Isotherms enthalpy recovery

Figure 4 shows the temperature dependence of the shift factors resulting from the reduction of the equilibrium creep data and the isothermal enthalpy recovery data shown in Figures 1, 2, and 3. The fit of equations 5 and 6 are also shown. Tj is taken as 207.5 C. The parameters used in the fits are, for equation 5 D/R = 1920 and T = 162 C and for equation 6 D/Ra =780, b/a = -1/560K and T2 = 151 C. Equation 6 seems to be better able to describe the temperature dependence in the glass due to the additional parameter. For modeling entMpy recovery, however, the results are expected to be identical with either equaticxi since the temperature range of interest is... 

The same techniques cannot be applied to the case of sPS/aPS blends, as the two components have similar Tg values (less than 10 °C difference). A higher resolution of close Tg values can be derived from the isothermal heat capacity curves, measured in the vicinity of Tg by modulated DSC [20]. Furthermore, sPS and aPS, when annealed separately below Tg, exhibit in both DSC and modulated DSC distinct endothermic transitions owing to the enthalpy recovery [20,21]. Both methods, when applied to the sPS/aPS blends, give a single temperature for all compositions in agreement with the presence of a miscibility between the components. 

For the case of isobaric temperature jumps or isothermal pressure jumps, these equations revert to the KAHR equations for volume recovery using (dS/dP)r = -Ak and (dS/dT)p = Aa. Ak is the difference in the compressibilities of the liquid and glass at Tg. For enthalpy recovery, (d5/dP)r = VTAa and (dS/dT)p = ACp. 

As pointed out earlier, most aging studies of blends have employed enthalpy relaxation and very few measurements have been made using volume relaxation. Notably, the volume changes during isothermal volume recovery are small, typically of the order of 1% or less and require high-precision measurements. 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

Owing to the fact that the glassy state is a nonequilibrium or metastable state, the thermodynamic properties of a glassy system (volume, enthalpy, etc.) in isothermal conditions will evolve toward thermodynamic equilibrium. The evolution of the volume is usually expressed in terms of = p — Ve)/Ve, where v and are, respectively, the specific volume at time t and at equilibrium. After a T-jump cooling experiment, the variation of 8 with time, in isothermal conditions, follows trends similar to those shown (18) in Figure 12.16. This process is known as structural recovery. 

A similar equation can be derived for the enthalpy. It turned out that this modification to give the retardation time a free volume dependence, was successful in describing one-step isothermal recovery but unsuccessful in describing memory effects. Kovacs, Aklonis, Hutchinson and Ramos (1979) attributed the latter to the contributions of at least two independent relaxation mechanisms involving two or more retardation times. These authors proposed a multiparameter approach, the so-called KAHR (Kovacs-Aklonis-Hutchinson-Ramos) model. The recovery process is divided into N subprocesses, which in the case of volumetric recovery may be expressed as ... 

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