Mechanical Resonance Dispersion in Crystalline Polymers at Audio-Frequencies

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Abstract
Measurements of the complex shear compliance (J*=J′—iJ″) of crystalline polymers (polytetrafluorethylene, linear polyethylene, and polyvinyl stearate) at closely spaced frequencies in the audio‐frequency range have resulted in the discovery of multiple frequency dispersions of the resonance type. Values of the loss compliance (J″) are found to have sharply defined maxima in the range from 100 to 5000 cps; values of the storage compliance (J′) rise to a maximum and then drop to a minimum as a narrow (50 to 100 cps wide) dispersion region is traversed in the direction of increasing frequency. The results indicate the presence of a dispersion mechanism different from those leading to the usual relaxation dispersions observed in connection with the mechanical properties of polymers. The magnitudes of the mechanical loss maxima and the frequencies at which the effect is observed depend on the static stress history and state of the sample material as well as temperature. Similar results obtained for pure polycrystalline metals (lead, indium, and aluminum) lead to speculation that stress‐induced motions of dislocations or other imperfections in the crystalline structure may be important in producing this type of dispersion. At present, however, there is no satisfactory explanation for these phenomena.
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