Determination of the Propagation Constants for Ultrasonic Waves in Melting and Molten Polyethylene

An extension of McSkimin's technique to polymer melts and soft amorphous polymers is reported. For 12-Mc/sec longitudinal waves, the modulus and internal friction are presented for the melting, super-cooling, and annealing or recrystallization of polyethylene. Maxima of internal friction are observed in the solid at −52°, 60°, and 100°–130°C. The last is associated with the presence of crystals, decreases sharply near the melting temperature, and is not present in the supercooled melt. At 129.5°C, the modulus increases linearly with the logarithm of recrystallization time and the internal friction decreases. Correlation of the modulus and lamellar thickness during recrystallization indicates that the effects above 100°C are related to recrystallization. For the melt between 140° and 290°C, the internal friction exhibits a low, nearly constant value. In this temperature range, the ratio of the thermal coefficient of velocity change to the thermal coefficient of volume expansion is −2.6. A decrease of modulus and increase of internal friction above 300°C is believed to be associated with thermal degradation of the polymer.