Crystalline polymers 656 INDEX

After a listing of some general definitions relating to crystalline polymers (Section 1), the subject is divided into sections dealing, successively, with local structural arrangements at the scale of a few bond lengths (Section 2), morphological aspects (Section 3), molecular conformation within polymer crystals (Section 4) and, finally, kinetic aspects of crystallization (Section 5). An alphabetical index of terms is provided for the convenience of the reader. 

The T of crystalline polymers may be determined by observing the first-order transition (change in heat capacity value) by DTA or by DSC (ASTM-D3418). Some comparative information on thermal properties of polyolefins may be obtained from the melt index. To determine the melt index, the weight of extrudate or strand under a specified load and at a specified temperature is measured. Melt index values are inversely related to the melt viscosity. 

Is a crystalline polymer in which the chains are present in helical conformations, with three repeating units per turn. Because of the added volume which is left between the pendant methyl groups, this opaque polymer has a low specific gravity (0.90) as cited in Table 112. Its index of refraction is 1.49. 

Because of their to-phase nature, semi-crystalline polymers are, in most cases, nontransparent, the difference in refraction index is responsible for strong light scattering. 

A polymer always has a higher average refractive index in the crystalline than in the amorphous state. However, since also the density of the crystalline polymer is higher, the molar refraction according to Lorenz-Lorenz and Gladstone-Dale remains practically constant. The molar refraction according to Vogel is not applicable to crystalline polymers, since it does not contain the polymer density. 

For crystalline polymers, however, not only the molecular weight and poly-dispersity index are of interest. Besides the degree of crystallinity, there is a specific influence of chain design, i.e., existence, length, and concentration of side chains on the resistance against environmental stress cracking, as will be pointed out in the next section. 

Optical Clarity Generally increases with density. Semi-crystalline polymers usually appear opaque because of the difference in refractive index of the amorphous and crystalline domains, which leads to scat- altering. Will depend upon crystallite size. 

For the crystalline polymer obtained with a-TiClj—Al(C2Hj)2Cl at Ti/Al ratio 1, a slight decrease in the index of polydispersity was found between 0 and 3000 minutes. This has been ascribed both to possible diffusion phenomena, even for low conversions, and to the kinetic behaviour of two different types of active centres. 

The level of sinkage/shrinkage is also influenced by other factors such as fillers, pigments, melt flow index, density and cooling rate. It should be noted that materials which are oriented, either purposely or accidentally (e.g. in a moulding, operation), will shrink significantly if they are heated above the temperature used for the orientation process. Amorphous materials show less shrinkage than crystalline polymers. 

Liquid crystalline polymers can be applied in the electro-optic modulator. Materials for such an application must be able to vary their refractive index according to the electric (optical) field, i.e.,... 

The permeability of a crystalline polymer to gases depends on its crystallinity index and degree of order. These depend on processing conditions, so the preparation conditions and thermal history of the samples to be measured must be clearly stated. 1... 

Molecules in anisotropic samples will be preferentially oriented along one direction, e.g., the nematic director for crystalline polymers or the fiber axis. The refractive index in the direction parallel to the orientation will differ from that in the perpendicular direction and, therefore, the sample is said to be birefringent. When polarized light passes through an anisotropic sample oriented in different directions, it is split into two components that are not in phase these will combine to give elliptically polarized light. Thus, oriented samples such as crystalline, liquid crystalline polymers and fibers are visible under polarized light. 

When we move on therefore, to a partially oriented semi-crystalline polymer such as HOPE and PP for which there are as yet many unsolved problems regarding the initial morphology, it is somewhat surprising that the simple scheme for molecular reorientation is applicable. It is not clear how a simple morphology of alternating crystalline and amorphous regions could always reform so that the direction of maximum refractive index was parallel to the direction of maximum elongation, for all specimen orientations and initial draw ratios, as is required by this scheme (Fig. 12). 

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