Dynamic behaviour of polymethyl methacrylate between 60 and 300 °K

Abstract

The resonant frequency and the coefficient of energy dissipation have been measured as a function of temperature in the interval (60÷300) °K for several specimens of polymethyl methacrylate of the plasticized and unplasticized type. The specimens have the shape of circular plates, and the measurements have been made for flexural vibrations in the frequency range (6÷40) kHz, with a strain amplitude smaller than 10−7. A thermally activated relaxation effect gives rise to a peak in the dissipation-temperature curve. For the above frequency range the temperature of the peak is about 110 °K. At the same temperature a slight inflexion is observed in the frequency-temperature curve, in accordance with the theory of linear dynamic deformations. The relaxation effect, which is probably due to the rearrangement of side chains, is superposed to another cause of energy dissipation which cannot be explained by means of the above theory in its present state, as the corresponding dissipation coefficient increases not only with temperature but also with frequency. The frequency-temperature curve exhibits aknee near 230 °K. This temperature is apparently independent of the vibration frequency, and the knee seems to be due to a second order transition.