Long Alkyl Side Chains Simultaneously Improve Mechanical Robustness and Healing Ability of a Photoswitchable Polymer

Mechanical robustness and healing ability are important properties of polymeric materials. However, the development of mechanically robust yet rapidly healable polymers is challenging. Herein, we demonstrate that long alkyl side chains simultaneously improve mechanical properties and the healing rate of a photoswitchable polymer. The photoswitchable polymer is a polyacrylate. Its side chain contains a hexamethylene spacer, a photoresponsive azobenzene group, and a long n-hexadecyl tail. The n-hexadecyl tail increases the decomposition temperature and glass transition temperature (T-g) of the polymer by 92 and 40 degrees C, respectively. Such an increase of T-g is opposite to textbook knowledge, which makes the polymer robust even at elevated temperatures. The photoswitchable polymer, which has increased T-g and improved mechanical properties, is a hard solid under normal usage conditions. It exhibits healing ability because it can be reversibly switched between a solid and a liquid via photoisomerization of the azobenzene groups. The polymer in the liquid state has significantly reduced viscosity for rapid healing. Importantly, the n-hexadecyl tail increases the healing rate of the polymer by 470 times. The introduction of long alkyl side chains is a new strategy for simultaneously improving mechanical properties and the healing rate of polymeric materials.