Plasticity of Semicrystalline Polymers

Abstract

Semi-crystalline polymers can be deformed up to a very high strain. The deformation process involves frequently a complete molecular rearrangement of the chain-folded lamellar morphology into a more or less chain-unfolded fibrillar microstructure. This transformation is likely to occur through an intermediate state of high molecular disorder at a local scale. It led to the formulation of a concept of strain-induced melting-recrystallization process as a main mechanism of the structure transformation. In contrast, several structural features occurring at moderate plastic strains are relevant to strictly crystallographic processes. The plastic deformation process of semicrystalline polymers and the micromechanisms involved are discussed. A critical discussion of experimental findings is made to point out the strength or the deficiency of the various argumentations. It is demonstrated that the crystallographic slip mechanisms, including slips: transverse and along the chains are the basic deformation mechanisms in the deformation sequence, active at all strain levels. Direct microscopic evidence of chain slip activity even at well advanced stages of the deformation process is presented. In contrary, the meltingrecrystallization seems to be restricted to the high-strain stage accompanied by chain unfolding and perhaps limited to only a small fraction of the crystalline phase. In addition the experimental results demonstrates clearly that the cavitation, necessary in the Peterlin's model, is really unessential in producing high deformation and appearance of the final highly oriented structure. This can be effectively accomplished with only crystallographic mechanisms employed. A very important role in the deformation sequence is played by the partially reversible shear deformation of amorphous interlamellar layers, producing not only high orientation of amorphous component but also influencing deeply the deformation of crystalline phase, since both phases are strongly connected and must deform simultaneously and consistently.

The structural evolution of high-density polyethylene during crazing in liquid medium

“…5A, 9B) that agrees with the data on the deformation of semicrystalline polymers (in particular, so called hardelastic polymers) in air [54]. The locking of the shear of amorphous layers due to straining of tie-molecules causes stress concentrations in lamellae (at interfaces around points of tie-molecules entrance into lamella) and results in lamellae bending according to the crystallographic mechanisms [10].…”

“…High-density polyethylene (HDPE) is a profoundly investigated semicrystalline polymer, and numerous works [1][2][3][4][5][6][7][8][9][10] have been focused on studying the mechanism of its deformation and the evolution of its structure upon stretching in air. At the same time, items made of HDPE are frequently exploited in contact with various liquid and gaseous media.…”

Structural mechanics of semicrystalline polymers prior to the yield point: a review

“…At present we have rather fixed conceptions on the origins of microstructure and deformation mechanisms of the meltcrystallized thermoplastic polymers [1][2][3][4][5][6][7]. Such polymers are composed of stacks of crystalline lamellae alternating with amorphous layers.…”

“…This is caused by a complex morphology of the materials as well as strong correlation between different structural elements under the given deformation conditions. Current understanding of these phenomena is based on manifold studies which were summarized in number of comprehensive reviews [1][2][3][4][5][6][7].…”

Crystalline plasticity in isotactic polypropylene below and above the glass transition temperature

“…A physical interpretation of deformation processes at the macroscopic scale can only be successful when both, micro-and mesoscopic deformation processes are considered. At the mesoscopic scale, rearrangements of lamellar crystalline blocks such as bending, rotation, fragmentation or shear band formation occur [1]. At the microscopic scale, basic micromechanisms of deformation within the crystalline and amorphous phase are active.…”