Strong extensional and shearing flows of a branched polyethylene.

Experimental measurements of stress in shear with an exponentially growing rate of strain are compared to stresses in steady planar extension, for a polyethylene, IUPAC X, with long side branches. We find that IUPAC X shows extreme strain hardening in planar extension, as is typical for polymers with long side branches in extensional flows, but does not show extreme strain hardening in exponential shear. Both planar extension and exponential shear are planar deformations and in both material lines are stretched exponentially in time. The major difference between the two deformation histories is that exponential shear is a rotational flow and planar extension is irrotational; thus the irrotational character of extensional flows seems to be an important requirement for producing extreme strain hardening.