Effect of the molecular network on high-strain compression of cross-linked polyethylene

A series of polyethylene samples cross-linked by electron irradiation with various doses was prepared and deformed by plane-strain compression. The stress observed in the neat and cross-linked samples was resolved into an elastic and visco-plastic part. The elastic part manifested as a quasi-stationary residual stress in relaxation experiments while the visco-plastic component was related to the relaxed part of the stress. The elastic stress component, found independent of the deformation rate, consists of elastic response of the crystalline skeleton and the response of the molecular network within amorphous phase. On contrary, the relaxing, visco-plastic component of the stress depends strongly on the deformation rate. It can be approximated by a sum of two relaxation processes: short around 200 s and long at the range of 104 s.The rubber-like network stress, being a sub-component of the observed elastic part of the stress, increases substantially above the true strain of e = 1.0. This stress component was found to be the main source of the strong strain hardening observed in all compressed samples at high strains.

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► The compression stress was resolved into an elastic and visco-plastic part.
► Elastic part includes a response of the molecular network within amorphous phase.
► This network stress, controlling strain hardening, depends on network density.
► Some entanglements are resolved and do not participate in network stress built-up.
► The relaxation time of the visco-plastic stress also depends on network density.