Tetrafluoroethylene/hexafluoropropylene copolymers (FEPs) are widely used in diverse fields due to their outstanding performances in chemical resistance, thermal stability, and insulation. However, their processsability is poor, exhibiting narrow stable flow region and remarkably early melt fracture. Herein, we tried to explore the origin of such poor processability in a rheological way, because melt rheology behaviors are highly related to their processing processes. The shear rheology results indicate that FEPs exhibit multiple flow regions. The flow curve of FEP608 was obtained, and its ?0 value was calculated to be 1.70 kPa s-1 at 360 degrees C. Extensional rheological data suggest that FEPs have much lower e and B values when compared with those of common polymers, suggesting their weaker elasticity during extrusion. Based on such rheological results, the poor processability of FEPs is ascribed to their high viscosity induced by special interchain interaction associating with F atom, which can easily cause accumulated elastic energy. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012