High elasticity, normal and shear stresses on shear deformation of low-molecular-weight polyisobutylene

Abstract

The rheological properties (shear and normal stresses and high-elastic deformations) of low-molecular-weight polyisobutylene, a typical viscoelastic liquid, were studied. Investigation of the character of development of stresses and deformation at a given constant shear rate show that the only criterion of attainment of steady-state flow condition is constancy of the normal stresses. Not one of the known theories gives a correct description of the relationship between the stresses and elastic deformations at transient (pre=steady=state) deformation regimes. A study of steady-state flow regimes has shown that within a wide range of shear rates, in both the linear and the nonlinear regions, the normal stresses are proportional to the square of the shear stresses.Lodges theory, which relates the halved ratio of normal to shear stresses with the elastic deformations, is valid only in the region of Newtonian flow. The question of the interrelation between the relaxation spectrum and the normal stresses (in the linear region) is discussed in detail, making use of independent measurements of the frequency dependence of the dynamic moduli. The temperature dependences of the parameters characterizing the viscoelastic properties of the liquid studied are discussed in detail, these parameters being: the initial (Newtonian) viscosity, the coefficient of normal stresses, and the high elastic deformations. The dependence of the high elastic deformations on the shear stress and the temperature are not described by the entropy theory of high elasticity owing to the change in internal energy of the system on development of viscous flow.