A Comparison of the Behavior of Concentrated Polyisobutylene Solutions and the Predictions of a New Constitutive Equation
A new constitutive equation is being proposed which contains as a special case the Bernstein, Kearsley and Zapas theory of an elastic fluid. In this equation, the non-linear moduli are corrected by a stiffening factor which depends on the previous strain history. The equation was applied in the description of the simple shearing behavior of a 19.3% solution of polyisobutylene (L-100) in cetane at 23°C. Having obtained the surface H(ί, t) of the single step stress relaxation response over a wide range of time t and strain ί, we proceeded to calculate the contribution of the siffening factor for a suddenly applied shear experiment where the rate of shear was 11.1 s-1. From all these data, we calculated the behavior as a function of time for rates of shear from 22.2 to 111 s-1. The comparison with experimental data was excellent. Very good agreement was obtained for stress relaxation after cessation of flow. The first normal stress difference behavior was also compared favorably for the same type of strain histories.
The two steps stress relaxation experiments where the first step was twice as big as the second step showed also a good agreement between theory and experiment for both shear and first normal stress behavior.
Consequences due to the presence of the stiffening factor will be discussed for experiments where data are not as yet available.