New self-healing epoxy-based polymers

Abstract

Cross-linked polymers with self-healing properties have become a very popular polymer research field in recent times. In this thesis, a new type of cross-linked self-healing polymer system based on the Diels-Alder (DA) system is developed, with adduct of furan and maleimide groups as the cleavable and reformable segments being incorporated into the amine-based cross-linker. Such a concept allows a large range of commercially-available and widely-used epoxy resins to be used as monomers for the production of self-healing polymers. Initially, one DA unit was incorporated into the structure of diamine cross-linker and its hydrochloride salt was obtained after several synthetic steps. However, the yield of its neutralised free amine was very low and product was impure, even after many methods were tried, including different neutralisation methods, different synthetic routes and after increasing the carbon chain length of the target diamine in an attempt to achieve better solubility. Given this issue, a new diamine cross-linker with two DA units in the structure was designed, successfully synthesised and its structure was verified by characterisations. The successfully synthesised diamine cross-linker was used to cure two different epoxy monomers, diglycidyl ether of bisphenol A (DGEBA) and triglycidyl p-amino phenol (TGAP), yielding polymers with different crosslink densities. The curing conditions were optimised based on near-infrared spectroscopy (NIR) and differential scanning calorimetry (DSC) tests. It was necessary that the temperature of the curing (polymerisation) cross-linked reaction should be lower than 80 °C , to minimise both the Retro Diels-Alder (RDA) and other side reactions, which may irreversibly destroy the DA unit. In addition, a high degree of cure is necessary for good mechanical properties of the cross-linked polymer. The polymers cured using optimised conditions were then characterised by Fourier transform infrared spectroscopy (FTIR), DSC, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The conditions of DA and RDA reactions, both in the synthesised diamine cross-linker itself, and in the cured epoxy polymers were investigated. The optimised conditions, which were also the self-healing conditions, were used to study the mechanism of thermal self-healing in the prepared epoxy polymers. By different methods, including the determination of the change in glass transition and swelling properties during the healing process, and the analysis of the components of scissioned polymer at temperatures above RDA reaction temperature, the thermal self-healing process of this type of epoxy polymer was understood. The self-healing process on the surface of the prepared polymers was observed by optical microscopy. Compared with the self-healing result from the control sample without DA units, the healed surface of a self-healing polymer that had been scratched demonstrated that the cleavage of the DA units and subsequent flow of the oligomer were the basis of the self-healing process. The flow activation energy of the formed oligomers during the healing process was calculated using the Arrhenius equation, and was found to be a little higher than that of the linear polymer, likely due to its highly branched structure.