Validation and application of a novel elongational device for polymer solutions

A novel elongational device is used to investigate the capillary thinning p rocess of threads of dilute and semidilute aqueous polymer solutions. It is shown that the end regions of the threads do not play an essential role in the thinning process, so that a simple theory describing the self-thinning of the liquid thread is appropriate to describe the experiments with polym er solutions carried out with this device. Aqueous solutions of four differ ent polymers (all polyacrylamide based) are studied using the elongational device. It is shown that the elasticity of the polymers is dominated by pol yacrylamide chains and that the effect of branching and topological structu re of macromolecules is negligible. The time-dependent decrease of the thre ad diameter can be divided into two stages. The first is a viscoelastic one where macromolecular coils are stretched by die elongational how, and the second is a quasi-Newtonian one, where full stretching has already been ach ieved, resulting in a very high but constant elongational viscosity. At the first stage the rheological behavior of the solutions studied is character ized by a constant relaxation time, whereas at the second one it is by a co nstant elongational viscosity. For polymer macromolecules of relatively low stability, mechanical degradation of the molecules is found during their s tretching in a self-thinning capillary thread. (C) 2000 The Society of Rheo logy. [S0148-6055(00)01003-8].