Cooling Equilibrium melting temperature

Fig. 17 B/E-p dependence of the critical temperatures of liquid-liquid demixing (dashed line) and the equilibrium melting temperatures of polymer crystals (solid line) for 512-mers at the critical concentrations, predicted by the mean-field lattice theory of polymer solutions. The triangles denote Tcol and the circles denote T cry both are obtained from the onset of phase transitions in the simulations of the dynamic cooling processes of a single 512-mer. The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...

Measurements by many researchers have shown that PTFE s equilibrium melting temperature is 327°C. ° Once it is heated above its melting temperature, its initial properties are irrecoverable. Compaction of PTFE powder and heating above 327°C results in a partially crystalline solid polymer composed of large crystals with a coexisting noncrystalline phase. Crystal size and perfection depend on die crystallization conditions slow cooling results in larger, more perfect crystals. On this point, we present detailed information from electron microscopy, corroborated by measurements of X-ray line breadth. 

Fig. 2. Determination of the equilibrium melting temperature, T/ °. (A) 26-80% blends of palm stearin in sesame oil, cooling rate 1°C/min, heating rate 5°C7min (adapted from Ref. 9) (B) 26 and 80% blends of palm stearin in sesame oil, cooling rates of 1, 10, and 30°C/min, heating rate 5°C/min. land Tm are defined in the text...

As a melt is cooled below its liquidus (the equilibrium melting temperature of the crystalline phases), the nucleation rate is initially small because AG /T is large. Further cooling reduces AG jT until it approximately equals Q/T. At this temperature, the maximum nucleation rate occurs. Further cooling increases Q/T relative to AG /T that is, diffusion becomes the limiting factor, and nucleation eventually stops. Because the maximum nucleation rate often occurs at a lower temperature than the temperature at which the crystal growth rate is maximum, numerous nuclei can form. 

After keeping the sample system at a temperature above the equilibrium melting temperature for a time sufficient to erase memory effects (Alfonso Ziabicki, 1995, Ziabicki Alfonso, 1994), the sample assembly was moved to the lower zone of the container where it was quenched by spraying a cooling fluid on both faces through two identical nozzles positioned symmetrically opposite to each face of the sample assembly (fig. la). 

When a crystallizable polymer is cooled below its equilibrium melting temperature, the hierarchical structure formed can be probed by in situ scattering. Crystallization mechanisms can be determined by comparing the time evolution of the degi ee of crystallinity (wc) determined from WAXS and the total integrated scattering intensity or invariant during crystallization [27,38] from SAXS. 

DSC experiments may be also a source of errors in the calculations of conversion degree. Indications concerning the proper protocol for DSC measurements are given in recently published works [30,60] and in Chapter 1. The recommended procedure is to heat up a sample above the equilibrium melting temperature to erase history, and then to cool it as fast as possible to the desired temperature of crystallization. The apparatus response should be subtracted from the signal and the baseline correctly determined. The beginning of crystallization may be delayed with respect to time at which the selected temperature was reached by a time called induction time. In this case, it is necessary to resolve the onset of crystallization, which is difficult because of negligibly small thermal effects produced by the nucleation. If the assumed onset time of crystallization differs from the true value by U, the Avrami analysis yields n instead of n ... 

The main effect of pressure on crystallization is to shift the equilibrium melting temperature to higher values. It can be explained by Equation (15.1) and Equation (15.2), but here the decrease of the entropy of fusion is due to the reduction of free volume. It means that during cooling, crystallization occurs at higher temperatures, which is observed experimentally (Fig. 15.3). For polypropylene, the increase of the equilibrium melting temperature can be described by the following equation [28] ... 

Here To is the initial melt temperature and V (r) is the time function for either cooling or heating. The simplest case to consider is a constant cooling rate where V (t) = (pt, being a constant. Using the Evans approach to isothermal crystallization kinetics, Ozawa showed that when a sample is cooled from the equilibrium melting temperature to the temperature T, at a constant rate (p, the volume fraction of transformed material can be expressed as (85)... 

These chemically reactive phases are prepared by slow cooling of melts with the appropriate composition under an inert atmosphere or vacuum. Equilibrium is slow to be attained at the low temperatures necessary to prevent dissociation at 6.9°C Na2K dissociates into a (solid solution of K in Na) and liquid (60/40 Na/K). The KjCs and K7CSJ phases are even less stable and result from cooling mixtures of the elements of the desired stoichiometry to — 100°C in a metal beaker under argon. ... 

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