Capillary breakup extensional rheometry (CaBER) on semi-dilute and concentrated polyethyleneoxide (PEO) solutions

Semi-dilute ( $c^\ast < c < c_{\rm e}$ ) as well as concentrated, entangled (c?>?c _e) solutions of PEO yield uniformly thinning, cylindrical filaments in capillary breakup extensional rheometry (CaBER) experiments. Up to c?≈ c _e thinning can be characterized by a single elongational relaxation time λ _E. Comparison with the longest shear relaxation time, λ _S reveals that λ _E/λ _S decreases with increasing concentration or molecular weight according to (cη])^???4/3. This is attributed to the large deformation the solutions experience during filament thinning. A factorable integral model including a single relaxation time and a Soskey or Wagner damping function accounting for the large deformation in CaBER experiments is used to calculate λ _E/λ _S and provides good agreement with experimental results. Irrespective of concentration or molecular weight a beads-on-a-string structure occurs prior to filament breakup at a diameter ratio D/D ₀?≈ 0.01. This instability is supposed to be closely related to a flow-induced phase separation.