Mechanical Properties of Polymers at Ultrasonic Frequencies

The mechanical properties of solid polymer materials are largely determined by what motions, parts or segments of the polymer chains can undergo. Toughness, mechanical impact strength and ultimate elongation depend on the facility with which the polymer molecule can be displaced. 122 MECHANICAL PROPERTIES OF POLYMERS 123 If only a small motion of the polymer chain can occur within the time of the measurement, the material has high elastic stiffness coefficients and acts similar to a rigid solid. On the other hand, if significant segments of the polymer chain can move at the frequency of measurement, the elastic stiffness is much lower and rubber-like behavior results. An intermediate case, which occurs when the significant motion of the polymer molecule is near the relaxation time at the frequency of measurement, is that of a damping material such as butyl rubber. Even "long time" qualities of plastics such as creep, stress relaxation and recovery depend on the integrated displacements of rapidly oscillating segments of the chain. One of the most promising methods for investigating these motions is to determine the reaction of mechanical waves on the polymer materials over a wide spectrum of wavelengths, eventually going to frequencies comparable with those of thermal vibrations of significant groups or segments in the macromolecules. If one wishes to understand the origins of these motions it is necessary to measure the molecules in the form of liquids or solutions since then the segments of the molecule are less restrained by their neighbors and can perform all the possible vibrations.