Plastic Deformation of Semicrystalline Polymers

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). 

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]. 

When the polymeric material is compressed the local deformation beneath the indenter will consist of a complex combination of effects. The specific mechanism prevailing will depend on the strain field depth round the indenter and on the morphology of the polymer. According to the various mechanisms of the plastic deformation for semicrystalline polymers 40 the following effects may be anticipated ... 

The initial process of plastic deformation in semicrystalline polymers is a result of deformation in ... 

Moreover, profuse microvoids were found in the LIB fibers by using a focus ion beam (FIB) method (The images can be found in Ref. [233]). Other than the crazes usually observed in the plastic deformation of glassy polymers, the microvoids in the LIB fibers were empty and aligned in the fiber direction. Such microvoids were similar to those observed during deforming semicrystalline... 

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.

Figure 12 represents all steps of craze formation in crystalline polymers in a single model. It is based on Hornbogen s model for a crack tip in a polymer crystal, under the utilization of individual block drawings by Schultz for the fine scale nature of plastic deformation in semicrystalline thermoplastics. The classification into four regions A to D (after ) helps to describe and imderstand the influence of molecular parameters on craze strength and craze breakdown. 

While due to their well-known plastic deformation properties glassy polymers provide excellent model systems for fracture studies, most engineering plastics are semicrystalline. Nevertheless, the molecular mechanisms of reinforcement of interfaces between semicrystalline polymers are much less well understood and the first systematic studies on the subject have only appeared recently [16, 30,96-99]. The reasons for this are mainly twofold ... 

Based on the facts presented above, the plastic deformation behavior of semicrystalline polymer materials and the structural changes accompanying the defor-matimi of such materials are craitroUed by the properties of both crystalline and amorphous phases. 

During elongation of a semicrystalline polymer several different processes, sudi as elastic deformation of the original spherulitic superstructure, tran ormation of a spherulitic into a fibrillar structure, plastic deformation of microfibrils by sliiqiage processes and elongation of molecular chains in the amorphous regions may occur simultaneously, successively or partly superimposed One of the advanta s... 

Toughened Polymers with Semicrystalline Matrix. It is well known that toughness of semicrystalline polymers such as PA (polyamide) and PP can be increased similar to the amorphous poljuners by the addition of relatively small amounts of rubber particles such as EPR or EPDM. As in HIPS and ABS, the modifier particles act as stress concentrators, initiating a plastic deformation of matrix strands between the particles as the main energy absorption step. In impact-modified PA and PP at room temperature, plastic deformation takes place through shear deformation (mechanism of multiple shear deformation). 

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