Lamellae Lamellar fragments

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. 

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.

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). 

Lamellar breakup occurs through chain pulling and unfolding the chains pulled still connect the fragments of the lamellae. 

The mechanisms of tensile deformation of semicrystalline polymers was a subject of intensive studies in the past [8-20]. It is believed that initially tensile deformation includes straining of molecular chains in the interlamellar amorphous phase which is accompanied by lamellae separation, rotation of lamellar stacks and interlamellar shear. At the yield point, an intensive chain slip in crystals is observed leading to fragmentation but not always to disintegration of lamellae. Fragmentation of lamellae proceeds with deformation and the formation of fibrils is observed for large strains [21-24]. 

---