Turning things around: from cationic/anionic complexation-induced nanoemulsion instability to toughened water-resistant waterborne polyurethanes

Abstract

Increasing eco-consciousness has promoted the development of waterborne polyurethanes (WPUs) with high mechanical performances and low volatile organic compound values. However, emulsion instability and poor water resistance severely restrict the applications of WPUs. Herein, we take the opposite way to construct hydrophobic thermoplastic WPU elastomers, that is, harnessing cationic/anionic electrostatic interaction that typically causes emulsion demulsification to enhance the mechanical strength and water resistance of various WPUs. The complexed WPU (termed WPU±) is prepared by mixing WPU+ (cationic PU) and WPU− (anionic PU) nanoemulsions simultaneously, and the resultant precipitates are hot-pressed into thermally reversible polymer films that maintain high tensile strength (over 60 MPa), breaking strain (∼1000%), and fracture toughness (249 kJ m⁻²) due to a collective reinforcement effect of electrostatic and hydrogen bonds. The reversible dynamic bonds afford self-healability at a mild temperature (60 °C). Due to the rapid reconstruction of hydrogen and ionic bonds, the WPU± elastomer demonstrates a long-lasting high adhesion strength in bonding various substrates with lap shear strength of 4.8 MPa under water, which is superior to those of underwater adhesives reported thus far. This cationic/anionic bonding strategy provides a versatile approach for fabricating water-resistant high-performance WPUs, showing great potential for expanding their application scope in wet conditions.