Design of High-Density Helical Polymer Brush on Silica Nanoparticles

Jan 8, 2018 - Even after 5× repeating process, the selectivity for C70 molecules remains at 99%. The Supporting Information is available free of char...
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Letter Cite This: ACS Macro Lett. 2018, 7, 148−152

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Design of High-Density Helical Polymer Brush on Silica Nanoparticles for the Size Recognition of Fullerene Molecules Masanao Sato,† Tomoki Kato,† Hitoshi Shimamoto,† Kazutaka Kamitani,‡ Noboru Ohta,§ Tomoyasu Hirai,*,†,‡,∥ and Atsushi Takahara*,†,‡,∥ †

Graduate School of Engineering, ‡Institute for Materials Chemistry and Engineering, and ∥International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: High-density syndiotactic poly(methyl methacrylate) (st-PMMA) brushes form a helical structure and encapsulate fullerene molecules in their helical cavities, leading to a PMMA brush/fullerene inclusion complex. The brushes recognize the size of guest molecules and spontaneously adapt their helical diameter to the guest molecules. Both polymer brush/C60 and polymer brush/C70 inclusion complex on the flat substrate were characterized on the basis of grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements, and it is revealed that the main chains oriented perpendicular to the substrate. Moreover, high-density st-PMMA brushes grafted onto nanoparticles efficiently separate C70 molecules from the mixture of C60 and C70 solution. Even after 5× repeating process, the selectivity for C70 molecules remains at 99%.

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PMMA and fullerene. We recently reported the synthesis of stPMMA brushes with a high graft density based on surfaceinitiated living anionic polymerization and the encapsulation of fullerene molecules in the helical cavity of polymer brushes.4−6 If the st-PMMA brush recognizes the molecular size and its internal helical cavity spontaneously changes in size, a novel functional surface with a molecular recognition ability could form. Here, we report molecular-size recognition using the internal cavity of a helical polymer brush to selectively separate fullerene molecules by size (Figure 1a). st-PMMA exhibits a helical structure and encapsulates C60 to form an inclusion complex, leading to the formation of a gel.15 To evaluate the effect of the molecular weight (Mn) of stPMMA on the formation of an inclusion complex and the encapsulation of C60, st-PMMA with Mn of 240,000 and 32,000 designated st-PMMA240 and st-PMMA32, respectively, were dissolved in a C60/toluene solution and annealed at 383 K for 30 min. Figure 1b,c show photo images of st-PMMA240 and stPMMA32 in the C60/toluene solution before and after annealing followed by centrifugation.11 In the case of st-PMMA240, the solution became a gel, which precipitated after centrifugation, while st-PMMA32 did not show any change. According to a previous report, st-PMMA forming a helical structure aggregates and entangles, leading to a network structure. The network structure grows large enough to precipitate by centrifugation for large Mn. In contrast, the aggregation and entanglement in st-PMMAs with low Mn are not significant

olymer brushes consisting of end-tethered polymer chains have attracted considerable attention as a powerful method for surface modification because they provide various functionalities based on the primary structure of the polymer grafted onto a surface.1−6 Polymer brushes forming a secondary structure such as a helical structure might enable to encapsulate other molecules in the internal cylindrical nanocavity, leading to a wide range of applications such as catalysts for organic synthesis and separation materials. Polymer brushes with a helical structure were first prepared using polypeptides.7 The polypeptide brush forms the α-helix structure based on intramolecular hydrogen bonding. However, the internal cylindrical cavity of the α-helix structure was too small to encapsulate other molecules. Moreover, the molecular design of the polypeptide brush was extremely limited because of the difficulties in modifying the N-carboxy anhydride (NCA) monomer that was used for surface-initiated ring-opening anionic polymerization.7,8 Vinyl-type polymers such as poly(methyl methacrylate) (PMMA) can be induced to form a helical structure by controlling their stereoregularity. In the presence of an organic solvent, syndiotactic PMMA (st-PMMA) is well-known to form a helical structure with a few nanometer internal cavity and to encapsulate functional molecules inside the cavity. 9−13 Furthermore, the helical structure of st-PMMA shows good flexibility and adaptability for changing size, which enables the recognition of various types of molecules.9,11,14 Thus far, these specific properties of st-PMMA have been applied for the selective separation of fullerene molecules.14 However, to use of bulk state of st-PMMA as a separation material, a tedious process is required to isolate fullerene from the mixture of st© XXXX American Chemical Society

Received: November 16, 2017 Accepted: January 4, 2018

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DOI: 10.1021/acsmacrolett.7b00906 ACS Macro Lett. 2018, 7, 148−152

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ACS Macro Letters

separated using st-PMMA brushes with a lower Mn than that of the bulk form, if it encapsulates C60 in its helical nanocavity. stPMMA brushes on silica nanoparticles with a 200 nm diameter were prepared following a method in the literature.4 During the polymer brush preparation process, polymers not tethered to the solid substrate, that is, free polymer was generated. The primary structures of the polymer brushes and free polymer were confirmed to be the same.4 The primary structure of stPMMA brushes is summarized in Table 1. To evaluate the Table 1. Summary of Primary Structure and Encapsulation Amount of C60 Molecules of st-PMMA Brush Grafted on SiO2 Particle tacticityb Mna

sample

(g/mol)

PDIa

mm

mr

rr

encapsulation amountc (wt %)

st-PMMA10 st-PMMA23 st-PMMA32

10000 23000 32000

1.03 1.04 1.11

0 0 0

12 15 12

88 85 88

4.90 5.67 7.05

a

Determined by SEC. bDetermined by NMR. cDetermined by UV measurement.

encapsulation behavior of the polymer brushes, silica nanoparticles with polymer brushes were dispersed in a 0.4 mg/mL C60/toluene solution, heated at 383 K for 30 min, and cooled to room temperature. Here, the concentration of st-PMMA component was 0.5 mg/mL. Subsequently, the particles were separated from the solution by centrifugation. The resulting solution were evaluated using UV−vis spectroscopy. As shown in Figure 1e, the absorption peak associated with C60 decreased after using the polymer brushes. This indicated that the low Mn polymer brushes could encapsulate C60. The extent of C60 encapsulation inside the polymer brushes was evaluated using eq 1:

Figure 1. (a) Schematic illustration of selective encapsulation of C70 molecules in st-PMMA brush on a SiO2 particle. Photograph showing (b) bulk st-PMMA (Mn: 240000) and (c) bulk st-PMMA (Mn: 32000). UV spectra of C60 toluene solution and supernatant after removing (d) bulk st-PMMA and (e) st-PMMA brushes.

enough to form large network structures. Hence, gel formation only takes place in the st-PMMA with high Mn. It should be noted that the inclusion complex consisting of PMMA/C60 could not be isolated from toluene without gel formation for the bulk state of PMMA. To investigate the concentration of C60 in solution before and after centrifugation, ultraviolet− visible (UV−vis) spectroscopy were performed (Figure 1d). In the case of st-PMMA240, the absorption peak at 340 nm associated with C60 decreased compared with that of the untreated C60/toluene solution (0.4 mg/mL), indicating that C60 encapsulated in the helical structure of st-PMMA was separated from the solution. In contrast, the absorption peak of st-PMMAs with Mn ranging from 10000 to 32000, which could not form a gel, did not change. These results supported the notion that C60 encapsulated in the st-PMMA precipitated as the gel when st-PMMAs with high Mn were used. The gel formation is essential for the separation of fullerene molecules from solution. st-PMMA with high Mn can separate C60 from the solution, while st-PMMA with a low Mn in the bulk state cannot form a gel, resulting in difficult selective separation, which is a significant disadvantage. Moreover, when C60 is isolated from the inclusion complex, a tedious process is required which often included column chromatography.14 To overcome this problem, we propose using st-PMMA brushes immobilized on silica nanoparticles to separate C60. The st-PMMA brushes prepared on the silica nanoparticles can be collected by centrifugation process without the gelation, because the polymer chain end is tethered to the silica particles that are not soluble in common organic solvents. In the case of the st-PMMA brushes, only the formation of the PMMA brush/ C60 inclusion complex is needed to separate C60 from the C60/ toluene solution. Hence, it is anticipated that fullerenes can be

W = (1 − A /A 0) × W0

(1)

where A0 and W0 are the intensity of the absorption peak at λmax and the initial mass of C60 in the C60/toluene solution, respectively, and A and W are the absorption peak intensity at λmax after centrifugation and the mass of C60 encapsulated in the polymer brushes. C60 molecules were clearly encapsulated in the st-PMMA brushes ranging from 4.90 to 7.05 wt %, and this amount increased with the increasing molecular weight of stPMMA brushes (Table 1). Although PMMA brushes with low molecular weights such as 10000, they could encapsulate fullerene molecules and could separate C60 from the solution. The values are smaller than that in st-PMMA bulk with high Mn. The values are closely related to a Mn of st-PMMA (Figure S2). The Mn of st-PMMA brushes is low, leading to a small accommodation. To evaluate molecular aggregation structure in the polymer brush encapsulating the fullerene molecules in their helical structure, grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements on a silicon wafer consisting of a flat and smooth substrate are necessary. st-PMMA400 brushes (Mn: 400000, polydispersity index (PDI): 1.31) were prepared on a silicon wafer according to a previously reported method.4 st-PMMA400 brushes were immersed in a 2 mg/mL C60 or C70 toluene solution and heated at 383 K for 30 min. Figure 2a−c show the GIWAXD patterns and one-dimensional (1D) line profiles along the equatorial direction of the st-PMMA400/C60 and st-PMMA400/C70 brushes, respectively. st-PMMA400/C60 149

DOI: 10.1021/acsmacrolett.7b00906 ACS Macro Lett. 2018, 7, 148−152

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ACS Macro Letters

The crystallinity of the st-PMMA32/C60 and st-PMMA32/C70 brushes on silica nanoparticles was evaluated using differential scanning calorimetry. The st-PMMA32/C60 and st-PMMA32/C70 brushes showed endothermic peaks corresponding to the melting temperature (Tm) of the inclusion complex (Figure S1). The enthalpy changes of the inclusion complexes of stPMMA32/C60 and st-PMMA32/C70 brushes were 6.80 and 0.04 J/g, respectively. This result also suggested that crystallinity of the st-PMMA/C70 brushes was less crystalline than the stPMMA/C60 brushes. In general, the melting temperature of the inclusion complex of st-PMMA/C60 could be observed around 220 °C. Whereas, the melting temperature of the polymer brushes consisting of st-PMMA/C60 and st-PMMA/C70 was observed at 237 and 245 °C, respectively. This can be attributed to the restriction of molecular motion in polymer brush on the solid substrate.17−19 The peak position associated with long-period structure of the inclusion complex shifted from q = 3.35 nm−1 (st-PMMA/ C60, d = 1.87 nm) to q = 3.23 nm−1(st-PMMA/C70, d = 1.94 nm) with the increasing fullerene size.15 Figure 2d, e, and f display a schematic illustration of a st-PMMA/C60 brush and the crystal structure of the st-PMMA/C60 and st-PMMA/C70 brushes, respectively. Kusuyama et al. reported that st-PMMA formed an inclusion complex under various solvent vapor conditions, and the diameter of the st-PMMA helix depended on the encapsulated molecules.9 Moreover, these kinds of phenomenon could be seen in bulk st-PMMA/fullerene system and it was concluded that bulk st-PMMA recognize the size of guest molecules and size of nanocavity changes dependent on the size.14,15 Taking these into account, it is clear that the peak shift of the inclusion complex was caused by the expansion of the helical diameter of the st-PMMA brush when the larger fullerene was encapsulated in the helical structure. These results indicate that st-PMMA brushes recognize the molecular size of guest molecules, and the size of the nanocavity changes depending on the guest molecule’s size. To determine the selectivity and the amount of encapsulated fullerene molecules in the polymer brush, UV−vis spectroscopy was performed (Figure 3a).14 The st-PMMA32 brushes were dispersed in a C60/C70 mixture toluene (0.6 mg/mL, 1/1 wt %) solution. The concentration of st-PMMA was 0.5 mg/mL. The solution was heated at 383 K for 30 min and cooled to room temperature. Subsequently, the solution was centrifuged, and

Figure 2. GIWAXD patterns of st-PMMA brushes prepared on a silicon wafer, including (a) C60 and (b) C70 molecules. (c) GIWAXD 1D line profiles of st-PMMA/C60 and st-PMMA/C70 brushes. (d) Schematic illustration of the st-PMMA/C60 brush. Model of the crystal structures of (e) st-PMMA/C60 and (f) st-PMMA/C70 brush.

brushes showed diffraction peaks at q = 3.35 nm−1 along the equatorial axis and at q = 7.79 nm−1 along the meridian axis. The former peak is associated with the long-period structure of the st-PMMA/fullerene inclusion complex, and the latter is associated with the helical pitch of st-PMMA.10,16 These results indicated that the st-PMMA400 brushes encapsulated C60 and formed inclusion complexes that were oriented vertically to the substrate but the packing lacks ordering. In the case of stPMMA400/C70, the GIWAXD pattern also showed a diffraction peak at q = 3.23 nm−1 along the equatorial axis. Clearly, the stPMMA400 brushes also formed inclusion complexes with C70, which were oriented vertically to the substrate. st-PMMA400/ C70 brushes did not show the diffraction peak at q = 7.79 nm−1, which is associated with the helical pitch of st-PMMA brushes. According to a previous report, st-PMMA/C70 is less crystalline than st-PMMA/C60,16 which implies that the asymmetric nature of C70 caused by disordered alignment of C70 molecules in stPMMA helix reduces the crystallinity of the inclusion complex.

Figure 3. (a) UV spectra of C60/C70 (0.6 mg/mL, 1/1 wt %) in toluene (solid line), free fullerene in the supernatant (dashed line) and fullerene extracted from st-PMMA brushes (red line). (b) Magnified UV spectra of extracted fullerene. UV spectra of C60 and C70 was used as a control. MALDI-TOF-MS spectra of (c) C60/C70 mixture (1/1 wt %), (d) free fullerene in the supernatant, and (e) fullerene extracted from st-PMMA brushes. 150

DOI: 10.1021/acsmacrolett.7b00906 ACS Macro Lett. 2018, 7, 148−152

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ACS Macro Letters the supernatant containing the free fullerene was removed by decantation. The amount of free fullerene in the supernatant was determined by UV−vis spectroscopy. Meanwhile, the stPMMA brushes were washed with toluene, which was subsequently removed by centrifugation followed by decantation. This process was repeated three times. Then, the fullerene encapsulated in the st-PMMA brushes was extracted using o-dichloro benzene, and centrifugation was performed to remove the st-PMMA brushes. Here, the st-PMMA brush also has advantage that when fullerenes are isolated from the inclusion complex, PMMA/C60, the polymer blush is not soluble in o-dichloro benzene. Therefore, the fullerene can be easily selectively separated from the inclusion complex. The amount of extracted fullerene was evaluated using UV−vis spectroscopy. Figure 3a shows the UV spectra of the C60/C70 mixture, free fullerene in the supernatant, and fullerene extracted from st-PMMA brush. In the case of the supernatant, the absorption peak at 380 nm, which is associated with C70, was lower than that of the C60/C70 mixture. Figure 3b shows the magnified spectrum of the extracted fullerene. The UV spectra of C60 and C70 are included for reference. The UV spectrum of the fullerene extracted from st-PMMA brushes was in good agreement with that of C70. This result implies that C70 molecules were selectively separated from the C60/C70 mixture using st-PMMA brushes. To confirm the selectivity of C70 separation, matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF-MS) was performed on the C60/C70 mixture (1/1 wt %), free fullerene, and extracted fullerene (Figure 3c−e). The MALDI-TOF-MS spectrum of the C60/C70 mixture (1/1 wt %) showed two peaks for C60 and C70 (m/z = 720 (C60 − M+), 840 (C70 − M+)) with a 1:1 ratio, while the spectrum of fullerene extracted from the st-PMMA brushes showed only the C70 peak. The selectivity for C70 molecules estimated from the integral intensity of the C60 and C70 peaks was 99%. This result strongly supported that selective separation of C70 was achieved using the helical st-PMMA brushes. The amount of C70 encapsulated in the st-PMMA brushes was estimated using eq 1 to be 5.1 wt %. This selectivity could be seen in bulk state of st-PMMA and the mechanism has been proposed.14 The diameter of C70 is larger than that of C60. The size of the helical diameter in st-PMMA flexibly changes depending on the encapsulated fullerene size. Taking the results into account, the formation of larger diameter is more thermodynamically stable. When C60 and C70 were encapsulated in the PMMA helical structure, the helical structure adapted to a size suitable for C70. During this process, C60 which is smaller, is likely replaced from the helical nanocavity, leading to selective separation. To investigate the reusability of the st-PMMA brushes, stPMMA brushes that were employed to separate the different size fullerenes were reintroduced into a C60/C70 mixture, and C70 was selectively separated using the same procedure. This procedure was repeated four times (Figure 4a−d). The MALDI-TOF-MS spectra exhibited only the peak corresponding to C70. Figure 4e shows the variation in the C70 content as a function of the number of cycles. Even after Cycle 5, the selectivity for C70 molecules remained at 99%. Moreover, the amount of C70 molecules encapsulated in the st-PMMA brushes during this process was almost constant. The highly selective separation of C70 through 5 cycle can only be archived using the st-PMMA brush. This approach is expected to be used for novel size separation materials.

Figure 4. MALDI-TOF-MS spectra of fullerene extracted from stPMMA32 brush at (a) Cycle 2, (b) Cycle 3, (c) Cycle 4, and (d) Cycle 5. (e) C70 content of all cycles calculated from the intensity of MALDI-TOF-MS spectra.

In conclusion, st-PMMA brushes recognized the size of fullerene molecules and adopted their structure to encapsulate guest molecules. Although the gelation of st-PMMA/fullerene inclusion complex is necessary to separate the encapsulated fullerene from free fullerene in the case of bulk st-PMMA, stPMMA/fullerene brushes immobilized on silica nanoparticles could be separated without gelation, and they were easily collected after a simple separation process. Moreover, the stPMMA brushes immobilized on nanoparticles could be recycled several times, maintaining their C70 separation performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00906. Synthesis of PMMA brush on the substrates and selective separation of fullerene (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +81-92-802-2516. Fax: +81-92-802-2518. E-mail: [email protected]. *Tel.: +81-92-802-2517. Fax: +81-92-802-2518. E-mail: [email protected]. ORCID

Tomoyasu Hirai: 0000-0002-6441-5163 Atsushi Takahara: 0000-0002-0584-1525 Notes

The authors declare no competing financial interest. 151

DOI: 10.1021/acsmacrolett.7b00906 ACS Macro Lett. 2018, 7, 148−152

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(15) Kawauchi, T.; Kumaki, J.; Kitaura, A.; Okoshi, K.; Kusanagi, H.; Kobayashi, K.; Sugai, T.; Shinohara, H.; Yashima, E. Encapsulation of Fullerenes in a Helical PMMA Cavity Leading to a Robust Processable Complex with a Macromolecular Helicity Memory. Angew. Chem., Int. Ed. 2008, 47, 515−519. (16) Kincer, M. R.; Choudhury, R.; Srinivasarao, M.; Beckham, H. W.; Briggs, G. A. D.; Porfyrakis, K.; Bucknall, D. G. Shear alignment of fullerenes in nanotubular supramolecular complexes. Polymer 2015, 56, 516−522. (17) Yamamoto, S.; Tsujii, Y.; Fukuda, T. Glass Transition Temperatures of High-Density Poly(methyl methacrylate) Brushes. Macromolecules 2002, 35, 6077−6079. (18) Savin, D. A.; Pyun, J.; Patterson, G. D.; Kowalewski, T.; Matyjaszewski, K. Synthesis and characterization of silica-graftpolystyrene hybrid nanoparticles: Effect of constraint on the glasstransition temperature of spherical polymer brushes. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2667−2676. (19) Tanaka, K.; Kojio, K.; Kimura, R.; Takahara, A.; Kajiyama, T. Surface relaxation processes of poly(methyl methacrylate) brushes prepared by atom transfer radical polymerization. Polym. J. 2003, 35, 44−49.

ACKNOWLEDGMENTS This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We also acknowledge support from the World Premier International Research Center Initiative (WPI) MEXT, Japan and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. Part of this work was supported by the Impulsing Paradigm Change through Disruptive Technologies (ImPACT) Program. The Xray diffraction measurements were performed at the BL02B2 and BL40B2 beamlines of SPring-8 under proposal numbers 2013B1171, 2014A1228, 2014A1222, and 2014B1286.

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REFERENCES

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DOI: 10.1021/acsmacrolett.7b00906 ACS Macro Lett. 2018, 7, 148−152