Self-Healing Process Based on Spontaneous Copolymerisation of Electron Rich and Electron Poor Monomers

Abstract

This project involves on developing highly efficient self-healing polymer systems based on spontaneous copolymerisation.
Several different self-healing systems, examples of micro-encapsulation processes, basic concepts of free radical polymerisation and copolymerisation, mechanism and examples of spontaneous copolymerisation, and fracture mechanics of polymeric materials were reviewed.
The linear spontaneous copolymerisation of electron donor and acceptor monomers was investigated. Linear copolymers were synthesised by the spontaneous copolymerisation of electron donor monomers (4-methoxy styrene and styrene) and acceptor monomers (maleic anhydride and N-methylmaleimide) in bulk at ambient temperature and at 50 °C. The reaction of 4-methoxy styrene with ethoxymethylene malononitrile was carried out at 50 °C and produced homopolymer of 4-methoxystyrene. The resulting linear polymers were found to be soluble in tetrahydrofuran and acetone and fully characterised by 1D and 2D NMR spectroscopy, SEC, and FTIR.
Cross-linked spontaneous copolymerisation of electron donor and acceptor monomers were synthesised. By adding divinylbenzene to 4-methoxy styrene, styrene, N-methylmaleimide, and maleic anhydride, cross-linked materials were obtained. Those materials, as expected, were completely insoluble in normal organic solvent. The cross-linked polymers were characterised by sol-gel technique and FTIR.
The micro-capsules were obtained by using urea-formaldehyde micro-encapsulation process. The liquid healing agents (4-methoxy styrene, styrene, divinylbenzene, and their mixtures) were encapsulated using a process involving polymerisation of urea-formaldehyde in oil-water emulsion. The average diameter of the micro-capsule was controlled by adjusting the agitation rate. Micro-capsules with diameter were selected using seivesfor self-healing specimen preparation.
The fracture toughness of epoxy matrix was investigated. Fracture toughness of pure epoxy matrix specimen was tested by compact tension geometry. The influence of micro-capsules and solid healing agents on the fracture toughness of epoxy matrix was investigated. The micro-capsules (5-20 wt. %) in the epoxy resin did not change the fracture toughness of matrix. However, solid healing agents MA and MeMal reduced the fracture toughness of epoxy resin as the amount of MA and MeMal added to the matrix. To keep the fracture toughness of specimen closed to that of pure epoxy resin, 10% solid healing was decided to add into the matrix for self-healing preparation. And the fracture toughness of the specimen did not change by adding EtOCN into the epoxy resin matrix.
The self-healing performance was assessed by fracture toughness recovery. The self-healing efficiency of those system designed in this project was first evaluated by injecting the liquid healing agents into the crack of the specimen containing solid healing agents. Then, the autonomous self-healing specimen was prepared by adding the micro-capsules and solid healing agents in the epoxy resin matrix. The specimens were subjected to fracture testing to establish healing efficiency. The self-healing system based on obtained good healing efficiency.