The dependence of shear and elongational viscosity on the molecular weightof poly(vinylidene fluoride)

The dependence of shear and elongational viscosity on the molecular weight of poly(vinylidene fluoride) has been studied using a capillary rheometer, The elongational viscosity was evaluated based on Cogswell's method with tw o types of capillaries: capillary length (L)/capillary diameter (D) = 10 mm /1 mm and LID = 0 mm/1 mm. We used the ratio P-0/P-L that indicates the con tribution of elongational flow to the total flow involving both the shear a nd elongational flows. P-L and P-0 are the pressure losses in the capillary and the converging flows, respectively. P-0/P-L increased with molecular w eight and shear rate. This corresponds to decreasing the number of entangle ments of molecular chain under a large displacement, especially high shear. Thus, we suggest using P-0/P-L as the parameter of the entanglement intera ction on the molecular chain under a large displacement. (C) 1999 John Wile y & Sons, Inc.