Critcal stress and recoverable shear for polymer melt fracture

Abstract The phenomenon of extrudate distortion, which is called melt fracture, was studied for polystyrene samples of narrow and broad molecular weight distribution, and commerical samples of polypropylene and linear and branched polyethylene. It was experimentally found that the shear stress at the onset of melt fracture (τ cr ) is of the order of 10 6 dynes/cm 2 and independent of the distribution of molecular weights. As the weight average molecular weight increases the shear stress τ cr decreases. For polystyrene extruded at τ cr the recoverable shear strain, which is defined to be half the ration (first normal stress difference/shear stress), was found proportional to the factor M z M z+1 / M w 2 which represents the distrubution of molecular weights. The proportionality is expected to hold for other polymer systems. The polymer behavior at the onset of melt fracture was explained in terms of Graessley's entanglement theory and his correlation between true and Rouse shear compliance.