Poly crystallites

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) ("ding" film), polyester, polyamide and polypropylene films. 

Hu W, Mao J, Xing B, Schmidt-Rohr K (2000) Poly(methylene) crystallites in humic substances detected by nuclear magnetic resonance. Environ Sci Technol 34 530-534... 

Outside of catalyst preparation, reaction of sucrose with metal nitrates has been used to prepare nanocomposite mixed oxide materials. Wu et al. [46] reported the synthesis of Mg0-Al203 and Y203-Zr02 mixed oxides by reaction of nitrate precursors with sucrose. The resulting powders had smaller particles than those prepared without sucrose. Das [47] used a similar method in the presence of poly vinylalcohol to produce nanocrystalline lead zirconium titanate and metal ferrierites (MFe204, M = Co, Ni, or Zn). The materials prepared using sucrose had smaller crystallites than those made without. Both authors observed an exothermic decomposition of the precursors during calcination. 

Optical compensation for polymers with chiral monomeric units may also occur when the racemic polymer consists of crystallites, each composed only of the rectus chains or only of the sinister polymer chains, and a same amount of optical antipode crystallites is present. This intercrystallite optical compensation211 has been found, for instance, in isotactic poly(propylene sulfide),212 poly ((3 -methy lpropiol ac (one),213 and poly(isopropylethylene oxide),214 where isochiral 2/1 helical chains are included in orthorhombic unit cells according to the space group P2 2 2. ... 

Porat et al. performed TEM (zero-loss bright field) studies of very thin Nation films that were cast from ethanol/water solutions, and some of the conclusions are as follows. It was suggested that the backbone had a planar zigzag conformation in large orthorhombic crystallites as in polyethylene, in contrast with the helical conformation found in poly(tetra-fluoroethylene). This is an interesting result, although there are no other studies that support this view. Sulfur imaging indicated the presence of sulfonate clusters that are 5 nm in size. 

For zirconia, the species in Scheme 11.4 have been proposed [67], accounting for both Br0nsted and Lewis acidity. These Br0nsted acid sites are proposed to be present only if sulfates, and possibly poly(sulfates), are also present on the top terminations of Zr02 crystallites [66]. 

Which would be more apt to produce crystallites (a) HDPE or (b) poly(butyl methacrylate) ... 

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

In addition to the excellent thermostability, which is due to their high crystallite melting points, poly(arylene ether ketone)s exhibit a very good chemical resistance because of their low solubility. 

After Little s proposal, many researchers have pursued such an exciting system in vain. Even metallic behavior was rarely seen in doped organic polymers, gels, and actuators. As mentioned in Sect. 3.4.4, MCso with linearly polymerized Ceo" exhibited one-dimensional (M = Rb, Cs) or three-dimensional (M = K) metallic behavior [144]. Recently a doped poly aniline was reported to exhibit a metallic temperature dependence for a crystalline polymer chemical oxidation of monomers grew crystallite polyaniline [329] early doping studies on polypyrrole (PFg) and poly(3,4-ethylene-dioxythiophene)X (X = PFg, BF4, and CF3SO3) prepared by electrooxidation at low temperatures also showed a metallic temperature dependence below 10-20 K (Scheme 16) [330, 331]. 

In view of the use of a similar repeating unit, tetramethylenecarboxyl, Seebach s approach should be included here. He incorporated oligomers and polymers of 3-hydroxybutanoic acid (3-HB) into planar bilayers and observed single-channel currents. It is known that poly(3-HB) forms lamellar crystallites with thickness in the range of 40 to 60 A when crystallized from dilute solutions. Therefore it is assumed that poly(3-HB) forms lamellar crystallites with a thickness of ca. 50 A. 

These hydrophobic crystallites fit properly into a planar bilayer by formation of H-bonds between the free end groups of the oligomers and the polar head groups of the phospholipids. In other words, a membrane contains islands of crystalline poly(3-HB) within the liquid crystalline phospholipid phase. A schematic representation is shown in Figure 8. Single-channel current fluctuations are assumed to... 

Figures, (a) Structure of oligomeric and poly(3-hydroxybutanoic acid), (b) Crystallite formation of oligo(3-HB) for the case n = 32 in planar phospholipid. The end group represents hydroxyl and carboxylic acid functionalities. ...

Other crystalline inorganic polymers such as poly(dichlorophosphazene), poly(aryloxyphosphazenes), liquid crystalline polysiloxanes and poly(dichloro-silane) have also been studied by X-ray diffraction methods, enabling the conformations in the crystallites in the solid state to be established. 

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