Behavior of supramolecular cross-links formed by host-guest interactions in hydrogels responding to water contents

Reversible or movable cross-links based on host-guest interactions in supramolecular materials create functions such as enhanced mechanical properties and stimulus responsiveness. A supramolecular complex on a polymer side chain achieves the described functions as a reversible cross-link. Generally, 1:1 complex is utilized as a cross-link for the host-guest interaction based-supramolecular materials, but herein, the 1:2 complex between γ-cyclodextrin (γCD) and methyl viologen terminated undecyl acrylate (VC11) was chosen to cross-link three polymeric chains. The obtained supramolecular hydrogels γCD-VC11 (x, y) (x and y represent the mol% of the γCD and VC11 units in feeding, respectively) was prepared by radical copolymerization with acrylamide as a mainchain monomer. Tensile test revealed that toughness (Gf) of γCD-VC11 (x, y) maximized at water content (Wc) ≈ 40 wt.%. The maxima of Gf depended on the amount of γCD and VC11 units. Through dynamic mechanical analysis, Arrhenius style plots between horizontal shift factors and temperature determined activation energies (Ea) of each γCD-VC11 (x, y) and MBAAm (0.5). Ea of MBAAm (0.5) monotonically decreased with the increasing Wc. On the other hand, slope of Ea of γCD-VC11 (x, y) changed at Wc ≈ 40 wt.%. Simultaneously considering results of the tensile tests and DMA, the reversible cross-links formed by γCD and VC11 in γCD-VC11 (x, y) function effectively at appropriate Wc.