Adhesion and surface interactions of a self-healing polymer with multiple hydrogen-bonding groups

The surface properties and self-adhesion mechanism of self-healing poly(butyl acrylate) (PBA) copolymers containing comonomers with 2-ureido-4[1H]-pyrimidinone quadruple hydrogen bonding groups (UPy) are investigated using a surface forces apparatus (SFA) coupled with a top-view optical microscope. The surface energies of PBA-UPy4.0 and PBA-UPy7.2 (with mole percentages of UPy 4.0% and 7.2%, respectively) are estimated to be 45-56 mJ m^-2 under dry condition by contact angle measurements using a three probe liquid method and also by contact and adhesion mechanics tests, as compared to the reported literature value of 31-34 mJ m^-2 for PBA, an increase that is attributed to the strong UPy-UPy H-bonding interactions. The adhesion strengths of PBA-UPy polymers depend on the UPy content, contact time, temperature and humidity level. Fractured PBA-UPy films can fully recover their self-adhesion strength to 40, 81, and 100% in 10 s, 3 h, and 50 h, respectively, under almost zero external load. The fracture patterns (i.e., viscous fingers and highly "self-organized" parallel stripe patterns) have implications for fabricating patterned surfaces in materials science and nanotechnology. These results provide new insights into the fundamental understanding of adhesive mechanisms of multiple hydrogen-bonding polymers and development of novel self-healing and stimuli-responsive materials.