Self-Healing Inside a Scratch of Oxetane-Substituted Chitosan-Polyuretheane (OXE-CHI-PUR) Networks
Originally published in Journal of Materials Chemistry, 2011, 21, 14473-14486
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These studies elucidate the origin of molecular and macroscopic processes of oxetane-substituted chitosan-polyurethane (OXE-CHI-PUR) crosslinked networks that occur inside a mechanically generated scratch during UV-initiated self-repair. The role of each network component, OXE, CHI, PUR, and dibutyltin dilaurate (DBTDL), inside the scratch of the OXE-CHI-PUR damaged network is determined using chemical and thermo-mechanical imaging probes. These studies show that the presence of controlled acidic environments determines whether self-healing will occur or not. The network remodeling during UV initiated repair involves activation of dormant oxonium ions that react with accessible and reactive macromolecular ends resulting from mechanical damage. Precisely controlled stoichiometry of the individual components giving pH = 6.8 induce cationic OXE ring opening as well as polyurea-to-polyurethane (PUA-to-PUR) conversions driven by a free radical process. This is accompanied by chair-to-boat conformational changes of glycosine units of CHI moieties. These orchestrated events trigger intra- and intermolecular crosslinking reactions resulting in self-repair via –C–O–C– bond formation. Deviations towards higher acidity will have adverse effects, and instead of repair, expansion of the scratch will occur. Upon mechanical damage, the glass transition temperature (Tg) inside the surface of the scratch diminishes, but upon repair a scar is formed, and the Tg is recovered. The scratch recovery during self-repair reactions is initiated at the lowest point of the scratch that exhibits the highest surface energy, thus forcing fragmented network components to flow to the top to form a scar. These studies show that minute localized chemical changes within a network have a tremendous impact on network rearrangements during repair processes.