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The influence of mechanical forces on structural changes in commercial grades of high and low density polyethylenes was investigated by using extremely high shear rates (>2 × 104S−1) during processing in a special injection molding machine and by long‐term loading at ambient temperature. Changes in molecular weight distribution (MWD) and the degree of long‐chain branching (LCB) were followed by gel permeation chromatography and viscometry. No effect of the mechanical forces was observed either in the long‐term loading or in the processing. At temperatures exceeding 280°C the small changes noted in MWD and LCB wrere found to be entirely due to thermal de radation.

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