Novel Self-Healable Thermosets and Their Carbon Fiber Reinforced Polymer (CFRP) Composites

Abstract

Intrinsically self-healing polymers allow repeatable damage repair of a fracture located within materials without the need for external additives. Self-healing polymers possess promising potential for numerous structural applications, such as the construction, automobile, and aerospace industries, due to their capability to reduce system maintenance requirements and increase the longevity and safety of the composite structures. The healing capacity of the polymers is derived from the formation of covalent bonds between reactive components at the fracture spot. These reactive components are typically varied in their chemical structures and are dependent on the employed healing chemistry, therefore a variety of new chemistries have been reported. However, current self-healing polymers either 1) undergo significant thermal degradation at elevated temperatures, or 2) lose their mechanical strength and stiffness above their typically low glass transition temperature and are thus limited by the healing chemistry. Developing a new healing chemistry which exhibits thermal stability and maintains mechanical properties and mechanical stability at a higher temperature range is of great importance for the development of high-performance self-healing materials for structural applications and has been a long lasting challenge in the composites research field. This dissertation details the discovery of a novel healing chemistry with stability at high temperature, based on which new self-healing materials with significantly improved high-temperature performance and mechanical stability have been developed. Initially, this new healing chemistry of isocyanurate-to-oxazolidone transformation is studied both in model compounds and in the polymer network, where the instability of isocyanurate in the presence of epoxide is confirmed. The isocyanurate-to-oxazolidone transformation is then embedded in isocyanurate-oxazolidone (ISOX) polymers and the polymerization mechanism of the ISOX polymers, including the initiation, cross-linking, and network propagation is further investigated. Based on the understanding of the polymerization mechanism, a two-step curing procedure is designed for the ISOX polymers to ensure the presence of a large amount of two reactive components, isocyanurate and epoxide, for the isocyanurate-to-oxazolidone transformation within the polymers. Combined chemical characterization through Fourier-transform infrared (FTIR) spectroscopy and carbon nuclear magnetic resonance (NMR) spectroscopy is performed in order to quantify the chemical composition of the ISOX polymers, including the isocyanurate fraction which can be controlled through both the nucleophilicity of the polymerization catalyst and the duration of the post-cure of the polymers. Secondly, the developed ISOX polymers are evaluated and shown to achieve damage repair in the presence of a macroscopic crack, yielding considerable recovery of polymer strength after thermal annealing which is repeatable over multiple damage-repair cycles. The study on structure-property relationship of the ISOX polymers shows that the material properties of the polymers are uncompromised with the addition of the self-healing functionality, as their excellent mechanical properties and high-temperature performance remain comparable to that of an engineering grade epoxy. Lastly, self-healing carbon fiber reinforced polymer (CFRP) composites are developed using the ISOX polymers as the matrix material. After multiple delamination events, repeatable strength recovery of the composites has been demonstrated with a first healing efficiency of up to 85% after thermal treatment. The strength and stiffness of the composites are comparable to those of engineering grade polymer matrix composites typically used in aerospace applications, while their thermal stability places them in the polybismaleimide performance region. This dissertation details the development of novel self-healing materials which have great potential for advanced structural applications in extreme environments.PHDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155070/1/zhalisha_1.pd