Lamellar crystals, microscopy

Figure 9.22. Scanning force microscopy images of polyethylene films formed on a model planar chromium polymerization catalyst. The small white stripes are lamellar crystals. These form the well-known spherulite superstructure upon crystallization from the...

Fig. 10.28 Height image of torsional tapping atomic force microscopy on a sheared PE film with the shear direction from down-right to up-left (Mullin and Hobbs 2011). The circle indicates the wedge-shaped profile at the growth front of lamellar crystals (Courtesy of Jamie Hobbs)...

Polyethylene. AFM imaging of thermoplastics has been widely used to corroborate and expand knowledge obtained using other structural methods, such as x-ray crystallography and electron microscopy. Direct observation of folded chain lamellar crystals of polyethylene (PE) was provided by afm. Spacings appropriate for the (known crystallographic) orthorhombic imit cell, and for the monoclinic unit cell that can be produced by mechanical deformation, were observed (84) as were boimdaries between regions of differently oriented folded chains (85) (see Ethylene Polymers, HDPE). 

Banded spherulites, eg in PE (179,180), poly(vinylidene fluoride) (PVDF) (181), or polyhydroxyalkanoates (182), have also been studied in detail. The periodic bands detected in optical microscopy correspond typically to periodic corrugations along the radial direction of the spherulite. Particularly interesting is the case of PVDF (181). On the basis of a careful analysis it was found that the slopes of the observed multilayer terraces of lamellar crystals are retained in each half of a banded spherulite. This evidence confirmed the macroscopic selection of one-handedness in the formation of spiral terraces in each growth direction of the sheaf at the center of banded spherulites of PVDF (181). 

Disregarding any detailed morphological models proposed for the lamellar crystals in semicrystalline polymers in the past half centiny, crystal imit cells in semicrys-talUne poisoners exactly obey the rules of classical crystallography, which consist of 7 crystal systems and 14 Bravais lattices (more precisely, there are only 6 crystal systems in polymer crystals, and the cubic system does not exist). Most of the unit cell determinations of the semicrystalline polymers rely on the wide-angle x-ray diffraction (waxd) experiments on oriented poisoner fibers and films. This is be-canse of small crystal sizes in polymers, which lead to a difficult experimental task to obtain single-crystal waxd results on semicrystalfine polymers. It is also possible to nse electron diffraction (ed) method in transmission electron microscopy (tern) to determine polymer lamellar crystal unit cell structures, dimensions, and symmetries. 

Once the objective of the experiment is known and the specimens selected for study, the next major step is the selection of the microscopy techniques and the specimen preparation methods required to image the polymer structures of interest (Table 7.2). If lamellar crystals must be... 

Once the objective of the experiment is known and the specimens selected for study, the next major step is the selection of the microscopy techniques and the specimen preparation methods required to image the polymer structures of interest (Table 6.2). If lamellar crystals must be evaluated, for instance, there is no point in considering most optical techniques as they will only provide an overview of these structures. Comparisons are made in this section regarding the various techniques, in both the text and tables, as an aid in this selection process. Observations of... 

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