An investigation into the relaxation response of a self-healing dual network polymer through controlled introduction of strain damage

Abstract

Self-healing ability in polymers is typically probed by monitoring restorative properties after localized macroscopic damage or repetitive cycles of destructive testing followed by a healing cycle (e.g. cut-and-heal tests). However, the restoration of homogeneously distributed damage before ultimate failure remains largely unattended in literature. This work investigates whether damage degree and damage restoration after plastic deformation in self-healing polymer networks can be identified via continuous relaxation spectra from rheology, which were recently successfully employed to identify energy contributions in self-healing polyurethanes. As a model system a self-healing epoxy-silane dual network with dynamic sulfur-sulfur bonds is used together with a non-healing reference epoxy. Three levels of deformation are probed: pristine, partially plastic (near yield), and fully plastic (near failure). Compared to the non-healing reference epoxy, the self-healing epoxy is observed to have faster dynamics of the main relaxation peak and to dissipate more energy at very fast (10-9 s) relaxation times. It is found that any level of plastic deformation (near yield or near failure) induces clear changes in the energy profile compared to the pristine state (e.g. relaxation peak height drop). Between the two levels of plastic deformation, no clear distinction could be made. An oven treatment, intended to repair any induced damage by stimulating network mobility through sulfur-sulfur bond reshuffling above Tg, was found to shift the energy profile to longer relaxation times for both pristine and plastically deformed samples not observed in the non-healing polymer. However, this time shift could be undone by allowing a one week recovery period after the oven treatment. It is proposed that the observed behavior can be explained by trapping, in self-healing polymers, a non-equilibrium polymer state through quenching. Despite the necessary further research, the results confirm the potential of continuous relaxation spectra to evaluate damage degree and damage restoration in plastically deformed self-healing polymers.