Changing polymer catechol content to generate adhesives for high versus low energy surfaces.

Adhesive bonding is commonly used to replace mechanical fasteners in many applications. However, the surface chemistry of different substrates varies, making adhesion to a variety of materials difficult. Many biological adhesives are adept at sticking to multiple surfaces with a range of surface chemistries. Marine mussels utilize a catechol moiety within their adhesive proteins to bring about surface binding as well as cohesive cross-linking. Mimicking this functionality in synthetic polymers has yielded high strength adhesives that can attach to both high and low surface energy materials, although not equally well. Here, the amount of catechol within a copolymer system was varied for potential tailoring to specific surfaces. Structure-function studies revealed differing trends of optimal catechol content for high energy aluminum versus low energy polytetrafluoroethylene (TeflonTM) surfaces. Adhesion strengths were optimized with ∼10 mol% catechol for aluminum and ∼41 mol% for TeflonTM. Varying the catechol incorporation also resulted in changes to wettability, failure modes, and mechanics on these substrates. When considering performance of the entire bulk material, the different surfaces required an altered adhesive-cohesive balance. Tailoring the composition of polymeric adhesives for different surfaces may aid future manufacturing in cases where joining a variety of materials is required.

Weak Bonds in a Biomimetic Adhesive Enhance Toughness and Performance.

Developing future high performance adhesives is predicated upon achieving properties including strength and ductility. However, designing tough materials that are simultaneously strong and soft is usually contradictory in nature. Biological materials including shells and wood achieve impressive toughness by using weak bonds to connect larger structures at several length scales. Here, we show that this toughness design approach can be applied to synthetic adhesives. A biomimetic adhesive polymer, poly(catechol-acrylic acid), was examined in conjunction with several compounds containing two organic functional groups. In a typical example, the diol ethylene glycol decreased the overall system modulus. Performance was seen to increase significantly. Spectroscopic and physical methods indicated that these bifunctional additives created an interpolymeric network of weak hydrogen bonds. Material toughness was enhanced when breakable bonds were available to dissipate mechanical stresses while leaving the surrounding matrix intact. These discoveries illustrate how a biological materials strategy of interplay between strength and ductility can be achieved with sacrificial bonds in an adhesive. Such an approach may be a general principle applicable to designing higher performance electronics, transportation, and aerospace systems.