Measurements of Mechanical Properties of Polyisobutylene at Audiofrequencies by a Twin Transducer

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Apparatus has been developed for measuring the dynamic viscosity and rigidity of soft rubber-like solids in small oscillating deformations. A plate rigidly attached between two identical coils in two permanent magnets shears a pair of disk-shaped samples when a driving current is passed through one coil. The open circuit voltage from the other coil is compared in amplitude and phase with the driving current by a method in which all measurements are in the form of settings of a potential divider. The apparatus has two advantages over the more familiar resonance devices: (a) the amplitudes of motion, which need not be measured directly, are extremely small, thus minimizing any non-linear effects, or temperature change due to heat dissipation; (b) a continuous range of frequencies, spaced as closely as desired, is available without adjusting masses. The dynamic rigidity and viscosity of two samples of polyisobutylene, of molecular weights 1.2 and 0.47 million, have been measured at 15, 25, and 35°C at frequencies from 20 to 600 cycles/ sec., with a reproducibility of five percent or better. The rigidity increases and the viscosity decreases with increasing frequency. Both decrease with increasing temperature, and are almost independent of molecular weight. The temperature dependence is quantitatively explained by the assumption that stresses relax by flow processes whose apparent activation energies are all identical with that characterizing the steady flow viscosity.