Deformation Instabilities and Lamellae Fragmentation during Deformation of Cross-linked Polyethylene.

Zbigniew Bartczak,Alina Vozniak

doi:10.3390/polym11121954

Abstract

The effect of the topology of the amorphous phase and phase interconnectivity on the stability of the deformation of semicrystalline polyethylene was investigated. The chain topology was modified by crosslinking the samples with electron beam irradiation. The samples were deformed by plane-strain compression, while the accompanying structural changes were monitored with X-ray and differential scanning calorimetry (DSC). At the true strain around of e = 0.3, the lamellar stacks parallel to the loading direction experienced microbuckling instability, which shortly led to the cooperative kinking of lamellae. Macroscopically, this showed up as the ‘second yield.’ Buckling is driven by the different stiffness levels of the hard and soft layers and their strong connectivity—for given layer thickness, the critical strain for buckling appeared proportional to the stiffness of the amorphous phase. Above e = 1.0, lamellae fragmentation was observed. This resulted from the localization of crystallographic slip, which was triggered by stress concentrations generated at lamellae faces by taut ‘stress transmitter’ (ST) chains. Accordingly, the fragmentation was found to be dependent on the surface fraction of STs at the amorphous-crystal interface: a low concentration of STs resulted in fewer but stronger stress concentrations, which led to earlier slip localization, followed quickly by lamellae fragmentation. The observed instabilities, either lamellae kinking or fragmentation, profoundly influenced the deformation process as well as the resultant structure. Both phenomena relieved much of the structural constraints imposed on deforming lamellae and make further strain accommodation easier.

Highlights

The outstanding mechanical performance of semicrystalline polymers, including their ability for large-scale plastic deformation, can be attributed to their unique morphology, consisting of crystalline and amorphous elements in the form of thin alternating layers
We postulated that the fragmentation was initiated in already highly deformed lamellae by the stress concentrations generated at the amorphous-crystal interface by stress transmitter chains, stretched and taut due to the interlamellar shear in amorphous layers that accompanies the deformation of crystalline lamellae
The deformation study of linear polyethylene, crosslinked in the amorphous phase by electron beam irradiation and highly deformed in compression, revealed several instabilities that accompany the plastic deformation process, occurring at various strains. These instabilities result in lamellae fragmentation and/or reorientation, which apparently enables the easier accommodation of the strain in an energy-minimizing way by opening new paths of relatively easy plastic deformation of the lamellar crystals

Summary

Introduction

The outstanding mechanical performance of semicrystalline polymers, including their ability for large-scale plastic deformation, can be attributed to their unique morphology, consisting of crystalline and amorphous elements in the form of thin alternating layers. Very important is the specific structure of layers—more or less regularly folded chains in crystalline lamellae vs highly entangled chains constituting the amorphous phase—and the robust phase interconnectivity, which is provided by numerous chains intersecting the amorphous-crystalline interface and ensuring the extremely strong covalent bonding of adjacent amorphous and crystalline layers. These chains allow for the load transfer between neighboring lamellae across the stack. Detailed knowledge, about the individual phases and the interplay between them, is essential for understanding the deformation habits of semicrystalline polymers.

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