Amorphous perfect crystal effect

G.30 B. E. Warren. X-Ray Diffraction (Reading, Mass. Addison-Wesley, 1969). Excellent advanced treatment, in which the author takes pains to connect theoretically derived results with experimentally observable quantities. Stresses diffraction effects due to thermal vibration, order-disorder, imr>erfect crystals, and amorphous materials. Includes a treatment of the dynamical theory of diffraction by a perfect crystal. 

Amorphous solids and polycrystalline substances composed of crystals of arbitrary symmetry arranged with a perfectly disordered or random orientation are elastically isotropic macroscopically (taken as a whole). They may be described by nine elastic constants, which may be reduced to two independent (effective) elastic constants. 

The Mechanism of Corrosion.—An attractive theory of the mechanism of corrosion has been outlined by Aitchison.2 Compact iron, when examined under the microscope (see Part III.), is seen to consist of crystals of ferrite separated from each other by an amorphous cement. It is reasonable to suppose that the solution pressure of this cement differs from that of the ferrite, for differences of this kind invariably occur between amorphous and crystalline varieties of substances. Upon immersion in an electrolyte, therefore, such as ordinary tap water or aqueous solutions of inorganic salts, a difference of potential exists leading to corrosion. If the cement is positive to the ferrite, it is the cement that will oxidise away and vice versa. In a perfectly annealed specimen, in which there is but little mechanical strain, the action will, in the main, be confined to that between the cement and ferrite. If, however, there is any appreciable potential difference between the crystals of ferrite themselves, this will increase the effect, the total observed corrosion being the sum of the two actions. 

An experimental relationship between the microhardness and elastic modulus (E) has been found for various PE materials with different crystallinity values (Flores et al.., 2000). It is important to realize that microhardness - the plastic deformation of crystals at high strains - primarily depends on the average thickness and perfection of the nanocrystals, whereas in the case of the modulus, the elastic response at low strains is dictated by the cooperative effects of both microphases, the crystalline lamellae and the amorphous layer reinforced by tie molecules. The... 

Evidence for the effect of chain ends on interfacial energy is provided by a number of experiments. The melting points of short PEO-containing diblocks [26] and triblocks [27,28], Tm=47-51 °C,is low compared to perfectly crystalline PEO (Tm=76 °C). This is due to the positive free energy of formation for the block copolymers of the amorphous layer from the melt, and of the crystal-line/amorphous interface. The end interfacial theory can be analyzed in terms of the theories for melting points of low molecular weight polymers [29,30]. 

Miller et al. have recently reported infra-red dichroic data obtained for high density polyethylene crystallised under the orientation and pressure effects of a pressure capillary viscometer. Their data for a number of crystalline bands (including the 1894 cm" absorption) showed that the crystal c-axes were almost perfectly oriented (f 1) in the initial extrusion direction. The amorphous orientation functions were generally lower, but corresponded to an extension ratio between 2 and 7 when compared with the above results of Read and Stein and of Glenz and Peterlin. Further evidence was also obtained for the relatively high orientation of the amorphous component of the 2016 cm" band (U = 0-66-0-72). 

Most of the methods for crystal pha.se investigation arc based on the essential structural dilTeience between the amorphous phase and the crystal one, but in the ca.se of polymer systems, these differences may prove to be insufficient for effective implcnieiitation of the classic procedures. The matter is that polymer crystals are much less perfect than those of low molecular compounds. 

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