Light-induced healing in azobenzene bridged silsesquioxanes

Abstract

Intrinsic healable polymers are materials capable of repairing itself through its chemical nature, in order to improve stability and durability and to restore the lost functionalities or properties. In this study, azobenzene (AZO)moieties were incorporated into a previously reported bridged silsesquioxane based on the reaction of isocyanatepropyltriethoxysilane (IPTS)with bisphenol A (BPA), by replacing a small fraction of BPA by a bisazophenol (4,4′-dihydroxyazobenzene, AZOH). The incorporation of these AZO moieties led to a material with UV light-induced healing abilities. The underlying healing mechanism is attributed to the intra-molecular conformational changes of the azo-chromophores that are induced by the trans to cis photoisomerization of the azobenzene. These changes temporary increased its mobility and allowed the material to flow in the damaged area, followed by a process of restoring the physical hydrogen bonds. This process was monitored following the viscoelastic properties during successive cycles of turning ON and OFF the UV irradiation. Remarkably, the resultant healed material has not significant observed mechanical differences with the original one.