Deformation mechanisms polymer crystals

The effect of mechanical deformation on polymer crystallization, which is a very complex topic at the frontiers of research on polymer processing, will be discussed briefly in Chapter 19. 

The importance of understanding the microscopic motion of matter has been expressed in the introduction of this article. The ramifications of dynamic disorder on the processes of polymer physics and chemistry are, indeed, very broad. In this regard, many defect types have in the past been proposed for polymer crystals (see the Appendix, Sect. 6). Often they were based on more or less extensive molecular mechanics calculations. Most of these defects were thought to explain some piece of the information needed for developing an understanding of the deformation of polymer crystals, but there were always some missing parts. 

Tashiro K and Tadokoro H (1977) Elastic modulus and mechanical deformation in polymer crystals, Kagaku (Kyoto) 32 848-850 (in Japanese). 

In contrast, for flexible-chain polymers, the transition into the ordered state is possible only if the flexibility can be decreased to values below fcr (in the absence of external deformational fields, the crystallization of flexible-chain polymers occurs by the mechanism of chain folding). 

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. 

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... 

In the past there have been several attempts to investigate the relationship between structure and mechanical properties in polyethylene. It has proved possible to produce samples of the polymer with a single-crystal texture ( ) but they are still polycrystalline and certain ambiguities remain concerning the detailed deformation mechanisms in such samples (9 ). There have also been reports of studies of the deformation of single crystals of polyethylene on extensible substrates (10,11). Because the polymer... 

Orientation of semicrystalline polymers below the melting point is often referred to as "cold drawing." Although some stress crystallization does occur, the process primarily involves the transformation of existing crystalline structures. A widely accepted model of the deformation mechanism is that provided by Peterlin (Figure 5) (41). Prior to necking, the crystal lamellae which... 

Y.M. Boiko, W. Brostow, A.Y. Goldman, A.C. Ramamurthy, Tensile, stress relaxation and dynamic mechanical behaviour of polyethylene crystallized from highly deformed melts. Polymer, 36 (7), 1383-1392,1995. 

The Deformation History. Polymer dynamics can be investigated by SANS via special phenomena such as demixtion observed by X-rays or crystallization. A more direct way is simply to observe a sample mechanically displaced out of equilibrium. A classical approach used in other techniques is a steady deformation, characterised by a constant rate of deformation s = 1/L dL/dt, where L is a distance. The investigation in time is related to the dependence on s. Other procedures involve time-dependent deformation histories, which relate to the actual time typical cases are a periodic deformation, e.g. oscillatory, and a stepstrain deformation. A time analysis is then needed. In the case of a periodic deformation, one can divide the period 2nj into small intervals of phase v /, i / -I- A , within which is measured. In the case of... 

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