Thermally induced disassembly mechanism of pseudo-polyrotaxane nanosheets consisting of β-CD and a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer

Abstract

A pseudo-polyrotaxane (PPR) is a topological supramolecule in which an axis polymer chain is included by multiple cyclic molecules such as cyclodextrins (CDs). We recently reported the formation of a pseudo-polyrotaxane nanosheet (PPRNS), which possesses a self-assembled nanosheet structure bearing a PPR consisting of β-cyclodextrins (β-CDs) and a carboxyl-terminated poly(ethylene oxide)₇₅-b-poly(propylene oxide)₂₉-b-poly(ethylene oxide)₇₅ triblock copolymer (COOH-EO₇₅PO₂₉EO₇₅). In this study, we investigated the disassembly mechanism of the PPRNS upon heating from the viewpoint of crystallinity, particle morphology, and PPRNS concentration through proton nuclear magnetic resonance spectroscopy, scanning electron microscopy, and X-ray scattering analysis. The results revealed that the PPRNS dissolved in two steps during heating. First, β-CDs of the PPRNS de-threaded from the axis and the PPRNS dissolved in the out-of-plane direction thinned while maintaining the rhombic structure. Upon further heating, both β-CD and the PPR dissolved, collapsing the rhombic structure of the PPRNS in the in-plane direction—this occurred upon the melting of the β-CD crystals of the PPRNS—thereby achieving an amorphous state. In conclusion, the PPRNS showed structural changes in two directions over different temperature ranges upon heating, owing to the anisotropy of the topological constraint of β-CD by the axis polymer.