Semi-crystalline polymers elastic deformation

This initial microcrack formation is reflected in a stress-strain curve by the deviation from the linear range of the elastic constants. In fact, the failure is analogous to the microcracks that form between spherulites when a semi-crystalline polymer is deformed. (Source Osswald, T.A. and G. Menges, Material Science of Polymers for Engineers, Hanser Publishers, New York, 1996). Refer also to Vulcanization, Peroxides, Peroxide Cross-Linking, Sulfur Vulcanization, and Vulcanizing Agents. 

Let s start by looking at a simple polymer, polyethylene, that has a lot going on in its stress/strain plots (Figure 13-38). Flexible, semi-crystalline polymers such as this (where the T of the amorphous domains is below room temperature) usually display a considerable amount of yielding or cold-drawing, as long as they are not stretched too quickly. For small deformations, Hookean elastic-type behavior (more or less) is observed, but beyond what is called the yield point irreversible deformation occurs. 

Gibson, A.G. at al. (1978). Dynamic mechanical behaviour and longitudinal crystal thickness measurements on ultra-high modulus linear polyethylene a quantitative model for the elastic modulus. Polymer, Vol. 19 (1978), pp. 683-693 Hong, K. et al. (2004). A model treating tensile deformation of semi-crystalline polymers Quasi-static stress-strain relationship and viscous stress determined for a sample of... 

The stress-strain curve in Fig. 7.24b first of all exhibits elastic and preplastic behaviour. It then reaches a maximum whose sharpness depends on the polymer and also the deformation rate. Beyond this point, the stress remains almost constant over a certain region, before suddenly increasing to fracture. This is brittle fracture, perpendicular to the load. Many semi-crystalline polymers, such as polyethylene, polypropylene, polyamide 6 and polyamide 6,6 exhibit this type of behaviour at ambient temperature. However, among amorphous polymers in the glassy state, polycarbonate is one of the rare examples to behave in this way. 

Let us consider the causes of the indicated discrepancy in more details. At present it has been assumed [1], that in case of semi crystalline polymers with devitrificated amorphous phase deformation in elasticity region, that is, at E value determination, the indicated amorphous phase is only deformed, that defines smaller values of both E and d This conclusion is confirmed by disparity between <7, values, calculated on the basis of mechanical characteristics (the Eqs. (1) and (2)) and crystallinity degree... 

Thus, the performed estimations demonstrated, that high values of reinforcement degree EIE for semi crystalline polymers, considered as hybrid nano composites (in case of studied HDPE EIE value is varied within the limits of 10-110) were due to recrystallization process (mechanical disordering of crystallites) inelastic deformation process and, as consequence, to contribution of crystalline regions in polymers elastic properties formation. It is obvious, that this mechanism does not work in case of inorganic nano filler (e.g., organoclay). Besides, a nano filler... 

Again, in crystalline and semi-crystalline polymers there are four regions of different stress-strain behavior. As before, in region 1 the polymer experiences elastic deformation and recoverable strain. In this region, the spatial arrangement of the crystal lamellae within spherulites becomes deformed as much as the reversible molecular conformation change of the inter-crystalline tie molecules will permit. 

Thin films of materials tested included two semi-crystalline polymers PE and PP above their corresponding glass transition temperatures Tg, and three fully amorphous polymers PS, PC, and PMMA well below The authors concluded The maximum penetration depth jc ax was found to scale linearly with the residual depth X, for all six polymers, irrespective of material strain rate sensitivity. The ratio Ix is related directly to the ratio of total impact energy W IW" dissipated via an elastic deformation of the material. 

There have been many efforts for combining the atomistic and continuum levels, as mentioned in Sect. 1. Recently, Santos et al. [11] proposed an atomistic-continuum model. In this model, the three-dimensional system is composed of a matrix, described as a continuum and an inclusion, embedded in the continuum, where the inclusion is described by an atomistic model. The model is validated for homogeneous materials (an fee argon crystal and an amorphous polymer). Yang et al. [96] have applied the atomistic-continuum model to the plastic deformation of Bisphenol-A polycarbonate where an inclusion deforms plastically in an elastic medium under uniaxial extension and pure shear. Here the atomistic-continuum model is validated for a heterogeneous material and elastic constant of semi crystalline poly( trimethylene terephthalate) (PTT) is predicted. 

Above amorphous liquid appears, while a semi-crystalline solid appears below Tm. Above Tg, the amorphous phase shows a high mobility leading to high elasticity and deformity. On the other hand, below Tg most polymers behave like rigid solids, and are frequently brittle (glass-like). Exten-sivity is drastically reduced, together with the capacity to resist the effect of blows and oscillation (impact strength). 

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