Mechanical Properties of Long Chain Molecule Liquids at Ultrasonic Frequencies

Abstract

Measurements have been made of the mechanical properties of long chain molecule liquids of the polyisobutylene type by means of shear and longitudinal waves in the ultrasonic frequency range. The shear waves show that these liquids behave as Maxwell relaxing liquids. The viscosity measurements in the range around 14 kilocycles check the viscosities measured by falling ball measurements within the experimental error. As the temperature decreases or the frequency increases, the reaction of the liquid shows that a shear elasticity of the Maxwell type comes into play with a shear elastic constant of from 5×${10}^6$ dynes/${\mathrm{cm}}^2$ to 5×${10}^7$. This elasticity increases with decreasing temperature and increases with chain length, and represents an intermediate type to the "frozen" type of elasticity or the "kinetic theory" type in that it has the high compliance of the "kinetic theory" type but the temperature variation of the "frozen" type of elasticity. It is suggested that this type of elasticity may be due to a composite motion of the chains, including hindered rotation within chains, as well as interaction of segments between chains. At very high frequencies this composite motion disappears and the shear stiffness becomes very high.