UV-initiated self-healing of oxolane–chitosan–polyurethane (OXO–CHI–PUR) networks

Abstract

These studies show that oxolane–chitosan–polyurethane (OXO–CHI–PUR) networks exhibit self-repairing behavior upon exposure to UV light. Spectroscopic analysis revealed that molecular level processes responsible for the self-repairing mechanism of OXO–CHI–PUR networks are driven by free radical catalyzed polyurea-to-polyurethane conversion, formation of linear –C–O–C– segments via ring opening of OXO rings, as well as chair-to-boat conformational changes of glycosine units of the CHI backbone macromonomer. Incorporation of the OXO five-member ring instead of OXE (four-member) into polyurethane networks facilitates slower but equally effective self-healing characteristics. The role of protic conditions inside the scratch showed that highly acidic environments favor scratch expansions instead of self-healing. Similar to self-healing of OXE–CHI–PUR networks, as a result of mechanical damage, glass transition temperature (T_g) is lowered within the damaged area, resulting in the formation of smaller macromolecular oligomeric entities with enhanced mobility. The repair mechanisms measured by the thermo-mechanical response inside and outside the scratch showed that upon UV light exposure the damaged area is repaired by the frontal growth reactions from the bottom of the scratch to the top, and the mass flow is facilitated by the lower glass transition temperature (T_g) inside the scratch. These studies also show that the presence of hindered amine light stabilizers (HALSs) retards self-healing, which can be compensated by elevated levels of the OXO component in OXO–CHI–PUR networks.