Multiaxial elongation of polyisobutylene with various and changing strain rate ratios

Abstract

The multiaxial elongational rheometer equipped with rotary clamps is modified such that in addition to simple, equibiaxial and multiaxial elongations also tests with new modes of elongation can be performed. As an example, polyisobutylene is elongated with a ratio of the principal strain rates of$m = \dot \varepsilon _{22} /\dot \varepsilon _{11} = 0.5$ and magnitudes of the maximum strain rate$\dot \varepsilon _{11} = 0.008$, 0.04 and 0.08 s⁻¹. As a test result, the first elongational viscosityµ ₁ (t) is obtained which follows closely the linear viscoelastic shear viscosity$\mathop \eta \limits^ \circ (t)$. In contrast, the second elongational viscosityµ ₂ (t) remains below$\mathop \eta \limits^ \circ (t)$. By means of a further modification of the rheometer, the test modes can be varied during the deformation period. This allows one to investigate the influence of a well-defined rheological pre-history on the following rheological behaviour. As an example a variation ofm = 0.5 → 2 was performed. The measured normal-stress differences superpose from the single steps of deformation similar to the linear viscoelastic prediction.