UC Berkeley

Adhesives are required to bond to wet surfaces for everyday applications including wound closure tapes, wearable devices, naval repairs, or even in the everyday exposure of commonplace adhesives to humid environments. Yet adhesion is notoriously difficult to wet surfaces – water weakens the intermolecular forces necessary for adhesion and creates interfacial layers separating the adhesive and substrate. Recent progress in improving underwater adhesion has come in part through the inclusion of multidentate groups in adhesives – chemical functionalities with multiple adjacent attachment points, which are believed to make the bond more stable. While molecular scale studies of multidentate groups have supported the macroscale findings of improved underwater adhesion for multidentate adhesives, we have yet to fully understand how the dynamic nature of these bonds impacts the overall adhesive strength of the material.

Connecting macroscale adhesion to the strength and dynamics of interfacial bonds is notoriously hard. Adhesive strength results from a complex combination of physics, including energy dissipated in breaking interfacial bonds, local elastic and plastic deformation near the crack tip, and viscous dissipation within the adhesive as it stretches. The convolution of these mechanisms makes it challenging for researchers to isolate the role of interfacial chemical bonds on overall adhesion, yet understanding each facet is crucial for the rational design and improvement of adhesives. Untangling the role of interfacial chemistry in adhesion using model systems will inform the development of future adhesives.

In this thesis we will develop tools and methods to elucidate the effects of dynamic chemical bonds at an interface on the macroscale adhesive strength and apply these methods to the study of a model tridentate hydrogen bonding epoxy adhesive. First we will present the development of a novel technique to fabricate reactive metal surfaces with extremely low surface roughness. Controlling surface roughness is crucial to measuring adhesion and interfacial forces, as surface roughness can obscure the nanoscale phenomena of interest and variations in contact area due to roughness can skew measured adhesive strengths. By thermally evaporating reactive aluminum onto a smooth mica template, followed by removal of the template in water, we obtain uniform Al/AlO3 surfaces of < 0.2 nm RMS roughness, rivaling the smoothness of the best noble metal2 films. We then demonstrate the applications for these films by improving the estimate of the surface energy of aluminum through adhesion measurements, measuring the surface potential of aluminum in LiCl electrolyte solutions, and reporting improved the corrosion resistance of the ultra-smooth films.

We next investigate the adhesion of a model epoxy adhesive (diglycidyl ether of bisphenol A, DGEBA) containing tridentate hydrogen bonding moieties (tris(hydroxymethyl)amino methane, Tris) to determine the mechanism underlying the strong underwater adhesion of DGEBA-Tris. We first present a model to relate the bond activation energy to the macroscale adhesive strength of a material, and then use self-arresting crack measurements to demonstrate that tridentate adhesives follow predicted adhesive behavior. We next utilize a Surface Forces Apparatus to show that Tris-epoxies exhibit robust adhesion to both mica and ultra-smooth aluminum substrates in water. Finally, we establish that the adhesive strength of DGEBA-Tris epoxies reveals a bond activation energy of 26.6 ± 0.03 kBT in air and 30.3 ± 0.6 kBT in water. These activation energies suggest that adhesion is dominated by tridentate Hydrogen bonds in both air and water, allowing the adhesive to maintain its bonding strength in water.

We subsequently extend our insight into multidentate bonding by exploring the adhesive behavior of DGEBA-Tris polymers as a function of curing and of measurement temperature. We first identify a characteristic threshold velocity, above which DGEBA-Tris polymers exhibit amplified adhesion strength with increased velocity. We then show that while the equilibrium adhesion only minorly increases with cure, the threshold velocity rises dramatically due the increased extensibility of the longer polymer chains after cure. Next, we observe a small growth in zero-velocity adhesion as the temperature is raised from 9 oC to 60 oC which is again eclipsed by the large increase in threshold velocity. The variation of threshold velocity with temperature is shown to follow an Arrhenius dependence, suggesting that the adhesive fracture proceeds through the activated rupture of interfacial bonds. The bond strength is then estimated to be 35±4 ??? at 20 oC, providing further evidence that DGEBA-Tris adhesives form cooperative tridentate hydrogen bonds.