Crystalline Distribution

Throughout the text we will relate polymer structure to the properties of the polymer. Polymer properties are related not only to the chemical nature of the polymer, but also to such factors as extent and distribution of crystallinity, distribution of polymer chain lengths, and nature and amount of additives, such as fillers, reinforcing agents, and plasticizers, to mention a few. These factors influence essentially all the polymeric properties to some extent including hardness, flammability, weatherability, chemical stability, biological response, comfort, flex life, moisture retention, appearance, dyeability, softening point, and electrical properties. 

Generate profiles with constant wall thickness. Constant wall thickness in a profile makes it easier to control the thickness of the final profile and results in a more even crystallinity distribution in semi-crystalline profiles. 

Theoretical treatments for the energy of mixing have been evaluated based on several models (i.e., unexpanded gas, quasi-crystalline, distribution function). However, similar results can be obtained as ... 

The most likely cause of the crystallinity variations in polypropylene spherulites is the accumulation of rejected atactic and low molecular weight impurities. This view is supported by the observation that adding increasing amounts of atactic material to polypropylene purified by octane extraction, leads to changes in the crystallinity distribution (Figure U). 

FIGURE 12.3 Variation in tensile strength of unvulcanized, compounded blends of ethylene-propylene copolymer due to differences in composition and crystallinity distribution. 

Fig. 25. An imaginary example of polyethylenes having the same MWD and crystallinity distribution but a different dependency on each distribution [IS]. Reprinted with permission...

The extrusion die, at the end of the extruder, gives the final shape to the polymer melt. It can be used to obtain, for example, films, sheets, pipes and tubular films, filaments, open and hollow profiles. There are some aspects to consider when designing an extruded plastic profile. Thick sections must be avoided because they are expensive, and hollow sections must be minimized because they make the die more difficult to clean. It is important to generate profiles with a constant wall thickness because this allows easier thickness control and a better crystallinity distribution of semicrystalline materials. 

The field ion microscope (FIM) has been used to monitor surface self-diflfiision in real time. In the FIM, a sharp, crystalline tip is placed in a large electric field in a chamber filled with Fie gas [14]. At the tip. Fie ions are fonned, and then accelerated away from the tip. The angular distribution of the Fie ions provides a picture of the atoms at the tip with atomic resolution. In these images, it has been possible to monitor the diflfiision of a single adatom on a surface in real time [15]. The limitations of FIM, however, include its applicability only to metals, and the fact that the surfaces are limited to those that exist on a sharp tip, i.e. difhision along a large... 

Toby B FI and Egami T 1992 Accuracy of pair distribution function analysis applied to crystalline and noncrystalline materials Aota Crystaiiogr.k 48 336-46... 

Obtaining high-quality nanocry stalline samples is the most important task faced by experimentalists working in tire field of nanoscience. In tire ideal sample, every cluster is crystalline, witli a specific size and shape, and all clusters are identical. Wlrile such unifonnity can be expected from a molecular sample, nanocrystal samples rarely attain tliis level of perfection more typically, tliey consist of a collection of clusters witli a distribution of sizes, shapes and stmctures. In order to evaluate size-dependent properties quantitatively, it is important tliat tire variations between different clusters in a nanocrystal sample be minimized, or, at tire very least, tliat tire range and nature of tire variations be well understood. 

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

A good compilation of the functions of fats in various food products is available (26). Some functions are quite subtle, eg, fats lend sheen, color, color development, and crystallinity. One of the principal roles is that of texture modification which includes viscosity, tenderness (shortening), control of ice crystals, elasticity, and flakiness, as in puff pastry. Fats also contribute to moisture retention, flavor in cultured dairy products, and heat transfer in deep fried foods. For the new technology of microwave cooking, fats assist in the distribution of the heating patterns of microwave cooking. 

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