Semicrystalline polymers elastic properties

It has been shown that at semicrystalline polymers with devitrificated amorphous phase consideration as natural hybrid nanocomposites their anormalous high reinforcement degree is realized at the expense of crystallites partial reciystallization (mechanical disordering), that means crystalline phase participation in these polymers elastic properties formation. It is obvious, that the proposed mechanism is inapplicable for the description of polynner nanocomposites with inorganic nanofillers. 

In a semicrystalline polymer, the crystals are embedded in a matrix of amorphous polymer whose properties depend on the ambient temperature relative to its glass transition temperature. Thus, the overall elastic properties of the semicrystalline polymer can be predicted by treating the polymer as a composite material... 

Above Tm, the viscosity of the melt has Arrhenius-type dependence, decreasing (exponentially) with increasing temperature. Therefore a sharp transition is observed in both mechanical and viscous properties of semicrystalline polymers at Tm, resulting in a physical situation that is closer to the classic melting interface of monomeric crystals where, on one side, there is a viscous liquid, and on the other side, an elastic solid. 

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]. 

The interest in multicomponent materials, in the past, has led to many attempts to relate their mechanical behaviour to that of the constituent phases (Hull, 1981). Several theoretical developments have concentrated on the study of the elastic moduli of two-component systems (Arridge, 1975 Peterlin, 1973). Specifically, the application of composite theories to relationships between elastic modulus and microstructure applies for semicrystalline polymers exhibiting distinct crystalline and amorphous phases (Andrews, 1974). Furthermore, as discussed in Chapter 4, the elastic modulus has been shown to be correlated to microhardness for lamellar PE. In addition, H has been shown to be a property that describes a semicrystalline polymer as a composite material consisting of stiff (crystals) and soft, compliant elements. Application of this concept to lamellar PE involves, however, certain difficulties. This material has a microstructure that requires specific methods of analysis involving the calculation of the volume fraction of crystallized material, crystal shape and dimensions, etc. (Balta Calleja et al, 1981). 

The Chow equations [5] and the Halpin-Tsai equations [8,9] are also useful in modeling the effects of the crystalline fraction and of the lamellar shape (see Bicerano [23] for an example) on the moduli of semicrystalline polymers. Grubb [24] has provided a broad overview of the elastic properties of semicrystalline polymers, including both their experimental determination and their modeling. Janzen s work in modeling the Young s modulus [25-27] and yielding [27] of polyethylene is also quite instructive. 

Fillers are introduced into semicrystalline polymers for various reasons. The filler is supposed to play a rather passive role lowering the polymer volume in the new material. The mechanical properties of such a system ought not to depart too much from the pure polymer i.e. its elasticity, plasticity, susceptibility to plastic deformation and impact strength should all be preserved. 

Amorphous polymers well above Jg behave either as liquids or, if they are cross-linked, as rubbers the properties of rubbers are discussed in the next section. In the region close to Jg the viscoelastic properties dominate even at small strains and relatively short times and these are considered in the next chapter. This means that the static small-strain properties of amorphous polymers can be discussed meaningfully only when the polymers are well below Tg. Semicrystalline polymers are really composite materials. At temperatures well below the Tg of the amorphous regions the material has small-strain elastic properties that depend on the proper-... 

Although there is enormous variability in mechanical properties because of local concentration and isotropy variation, the Halpin-Tsai model has been shown to provide reasonably good approximations of the elastic modulus of randomly distributed CNT-based nanocomposites [322,323]. The Halpin-Tsai model (Equations 5.7, 5.8) is specifically for glassy matrix polymers, but it has also shown to be fairly effective for semicrystalline polymers as well [324]. 

Due to the reversibility of the piezoelectric effect, materials exhibiting such an electromechanical coupling may be used to handle actuation as well as sensing tasks. The different piezoelectric materials are able to provide these properties in a frequency spectrum ranging beyond the level of acoustics. On the one hand, there are several monocrystals and polycrystalline ceramics, which are hard and brittle and therefore are suitable only for relatively small strains. On the other hand, there are semicrystalline polymers, which are soft and elastic but show less pronounced coupling properties. Another kind of... 

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