Isochronal experiments

A coincidence between r and Tf, accompanied by the formation of a maximum of energy losses (peak of the loss modulus G" or of the loss factor tan <5 = G7G ), can be achieved by varying either the period Tf at constant temperature (isothermal experiment) or t (by changing the temperature) at the constant period Tf (isochronous experiment). The latter procedure is experimentally easier to implement and is therefore more frequently used. 

Dynamic Mechanical Analysis. A Rheometrics Dynamic Mechanical Thermal Analyser (DMTA) Mk III was used in a single cantilever arrangement. Both and tan 5 were measured as a function of temperature at a frequency of IHz over the temperature range -145 to +150 C and a heating rate of 2 C/min for the isochronal experiments. 

For constant-frequency (sometimes referred to as isochronal) experiments on amorphous polymers, the highest temperature relaxation is the glass transition, also referred to as a, while secondary p and y relaxations are commonly observed. There also may be a 8 process. In this case all the relaxations are associated with the amorphous phase. The y and 8 relaxations (and in certain cases also the p relaxation) are below -100 °C. These relaxations would be missed if the only cooUng device available were a mechanical chiller that would cool to only -100 °C. Figure 5.18 illustrates the various loss peaks associated with the transitions in amorphous and crystalUne polymers. 

When the samples were etched mildly, the anomalous increase upon annealing was not observed. In an isothermal annealing experiment performed at 423 K for As—H complexes, the exponential decay given by Eq. (3) was verified for a 50 times reduction in concentration. In Fig. 11 the results of a series of 30 min isochronal anneals are shown for each of the donor-H complexes. The curves are given by Eq. (3) with an assumed attempt frequency of 1013 s-1 and binding energies of 1.32 eV for P—H and 1.43 eV for As—H and Sb—H. 

In order to know how is the variation of the mechanical properties of the polymers with temperature it is necessary to know the time of the measurements. In fact, E and D values obtained at different temperatures are comparable themselves if the time considered for the experiment is the same. Therefore the comparison of the experiments at different temperatures at the same time are isochrones [1-7,15-20], It is interesting to analyze the effect of the temperature on the elastic modulus. The classical schematic representation of this behaviour is shown on Fig. 2.4 ... 

Besides, the average of the migration energies for two time constants estimated from the data on heat capacity in Ref. [6] is the same as that determined in the isochronal residual resistivity recovery experiments [9], i.e. 0.27 eV. Moreover, relaxation times of residual resistivity for LuHq.iso or LuHo.254 and heat capacity for LuHo.i48 are similar to each other for the given low (below 200 K) temperatures (see Fig. 2). These facts once again suggest that, for low temperatures, these two quite different experiments involve the same phenomenon and are conditioned by the same nature —short-range order relaxation. 

Figure 2 shows the results of isochronal annealing experiments on ZnO samples treated with a hydrogen plasma at 350 °C. It follows from the figure that the H-I defect anneals out at 500 °C, whereas H-II is more stable and disappears from the spectra at 600 °C. Parallel to the annealing of H-I and H-II, another line at 3191.6 cm grows in the spectra. It represents a stretch LVM of the well-known Cu-H defect. It reaches its maximum intensity at 600 °C, when both H-I and H-II are already gone from the spectra. 

A special case of heteronuclear interations exists with coupling between Li and Li, which has most recently been detected by a 2D heteronuclear Li, Li shift correlation experiment [129], (see Section 3.2) after earlier attempts to find such couplings for partially labelled compounds [130] were unsuccessful. Such interactions are of interest because they can be observed also for chemically equivalent Li nuclei which are isochronous in the homonuclear case. From the known magnitude of a Li, Li coupling (see above) estimated values are ca 0.4 Hz. 

Finally, the Li/Li correlation experiment (Figure 18), which has recently been performed for the first time [129], is of interest because it allows detection of coupling between lithium nuclei which are isochronous and may open up a new way to determine cluster sizes. 

Enantiotopic nuclei or groups are capable of fulfilling all or, at least, most of the foregoing symmetry-related expectations. Their chemical shifts depend, in addition, on both the medium in which the NMR experiment is conducted and the spectral resolution of the spectrometer. The latter is influenced by, for example, the magnetic-field strength. Enantiotopic groups are isochronous in achiral or racemic media and constitute A2,X2, etc., systems. Moreover, they are potentially anisochronous in chiral media. 

The quantity in the brackets is a function of time only, so will be a constant for the isochronal stress-strain experiment. Thus the isochronal stress-strain curve for a linear material will be straight. 

It is evident from these expressions that the isochronal stress-strain response depends on the type of experiment. For example, at t t = 1, the isochronal stress-strain curves will be cr = 0.632 Gy, cr = 0.5 Gy and cr = 0.368 Gy respectively. 

The slope is CR, the product of the initial iodine isotope ratio and the conversion factor. The real point of the experiment is to determine R, the initial iodine isotogm composition. To do so, however, it is necessary to know C, the efficiency with which is converted to Xe. This is normally done by including an irradiation monitor in the nuclear irradiation, a sample with a known ( 1/ I)o ratio. The data from the monitor are then plotted on the same type of isochron plot, but for the monitor, the unknown is C, the conversion factor for that particular experiment. Then, knowing the conversion factor, R can be determined for the other samples irradiated at the same time. 

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