Refractive-index variations with temperature of PMMA and polycarbonate.

TLDR: A relevant discontinuity of the coefficient of thermal expan­ sion has been observed for PMMAs various tacticities and a so-called α peak was observed at ~60°C and was widely discussed by Neki and Geil.

Abstract: J. M. Cariou, J. Dugas, L. Martin, and P. Michel Laboratoire de Physique des Solides, Associé au CNRS, Université Paul Sabatier, 31062 Toulouse CEDEX, France. Received 30 July 1985. 0003-6935/86/030334-03$02.00/0. © 1986 Optical Society of America. For polymers, the thermal volume expansion coefficient is always much higher than the coefficient for inorganic materials. We have recently shown that the variation of the refractive index of polymers vs temperature is essentially due to the variation of density. From the Lorentz-Lorenz relation, the function (n + 2)/(n 1) appears perfectly linear in the various temperature ranges where no phase transition occurs. The slope of the representative curve exactly follows the thermal expansion coefficient α which keeps a constant value in each temperature range. Thus, α may be easily deduced from the refractive-index measure­ ment at various temperatures. At the same time, the tem­ peratures where discontinuities of (n + 2)/(n 1 ) or its derivative appear conveniently fit with the transition tem­ peratures which may be so determined. A correct observation of such behavior is that the tempera­ ture variations are so slow that the material is continuously kept in thermal equilibrium. Indeed, it is well known that the various molecular processes which induce the transitions are characterized by very large relaxation times. If the material undergoes temperature changes too fast, it remains in a metastable state which is not a true equilibrium state. Waxier et al. have interferometrically measured some optical properties of Plexiglas and Lexan and determined the evolution of dn/dt, with the temperature between -160°C and +60°C. They obtained irregular but continu­ ous curves which do not evidence, even in a crude way, any systematic change in their behavior which may be related to a phase transition. Nevertheless, in the temperature range studied, at about -30°C PMMA (Plexiglas) undergoes the so-called β transition which is attributed to the beginning of the rotation of lateral chains of methacrylate radicals. A relevant discontinuity of the coefficient of thermal expan­ sion has been observed for PMMAs various tacticities. In the same way, for polycarbonate, the glass transition at 125°C is out of the temperature range studied by these authors. However a so-called α peak was observed at ~60°C and was widely discussed by Neki and Geil. No noticeable anomaly appears in the Waxier results. We have measured the refractive index of commercial samples of PMMA (Altuglas) and polycarbonate (Lexan) between about -100°C and +150°C. To avoid any stress which could take place by contact with another material, the measurement method chosen was the minimum deviation of a prism. The PMMA prism was drawn from bulk material