Orientation hardening crystalline

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

If a material shows work-hardening, as in many crystalline polymers (due to orientation during the plastic deformation), the ultimate strength (if calculated on the basis of original cross section) will of course be higher than the yield stress. 

This polyimide precursor forms a polymer coating on the glass surface, onto which alignment may be introduced by rubbing, before the coating has hardened such rubbed films induce orientation in liquid crystalline materials. 

The very last portion of the stress-strain curve indicates strain hardening, induced mainly by further chain orientation. In semi-crystalline polymers, one detects an increase in crystallinity and orientation—both leading to an increase of stress. In these cases, Sb > Sy. 

Rotational injection molding n. A modified injection-molding process applicable to hollow, axisymmetrical articles such as cups and beakers, in which the male half of the mold is rotated during the molding cycle until the material has hardened to a pre-chosen degree. The rotation produces orientation and increased crystallinity of some polymers, resulting in improved toughness and stress-craze resistance. 

In an isotropic polycrystalline polymer whose microstructure consists of stacked lamellae arranged in the form of spherolites, the slip systems activated depend on the local orientation of the lamellae with respect to the applied stress and, as deformation proceeds, these orientations are modified. To calculate the evolution of the crystalline texture, one can consider the polymer to behave as a crystalline aggregate. Although the entropic contribution of chain orientation in the amorphous regions may also need to be considered, the major contribution to work hardening in tension is rotation of the slip planes toward the tensile axis, so that the resolved shear stress in the slip direction diminishes. This results in a fiber texture in the limit of large deformations, such that the crystallites are oriented with their c axis (the chain axis) parallel to the stretch direction. Despite the relative success of such models, they do not explicitly address the micro-mechanisms involved in the transformation of the spherulitic texture into a fiber texture. One possibility is that the... 

Uniaxial tensile tests of poly (ethylene terephthalate) (PET)/montmorillonite(MMT) nanocomposites were preformed over a temperature range of 85°C-105°C and stretch rate of 7.5mm/s-12.5mm/s. The stress-strain curves consisted of three regions the hnear visoelasticity, the rubbery plateau and the strain hardening. The effects of temperature and stretch rate on stress-strain behavior were discussed. The results of differential scanning calorimetry (DSC) measurements indicated that the stretch lead the increase of the crystallinity degree of specimens. The wide angle X-ray diffraction (WAXD) measurements revealed that the more perfect crystal structures were obtained with the increase of temperature and oriented along the stretch direction. 

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