Cross-slot extensional flow birefringence observations of polymer melts using a multi-pass rheometer

This paper reports initial experimental flow birefringence studies of cross-slot flow for a range of molten polyethylenes using a multipass rheometer (MPR). An MPR was modified to accommodate a cross-slot center section insert thereby enabling small quantities of molten polymer to be evaluated in a near two-dimensional pure shear extensional flow. The results show that it was possible to obtain precise processing data for cross-slot flow using 10 g quantities of polymer. Experimental observations on different polyethylenes showed that in all cases the flow commenced from a Newtonian situation where the optical birefringent fringes around the central stagnation point were essentially symmetric. As the flow developed, the fringe pattern became progressively more asymmetric about the exit symmetry plane and the extent of the viscoelasticity of the polymer could be quantified by the observed level of anisotropy between the birefringence fringes on the entry and exit symmetry planes of the flow. The progressive development of anisotropy and stress with increasing extensional flow rate and type of polymer is presented for a number of flow conditions and it was found that increasing viscoelasticity of the polymer resulted in a progressive increase in viscoelastic center-line extensional stresses. The data provide benchmark large strain extensional flow experiments for certain polymer melts that can in the future be used to compare with numerical simulation.