Letter Cite This: ACS Macro Lett. 2019, 8, 140−144
pubs.acs.org/macroletters
Drastic Change of Mechanical Properties of Polyrotaxane Bulk: ABA−BAB Sequence Change Depending on Ring Position Shuntaro Uenuma, Rina Maeda,* Kazuaki Kato, Koichi Mayumi, 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: Polyrotaxane (PR), consisting of many ring molecules and an axis polymer, is a typical supramolecular structure with unique topological characteristics. In this study, we demonstrated the drastic change of the macroscopic mechanical properties depending on the ring position of PR in bulk. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymer was employed as an axis polymer to control the position of βcyclodextrin (β-CD). To transfer the β-CD positions, hydroxypropyl groups (HPPR) and hydrophobic trimethyl silyl groups (TMS-HPPR), which have hydrophilic and hydrophobic β-CD, respectively, were synthesized. β-CDs in HPPR were localized on a central PPO segment and formed crystal domains. The axis polymer of HPPR could not bridge β-CD crystal domains, resulting in a melt state at high temperature. On the other hand, β-CDs in TMS-HPPR were transferred to both PEO segments and formed crystal domains. The axis polymer in TMS-HPPR could bridge the β-CD crystal domains, resulting in an elastic state even at high temperature. We succeeded in demonstrating the potential ability of PR: the macroscopic mechanical properties of PR can be changed from a melt state to an elastic one by manipulating the ring positions.
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In the present study, we dedicated to demonstrate the drastic change of mechanical properties of PR bulk based on the model mentioned above. PR consisting of 14 β-cyclodextrins (β-CDs) and poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymer (PPR; Scheme 1)17 was used as the main structure. Since PPR has PPO that works as a station for β-CDs18,19 in its axis, it is easier to make a difference in β-CD position than in the case of conventional PRs that do not have any station. In the past report, we succeeded in making a difference in the βCD position by changing the way of sample preparation, although the mechanical properties are less changed due to a small difference.17 In order to transfer the ring position more largely, the chemical properties of the rings’ outer wall were totally altered from hydrophilic to hydrophobic. Since the ring position is determined by a variety of interactions working in the system,20−22 altering the chemical properties of the rings’ outer wall was expected to lead to the change of all interactions in the system, resulting in a largely different ring position. The chemical structures and the synthetic scheme of newly synthesized PRs are shown in Scheme 1 (experimental details
olyrotaxanes (PRs) have unique properties due to the mobility of ring components that are not covalently bonded but are topologically constrained to an axis chain. The abilities of ring components in PR are their rotations and location changes along an axis polymer.1,2 Utilizing the rings’ abilities has attracted much interest of researchers and yielded various functional materials.3−13 Recently, aiming to expand the possibility of the rings’ abilities, methodologies of the manipulation of ring positions with the various types of stimulation have been developed.14−17 However, the change of macroscopic properties depending on ring positions has still been challenging, especially in bulk. Although there are a few reports about the research of molecular muscles in a bulk state,11 the drastic change from a melt state to an elastic one has not been achieved. With the aim to utilize the rings’ abilities to drastically change the macroscopic mechanical properties in PR bulk, we here propose the following molecular model: PR can be regarded as a sequence-changeable copolymer from ABA to BAB triblock copolymers depending on the ring positions (Figure 1). When B segments piled with rings form a hard domain, a backbone polymer of BAB-type PR can bridge the hard B domains (Figure 1b). In this case, the system has network structure and elasticity. On the other hand, PRs lose the network structure and gain a fluidity by switching the sequence from BAB to ABA (Figure 1a). © XXXX American Chemical Society
Received: November 18, 2018 Accepted: December 19, 2018
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DOI: 10.1021/acsmacrolett.8b00896 ACS Macro Lett. 2019, 8, 140−144
Letter
ACS Macro Letters
Figure 1. PR can be regarded as a sequence-changeable copolymer from ABA to BAB triblock copolymer. The configurations of axis chain can transform from an unbridged state (a) to a bridged state (b).
Scheme 1. Chemical Structures of PPR and Synthetic Scheme of HPPR and TMS-HPPR
of the synthesis are described in the Supporting Information). First, PPR was chemically modified with hydroxypropyl groups (HPPR). β-CDs in HPPR have more hydrophilicity and less hydrogen bonding ability than unmodified β-CDs.23 Next, chemical modification of HPPR was performed with hydrophobic trimethyl silyl groups (TMS-HPPR). The removal of free axis polymer and β-CDs was confirmed using sizeexclusion chromatography (SEC), as shown in Figures S1 and S2. The modification ratio of hydroxyl groups of β-CD with hydroxypropyl groups was quantitatively determined to be nearly 100% using 1H NMR analysis (Figure S3). Furthermore, it was found that all the hydroxyl groups of HPPR were modified with trimethyl silyl groups (Figure S4). First, the selfassembled structures were analyzed in order to deduce the ring positions of HPPR and TMS-HPPR. Unmodified β-CDs in an inclusion complex with polymers are known to form well-defined monoclinic crystal domains due to the multipoint hydrogen bonds among β-CDs.19 Therefore, chemical modifications of β-CDs interrupt the hydrogen bonds and likely affect the self-assembling behavior of β-CD.23 For investigating the structures of β-CDs in HPPR and TMS-HPPR, wide-angle X-ray diffraction (WAXD) was used. In the profile of TMS-HPPR, three peaks at q* = 0.33 Å−1 (corresponding to a d-spacing of 19.0 Å; q* is the primary peak position), √3q*, and 2q* (filled arrows) were observed (Figure 2a). The positions of the peaks indicate the formation of the hexagonal columnar structure. Since the periodic distance was 19.0 Å, it is reasonable to conclude that the βCDs were hexagonally packed in a radial direction of β-CD with the expansion of TMS-HP groups (cf. the diameter of unmodified β-CD is 15.3 Å24). Furthermore, a shoulder observed at 0.44 Å−1 (corresponding to a d-spacing of 14.2 Å; open arrow) supports that β-CDs were piled along the axis
Figure 2. WAXD, DSCm and SAXS profiles of (a, c, e) TMS-HPPR and (b, d, f) HPPR.
with the expansion of TMS-HP groups (cf. the height of unmodified β-CD is 9.0 Å24) . It was also found that the β-CD crystal in TMS-HPPR was not deconstructed by heating to 80 °C. On the other hand, the profile of HPPR (Figure 2b) showed only two small peaks at q = 0.70 Å−1 (open arrow) and 141
DOI: 10.1021/acsmacrolett.8b00896 ACS Macro Lett. 2019, 8, 140−144
Letter
ACS Macro Letters
Figure 3. Schematic illustration of the phase behavior of (a−c) HPPR and (d−f) TMS-HPPR. The Tg of PR glass of HPPR and TMS-HPPR is around 15 °C and −25 °C, respectively. The Tm of PEO crystal is around 40 °C in both HPPR and TMS-HPPR. The β-CD crystal domains are not deconstructed even at 80 °C in both HPPR and TMS-HPPR.
0.40 Å−1 (filled arrow), corresponding to a d-spacing of 9.0 and 15.7 Å, respectively. These peaks should be derived from the βCD crystals since the profile was not changed by heating to 80 °C (PEO and PPO do not have any specific structure at this temperature). These d-spacings of the peaks could be assigned to the periodic distances of a height and radial direction of the β-CD crystal structure, which is expanded with HP groups (Figure S5). It is reasonable that the periodic distance is smaller than that of TMS-HPPR. It should be noted that β-CD crystal structure was formed and maintained even at 80 °C in both TMS-HPPR and HPPR. From DSC measurement, we could investigate the thermal properties and the phase states existing in TMS-HPPR and HPPR systems. Baseline shifts based on glass transitions around −25 and 15 °C were observed in the profiles of TMSHPPR (Figure 2c) and HPPR (Figure 2d), respectively. Since the weight fraction of PPO was very small (
