Deformation of Semicrystalline Polymers

Men Y, Rieger J, Strobl G (2003b) Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys Rev Lett 91 955021-955024 Men Y, Strobl G (2002) Evidence for a mechanically active high temperature relaxation process in syndiotactic polypropylene. Polymer 43 2761-2768 Plazek DJ, Chay I, Ngai KL, Roland CM (1995) Visoelastic properties of polymers. 4. Thermo-rheological complexity of the softening dispersion in polyisobutylene. Macromolecules 28 6432-6436... 

It is commonly believed that the function of the amorphous phase, above the glass transition temperature, in yielding during tensile deformation of semicrystalline polymers is relatively small and is limited to transfer the stress between adjacent crystals (Seguela and Darras 1994). The stress is transmitted through... 

J.M. Haudin, Plastic deformation of semicrystalline polymers, in Plastic Deformation of Amorphous and Semi-crystalline Materials, ed. by B. Escaig, C. G Sell (Les Editions de Physique, Paris, 1982), p. 291... 

The mechanisms of plastic deformation at microscopic level of amorphous polymers are mainly crazing and shear yielding [3-5]. In semicrystalline polymers, although the glass transition temperature, density, infrared spectrum and other properties of the amorphous phase interdispersed between the crystalline lamellae are close to those of bulk amorphous polymers, the mechanisms of plastic deformation are very different from those of the amorphous materials, since also the crystalline phase plays a key role [Ij. However, because of the presence of the entangled amorphous phase, the mechanisms of plastic deformation of semicrystalline polymers are also different from those of other crystalline materials (for instance metals). 

R. Seguela, Dislocation Approach to the Plastic Deformation of Semicrystalline Polymers Kinetic Aspects for Polyethylene and Polypropylene , J. Polym. ScL, Part B Polym. Phys. 40, 593-601 (2002). 

Hong K, Rastogi A, Strobl G (2004a) A model treating tensile deformation of semicrystalline polymers quasi-static stress-strain relationship and viscous stress determined for a sample of polyethylene. Macromolecules 37 10165... 

Men Y, Rieger J, Strobl G (2003) Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys Rev Lett 91 1... 

Seguela R (2002) Dislocation approach to the plastic deformation of semicrystalline polymers kinetic aspects for polyethylene and polypropylene. J Polym Sci B Polym Phys 40 593 Seguela R (2005) On the strain-induced crystalline phase changes in semi-crystalline polymers mechanisms and incidence on the mechanical properties. J Macromol Sci C Polym Rev 45 263-287... 

Also, if the plastic deformation of semicrystalline polymers is often described with changes in the lamellae, it should be necessary to emphasize that the mechanical behavior is also greatly affected by the state and mobility of their amorphous phase [17,18]. 

The mechanisms of tensile deformation of semicrystalline polymers was a subject of intensive studies in the past [8-20]. It is believed that initially tensile deformation includes straining of molecular chains in the interlamellar amorphous phase which is accompanied by lamellae separation, rotation of lamellar stacks and interlamellar shear. At the yield point, an intensive chain slip in crystals is observed leading to fragmentation but not always to disintegration of lamellae. Fragmentation of lamellae proceeds with deformation and the formation of fibrils is observed for large strains [21-24]. 

Deformation of semicrystalline polymers is more difficult to understand due to complexity of structures made of an amorphous phase and a crystalline phase yet, a simple explanation... 

Figures 10.9 to 10.11 illustrate how stretching curves and critical strains vary with temperature, again with results for PEVA12, and with the crystallinity here polyethylenes with different crystallinities are compared. Curves demonstrate a further general property of semicr3 talline pol5oners. While the stresses vary in systematic manner, there is no effect on the critical strains for softening (en 0.1) and hardening (en 0.6) and virtually no change in the elastic-plastic composition of the strains. Hence, tensile deformation of semicrystalline polymers is strain-controlled and changes the mechanism at two critical strains that are temperature and crystallinity invariant.

The development of thermoplastics of high tensile modulus has been the subject of intense research in recent years. This can be achieved by introducing a high degree of chain orientation and extension by solid state deformation of semicrystalline polymers by the processes of drawing (1) and extrusion (2). 

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