Isothermal scans

Isoperibolic instruments have been developed to estimate enthalpies of reaction and to obtain kinetic data for decomposition by using an isothermal, scanning, or quasi-adiabatic mode with compensation for thermal inertia of the sample vessel. The principles of these measuring techniques are discussed in other sections. 

Rapidly solidifying compositions used for reactive injection molding place some restrictions on measurement. In the time required to prepare the reaction mixture, place a sample in the measuring cell of an instrument, and achieve a steady state in the sample and the measuring system at a preset temperature, chemical conversion of the material may advance considerably, making viscosity measurements meaningless. The volume of lost information depends on the ratio of the transient time necessary to achieve a steady state in the sample and the characteristic time of the chemical reaction. The sensitivity of the reaction rate to temperature is also important. In order to avoid the necessity to maintain isothermal conditions for the measurements, a non-isothermal scanning method for viscosity measurements was proposed.156... 

Figure 5.1 shows isothermal scans for formulations containing different amounts of EGDMA, and Fig. 5.2 shows the polymerization rate as a function of conversion for the same compositions. 

Figure 4-26 Isothermally scanned Raman spectra of phases I, II, III and IV of ammonium nitrate together with the DSC thermogram Characterizing the phases (with permission of Ref 48).

As already stated, the usual output of a DSC instrument is a thermogram. This is a plot of differential heat flow rate (differential power) versus temperature for a temperature scan, or a plot of differential heat flow rate versus time for an isothermal scan. The thermogram is most logically plotted (by the instrument s software) such that a peak represents an exothermic event in the sample while a trough represents an endothermic event. However, the thermogram from a PC instrument is sometimes plotted in the... 

Detailed examination of the relaxations requires isothermal scans of relative permittivity and dielectric loss factor as a function of frequency/ so that effective dipole movements and activation energies of relaxation times may be obtained. A typical pair of plots of d and e" values against log/is shown in Fig. 3.7. Graphs of dielectric data of this kind are sometimes called, rather... 

Figures 14a and 14b show the logarithm of the storage, E , and loss, E", moduli vs. termperature for specimens cured at 124°C for 5 and 11.5 hours, respectively. The 5 hour sample, as mentioned above, corresponds to a time between the two DSA peaks at 3.5 Hz isothermal scans while the 11.5 hour sample corresponds to a time longer than those of both DSA loss tangent peaks. The E shows an initial decrease (greater in the case of the 5 hour sample than the 11.5 hour sample) followed by an increase and a final decrease at about 250°C. E exhibits three dispersion regions in order of increasing temperature. The first is associated with the softening manifested by the decrease in E", followed by a peak due to chemical reaction, and finally a peak associated with the glass transition of the fully cured resin.

The first mesoporous zeolite single crystal (ZSM-5) was synthesized by Jacobsen et al. [20] and since that time a particular attention has been devoted to characterization of this material [ 83, 84 ]. Combination of nitrogen adsorption isotherms, scanning electron... 

In this contribution we report on the incorporation of tin into the mesoporous molecular sieves MCM-41 and MCM-48 comparing traditional hydrothermal approach with microwave synthesis of these materials. X-ray powder diffraction, nitrogen adsorption isotherms, scanning electron microscopy and FUR of probe molecules were employed to characterize these molecular sieves. Oxidation of adamantanone with hydrogen peroxide was used as model reaction. 

Figure 11. Isothermal scan at 150 degrees C for UV cured films.

In many TG experiments, the temperature of the furnace is raised at a constant rate. This type of experiment is referred to as non-isothermal, scanning or rising temperature. An alternative experimental technique is available, and is often used in kinetic studies. Instead of raising the temperature at a constant rate, the temperature is held constant and the mass loss (or mass gain) observed at this fixed temperature. The results are then presented as mass loss against time, t. In practice the sample has to be placed on the thermobalance and the furnace at first left away from the sample. The furnace is then run up to the required temperature and left to stabilise. When the furnace temperature is constant at the required value, the furnace has to be moved quickly around the sample. There are a number of difficulties with this technique. The sample, crucible, thermocouple and cradle have to move rapidly from room temperature to the experimental temperature. They all have a finite thermal capacity, so cannot heat instantaneously. There is a thermal lag while the sample temperature rises. The first part of this rise does not matter, because the reaction being studied will not occur rapidly at lower temperatures. However, as the reaction temperature is approached, some reaction will... 

Crystallization kinetics Heat of reaction 11357-7 D3417 Isothermal scans at different cooling rates at three temperatm s in the crystallization range. 

Randzio, S.L., Grolier, J.-P.E., and Quint, J. R. (1994) An isothermal scanning calorimeter controlled by linear pressure variations from 0.1 to 400 MPa. 

Control of the Sample Environment High-vacuum conditions or constant inert gas flow are necessary for studying intrinsic relaxation phenomena in hygroscopic materials. A chemically inert atmosphere is also desirable in studies of polymer solutions, biomacromolecules, and other biological substances, where precise control of the hydration levels is vital. In isothermal scans fluctuations of the sample temperature should be as small as possible (below 0.1 K), such fluctuations are usually controlled by most commercial temperature stabilization/regulation systems (see Section 6.4.1.2). The same recommendation applies for the heating rates typically used in nonisothermal dielectric techniques. 

Figure 6 24 DEA results for thermoplastic polyurethane films with various metals in the chain extender (a) comparison between c(f) and M"(f) spectra recorded at 25 ""C (b) M (f) plot for a polyurethane sample PU(Cu ) placed in a brass electrode capacitor. Isothermal scans were performed between -55 °C and 40 °C, in 5 ° increments. The progressive upshift of various relaxations with increasing temperature of the scan is a typical behavior. [Adapted from Kalogeras, I. M., Roussos, M., Vassilikou-Dova, A., Spanoudaki, A., Pissis, P, Savelyev, Yu. V., Shtompel, V. L, and Robota, L. P. (2005), Eur. Phys. J. E 18,467. Copyright 2005. With kind permission of Springer Science and Business Media.l...

In a similar fashion, DSC isothermal scans were recorded in order to study the crystallization kinetics of the PPDX homopolymer after melting the samples for 3 min at 150 °C and quenching them (at 80 °C/min) to the desired crystallization temperature (7(.). After the crystallization was complete, the inverse of the half -crystallization time, (i.e., the time needed for 50% relative conversion to the crystalline state [31,60]), was taken as a measure of the overall crystallization (nucleation and crystal growth) rate and its dependence on the crystallization temperature was analyzed. 

DSC isothermal scans were also recorded in order to study the crystallization kinetics of PPDX after self-nucleating the sample at 117 °C. This self-nucleation temperature was determined by analyzing the self-nucleation domains and choosing an intermediate temperature within Domain II (more on the self-nucleation protocol can be found in Ref [52,60,91]). 

Dielectric Spectroscopy - Surface Characterization ofLiquid-borne Colloids When an oscillatory electric field is applied to a colloidal suspension, the electric double layer around the particle will be polarized. The complex dielectric properties of the suspension, the loss factor, and the relative permittivity are determined by performing several isothermal scans as a function of frequency in the range of 100-10 ° Hz. The dielectric relaxation of particles can be determined through the dielectric spectroscopy and the... 

The problem of design of the Isotherms scanning the loop of type H2 is reduced to mixed bond-site percolation problem (ref. 6). 

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