TENSILE YIELD OF POLYETHYLENE IN RELATION TO CRYSTAL THICKNESS

Tensile yield stress measurements have been performed on a serie of four ethylene/1-butene copolymers covering the crystallinity range 0.33-0.74. Samples have been prepared from melt-crystallization and from solution-crystallization at various concentrations in decalin in order to span a wide range of crystal thickness. It is shown that tensile yield stress depends on crystal thickness rather than on crystallinity. This result is perfectly consistent with a plasticity model based on the nucleation of dislocations. Besides, it indicates that the main role of the amorphous phase is only to transmit the load upon the crystals, irrespective of its proportion. The yield stress dependence on crystallinity reported previously relies on the direct correlation of crystal thickness and crystallinity, in the case of melt-crystallized samples only. Discrepancies with the model are amply discussed.