Towards the Detection of Microcracks: The Use of Cyanine Acrylates to Functionalise Carbon Fibre

CFRP (carbon fibre reinforced polymers) composites are an invaluable asset to the aerospace industry. However, when put under stress these composites can undergo debonding causing microcracks at the interface. These cracks are expensive to identify and manage due to the current methods of ultrasonic identification. Herein, it is proposed to introduce a fluorescent cyanine dye to the polymer at the interfacial surface, so that when a microcrack appears a difference in emission wavelength of the dye can be detected, reducing the labour needed to monitor these cracks. Previous literature on cyanine dyes often focuses on the synthetic conditions required for specific application but few recent studies have been appeared on the general methods. Because of this, a myriad of conditions has been used, which greatly complicates the use of these dyes due to the convoluted nature of the literature. The first aim of this project was to determine and optimise reaction conditions for cyanine synthesis, to provide a clear starting point. Herein new reaction conditions for the synthesis of symmetrical Cy5 and Cy7 dyes are described. The optimised conditions require 1.5-2 eq. of DMAP and 10 eq. of acetic anhydride, with the reaction stirred at ambient temperature for 4 hours. These conditions provide a starting point for all future synthesis and hopefully will lead to these dyes being more widely available to the scientific community. These reaction conditions were utilised to synthesise two novel cyanine dyes being a Cy5 acrylate and Cy5 hexene, compound 9 and 6, respectively, and two novel intermediates compound 7 and 8. Characterisation of these compounds were completed by via 1H NMR, 13C NMR, ESI-MS and FTIR spectroscopy. Both compounds contain functional groups that allow for polymerisation to occur which is essential if they are to be introduced to CFRPs. iv The capacity of these dyes to be polymerised was first successfully completed on gold on silica substrate. The SEEP (surface electroinitated emulsion polymerisation) method of polymerisation was performed, using each of these dyes as the monomer and a 4-nitrodiazonium salt as the initiator. Both dyes were able to successfully functionalise a gold on silica surface, which was confirmed via water droplet contact angle measurements, AFM (atomic force microscopy) and IR spectroscopy. Based on this data, functionalisation of a carbon fibre surface was attempted. Initial results from SEM suggests that polymerisation occurred, but further analysis is required to confirm this.