Thermal variations of refractive index of PMMA,polystyrene, and poly (4-methyl-1 -pentene)

The refractive indexes of PMMA, polystyrene, and poly-(methyl pentene), three transparent polymers having various morphologies, have been measured between about -100°C and +150°C. The prism method was used in order to make the sample quite free of any contact and mechanical interaction. Thermally stimulated current experiments were concurrently performed to observe and to confirm the existence of the transitions occurring in the various samples. It is verified that the refractive index variations, on the one hand, perfectly follow the Lorentz-Lorenz law and, on the other hand, systematically exhibit anomalies every time a morphological change temperature is overstepped by the material. So, it is shown that measurement of the thermal variations of the refractive index of polymers is another means to follow the behavior of their specific volume while avoiding the application of any stress from the experimental device.