AbstractLongitudinal and shear sound speeds and absorptions were measured for a phenolic polymer and a poly(phenylquinoxaline) (PPQ) polymer, from room temperature to 70°C, at a frequency of 1.8 MHz. Phenolic specimens were cured at a maximum temperature of either 120°C, 135°C, or 180°C. The specimens varied in density from 1.217 to 1.229 g/cm3with no correlation to curing temperature. Longitudinal sound speed in phenolic was found to depend only on density (or specific volume) and not directly on temperature or degree of crosslinking. Sound speeds and elastic constants, in phenolic, are not sensitive to cure temperature but sound a bsorption is: the higher the cure temperature, the lower the absorption. Hence ultrasonic absorption is a possible way to monitor or measure degree of crosslinking. Sound absorption is higher in phenolic than in PPQ, contrary to what is expected for a polymer with high sound speed. This high absorption indicates an additional mechanism of absorption in phenolic not found in PPQ. Also, the sound absorption increases with temperature in phenolic but is constant in PPQ. The Gruneisen parameter of phenolic appears high and that of PPQ about right when compared with other polymers. All of these results suggest there is a molecular transition in phenolic, but not PPQ. Further evidence for a transition comes from specific volume measurements as a function of temperature. The exact temperature of the transition and the molecular process responsible cannot, however, be determined from the measurements reported h
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