Endothermic effect fusion

In some works [3, 4] it is supposed, that PETP can have two morphological forms of crystals which define double endothermic effect in the field of fusion. Form I, to which more high-temperature corresponds originally endothermic peak of fusion, has been attributed folded structure, and for the form II responsible for more low-temperature peak of fusion, the crystal structure from more extended circuits has been offered. 

Displacement of peak of fusion on curves DTA in area of smaller temperatures specifies that in the modified fibres crystals have mainly morphological form II (the extended circuits of polymers incorporated into crystallites) while in initial PETP crystals mainly have morphological form I (folded structure). Therefore for initial PETP it is observed endothermic effect at temperature 269°C - speaking by fusion flat folded crystallites (morphological form I). At modified PETP - fibers this effect is observed at temperature 245 - 263°C, it speaks fusion of spherallite (the morphological form II). 

Differential thermal analysis (DTA) is a thermal technique in which the temperature of a sample, compared with the temperature of a thermally inert material, is recorded as a function of the sample, inert material, or furnace temperature as the sample is heated or cooled at a uniform rate. Temperature changes in- the sample are due to endothermic or exothermic enthalpic transitions or reactions such as those caused by phase changes, fusion, crystalline structure inversions, boiling, sublimation, and vaporization, dehydration reactions, dissociation or decomposition reactions, oxidation and reduction reactions, destruction of crystalline lattice structure, and other chemical reactions. Generally speaking, phase transitions, dehydration, reduction, and some decomposition reactions produce endothermic effects, whereas crystallization, oxidation, and some decomposition reactions produce exothermic effects. 

The DSC heating-trace in Fig. 3.16 shows several comphcations during the nonisothermal reaction. First, there is the melting endotherm of the LiH2P04 monomer, beginning at about 470 K (AT in the endothermic direction is proportional to the consumed heat to raise temperature). In addition to the fusion, other endothermic effects are due to the evaporation of water evolved in the chemical reaction. The TGA trace of Fig. 3.17 registers the changes in mass, and records quantitatively the... 

This endothermal effect shields the glass transition taking place in the same temperature range. The second endothermal transition determines the heat of fusion and the glass transition temperature Tg of the hard crystalline... 

The effects produced by added fats are simpler to check, since most of them do not interact with starch and therefore produce a reversible endotherm of fusion in the 30-50 °C range, while a small fraction is involved in the formation of amylose-lipid complexes throughout the temperature range of starch gelatinization and therefore enhances the signal at 110 °C a semiquantitative evaluation of both effects is indeed possible [78],... 

Melting/recrvstallisation temperature determinations Semi-crystalline polymers generally melt over a wide temperature range. This behaviour is related to imperfections in the crystallites and non-uniformity in their size the smaller and/or less perfectly formed crystallites will melt at lower temperatures. The endothermic fusion effect as measured by the DSC is in many cases indicated by the temperature of the maximum heat flow (the Tm-value) and by the total heat involved in the fusion process (the Hf-value). Often reported is also the Te-value i.e. the temperature at which the last crystallite has fused. 

Thermotropic liquid crystalline polymers (LCP) show during heating one or more mesophase transition effects before they change after an endothermic fusion maximum into an isotropic melt. These transition effects are usually indicated by Tm,... 

A purified DPM reference sample (p,p-DPM > 80 %wt.) showed two endothermic fusion effects (Tmi l58°C/Hfi = 124 J/g, Tm2 -108°C/Hf2 = 14 J/g) and a clear recrystallisation effect (Tc = 120°C/Hc = 88 J/g) during heating and cooling scans at 20°C/minute in the DSC. The difference between the Hfl-value (124 J/g) and the Hc-value (88 J/g) shows already that recrystallisation from the melt for this relative pure sample clearly occurs slower than recrystallisation from solution. 

A raw DPM sample recrystallised during storage at 20°C showed an endothermic fusion effect between about 40°C and 140°C, with maxima at 125°C and 94°C and a Hf-value of 105 J/g. Recrystallisation effects during cooling scans were, as expected, not detected during these experiments. 

Contact between a sample and crucible in fusion experiments can be improved by pre-melting the sample to give a uniform coating over the base of the crucible. This has the effect of enhancing the shape of the subsequent endotherm with the proviso that no reaction has occurred. However, even here care is needed the opposite effect is observed when powdered gold is melted to give globules which have poor thermal contact with the crucible. 

The experimental thermograms obtained, however, indicate that KNbCl6 formation is probably more complex than a simple dissolution of solid KCl into NbCl5 on heating, an endothermic peak is observed at 478 K, which corresponds to the fusion of NbCl5, then the thermal effect becomes highly exothermic and ends near 498 K after an... 

Because the differing structures and interaction effects between the molecular chains in amorphous and crystalline states require different quantities of heat to melt, it is possible to calculate the crystallinity of a sample from the energy required to produce the melting endotherm. Thermal data from a differential scanning calorimeter (DSC) scan can be used to calculate the level of crystallinity in a p>olymer by using both the latent heat of melting obtained from the scan and the enthalpy of fusion for a 100% crystalline sample of the polymer. 

---