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Stimuli-responsive polymers that are sensitive to certain triggers from the external environment have been attracting many researchers’ interests. Stimulus that can be used to trigger these materials have been varied from temperature, light, electrical or magnetic fields, to chemicals. However, some applications require new triggers such as force. We thought that polyrotaxane-based gel is a new candidate of force-responsive material. Polyrotaxane is a type of mechanically interlocked supramolecule consisting of polymer axle and ring molecules, in which multiple rings are threaded onto a polymer axle and prevented from dethreading by bulky end groups. When a force is applied to polyrotaxane gels, in which polyrotaxanes are crosslinked through ring molecules, ring molecules slide along an axle polymer to evenly distribute the applied force. Recently we created block polyrotaxane whose axle is a triblock copolymer poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPO-PEO).[1] β-Cyclodextrin (β-CD) form inclusion complex with PPO but not with PEO in water. We hypothesized that chemical property of the hydrogels obtained from this block polyrotaxane would be converted to hydrophobic by force. β-CDs should stay on PPO segment without loading, while β-CDs would slide from PPO to PEO segment to expose hydrophobic PPO segment when a force is applied. In order to confirm this concept, we studied hydrogels based on polyrotaxane made of β-CDs and PEO-PPO-PEO with several molecular weight and PEO/PPO ratio. Three polyrotaxanes compared in this study are followings; PEO60-PPO30-PEO60 with 17CDs (PPR_7k), PEO80-PPO70-PEO80 with 13CDs (PPR_11k), PEO110-PPO55-PEO110 with 12CDs (PPR_13k) (subscripts denote a number of repeating unit of each segment). 1H NMR measurements were conducted to investigate a position of β-CDs of a series of polyrotaxanes in water. It was found that β-CDs in the PPR_13k shuttles both on PEO and PPO segment but β-CDs in PR_7k and PR_11k stayed on PPO. Small-angle X-ray scattering (SAXS) was conducted to the hydrogels obtained from cross-linked polyrotaxane. Figure shows SAXS 1D profiles of the hydrogel obtained from PPR_11k. Before stretching, no peak was observed based on a periodic structure. Then this hydrogel was extended to 1.8 times of the original state to show a peak corresponding to a periodic distance of 29 nm. These results indicate PPO segments were exposed after loading to form hydrophobic periodic domains in PPR_11k. Other hydrogels obtained from PPR_7k and PPR_13k did not show any periodic structures even after loading. We are hoping that this new force-responsive block polyrotaxane system can be applied to the materials that can alter materials permeability by an applied force. [1] S. Uenuma et al. Chem. lett., 45 (8), 991-993 (2016) Figure 1

To overcome the trade-off relationship between stiffness and toughness of conventional polymer gels with covalent cross-links, introducing dynamical cross-links into polymer network is a promising strategy. A typical example of dynamically cross-linked gels is slide-ring gel, in which polymer chains are cross-linked by rings and the cross-linking points can slide along polymer chains. When slide-ring gels are deformed, the network structure of slide-ring gels is homogenized due to the sliding of the ring molecules, which causes their high deformability [1]. We have recently performed crack propagation tests on slide-ring gels and found that their fracture energy is dominated by the sliding distance of the movable cross-links [2]. In this work, we investigate the relationship between the macroscopic fracture toughness of slide-ring gels and nano-scale sliding dynamics of rings on polymer chains. Slide-ring gels are prepared by cross-linking ring molecules of polyrotaxanes, necklace-like supramolecules consisting of cyclodextrins and polyethylene glycol. The fracture energy of slide-ring gels keeps constant at strain rates below a certain value, and decreases at a high strain rate, where the movable cross-linking points cannot slide and behave like covalent cross-links. From the strain rate dependence of the fracture energy, we have estimated the time scale for the sliding dynamics of cross-links in slide-ring gels. References [1] Ito, K., Polym. J., 39, 489 (2007). [2] Liu, C., Kadono, H., Mayumi, K., Kato, K., Yokoyama, H., Ito, K. ACS Macro Letters, 6, 1409 (2017). Figure 1

The slide-ring (SR) gel is made from polyrotaxane (PR), in which cyclic cyclodextrins (CDs) are threaded on an axial poly(ethylene glycol) (PEG) polymer chain capped by bulky end groups, through intermolecular crosslinking between the CDs. The CDs form mobile cross-linking junctions that slide along the PEG, so that these SR gels exhibit unique characteristics, ”pulley effect ”, such as unique swelling and mechanical properties. This research was directed to use SR gel to achieve polymer gel electrolytes with both high ionic conductivity and mechanical ductility, which was one of the indexes of mechanical strength, at the same time. By using the SR gels swollen with propylene carbonate containing lithium ion, this research explores the use of the SR gel to achieve polymer gel electrolytes with both high ionic conductivity and mechanical ductility. First, by modifying the hydroxyl groups of the CDs by methyl groups, the affinity with several solvents, particularly mixed electrolysis solution (ES) of propylene carbonate (PC) and lithium salt, was improved, and the SR gel was swollen with ES. In the present research, we report the ionic conductivity of PR or methylated polyrotaxanes (MePR)-SR gels containing lithium ES with varying the crosslinking density of SR gels to clarify the ion transport of ES in the SR gel network. Furthermore, we report compressive measurement of SR gel containing ES to show that SR gels have high ionic conductivity as well as high mechanical ductility. MePRs were prepared with a degree of substitution (DS) at 27.5 and 74.2 %. Subsequently, MePR-SR gels were prepared with various number ratios of crosslinking reagents, such as divinyl sulfone where the sum of hydroxyl and methyl group of MeCD was varied from 6 to 10 %. The SR gel was swollen with pure PC or ES, which was 1.0 M PC solution of LiTFSI, followed by evaluation of the swelling ratio of the SR gel. For investigating electrical properties, we measured ionic conductivity, molar conductivity and potential window of SR gels swollen with ES. Furthermore, the relationship of ionic conductivity and temperature of those swollen gel was examined to obtain activation energy. Moreover, by the compaction test of MePR-SR gel with DS of 74.2 %, the mechanical property was investigated. A pure PC could hardly make the SR gel swell both for PR-SR gel and MePR-SR gel with DS of 27.5 %, while MePR-SR gels with DS of 74.2 % could be slightly swollen with pure PC. Since PEG, which is component of PR, can be soluble in PC, the ability of swelling depends on the affinity of PC to CDs or MeCDs. Increasing DS of MeCDs make MePR soluble to pure PC, and consequently MePR-SR gels with DS of 74.2 % was slightly swollen with pure PC. Next, ES could not make PR-SR gel swell at all, while MePR-SR gels could be swollen with ES very well. From the results of conductivity measurements, MePR-SR gel with DS of 74.2 % and the number ratio of crosslinking reagent of 6 mol% had 95 % of molar conductivity for pure ES, and decreased with increasing number ratio of crosslinking reagent, which confirmed that the polymer matrix distrurbed the ionic transport of the ES. Whereas, the potential window of MePR-SR gels with DS of 74.2 % were identically 5.5 V, which agreed with the value of PC-LiTFSI. This result suggested that SR gels do not affect potential window. The mechanical property of MePR-SR gel with DS of 74.2 %, in all gels, Young’s modulus was quite small that meant that these gels were quite soft. With swelling ratio, that is, decreasing number ratios of crosslinking reagents, Young’s modulus decreased and gel became a bit soft. MePR-SR gels with high number ratios of crosslinking reagents were fractured, while MePR-SR gel with low number ratios of crosslinking reagents did not fractured under the compression of almost half thickness than its original thickness. This result suggested that gels with a small crosslinking density were very soft, hence the stress was dispersed and cracks were difficult to generate, so gels were not fractured under high compression. Figure 1