Melting zero-entropy-production

Figure 4.8. Zero-entropy-production melting temperatures of lamellar crystals of...

The opposite behavior on fusion of a macromolecule is shown in Fig. 3.10. Thin, lamellar polyethylene crystals decrease in melting temperature on faster heating because of less reorganization. Finally, the zero-entropy-production melting could be established at a heating rate of more than 50 K/min, permitting the use of Eq. (5) of Fig. 4.28 for analysis. 

How can one distinguish the case of equilibrium melting fi om that of zero-entropy-production melting of a metastable crystal ... 

Figure 5.27 seems to verify Eq. (4). A more careful analysis, however, shows that copolymer crystals are never close enough to equilibrium to follow Eq. (4). For example, if one extrapolates the curve described by Eq. (4) to concentration equal to 1, one never reaches the equilibrium melting temperature. The temperature observed is typically 5 to 10 K below the equilibrium melting temperature, indicating that even the largest crystals of the copolymer are metastable and must be treated as thin lamellae, as is discussed in Fig. 4.28. This raises the question How well can a metastable equilibrium be expressed by an equilibrium equation At best one can hope that the measured free enthalpy is parallel to the equilibrium curve (see Figs. 4.29 and 4.30). Then, one can introduce ATj j, the measured lowering of melting point from the zero-entropy-production melting temperature of the homopolymer, into Eq. (4) for the approximate evaluation of the influence of...

For many crystals (but not all see Sects. 4.7.1-4.7.2) the superheating on melting is much less serious than the supercooling on crystallization, so that melting may approach equilibrium as long as one starts with an equilibrium crystal. For nonequilibrium crystals it is, at best, possible to analyze the zero-entropy-production melting described in Sect. 4.7.1. 

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