(583h) Orientation Number in Elongational Flow of Polymer Melts and Solutions

A modified Weissenberg type number, Orientation number, is suggested to explain the agreement and disagreement between the elongational viscosity data measured by different techniques. The suggested Orientation number is the product of the elongational strain rate, the Hencky strain and the average relaxation time. Elongational viscosity of different polymer melts was measured using three different techniques and in different laboratories, namely hyperbolic convergent die technique, a Meisner type device and an Instron tensile tester. When the Orientation number is less than one it appears that the elongational flow is in a relaxation dominant mode in which orientation is relaxed nearly as fast as it develops. At Orientation numbers greater than one an orientation dominant mode results and significant orientation can develop. The degree of orientation induced in the orientation dominant regime should increase with Hencky strain, elongational strain rate, and elongational viscosity. The elongational viscosity is a function of the molecular architecture and therefore indirectly related to the average relaxation time. In the hyperbolic dies, slip at the wall is apparently induced when the orientation number exceeds one. When the Orientation number is near one an apparent transition region occurs giving flow instabilities that in the hyperbolic dies cause some scatter in the data and extrudates surface roughening, but in unconstrained techniques may be responsible for deviation from strain hardening and ultimate failure. When the orientation number passes through the transition region at low Hencky strains the data from different techniques are in agreement.