Adhesion between Semihard Polymer Materials Containing Cyclodextrin and Adamantane Based on Host–Guest Interactions

The adhesion between hard materials without any glue is difficult but challenging topic because hard materials inhibit molecular mobility and have a large gap on the adhesive interface. Here we show that host–guest interactions realize a strong adhesive strength between semihard materials based on polyacrylamide xerogel by using a minimal amount of water. The adhesive strength between βCD xerogel(x) and Ad xerogel(y), where βCD and Ad denote β-cyclodextrin and adamantane, respectively, increases as the molar ratio (mol %) of host (x) or guest (y) molecules increases, which indicates that adhesion between βCD xerogel(x) and Ad xerogel(y) is due to complexation between the βCD and the Ad units. Adhesion between βCD xerogel(10) and Ad xerogel(5) displays a robust adhesive strength (5.1 MPa). Competitive experiments using small molecules confirmed the importance of host–guest interactions in adhesion. Optical microscopic observations of the adhesion interfaces reveal that both sides of βCD xerogel(x) and Ad xerogel(y) pull together, reflecting the specific adhesive nature between the two materials. Furthermore, a self-healable semihard material, βCD-Ad xerogel(0.3, 0.4) modified with both 0.3 mol % of βCD and 0.4 mol % of Ad, shows an 88% stress recovery ratio. These results indicate that βCD and Ad moieties function as glue on the molecular level in the solid state.