Healable, Glassy Azobenzene Polymers at Room Temperature Based on Photoinduced Reversible Solid-to-Liquid Transitions

Abstract

Designing healable, glassy polymers at room temperature remains a significant challenge due to the higher glass transition temperature (T_g) of glassy polymers than room temperature restricting the molecular or segmental mobility at room temperature. Glassy polymers present low healing efficiency although enabling self-healing through secondary transition of small-sized structural units in molecular chains. Herein, the healable, glassy azobenzene polymers (PnC-AZO) with the tunable methylene-based spacer lengths (n = 0, 3, 6, 9, 12) and the designed number-average molecular weight (M_n < 10 kDa) were synthesized and developed by free radical polymerization of azobenzene monomers. The PnC-AZO (n = 6, 9, 12) with trans-azobenzene structures shows the characteristic transition temperature (T_trans) in rheology and crystallization melting temperature (T_m) at ∼50 °C much higher than room temperature and yet presents efficient healing ability at room temperature by alternating ultraviolet light and green light irradiation, resulting from the photoinduced reversible solid-to-liquid transitions along with reversible isomerization between thermodynamically stable trans-configuration and metastable cis-configuration of azobenzene units in PnC-AZO. This will provide an alternative toward healable, glassy polymers at room temperature for future sustainable material design.