Abstract
Several optical and piezo-optical properties of PMMA have been studied and results are now presented. The refractive index shows a small dispersion for wavelengths in the visible region and decreases considerably for temperatures T>50° C. A similar behaviour is shown by the piezo-optical coefficient. Its dispersion in the visible and near ultraviolet has been analysed in the light of the models proposed by Kuhm and Grumm, and Wemple and Di Domenico. From them the influence of temperature on the molecular polarizability and on the “interband transition”, energy splitting has been obtained. Several differences have been found between the optical and the mechanical relaxations. Their temporal dependence seems to obey simple logarithmic laws although they depend on temperature. At temperatures higher than the glass transition temperature Tg, the above picture is seriously modified. A qualitative explanation of this behaviour is tentatively proposed.
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