Deformation Behavior of Semicrystalline Polymers

By using the energy criteria for a brittle-to-ductile transition of amorphous polymers, Meijer derived the quantitative relation between the critical thickness and v, showing that the larger the v value, the larger the critical thickness. Thus, polymers that tend to craze due to low v value (e.g., PS) need thinner layers to develop large macroscopic extension than polymers that tend to shear due to high v value (e.g., PC). 

Many interesting results can be found in their treatments and readers are encouraged to read their original papers for more details [3-8]. The effect of reduced layer thickness and the resulting ductile behavior is later identified for block copolymers, too, in which thin PS layers in lamellar morphology deform plastically when the thickness of the PS layer is smaller than the critical value (named as thin-layer yielding ) [28](see Section 18.4). 

Crystalline polymers show various morphologies, ranging from single crystals (100% crystalline) to semicrystalline polymers (made of a crystalline phase and an amorphous 

As described above, semicrystalline polymers are more complex, including the two-phase stiucture, with the crystalline phase having various degrees of crystallinity and 

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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. 

The non-trivial role of the crystalline phase in the deformation behavior of semicrystalline polymers is here illustrated in the case metallocene-made isotactic polypropylene (iPP). 

Schrauwen BAG, Janssen RPM, Govaert LE, Meijer HEH (2004) Intrinsic deformation behavior of semicrystalline polymers. Macromolecules 37 6069-6078 Schultz JM (1974) Polymer materials science. Prentice Hall, Englewood Cliffs, NJ Schultz JM (1984) Microstructural aspects of failure in semicrystalline polymers. Polym Eng Sci 24 770-785... 

Although understanding of the deformation behavior of amorphous polymers is well established in terms of molecular structure, as described in Section 18.2, the deformation behavior of semicrystalline polymers, in particular, large-strain behavior. 

K. Banik and G. Menning, Process-induced long-term deformation behavior of semicrystalline PBT, Polym. Eng. Sci., 46 882-888,2006. 

For many applications, however, the small-strain behavior is at least as important as the large deformations. As the prediction of elastic behavior of heterogeneous materials is the most known and well-established, it is worth to know if the cmrent micromechaiiical models are able to predict the behavior of semicrystalline polymers. Firstly, the elastic properties of the composite components should be determined. 

The deformation behavior of amorphous polymers has been studied extensively, partly because the structure is rather simple as compared with semicrystalline polymers thus, the relationship between structure and properties can be established with relative ease. It is well known that two major micromechanisms are involved in the deformation and subsequent fracture of glassy polymers [1,2,13] (see Figs. 18.1 and 18.2). These are crazing and shear yielding, and both involve localized plastic deformation and some energy is dissipated during the deformation. In a craze, polymer chains are stretched along the stress direction and... 

Chang and co-workers (52,53) studied the large-scale deformation behavior and recovery behavior of semicrystalline ethylene-co-styrene polymers as a function of temperature, comonomer content, and crystallinity and compared them to the behavior of metallocene-produced ethylene/l-octene copolymers. Chen and co-workers (54) have provided an in-depth comparison of the morphological structure and properties of copolymers and confirmed that aspects of deformation that depended on crystallinity, such as yielding and cold drawing, were determined primarily by comonomer content for both sets of copolymers. 

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]. 

As it is known [7], parameter x, effects essentially on deformational behavior and mechanical properties of semicrystalline polymers. In Fig. 14.2, the dependence XjPr) is adduced for the studied nanocomposites, which turns out to be hnear, that allows to describe it analytically as follows ... 

At room temperature (i.e., above Tg for many semicrystalline polymers), semicrystalline polymers are tough and show large plastic deformation before fracture. A typical morphology (at the highest level) found in semicrystalline polymers is spherulite, which influences the deformation behavior of polymers. 

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