Elastic measurements of layered nanocomposite materials by Brillouin spectroscopy

Surface Brillouin spectroscopy makes it possible to measure surface elastic wave propagation parameters at frequencies up to 20 GHz or more. This enab les us to measure the elastic properties of surface layers only a small fra ction of a micrometre thick. The wavelength and incident angle of the light determine the wavenumber of surface elastic waves (SAW) that scatter the l ight inelastically, and their frequency can be found by measuring the chang e in wavelength of the scattered light. By analysing the elastic wave modes present in the surface, the elastic properties can be deduced. We have use d this technique to measure the elastic properties of layered nanocomposite materials, which are widely used in the packaging industry. 12 mu m polyme r films (PET) were coated with glass oxide layers of thickness as little as 25 nm, to give transparent nanocomposite structures with excellent gas bar rier properties. In order to understand and model the behaviour of these fi lms under deformation, it is necessary to determine the elastic properties of the different layers. Evaluation of the elastic properties presents seve ral challenges. First, the oxide layers are much thinner than the wavelengt hs of the surface phonons in surface Brillouin spectroscopy (and hence the depth probed), which usually lie in the range 250-500 nm. The anisotropic e lastic properties of the PET substrate must therefore be measured accuratel y, and this can be done using bulk Brillouin spectroscopy. Second, a thin l ayer of metal (usually 10-20 nm) must be deposited on the glass surface so that the surface phonons scatter the light effectively. The elastic propert ies of the glass layer can then be deduced from surface Brillouin spectrosc opy measurements, by simulating the surface wave modes of the metal/glass/p olymer composite, and adjusting the parameters to give the best fit. In thi s way it is possible to observe how the properties of the glass vary as a f unction of thickness, and in turn to understand how to improve systematical ly the properties under deformation. (C) 2000 Elsevier Science B.V. All rig hts reserved.