Structure and dynamics of polyrotaxane-based sliding graft copolymers with alkyl side chains

Abstract

Polyrotaxanes (PRs) with side chains grafted onto cyclic molecules (sliding graft copolymers, SGCs) can be categorized as a new type of graft copolymer, in the sense that the grafting point of the side chains can move along the main chain. In this work, alkyl oligomers were employed as side chains for the precise control of side chain length. Several SGCs with different side chain lengths were prepared from the PR of polyethylene glycol and α-cyclodextrin (α-CD). The solid-state structures of the SGCs were investigated by X-ray scattering. The SGCs formed periodic structures whose sizes increased linearly with the side chain length. SGCs with crystalline side chains exhibited crystalline structures of side chain and packing structures of the α-CDs; these were synchronously driven by the sliding and rearrangement of the α-CDs with the side chains on the main backbone chain. Viscoelastic measurements of solid-state SGCs revealed that SGCs with non-crystalline side chains commonly had a broad relaxation process corresponding to the cooperative fluctuation of the CDs and PEG backbone, and some of them also exhibited an anomalous shoulder. This new viscoelastic mode was presumably due to the sliding motion of CD molecules with the side chains along the backbone.