Enthalpy relaxation correspondence

Twin models. Figure 2 illustrates the temperature dependence of heat capacity for the two twin models and Table I gives the corresponding numerical data. Figure 2 typifies the Cp(T) curve of conventional glasses with a well defined enthalpy relaxation peak and smooth solid and liquid lines. From the extrapolated solid and liquid lines we can measure the heat capacity jump at Tg, by equation 1. Within our experimental range, the data fit a straight line with slopes (B) as listed in Table n. 

Analysis of the enthalpy relaxation the enthalpy relaxation time and the activation energy were calculated by KWW in accordance with the previous work (Kawai et al., 2004). The KWW theory was originally proposed in dielectric relaxation study by Williams and Watts (1970), then applied in the form of nonexponential function such as the enthalpy relaxation. In KWW theory, the enthalpy relaxation, AH eiax/ which corresponds to the peak area given from the enthalpy relaxation is expressed by the equation... 

Enthalpy relaxation for PC and PET proceeded cQmost comparably at corresponding tenq)erature Intervals belov their respective T s (T - T,). Furthermore, a comparison of these data vith th PS risults obtained previously (2 ) revecLLs no notable differences. The relaxation rates at comparable temperature intervals belov T fall in the order PET> PS> PC — an order... 

The effect of molecular structure on the enthalpy-relaxation processes can be explored in greater detail by considering, as pointed out in previous studies of relaxation processes for glasses (1-6), that the rate of enthalpy relaxation is a functlcn of the extent of the displacement of the system from its corresponding equilibrium state well as a function of... 

Figure 4.54 is a quantitative quasi-isothermal MTDSC trace for quenched, poorly crystallised PTT. The corresponding semiquantitative MTDSC is depicted in Figure 4.38. The cold crystallisation at about 325 K, the recrystallisation, 450 K, and the small enthalpy relaxation at 320 K are seen to be fully irreversible, and as in PET, the kinetics of the glass transition and the cold crystallisation can be further analysed quantitatively making use of the reversing heat capacity. It is also clear that during the standard DSC measurement, the cold crystallisation never stops completely between the two peaks and considerable errors in the crystallinity may result from choosing a baseline without MTDSC data.

For small enough temperature steps (< lOK) during small step annealing the vacancy concentration practically remains constant and corresponds to the instantaneous aimealing temperature. This allows for an easy analysis of SRO-kinetics yielding SRO-relaxation times and SRO-activation enthalpies, which by usual interpretation correspond to H +Hf. 

The ultimate goal of kinetics studies is the identification of a (unique) chemical kinetic mechanism, which consists of a reaction scheme such as the one shown in Figure 1.3 and the corresponding numerical values of the rate coefficients, k, which incorporate entropy and enthalpy differences. This is an inverse problem, since only the concentration profile or, in less favorable conditions, only the relaxation times can be observed, and the reaction mechanism must be deduced from this information. Any experimental method that establishes a connection between the signal and the concentration of molecules can be used to investigate kinetics. However, it is necessary that the method has sufficient time resolution since time is the crucial parameter in kinetic experiments. 

The available evidence thus suggests that relaxation times for planar-tetrahedral equilibria in nickel(II) complexes in solution at room temperature fall in the range 0.1-10 /isec, corresponding to rate constants of the order 105-107 sec-1. These relaxation times are several orders of magnitude longer than those observed for octahedral spin equilibria. The reaction coordinate for the planar-tetrahedral equilibria is characterized by large enthalpies of activation for the reaction in both directions, in contrast with a relatively low enthalpy of activation for the high-spin to low-spin process in octahedral iron complexes. 

An Arrhenius-type analysis of temperature dependence can be used to calculate the enthalpy and entropy of activation for the relaxation process. For liquid water, the enthalpy of activation is 19 kjmol-1, which corresponds approximately to the energy required to break one hydrogen bond. For ice, the equivalent enthalpy is 54 kj mol-1,... 

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