Pressure-sensitive adhesives with squaramide as a building block

Pressure-sensitive adhesives (PSAs) with squaramide as a building block are reported herein, presenting a modular approach to the rational design of supramolecular adhesives. A poly(butyl acrylate-co-ethyl squaramate methacrylate) scaffold is synthesized and quantitatively functionalized with a library of primary amines (alkyl, mono-hydroxyl, and di-hydroxyl) through highly selective squaramate aminolysis to install diverse side-chain squaramide hydrogen-bonding networks. With the main-chain sequence distribution and molecular weight fixed, the macroscopic viscoelasticity of the resulting squaramide PSAs is directly dictated by the side-chain hydrogen-bonding capacity. As such, the balance between adhesion and cohesion is tuned through the side-chain structure across the functionalized series. Macroscopic evaluations reveal that the alkyl (propylamine) modification provides moderate cohesive enhancement through the squaramide hydrogen-bonding network, the mono-hydroxyl (ethanolamine) modification affords a favorable adhesion–cohesion balance with high lap shear strength (183.3 kPa), high static shear resistance (7920 min under 500 g loading), and high 180° peel strength (11.1 N/25 mm), and the extended mono-hydroxyl (4-amino-1-butanol) and di-hydroxyl (3-amino-1,2-propanediol) modifications furnish high lap shear strength (197.8 and 265.6 kPa) and high cohesive shear resistance (no failure after 7 days under 500 g loading). The squaramide PSAs developed herein showcase squaramide as a versatile and modular building block for the rational design of supramolecular adhesives.