Self-Assembly of Hierarchical Structures Using Cyclotriphosphazene

Dec 15, 2017 - Since the synthesis and properties of poly(substituted methylene) BCPs have yet to be fully investigated, to develop a novel class of B...
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Letter Cite This: ACS Macro Lett. 2018, 7, 37−41

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Self-Assembly of Hierarchical Structures Using CyclotriphosphazeneContaining Poly(substituted methylene) Block Copolymers Fuminobu Kato,†,∥ Alvin Chandra,†,∥ Masatoshi Tokita,† Hironori Asano,‡ Hiroaki Shimomoto,‡ Eiji Ihara,*,‡ and Teruaki Hayakawa*,†,§ †

Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-S8-36 O-okayama, Meguro-ku, Tokyo 152-0033, Japan ‡ Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, 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: The cyclotriphosphazene-substituted diazoacetate homopolymer (polyPNDA′) (PNDA′ = hexaphenoxysubstituted phosphazene-containing methylene) and a novel poly(substituted methylene) block copolymer, polyPNDA′block-poly(hexyloxycarbonylmethylene) (polyPNDA’-b-polyHDA′), were synthesized, and the self-assembly behavior of these polymers was studied in detail. A hexagonally packed aggregated structure was observed in the self-assembled structure of polyPNDA′, whereas a lamellar structure was observed in the microphase-separated nanoassembly of polyPNDA′-b-polyHDA′. These results indicate that a hierarchical structure composed of highly regular polyPNDA′ nanoaggregates and the long-range microphase-separated polyPNDA′ and polyHDA′ domains had formed.

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Scheme 1. Substitution Patterns of Polymers Synthesized via Vinyl Polymerization and Poly(substituted methylene) Synthesis (PSMS)

inyl polymerizations represent one of the most important methods for synthesizing polymers consisting of a carbon−carbon (C−C) main chain.1−5 Furthermore, polymers with “living” characteristics, i.e., linearly increasing molecular weights, low polydispersities, and highly controllable tacticity, can be synthesized via well-studied methods such as anionic polymerization,1 cationic polymerization,2 controlled radical polymerizations,3,4 coordination polymerizations,5 etc. However, despite significant advances in such polymerization techniques, there remains numerous limitations faced by vinyl polymerizations. One such setback is that the distribution of substituents along the main chain is relatively fixed and that substituents can only be introduced onto every other carbon on the main chain. One highly promising method for further controllability of the substituent distribution is poly(substituted methylene) synthesis (PSMS) (Scheme 1). Although typical vinyl polymerizations use 1,1-disubstituted or monosubstituted vinyl monomers to synthesize a polymer with a C−C main chain that contains substituents located on every other carbon atom, polymers synthesized via PSMS yield densely substituted polymers with side chains located at every carbon along the C−C main chain. Liu et al.6 and Ihara et al.7 first succeeded in using the PSMS method to synthesize poly(substituted methylene)s from diazoacetates with Cu and PdCl2 catalysts, respectively. However, the number-average molecular weights © XXXX American Chemical Society

(Mn’s) of the resulting poly(alkoxycarbonylmethylenes)s were only ca. 1−3 kg mol−1. On the other hand, de Bruin et al. could synthesize poly(ethoxycarbonylmethylene) from ethyl diazoReceived: October 5, 2017 Accepted: December 4, 2017

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DOI: 10.1021/acsmacrolett.7b00789 ACS Macro Lett. 2018, 7, 37−41

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ACS Macro Letters acetate with Mn’s around 100−200 kg mol−1 using Rh-complex catalysts,8 which also resulted in a highly stereoselective polymerization. Likewise, an (NHC)Pd/borate (NHC = Nheterocyclic carbene) initiating system was found to afford relatively high Mn (around 20−30 kg mol−1) polymers with a nonstereoselective propagation.9,10 Meanwhile, another Pdbased initiating system effective for the polymerization of diazoacetates, specifically a (η3-C3H5)-PdCl-based system,11−14 was reported to promote controlled polymerizations of diazoacetates with a sterically bulky ester substituent. Indeed, Ihara et al. demonstrated that by using the Pd-based initiating system the polymerization of diazoacetate monomers bearing a bulky cyclotriphosphazene moiety successfully yielded products with narrow molecular weight distributions (∼1.15) and welldefined block copolymers. Realization of the controlled polymerization of cyclotriphosphazene-containing diazoacetates is quite significant in terms of the development of novel polymeric materials. This is because cyclotriphosphazenes15−18 have been shown to exhibit unusual thermal, chemical, and dielectric properties such as flame retardancy and self-extinguishability. Furthermore, the densely packed arrangement of the cyclotriphosphazene moieties along the C−C main chain is expected to enhance the above-mentioned characteristics. Indeed, the poly(hexaphenoxy-substituted cyclotriphosphazene-containing methylene) (polyPNDA′, Scheme 2a) exhibited a higher degradation temperature (Td5 = 347 °C) than the analogous vinyl polymerized polymer (Td5 = 321 °C).11

properties such as enantioselective permeation, selective metal ion capturing, and lithium ion conductivity,19 the nano-order self-assembled structures of well-defined cyclotriphosphazenecontaining poly(substituted methylene) BCPs hold great potential in advanced engineering applications. Therefore, in this article, we synthesized a novel poly(substituted methylene) block copolymer with high thermal degradation properties for the development of regular nanostructures of cyclotriphosphazene-containing block copolymers (BCPs). The cyclotriphosp h a z e n e -c o n t a i n i n g B C P , p o l y P N D A ′- b l o c k -p ol y(hexyloxycarbonylmethylene) (polyPNDA′-b-polyHDA′, Figure 1b), was synthesized via PSMS, and the hierarchical selfassembled nanostructure formed was characterized in detail.

Scheme 2. Reaction Scheme of (a) the Homopolymer, polyPNDA′, and (b) the Block Copolymer, polyPNDA′-bpolyHDA′ Figure 1. (a) Polarized optical microscopy (POM) images, (b) differential scanning calorimetry (DSC) curves, and (c) themogravimetric analysis (TGA) curve of polyPNDA′-b-polyHDA′.

The cyclotriphosphazene-containing diazoacetate monomer for the polymerization of polyPNDA′ was synthesized using hexachlorocyclotriphosphazene as the raw material in a fourstep transformation (Scheme S1).11 Subsequently, the PSMS of the cyclotriphosphazene-containing diazoacetate monomer (PNDA) was carried out utilizing a (η3-C3H5)-PdCl-based initiating system to yield a polymer with a narrow molecular weight distribution. The details of the polymerization reaction mechanism can be found in these references.11−14 The polymerization of PNDA using (η3-C3H5)PdCl/NaBPh4 in tetrahydrofuran (THF) was conducted at −20 °C to yield a polymer with Mn = 13.7 kg mol−1 and a yield of 66%. The block copolymerization of PNDA and n-hexyl diazoacetate (HDA) was carried out through the sequential addition of the comonomers in THF at −20 °C. The resulting block copolymer had an Mn = 19.3 kg mol−1 and a yield of 67%. The molecular weights were determined by gel permeation chromatography (GPC) using THF as the eluent and poly(methyl methacrylate) standards. 1H nuclear magnetic resonance (NMR) analysis showed that polyPNDA′-b-polyHDA′ had a weight ratio of 83:17 PNDA′:HDA′. The reaction scheme of the polymers synthesized in this study is shown in Scheme 2. Bulk samples of polyPNDA′ and polyPNDA′-bpolyHDA′ were prepared by allowing a chloroform solution to evaporate at rt for several days. The resulting bulk samples were annealed at 100 °C for 12 h prior to characterization.

With this background in mind, we reasonably anticipated that the precise synthesis of well-defined cyclotriphosphazenecontaining poly(substituted methylene)s will lead to the development of a variety of unique polymeric materials. Since the synthesis and properties of poly(substituted methylene) BCPs have yet to be fully investigated, to develop a novel class of BCPs, the synthesis and fundamental higher-ordered selfassembly behavior of a cyclotriphosphazene-containing poly(substituted methylene) BCP was attempted. In addition, as the cyclotriphosphazene side chains were reported to show 38

DOI: 10.1021/acsmacrolett.7b00789 ACS Macro Lett. 2018, 7, 37−41

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ACS Macro Letters Additionally, fiber samples were prepared by melting the bulk samples at 150 °C and anisotropically stretched. To investigate the thermal properties of polyPNDA′-bpolyHDA′, polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimetric analyses (TGA) were conducted. The optical textures of the ordered nanostructures of polyPNDA′-b-polyHDA′ were observed during a heating cycle from room temperature (rt) to 300 °C on a POM (Figure 1(a)). The POM images indicate that optically anisotropic structures had formed and remain stable at 300 °C. Dark-field images were obtained at 335 °C. Only a single baseline shift could be observed in the heating and cooling cycles of the DSC analysis of polyPNDA′-b-polyHDA′. A DSC measurement was first carried out using the polyHDA′ homopolymer from −80 to 200 °C; however, a baseline shift corresponding to the glass transition temperature (Tg) of polyHDA′ could not be observed. As polyHDA′ is a viscous liquid at room temperature, it is likely that the Tg is below −80 °C and could not be detected in the DSC analysis. In addition, a single baseline shift corresponding to a Tg of 27 °C was observed in the second heating cycle of the DSC analysis of the polyPNDA′ homopolymer. Meanwhile, in the DSC analysis of the BCP carried out at −50 to 200 °C, a single baseline shift was observed. Since the baseline shift corresponds to a Tg of 26 °C, it is likely the Tg of the polyPNDA′ (Figure 1(b)) segment of the BCP. Furthermore, TGA analysis revealed a two-step weight loss of about 40% and 20% of the original weight, respectively, and yielded a final residue that corresponded to approximately 40% of the original weight (Figure 1(c)). The initial weight loss of 40% could likely be attributed to the degradation of the main polymer chain which contributes to ca. 34 wt % of the BCP, whereas the second weight loss of 20% is resultant of the partial decomposition of the phenyl groups on the phosphazene moieties (43 wt %). The final residue (40 wt %) that remains even at high temperatures likely corresponds to the thermally stable phosphazene moieties that have partially cross-linked structures.18,20 Thus, the expected weight loss from the degradation of the main chain and cyclotriphosphazene moiety is in good agreement with the weight loss shown in the TGA analysis. These thermal analyses suggest that an optically anisotropic, ordered structure that is stable at higher temperatures (up to ca. 300 °C) had formed in polyPNDA′-bpolyHDA′. To determine if the optically anisotropic structures had formed because of the microphase separation and self-assembly of the BCP, wide-angle X-ray diffraction (WAXD) analyses were conducted on the bulk and fiber samples of both the homopolymer, polyPNDA′, and the BCP, polyPNDA′-bpolyHDA′. In the 2-dimensional WAXD pattern of the bulk samples, ring-shaped isotropic diffraction signals could be detected. However, anisotropic diffraction patterns corresponding to 25 Å, 14 Å, 9.4 Å, and 4.4 Å could be observed in the fiber axis direction of the 2-dimensional WAXD analysis of the fiber samples of both the homopolymer and the BCP (Figure 2(a) and (b)). Therefore, since the peaks could be observed in the WAXD profiles of both the homopolymer and the BCP, the optically anisotropic structures are likely derived from polyPNDA′. In particular, the peaks at 25 Å, 14 Å, and 9.4 Å correspond to q value ratios of 1:√3:√7 in the small-angle region (Figure 2(c) and (d)). On the other hand, the broad peaks corresponding to 4.4 Å in the wide-angle region are a result of isotropic scattering. These results suggest that an ordered aggregated structure with a hexagonal packing and an

Figure 2. Two-dimensional wide-angle X-ray diffraction (WAXD) pattern of (a) polyPNDA′ and (b) polyPNDA′-b-polyHDA′. The arrows indicate the fiber axis, and the fan shape represents the integrated range of the one-dimensional patterns of (c) polyPNDA′ and (d) polyPNDA′-b-polyHDA′.

intercenter distance of 29 Å had formed within the polyPNDA′ domain, as illustrated in Figure 3. The distance of 29 Å between

Figure 3. Schematic illustration of the ordered and aggregated nanoassemblies of polyPNDA′ within the homopolymer and the block copolymer polyPNDA′-b-polyHDA′.

neighboring backbones is smaller than twice the length of a side chain (17 Å) in the most extended conformation, suggesting that the phenyl moieties are overlapping with each other at the end of the side chains. Hirai et al. had reported on the formation of similar hexagonally packed structures in rod−coiltype organic−inorganic block copolymers which contained polyhedral oligomeric silsesquioxane (POSS) moieties.21 The bulky, rigid, and well-defined cage structures of POSS spontaneously assemble around the flexible C−C main chain of the polymer to produce a helical staircase-like structure with the POSS moieties arranged in a spiral along the stretched main chain. This stretched main chain produces a rod-like structure which then packs in a hexagonal arrangement. In the case of polyPNDA′, it is highly likely that a similar aggregated rod-like structure had formed due to the arrangement of the bulky cyclotriphosphazene moiety around the polymer main chain. As the bulky side chains coil around the main chain to avoid 39

DOI: 10.1021/acsmacrolett.7b00789 ACS Macro Lett. 2018, 7, 37−41

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ACS Macro Letters

polyPNDA′ structure formed by the stacking of the rigid cyclotriphosphazene moieties tend to aggregate into flat hexagonally packed layers. The flexible polyHDA′ block, on the other hand, simply fills the space between the layers of polyPNDA′, leading to the high stability of the lamellar phase even at asymmetric volume fractions. In conclusion, the poly(substituted methylene) synthesis of the cyclotriphosphazene-containing homopolymer and diblock copolymer, polyPNDA′ and polyPNDA′-b-polyHDA′, had been successfully synthesized. Thermal (i.e., POM, DSC, TGA) and wide-angle X-ray diffraction (WAXD) analyses revealed that a hierarchically ordered, aggregated structure had formed within the polyPNDA′ domain which then develops a hexagonally packed arrangement with an intercenter distance of 29 Å. Furthermore, small-angle X-ray scattering (SAXS) and scanning transmission electron microscopy (STEM) analyses revealed that a higher-ordered lamellar structure had been formed by the self-assembly of the microphase-separated polyPNDA′-b-polyHDA′ diblock copolymer. Therefore, these results indicate that a hierarchical structure had formed in polyPNDA′-b-polyHDA′: the rod-like structures formed by the stacking of polyPNDA′, the hexagonal packing of the polyPNDA′ columnar rods into domains that form flat layers, and the alternating lamellar structure derived from the microphase-separated polyPNDA′ and polyHDA′ domains of the BCP. The newly synthesized poly(substituted methylene) BCP and the detailed characterization of its self-assembly behavior represent an important first step in the development of a novel class of poly(substituted methylene) BCPs that can be expected to show improved properties over vinyl polymers for highly advanced engineering applications.

contact with each other, the main chain gets stretched, resulting in a columnar rod-like polyPNDA′ structure. Further analysis of the self-assembled polyPNDA′-b-polyHDA′ fiber and bulk samples was carried out using small-angle X-ray scattering (SAXS) and scanning transmission electron microscopy (STEM), respectively. SAXS analysis of the fiber sample revealed peaks with q ratios of 1:2, and a d-spacing of 17.8 nm could be observed (Figure 4(a)). An anisotropic

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Figure 4. (a) Small-angle X-ray scattering (SAXS) profile and (b) twodimensional SAXS profile. The arrow represents the fiber axis. (c) Scanning transmission electron microscopy (STEM) image and (d) schematic illustration of polyPNDA′-b-polyHDA′ self-assembled nanostructures.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00789. Materials, polymerization procedures, characterization, and Figures S1−S3 (PDF)

scattering pattern could be observed in the 2-dimensional (2D) SAXS pattern, showing good agreement with our earlier WAXD results. Furthermore, line-like structures with low long-range ordering could be observed in the STEM images of polyPNDA′-b-polyHDA′. As regions with increased electron densities appear darker in STEM images, it is likely that the darker region corresponds to the polyPNDA′ domain, whereas the lighter regions correspond to the polyHDA′ domain. According to the SAXS and STEM results, a lamellar structure had formed in the microphase-separated nanostructure of the block copolymer, polyPNDA′-b-polyHDA′. Polymers which contain rigid side chains fall under a category of polymers known as rod−coil block copolymers, and unlike typical coil−coil block copolymers which consist of polymers behaving as flexible coils, the self-assembly behavior of rigid rod-flexible coil block copolymers is starkly different, resulting in highly asymmetric phase diagrams.21−29 Although the weight fraction of the polyHDA′ domain was extremely low (17 wt %), the lamellar morphology continues to remain as the most thermodynamically stable nanostructure. On the other hand, typical coil−coil block copolymers tend to go through an order-to-order phase transition from lamellae to gyroids or cylinders at approximately 37−41 vol %.25 However, since polyPNDA′-bpolyHDA′ exhibits lamellar morphologies even at 83 wt % polyPNDA′, these results indicate that the stretched, rod-like

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Masatoshi Tokita: 0000-0002-4534-7337 Eiji Ihara: 0000-0002-0279-5105 Teruaki Hayakawa: 0000-0002-1704-5841 Author Contributions ∥

F.K. and A.C. authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Ryohei Kikuchi of the Tokyo Institute of Technology Center for Ascended Materials Analysis for providing the STEM measurements. This work was funded by the Japan Science and Technology Agency (JST), the Precursory Research for Embryonic Science and Technology (PRESTO) on the Molecular Technology and Creation of New Functions, a Grant-in-Aid for Scientific Research (B) (JSPS 40

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(14) Ihara, E.; Akazawa, M.; Itoh, T.; Fujii, M.; Yamashita, K.; Inoue, K.; Itoh, T.; Shimomoto, H. π-AlplylPdCl-Based Initiating Systems for Polymerization of Alkyl Diazoacetates: Initiation and Termination Mechanism Based on Analysis of Polymer Chain End Structures. Macromolecules 2012, 45 (17), 6869−6877. (15) Inoue, K.; Nakano, M.; Takagi, M.; Tanigaki, T. Radical Polymerization of Vinyl Monomers Containing Cyclotriphosphazene and Thermal Behavior of Their Polymers. Macromolecules 1989, 22 (4), 1530−1533. (16) Brown, D. E; Ramachandran, K.; Carter, K. R.; Allen, C. W. Poly[(vinyloxy)cyclophosphazenes]. Macromolecules 2001, 34, 2870− 2875. (17) Allen, C. W. Electronic Structure and Reaction Mechanism as Guides to the Design of Hybrid Inorganic-Organic Polymers. Inorg. Chim. Acta 2011, 372 (1), 32−36. (18) Allcock, H. R.; Hartle, T. J.; Taylor, J. P.; Sunderland, N. J. Organic Polymers with Cyclophosphazene Side Groups: Influence of the Phosphazene on Physical Properties and Thermolysis. Macromolecules 2001, 34 (12), 3896−3904. (19) Inoue, K.; Itaya, T. Synthesis and Functionality of Cyclophosphazene-Based Polymers. Bull. Chem. Soc. Jpn. 2001, 74, 1381− 1395. (20) Haw, J. F.; Maynard, S. J.; Sharp, T. R. Thermal Degradation Chemistry of Poly(diphenoxyphosphazene). Macromolecules 1991, 24 (10), 2794−2799. (21) Hirai, T.; Leolukman, M.; Jin, S.; Goseki, R.; Ishida, Y.; Kakimoto, M. A.; Hayakawa, T.; Ree, M.; Gopalan, P. Hierarchical Self-Assembled Structures from POSS-Containing Block Copolymers Synthesized by Living Anionic Polymerization. Macromolecules 2009, 42 (22), 8835−8843. (22) Olsen, B. D.; Segalman, R. A. Materials Science and Engineering R. Mater. Sci. Eng., R 2008, 62, 37−66. (23) Dolezel, S.; Behringer, H.; Schmid, F. A Model for Rod-Coil Block Copolymers. Polym. Sci., Ser. C 2013, 55 (1), 70−73. (24) Müller, M.; Schick, M. Ordered Phases in Rod−Coil Diblock Copolymers. Macromolecules 1996, 29 (27), 8900−8903. (25) Bates, F. S.; Fredrickson, G. H. Block CopolymersDesigner Soft Materials. Phys. Today 1999, 52 (2), 32. (26) Cochran, E. W.; Garcia-Cervera, C. J.; Fredrickson, G. H. Stability of the Gyroid Phase in Diblock Copolymers at Strong Segregation. Macromolecules 2006, 39 (7), 2449−2451. (27) Kato, F.; Chandra, A.; Horiuchi, S.; Hayakawa, T. Morphological Dependence on the Addition of a Soft Middle Block Segment to Rigid POSS-Containing Triblock Copolymers for Forming Cylindrical Nanostructures. RSC Adv. 2016, 6, 62172−62180. (28) Kato, F.; Sugimoto, S.; Chandra, A.; Hayakawa, T. Morphology Control and Inducing a Curvature on the Domain Interface by Increasing the Steric Bulk of Polymethacrylate in POSS-Containing Diblock Copolymers. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (13), 2234−2242. (29) Nakatani, R.; Takano, H.; Chandra, A.; Yoshimura, Y.; Wang, L.; Suzuki, Y.; Tanaka, Y.; Maeda, R.; Kihara, N.; Minegishi, S.; Miyagi, K.; Kasahara, Y.; Sato, H.; Seino, Y.; Azuma, T.; Yokoyama, H.; Ober, C. K.; Hayakawa, T. Perpendicular Orientation Control without Interfacial Treatment of RAFT-Synthesized High-χ Block Copolymer Thin Films with Sub-10 Nm Features Prepared via Thermal Annealing. ACS Appl. Mater. Interfaces 2017, 9 (37), 31266−31278.

KAKENHI Grant Number 17H03113), a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No.2401)” (JSPS KAKENHI Grant Number 15H00755), and “Studying the Function of Soft Molecular Systems by the Concerted Use of Theory and Experiment (No.2503)” (JSPS KAKENHI Grant Numbers 26104525 and 16H00841), a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI Grant Number 15K05521), and a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI Grant Number 16K17916). We would also like to extend our gratitude to the Applied Protein Research Laboratory in Ehime University for their assistance in carrying out NMR analyses.

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DOI: 10.1021/acsmacrolett.7b00789 ACS Macro Lett. 2018, 7, 37−41