Transition Temperatures of Methyl Methacrylate Polymers at Ultrasonic Frequencies

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Abstract
The velocity of sound in methyl methacrylate polymers (Plexiglas) has been measured at temperatures from 24° to 90°C, and frequencies from three to eleven megacycles. Optical diffraction methods were used to determine the sound velocity within a maximum error of 1 percent. This low experimental error made possible a detailed study of the sound velocity-temperature relationships. A previously unreported transition of a new type has been found. The sound velocity decreases linearly with increasing temperature, with an abrupt increase of slope occurring when a transition temperature is reached. The transition temperature is found to decrease linearly with frequency, falling from 64°C at three megacycles to 49°C at eleven megacycles. The observed transitions apparently are not manifestations of elastic relaxational effects and are tentatively attributed to the excitation of some mode of molecular motion, for example, molecular vibrations.