Polymeric crystallites types

There are several types of polymeric crystallites. Of these, the following need to be mentioned ... 

A strain energy in the polymer crystallites caused by molecular displacement during polymerization is observed. In some cases, this is due to the fact that the polymer crystallites show interference color patterns under a polarizing microscope20. Such an accumulated strain energy is characterized as a distinctive feature of polymer property. This type of strain energy may play an important role in the crystal growth of natural polymers. 

The polymer produced in highest volume is polyethylene. Invented by the British, who call it polythene, and put into production in 1939, it could for a long time only be produced by the oxygen-catalyzed polymerization of ethylene at pressures near 40,000 Ib/in.. Such pressures are expensive and dangerous to maintain on an industrial scale. The polyethylene produced has a low density and is used primarily to make film for bags of all types— from sandwich bags to trash can liners. The opaque appearance of polyethylene is due to crystallites, regions of order in the polymer that resemble... 

The most common type of uniaxial extension can take place during simple tensile drawing of cast polymeric sheets. In this case, the two ends are gripped and the material is pulled to extend, and then chain extension and crystallite rearrangement takes place. 

Unlike molecular solids, polymeric solids inherently have a very large density of defects, making the establishment of an accurate value for Tm far more difficult. For semicrystalline polymers, with the very rare exceptions of special types of polymers made by the solid state polymerization of some rather unique monomers, the key properties of the crystalline phase (Tm and AHm) must be extracted from data obtained by using samples which have at least two phases. There may be more than one type of morphology present in the crystalline phase, as well as crystallites of different sizes and levels of perfection, and possibly even interfaces between the amorphous and crystalline domains. The presence of chains of different lengths provides additional complexities, as does the presence of chain defects, deviations from the predominant stereo regularity (tacticity) of the chains, and oxidation sites along the chains. 

Type (a) behavior is observed for many first generation catalyst systems, e.g., or-TiCls, VQ3, etc. with diaUcylaluminum hahdes as cocatalysts in the polymerization of propylene in hydrocarbon media. During an initial acceleration period, which is of 20-60 minutes duration for many propylene polymerizations at 1 atm pressure in the temperature range 50-70°C, the rate increases from the beginning to reach a more or less steady value. Natta and Pasquon (1959) attributed this behavior to the breakdown of the or-TiCls matrix to smaller crystallites due to the pressure of the growing polymer chains in the initial stages, leading to exposure of fresh Ti atoms and creation of new active centers with consequent increase in... 

The recent interest in using stiff nanometric particles as reinforcement materials in polymeric matrixes, composites, or nanocomposites has been increasing. Two good examples of these types of particles are carbon nanotubes and cellulose nanofibers. Cellulose nanofibers, also reported in the literature as whiskers, nanocrystals, cellulose crystallites, or crystals, are the crystalline domains of ceUulosic fibers, isolated by means of acid hydrolysis, and are called in this way due to theb physical characteristics of stiffness, thickness, and length (Souza and Borsali 2004). 

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