Melt rheology of poly(vinylidene fluoride) (PVDF)/low density polyethylene (LDPE) blends

The aim of this work is to study the rheological properties of PVDF/LDPE blends in the molten state, in relation to shear rate, shear stress and temperature. Blends of LDPE and PVDF at different compositions, namely PVDF/LDPE = 0/100, 30/70, 50/50, 70/30, 100/0 were realized. The blends were prepared by using brabender plastograph and measurements were carried out by using capillary rheometer. For studying the viscous properties non-Newtonian index n, true viscosity η and the flow activation energy E were determined. The melt elasticity has been studied in term of die swell ratio B. The results show that PVDF/LDPE blends behave in pseudo-plastic manner (n < 1), plots of true viscosity versus blending ratio go through a minimum nearly at blending ratio (50/50), and the flow activation energy decreases with increasing shear rate at the wall γ but it increases with increasing true shear stress τ. The results of die swell ratio show that, B increases linearly with increasing true shear stress τ, while B decreases with increasing L/R. It was also found that a maximum of elastic properties is observed at certain blending ratio. This phenomenon can be attributed to the amount of elastic deformation energy stored in the melt flow and its transition due to the viscoelastic difference between the two phases.