Enhanced Adhesion and Cohesion of Bioinspired Dry/Wet Pressure-Sensitive Adhesives

Abstract

The byssus-mediated adhesion of marine mussels is a widely mimicked system for robust adhesion in both dry and wet conditions. Mussel holdfasts are fabricated from proteins that contain a significant amount of the unique catecholic amino acid dihydroxyphenylalanine, which plays a key role in enhancing interfacial adhesion to organic and inorganic marine surfaces and contributes to cohesive strength of the holdfast. In this work, pressuresensitive adhesives (PSAs) were synthesized by copolymerization of dopamine methacrylamide (DMA) with common PSA monomers, butyl acrylate and acrylic acid, with careful attention paid to the effects of catechol on adhesive and cohesive properties. A combination of microscopic and macroscopic adhesion assays was used to study the effect of catechol on adhesion performance of acrylic PSAs. Addition of only 5% DMA to a conventional PSA copolymer containing butyl acrylate and acrylic acid resulted in 6-fold and 2.5-fold increases in work required to separate the PSA from silica and polystyrene, respectively, and a large increase in 180°peel adhesion against stainless steel after 24 h storage in both ambient and underwater conditions. Moreover, the holding power of the catechol PSAs on both steel and high-density polyethylene under shear load continuously increased as a function of catechol concentration, up to a maximum of 10% DMA. We also observed stark increases in shear and peel adhesion for the catecholic adhesives over PSAs with noncatecholic aromatic motifs, further underlining the benefits of catechols in PSAs. Overall, catechol PSAs perform extremely well on polar and metallic surfaces. The advantage of incorporating catechols in PSA formulations, however, is less straightforward for peel adhesion in nonpolar, organic substrates and tackiness of the PSAs.

Cooperativity of Catechols and Amines in High‐Performance Dry/Wet Adhesives

“…Molecular weight and T g values of polymers were measured using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) and are listed in Table (Supporting Information, Table S1, Figures S40, S41). Compared to the control PSA polymer (BA 91.4 % :AA 8.6 % ), the functional polymers had higher M w , which can be attributed to the enhanced non‐covalent and covalent interactions between the polymer chains owing to the presence of catechol groups, as described previously in more detail . The small variations observed between the molecular weight and T g of the functional polymers can further underline that physical properties of PSAs are not expected to be a major contributor to the differences noticed in their adhesive performance…”

“…In a previous effort to improve their wet adhesion, we applied the mussel inspiration principles to the base PSA polymers and showed that catechol PSAs outperformed conventional benchmarks in different industry standard tests even at a low catechol content (ca. 3 mol %) . Apart from the inferior underwater performance, another major shortcoming of conventional PSAs is their poor cohesive strength and low toughness.…”

“…However, the low modulus can lead to poor cohesive strength which can substantially limit the use of PSAs to low stress applications . Although a few strategies have been previously proposed to improve the cohesive strength of PSAs, optimizing the adhesive and cohesive properties of PSAs has proven to be a challenging task, as improving one property often comes at the expense of another …”

“…To isolate the effect of composition on adhesion, we chose to keep the functional monomer feed ratio fixed at 2.85 mol % (Table ), as this concentration of catechol was demonstrated to be sufficient for boosting the adhesive performance of acrylic PSAs . Calculations based on the integration of area under the peaks from functional groups in the NMR spectra showed that the compositional drifts from the intended feed ratio in the polymerization were not significant (Supporting Information, Table S2 and Figures S29–S34).…”

“…An alternative approach to estimate the magnitude of the overall adhesive strength between the polymer film and the substrate in the contact area is to integrate the area under the retraction trace in the F ‐ D curves . The calculated quantity has the units of energy and will be referred to as separation work ( W ) herein . The separation work calculated herein is related to the energy that is required to detach the probe from the substrate …”