Mechanical Properties of Supramolecular Polymeric Materials Formed by Cyclodextrins as Host Molecules and Cationic Alkyl Guest Molecules on the Polymer Side Chain

Supramolecular polymeric materials have received much attention in recent years due to the fabrication of supramolecular materials with mechanical properties and stimuli-responsive properties in response to polymeric designs based on noncovalent cross-linkers. Host-guest interactions are useful noncovalent interactions that can be employed to realize supramolecular materials. To understand the relationship between the mechanical properties of supramolecular polymeric materials based on host-guest interactions and the molecular structure of guest molecules on the polymer side chain, we prepared supramolecular hydrogels cross-linked by inclusion complexes between cyclodextrin (CD) and cationic alkyl guests on the polymer side chain. The mechanical properties were influenced by an electric barrier due to the cationic group, CD cavity size, length of the guest unit, charge number of the electric barrier, and reduction responsiveness. Although the introduction of an electric barrier to the end of the alkyl guest moiety increased the rupture stress, the rupture strain decreased due to the electric repulsion of the CD unit and the cationic group. The rupture stress of the alpha CD-cationic alkyl hydrogel was higher than that of the beta CD-cationic alkyl hydrogel because electric repulsion between the alpha CD unit and the cationic group is higher than that of the beta CD unit due to the relatively small cavity size of alpha CD. The fracture energy increased as the alkyl chain length or charge number of the cationic guest group increased. We demonstrated the change in the mechanical properties by reduction stimulus.