Lamellar fragmentation

Figure 1.7. Specimen of PTFE drawn to high elongation. Note the three lamellar fragments in a row near the center. The draw direction is horizontal. Scale har = 0.5 pm.

Through the use of multiple experimental techniques, we have shown how both the NXL and XL phases of PILE interact and respond to applied tensile deformation. Strains transmitted to PILE crystals lead to two distinct slip modes and, at higher strains, to the breakup and alignment of lamellar fragments. In our experiments, crystallites in PTFE orient fuUy with respect to the draw direction at strains between 70 to 200%. With increasing strain, some chains originally in the XL phase are transformed to NXL material. Noncrystalline chains continue to orient until macroscopic failure is reached. This could be a fairly general microstructural response for semicrystalline polymers. 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Cellotetraose. Despite several years of steady effort, no single crystal adequate for conventional X-ray crystallography could be grown. Fig 8. shows a powder diffraction photograph the wide angle neutron diffractogram is in Fig. 9. Crystal data are in Table V. Fig. 10 shows both a lamellar fragment of cellotetraose... 

The break-up of crystallites and the reformation of the lamellar fragments into microfibrUs is the basis of a theory for the cold-drawing of isotropic semi-crystalline polymers due to Peterlin. " (See also Hosemann et and Robertson .) Both Peterlin and Hosemann assert that the main mechanism is the break-up of each crystallite into approximately twenty smaller units which lie like pearls on a string with their chain axes parallel to the IDD. Many aspects of these theories would seem to be relevant to the deformation of oriented polymers of modest draw ratios. 

Under the condition of fibre symmetry signals in the pattern vertical to the tensile direction report highly oriented lamellae or lamellar fragments in tensile direction. This final pattern remains mainly constant also during further heating to about 160 °C (below the melting point) under load as well as unloaded. 

Fig. 8. Pattern during recrystallisation of a stretched iPP sample at temperatures below 140 °C Isotropic pattern after crystallisation from the melt, a slight preferential orientation may be observed (creation of crystallites with a wide range of orientation) crystallisation of the common stretched lamellar fragments generation of a meridional double-reflex (daughter lamellae) creation of a new series of (h k l)-crystallites (from left to right). The stretching direction is vertical.

Stretching above about 80 °C causes a destruction of the initial morphology without cavitation. The crystalline/lamellar fragments were oriented via the stress transferred by the... 

The particulate material obtained by sedimentation of broken chloroplasts consists of lamellae and lamellar fragments. Park (1963) has suspended this green precipitate in water and then precipitated it according to the critical point method of Williams (1953). When the precipitated material was dried down on a screen, shadowed with heavy metal for viewing in the electron microscope, and photographed, the material was found to be clearly lamellar in structure (Fig. 2). 

On the other hand, PP shows shear yielding without craze formation and yielding behavior (i.e. necking) with a plateau in a plane stress state. The stress for yielding is also affected by the density of tie molecules because the tie molecules are pulled out from the lamellar fragment during the yielding process [4]. 

Lv R, Xu W, Na B, Zhang Q and Fu Q (2008) Large tensile deformation behavior of oriented high-density polyethylene A correlation between cavitation and lamellar fragmentation, J Polym Sci Polym Phys 46 1202—1206. 

The processes of lamellar perfection, for example, lamellar thickening and merging of lamellar fragments, are revealed during the secondary aystallization stage. ... 

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