Incorporation of Luminescent Zinc Oxide Nanoparticles into Polystyrene
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Incorporation of Luminescent Zinc Oxide Nanoparticles into Polystyrene
Rui Li1, George R. Fern, Robert Withnall1(the late), Jack Silver1, Peter Bishop2, Benedicte Thiebaut2 1 2
Wolfson Centre for Materials Processing, Brunel University, Uxbridge, UB8 3PH, UK Johnson Matthey Technology Centre, Reading, Berkshire, RG4 9NH, UK
ABSTRACT Zinc oxide (ZnO) nanoparticles and nanoparticles of luminescent zinc oxide (ZnO:Zn) phosphor were successfully synthesised and well characterised. A transparent polystyrene composite sheet containing ZnO:Zn nanoparticles was prepared by a solvent casting method. The sheet manifested comparable transmission to a virgin polystyrene film due to very uniform dispersion of the ZnO:Zn nanoparticles into the polystyrene. Evidence for uniform dispersion was evident in both its luminescent properties and in a SEM image. The photoluminescent characteristics of the ZnO:Zn, both as a pure powder and embedded in a polystyrene matrix, are reported. The uniformity of the photoluminescence of the composite sheet under near ultraviolet excitation is demonstrated. The luminescent ZnO:Zn nanoparticles are shown to have applications for use not only as an inhibitor of the ultraviolet degradation of polymers, but also for providing polymers with light emitting functionality. INTRODUCTION Many polymers degrade in-service when they are exposed to near ultraviolet light or sunlight. The location/situation of the polymeric material can vastly affect its life out of doors, and a general rule is that the closer to the equator the location the shorter the period before degradation begins. Common synthetic polymers which suffer such degradation include those where tertiary carbon-hydrogen bonds are the primary focus of attack. Such degradation can involve photo-oxidation reactions that proceed via free radical mechanisms causing polymer chain scission and branching [1]. Currently the preferred way of defending the polymers is by including UV radiation stabilisers in the plastics to reduce or prevent degradation by UV light. Examples of such stabilisers include benzophenones (these absorb the UV light and so limit the formation of free radicals), and hindered amine light stabilisers (these quench oxygen-polymer charge transfer complexes which are the initiators of the photo-oxidative degradation [1,2]). Two drawbacks to this approach are:- 1) that such stabilisers are expensive, and 2) they are sacrificial molecules that can only be used once each with the result that when the amount present in any location in the polymer is used then the degradation begins. It is obvious that what is really needed is an included material which will protect as long as required so long as it is not degraded. To achieve this we have developed nanometre sized phosphor particles to protect the polymers by converting the UV to other wavelengths [3]. An example of this use of downconverting phosphor nano-particles is in the protection of transparent polymers. Nanoparticle phosphor powders have an advantage in such applications since the
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