Shear Deformation in Semicrystalline Polymers

Shear Deformation in Semicrystalline Polymers elastic viscous... 

The dominant mechanism of deformation depends mainly on the type and properties of the matrix polymer, but can vary also with the test temperature, the strain rate, and the morphology, shape, and size of the modifier particles (Bucknall 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). Properties of the matrix determine not only the type of the local yield zones but also the critical parameters for toughening. In amorphous polymers with the dominant formation of crazes, the particle diameter, D, is of primary importance, while in some other amorphous and in semicrystalline polymers with the dominant formation of dilatational shear bands or intense shear yielding, the interparticle distance ID, i.e., the thickness of the matrix ligaments between particles, seems to be also an important parameter influencing the efficiency of toughening. This parameter can be adjusted by various combinations of modifier particle volume fraction and particle size. 

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). 

In an amorphous glassy polymer, work hardening is considered to correspond to stretching of the entanglement network invoked to account for the rubbery plateau above Tg (see Section 14.3.3). This explains the recoverability of the deformation above Tg (in the absence of an applied force, the network retracts to its equilibrium conformation) and it is borne out by the observed correlation between the value of X in the neck and the maximum extensibility of the network, X ax In semicrystalline polymers, the evolution of the crystalline texture may also contribute to work hardening, because the resolved shear stress on activated slip systems tends to decrease as deformation proceeds at constant stress, as will be discussed further (see Section 14.4.3). 

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]. 

There are three, currently recognized, principal modes of deformation of the amorphous material in semicrystalline polymers interlamellar slip, interlamellm-separation and lamellae stack rotation [84,85]. Interlamellar slip involves shem-of the lamellae parallel to each other with the amorphous phase undergoing shear. It is a relatively easy mechanism of deformation for the material above Tg. The elastic part of the deformation can be almost entirely attributed to the reversible interlamellar slip. 

When a solid undergoes shear yielding, the local packing of its constituent units—atoms, molecules, or ions—changes to a new configuration that is stable in the absence of stresses. In glassy and semicrystalline polymers the plastic deformation takes place by means of local shear strains, without any appreciable changes in volume or density. 

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 octahedral shear stress criterion has some appeal for materials that deform by dislocation motion In which the slip planes are randomly oriented. Dislocation motion Is dependent on the resolved shear stress In the plane of the dislocation and In Its direction of motion ( ). The stress required to initiate this motion is called the critical resolved shear stress. The octahedral shear stress might be viewed as the "root mean square" shear stress and hence an "average" of the shear stresses on these randomly oriented planes. It seems reasonable, therefore, to assume that slip would initiate when this stress reaches a critical value at least for polycrystal1ine metals. The role of dislocations on plastic deformation in polymers (even semicrystalline ones) has not been established. Nevertheless, slip is known to occur during polymer yielding and suggests the use of either the maximum shear stress or the octahedral shear stress criterion. The predictions of these two criteria are very close and never differ by more than 15%. The maximum shear stress criterion is always the more conservative of the two. 

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