Smart Elastomers for Use in Security Applications

Smart polymers are utilised for a variety of functions. This project aimed to produce a polymer which exhibits fluorescent emission in response to tensile stimuli. Previous fluorescence studies have shown this to be theoretically possible and fluorescence spectroscopy techniques can be used to establish the existence of such responses. A wide range of elastomers (styrene butadiene rubber, polyacrylates, polyurethane acrylates and polyurethanes) have been synthesised with aliphatic polyurethane elastomers being the most promising thus far.
Fluorescent labels (9-vinylanthracene, fluorescein and 1,5-dihydroxynaphthalene) have been successfully incorporated into elastomeric products as have heavy atom containing species (4-bromostyrene, 3-bromo-1,2-propane diol and 4-bromoresorcinol).A library of fluorescent and quenching prepolymers was built up which was then used in a wide range of combinations to produce elastomers containing both fluorophore and heavy atom containing species.
Quenching of the excited state was observed in solution for most fluorophore/heavy atom combinations. 4-Bromostyrene was shown to quench 9-vinylanthracene within solid cross-linked elastomers. 1,5-dihydroxynaphthalene was also successfully quenched within polymer sheets however the emission wavelength of 1,5-dihydroxynaphthalene is not visible to the naked eye. Fluorescein emits visible light but has not been successfully quenched within cross-linked elastomers, with the lifetimes actually increasing in some samples. This observation was attributed to delayed fluorescence resulting from triplet-triplet annihilation promoted by the intersystem crossing to the triplet state brought about by the heavy atom.
Generally speaking, the incorporation of fluorescent labels into solid elastomeric sheets had the effect of increasing the lifetime of the associated excited state. This is probably due to the polymer protecting the excited states from quenching due to atmospheric oxygen.