Amorphous endothermic reaction

Enthalpy changes are derived from the peak areas of DSC curves taking the melting heat of DSP (12.0 kcal/mol) as a standard66. From the relative enthalpy levels of molten and crystalline DSP, and amorphous and crystalline poly-DSP, the topochemical photopolymerization of DSP is found to be an endothermal reaction with an enthalpy increase of about 3.7 kcal/mol. 

It is shown in fig. 129 that b,f of the amorphous Laj5Al25Ni2o ribbon remains unchanged in the T, range below 406 K, decreases from 1.0 to 0.04 in the narrow Tg range between 406 and 434 K and shows a nearly constant value of about 0.03 in the Tg range above 434 K. The change in b,f with 7), indicates that the amorphous ribbon becomes brittle at 416 K in the case of an annealing time of 1.8 ks. The exothermic reaction for the as-quenched sample as well as the endothermic reaction for the annealed sample is a reversible process for the La-Al-Ni amorphous alloy with a wide supercooled liquid... 

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. 

Binary amorphous silicon/PTFE pyrolants have been investigated by Cudzilo eta . [25, 39]. A combined TG/DTA trace for (Si/PTFE) (50/50) is displayed in Figure 5.31. Following the PTFE melting endotherm at 330°C, the exothermic reaction commencing at 500°C is due to formation of volatile Sip4 causing the observed mass loss of 50%. 

Soils and clays, in general, when calcined give off adsorbed, interlayer, and hydrated types of water. These effects produce endothermal peaks or loss of weight in DTA and TG, respectively. The endothermal peaks are followed by exothermal peaks that are caused by re-crystalliza-tion. Although many types of clay minerals such as montmorillonite, illite, and some shales show these effects, they are not suitable as pozzolans in concrete. Metakaolin, formed by heating kaolinite, seems to be the most suitable additive material for cement. Heating of kaolinite involves removal of adsorbed water at about 100°C and dehydroxylation at above 600°C, followed by the formation of metakaolinite, an almost amorphous product. The sequence of reactions is as follows ... 

---