On the Dynamic Mechanical Properties of Polymers at Ultrasonic Frequencies in Relation to Their Glass Transition Phenomena

Abstract

Complex Young's modulus, rigidity and bulk modulus are measured at 33 kc/s, 66 kc/s and 1 Mc/s for polystyrene, polymethyl methacrylate, polyvinyl acetate and phenol resin. In each material, the storage modulus-temperature curve has a discontinuity in the slope, irrespective of frequency and type of modulus, at the glass transition temperature T _g determined from thermal expansion. This discontinuity takes place through a strong dependence of the modulus on the specific volume. Besides, the first three materials exhibit secondary discontinuities at T _g ', about thirty degrees below T _g , both in thermal expansion and modulus-temperature curve. Some possible interpretations are given to this secondary discontinuity, resulting in the conclusion that the glass transition occurs over a rather wide range of temperature and is completed at T _g with increasing temperature. The ratio of the temperature coefficients of the modulus, taken below and above T _g , agrees well with that of volume expansion curve. This is also the case about T _g ', being indicative of predominantly energetic nature of elasticity. The loss factor begins to rise slightly at T _g ', then rapidly at T _g as the temperature is elevated, suggesting that local motions of molecular chains are first encountered at T _g '. The real part of Poisson's ratio seems nearly constant through the temperatures studied, including the glass transition temperature, whereas the imaginary part is found to vanish.