Semicrystalline polymers yield stresses

Hence, the stated above results demonstrate nonzero contribution of noncrystalline regions in yield stress even for such semicrystalline polymers, which have devitrificated amorphous phase in testing conditions. At definite conditions noncrystalline regions contribution can be prevailed. Polymers yield stress and elastic constants proportionality is not a general rule and is fulfilled only at definite conditions. 

When we begin to stretch a semicrystalline polymer it deforms affinely, that is, each element of the sample within the gauge region experiences identical stress and strain. As we continue to stretch the sample, we reach a point at which affine deformation ceases and the sample yields. At this point, it typically develops a local region of reduced cross-sectional area, known... 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

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. 

Semicrystalline polymers must be considered two-phase mixtures of amorphous regions between lamellar crystals. It has been demonstrated that the yield stress increases with increasing crystallinity when the deformation process occurs at temperatures above the glass transition temperature of the amorphous phase and below, but close to, the melting... 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

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. 

The restraining influence of the crystallite alters the mechanical behavior by raising the relaxation time T and changing the distribution of relaxation and retardation times in the sample. Consequently, there is an effective loss of short T, causing both the modulus and yield point to increase. The creep behavior is also curtailed and stress relaxation takes place over much longer periods. Semicrystalline polymers are also observed to maintain a relatively higher modulus over a wider temperamre range than an amorphous sample. 

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... 

The localization of yield can be explained in terms of density fluctuations. On cooling from the melt, low density regions are frozen in, the volume of these regions increasing with cooling rate. Also, in semicrystalline polymers, such as iPP, the relative amounts of different crystal phases and the stresses between phases need to be considered when calculating bulk properties. 

Increasing crystallinity of a semicrystalline polymer is accompanied by increases in modulus, stiffiiess, density, yield stress, chemical resistance, melting point, glass-transition temperature, abrasion resistance, creep resistance, and dimensional stabihty, and by reduction in impact resistance, elongation, thermal expansion, permeabihty, and swelling. 

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