Stress-strain curves polymer morphology

As noted in Fig. 14.1 (a), commercial fibers of semicrystallme polymers are always cold-drawn after spinning to achieve further structuring through further macromolecular orientation and crystalline morphological changes, many of which are retained because of the low temperature of the cold-drawing processes. A typical stress-strain curve for a polycrystalline polymer at a temperature Tg < T < Tm appears in Fig. 14.6. 

The discussion of mechanical properties comprises the various contributions of elastic, viscoelastic and plastic deformation processes. Often two characteristic stress levels can be defined in the tensile curve of polymer fibers the yield stress, at which a significant drop in slope of the stress-strain curve occurs, and the stress at fracture, usually called the tensile strength or tenacity. In this section the relation is discussed between the morphology of fibers and films, made from lyotropic polymers, and their mechanical properties, such as modulus, tensile strength, creep, and stress relaxation. 

The different morphologies generated in the bulk polymer samples prepared using catalyst 10a in combination with either MAO or borate activation (A and B series. Table 9.2) can be implicated in variations in mechanical behavior. To study the influence of the morphology on macroscopic mechanical behavior, melt-pressed samples were subjected to uniaxial stretching until they failed (A series. Figure 9.28 B series. Figure 9.29). For mmmm contents below 40%, stress-strain curves are observed that are characteristic of elastomeric materials. 

Figure 18.11 Schematic stress-strain curves in different morphological forms of crystalline polymers. Kinloch and Young [10]. Reproduced with permission of Elsevier.

One of the oldest ways of characterizing polymers, their behavior in stress-strain, seems an unlikely way to follow changes in morphological and fundamental molecular properties. In fact, with PTFE at least, the shape of a tensile strength curve can be very revealing. Initial reports on this approach are in our ACS paper frcm 1956 [30]. 

Solids of different classes, including polymers, are characterized typically with a complex non-uniform structure on various morphological levels and the presence of different local defects. The theoretical approaches describe the deformation of solid polymers via local defects in the form of dislocations (or dislocation analogies ) and disclinations, or in terms of dislocation-disclination models even for non-crystalline polymers [271-275, 292]. In principle, this presumes the localized character and jump-like evolution of polymer deformation at various levels. Meantime, the structural heterogeneity and localized microdeformation processes revealed in solids by microscopic or diffraction methods, could not be discerned typically in the mechanical (stress-strain or creep) curves obtained by the traditional techniques. This supports the idea of deformation as a monotonic process with a smoothly varying rate. Creep process has been investigated in the numerous studies in terms of average rates (steady-state creep). For polymers, as the exclusion. 

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