Crystallinity influence, cohesion

One may expect that with increasing temperature the thermal expansion in the crystalline regions will lead to an enlargement of the chain cross-section in the crystalline phase which in turn will induce a decrease in the cohesion energy of the crystals thus causing a gradually lower resistance to plastic deformation. In order to minimize the effect of the surface layer, the influence of temperature on microhardness has been investigated in PE crystallized at 260 °C under a pressure of 5 Kbar 28). The decrease of MH with temperature for the above chain extended PE material is depicted in Fig. 11. The hardness decrease follows an exponential law... 

The influence of substituents on the solubility of molecules in water can be due to their effect on the properties of the solid or liquid (for example, on its molecular cohesion) or to the effect of the substituent on its interaction with water molecules. It is not easy to predict what effect a particular substituent will have on crystal properties, but as a guide to the solvent interactions, substituents can be classified as either hydrophobic or hydrophilic, depending on their polarity (see Table 5.4). The position of the substituent on the molecule can influence its effect, however. This can be seen in the aqueous solubilities of o-, m- and p-dihydroxy-benzenes as expected, all are much greater than that of benzene, but they are not the same, being 4, 9 and 0.6 mol dm, respectively. The relatively low solubility of the para compound is due to the greater stability of its crystalline state. The melting points of the derivatives indicate that is so, as they are 105°C, 111°C, and IZO C, respectively. In the case of the ortho... 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

The X-ray powder diffraction study published by Tourne and Tourne is the main reference for the metal-substituted Keggin anionsJ" " These authors studied the K+, Rb+ and NH4+ salts and verified that they crystallized in a small number of structural types, depending on the number of cations in the molecular formula. The water molecules of crystallization also have a small role, as they reinforce the crystal cohesion and influence the orientation of the anions. Loss of water leads to loss of crystallinity and may alter the lattice parameters. 

In semicrystalline polymers, the crystallites may be regarded as physical cross-links that tend to reinforce or stiffen the structure. Viewed this way, it is easy to visualize that Tg will increase with increasing degree of crystallinity. This is certainly not surprising since the cohesive energy factors operative in the amorphous and crystalline regions are the same and exercise similar influence on transitions. It has been found that the following empirical relationship exists between Tg and Tj . 

The glass transition temperature is mainly influenced by the barriers to rotation around bonds within the chain and by the cohesive energy of the chains. It is also influenced by other factors that change the ability of the molecules to move fillers, plasticizers and crystallinity are all important. 

W.M. Ord On the Influence of Colloids upon Crystalline Form and Cohesion Stanford, London 1879... 

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