Comparison of Rheological Properties of Polymer Melts Under Shear Flows in Closed Cavity and Open Boundary Rheometers

Shear flows involving a number of different geometries provide the basis for commercially available dynamic shear rheometers that have proven very useful in the study of molten polymers. Two different technologies exist and are referred to as Combined Motor (CMT) and Separate Motor Technologies (SMT) each having their own advantage and limitations for measuring material rheological parameters. The main limiting factor for measuring melt viscoelasticity though is the occurrence of flow irregularity at the edge which appears due to the melt elasticity at higher shear rates. With the introduction of closed cavity oscillatory rheometers, Rubber Process Analyzers (RPA) descriptive testing of thermoplastic polymers is now easy and repeatable. These Instruments provide the ability to test material mechanical properties as a function of frequency and applied strain at various temperatures both in the linear and nonlinear viscoelastic regimes albeit with the luxury of having a closed boundary condition avoiding edge fracture which typically occurs during molten polymer characterization. In this work we present an experimental study measuring key rheological parameters in both linear and branched polymers and present differences and limitations obtained while using an open boundary or closed cavity rheometer.