Structural Nanospace Feature and Substrate Contribution to

Note pubs.acs.org/Macromolecules

Structural Nanospace Feature and Substrate Contribution to Maintaining Stable Porosity of Polymer Chain in Layer-by-Layer Assembled Isotactic Poly(methyl methacrylate) Films Hiroharu Ajiro,†,‡ Chizuru Hongo,† Masumi Maegawa,† Daisuke Kamei,† Sono Sasaki,§ Hiroki Ogawa,§ Hiroyasu Masunaga,§ Yukie Takemoto,† Kazuyuki Horie,§ and Mitsuru Akashi*,†,‡ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871 Japan ‡ The Center for Advanced Medical Engineering and Informatics, Osaka University, 2-2 Yamada-oka, Suita, Osaka 565-0871 Japan § Japan Synchrotron Radiation Reserch Institute/Spring-8, Sayo, Hyogo 679-5198, Japan

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INTRODUCTION

In order to elucidate the effects of the substrate on LbL assembled nanoporous films, we employed three samples in this study: the original porous it-PMMA film deposited on the substrate by the LbL approach, the bulk it-PMMA sample without any substrate, and a spin-coated it-PMMA film on the substrate. It is noteworthy that the bulk it-PMMA sample was successfully prepared after the removal of the st-PMAA from the bulk it-PMMA/st-PMAA stereocomplex by alkaline treatment without any substrate (Figure 1b), in order to clarify the role of the substrate. Furthermore, a spin-coated it-PMMA film on the substrate was prepared in order to clarify the necessity of preliminary stereocomplex formation and extraction of the stPMAA for nanospace creation (Figure 1c). Next, the films were analyzed by XRD tools, including grazing-incidence wide-angle X-ray diffraction (GIWAXD) and grazing-incidence small-angle X-ray diffraction (GISAXD), as well as by transmission XRD. The structural features of the porous it-PMMA are also discussed.

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A large number of studies on the stereocomplex of isotactic (it-) poly(methl methacrylate) (PMMA) and syndiotactic (st-) PMMA have been performed since it was discovered in 1958 by Fox et al.,2 due to the specific polymer−polymer recognition phenomenon in synthetic macromolecules. In addition to their usual behaviors in organic solvents, the complementary interactions of the it-PMMA/st-PMMA stereocomplex were investigated in ionic liquids,3 in supercritical CO2,4 and as the stereoblock copolymer of it-PMMA and st-PMMA.5 As a membrane, the it-PMMA/st-PMMA stereocomplex could be considered to be a biomaterial.6−8 By skillfully exploiting this complexation ability, several novel materials have been invented, such as silica hybrids,9 fullerene assemblies,10 nanoto-microsized assemblies using core cross-linked star stPMMA11 and hollow capsules by layer-by-layer (LbL) assembly.12 Structural analyses of these stereocomplexes have been intensively studied. Although several approaches were attempted for it-PMMA/st-PMMA stereocomplex analyses, such as SEC,13 fluorescence,14 light scattering,15 infrared,16 and high-resolution AFM methods,17,18 XRD has been employed to clarify the structure of it-PMMA/st-PMMA19,20 and it-PMMA/ st-poly(methacrylic acid) (PMAA)21 stereocomplexes with stretched fibers. It is known that st-PMAA, which is a derivative of st-PMMA, forms a stereocomplex with it-PMMA,22 and an alternative LbL technique23 to deposit them onto a substrate results in an ultrathin film.24 More importantly, the different solubilities of it-PMMA and st-PMAA enable st-PMAA to be extracted from the stereocomplex film to prepare molecularly porous it-PMMA films (Figure 1a).25 Furthermore, stereospecific template polymerization with a radical method was used to develop porous it-PMMA films,26−29 although the template effect was limited in solution without any substrate.30,31 Due to such an attracting phenomenon, we recently focused on the structural analysis of the porous it-PMMA film on substrates by AFM32,33 and transmission X-ray diffraction (XRD) analyses. However, the available information was limited to wide-angle X-ray diffraction (WAXD).32,34 We expected that the stereocomplex would lie flat on the substrate as an LbL assembled film, which would lead to a suppression of thermal mobility as well as an anisotropic nature. © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis of Stereoregular Polymers. it-PMMA in this study was synthesized by anionic polymerization35 of mentyl methacrylate (Tokyo Chemical Industry, Japan) in toluene at −78 °C with tBuMgBr (mm:mr:rr = 96:2:2 Mn = 23 000, PDI = 1.21. mm:mr:rr = 94:4:2 Mn = 10 000, PDI = 1.31 was used for the bulk stereocomplex). st-PMAA in this study was synthesized by anionic polymerization36 of trimethylsilyl methacrylate (Aldrich Co. Ltd.) in toluene at −78 °C with t-BuLi/bis(2,6-di-tert-butylphenoxy)methylaluminum and the subsequent hydrolysis. The obtained PMAA was methylated by diazomethane to be characterized (mm:mr:rr = 1:5:94 Mn = 34 000, PDI = 1.45). LbL Assembled Porous it-PMMA Film. A silicon wafer was cut into square (1 cm × 1 cm), and it was immersed in an it-PMMA solution in acetonitrile (0.017 M) for 5 min at 25 °C and rinsed gently with acetonitrile two times. The substrate was immersed in a st-PMAA solution in acetonitrile/water (4:6 v/v; 0.017 M) for 5 min at 25 °C and rinsed similarly with acetonitrile/water (4:6 v/v). Alternate immersions were repeated for 16 steps (8 cycles) or 100 steps (50 cycles). The selective extraction of st-PMAA in a 10 mM NaOH aqueous solution for 5 min was performed to obtain the porous itPMMA film. Received: July 10, 2012 Revised: August 30, 2012 Published: September 14, 2012 7660

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Figure 1. Schematic illustration of stereocomplex: (a) the extraction of st-PMAA from it-PMMA/st-PMAA stereocomplex by the alkali treatment; (b) the bulk stereocomplex and the alkali treatment cause the disturbed structure; (c) the stereocomplex and alkali treatment on the substrate. Bulk it-PMMA Sample Prepared from Stereocomplex. 10 mg of it-PMMA and st-PMAA was separately dissolved in acetonitrile/ water (78:22 v/v, 18 mL), respectively. After mixing these solutions, it was placed for a few days at 25 °C. The precipitate was collected by decantation, and washed adequate amount of acetonitrile. The obtained bulk it-PMMA/st-PMAA stereocomplex (7.4 mg) was then extracted by dipping into 10 mM NaOH aqueous solution.37 These samples were used for transmission XRD, grazing-incidence WAXD (GIWAXD), and grazing-incidence small-angle X-ray diffraction (GISAXD) measurements. Spin-Coated Film of it-PMMA. 150 mg of it-PMMA was dissolved in 1 mL of acetonitrile, and a drop of 10 μL was used for coating. The it-PMMA solution was dropped into the glass plate with spin-coater with 3000 rpm for 30 s (Spincoater 1H-D7, Mikasa Co., Ltd., Japan). After the coating, the spin-coated sample was vacuumed under reduced pressure to remove acetonitrile overnight. The thickness of spin-coated film was confirmed by the observation of a laser scanning microscope (VK-9700, Keyence Co. Ltd., Japan) and set to the same value of thickness as that of LbL film. The spin-coated films were separately heated at the set temperature for 4 h before measuring XRD. Transmission XRD. Transmission XRD measurements were carried out at 25 °C using an imaging plate installed at a BL-40B2 beamline in SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan) for LbL assembled films and the bulk samples. The value of λ was equal to 0.1025 nm. A Rigaku RINT2000 was used for the spin-coated film measurements. Cu Kα (λ = 0.154 nm) was used as the X-ray source and operated at 40 kV and 200 mA with Ni filter (Rigaku ultra X18). Grazing-Incidence XRD. GIWAXD data were collected with time was 2 s per step, and the angular interval between steps 0.05° with an imaging plate installed at a BL-40B2 beamline in SPring-8. The data were collected as in plane. The flat panel for GIWAXD (camera length: 112 mm) and CCD camera for GISAXD (camera length: 780 mm) were installed for coincidence measurements. The CCD camera was set at 300 mm length in cases of the normal GIWAXD reflection collection of the LbL stereocomplex thin films.

on a substrate,26 so the presence of the substrate should be considered an important factor. In order to clarify the role of the substrate in supporting the porous film, we prepared compatible samples by extracting the st-PMAA from the bulk itPMMA/st-PMAA stereocomplex without any substrate (Figure 1b). Although a large number of it-PMMA/st-PMMA stereocomplex studies have been reported, little data about itPMMA/st-PMAA stereocomplexes are available. DMF and EtOH/water (83/17 vol %) have been used to prepare itPMMA/st-PMAA from solution.21,22 However, we selected acetonitrile/water to conform to the solvent conditions for the LbL assembly on the substrate.34 After preliminarily titrating the acetonitrile/water ratio and concentration, the it-PMMA was fully soluble in acetonitrile/water (78:22, v/v) at 0.56 mg/ mL, and st-PMAA was also soluble under the same conditions. Thus, the it-PMMA/st-PMAA stereocomplex was prepared by mixing the it-PMMA plus the st-PMAA solutions. The precipitates appeared after about 2 days of still standing (Figure 1b). For the first set of analyses, coincidence measurements of the GISAXD and GIWAXD of an it-PMMA/st-PMAA stereocomplex film on a silicon wafer were performed to survey where the peaks appeared for our original sample, an itPMMA/st-PMAA thin film prepared by the LbL approach. Figure 2 shows the results of the GISAXD for the it-PMMA/stPMAA stereocomplex film on a silicon wafer prepared by LbL. One hundred steps of LbL assembly gave a weak halo around 2 nm on the CCD camera, although no specific patterns were observed from the samples with 2 and 16 steps. This value

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RESULTS AND DISCUSSION Although the stereoselectivity of the template polymerization of methyl methacrylate in the presence of st-PMMA in DMF at 25 °C was achieved in a 90% isotactic triad, it has been limited to a lower than 4% conversion in solution.30,31 Recently, we prepared nanospaces by cross-linking helical st-PMAA after stereocomplexation, but the it-PMMA incorporation ability of the nanospace was unstable in a suspension state. Thus, it would be difficult to suppress the thermal mobility of polymers both in solution and in suspension.37 In contrast, it has succeeded in the stereospecific polymerization of an LbL film

Figure 2. GISAXD image of it-PMMA/st-PMAA stereocomplex on substrate obtained by the LbL assembly. 7661

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corresponded to that previously reported for it-PMMA/stPMAA,21 which was measured with stretched fibers of the stereocomplex. Together with the reported results from transmission XRD,34 it could be considered that the thin film deposited on a substrate by the alternative LbL assembly of itPMMA/st-PMAA possesses a similar structure as the bulk stereocomplex sample formed in solution. On the other hand, the GIWAXD which was performed at the same time over the region from 0.4 to 1 nm showed no images, probably due to the large amount of diffraction noise without the slit on the measurements. Moreover, the separately measured GISAXD with a 1600 mm camera length also resulted in no peaks, which showed that almost no other domains were formed in the region from 5 to 80 nm. Thus, we focused on measurements over the region from 0.5 to 2 nm using GIWAXD. Figure 3 shows in-plane GIWAXD profiles of the it-PMMA/ st-PMAA stereocomplex and the porous it-PMMA film. The

washing treatment of the bulk stereocomplex sample negated the three peaks (Figure 4b), while the same treatment on the LbL stereocomplex thin film resulted in the specific spectral pattern at q = 10.0 nm−1 (2θ = 13.7°, d = 0.63 nm) (Figure 3b).32 This finding validates the presence of the substrate and strongly supports the structure of the porous it-PMMA thin film, which was obtained by the extraction of st-PMAA from the it-PMMA/st-PMAA stereocomplex thin film by the LbL approach. Finally, the structural changes of it-PMMA chains in the porous it-PMMA film were investigated by heating to compare with the spin-coated it-PMMA film. We previously reported that the porous it-PMMA film prepared by the extraction of stPMAA from an it-PMMA/st-PMAA stereocomplex LbL film was stable at 70 °C for 4 h but allowed the it-PMMA crystallization at 90 °C. This can be readily confirmed by transmission XRD because the peak (2θ = 13.7°, d = 0.63 nm) changed into two characteristic peaks at 2θ = 8.6° and 14° (d = 1.00 and 0.62 nm), indicating that the porous it-PMMA structure changed into a double-helical structure following crystallization. Figure 5 shows the results from the transmission XRD patterns of spin-coated it-PMMA films. Compared to the

Figure 3. GIWAXD patterns of it-PMMA/st-PMAA stereocomplex thin film on substrate (a) and the alkali treatment (b).

crystalline diffraction peaks at q = 8.79 and 11.0 nm−1 (2θ = 12.0°, d = 0.72 nm; 2θ = 15.1°, d = 0.58 nm) (Figure 3a) corresponded to previous reports obtained by transmission XRD,22,34 and the porous it-PMMA LbL film has shown a specific pattern at q = 10.0 nm−1 (2θ = 13.7°, d = 0.63 nm) (Figure 3b). The results showed that the porosity develops similarly on the film surface as per the interior of the film, when the st-PMAA was extracted from the it-PMMA/st-PMAA stereocomplex. This is suported by the corresponding patterns with the different measurement approaches from transmission XRD34 and reflective GIWAXD. Next, we analyzed the bulk samples by transmission XRD as shown in Figure 4 in order to examine the substrate effect on

Figure 5. Transmission XRD patterns of the partially crystallized itPMMA (a), the spin-coated it-PMMA film on the glass plate at room temperature (b), and the it-PMMA films after heat process for 4 h at 50 (c), 70 (d), and 90 °C (e).

partially crystallized it-PMMA as a starting material with peaks at 2θ = 8.6° and 14° (d = 1.00 and 0.62 nm) (Figure 5a), the spin-coated it-PMMA on a glass substrate showed a broadened peak around (2θ = 13.7°, d = 0.63 nm), similar to that of the porous it-PMMA film. The XRD pattern was retained after the heating processes at 50 °C (Figure 5c) and at 70 °C (Figure 5d) for 4 h, similar to the previously reported porous it-PMMA film.38 Even at 90 °C (Figure 5e), the pattern looked almost the same as that before the heating process, with a slight emerging peak at 2θ = 8.6°. In contrast, two peaks appeared at 2θ = 8.6° and 14° for the porous it-PMMA film.38 This observation suggests that the polymer chains in the spin-coated it-PMMA film were not sufficiently mobile to be crystallized at 90 °C on the film, although the LbL assembled it-PMMA film obtained after the extraction of st-PMMA from the it-PMMA/st-PMAA stereocomplex film was crystallized, owing to its maintained

Figure 4. Transmission XRD patterns of the bulk it-PMMA/st-PMAA stereocomplex (a) and the alkali treatment sample (b).

the LbL-assembled film. The bulk stereocomplex sample from solution showed a q value at 2.56 nm−1 (2θ = 3.64°, d = 2.36 nm) (Figure 4a), which is assignable to the distance of the chain−chain lateral spacing by AFM.17 The crystalline diffraction peaks at q = 8.79 and 11.0 nm−1 (2θ = 12.0°, d = 0.72 nm; 2θ = 15.1°, d = 0.58 nm) also corresponded to the previous reports (Figure 4a).22,34 However, the alkaline 7662

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molecularly porous spaces at room temperature. This result implies that the polymer chains in the porous it-PMMA film on the substrate are bound more loosely than those in the spincoated it-PMMA film on the substrate as well as the bulk itPMMA sample. In conclusion, we demonstrated that the presence of the substrate was necessary to maintain the porous structure of itPMMA films, in comparison to a bulk it-PMMA sample prepared without any substrate. These results suggest that the substrate is necessary for the template polymerization of methacrylate in the presence of stereoregular polymers. The porosity which developed after st-PMAA extraction from the stereocomplex film on the substrate was homogeneously generated on the surface and inside the film, which were revealed by GIWAXD. It was also confirmed that the porous itPMMA film was easier to crystallize than the spin-coated itPMMA film during a heating process because the porosity would enable the polymer chains in the film easily to improve polymer−polymer interaction. These observations should serve as a basis for the application of porous it-PMMA film as a nanospace reaction field.

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AUTHOR INFORMATION

Corresponding Author

*Tel +81-6-6879-7356, Fax +81-6-6879-7359, e-mail akashi@ chem.eng.osaka-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the helpful discussion to Drs. T. Kida, M. Matsusaki, and T. Akagi. This work was partly supported by a Grant-in-Aid for Scientific Research (S) from the Ministry of Education, Culture, Sports, Science and Technology (23225004). This work was also supported in part by the MEXT project, “Creating Hybrid Organs of the Future”, at Osaka University.

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REFERENCES

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dx.doi.org/10.1021/ma301418u | Macromolecules 2012, 45, 7660−7663