Influence of molecular structure on secondary flow of polyolefin melts as investigated by laser-Doppler velocimetry

Laser-Doppler velocimetry (LDV) is applied to investigate velocity profiles in the entrance region of a slit die. Due to the high spatial resolution o f the device and the accuracy of the velocity measurements the secondary fl ow patterns of different polyolefins have quantitatively been analyzed for the first time. A linear polyethylene is compared with two long-chain branc hed polyethylenes and a conventional linear polypropylene with a long-chain branched one. All materials are rheologically characterized with respect t o their viscosity functions, elasticity, and elongational properties. For t he two linear materials no indication of secondary flow is found, but the t hree long-chain branched polymers (two polyethylenes and one polypropylene) exhibit pronounced vortices. Neither viscosity nor elasticity seem to be d ecisive for the occurrence of secondary flow. The viscosity has an influenc e, however, on the size of the vortices and the velocities within theta. Al l of the three long-chain branched polymers are strongly strain hardening w hich gives rise to the conclusion that this behavior may be a necessary con dition for the formation of vortices. The linear polypropylene does not sho w any indication of strain hardening. The linear polyethylene, surprisingly , is significantly strain hardening, but it becomes less pronounced with hi gher strain rates. As most of the deformation in the entrance region takes place at elongational rates at which the strain hardening of the linear pol yethylene is not significant, the findings on the linear polyethylene do no t contradict the hypothesis that strain hardening and vortex formation in e ntrance flow may be related to each other.