Amorphous exothermic peak

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

Quantitative estimation of the sulphate content by the fusion method was found to be difficult because of the low percentage of the impurity. The anatase, thus prepared, was amorphous. The surface area of this anatase sample (B.E.T.) was 54 m2/g and the differential thermal analysis curve of the anatase sample is shown in fig. 2a. Although no exothermic peak due to crystallization was observed, the endothermic peak shows a definite splitting around... 

Figures 6 and 7 show DSC cooling scans of the pure rubbers, LDPE, and blends of rubber with 50 phr LDPE. The crystallization temperature (Tcr) of LDPE (largest exothermic peak) is decreased six kelvins in the presence of the amorphous EPDM and is decreased 12 kelvins in the presence of the crystalline EPDM. This Tcr decrease could be attributable to partial solubility of EPDM rubber and LDPE. The soluble EPDM chains would have to be expelled from the LDPE crystal-...

The peak located at 450 °C in the DTA curve was a very small exothermic peak, but no weight loss above 400 C could be observed in the TGA. Therefore we can know that the reactions in the xerogel has basically finished and the LiNi,.,exothermic peak located at 450 °C in the DTA curve can be interpreted as a kind of transformation of LiNi,. Co,V04 from amorphous solid into crystal solid. 

In the case of solid supports having crystalline structures, like quartz, zirconium dioxide and o-alumina, the shape of the enthalpic curve is quite different and exhibits a distinct exothermic peak in the vicinity of the cmc (e.g., Figs. 6a and 6b). At first sight, the phenomenon seems strange, especially if all other curves are similar to those for the amorphous silica (see for example Figs. 2b, 3a and 7). An explanation based on the specific model of surface topography is advanced in this paragraph. 

A differential scanning calorimeter (DSC 1-B, Perkin-Elmer Corporation) was used to determine the extent of cure 10-mg to 20-mg specimens were tested at a scanning rate of 10°C/min. An exothermic peak on the thermograph indicates the heat of reaction whereas an endothermic peak in the amorphous polymer indicates the presence of residual stresses or the occurrence of a transition such as the glass transition. The presence of an exothermic peak in the DSC-scan of a pre-cured sample is an indication of incomplete curing. 

Organic materials can also exist in crystalline and amorphous forms. An amorphous material can give rise to several transitions as it is heated, as indicated in Figure 2.2. The first transition can be observed as an endothermic shift in the baseline. This is the glass transition temperature. As the temperature increases an exothermic peak may be observed if the sample can orientate into a more crystalline form. Finally, any crystalline material present may melt or (as in this case) melt and decompose at higher temperatures. 

These interpretations are supported by DTA measurements already reported in detail [144, 145]. By using the model proposed by Marotta et al. [151] and further developed by Ray et al. [152, 153] the exothermic peak of crystallization, i.e. of the optimum crystal growth in the amorphous matrix, has been used to investigate the kinetic of crystallization. For our amorphous SiC system this peak is observable in the range 1050-1130 °C, and its exact position Tp as well as its height Tp depends on parameters like the temperature of nucleation treatment and the heating rate during the DTA experiment. 

The sample investigated was amorphous to X-rays. Its crystallisation is represented by the sharp exothermic peak centred at 492 C. During crystallisation, the residual species trapped in the amorphous network (water (m/z=18) and a small peak of CO2 - its magnification marked as A in the inset in the figure) were evolved. The amount of the carbon released during the crystallisation of the zirconia aerogel amounts to 0.007 mass%. 

In the case of polyesters, the phenomenon of cold crystallization appears, explained by the approaching of segments from the chain in the amorphous regions of the polymer, without any molecular rearrangements [765]. The exothermal peak characteristic of this phenomenon disappears or gets smaller, if the polymer sample suffers a treatment which increases its crystallinity. 

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