Self-healing materials have the ability to repeatedly repair damages that occur before complete failure of materials. The development of stimuli-responsive self-healing materials has been in demand recently for composite structures, since their failure is relatively uncertain and can result in major expenses. Using such materials can enhance damage tolerance, leading to greater asset reliability - it also limits expenditure and keeps the need for human interventions to a minimum. This work investigates Fe3O4 magnetic polymer nanocomposites that can be used to intrinsically heal composites through thermal stimuli, followed by self-healing of glass fibre reinforced polymer (GFRP) composites, which are fabricated by embedding the healable polymer nanocomposite as one of the sacrificial layered matrices. However, performance of nanocomposites depends on various parameters, including nanoscale dispersion of nanoparticles. Specifically, a lack of hierarchical dispersion of nanoparticles in three-dimensional polymer matrices prevents electron tunnelling and deteriorates the nanocomposites' ability to conduct heat stimuli or otherwise lead to pyrolysis. To address this issue, two functionalisation techniques - viz. silica (Stöber method for lower silica loading and tri-phasic reverse emulsion method for higher silica loading), and oleic acid (22%, 33%, 44%, and 55% w/w of nanoparticles) variations - were experimentally investigated as capable of changing hydrophobic characteristics for facilitating uniform dispersion of the Fe3O4 magnetic nanoparticles (MNPs). The main focus of the presented work is to understand the role of functionalisation routes in the particle-polymer interface in forming a uniformly dispersed and hierarchical network of MNPs in a polymer matrix. Emphasis was on understanding and optimising the role of activator and initiator proportions in controlling the in-situ polymerisation of PA6, capturing the MNPs dispersion state. The resulting dispersion state due to functionalised Fe3O4 MNPs determined the properties of magnetic PA6 nanocomposites (PMC) to help achieve a generic set of principles for designing the desired materials for stimuli-induced self-healing of GFRP composites. The method used to achieve this involved firstly undertaking anionic ring-opening in-situ polymerisation of PMC by experimental synthesis within the laboratory, with optimised EtMgBr (activator) and NACL (initiator) proportions for improved degree of crystallinity, and capturing the MNPs dispersion state attained by probe ultrasonication of the melt monomer & MNPs solution mixture. Based on this, 50% EtMgBr (activator) and 30% NACL (initiator) were assessed as the optimised proportions for giving the highest possible crystallinity amongst all the prepared PMC variations. Secondly, as per the functionalisation type of the MNPs, the prepared PMC samples were tested based on chemical, thermal, structural and magnetic characterisations, for the purpose of assessing their self-healing capability by microwave stimuli. The physical characterisation results were also used to train a simulation model to create the 3D dispersion state, for better studying the dispersion state and interaction region defined by the interaction radius (IR) of each MNP/agglomerate of the MNPs. Based on this overall comparison, the most suitable PMC of 22 w/w % OA loading was selected and formed into thin films. Sandwiched tensile testing samples were then prepared using this PMC film as a sacrificial layer between GFRP tapes. Both bare and modified Fe3O4 MNPs PMC exhibited paramagnetic behaviour, with average particle sizes ranging from 30-60 nm. The saturation magnetisation (Ms) of the unmodified MNPs PMC was around 65% and that of the selected PMC with 22 wt/wt % OA loading was 47%. The self-healing concept was demonstrated with the prepared composite samples' microwave induction heating, and the efficiencies based on strength recovery were calculated as 84%, 58% and 34% after first, second and third healing, respectively. This can essentially increase the life-cycle viability of the composite structure by over 175% (with 60% certainty) compared with that of an otherwise damaged structure, hence promising cost saving by extending the structural life.
GUPTA, R. 2021. Magnetic polyamide 6 nanocomposites for increasing damage tolerance through self-healing of composite structures. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-1987943