Crystal texture effect

The number of crystals comprising the sample has to be very large and their orientation has to be random (see section 4.2). It sometimes happens, however, that this condition is impossible to fulfill, resulting in discrepancies between the integrated intensities of the peaks in the pattern and the values strictly related to structural parameters. When the nature of this preferential orientation is known, it is possible to take it into account in the refinement. This led several authors [ALT 96, PES 95] to incorporate, into their Rietveld refinement programs, preferential orientation parameters based on the use of the March function which was first introduced by Dollase [DOL 86], Nonetheless, it is still better to avoid any texture effects. 

Polymer crystals are frequently small and imperfect, so that the diffraction spots are fuzzy, or they are arcs from an oriented polycrystalline texture. The degree of perfection of crystals of known structure can be determined from measurement of diffraction line widths and intensities. The analysis used for X-ray diffraction [25, 39, 40] can be transferred directly to electron diffraction. One can distinguish between crystal size effects and the effects of disorder within the crystals, but often a simple estimate of the mean crystal size, 0.9A/(angular breadth), is used. Any measure of disorder can only be an upper limit, unless careful precautions are taken to account for the effects of radiation damage. 

The Effect of Crystal Texture. Three samples of P.T.F.E. were examined. One was sintered and slowly cooled (band width -3000A). 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

The growth of lactose crystals in ice cream results in a serious texture defect known as sandiness. It is usually caused by temperature fluctuations, high serum solids in the mix, and high cabinet storage temperatures. The ratio of serum solids and moisture must be controlled. Hydrocolloids have some effect on controlling sandiness, but not as much as proper handling techniques and formulation. 

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