Molecular orientation, effect crystallization

Effect of Molecular Orientation on Crystallization of Flexible-Chain Polymers. 217... 

This technique is suitable for amorphous materials, such as waxes, tarry solids, hot melt adhesives, and some low-melting solids. Rubbery materials often have a tendency to retract, since the applied pressure is normally only by hand, and the assembly may need to be clamped. For rubbers it is probably better to prepare a thin film by the melt method described in the section Hot compression molded films. This melt approach is not recommended for materials which will crystallize on solidification, since scatter and molecular orientation effects will lead to both poor spectral contrast and irreproduci-bility. 

Y. W. Leong, M. Kotaki and H. Hamada, Effects of the molecular orientation and crystallization on film-substrate interfacial adhesion in poly(ethylene terephthalate) film-insert moldings, Journal of Applied Polymer Science, 104, 2100-2107... 

Polyimide fibres have also been evaluated from the standpoint of the effect of molecular orientation and crystallization on mechanical properties. "... 

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

We carried out thermodynamic studies on the crystallization from melts of flexible-chain polymers uniaxially stretched at various degrees of molecular orientation in the melt and studied the effect of the stretching stress on thermodynamic parameters such as degree of... 

NAJM Van Aerie, M Barmentlo, and RWJ Hollering, Effect of rubbing on the molecular orientation within polyimide orienting layers of liquid-crystal displays, J. Appl. Phys., 74 3111-3120, 1993. 

Fig. 24. Three principal types of orientational effects induced by electric (E) and magnetic (H) fields in nematic low molecular liquid crystals. At the top of the figure the initial geometries of molecules are shown. Below the different variants of the Frederiks transition — splay-, bend- and twist-effects are represented...

It is important, that for all of the three effects exhibited, there is no need for a current to flow through a layer of a liquid crystal, i.e. the named effects are purely field effects. Besides, as is seen from Fig. 24, the initial molecular orientation is strictly predetermined. 

One of the manifestations of orientational effect in LC polymers is presented by a so called guest-host effect, which is well-known for low-molecular liquid crystals. 

Not only may the rate of cooling be slow it also may vary from one part of the moulding to another, and this can have marked effects on the crystalline morphology, molecular orientation, shrinkage—and consequently on the qualities and appearance of products. If, for example, the rate of crystallization differs in different parts of a moulding the sizes of crystals also will differ, and its clarity will be impaired. 

Figure 3.23. Model of surface plane for the evaluation of the surface-molecule reorientation and its effects on the first substrate planes. The changes in the molecular orientations are replaced by a compression or dilation of a set of two planes connected elastically the missing forces are assumed to act only on the hatched planes. The parameters of the model are the distance d between two hatched planes and the distance a separating two nearest-neighbor hatched planes belonging to two different crystal planes. The interaction forces between planes of different "molecules" are indicated with the notation in the text [cf. (3.38)].

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