Local strain-induced energy storage as driving force for autogenous scratch closure

Autonomous through-thickness scratch repair (healing) in coatings requires scratch closure and interfacial molecular sealing. Although qualitative aspects of the first stage of self-healing have been addressed, quantitative description enabling the control over the healing process need further understanding. In this work the polymer-architecture-dependent stored entropic energy during deformation is quantified using the rubber elasticity theory and correlated to the scratch closure degree experimentally observed in microscopic measurements. Using well-defined thermoplastic healing polyurethanes with variable soft phase fraction contents these studies show that pressure-free damage closure of scratches maintaining mechanical integrity during healing is governed by the capability of the polymer to store entropic energy during damage. The storage (and release) of energy is controlled by varying the damage and healing temperatures in relation to the specific viscoelastic length transition (TVLT) and the glass transition temperature (Tg). Damage closure increases linearly with the entropy release and is controlled by two parameters of the network, the junction density and damping factor. If mechanical damage does not lead to storage of mechanical energy healing does not occur.