Effects of Ring Size on the Dynamics of Polyrotaxane Glass

Abstract

The dynamics of two polyrotaxanes with different ring component sizes were compared through viscoelastic measurements. An improved polyrotaxane synthesis was used to develop a glass-forming polyrotaxane composed of γ-cyclodextrin (γ-CD), which is a larger cyclic molecule than α-cyclodextrin (α-CD)—a component of a comparative polyrotaxane. In the glass state, both polyrotaxanes exhibited a large secondary relaxation attributed to the confined motion of the threading polyethylene glycol as the main chain, and their dynamics were not affected by the ring size. However, the glass-transition dynamics were clearly steepened by the enlargement of rings. The analysis of the glass-transition dynamics revealed that the segment motion of polyrotaxane, which is the translation of the threading polymer with several rings threaded cooperatively, was released near the glass-transition temperature determined by differential scanning calorimetry, Tg(d), in the case of polyrotaxane with γ-CD, whereas the release was relatively delayed in the case of α-CD. This finding suggests that the enlargement of the rings weakens the topological constraints that prolong the onset of the segment motion of polyrotaxanes. Positron annihilation lifetime measurements showed a negligible difference in the free volume of polyrotaxane glasses with different ring sizes. This finding indicates that the larger ring did not provide an additional space for the segment motion but probably an undetectable small gap between the inner wall of the ring and the threading polymer to facilitate the ring-chain cooperative motion.