Thiourea as a "Polar Hydrophobic" Hydrogen-Bonding Motif: Application to Highly Durable All-Underwater Adhesion

Here, we report that, in contrast to urea, thiourea functions as a "polar hydrophobic" hydrogen-bonding motif. Although thiourea is more acidic than urea, thiourea exchanges its N-H protons with water at a rate that is 160 times slower than that for urea at 70 degrees C. This suggests that thiourea is much less hydrated than urea in an aqueous environment. What led us to this interesting principle was the serendipitous finding that self-healable poly(ether thiourea) adhered strongly to wet glass surfaces. This discovery enabled us to develop an exceptionally durable all-underwater adhesive that can maintain large adhesive strength for over a year even in seawater, simply by mechanically mixing three water-insoluble liquid components on target surfaces. Because thiourea is hydrophobic, its hydrogen-bonding networks within the adhesive structure and at the adhesive-target interface are presumed to be dehydrated. For comparison, a reference adhesive using urea as a representative "polar hydrophilic" hydrogen-bonding motif was durable for less than 4 days in water. Highly durable all-underwater adhesives are needed in various fields of marine engineering and biomedical sciences, but their development has been a major challenge because a hydration layer that spontaneously forms in water always inhibits adhesion.