Capillary Flow of Polypropylene

III. Influence of Molecular Weight and Molecular Weight Distribution on Flow Behavior

Abstract

Seven fractions of isotactic polypropylene ranging in molecular weight from 1.1×10⁵ to 7.7×10⁵ and five blends with molecular weight of approximately 1.7×10⁵ were used to investigate the melt flow behavior at 250°C through capillary. A non-newtonian parameter, N(=d ln η/d ln D) increases with increasing molecular weight and molecular weight distribution (MWD), and another parameter proposed in this paper a(=d{1/ln η}/dln τ) strongly depends upon MWD. The three methods of determining the zero-shear stress viscosity η₀ were discussed.η₀ from method 1 (plots of ln 1/4η_a vs.τ) and method 2 (plots of In η vs.τ) were less precise than that from method 3 (linear plots of 1/ln η vs.τ) because of the curvature of the former plots. The following equations were obtained between η₀, and viscosity average molecular weight M_v, respectively
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and
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Melt viscosity of blends is usually smaller than that of the fractions having the same M_v. Irregular shapes of the extruded filaments appear at constant shear stress τ_c≈1×10⁶ dyne/cm² for the fractions irrespective of M_v.τ_c for the blends is greater than that of the fractions however. Dependence of Barus effect on molecular weight was well explained with the theory proposed in the preceding paper.