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Highly Ordered Cylinder Morphologies with 10 nm Scale Periodicity in Biomass-Based Block Copolymers Takuya Isono,*,† Brian J. Ree,‡ Kenji Tajima,† Redouane Borsali,§ and Toshifumi Satoh*,† †

Faculty of Engineering and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan Univ. Grenoble Alpes, CNRS, CERMAV, 38000 Grenoble, France

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S Supporting Information *

ABSTRACT: Microphase-separated structures of block copolymers (BCPs) have attracted considerable attention for their potential application in the bottom-up fabrication of 10 nm scale nanostructured materials. To realize sustainable development within this field, the creation of novel BCP materials from renewable biomass resources is of fundamental interest. Thus, we herein focused on maltoheptaose-b-poly(δdecanolactone)-b-maltoheptaose (MH-b-PDL-b-MH) as a sustainable alternative for nanostructure-forming BCPs, in which both constitutional blocks can be derived from renewable biomass resources, in the case, δ-decanolactone and amylose. Well-defined MH-b-PDL-b-MHs with varying PDL lengths were synthesized through a combination of controlled/living ring-opening polymerization and the click reaction. The prepared MH-b-PDL-b-MHs successfully self-assembled into highly ordered hexagonal cylindrical structures with a domainspacing of ∼12−14 nm in both the bulk and thin film states. Interestingly, the as-cast thin films of MH-b-PDL-b-MHs (with PDL lengths of 9K and 13K) form horizontal cylinders, with thermal annealing (180 °C, 30 min) resulting in a drastic change in the domain orientation from horizontal to vertical. Thus, the results presented herein demonstrated that the combination of oligosaccharides and biomass-derived hydrophobic polymers appears promising for the sustainable development of nanotechnology and related fields.

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INTRODUCTION

Currently, several BCP systems, such as poly(styrene)-bpoly(ethylene oxide),12 poly(styrene)-b-poly(methyl methacrylate),10 and poly(styrene)-b-poly(butadiene),13 are widely employed for nanopatterning purposes. However, such BCPs are prepared from fossil fuel-based resources that are being rapidly depleted. As such, the increasing demand for BCP materials would result in appreciable consumption of fossil resources and lead to nondegradable waste accumulation. Thus, the design and creation of sustainable alternatives to BCP materials for application in the sustained development of nanotechnology and related fields are a significant challenge. However, to date, little attention has been paid to the development of biomass-based BCPs for the preparation of nanostructures, although several research groups have reported novel BCP systems consisting solely of renewable resources, such as poly(lactide)-b-poly(mentide)-b-poly(lactide),14 poly(α-methylene-γ-butyrolactone)-b-poly(mentide)-b-poly(αmethylene-γ-butyrolactone), 15 and poly(L-lactide)-b-poly(myrcene)-b-poly(L-lactide).16 Although such BCPs are indeed able to microphase separate into certain morphologies, previous studies have focused mainly on their bulk material applications,

Block copolymers (BCPs) consisting of more than two chemically distinct polymer segments are an important class of polymeric material due mainly to their ability to selfassemble into periodic ordered nanostructures, such as microphase-separated structures in both the bulk and the thin film states. Examples of such structures include body-centered cubic spheres, hexagonally close-packed cylinders, gyroids, and lamellae.1−3 To date, considerable research efforts have been focused on utilizing such microphase-separated nanostructures for the fabrication of various nanostructured materials.4−6 One of the most important applications of these periodic nanostructures is in next-generation nanolithography on the sub-10 nm resolution, in which a microphase-separated BCP thin film on a semiconductor substrate can serve as a lithographic template.7−9 Through combination with directed self-assembly technologies, the lithographically interesting patterns can be transferred to the underlying substrate through the selective etching of the block. In addition, microphaseseparated nanostructures are useful in the preparation of nanopatterned inorganic materials10 as well as nanoporous materials.11 For such nanotechnology applications, BCP materials capable of forming microphase-separated structures with a domain spacing of 180 °C. We expect that this is due to heating F

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Figure 7. Synchrotron GISAXS data of (a) the as-cast film of MH-b-PDL9k-b-MH measured at an incidence angle (αi) of 0.135°, (b) the as-cast film of MH-b-PDL13k-b-MH measured at an αi of 0.119°, (c) the as-cast film of MH-b-PDL23k-b-MH measured at an αi of 0.158°, (d) the thermally annealed film of MH-b-PDL9k-b-MH measured at an αi of 0.176°, (e) the thermally annealed film of MH-b-PDL13k-b-MH measured at an αi of 0.186°, and (f) the thermally annealed film of MH-b-PDL23k-b-MH measured at an αi of 0.141°.

orientation, which is indicated by the appearance of narrow scattering peaks vertically rising from the out-of-plane direction. This orientational switching of the cylinders was also confirmed by the AFM images. As in the case of the as-cast thin films, the scattering peaks from the MH-b-PDL13k-b-MH film were stronger and more clearly defined than those of MH-bPDL9k-b-MH. In addition, three scattering peaks originating from the {100}, {110}, and {200} reflections were observed for the MH-b-PDL13k-b-MH film (Figure 7e), while for the MH-bPDL9k-b-MH film, only the {100} reflection could be assigned (Figure 7d). As such, it could be concluded that despite bearing a longer PDL block, MH-b-PDL13k-b-MH forms more a defined microstructure in both the as-cast and the thermally annealed films compared to MH-b-PDL9k-b-MH. The thin film morphologies of MH-b-PDL9k-b-MH and MHb-PDL13k-b-MH before and after thermal annealing are shown schematically in Figure 8. As previously indicated by the AFM and GISAXS analyses, the as-cast thin films consist of horizontally orientated HEX structures, with thermal annealing above 180 °C inducing a drastic change in morphology from horizontal to vertical cylinders. It should be emphasized that MH-b-PDL-b-MH formed vertical cylinders through simple

cooling process (Figure 6). Therefore, the amorphous nature of the PDL block should be one of the key factors for obtaining highly ordered nanostructured thin film. To further confirm the microphase-separated structures of MH-b-PDL-b-MHs inside the thin film samples employed for the AFM observations, grazing incidence small-angle X-ray scattering (GISAXS) experiments were performed. As shown in Figure 7, the as-cast films of MH-b-PDL9k-b-MH and MH-bPDL13k-b-MH revealed horizontally oriented HEX morphologies consisting of cylinders formed by the MH moieties and the surrounding PDL matrix. This was confirmed by splitting of the scattering peaks into pairs of transmission and reflection modes, where the scattering peaks could be indexed according to the conventional hexagonal structure.32−34 It is important to note that the MH has a strong tendency to phase separate from PDL and form stable cylinders, as observed from the second-order scattering peaks in both Figures 7a and 7b. On the basis of a qualitative evaluation, it appeared that the scattering peaks from the MH-b-PDL13k-b-MH film were stronger and slightly more defined than those of MH-b-PDL9k-b-MH, indicating that MHb-PDL13k-b-MH forms more regular and stable cylindrical structures than MH-b-PDL9k-b-MH. In contrast, the as-cast film of MH-b-PDL23k-b-MH exhibited a featureless GISAXS profile (Figure 7c). This difference in behavior may originate from the longer PDL block of MH-b-PDL23k-b-MH compared to those of MH-b-PDL9k-b-MH and MH-b-PDL13k-b-MH, which could result in a significantly greater chain entanglement effect in the case of MH-b-PDL23k-b-MH. The resulting reduced chain mobility of the triblock copolymer would then impair formation of the MH cylinders. The thermally annealed thin films present a surprising set of results that reflect the interesting properties of the BCPs. For example, as shown in Figures 7d and 7e, the HEX morphologies of MH-b-PDL9k-b-MH and MH-b-PDL13k-bMH, respectively, switched from the horizontal to the vertical

Figure 8. Schematic representations of the presumed morphologies of the MH-b-PDL(9k and 13k)-b-MH thin films on a silicon substrate. The green and blue portions represent the PDL matrix and the MH cylinder domain, respectively. G

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Macromolecules thermal annealing despite the MH and PDL blocks having highly dissimilar surface free energies. In general, BCPs exhibiting high χ values tend to produce horizontally orientated cylinders (or lamellae) in the thin film state due to the large difference between the surface free energies of the two constitutional blocks. Indeed, this constitutes the first example of vertical cylinder formation in sugar-based BCP thin films imparted through simple thermal annealing. Importantly, both the horizontally and vertically orientated cylindrical structures with 10 nm periodicity are of particular interest in the context of nanolithographic applications to produce various nanopatterns on a semiconductor substrate.4−10 In addition, as we previously reported, such control over the orientation of the sugar domain is also beneficial in the manufacture of highperformance flash memory devices.35 In the case of the thermally annealed MH-b-PDL23k-b-MH film, the scattering data indicated the presence of a well-defined horizontally oriented hexagonal cylinder structure. Unlike the triblock copolymers bearing lower molecular weight PDL blocks, which switched in orientation while retaining the HEX morphology, MH-b-PDL23k-b-MH developed into a wellordered HEX morphology in the horizontal orientation upon thermal annealing. In addition, the raw scattering profile (Figure 7f) contains sharper scattering peaks than that of the as-cast films of MH-b-PDL9k-b-MH (Figure 7a) and MH-bPDL 13k-b-MH (Figure 7b), which is indicative of the horizontally oriented HEX morphology in the thermally annealed film of MH-b-PDL23k-b-MH exhibiting enhanced structural ordering compared to the as-cast films of MH-bPDL9k-b-MH and MH-b-PDL13k-b-MH. Although extremely unique, this result appears to conflict with the bulk X-ray scattering and AFM results, where MH-b-PDL23k-b-MH shows a little improvement in its bulk X-ray profile upon annealing, and the AFM image of the annealed film displays vertically oriented spherical structures that hint at the formation of vertical cylinders. This apparent conflict with the bulk X-ray profile may be the result of the nanoscale thin film dimension imposing geometrical confinement effects that positively influence cylinder formation upon thermal annealing, whereas the bulk state does not exhibit such factors that allow high structural order. Moreover, it is important to note that AFM gives details regarding the surface structure of the thin film, while GISAXS measurements provide information regarding the internal structures. It is therefore possible that the MH-bPDL23k-b-MH thin film presents vertically oriented cylinders at and near the film surface, whereas the remainder of the film consists of horizontally oriented cylinders (Figure S8). Considering the high structural ordering of both the surface and the internal structures of the film, we could conclude that this is a unique phenomenon, which requires further investigation in the future.

packed cylindrical (HEX) morphologies with small domainspacing values of 12−14 nm. More importantly, the MH-bPDL-b-MHs also formed highly ordered HEX morphologies in the thin film state. Remarkably, the thin films of MH-b-PDL9kb-MH and MH-b-PDL13k-b-MH produced horizontally orientated HEX morphologies prior to thermal annealing, while annealing at 180 °C for 30 min resulted in the formation of highly ordered vertically orientated HEX morphologies. Such switching of the cylindrical domain orientation following thermal annealing is particularly appealing, as both the horizontal and vertical cylinders are potentially useful for nanolithographic applications on the sub-10 nm scale, in which they are suitable for the fabrication of line-and-space and contact hole patterns, respectively. In addition, such control over the sugar domain orientation is also of interest in the context of high-performance flash memory applications. Thus, we herein demonstrated that the combination of oligosaccharides and biomass-derived hydrophobic polymers appears promising for the sustainable development of nanotechnology and related fields. Further studies into this novel BCP system from both a fundamental and an application point of view are currently underway in our laboratory, and the results will be presented in due course.

CONCLUSIONS We successfully synthesized ABA-type triblock copolymers consisting of maltoheptaose (MH, A block) and poly(δdecanolactone) (PDL, B block), i.e., MH-b-PDL-b-MH, as a sustainable alternative for nanostructure-forming block copolymers (BCPs). These novel triblock copolymers were fully obtainable from renewable biomass resources (i.e., δdecanolactone and amylose) via a straightforward three-step synthesis involving ring-opening polymerization, end-functionalization, and a final click reaction. In the bulk state, MH-bPDL-b-MHs successfully self-assembled into hexagonally close-

This work was financially supported by a MEXT Grant-in-Aid for Scientific Research (B), a research grant from the Ogasawara Foundation for the Promotion of Science & Technology, and a research grant from the Tonen General Foundation.

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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/acs.macromol.7b02279. Experimental; Table S1 (DPP-catalyzed ROP of δ-DL using BDM as the initiator); Figure S1 (SEC traces of the HO-PDL-OHs); Table S2 (preparation of the N3-PDLN3s); Figure S2 (FT-IR spectra); Figure S3 (TGA profiles); Figure S4 (partial DSC thermograms of the triblock copolymers and their azido-functionalized PDL); Figures S5 and S6 (SAXS profiles); Figure S7 (variation in the primary scattering peak intensity upon heating); and Figure S8 (schematic illustration of MH-b-PDL23k-bMH thin film) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(T.I.) E-mail: [email protected]. *(T.S.) E-mail: [email protected]. ORCID

Takuya Isono: 0000-0003-3746-2084 Kenji Tajima: 0000-0002-3238-813X Redouane Borsali: 0000-0002-7245-586X Toshifumi Satoh: 0000-0001-5449-9642

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Funding

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. I. Otsuka and Dr. Y. Nishiyama (CNRSCERMAV, France) for their support in the synchrotron X-ray H

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experiments at the European Synchrotron Radiation Facility (ESRF) and Mr. K. Yoshida (Hokkaido University, Japan) for the graphical drawing of Figure 8 and Figure S8. T.I. and. B.J.R. gratefully acknowledge the Nanotech CUPAL NRP program and the JSPS Fellowship for Young Scientists, respectively.

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ABBREVIATIONS AFM, atomic force microscopy; BCC, body-centered cubic; BCP, block copolymer; BDM, 1,4-benzenedimethanol; DL, δdecanolactone; DMAP, 4-(dimethylamino)pyridine; DPP, diphenyl phosphate; EDC, 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide; FFT, fast Fourier transform; FT-IR, Fourier transform infrared; GISAXS, grazing incidence smallangle X-ray scattering; HEX, hexagonally close-packed cylindrical; MH, maltohepotaose; MH-b-PDL-b-MH, maltoheptaose-b-poly(δ-decanolactone)-b-maltoheptaose; PDL, poly(δ-decanolactone); PMDETA, N,N,N′,N″,N″pentamethyldiethylenetriamine; ROP, ring-opening polymerization; SAXS, small-angle X-ray scattering; SEC, size exclusion chromatography; TGA, thermogravimetric analysis.

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