Molecular Level Design of Self-Healing Polyurethanes

Self-healing synthetic materials emerged a couple of decades ago and continue to attract scientific community driven by the opportunity to develop life-like materials and sustainable technology. Inspired by living organisms which utilize metabolic processes to achieve adaptive, reproductive, and self-healing functions, self-healing synthetic materials require dynamic and precise control of sequential chemical and physical events similar to biological systems. The study presented in this dissertation is inspired by the role of carbohydrates in biological systems as energy storage molecules and structural components. The objective is to obtain self-healing by incorporating carbohydrate molecules into crosslinked polyurethane networks with sophisticated network design, and study the self-healing mechanisms from both chemical and physical perspective. Two self-healing systems were developed containing carbohydrate molecules, such as methyl-α-D-glucopyranoside (MGP), that are crosslinked into polyurethane (PUR) networks. When network formation is catalyzed by dibutyltin dilaurate (DBTDL), these materials are capable of self-repairing in air by reacting with atmospheric amounts of CO2 and H2O, thus resembling plant’s behavior of carbon fixation during photosynthesis cycle. However, when organotin catalyst was replaced by zinc acetate (Zn(OAc)2), similar network composed MGP and PUR exhibited completely different self-healing behavior and chemical re-bonding mechanism. Self-healing proceeds at elevated temperatures, is independent of CO2 or H2O, and is attributed to reaction of damage-induced amines to reform covalent linkages. As stated earlier, self-healing synthetic materials require dynamic and precise control of sequential chemical and physical events. Therefore, the physical driving force for self-healing was also explored by mechanical analysis and thermodynamic studies. A three-step process is proposed which can lead to self-healing of polymers: 1) shape memory effect induced damage closure, which generates interfacial contacts; 2) chain rearrangements within the interface along with 3) subsequent covalent bond reformations lead to recovery of mechanical strength. Using the three-step self-healing concept as a platform, a multi-functional material encoded with color change, shape memory, and self-healing attributes in one is designed and synthesized. Phase-separated morphologies are achieved by a chemical makeup and directional drawing that lead to fibrous morphologies of polycaprolactone and polyurethane. The built-in heterogeneity and interphase structure are critical in achieving enhanced shape memory and self-healing attributes