Enthalpy relaxation measurements

The rates of enthalpy relaxation measured for the polymers can be expressed in terms of temperature and excess enthalpy according to relationships of the form described previously (1.2),... 

Table 14.16. Epoxy resin blends with polyethersulphone data derived using the C-F model from enthalpy relaxation measurements [Breach (1992)]...

Cowie, J. M. G., and Ferguson, R., The ageing of poly(vinyl methyl ether) as determined from enthalpy relaxation measurements, Polym. Commim., 27, 258-260 (1986). 

Dependence of the fragility index, m, on the average coordination for Ge-Se and Ge-As-Se glasses. The fragility index was derived from the activation energy for enthalpy relaxation measured by DSC using Moynihan s method. 

As mentioned earlier, the relaxation enthalpy was measured by superimposing the first and the second DSC scans for each specimen. Figure 23 shows the relaxation-enthalpy loss versus logarithmic sub-Tg annealing time at 80°, 110°, and 140 °C. There is clearly a linear relationship between the enthalpy relaxation process and the logarithmic aging time, Ta. 

The problems associated with freeze drying of peptides and proteins for therapeutic use have also received calorimetric attention recently - particularly, attempts to understand and interpret the dynamics of amorphous solids. Structural relaxation time is a measure of molecular mobility involved in enthalpy relaxation and thus is a measure of the dynamics of amorphous (glassy) solids. These dynamics are important in interpretation of the physicochemical properties and reactivities of drugs in amorphous formulations. The authors conclude that microcalorimetry may provide data useful for rational development of stable peptide and protein formulations and for control of their processing . 

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. 

Enthalpy relaxation is one of the most widely studied in the context of both non-linearity and non-exponentiality of the measured glass properties. A convenient technique for these studies is scanning calorimetry. In simple cooling and heating experiments, heat capacity curves exhibit normal increase with characteristic hump of Cp above the glass transition as represented in Figure 9.08(A). The fictive temperature,... 

The experimental glass transition is, therefore, associated with a relaxing property - the enthalpy. The enthalpy/specific heat modes, in principle, couple to all phenomena which occur in the glass transition region. Enthalpy relaxation can be studied by applying a sinusoidal temperature pulse, much like an alternating electric field, which is applied to study dielectric relaxation. The specific heat is therefore treated as a frequency (of thermal field) dependent property. The traditional adiabatic technique cannot be applied because the time required for heat diffusion across the sample has to be short compared to the measurement time and the former is determined by the thermal diffusivity, which is low for most solids and is of the order of 10 cm s. ... 

Enthalpy relaxation time, determined by differential scanning calorimetry,608 and mechanical relaxation, determined by dynamic mechanical analysis,609 can also be used as measures of molecular mobility of amorphous pharmaceutical solids. 

The DSC is widely used to measure the glass-rubber transition temperature (Tg-value), which is an important parameter for polymer characterisation. The Tg-value represents the temperature region at which the (amorphous phase) of a polymer is transformed from a brittle, glassy material into a tough rubberlike liquid. This effect is accompanied by a step-wise increase of the DSC heat flow/temperature or specific heat/ temperature curve. Enthalpy relaxation effects can hamper the... 

At atmosperic pressure, P and V are sufficiently small for the left-hand side of Eq. (9.8) to be set at zero. Equations (9.8) and (9.9) allow the prediction of volume relaxation from experimental measurements of the enthalpy relaxation, and vice versa. This is a significant step toward the desirable goal of being able to correlate the rate of aging measurements for different physical properties. A simplified form of the Jain-Simha equations developed by Cowie et al. [1998b] can be written as... 

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