Measurement of Ionic Conductivity and Mechanical Property of Slide-Ring Gel Swollen with Propylene Carbonate Including Lithium Ions
Authors: Sugihara Naoki , Kanehashi Shinji , Mayumi Koichi , Tominaga Yoichi , Shimomura Takeshi , Ito Kohzo 
Published: February 2020

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