Inherent viscosities of PAAs were measured by an Ostwald viscometer. bGlass transition temperatures (Tg) of polyimides were determined by DSC analysis. cComposition ratios of PAA/F127 = 50/50 wt %. dDomain spacings (d0) and morphologies were estimated by SAXS. rate of 1 °C min−1. In the case of PAA(ODPA-MAPB)/PS-b-PMA/R, the degradation was performed at 380 °C. Characterization. The inherent viscosities of PAAs were measured with an Ostwald viscometer using 0.5 g dL−1 of DMAc solutions at 30 °C. Thermogravimetric analysis (TGA) was performed with an EXSTAR TG/DTA 7300 (Seiko Instrument Inc., Tokyo, Japan) at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was conducted with an EXSTAR DSC7020 (Seiko Instrument Inc., Tokyo, Japan) by heating the polymers at a rate of 5 or 10 °C min−1. SAXS measurements were studied with a Bruker NanoSTAR (Bruker AXSK.K., Kanagawa, Japan, 50 kV per 50 mA) with the 2D-PSPC detector (camera length 1055 mm). The X-ray wavelength (λ) was 1.54 Å using Cu Kα radiation. Transmission electron microscopy (TEM) was performed using a JEOL JEM1010BS (JEOL, Tokyo, Japan) or a H-7650 Zero A (Hitachi, Ltd., Tokyo, Japan) microscope at an accelerating voltage of 80 and 100 kV, respectively. Fast Fourier transform (FFT) images were obtained by processing TEM images using ImageJ software.45,46 FE-SEM was performed with a SU9000 (Hitachi High-Technologies Corporation, Tokyo, Japan) at 3 kV accelerating voltage. STEM was performed with the TECNAI Osiris (FEI Company, Hillsboro, OR) instrument with four windowless silicon drift energy-dispersive X-ray spectroscopy (EDX) detectors (FEI Super X) at an accelerating voltage of 200 kV. EDX elemental mapping was performed using an electron probe of 0.83 nm diameter and 0.6 nA current with a dwell time of 30 μs/ pixel. For 20 min scanning, 300 × 300 pixel maps were obtained.

reprecipitated into an excess amount of methanol, vacuum filtered, and dried in a vacuum oven at 50 °C for 24 h to yield a pale-yellow powder (2.067 g, 99% yield, ηinh = 0.47). 1H NMR (400 MHz, DMSO-d6, δ, ppm): 7.04 (d, J = 8.8 Hz, 4H, −NH−Ar−O−), 7.71 (d, J = 8.0 Hz, 4H, −NH−Ar−O−), 7.97 (s, 1H, −Ar−), 8.33 (s, 1H, −Ar−), 10.55 (s, 2H, amide). 13C NMR (100 MHz, DMSO-d6): δ (ppm): 118.8, 121.5, 128.8, 130.7, 133.1 134.8, 139.1, 152.9, 165.8, 166.7. The synthetic conditions of other PAA are summarized in Table S2. Preparation of PAA/F127 Composite Films. A series of PAA/ F127 composite films were prepared through solution casting (Figure 1 and Table 1). The composite films were cast in a silicone mold from DMF solutions of 10 wt % polymer mixture of PAA and F127 with a composition rate of 50/50 wt %, followed by solvent evaporation at 50 °C for 24 h and drying at 50 °C for another 24 h under vacuum. The obtained composite films were characterized by SAXS (Figure 2). Preparation of Composite Films Based on PAA(ODPAMAPB)/BCP/R. The composite films based on BCP, PAA(ODPAMAPB)/BCP, and PAA(ODPA-MAPB)/BCP/R were prepared through solution casting (Figure 3). The composite films with the composition ratios summarized in Table 2 were cast in a silicone mold from DMF solutions of 10 wt % polymer mixture, with the exception of PS-b-PMA, for which a mixture of DMF and THF (50/50 vol %) was used. Subsequently, solvent evaporation at 50 °C for 24 h and drying at 50 °C for another 24 h under vacuum were carried out. Preparation of Porous Polyimide Films Based on PAA(ODPA-MAPB)/BCP/R. The porous polyimide films were fabricated by the thermal treatment of PAA(ODPA-MAPB)/BCP/R composite films (Figure 3). The PAA(ODPA-MAPB)/BCP/R composite films were thermally annealed at 100 °C for 24 h for cross-linking of resol, followed by imidization of PAA and thermal decomposition of templating BCP at 350 °C under nitrogen gas flow for 1 h at a heating

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RESULTS AND DISCUSSION Characterization of PAAs. A series of PAAs were synthesized via condensation polymerization of the diamine and tetracarboxylic dianhydride monomers listed in Figure 1.

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Self-Assembly of F127 with Various PAAs. The effect of the chemical structure of PAA on the nanostructure of the composite films was studied by investigating various PAAs blended with a representative BCP, F127, which is a commercially available ABA-type triblock copolymer consisting of a hydrophilic A segment (PEO) and a hydrophobic B segment (PPO). The PAAs possessing polar functional groups (carboxyl group and amide group) were expect to exhibit a strong interaction with the hydrophilic segment of F127, which would result in the formation of a microphase-separated nanostructure, segregated from the hydrophobic segment. The composite films were prepared as follows. A solution of the polymers (PAA:F127 = 50:50 wt %) in DMF was cast in a silicone mold, and then the solvent was slowly evaporated at 50 °C for 24 h. The resulting film was dried at 50 °C for 24 h under vacuum. The obtained composite films were characterized by SAXS, and the results are shown in Table 1 and Figure 2. The intensities of the SAXS profile were calculated by azimuthal integration of the scattering pattern as a function of the scattering vector q = 4π sin(θ/2)/λ, where 2θ is the scattering angle and λ is the X-ray wavelength. The SAXS profiles of PAA/F127 composite films with PMDA-ODA, ODPA-ODA, ODPA-PAPB, ODPA-MAPB, and PMDA-DDM showed higher order scattering peaks with q/q* ratio of 1:√2:√3:√4, indicating the formation of body-centered cubic (BCC) structures, where q* was the position of the first-order scattering peak. The SAXS profiles for BPDA-ODA, TPDA-ODA, and ODPA-PPDA showed peaks with q/q* ratio of 1:√3:√7, which indicated the formation of hexagonal packed (HP) structures. In contrast, the profiles for PMDAPPDA, BPDA-PPDA, TPDA-PPDA, PMDA-DAPT, BPDADAPT, and TPDA-DAPT did not show any clear higher order peaks, and only a broad primary peak was observed, which suggested that they had disordered structures. The domain spacings of the ordered nanostructures (d0) were in the range 13.0−16.4 nm, which were calculated from d0 = 2π/q*. The differences in the SAXS profiles reflect the nature of the PAAs. The glass transition temperatures (Tg) of the polyimides after imidization seemed to have a correlation with the resulting nanostructures (Table 1). The composite films using the PAAs with polyimides with Tg below 450 °C resulted in well-ordered nanostructures. Furthermore, these composite films were visually uniform and transparent. The main chains

Figure 2. SAXS profiles of the PAA/F127 composite films (50/50 wt %) with (a) disordered structure and (b) ordered structure prepared in the manner shown in Figure 1.

The synthesized PAAs were characterized by viscosity measurements using an Ostwald viscometer (Table 1). The inherent viscosities of the synthesized PAAs (listed in Table 1) were in the range 0.36−0.99. The glass transition temperatures (Tg) were determined from the second heating curve of DSC measurement conducted after heat-treating the polymers at 100 °C for 1 h and then at 200 °C for 1 h for thermal imidization (Table 1). The correlation of these properties and the nanostructures will be discussed later.

Figure 3. Schematic illustration of the fabrication of the porous polyimide films. 1212

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ACS Applied Polymer Materials Table 2. List of the Composite Films before and after Thermal Treatment weight fraction entry

samplesa

1 2 3 4 5 6 7 8 9 10 11 12

F127 PS-b-P2VP PS-b-P4VP PS-b-PMA PAA/F127 PAA/PS-b-P2VP PAA/PS-b-P4VP PAA/PS-b-PMA PAA/F127/R PAA/PS-b-P2VP/R PAA/PS-b-P4VP/R PAA/PS-b-PMA/R

PAA [%]

BCP [%]

50 50 50 50 40 40 40 40

100 100 100 100 50 50 50 50 50 50 50 50

before thermal treatment

after thermal treatment

resol [%]

d0b [nm]

morphologyb

d0b [nm]

morphologyb

10 10 10 10

20.7 52.5 49.3 36.9 13.0 36.8 36.8 36.4 15.2 36.8 36.8 34.0

lamella lamella lamella lamella BCC sphere disordered sphere disordered sphere macrophase-separated structure HP cylinder disordered sphere disordered sphere HP cylinder

16.4 44.2 34.0 29.4

disordered disordered disordered sphere HP cylinder

a

PAA: ODPA-MAPB. bDomain spacings (d0) and morphologies were estimated by SAXS and/or TEM.

Figure 4. (a) SAXS profiles of composite films based on F127 and PAA(ODPA-MAPB) prepared as shown in Figure 3. (b) TEM image and (c) corresponding FFT image of PAA/F127 before thermal treatment stained with RuO4. (d) TEM and (e) FFT images for PAA/F127/R after thermal treatment without stain.

of these PAAs were considered to be relatively flexible because these included ether or methylene linkages. Such flexibility probably contributed to the solubility of PAAs in the cast solution, which should be guaranteed throughout the fabrication process for the composite films. In contrast, the composite films with relatively rigid PAAs did not show wellordered nanostructure in SAXS, and some of the composite films appeared turbid. These results might reflect the poor solubility of PAAs derived from the rigidity of main chains. Therefore, it was concluded that PAAs with relatively flexible main chains were promising precursors of well-ordered mesoporous polyimide films fabricated by the soft-template method.

Self-Assembly of Various BCPs with PAA(ODPAMAPB). The chemical structure of the BCP template was also expected to affect the nanostructure of the composite film. This effect was studied by investigating various composite films comprising ODPA-MAPB (PAA) and four different BCPs with different hydrophilic functional groups, namely, F127, PS-bP2VP, PS-b-P4VP, and PS-b-PMA, as shown in Figure 3 and Table 2. The nanostructures were investigated by SAXS and TEM. The TEM samples of PAA/F127 and PAA/PS-b-PMA were stained with RuO4, and those of PAA/PS-b-P2VP and PAA/PS-b-P4VP were stained with iodine for obtaining good contrast in the images. The results for the PAA/F127 composite film are summarized in Figure 4. The SAXS profile with pristine 1213

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Figure 5. (a) SAXS profiles of composite films based on PS-b-P2VP and PAA(ODPA-MAPB) prepared in the manner shown in Figure 3. (b) TEM image and (c) corresponding FFT image of PAA/PS-b-P2VP before thermal treatment stained with iodine. (d) TEM and (e) FFT images for PAA/PS-b-P2VP/R after thermal treatment without stain.

Figure 6. (a) SAXS profiles of composite films based on PS-b-P4VP and PAA(ODPA-MAPB) prepared in the manner shown in Figure 3. (b) TEM image and (c) corresponding FFT image of PAA/PS-b-P4VP before thermal treatment stained with iodine. (d) TEM and (e) FFT images for PAA/PS-b-P4VP/R after thermal treatment without stain.

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Figure 7. (a) SAXS profiles of composite films based on PS-b-PMA and PAA(ODPA-MAPB) prepared in the manner shown in Figure 3. (b) TEM image and (c) corresponding FFT image of PAA/PS-b-PMA before thermal treatment stained with RuO4. (d) TEM and (e) FFT images for PAA/ PS-b-PMA/R after thermal treatment without stain.

F127 showed higher order scattering peaks with q/q* ratio of 1:2:3:4, indicating the formation of a lamella structure with a domain spacing of 20.7 nm. The SAXS profile of the PAA/ F127 composite film showed a different scattering pattern with a q/q* ratio of 1:√2:√3:√4 in the peaks, indicating a highly ordered BCC structure with a domain spacing of 13.8 nm. The TEM image (Figure 4b) showed a pattern of dark dots in a white matrix, which likely corresponded to the PPO sphere domain and a PEO+PAA matrix. The average center-to-center distance of the dots was 14 nm, which was in good agreement with the domain spacing from SAXS measurements. The SAXS and TEM results for the PAA/PS-b-P2VP composite film are summarized in Figure 5. The SAXS profile with pristine PS-b-P2VP showed higher order scattering peaks with the q/q* ratio of 1:2:3:4, indicating the formation of a lamellar structure with a domain spacing of 52.5 nm. The SAXS profile of the PAA/PS-b-P2VP composite film showed a different scattering pattern with higher order scattering peaks having a domain spacing of 36.8 nm, although these scattering peaks could not be assigned to typical well-known ordered structures. The TEM image (Figure 5b) showed a pattern of white dots in a dark matrix which corresponds to a disordered PS sphere domain in a P2VP+PAA matrix. The average centerto-center distance of the dots was 36 nm, which was in good agreement with the domain spacing from SAXS measurements. The results for the PAA/PS-b-P4VP composite film (Figure 6) were similar to those obtained for PS-b-P2VP. The domain spacing of pristine PS-b-P4VP film from SAXS was 49.3 nm. The domain spacing of PAA/PS-b-P4VP composite film from SAXS and the average center-to-center distance from TEM were 36.8 and 38 nm, respectively.

The SAXS and TEM results for the PAA/PS-b-PMA composite film are given in Figure 7. The SAXS profile with pristine PS-b-PMA showed higher order scattering peaks with q/q* ratio of 1:2:3:4, indicating the formation of a lamellar structure with a domain spacing of 36.9 nm. The SAXS profile of the PAA/PS-b-PMA composite film showed higher order scattering peaks with a domain spacing of 36.4 nm, although these scattering peaks could not be assigned to typical wellknown ordered structures. The TEM image (Figure 7b) showed a mix of lamellar regions and a macrophase-separated region. The former likely corresponded to the PS domain and the PAA+PMA domain, while the latter could be attributed to the excess PAA domain that was not miscible with the PMA domain. The results of pristine BCP films and PAA/BCP composite films revealed that F127, PS-b-P2VP, PS-b-P4VP, and PS-bPMA were promising soft-template candidates for forming periodic nanostructures in the PAA, although only the composite film with PS-b-PMA partly formed the macrodomain of PAA. Fabrication of Porous Polyimide Films. Mesoporous polyimide films were fabricated by thermal treatment of the PAA(ODPA-MAPB)/BCP composite films, as illustrated in Figure 3. The thermal treatment should be performed at a temperature where the templating BCP decomposes but the polyimide is stable. Furthermore, the microphase-separated structure should be retained throughout the thermal treatment to successfully obtain well-ordered mesoporous polyimide films. The TGA curves of F127, PS-b-P2VP, PS-b-P4VP, PS-bPMA, and PAA(ODPA-MAPB) are shown in Figure 8. The TGA curves for F127, PS-b-P2VP, PS-b-P4VP, and PS-b-PMA indicated that they started to decompose at 280, 320, 320, and 1215

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in Figure 6a suggest that the addition of resol did not affect the nanostructure before the thermal treatment. The higher order peaks of the SAXS profile after the thermal treatment were very weak, which suggested a partial collapse of the nanostructure. The TEM image of the film (Figure 6c) revealed disordered white dots in a dark matrix, suggesting that the nanostructure was partly retained and a disordered spherical structure had formed. The PAA/PS-b-PMA/R composite film was fabricated in a similar manner, i.e., by addition of 10 wt % of resol and heat treatment at 100 °C for 24 h and then at 380 °C for 1 h (Table 2, entry 12). The fourth spectrum from the top in Figure 7a is the SAXS profile of PAA/F127/R before the thermal treatment and indicates the formation of an HP structure by showing a q/ q* ratio of 1:√3:2:3. This structure was different from that of PAA/PS-b-PMA, which meant that the addition of resol somehow affected the microphase-separated structure. The fifth spectrum from the top in Figure 7a is the SAXS profiles of PAA/PS-b-PMA/R after the thermal treatment and indicates the retention of the HP structure throughout its thermal treatment by showing a q/q* ratio of 1:√3:2:3. The TEM image of PAA/PS-b-PMA/R after it was subjected to heat is shown in Figure 7c. This TEM image revealed HP white dots in a dark matrix, indicating HP mesopores in the polyimide matrix. The hexagonal structure was highly regular in the tested composite films. Indeed, the HP pattern was uniformly observed in the range of a few micrometer squares (Figure S2). The distance between the arrays was ∼30 nm, which is close to the domain spacing determined from the SAXS profile (29.4 nm). Characterization of Porous Polyimide Films. Further characterization of the porous polyimide film based on PAA/ PS-b-PMA/R was performed using FE-SEM, STEM-EDX, TGA, and DSC. The cross-sectional FE-SEM images of the porous polyimide films (Figure 9) showed oriented cylindrical mesopores in the film, which were in good agreement with the results of SAXS and TEM measurements. The pore diameter was ∼19 nm, which corresponds to 26% of porosity. STEMEDX analysis was performed to obtain high-resolution elemental distribution images. The EDX detectors were positioned symmetrically around the optical axis near the specimen area, which significantly enhanced the EDX detection sensitivity and enabled the rapid detection of small amounts of light elements in polymer materials.47,48 In the STEM high-angle annular dark field (HAADF) images as shown in Figure 10a, the brighter region corresponded to the polymer domain with a higher elastic scattering than other parts in the specimen. Figures 10b−d show the EDX mapping for carbon, nitrogen, and oxygen, respectively. These EDX mapping images indicated the polyimide constituent of the polymer matrix, and the dark region represented the uniform distribution of pores. The thermal properties of the obtained porous film were evaluated by TGA and DSC as shown in Figure 11 and Figure S3, respectively. The 5 wt % decrease temperature of the porous polyimide film was 326 °C, and the weight loss at 500 °C was less than 15%, suggesting that the porous film retains the high thermal stability of polyimide. Thus, it was concluded that well-ordered mesoporous polyimide films had been successfully obtained via the softtemplate method using PS-b-PMA as the template BCP. Note that the well-ordered structure has been immobilized with a relatively small amount of resol cross-linker (10 wt %), whereas our previous study with low molecular weight oligo(amic acid)

Figure 8. TGA curves of BCPs and PAA (heating rate = 10 °C min−1).

350 °C, respectively, while PAA was stable up to 500 °C. Therefore, the thermal treatment temperature was set as 350− 380 °C. Before such thermal treatment was tested, the composite films were treated at 100 °C for 12 h to tentatively evaluate the thermal stability of the microphase-separated structures. The SAXS profiles of the heat-treated films are shown as the third spectra from the top in Figures 4−7a. The peaks in the SAXS profiles of the PAA/BCP composite films disappeared or decreased, indicating that the nanostructures had disappeared or partly collapsed. These results suggested that the microphase-separated structures needed to be further immobilized to obtain well-ordered mesoporous polyimide films. Therefore, cross-linking of the polymer chains by resol was attempted. To prevent the nanostructure from collapsing during the thermal treatment, a PAA/F127/R composite film was fabricated by adding 10 wt % of resol (Table 2, entry 9). It was rationalized that resol would be miscible with the hydrophilic domain of the composite film and immobilize the nanostructure by cross-linking the polymer chains. The composite film was heat-treated at 100 °C for 24 h and then at 350 °C for 1 h. The fourth and the fifth spectra from the top in Figure 4a show the SAXS profiles of PAA/F127/R before and after the thermal treatment, respectively. As PAA/F127/R showed an almost similar profile to that of PAA/F127 before thermal treatment, it was assumed that the addition of 10 wt % of resol did not affect the nanostructure. However, the scattering peaks after the heat treatment were very weak, suggesting that the ordered nanostructure had almost collapsed. A TEM image of PAA/F127/R after the thermal treatment is shown in Figure 4c. The TEM image shows disordered white areas in a dark matrix, indicating that a wellordered mesoporous polyimide film was not obtained. The PAA/PS-b-P2VP/R composite film was fabricated by adding 10 wt % of resol and heat-treated at 100 °C for 24 h and then at 350 °C for 1 h (Table 2, entry 10). The results shown in Figure 5 are similar to those of PAA/F127/R. The addition of resol did not affect the nanostructure, but the nanostructure was not retained after heat treatment at 350 °C. The PAA/PS-b-P4VP/R composite film was also fabricated by adding 10 wt % of resol and heated at 100 °C for 24 h and then at 350 °C for 1 h (Table 2, entry 11). The SAXS profiles 1216

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Figure 11. TGA curves of PS-b-PMA, polyimide (ODPA-MAPB), and porous polyimide (heating rate = 10 °C min−1).

microphase-separated structure of an amphiphilic BCP. It was revealed that the PAA with flexible domains such as ether or methylene groups tended to form well-ordered nanostructures. Remarkably, after thermal treatment, PAA(ODPA-MAPB)/ PS-b-PMA/resol formed well-ordered hexagonal-packed cylindrical mesopores. The domain spacing, as determined by SAXS, was 29.4 nm, and the pore diameter (from FE-SEM measurements) was ∼19 nm. Other BCPS, F127, PS-b-P2VP, and PS-b-P4VP were also effective to obtain well-ordered structures, but such structures were not retained after the thermal treatment. This problem might be solved after further optimization of the thermal treatment conditions. The softtemplate method described in this study allows controlling of the morphology and pore size by optimizing the structure of the template BCP and the composition rate with the PAA. Therefore, by use of this method, polyimides can be further developed for high-performance applications as catalysts, absorbents, and low-dielectric materials. Besides, the softtemplate method also enables the formation of well-ordered mesoporous structures for other polymer matrixes that are hard to obtain; indeed, this method paves the way for the development of various advanced materials.

Figure 9. Cross-sectional FE-SEM images of PAA/PS-b-PMA/R after thermal treatment at (a) ×30.0K and (b) ×100K magnification.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00211. Synthetic procedure and characterization of PS-b-PMA, summary of commercial sources and purification method of monomers of PAAs, summary of synthetic conditions of PAAs, and TEM image and DSC curve of porous polyimide based on PAA/PS-b-PMA/R (PDF)

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Figure 10. (a) STEM-HAADF image and STEM-EDX elemental maps for (b) carbon, (c) nitrogen, and (d) oxygen of PAA/PS-bPMA/R after thermal treatment.

AUTHOR INFORMATION

Corresponding Author

*(T.H.) Phone +81-3-5734-2421; e-mail hayakawa.t.ac@m. titech.ac.jp. ORCID

required ∼50 wt % of cross-linker.35 The present method is more promising in terms of retaining excellent property of polyimides.

Takahiro Komamura: 0000-0002-3736-4874 Shin Horiuchi: 0000-0001-8256-0498 Yuta Nabae: 0000-0002-9845-382X Teruaki Hayakawa: 0000-0002-1704-5841

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CONCLUSIONS Well-ordered mesoporous polyimide films were successfully fabricated via the soft-template method that utilized the

Notes

The authors declare no competing financial interest. 1217

DOI: 10.1021/acsapm.9b00211 ACS Appl. Polym. Mater. 2019, 1, 1209−1219

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ACKNOWLEDGMENTS We are grateful to Ryohei Kikuchi (Ookayama Materials Analysis Division, Tokyo Institute of Technology) for the TEM and FE-SEM observations. This research was supported in part by the Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO) on the Molecular Technology and Creation of New Functions, and a Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number 17H03113), JSPS KAKENHI for Scientific research on Innovative Areas “Materials Science on Mille-feuille Structure (MFS)” (Grant Number JP18H05482), and the Ogasawara Foundation for the Promotion of Science & Engineering.

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