Stress-aided thermal activation of crack propagation in multidentate hydrogen bonding adhesives.

Abstract

Adhesives containing multidentate hydrogen bonding moieties are gaining prominence for their ability to adhere strongly underwater. Previous studies attributed their remarkable underwater adhesion to the multiple adjacent attachment points within a moiety stabilizing the bond, enabling cooperative hydrogen bonding. However, as adhesion involves multiple coupled phenomena, isolating the contribution of individual bonds to the adhesive strength remains challenging. Here we investigate the relationship between peeling velocity and adhesion over a range of temperatures to estimate the activation energy of the chemical bonds that fracture at the adhesive interface. We utilize a model epoxy modified by the addition of tridentate hydrogen bonding moieties (DGEBA-Tris). We report on the effect of curing, debonding temperature, and crack velocity on the adhesive strength at the DGEBA-Tris/mica interface. Adhesion is measured using self-arrested crack propagation to probe the threshold velocity above which the energy release rate transitions from velocity-independent to increasing sharply with crack velocity. We measure a shift by two orders of magnitude in the threshold velocity as the debonding temperature increases from 9 °C to 60 °C, consistent with a thermally activated process. The increase in threshold velocity with an increase in temperature follows an Arrhenius dependence revealing a bond activation energy of 14 ± 2 × 10^-20 J (or 35 ± 4<i>k</i>_B<i>T</i> at 20 °C). The bond energy and associated temperature dependence of the energy release rate suggest that adhesion is dominated by cooperative tridentate hydrogen bonds, and that adhesive fracture of these bonds proceeds through an activated process.