Effect of Interfacial Adhesion on Viscoelastic Relaxation Processes in Thin Polymer Film Indentation

Polymer coatings are dependent on strongly bonded buried interfaces to maintain adhesion and protective properties over long application lifetimes. In this work, we detail the detection of buried interfaces with deviations from perfect bonded or perfect slip interfaces using surface indentation on thin films with large contact areas. The interfacial interactions between photopolymerized methacrylate films and a glass substrate were tailored using silane chemistry to create an interface that either easily releases from or cross-links with the polymer network. Creep compliance measurements on the polymethacrylate films were compared with predicted contact measurements for literature models of ideal interfaces. Nonideal contributions from interfacial effects were detected during experiments with high confinement. The extent of these effects varied with polymer network structure and polymer/substrate interface strength, with fluorinated interfaces exhibiting up to a 25% increase in indentation contact area as compared to an ideal bonded indentation due to the presence of a weak interface. The ability to probe the response of a buried interface under low indentation loads is attractive for testing and validating the interfacial properties and performance of coatings and films. This approach could be used to interrogate the fidelity of an interface in critical areas such as corrosion protection and encapsulation.