Analysis of the effects of local stress transmission on yielding properties of the polyethylenes

To study the influence of the microstructure on the mechanical properties of the semicrystalline polymers, heat treatments were performed on polyethylenes in order to, either vary the microstructural parameters (crystallinity, crystallite thickness...) keeping the same molecular parameters, or keep similar structural parameters with different molecular parameters. Tensile tests have been carried out to evaluate qualitatively the presence of tie molecules and entanglements. From these characterisations, it has been attempted to link these different parameters to mechanical properties. It has been shown that tie molecules, as the interface which participates in mechanical coupling between amorphous and crystalline phase, also affect small deformation behaviour of the polymers. It has appeared that for linear polyethylenes, yield stress seems to follow classical correlation with crystallite thickness. On the contrary, polyethylenes with large co-units rate do not obey to this correlation. © Springer/ESAFORM 2008.