Rational Design of Supramolecular Polymers with Tunable Mechanical Robustness and High Adhesive Strength


Student thesis: Doctoral Thesis

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Award date22 May 2023


Adhesives are typically made from cyanoacrylate, epoxy resin, methacrylate, polyurethane, and polyvinyl alcohol and are widely used in both everyday life and industrial purposes. Although these adhesives have demonstrated exceptional adhesion performance on specific substrates, their permanent bonding or cured nature prevents them from being reused or recycled, limiting their practical applications and increasing their economic costs. Taking advantage of supramolecular strategies in regulating molecular arrangement and mechanical integrity, supramolecular adhesives display various characteristics such as stimuli responsiveness, sticking and detaching capability, and recyclability. However, achieving strong yet reversible adhesion remains difficult and has not been widely reported. Also, high adhesion strength and durability in various conditions, such as temperature, pH, solvent, salt, and chemicals, are less recognized. Furthermore, adhesion mechanisms are not fully understood. This thesis develops a series of supramolecular structural adhesives with improved mechanical strength, and exceptional wet adhesion strength and stability, as well as robust adhesion over a wide temperature or oil inclusion range. Moreover, this thesis highlights the significance of the molecular engineering in the interfacial adhesion and the bulk cohesion of supramolecular adhesives, as well as elucidates the underlying mechanistic insight of mechanical robustness and adhesive strength.

The first chapter introduces the fundamentals of adhesives, and covers the design principles and requirements of supramolecular adhesives, as well as summarizes the emerging applications of supramolecular adhesives.

In chapter two, high-strength supramolecular adhesives with controlled oil inclusion are designed for antimicrobial and antiviral applications. Bifunctional monomers are synthesized and assembled into linear polymers with semi-crystalline stackings by hierarchical hydrogen bonds, where incorporated oil molecules could regulate the semi-crystalline stackings into nanosized crystalline domains via intermolecular hydrogen bonds. The abundant bonding motifs provided by the supramolecular crosslinked networks could accommodate oil molecules with high inclusion capacity and provide additional interfacial binding sites with high adhesion strength, and nanosized crystalline domains could stabilize the organogel network and compensate the interactions with oil molecules to enhance structural and mechanical stability. Besides, healable properties, robust adhesion, and antibacterial and antiviral activity are also demonstrated.

Third, a unique strategy of localized hydration is presented to create supramolecular adhesive tapes with exceptional wet adhesion. The rational molecular design satisfies the requirements for strong wet adhesion: a large amount of interfacial water was removed by hydrophobic polymer chains; the hydrated and softened interfacial layer was mechanically compliant with substrates; high density of hierarchical hydrogen bonding interactions may generate robust adhesion; bulk phase without additional hydration can maintain mechanical robustness. The resultant supramolecular adhesive tapes outperform previously reported and commercial underwater adhesives. Furthermore, two-in-one waterproof tapes and double-sided tapes were developed for diverse underwater applications.

Next, supramolecular structural adhesives with mechanical robustness, high adhesion strength at elevated temperatures, and thermally regulated switchable adhesion are developed. In particular, hierarchical hydrogen-bonding motifs provide strong bonding with hydrophilic and hydrophobic substrates, semi-crystalline and microphase-separated structures offer high bulk mechanical strength and thermal stability, and thermal response of physical dissociations and phase transitions allow for controlled adhesion behaviors. The adhesion strength and stability of such adhesives are equivalent to those of thermoset polymers and structural adhesives.

In summary, this thesis exhibits the rational design and simple synthesis of high-performance supramolecular adhesives using synergistic considerations of interfacial adhesive chemistry and bulk cohesive robustness from the perspective of molecular architectures and aggregation engineering to meet the specific requirements in diverse conditions. This thesis not only advances the fundamental understanding of molecular mechanisms of supramolecular adhesives in high-strength adhesion and excellent underwater adhesion, but also provides new insights into the design and preparation of supramolecular polymers, which will benefit their potential applications in soft robotics, flexible electronics, automotive, construction, and biomedical applications.