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

Semi-dilute (c* < c < c(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 approximate to c(e) thinning can be characterized by a single elongational relaxation time lambda(E). Comparison with the longest shear relaxation time, lambda(S) reveals that lambda(E)/lambda(S) decreases with increasing concentration or molecular weight according to (c[eta])(-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 lambda(E)/lambda(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 approximate to 0.01. This instability is supposed to be closely related to a flow-induced phase separation.