The challenges of obtaining mechanical strength in self-healing polymers containing dynamic covalent bonds

Although the use of dynamic bonds for the development of self-healing polymers has been widely studied, their concomitant influence on the mechanical properties and the fundamental challenges associated with their implementation in mechanically strong materials have often been ignored. In this work, the underlying role of dynamic covalent bonds on the mechanical properties and self-healing behavior in mechanically strong materials is explored, using as a case study a series of films cast from waterborne, aromatic disulfide-containing, poly(urethane-urea)s (PUUs). Both linear and cross-linked waterborne PUU dispersions were synthesized containing either dynamic S–S bonds or comparable “static” C–C bonds. These materials were compared by means of thermal, mechanical, rheological and fracture mechanical analysis. It is shown that, as expected, dynamic bonds significantly contribute to reducing the time scale of healing and allow for higher healing degrees in both linear and cross-linked polymers. However, the presence of dynamic bonds simultaneously diminishes the inherent mechanical strength of the PUUs. Therefore, the dynamic nature of the disulfide bond leads to both the desired enhancement of chain mobility and to the undesired weakening of the material. It is clear that these two contrasting effects must be carefully balanced in the design of self-healing materials.