Non-swellable self-healing polymer with long-term stability under seawater†
Polymers are widely used in marine environments due to their unique thermal/mechanical properties. Imparting self-healing ability to those polymers would be a beneficial strategy for improved durability; e.g. extending the life time and reducing the maintenance cost. However, most of the existing self-healing polymers suffer from swelling-induced mechanical instability and loss of self-healing ability because of a severe water uptake in fully submerged conditions. Thus, a judicious design principle to prevent the swelling-induced deterioration of the self-healing polymers is greatly required for the long-term use in water-related applications. In this study, we report a polymer that is non-swellable (<2 wt% of swelling) and highly self-healable (91% based on toughness) under seawater. Dynamic crosslinking of catechol-functionalized polymers with p-phenyldiboronic acid (PDBA) through non-ionic boronate ester bonds is the key to realizing these two properties simultaneously. Atomic force microscopy and transmission electron microscopy reveal that the ionically crosslinked catechol polymers contain hydrophilic aggregates of metal (Ca2+)–catechol complexes, referred to as ion-clusters. In contrast, we find the non-ionic boronate ester crosslinkers are not aggregated but finely dispersed in hydrophobic catechol polymer matrix, allowing materials to self-heal with high water stability. Even in seawater, the PDBA-containing catechol polymer retained its mechanical properties for at least 1 month. The polymer described here could provide a valuable strategy for further development of polymeric materials used in marine industries.