Metastable crystals, heating

As a result of the strong drive to equilibrium, it is usually difficult to quench the isotropic melt to an amorphous glass when liquid crystal formation is possible. Extraordinary quenching techniques may be needed. Once produced, the amorphous state loses its metastability on heating above the glass transition Tga. The melt is quite unstable, so that it may not be possible to keep the melt from changing to the mesophase 21. ... 

The use of faster crystallization rate in TREF may result in formation of metastable crystals that re-crystallize during the heating cycle and result in anomalous double peaks, as shown in Fig. 16 for the analysis of an HDPE resin at different crystallization and heating rates. Increasing the heating rate can also overcome this effect by not giving time for re-crystallization. 

Reorganization of the metastable crystals formed during heating resulting in crystal perfection and/or crystal thickening. 

Finally, the left curves of Fig. 2.45 show that above about 260 K, melting of small, metastable crystals causes abnormal, nonlinear deviations in the heat capacity versus crystallinity plots. The measured data are indicated by the heavy lines in the figure. The thin lines indicate the continued additivity. The points for the amorphous polyethylene at the left ordinate represent the extrapolation of the measured heat capacities from the melt. All heat capacity contributions above the thin lines must thus be assigned to latent heats. Details of these apparent heat capacities yield information on the defect structure of semicrystalline polymers as is discussed in Chaps. 4-7. 

Further study shows that this LC-PI exhibits double melting behavior, which can be induced by annealing or crystallization at lower temperatures [81]. Double melting phenomenon is also observed in some other thermotropic liquid crystalline polymers [82,83]. Literature indicated that the crystallization in solid polymers was the same as that from melt. To obtain the metastable equilibrium heat of fusion, an extrapolation of versus (logfa) for the newly... 

Poly(oxymethylene) hedrites analyzed at different heating rates. Are the three endotherms and one exotherm separate transitions It can be shown that all effects are caused by rearrangement of metastable crystals of the same structure ... 

Although many of these analyses have only been carried out qualitatively, many can also be carried out quantitatively in heat of fusion (crystallinity, see Chapter 5). It was shown in this section that the thermal analysis of metastable crystals, although initially more complicated, can yield important additional information on the history of the material. Modern materials analysis is unthinkable without this information. 

These examples of time-dependent DTA have shown that much information needed for modern materials analysis can be gained by proper choice of time scale. The thermal analysis with controlled cooling and heating rates has also been called dynamic differential thermal analysis (DDTA). Adding calorimetric information, as is described in Chapter 5, extends the analysis even further. All of this work is, however, very much in its early stage. No systematic studies of metastable crystal properties or information on hystereses in glasses have been made. 

Metastable crystals show usually much less chain extension. Their molecular chains are folded every 50 to 500 A, rather than every 10000 to 100000 A. In accord with this, superheating is frequently absent at heating rates presently possible with dynamic calorimetry. The effect which is observed is reorganization. Qualitativdy, the following different effects have been observed by dynamic calorimetry ... 

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