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The phenomenon of extrudate distortion, which is called melt fracture, was studied for polystyrene samples of narrow and broad molecular weight distribution, and commercial samples of polypropylene and linear and branched polyethylene. It was experimentally found that the shear stress at the onset of melt fracture ( T~~) is of the order of lo6 dynes/cm2 and independent of the distribution of molecular weights. As the weight average molecular weight increases the shear stress rCr decreases. For polystyrene extruded at T~~ the recoverable shear strain, which is defined to be half the ratio (first normal stress difference/shear stress), was found proportional to the factor @, z,+ 1/aw2 which represents the distribution 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.Ltd. ) ; high density ( linear ) polyethylene ( Union Carbide Canada Ltd. and Dow Chemical of Canada Ltd. ) ; polypropylene ( Shell Canada Ltd. ) ; and blends of narrow distribution polystyrene (Pressure Chemical Co., Pittsburgh, Pa. ). The blends were prepared by mixing and dissolving the components in tetrahydrofurane ( a t room temperature), then precipitating with purified methanol and filtering.The average molecular weights of the blends were determined from the blending ratios using the following equations :

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