Fabrication and Modification of Ordered Nanoporous Structures from

Dec 7, 2012 - We report facile preparation of nanoporous thin films by rinsing out a metal salt from nanophase-separated hybrid films composed of a bl...
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Fabrication and Modification of Ordered Nanoporous Structures from Nanophase-Separated Block Copolymer/Metal Salt Hybrids Yoshio Sageshima,† Shigeo Arai,‡ Atsushi Noro,*,† and Yushu Matsushita*,† †

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan ‡ Ecotopia Science Institute, Nagoya University, 464-8603, Japan S Supporting Information *

ABSTRACT: We report facile preparation of nanoporous thin films by rinsing out a metal salt from nanophase-separated hybrid films composed of a block copolymer and a watersoluble metal salt. Nanophase-separated hybrids were prepared by mixing polystyrene-b-poly(4-vinylpyridine) (PS-P4VP) and iron(III) chloride in a solvent of pyridine, followed by solventcasting and thermal-annealing. Film samples with a thickness of ca. 100 nm were fabricated from the nanophase-separated hybrids by using a microtoming technique. Metal salts in the films were removed by immersion into water to fabricate nanopores. Morphological observations were conducted by using transmission electron microscopy (TEM). Ordered cylindrical nanopores were clearly observed in the thin films prepared from the water-immersed hybrids which originally present cylindrical nanodomains. These nanoporous films were modified by loading another metal salt, samarium(III) nitrate, into the nanopores on the basis of the coordination ability of P4VP tethered to the pore walls. The samples after loading treatment were evaluated by TEM observation and elemental analysis with energy dispersive X-ray spectroscopy.

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INTRODUCTION

Recently, more functional nanopores have been reported by introducing a supramolecular technique into nanophaseseparation of block copolymers, which can be an effective approach to produce nanoporous materials because additives incorporated into nanodomains via noncovalent bonding can be extracted just with immersion treatment into solvents. Stamm and co-workers reported preparation of nanoporous thin films by removing 2-(4′-hydroxybenzeneazo)benzoic acid from supramolecular complexes composed of polystyrene-bpoly(4-vinylpyridine) and the acid.39−41 Ikkala and ten Brinke et al. also prepared nanoporous structures from similar complexes composed of polystyrene-b-poly(4-vinylpyridine) and pentadecylphenol.42−44 Because such a supramolecular technique does not require a decomposition procedure and the additives can be collected after removal, this is a promising approach compared with conventional methods. Recently, several research groups have reported preparation of block copolymer/metal salt hybrids via metal-to-ligand coordination,45−49 which represent clear nanophase-separated structures composed of an organic phase and a hybrid phase containing metal salts. Because solubility of metal salts in water is typically high while many polymers are insoluble in water, nanoporous materials could be fabricated quickly through a

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Multicomponent soft materials, particularly block copolymers, have received significant attention due to their ability to selfassemble into periodic structures at the nanoscopic length scale, i.e., “nanophase-separated structures”.2,3 Because fascinating morphologies can take place depending on various molecular characteristics such as molecular weight,4,5 composition,6 polydispersity,7−10 or chain connectivity,11,12 nanophaseseparated structures of block copolymers are expected to become scaffolds and templates for nanoscopic structure devices.13−16 Indeed, many studies have been directed toward achieving highly functional nanomaterials from nanophase-separated block copolymers.17−25 One example is among nanoporous materials, which have well-ordered nanopores with a uniform size. A well-known nanoporous material is a mesoporous silica prepared by calcination,26 where a nanophase-separated structure of a block copolymer was used as a burnable template.27,28 Another strategy to prepare nanoporous materials from block copolymer films includes a polymer degradation technique such as ozonolysis29 or hydrolysis.30,31 Such procedures together with decomposition have been widely applied both in bulk32−35 and in thin films;36−38 however, they are time-consuming and require an irreversible process to decompose constituent blocks, which means they are hardly recyclable. © 2012 American Chemical Society

Received: October 23, 2012 Revised: November 23, 2012 Published: December 7, 2012 17524

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convenient process such as rinsing thin films with water, where metal salts incorporated into periodic structures are removed by disassembling metal-to-ligand complexes. Nanopores prepared in this procedure are also expected to be modified by loading other compounds, such as metal compounds or organic molecules, because functional polymer chains with coordination ability are utilized for nanodomain formation. In this study, we report facile preparation to achieve nanoporous thin films by rinsing out metal salt from block copolymer/metal salt hybrid films as schematically shown in Figure 1. A polystyrene-b-poly(4-vinylpyridine) (PS-P4VP) Figure 2. GPC chromatograms of (a) PS-P4VP-1 and (b) PS-P4VP-2. Solid lines and dashed lines in each graph correspond to a PS-P4VP block copolymer and a PS precursor, respectively. inhibiting the strong interaction between P4VP and FeCl3. The films were dried to remove residual solvent and annealed at 165 °C for 60 h in vacuo. Four hybrid samples were prepared, and their codes and characteristics were summarized in Table 2. Preparation of Thin Films with Nanopores by Microtoming. The procedure to attain nanoporous films is as follows. First, bulk films of hybrids were embedded in an epoxy resin and cut into thin sections with a thickness of ca. 100 nm by using a microtome, Leica Ultracut FCS, which has a water reservoir under a diamond knife. Thin sections with the area size of ca. 0.5 mm × 5 mm were floated on water due to the high surface tension of polymers (PS and P4VP) to water. Then the thin sections were placed on a copper grid and immersed into water for 30 s. Note that most of the cylinders in the bulk film are basically aligned parallel to the film surface. The bulk film was sliced perpendicularly to the film surface by a diamond knife; hence, cylinders are aligned vertically in the thin sections (see also Figure S1 in Supporting Information). These water-immersed samples are coded as i-X, where X represents the code of a hybrid in Table 2. Hybrids and neat PS-P4VP were also cut into thin sections in a dry atmosphere without using water as control experiments. Modifying Nanopores with Another Metal Salt. Nanoporous thin films placed on a copper grid were immersed in a 2 wt % Sm(NO3)3 aqueous solution for 5 min to refill nanopores with this salt. Sm(NO3)3 was used because it interacts with a P4VP block via metal-to-ligand coordination. Morphological Observation. Morphological observation was carried out by using a transmission electron microscope (TEM) of JEM-1400(JEOL) under an acceleration voltage of 120 kV. Thin films of neat PS-P4VP and i-2C were exposed to iodine vapor at 50 °C for 40 min to stain the P4VP phase, whereas sections of the other samples were not stained at all. An energy-dispersive X-ray (EDX) spectrometer (Model 550i Hardware, IXRF system) installed to a TEM (H-800, Hitachi) was also used to detect the characteristic Xrays from metal elements in hybrids generated by electron beams. Small-angle X-ray scattering (SAXS) was also conducted for bulk hybrid samples at the beamline 6A in the Photon Factory of KEK, Tsukuba, Japan.

Figure 1. Schematic illustration of a preparation procedure for a nanoporous film from a block copolymer/metal salt hybrid.

block copolymer and a water-soluble metal salt, iron(III) chloride (FeCl3), were used for hybrid formation, where a P4VP block and FeCl3 are connected via metal-to-ligand coordination in a bulk state. Hybrid thin films were prepared by a microtoming process, and they were immersed into water to rinse out the metal salt from hybrid nanodomains. Nanoporous thin films thus obtained were reimmersed into a samarium(III) nitrate (Sm(NO3)3) aqueous solution to functionalize the nanopores with Sm(NO3)3. Morphological observation and elemental analysis were conducted by using transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy.

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EXPERIMENTAL SECTION

Materials. 4-Vinylpyridine (4VP) was purchased from Aldrich, while the other reagents including FeCl3 and Sm(NO3)3 were purchased from Kishida Regents Chemicals Co. Ltd. Styrene and 4VP for polymerization were purified by passing through an aluminum oxide column before use. Pyridine as a solvent for casting was used after removing water with molecular sieves. The others were used as received. Synthesis and Characterization of Block Copolymers. Two PS-P4VP block copolymers were synthesized via a reversible addition−fragmentation chain transfer (RAFT) polymerization technique. Details on polymer synthesis were reported in previous papers.45,50 Polydispersity indices (PDIs) of PS-P4VP were estimated by gel permeation chromatography (GPC, Figure 2), which was performed by using three TSK-GEL G4000HHR columns combined with a DP-8020 dual pump and a UV detector, where the wavelength was set at 220 nm. The eluent was dimethylformamide (DMF), and the flow rate was 1 mL/min. A calibration curve of molecular weight by polystyrene standards was basically used to estimate the PDI of each sample. A total number-average molecular weight (Mn) and a volume fraction (ϕs) of a PS block in PS-P4VP were determined by 1H NMR (Varian, 500 MHz). The molecular characteristics of each PSP4VP were summarized in Table 1. Note that two block copolymers with relatively low PDIs7−10 were used for single morphology formation as shown in Table 1 and Figure 2. Preparation of Hybrid Bulk Films. Nanophase-separated hybrids were prepared via a solvent-casting procedure by using a solvent with coordination ability, which was reported previously.45 PS-P4VP and FeCl3 were mixed in a solvent of pyridine, followed by solvent-casting on a hot plate at 50 °C for 24 h. Pyridine was used to prevent P4VP blocks from forming aggregates with FeCl3 via coordination by

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RESULTS AND DISCUSSION Preparation of Nanoporous Thin Films from Hybrid Films by Microtoming. Figure 3a shows a TEM image of neat PS-P4VP-1, where darker P4VP spheres are observed in a brighter PS matrix. Figure 3b is the TEM image of a 1C hybrid, which represents a cross-section of a hexagonally packed cylindrical structure with an interdomain spacing of ca. 42 nm. These images indicate that addition of FeCl3 induced morphological transition from a spherical structure to cylindrical one, which was also confirmed by SAXS measurements (see Figure S2 in Supporting Information). Note that Figure 3b shows good contrast between cylinders and matrix regardless of staining. This is because FeCl3 with high electron 17525

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Table 1. Molecular Characteristics of Two PS-P4VP Block Copolymers code

Mn(total)a

Mn(PS)b

Mn(P4VP)c

ϕPSd

PDIe

morphologyf

D, nmg

PS-P4VP-1 PS-P4VP-2

37000 54000

28000 32000

9000 22000

0.79 0.62

1.13 1.20

sphere cylinder

24 53

a Total number average molecular weight calculated by using Mn(PS), ϕPS of PS-P4VP, and bulk densities of component polymers, i.e., 1.05 for PS and 1.17 for P4VP. bNumber average molecular weight of a PS block determined with 1H NMR by end-group analysis. cNumber average molecular weight of a P4VP block. dVolume fraction of PS determined by 1H NMR. Bulk densities of component polymers were used for calculation. e Polydispersity index determined by GPC as shown in Figure 2. Calibration was performed with polystyrene standards. fMorphology of a neat block copolymer in a bulk state determined by both TEM and SAXS. gDistance between nanodomains for each morphology at room temperature. The values were determined on the basis of SAXS data.

Table 2. Codes and Characteristics of PS-P4VP/FeCl3 Hybrids code

polymer

wPS‑P4VP:wFeCl3a

n4VP:nFeCl3b

ϕPS:ϕP4VP:ϕFeCl3c

morphologyd

D, nme

1S 1C 1L 2C

PS-P4VP-1 PS-P4VP-1 PS-P4VP-1 PS-P4VP-2

93:7 85:15 80:20 85:15

100:20 100:50 100:70 100:20

0.75:0.22:0.03 0.73:0.21:0.06 0.71:0.20:0.09 0.57:0.36:0.07

sphere cylinder lamella cylinder

22 44 41 46

a Blend weight ratio of PS-P4VP:FeCl3. bBlend molar ratio of 4VP:FeCl3 cVolume fraction of PS:P4VP:FeCl3, which was calculated by using bulk densities of components, i.e., 1.05 g/cm3 for PS, 1.17 g/cm3 for P4VP, and 2.80 g/cm3 for FeCl3. The calculation was based on the assumption of incompressibility after mixing of PS-P4VP and FeCl3. dMorphology in the bulk state determined by both TEM and SAXS. eDistance between domains for each morphology at room temperature. The values were determined on the basis of SAXS data.

Metal Salt Removal from Hybrids with Different Morphologies. As reported previously, morphologies of block copolymer/metal salt hybrids can also be tuned by the amount of metal salt added.45 Taking into account the results from this previous report, we prepared other nanohybrids with different amounts of FeCl3. Figure 4a and 4b shows TEM

Figure 3. TEM images of (a) neat PS-P4VP-1, (b) 1C, and (c, d) water-immersed i-1C. Panel c is a cross-sectional view of cylindrical nanopores, while panel d is a lateral view of cylindrical nanopores. Neat PS-P4VP-1 was stained with iodine vapor, whereas 1C and i-1C were not stained with any agents. All scale bars are 100 nm.

density was dispersed uniformly in a P4VP phase, which was confirmed by an elemental-mapping TEM observation and EDX measurements in a previous report.45 The TEM image of a thin film after immersion is shown in Figure 3c. A cylindrical structure was observed again; however, an image with an inverse contrast was seen, which means that the darker cylindrical nanodomains turned to a brighter phase due to the generation of nanopores. This observation indicates that water-soluble FeCl3 in cylindrical nanodomains probably dissolved out to water from insoluble cylindrical P4VP nanodomains with the PS matrix. Figure 3d also shows the TEM image of a lateral view of nanopores in i-1C, which strongly supports the fabrication of cylindrical nanopores.

Figure 4. TEM images of PS-P4VP-1/FeCl3 hybrids prepared under dry conditions and samples after immersion in water: (a) 1S; (b) 1L; (c) i-1S; (d) i-1L. None of the samples were stained. All scale bars are 100 nm.

images of 1S and 1L, respectively, which were prepared in a dry atmosphere. Spherical and lamellar structures with a good contrast between two phases were clearly observed despite not being stained, suggesting the selective incorporation of FeCl3 into P4VP nanodomains. On the other hand, Figure 4c and 4d represents TEM images of water-immersed samples from 17526

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nanopores in a larger area (ca. 2.5 μm × 2.5 μm), as shown in Figure S5. On the basis of TEM images in Figure 5a and 5b, average diameters of darker cylindrical nanodomains and the brighter nanopores obtained were roughly estimated as 25 nm and 19 nm, respectively. Taking the preparation procedure of nanoporous thin films into consideration, insoluble P4VP blocks inside cylinders should remain on the wall of the PS matrix; therefore, it is reasonable that the average diameter of the nanopores was quantitatively smaller than that of the hybrid cylindrical domains. To confirm the presence of P4VP chains around the nanopores, an i-2C nanoporous thin film was exposed to iodine vapor to stain the P4VP chains. Figure 5c shows the TEM image of iodine-stained i-2C, and the highmagnification image is also shown in the inset. Although there is an excessive staining effect on the film, the darker phase is clearly observed at the surface of the nanopores, which corresponds to the P4VP phase. Modification of Nanopores with Another Metal Salt. Because nitrogen atoms on P4VP chains have coordination ability, the nanopores could be refunctionalized by loading other metal salts, metal nanoparticles, or other functional compounds. Consequently, a nanoporous film was immersed in a 2 wt % aqueous solution of Sm(NO3)3 for approximately 5 min to functionalize the nanopores, and the TEM image of the film after immersion is shown in Figure 6a. While the darker cylindrical domains are seen in the lower left area of Figure 6a, brighter cylinders of nanopores can still be observed in the upper right area. Because the blend of a block copolymer and Sm(NO3)3 conveys a clear two-phase image of a nanophaseseparated hybrid via metal-to-ligand coordination shown in the TEM image (see Figure S6, Supporting Information), darker cylinders in the lower left area of Figure 6a should be a hybrid phase with high electron density due to Sm(NO3)3 loaded in the nanopores. Although the TEM image including both a Smdoped area and a nanoporous area is shown in Figure 6a for the purpose of clear understanding of loading Sm(NO3)3, it should be noted that the degree of modified area can be tunable by changing the concentration of a Sm(NO3)3 aqueous solution. Elemental analysis of these darker cylinders was also conducted by using EDX to confirm the presence of Sm(NO3)3. Although a clear elemental-mapping TEM image was not acquired because of unsatisfactory resolution, elemental analysis was conducted for the area where all the nanopores seemed to be filled with Sm(NO3)3, as shown in Figure S6. Figure 6c shows an EDX spectrum of i-2C after the immersion treatment. The peaks at 5.64 keV and 6.20 keV, shown as red regions in Figure 6c, represent characteristic X-rays of Sm Lα and Sm Lβ, respectively. Although there are also small peaks at 6.40 keV and 7.06 keV, which represent characteristic X-rays of Fe Kα and Fe Kβ, respectively, they are negligibly small (see Figure S8 for the detail on EDX analysis). This fact strongly suggests that most of the nanopores were filled with Sm(NO3)3. These results also imply that our nanoporous thin films can be functionalized easily by loading various kinds of metal compounds inside the nanopores, which can provide films with desirable properties.

nanohybrids (i-1S and i-1L, respectively). In contrast to the case of i-1C with a cylindrical structure, no significant differences were found between the 1S hybrid film (Figure 4a) and the water-immersed film of i-1S (Figure 4c), even if the contact time with water was 10 min or longer. As for i-1L with a lamellar structure, peeled-off lamellae with a brighter contrast were seen in darker lamellae after immersion in water as shown in Figure 4d, indicating that the lamellar structure had been partly destroyed. These results suggest that metal salts can be rinsed out easily from the continuous nanodomains as cylinders or lamellae in thin films by merely immersing into water even for a short time of 30 s. The results also suggest that noncontinuous structures such as spheres are not appropriate for preparing nanoporous films. Among the continuous structures, a cylindrical structure is the most suitable for nanopore preparation because the cylindrical shape is maintained by the support of the hydrophobic PS matrix, whereas the lamellar structures are apt to fall apart from lack of such strong support (see also the large-size image of i-1L in Figure S3, Supporting Information). Structural Analysis of Nanopores. For further investigation of nanopores, another nanoporous film with larger nanopores was also prepared by extracting FeCl3 from a film of 2C with larger cylinders. Figure 5a is a TEM image of a 2C

Figure 5. TEM images of a hybrid and a water-immersed sample: (a) 2C (no staining), (b) i-2C (no staining), and (c) i-2C stained with iodine vapor, with a high-magnification image of i-2C in the inset. Scale bars in panels a−c represent 200 nm, and the scale bar in the inset represents 30 nm.

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CONCLUSION We demonstrated the simple preparation of nanoporous thin films from a block copolymer/metal salt hybrid by using a microtoming technique under wet conditions where a metal salt such as FeCl3 in a specific phase of nanophase-separated structures was rinsed out. By comparison with water-immersed

hybrid which presents a darker cylindrical structure, while Figure 5b is the image of immersed i-2C with brighter cylindrical nanodomains. These images indicate that the fabrication of i-2C nanopores proceeded just as well as that for i-1C. (More information on neat PS-P4VP-2 and 2C is shown in Figure S4.) This sample provided highly ordered 17527

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Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS The authors are grateful for financial support of KAKENHI grant no. 22245038 (Y.M.), no. 23655123 (A.N.), and no. 24685035 (A.N.) from JSPS and MEXT, Japan. The authors also thank the Global COE program in Chemistry at Nagoya University and the Program for Leading Graduate Schools at Nagoya University. Use of synchrotron X-ray source was supported by Photon Factory, KEK, in Tsukuba, Japan (no. 2010G59 and no. 2012G176 (A.N.))

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Figure 6. (a) The TEM image of i-2C after immersion into a Sm(NO3)3 aqueous solution. The film was not stained with any agents. The scale bar is 200 nm. (b) Schematic illustration of modification of the nanopores with another metal salt. (c) The EDX spectrum of i-2C after the immersion treatment.

samples of different morphologies, a cylindrical structure was found to be the most suitable for nanopore preparation. Furthermore, these nanoporous films could be functionalized by loading another metal salt such as Sm(NO3)3 into the nanopores because P4VP chains tethered to the nanopore wall possess coordination ability. Although the films should be prepared with a larger area for practical application, this study still suggests that this procedure for the preparation of nanoporous structures could provide highly functional materials such as highly efficient catalysts, photovoltaic films, and photonic devices.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images, SAXS profiles, and EDX spectra in Figures S1− S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. 17528

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dx.doi.org/10.1021/la3042023 | Langmuir 2012, 28, 17524−17529