Dynamic Mechanical Properties of Polyvinyl Stearate at Audio-Frequencies

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Abstract
Measurements of the complex shear compliance (J*=J′−iJ″) of polyvinyl stearate at frequencies from 50 to 5000 cps have resulted in the discovery of several sharp resonances in the compliance as previously described. The effects of temperature and static stress normal to the direction of the dynamic shearing stress have now been studied for this material. The two principal resonances observed at 25°C were located at (1) 180 and (2) 500 cps. At successive temperatures of 30, 36, and 42°C the first resonance remained essentially unchanged, but the second resonance shifted to higher frequencies and decreased in magnitude until it virtually disappeared at 42°C. A return to 25°C and a reapplication of a slight static stress resulted in the return of the second resonance (at 525 cps). At successively increasing static stresses corresponding to compressions up to 1.8% the second resonance now decreased in magnitude and shifted to higher frequencies. The first resonance disappeared at a static compression of 0.8%. Similar effects of static stress were observed at room temperature (∼25°C) on a second set of samples of different dimensions. The reversibility of the effects of static compression were demonstrated by the second set of samples where for example, a resonance originally present at 2.8% static compression disappeared at 3.7% and reappeared when the static compression was reduced to 3.1%. The reversible nature of the effects of static stress seems to preclude the possibility of an explanation of the resonances in terms of changes in dislocation density.