Semi-melting temperature

Figure 15.2 shows some typical hardness data for a typical metal (copper) as a function of temperature. It indicates that there are usually two regimes one above about half the melting temperature and one below. Both tend to be exponential declines, so they are linear on semi-logrithmic graphs. The temperature at which the break occurs is not strictly fixed, but varies from one metal to another, with the purity of a metal, with grain size, and so on. 

For compounds the hardness-temperature curves are similar to those for the pure metals. Semi-logarithmic graphs of the data show two straight lines with the knees at about half the melting temperatures. For a dozen aluminides, Petty (1960) shows this. 

Semicrystalline polymers, 20 351, 352, 588 toughness of, 20 354 Semicrystalline resins, 19 537 Semicrystalline thermoplastics, melting temperature of, 19 538t Semiefficient (semi-EV) sulfur... 

The answer is B. Saturated fatty acids and trans fatty acids are structurally similar their hydrocarbon tails are relatively linear. This allows them to pack tightly together in semi-crystalline arrays such as the membrane bilayer. Such arrays have similar biochemical properties in terms of melting temperature (fluidity). Although some of the other properties listed are also shared by saturated and trans fats, they are not thought to account for the tendency of these fats to contribute to atherosclerosis. 

A common example of a copolymer is an ethylene-propylene copolymer. Although both monomers would result in semi-crystalline polymers when polymerized individually, the melting temperature disappears in the randomly distributed copolymer with ratios between 35/65 and 65/35, resulting in an elastomeric material, as shown in Fig. 1.19. In fact, EPDM rubbers are continuously gaining acceptance in industry because of their resistance to weathering. On the other hand, the ethylene-propylene block copolymer maintains a melting temperature for all ethylene/propylene ratios, as shown in Fig. 1.20. 

A decrease in thermal diffusivity, with increasing temperature, is also observed in semicrystalline thermoplastics. These materials show a minimum at the melting temperature as demonstrated in Fig. 2.18 [24] for a selected number of semi-crystalline thermoplastics. It has also been observed that the thermal diffusivity increases with increasing degree of crystallinity and that it depends on the rate of crystalline growth, hence, on the cooling speed. 

Figure 7.3 shows a typical specific heat measurement of a semi-crystalline thermoplastic (PA6) with a melting temperature around 220°C. Let us assume that we can obtain this continuous curve from a continuous equation. The increase in the Cp represents the heat of fusion of the transition between the semi-crystalline solid to a melt, and is represented with N measurements or discrete points (see Table 7.1). 

Those which do crystallise invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterised by their melting temperature (TJ. Many thermoplastics are, however, completely amorphous and incapable of crystallisation, even upon annealing. Amorphous polymers (and amorphous phases of semi-crystalline polymers) are characterised by their glass transition temperature (T), the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft). This transition corresponds to the onset of chain motion below T the polymer chains are unable to move and are frozen in position. Both T and T increase with increasing chain stiffness and increasing forces of intermolecular attraction. 

Table 15.2 Typical polymer glass transition and semi-crystalline melting temperatures...

If the solid can be melted, its surface tension is measured while it is a liquid, at varyious temperatures, and the derived experimental data are extrapolated to room temperature by means of suitable semi-empirical equations. Since the surface enthalpy of the solid is 10-20% larger than its near-melt temperature values, this fact is also considered in these calculations. 

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