JOURNAL OF MATERIALS SCIENCE 15 (1980) 53-60 Piezobirefringence of PMMA: Optical and mechanical relaxations and influence of temperature F. MESEGUER Departamento de Optica V Estructura de la Materia, and Instituto de Fisica de/Estado Solido (C.S.I.C.), Universidad Autonoma de Madrid Cantoblanco, Madrid, Spain C. SANCHEZ FEMSA, D.E.P., Hnos. Garcia Noblejas, 19, Madrid- 17, Spain 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 decreasesconsiderably 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 Gr0mm, and Wemple and Di Domenico. From them the influence of temperature on the molecular polariz- ability 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. 1. Introduction Polymethyl-methacrylate (PMMA) is nowadays a very useful material from an optical point of view. Therefore, a better knowledge of the optical and piezo-optical properties of PMMA is desired. The mechanical and optical properties of polymers have been extensively studied and detailed reviews have been published [1-5]. Both applied and basic information is available especially on PMMA [6]. We have applied uni- axial mechanical stresses to measure the piezo- birefringence (PB) coefficient of PMMA as a function of wavelength and temperature (be- tween room temperature and 90 ~ C). Two alter- native theories (Wemple and Di Domenico [7] and Kuhn and Grikmm [8,9]) have been used to analyse the results. The refractive index of PMMA has been measured in the visible region between 20 and 70 ~ C. Our results at room temperature show good agreement with those already pub- lished [3]. On the other hand, the time dependence of 0022-2461/80/010053-08502.80/0 birefringence (optical relaxation) has been studied by using mechanical models and applied to organic [3, 10] and inorganic polymers [11, 12]. We have studied the optical and mechanical relaxations and their time dependence has been analysed by accepting logarithmic kinetics. 2. Experimental details Samples used throughout this work were prepared by cutting and polishing commercial PMMA sheets. Pieces from different suppliers have been studied and only small differences have been found among them. To measure the refractive index, an Abbe refractometer from Carl Zeiss was used. Tempera- tures higher than 20~ were obtained by a con- trolled hot-water flow through the prisms of the refractometer. With this system the sample tem- perature remains constant with variations smaller than + 2 ~ C. We have used an intensity method [13-15] for measuring the stress-induced birefringence. The 1980 Chapman and Hall Ltd. 53