Layers, transcrystalline oriented

The two-component system—crystal lamellae or blocks alternating with amorphous layers which are reinforced by tie molecules— results in a mechanism of mechanical properties which is drastically different from that of low molecular weight solids. In the latter case it is based on crystal defects and grain boundaries. In the former case it depends primarily on the properties and defects of the supercrystalline lattice of lamellae alternating with amorphous surface layers (in spherulitic, transcrystalline or cylindritic structure) or of microfibrils in fibrous structure, and on the presence, number, conformation and spatial distribution of tie molecules. It matters how taut they are, how well they are fixed in the crystal core of the lamellae or in the crystalline blocks of the microfibrils and how easily they can be pulled out of them. In oriented material the orientation of the amorphous component (/,) is a good indicator of the amount of taut tie molecules present and hence an excellent parameter for the description of mechanical properties. In fibrous structure it directly measures the fraction and strength of microfibrils present and therefore turns out to be almost proportional to elastic modulus and strength in the fibre direction. 

A transcrystalline layer (TCL) is the supermolecular crystalline stmcture, induced by an oriented growth in the presence of the foreign surface. Transcrystallization occurs when the nucleation density of a solid filler that is in contact with melted... 

The first extensive SEM investigation of PA6/PET-based MFCs and their precursors performed by Evstatiev et al. [82] undoubtedly showed the fibrillar structure of the PET reinforcements preserved after the PA6 matrix isotropization. Since then, electron microscopy has been used to visualize the orientation and morphology of the matrix and reinforcing components in almost every report on MFCs. It is worth noting some more recent studies on MFCs comprising LDPE and PET as matrix and reinforcement, respectively [30,31]. Several microscopic techniques were used, e.g., SEM, polarizing light microscopy (PLM) and TEM. Thus, by SEM it was demonstrated that the isotropic LDPE matrix embedded PET microfibrils with random orientation. Thin slices of PLM and TEM showed the orientation in the machine direction. The latter method also revealed the formation of transcrystalline layers of LDPE on the oriented PET microfibrils. 

Both SEM and SAXS studies of UDP MFC materials reinforced by PA6 or PA12 fibrils gave evidence that the reinforcing fibrils most probably have a layered, coaxial structure a core of oriented PA and a shell of oriented, transcrystalline HDPE. The WAXS measurements supported and allowed a further development of this hypothesis. 

Oriented or crystalline structures such as transcrystalline layers can easily be seen in crossed polars. Many other examples of polarized light images of polymers are shown in Chapter 5 of Sawyer et al When counting randomly oriented birefringent particles such as polymer wear particles in tissue, some would be in their extinction orientation and not be visible under crossed polars. The solution is to use crossed circular polars. A quarter-wave plate, that is, one with a retardation of 2/4, placed after the linear polarizer and set at +45 °... 

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