Annealing endotherm

For PUs obtained with MDI/BDO based HS, Saiani also showed that PUs usually display three endotherms [186, 187]. A first annealing endotherm was observed at 20-30°C, higher than the applied annealing temperature. The two other melting endotherms were observed at higher temperatures (up to 250°C). This is consistent with the results obtained by us on the MDI based PUs achieved with various diol chain extenders [60]. 

Chen et al [236] investigated the origin of the so-called annealing endotherm . The authors suggested that this endotherm was related to some relaxation effects of the polymer chain in the HS phase due to the physical aging of the sample. 

Decompositions may be exothermic or endothermic. Solids that decompose without melting upon heating are mostly such that can give rise to gaseous products. When a gas is made, the rate can be affected by the diffusional resistance of the product zone. Particle size is a factor. Aging of a solid can result in crystallization of the surface that has been found to affect the rate of reaction. Annealing reduces strains and slows any decomposition rates. The decompositions of some fine powders follow a first-order law. In other cases, the decomposed fraction x is in accordance with the Avrami-Erofeyev equation (cited by Galwey, Chemistry of Solids, Chapman Hall, 1967)... 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

Lattice constants for Pd-Rh alloys quenched from 1300°C vary smoothly with composition, showing only a small positive deviation from Vegard s law, but prolonged vacuum annealing below 850°C revealed the existence of a wide miscibility gap 151). Figure 25 shows the limits of the miscibility gap calculated from the lattice constants of the two-phase system between 825° and 600°C. Recent thermodynamic data (Table I) confirm the tendency to phase separation. The enthalpies of formation are endothermic... 

Carbene lv is photolabile, and 400 nm irradiation produces a mixture of products.108 By comparison with calculated IR spectra the major product was identified as cyclopropene 3v. The formation of 3v is irreversible, and it cannot be thermally (by annealing the matrix) nor photochemically converted back to carbene lv. The lv -> 3v rearrangement is calculated (B3LYP/6-31G(d) + ZPE) to be endothermic by only 5.4 kcal/mol with an activation barrier of 18.2 kcal/mol. Due to the two Si-C bonds in the five-membered ring of 3v this cyclopropene is less strained than 3s, which is reflected by the smaller destabilization relative to carbene lv. The thermal energy available at temperatures below 40 K is much too low to overcome the calculated barrier of 12.8 kcal/mol for the rearrangement of 3v back to lv, and consequently 3v is stable under the conditions of matrix isolation. 

FIGURE 5.11 DSC endotherms of rice starch annealed at 70 °C (times are indicated next to each profile) [adapted from Nakazawa et al. (1984) reproduced with permission from Japan Science and Technology Agency]. 

Figure 7.5 shows the relative endotherms of PVDF/PMMA blends as a function of PVDF content. A relative endotherm is defined here as the ratio of a melt endotherm at a certain PVDF blend composition to the endotherm of the pure annealed PVDF in DSC measurements. The DSC measurements are at a constant heating rate of lOC/min. For annealed and solution-cast blends, the relative endotherms are in general greater than those of quenched blends of the same composition. Endotherms can be detected at all compositions when the blends are annealed. The quenched PVDF shows only 75% endotherm relative to the annealed PVDF. No endotherm can be detected when the PVDF content is lower than 50 wt % in quenched PVDF/PMMA blends. Obviously the thermal history of the blends has a tremendous effect on the final performance of the material. 

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