Zwitterionic-Cationic, Catechol-Based Polyesters Reinforced by In Situ Sol–Gel Inorganic Nanoparticles for Rebondable Waterborne Adhesion

Abstract Unlike conventional solvent-borne systems, which raise concerns regarding volatile organic compound (VOC) emissions and toxicity, the waterborne polymer adhesives developed in this study incorporate marine mussel-inspired catechol functionalities and zwitterionic/cationic moieties into a terephthalate-based polyester backbone. The resulting polymers, PSBD0–PSBD4, exhibit excellent hydrophilicity and rebondable adhesion. The optimal catechol-bearing formulation, PSBD2, shows a lap-shear strength of 2.09 ± 0.09 MPa, an 83% increase over the catechol-free PSBD0. The polymers are further reinforced by in situ generated Ti- or Si-based sol–gel inorganic nanoparticles. The corresponding PSBD composites achieved average lap-shear strengths of 2.47 ± 0.22 MPa (Ti-based) and 2.72 ± 0.54 MPa (Si-based), corresponding to 18% and 30% increases over neat PSBD2, and the highest single cycle lap-shear strengths reached 3.27 ± 1.18 MPa (Ti-based) and 3.34 ± 0.78 MPa (Si-based) under the best conditions. Furthermore, the polymer can be hydrolyzed under mild basic conditions. SEM and EDX analyses reveal that catechol incorporation enhances the morphological and sol–gel distribution homogeneity of the materials, which likely accounts for the observed enhancement in macroscopic adhesion strength. TEM analysis reveals continuous particle growth within the composites during the repeated wetting/drying processes, and solid-state 29Si NMR analysis was used to characterize the corresponding chemical states. Finally, a scheme interpreting the reinforcement mechanism and the structure-performance relationship is proposed. This study demonstrates a viable molecular design strategy for developing waterborne polymer adhesives.