Thermoplastic polymers deformation behavior

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

R. A. Mendelson, Miscibihty and deformation behavior in some thermoplastic polymer blends containing poly(styrene-co-acrylonitrile), /. Polym. Sci. Polym. Phys. Ed., 23, 1975, 1985. 

Na B, Zhang Q, Fu Q, Men Y, Hong K and Strobl G (2006) Viscous-force-dominated tensile deformation behavior of oriented polyethylene. Macromolecules 39 2584-2591. Friedrich K (1983) Crazes and sheai bands in semi-crystalline thermoplastics, Adv Polym Sci 52/53 225-274. 

Lee H S, Yoo S R and Seo S W (1999) Domain and segmental deformation behavior of thermoplastic elastomers using synchrotron SAXS and FTIR methods, J Polym Sci Part B Polym Phys 37 3233-3245. 

All thermoplastic polymers start out with a glassy behavior below the glass transition temperature Tg. As T is increased above Tg, they all become more ductile or leathery and later become softer or more rubber-like. A semicrystaUine isotactic pol)nner will retain its strength longer than an amorphous polymer because its crystalline structure permits more elastic deformation while the others deform viscoelactically. Eventually, semicrystalline isotactic polymer as well as the amorphous polymer will turn into a viscous liquid as they eventually melt. A cross-linked polymer softens somewhat at the glass temperature but does not melt because of the strong covalent cross-link bonds. It will decompose before these bonds are broken. 

In practice polymeric materials are often subjected to mechanical loads. Therefore, it is essential to know how polymers respond to the mechanical load. Figure 1.3 presents the common behavior of thermoplastic polymers under uniaxial deformation. On the stress(a)-strain(e) curve four regions [7] can be distinguished in region I the material shows an elastic behavior. In this region the material is characterized by its Young s modulus of elasticity. In region II the material yields. The slope of... 

SAXS and WAXS are particularly efficient in the study of amorphous polymers including microstructured materials, hence their use in block copolymers (see also Chapters 6 and 7). The advent of synchotron sources for X-ray scattering provided new information, particularly on the evolution of block copolymer microstructures with time resolution below one second. In particular, the morphology of TPEs is most often studied with these techniques Guo et al. [108] applied SAXS to the analysis of the phase behavior, morphology, and interfacial structure in thermoset/thermoplastic elastomer blends. WAXS is often associated with SAXS and some other methods, such as electron microscopy, and various thermal and mechanical analyses. It is mainly used in studies of the microphase separation [109,110], deformation behavior [111], and blends [112]. 

Fakirov S, Fakirov C, Fischer E W and Stamm M (1991) Deformation behavior of poly(ether ester) thermoplastic elastomers as revealed by small-angle X-ray scattering, Polymer 32 1173-1180. 

Section II B of Chapter 2 gave a description of the uniaxial deformation behavior of an unoriented thermoplastic polymer. It was indicated that — depending on experimental and material parameters - failure could occur at any of the different stages of a tensile loading process ... 

Therefore, a different approach was followed in the present paper in order to improve the understanding of the relationship between the structure and the behavior of crosslinked polymers. A series of directly comparable model polymers were prepared with crosslink densities varying from high (thermoset) to zero (thermoplastic). Five polymers with well defined crosslink densities [11] were tested at various levels of deformation. This approach produced a small but assessable and fairly consistant body of results. Basic relationships derived from these results were related to corresponding results from the literature. 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

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