Thermal history, of sample

A dsc scan of a typical commercial ionomer shows two endotherms at about 50 and 98°C, respectively. The size of the lower peak can be correlated with stiffness and yield point. The thermal history of the sample influences the relative size of the lower peak and moves it to higher temperatures, while the upper peak decreases in size but remains at the same temperature. Room temperature aging also increases the size of the lower endotherm. 

Electrical Properties. The electrical properties of siUcon carbide are highly sensitive to purity, density, and even to the electrical and thermal history of the sample. 

Thermal transitions can be studied by DSC. The crystallization transition is usually sharp with a good baseline. The melting transition is more complex and often not a single transition (Fig. 3.19)48 as it depends on the thermal history of the sample and the structural changes that can take place upon heating. In warming, solid-state transitions can take place in the unit cell, the lamellae can thicken, and secondary crystallization can also take place. The heats of crystallization and... 

Sample Preparation. Samples for mechanical studies were made by compression molding the polymers at 150°C between Teflon sheets for 15 minutes followed by rapid quenching to room temperature in air. These will be referred to as PQ (press-quenched or simply quenched) samples. The thickness of the PQ samples was around 10 mils (0.25 mm). The thermal history of all of the PQ samples (HBIB, HIBI, and LDPE) were essentially the same. They were used within one week after they were pressed. Samples for morphology, SALS and SEM studies were prepared from toluene solutions. These films were cast on a Teflon sheet at 80 C from a 1% (by weight) solution in toluene. These films were about 5 mils in thickness. When the polymer films had solidified (after 5 hrs), they were stored in a vacuum oven at 80°C for two days to remove residual solvent. These samples will be designated by TOL (solution cast from toluene). 

The cracking susceptibility of a micro-alloyed HSLA-100 steel was examined and compared to that of a HY-100 steel in the as-received condition and after heat treatment to simulate the thermal history of a single pass weld. Slow strain rate tensile tests were conducted on samples of these alloys with these thermal histories in an inert environment and in an aqueous solution during continuous cathodic charging at different potentials with respect to a reference electrode. Both alloys exhibited reduced ductilities at cathodic potentials indicating susceptibility to hydrogen embrittlement. The results of these experiments will be presented and discussed in relation to the observed microstructures and fractography. 

DSC indicates a gradual increase in Tg with increasing PHBA/diol ratios except for the 60/40 ratio. Otherwise DSC scans of these polyols are complex and are strongly affected by the thermal history of the materials. The 40/60 and 50/50 PHBA/diol ratio materials often have two endothermic peaks, one near 10 and the other near 70°. The relative size of these peaks vary depending on sample history. Small peaks at about 100 are also sometimes observed. 

The observation and the exact position of Tg, and especially of and T , are strongly dependent on the thermal history of the sample. Therefore, often several DSC temperature cycles are carried out (maximum temperature must be clearly below decomposition temperature) with analyzing the thermal transitions usually in the second and not in the first run after a defined cooling cycle. 

Figure 7.7 Thermal history of the product and of air temperature in unpackaged samples subjected to 4 and 16 fluctuations in the range —18 to —6°C.

The area of a DSC peak can be used to estimate the enthalpy of transition, AH, provided the thermal history of the sample is considered [29]. Calibration with respect to enthalpy requires an area that corresponds to a well-defined enthalpy change—a heat of fusion AH(Tm) is commonly used, especially that of indium [3]. 

The temperature-dependent irreversibility demonstrates that the ion-exchange behavior of NaX towards bivalent cations depends strongly upon the thermal history of the sample. The rather pronounced differences in behavior of transition-metal ions, also observed in synthetic zeolite 4 A (9) is in very sharp contrast with the nearly identical, either hydrated or crystallographic, dimensions of these ions (10). Obviously, this observation raises important questions as to the value of the current interpretation (nearly) exclusively in terms of physical dimensions of ions and pore width. In contrast, the similarity of behavior in mont-morillonite is remarkably close the AG0 value for the replacement of Na by either Ni, Co, Cu, or Zn is —175 cal ( ll)/equivalent, irrespective of the nature of the cation (11). Therefore, the understanding of their difference in behavior in zeolites must take other effects into consideration. 

In many of the above polymers characteristic temperature drifts of the calorimeter were observed above and below Tg depending upon the previous thermal history of the sample. In the case of the synthetic rubber, GR-S, Rands,... 

These results show the dependence of the plastic deformation of PMMA on the thermal history of the sample. The relation with the cooperativity of the ft transition motions is addressed later in Sect. 3.1.1.6. 

In the solid state, polysilanes present a far more complicated picture than in solution. Many different phases may be possible for a single polymer, and the transformations between them may be slow, so that the conformation and spectroscopic behavior often depends on the thermal history of the sample. In partial compensation, additional techniques for studying the polysilanes become available for the solids, especially X-ray scattering. Nevertheless the conformations and structures of solid polysilanes are not well understood. 

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