Thermal History Effects

While the densities of crystalline materials are not particularly sensitive to the thermal history of the sample, densities of glasses are always dependent upon the thermal history of the particular sample measured. Although the differences in density which result from changes in thermal history are not particularly large, they can be very important in certain applications, especially those requiring highly reproducible values of the refractive index of glasses. 

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The measurements were performed on a DSC Mettler Toledo (DSC822e/400). About 10 mg of each film-forming component or film conditioned at 25°C and 58% RH for 2 days previously was sealed in a standard aluminum pan. The first scan, from 20 to 150°C was applied to remove any thermal history effects. This first scan was stopped before the material melted, and samples... 

Since the heat capacity is measured using a differential scanning calorimeter, the dependence of Cp on time and temperatures is complicated because Cp is measured continuously while the sample is being heated or cooled at a constant rate. Therefore, the time for the heat capacity measurement is not well defined, and thermal history effects complicate the shape of the step at 7. To overcome this difficulty, one can approximate the real situation by changing the temperature in small discrete steps AT at time intervals At. Stephens and Moynihan et al. follow this procedure and calculate the change in the sample s enthalpy H and its heat capacity from Cp =LH/l T. However, we already have an explicit expression for the equilibrium enthalpy and heat capacity. Since the result for Cp, (8.2), depends partially on the rate of change of p with respect to T, we need only include the effect of p falling out of equilibrium (i.e., dp/dT tCS) for T< 7 in that result. ... 

Measurement by IR spectroscopy is potentially promising and semi-quantitative evaluations are possible. IR methods suffer from two main drawbacks absence of suitable calibration, and thermal history effects. The second limitation is the change induced in the infrared spectrum hy thermal history effects. Thus, the IR bonds used to measure isotacticity are bonds sensitive to the formation of regular isotactic helices rather than isotacticity. As such, IR absorbance will be dependent on pretreatment and annealing, which determine the conformation and morphology of polymer chains. 

Bfect of Environment olubilization and Thermal History Effect of Low Molecular Weight Monomers onformational Changes and Solvent Effect Viscosity and Conformational Effects End-Group Analysis Free Volume Copolymer Architecture pPolymer Blends Ofects of Additives, Conformational Analysis 1 Properties chanical Properties Stability of Polymers... 

Various amounts of nanoclay were used (2.5, 5, and 7.5 wt.% based on the total amount of PVC in the mixer). For comparison with nanoclay-reinforced PVC matrix, a pre-blended PVC without nanoclay was also run in the mixer as a control matrix. This was accomplished to eliminate the thermal history effect between the two matrices (with and without nanoclay). 

Fig. 4. Effect of thermal history on low temperature thermal expansion of vitreous siUca (143), where (-), (---), and (—) represent glasses having ftctive...

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

Effect of Thermal History. Many of the impurities present in commercial copper are in concentrations above the soHd solubihty at low (eg, 300°C) temperatures. Other impurities oxidize in oxygen-bearing copper to form stable oxides at lower temperatures. Hence, because the recrystallization kinetics are influenced primarily by solute atoms in the crystal lattice, the recrystallization temperature is extremely dependent on the thermal treatment prior to cold deformation. 

Among these three polybibenzoates, PTEB has a smectic mesophase stable during several days at any temperature below its isotropization point, although the transformation into a three-dimensional crystal can be attained by annealing at the appropriate temperatures, thus making it possible to analyze the effect of the thermal history on the dynamic mechanical relaxations of PTEB [27]. 

On the contrary, the phase structure and the thermal history do not have important effects on the location and intensity of the /3 relaxation. This relaxation is very broad in all the samples and overlaps the y relaxation. The activation energy of the /3 peak is about 85 kJ mol for the three samples, of the same order of magnitude as that of other polyesters [38,40]. Finally, the y relaxation is found in the three samples of PTEB with no remarkable influence of the thermal history. 

In conclusion, the different thermal histories imposed to PTEB have a minor effect on the /3 and y relaxations, while the a. transition is greatly dependent on the annealing of the samples, being considerably more intense and narrower for the specimen freshly quenched from the melt, which exhibits only a liquid crystalline order. The increase of the storage modulus produced by the aging process confirms the dynamic mechanical results obtained for PDEB [24], a polyester of the same series, as well as the micro-hardness increase [22] (a direct consequence of the modulus rise) with the aging time. 

The number of crosslinks had no significant effect on the outcome of the thermal history in the present materials. 

In conclusion, is it necessary to obtain a microwave athermal effect to justify microwave chemistry Obviously not - it is not necessary to present microwaves effects in a scientific disguise. There are many examples in which microwave heating results in specific time-temperature histories and gradients which cannot be achieved by other means especially for solid materials. Hence, rather than claiming nonther-mal effects it is better to claim a means or a tool which induces a specific thermal history. 

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