Formation of Isolated Pseudo-Polyrotaxane Nanosheet Consisting of α

May 14, 2019 - Nanosheet materials have recently attracted the attention of researchers because of their unique physical properties. In the present st...
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Formation of Isolated Pseudo-Polyrotaxane Nanosheet Consisting of α‑Cyclodextrin and Poly(ethylene glycol) Shuntaro Uenuma, Rina Maeda,* Hideaki Yokoyama, and Kohzo Ito* Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa-City, Chiba 277-8561, Japan

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S Supporting Information *

ABSTRACT: Nanosheet materials have recently attracted the attention of researchers because of their unique physical properties. In the present study, we have reported a novel methodology to fabricate isolated nanosheet materials, called pseudopolyrotaxane (PPR) nanosheets, which are formed by biocompatible complexation between α-cyclodextrin (CD) and poly(ethylene glycol) (PEG) for the first time. When the molecular weight of axis PEG was changed in the range between 2k and 6k, isolated PPR nanosheets were obtained with thicknesses ranging from 14.6 to 33.8 nm, depending on the axis PEG length. We found that uncovering the axis ends led to the formation and isolation of PPR nanosheets when the binding constant between the axis ends and α-CD was changed. Additionally, the PPR structures consisting of multiarm PEG indicated that the uncovered parts of the axis near the branched point also promoted the formation of isolated PPR nanosheets. Uncovering the parts of the axis polymer is essential for fabricating the isolated PPR nanosheets because the uncovered parts of the axis suppress the crystal growth of α-CD in the axis direction and results in the isolation of PPR nanosheets.

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between the α-CD’s inner wall and the axis PEG and on the hydrogen bonding among the hydroxyl groups of adjacent αCDs.16−18 After the formation of the inclusion complex, they can be dissolved in water by heating.15 Since it is known that CDs can incorporate the drug molecules in the CD’s cavity and in the spaces among the outer wall of CD’s columnar crystals,11−14 the complexes of CDs are expected to be used as drug carriers. On the basis of the fact that α-CD can form a hexagonal columnar crystal because of hydrogen bonding between adjacent CDs, some researchers have tried to control the self-assembled structure of PPR through designing the components over the past few decades. Designing axis polymer structures is one potential method that can be used to control the self-assembled structures of PPR. Takahashi et al. reported the formation of a thin α-CD crystal layer with a thickness of 10 nm using PEG brushes grafted on an Au substrate.19 Wang

INTRODUCTION Two-dimensional (2D) nanomaterials, called nanosheets, have an ultrathin nanoscale thickness and lateral dimensions of over a few hundred nanometers. Nanosheets are used in a wide range of applications owing to their unique physicochemical properties, such as anisotropic electronic and thermal conductivities.1−5 With the aim to develop nanosheet engineering, researchers have given attention to the selfassembly approach. Nanosheet materials fabricated via bottomup are expected to possess mass producibility and controllable size/shape depending on environmental conditions.6−10 However, the fabrication of self-assembled nanosheets usually requires toxic compositions and/or complicated synthetic processes. The self-assembly of pseudo-polyrotaxane (PPR), which is formed by complexation between cyclodextrin (CD) and a polymer, has great advantages when used in biomaterials because CD is an accessible natural product. The complexation between α-CD and poly(ethylene glycol) (PEG) was first reported by Harada et al. in 1990.15 The threading phenomena are considered to be based on van der Waals interaction © XXXX American Chemical Society

Received: March 11, 2019 Revised: April 20, 2019

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DOI: 10.1021/acs.macromol.9b00491 Macromolecules XXXX, XXX, XXX−XXX

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plane indicates that the periodic distance in the c axis direction of the PPRs in this study was slightly larger than that reported in the past.31 This is likely because water molecules were incorporated in the crystals when the crystal structure of the PPR nanosheets was measured in water in this study, not in the dry state. From the WAXS measurement, it was confirmed that the crystal structures similar to the hexagonal crystal reported previously were formed in these samples in water. Next, SAXS experiments were conducted to analyze the selfassembled structures of these PPRs in water (Figure 1). Clear

and Zhang showed the formation of a nanosheet consisting of α-CDs and a long PEG chain, although the nanosheets are bridged through the axis polymer and are not isolated.20 By using a block copolymer as an axis polymer, CDs selectively cover a certain segment, and thereby a lamellae-type microphase-separated structure was formed.21 The use of sodium dodecyl sulfate as axis molecules could control the morphology of self-assembled PPR structures.13,22 Recently we reported the fabrication of an autonomously isolated PPR nanosheet using a β-CD and poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (PEG−PPG− PEG) triblock copolymer.23 The topology of axis polymers can also affect the self-assembled structure. The formation of a nanoplatelet with a thickness of 9 nm and a diameter of 30 nm was observed using α-CD and four-branched star poly(εcaprolactone).24 More examples are available in the published refs.25−28 In particular, the formation and isolation of a PPR nanosheet are still challenging. The isolation is difficult because of the strong van der Waals interaction between the nanosheets but necessary for realizing the unique functions of a nanosheet material because stacking deactivates their intrinsic properties, such as the large surface area.30 Although we reported the isolation of a PPR nanosheet by introducing the ionic group (carboxyl groups) on the axis end to generate Columbic repulsion between the nanosheets,23 a fabrication method for isolated PPR nanosheets with nonionic axis polymers has never been investigated. Furthermore, the strategy for obtaining the isolated PPR nanosheet has never been established. Fabrication of isolated PPR nanosheets other than those with ionic groups is important because the biocompatible and decomposable nanosheet materials are expected to become a new kind of drug carrier that has a high adhesion property based on the large-area interactions.29 Herein, we report a methodology for isolating a PPR nanosheet with a nonionic PEG axis. We analyze the PPR nanosheet structure and isolation by wide-angle and smallangle X-ray scattering (WAXS and SAXS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) while changing the length, axis end group, and topology of PEG. From the results, it is found that the uncovered parts of the axis play a crucial role in fabricating isolated PPR nanosheets.

Figure 1. SAXS profiles of PPR consisting of α-CD and PEG with a variety of Mn in water.

fringes were observed in the profiles of PPR2k, PPR3.4k, PPR4k, PPR4.6k, and PPR6k. By fitting the profiles with the nanosheet form factors32 equation, it was found that isolated nanosheets with thicknesses of 14.6, 25.5, 29.0, 33.8, and 20.8 nm were formed (the fitting is shown in the Supporting Information 2). These values reflect the thicknesses of the αCD columnar layers because X-ray scattering is mainly caused by the α-CDs because of their higher electron densities than those of polymer chains. Therefore, it was obvious that the αCD crystal in PPR formed the nanosheet structures. In contrast, the profiles of PPR400 and PPR600 showed no fringes like that of PPR1k, and the profiles of PPR10k, PPR12k, PPR20k, and PPR35k were similar to that of PPR8k, which have almost no fringes, as shown in the Supporting Information 3. These results indicate that the molecular weight of PEG in a certain range (from 2k to 6k) was essential to form PPR nanosheets. To confirm the nanosheet structures in a real image, SEM observations were conducted for the dried state of PPR4k and PPR6k, where many nanosheets were observed (Figure 2). The thicknesses were measured using AFM and were found to be 32.6 and 22.5 nm for PPR4k and PPR6k, respectively. These values were almost the same as the thicknesses obtained by SAXS measurements. The thickness of the PPR nanosheet is dependent on the axis chain length. It is known that the axis PEG takes all trans conformation when it is fully covered with α-CD.33 The lengths of PEG2k, PEG3.4k, PEG4k, PEG4.6k, and PEG6k with all trans conformations are 16, 27, 32, 36.5, and 48 nm, respectively. By comparing the thicknesses of the PPR2k,

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RESULTS AND DISCUSSION Effect of Axis Polymer Length. To investigate the effect of the axis polymer length on the self-assembled structures of PPR, the PPRs consisting of PEG400, PEG600, PEG1k, PEG2k, PEG3.4k, PEG4k, PEG4.6k, PEG6k, PEG8k, PEG10k, PEG12k, PEG20k, and PEG35k (the numbers included approximately indicate number average molecular weights) were prepared and named as PPR400, PPR600, PPR1k, PPR2k, PPR3.4k, PPR4k, PPR4.6k, PPR6k, PPR8k, PPR10k, PPR12k, PPR20k, and PPR35k, respectively. The structures in water were analyzed and compared. First, α-CD crystal structures were investigated using WAXS measurements (Supporting Information 1). The characteristic peaks at 2θ = 5.0°, 7.4°, 10.1°, and 12.7° were observed. These values are almost the same as those reported in a past paper:31 5.30° (001), 7.41° (100), 10.9° (002), and 12.8° (110) obtained from the dried PPR crystal consisting of α-CD and PEG (hexagonal lattice with unit cell parameters a = b = 13.65 Å and c = 16.4 Å). The difference in the 2θ value of the (001) B

DOI: 10.1021/acs.macromol.9b00491 Macromolecules XXXX, XXX, XXX−XXX

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with α-CDs. The uncovered axes were probably located near the ends of the axis PEG because CDs are stacked along the axis to form columnar structures because of the strong hydrogen bonds between adjacent CDs,34 and the ends of a polymer chain generally tend to be uncovered owing to freer motion compared to the other parts (Figure 3b). The uncovered free motion of the axis ends should suppress the α-CD crystal growth in the axis direction while α-CD columnar crystal easily grows in the radial direction, resulting in the formation of the 2D nanosheet structure. This means that uncovering the axis end is important for fabricating the PPR nanosheet. Interestingly, the thickness of PPR6k (20.8 nm) was largely different from the length of the axis PEG6k chain (48 nm) and approximately the half length of PEG6k (24 nm). This can be explained by the axis chain folding (Figure 3c). As reported previously, the long axis PEG brushes on the Au substrate fold more than twice in the process of complexation.19 If the α-CD totally covers the long chain and the chain takes all trans conformation, the entropic reduction becomes too large. Therefore, the axis chain tends to fold to gain entropy, when the axis chain is long. The uncovered chains near the folding points and the uncovered axis ends are probably what lead to the thickness of the nanosheet being shorter than the half length of the extended PEG6k axis. In the case of PPR8k, the fringes in the SAXS profile almost disappeared though some nanosheet structures were still observed in the SEM images (Figure 2f). These SAXS and SEM results suggest that the PPR nanosheet of PEG of 8k or more in Mn had various structures. As mentioned before, the long axis PEG brushes on the Au substrate fold more than twice in the process of complexation,19 which probably caused the nonuniform nanosheet thickness (Figure 3d). The aggregated structures were actually observed in the SEM images of PPR8k and PPR20k (Supporting Information 4). Furthermore, the fringes were not observed for the PPR consisting of PEG below 1k in Mn. The SEM images of PPR400, PPR600, and PPR1k showed a block structure with a thickness of over 1 μm (SEM images of PPR400 and PPR600 are shown in the Supporting Information 5 and that of PPR1k

Figure 2. SEM images of (a) PPR4k and (b) PPR6k. AFM images and height profiles of (c) PPR4k and (d) PPR6k. SEM images of (e) PPR1k and (f) PPR8k.

PPR3.4k, PPR4k, and PPR4.6k nanosheets with those of the axis chain lengths, it was found that the thicknesses of all PPR nanosheets were slightly (from 1.5 to 3.0 nm) shorter than those of the axis PEG lengths with all trans conformation. This means that a small amount of axis PEG chain is not covered

Figure 3. Schematic illustrations of PPR self-assembling mechanisms. (a) α-CDs can cover the ends of PEG400-1k and the α-CD crystal grows without limitation of axis length. (b) Ends of PEG2k−4.6k completely tend to be uncovered by α-CDs because of the entropic gain by the free motion of the axis, which leads to the formation of isolated and uniform thickness PPR nanosheets. (c) PEG6k axis folds to gain the chain entropy not only at the ends but also at the folding points. The isolated and uniform thickness nanosheets are also obtained. (d) In the case of PEG8k−35k, the axis chains fold many times and form a bridge between the PPR nanoplatelet clusters. C

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no fringes were observed for PrPPR2k, nBuPPR2k, and nPePPR2k, shallow but clear fringes were observed in both iBuPPR2k and PiPPR2k. Furthermore, NH2PPR2k and FBuPPR2k showed clearer fringes than other PPR nanosheets. The depths of the fringes correspond to the reverse order of the binding constant of the functional groups. This means that the formation of PPR nanosheets was promoted more easily when the binding constant of the axis end groups was lower (Figure 5). These results strongly support the notion that

in Figure 2). These results indicate that when the whole axis chain, including the ends, is totally covered with α-CD for the axis chains shorter than 1k in Mn, the α-CD crystal can grow without limitation of the axis length (Figure 3a). This study clarified the self-assembling behavior of PPR, which depends on the axis PEG length systematically and the formation mechanism of PPR nanosheets (Figure 3). When the PEG molecular weight was in a range between 2k and 4.6k, isolated PPR nanosheets could be obtained, and the thickness was determined by the axis length. As for PPR6k, the axis in the PPR nanosheet was folded, and the uncovered axis near the folding point also led to the isolation of the PPR nanosheet. It is noteworthy that the formation and isolation of the PPR nanosheet are due to the uncovered parts of the axis. Effect of the Axis End Groups. Next, we investigated the effect of the axis end groups on the PPR structure to confirm the validity of the findings, which suggested that the uncovered axis ends played an important role in forming an isolated nanosheet. It has been reported that the binding constant between α-CD and a functional group is systematically reduced when the functional group gets bulky.35 Furthermore, the polar NH2 group and the small FBu group, which are much smaller than the inner diameter of α-CD, are known to hardly be incorporated into the cavity of α-CD.36,37 In this study, the PPRs consisting of the end-functionalized PEG2k with propanoyl (Pr) groups (PrPEG2k), n-butyryl (nBu) groups (nBuPEG2k), n-pentanoyl (nPe) groups (nPePEG), isobutyryl (iBu) groups (iBuPEG2k), pivaloyl (Pi) groups (PiPEG2k), amino (NH2) groups (NH2PEG2k), and perfluorobutyryl (FBu) groups (FBuPEG2k) were prepared; they were named as PrPPR2k, nBuPPR2k nPePPR2k, iBuPPRPEG2k, PiPPR2k NH2PPR2k, and FBuPPR2k, respectively, and those structures were analyzed. The expected order of the binding constants of chemical groups is estimated from their sizes and hydrophobicity as follows: nPe > nBu > Pr > iBu > Pi ≫ NH2, FBu. First, it was confirmed that these PPRs formed the same crystal structures as the unmodified PPR by WAXS measurement (Supporting Information 6). The SAXS profiles for PrPPR2k, nBuPPR2k, nPePPR2k, iBuPPR2k, PiPPR2k, NH2PPR2k, and FBuPPR2k are shown in Figure 4. Although

Figure 5. Schematic illustration of the end group effect on PPR selfassembling. (a) When the binding constant between the axis ends and α-CD is high, α-CD can cover the ends of the PEG axis, and the αCD crystal grows without the limitation of the axis length. (b) When the axis ends have the low binding constant with α-CD, they are uncovered by α-CDs, which results in the formation and isolation of uniform thickness PPR nanosheets.

uncovering the end groups encourages the formation and isolation of PPR nanosheets because the low binding constant between α-CD and the functional group leads to the uncovering of the axis end. The thicknesses of iBuPPR2k, NH2PPR2k, and FBuPPR2k were analyzed by fitting (see Supporting Information 7) and were all determined to be 16 nm, which is equal to the length of PEG2k with all trans conformation. These results indicated that the PEG2k backbone of NH2PPR2k and FBuPPR2k was fully covered with CD, but the end groups were not covered. In contrast, the thickness of PiPPR2k was 9 nm. This is probably because the number of α-CDs in PiPPR2k is much smaller than that in other PPR nanosheets. Kato et al. reported that the bulky axis ends decreased the number of α-CDs in the inclusion complex formation. 38 The nanosheet formations of PiPPR2k, NH2PPR2k, and FBuPPR2k were confirmed in real images by SEM observations, which are shown in the Supporting Information 8. In this section, we have revealed that a low binding constant between the end of axis polymer and α-CD induced the formation and isolation of PPR nanosheets. The lower the binding constant of the axis ends, the more the PPR nanosheet formation was promoted. Effect of Branched Axis. It was revealed that uncovering the axis ends was important for forming PPR nanosheets, as mentioned above. To develop a new way to form PPR nanosheets, we tried to fabricate PPR nanosheets using the branched axis polymer. It is known that α-CD cannot cover the neighborhood at the branched point owing to the steric hindrance,24 which may affect the formation of isolated PPR nanosheets. The PPRs consisting of four arm PEG1.25k (PEG1.25k × 4), four arm PEG2.5k (PEG2.5k × 4), and eight

Figure 4. SAXS profiles for nPePPR2k, nBuPPR2k, PrPPR2k, iBuPPR2k, and PiPPR2k in water. D

DOI: 10.1021/acs.macromol.9b00491 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules arm PEG1.25k (PEG1.25k × 8) were prepared and named as PPR1.25k-4, PPR2.50k-4, and PPR1.25k-8, respectively. From the WAXS measurement, it was confirmed that α-CD formed a columnar crystal in the PPR nanosheet (Supporting Information 9). The SAXS profiles are shown in Figure 6. The

the others are outside the PPR nanosheet and (2) some chains of multiarm PEG are partially incorporated in the α-CD layer from both the sides. In any case, the α-CD layer in the PPR nanosheet is surrounded by arms and branched points of PEG. It should be noted that linear PEG1k did not form PPR nanosheets, whereas PEG1.25k-4 and PEG1.25k-8 yielded isolated PPR nanosheets although the lengths of their PEG chains are almost the same. Thus, the branched points likely assist the formation of an isolated PPR nanosheet. When the branched PEG was used for the formation of PPR nanosheets, the thicknesses of the PPR nanosheet were almost the same (≑10 nm), which implies that there may be a threshold thickness of crystallization to overcome the interfacial energy between the α-CD crystal and water. The formation of PPR nanosheets consisting of the branched PEGs was confirmed by SEM observation (Supporting Information 11). The formation of a PPR nanosheet was also achieved by introducing a branched point into the axis polymer. The α-CD crystal did not grow in the axis direction because of the steric hindrance of the branched point and the uncovered polymer ends. The branched point, which is not susceptible to α-CD covering, surrounds the surface of PPR nanosheets and thereby leads to the isolation of PPR nanosheets.

Figure 6. SAXS profiles for PPR1.25k-4, PPR2.50k-4, and PPR1.25k8 in water.

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fringes are observed for all water dispersion of PPR, which indicates the formation of the isolated nanosheet structure. The thicknesses of PPR1.25k-4, PPR2.50k-4, and PPR1.25k-8 were found to be 10, 9, and 9 nm, respectively (the fitting results are shown in the Supporting Information 10). PPR2.50k-4 consists of four arms of PEG2.5k, and each PEG length is about 20 nm. Therefore, we concluded that the axis ends were uncovered to gain the chain entropy because of their relatively long axis (Figure 7a). The branched points also

CONCLUSIONS In the present study, we reported the general methodology for the fabrication of isolated PPR nanosheets consisting of α-CD and PEG for the first time. The uncovered parts of the axis chain played an important role in fabricating an isolated PPR nanosheet by suppressing the α-CD crystal growth. When the PEG molecular weight is in a range between 2k and 4.6k, an isolated PPR nanosheet with a thickness determined by its axis length was obtained by uncovering the axis ends. In the case of PPR6k, the axis in the PPR nanosheet was folded, and the uncovered axis near the folding point also promoted the isolation of the PPR nanosheet. The suggested mechanism was confirmed by changing the binding constant between the axis ends and α-CD. The formation of isolated PPR nanosheets was prominently promoted when the binding constant at the axis ends with α-CD was low. We also investigated the PPR structure consisting of multiarm PEG and found that the uncovered chain of the axis near the branched point encouraged the formation of isolated PPR nanosheets. PPR nanosheet is a novel nanosheet material that has biocompatibility and decomposability. We have been giving attention to PPR nanosheets because they have significant potential to become a new kind of drug carrier with the expectation of high adhesive property based on the large-area interaction with the targeted objects.

Figure 7. Schematic illustrations of the self-assembled structures of PPR2.50k-4 and PPR1.25k-4.

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caused the suppression of α-CD crystal growth in the axis direction and the isolation of PPR nanosheets because α-CDs cannot cover the neighborhood of the branched point. For PPR1.25k-4 and PPR1.25k-8, which consists of four or eight arms of PEG1.25k with the length 10 nm of each PEG arm with an all trans conformation, the thickness of the PPR nanosheet should be less than 10 nm because α-CDs cannot cover the neighborhood of the branched point. However, the PPR nanosheet thickness was determined to be 10 nm. To explain this thickness, some nanosheet formation models can be considered as shown in Figure 7b: (1) a chain of the multiarm PEG is fully covered with α-CD and extended and

METHODS

Materials. α-CD was purchased from Nihon Shokuhin Kako Co. Ltd. PEG400, PEG600, PEG1k PEG2k, PEG3.4k, PEG4k, PEG4.6k, PEG6k, PEG8k, PEG10k, PEG12k, PEG20k, and PEG35k were purchased from Sigma-Aldrich and Wako Pure Chemical Industries. PEG2k with amino (NH2) groups (NH2PEG2k) was purchased from NOF Corp. The poly dispersity indices of PEG reagents between 2k and 4.6k of Mn were less than 1.1, and those of PEG6k, PEG8k, PEG10k, PEG12k, PEG20k, and PEG35k were 1.2, 1.3, 1.3, 1.8, 1.5, and 1.8, respectively. The multiarm PEG samples consisting of PEG1.25k × 4, PEG2.5k × 4, and PEG1.25k × 8 were all purchased from Creative PEGworks. All reagents were used as received. The reagents for the synthesis of the end-functionalized PEG (propionyl chloride, n-butyryl chloride, n-pentanoyl chloride, isobutyryl chloride, E

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pivaloyl chloride, perfluorobutyryl chloride, Na2SO4, dichloromethane, and methanol) were purchased from Tokyo Chemical Industry Co., Ltd. and Wako Pure Chemical Industries. All reagents were used as received. Preparation of PPR. α-CD was dissolved in deionized water (pH was around 7) at 23 ± 1 °C. The α-CD concentration was 0.12 g/mL. One gram of solid PEG reagents (linear PEGs, PEG derivatives, and multiarm PEGs) was added into 33.2 mL of the water solution of αCD, and the solutions were vigorously shaken by a vortex for 3−10 min. Then, the solutions were put on the shaker and aged for 1 week with continuous shaking. Although the times to be turbid were different, 1 week was enough for aging because the phase separation between the white powders and the transparent solution occurred in the solutions except for PPR with the multiarm PEGs. The aqueous PPR solution with the multiarm PEGs formed a thixotropic gel, which was used for the structural analysis after aging for the same time as those of the other PPR samples. Synthesis of End-Functionalized PEG. The general procedure for the synthesis of the end-functionalized PEG2k is as follows: 1.0 g (0.5 mmol) of PEG2k and 12 mmol of trimethyl amine were dissolved in 19 mL of dichloromethane. Then, 10 mmol of an acid chloride reagent was added to the solution at 0 °C. The solution was mixed for 5 h at room temperature. The organic phase was washed with distilled water and then dried with Na2SO4. The solution was poured into 150 mL of ethanol. The white solid was precipitated by storing the ethanol solution at −10 °C. The white powder product was obtained by centrifuge and dried under vacuum conditions. By changing the acid chloride reagent, PrPEG2k, nBuPEG2k, nPePEG, iBuPEG2k, PiPEG2k, and FBuPEG2k were synthesized. The yields of all products were over 80%. The obtained products were characterized by using proton nuclear magnetic resonance (1H NMR) spectrum (Supporting Information 12). Sample Preparation for Structural Analysis. The samples for AFM and SEM analyses were prepared by dipping a silicone oxide substrate into the water dispersion of the PPR. The water dispersion of the PPR was poured into a glass capillary for X-ray measurements (WJM-glass/Muller GmbH boro-silicate capillary: φ = 2.0 × length = 80 mm) and used for SAXS and WAXS measurements. Measurements. WAXS and SAXS experiments were carried out using a Rigaku NANOPIX instrument with a Hypix-3000 detector. The sample-to-detector distance was calibrated with a silver behenate diffraction peak. SEM observation was conducted with a JEOL JSM7800F microscope. AFM experiment was carried out under ambient conditions using a Bruker Nano MultiMode 8 microscope operating under the tapping mode. Antimony-doped silicon cantilever tips (Bruker RTESPA-300) with a resonant frequency of around 300 kHz and a spring constant of 40 N m−1 were used. NMR (1H, 400 MHz) spectra were recorded in CDCl3 on a JEOL JNM-AL400 instrument.

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This work was supported by the ImPACT Program (Cabinet Office, Government of Japan), by a Grant-in-Aid for Young Scientists (B) (no. JP16K17909), and by OPERANDO-OIL. Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00491.

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Characterization of the series of the end-functionalized PEG2k, WAXS results of the PPR, additional SEM image of PPRs, and fitting of the nanosheet form factor for the SAXS profiles of PPR (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.M.). *E-mail: [email protected] (K.I.). ORCID

Shuntaro Uenuma: 0000-0003-0693-9310 Hideaki Yokoyama: 0000-0002-0446-7412 F

DOI: 10.1021/acs.macromol.9b00491 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.9b00491 Macromolecules XXXX, XXX, XXX−XXX