Deformation mechanisms semicrystalline polymers

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 ... 

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... 

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). 

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]. 

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 SFA, originally developed by Tabor and Winterton [56], and later modified by Israelachvili and coworkers [57,58], is ideally suited for measuring molecular level adhesion and deformations. The SFA, shown schematically in Fig. 8i,ii, has been used extensively to measure forces between a variety of surfaces. The SFA combines a Hookian mechanism for measuring force with an interferometer to measure the distance between surfaces. The experimental surfaces are in the form of thin transparent films, and are mounted on cylindrical glass lenses in the SFA using an appropriate adhesive. SFA has been traditionally employed to measure forces between modified mica surfaces. (For a summary of these measurements, see refs. [59,60].) In recent years, several researchers have developed techniques to measure forces between glassy and semicrystalline polymer films, [61-63] silica [64], and silver surfaees [65,66]. The details on the SFA experimental procedure, and the summary of the SFA measurements may be obtained elsewhere (see refs. [57,58], for example.). 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

In addition, most semicrystalline polymers, particularly those produced commercially, are partially oriented i.e. their chains have an overall alignment that may impart to the bulk polymer certain advantageous properties, e.g. increased mechanical strength or dielectric polarizability. Molecular orientation, whether arising from crystallization under stress or deformation of a solidified polymer, or in naturally occurring oriented crystalline polymers such as cellulose or keratin, is always associated with an orientational morphology. 

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 ... 

During polymer processing non-isothermal crystallization conditions, mechanical deformation, and shear forces may alter the morphology and orientation of polymers both at the surface and in the bulk. In addition, orientation effects of semicrystalline polymers that crystallize in contact with solids are considered. 

The deformation of long chain polymer molecules has always been of great industrial interest as more value can be placed on a material that has improved properties. Molecular extension, or alternatively molecular orientation, is of particular interest as it can enhance mechanical properties of an otherwise weak polymer. Oriented materials are inherently anisotropic. These anisotropic regions can be found directly in semicrystalline polymers where chains organize themselves into crystalline domains. 

The effects of orientation via mechanical deformation on Tg have been reviewed [65]. Tg increases in those amorphous regions of a semicrystalline polymer that are either attached to crystallites or so close to them that their chain segment mobilities are hindered because of the interference of the crystallites. On the other hand, orientation has little effect on Tg in amorphous regions far away from crystallites as well as in completely amorphous polymers. 

The optical properties of semicrystalline polymers are often anisotropic. On the other hand, amorphous polymers are normally isotropic unless directional stresses are frozen in a glassy specimen during fabrication by a process such as injection molding. Anisotropy can often be induced in an amorphous polymer by imposing an electric field (Kerr effect), a magnetic field (Cotton-Mouton effect), or a mechanical deformation. Such external perturbations can also increase the anisotropy of a polymer that is anisotropic even in the absence of the perturbation. 

Orientation of semicrystalline polymers below the melting point is often referred to as "cold drawing." Although some stress crystallization does occur, the process primarily involves the transformation of existing crystalline structures. A widely accepted model of the deformation mechanism is that provided by Peterlin (Figure 5) (41). Prior to necking, the crystal lamellae which... 

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