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

Abstract

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 (G_f) of γCD-VC11 (x, y) maximized at water content (W_c) ≈ 40 wt.%. The maxima of G_f 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 (E_a) of each γCD-VC11 (x, y) and MBAAm (0.5). E_a of MBAAm (0.5) monotonically decreased with the increasing W_c. On the other hand, slope of E_a of γCD-VC11 (x, y) changed at W_c ≈ 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 W_c.