Relaxation calorimeter

A melted sample of the terpolymer was cooled in the calorimeter at 320°/min to — 40°C and was allowed to remain at — 40°C for 16 hr after which the temperature was lowered to — 150°C. Upon reheating, the Tg of the annealed sample increased to — 31 °C moreover, an additional adsorption of thermal energy was superimposed upon the normal increase in Cp during the glass transition. The additional increase in enthalpy, 0.5 cal/g, is the result of enthalpy relaxation occurring during... 

Figure 12, Brillouin splittings Aw d vs, temperature near the glass-rubber relaxation for PMMA, 10,000 molecular-weight PS and 2100 molecular-weight polystyrene. The arrows indicate the value of T(g) determined uHth a differential scanning calorimeter.

No serious experimental problems were encountered with the boron/alu-minum composite, but the resin-based materials were difficult to measure with precision, due to long thermal relaxation times and what appeared to be an entrapment of helium gas within the specimens. The evolution of helium, which provided a thermal coupling between the specimen and calorimeter assembly, was particularly troublesome below 4 K and frequently limited the precision of measurement. Any poor data resulting from sporadic bursts of outgassing were manually screened out at an early stage and not brought forward into the final analysis. 

Calorimetry has been one of the most commonly used techniques to follow the enthalpic relaxation during aging. Measurements are readily carried out at temperatures below Tg using a differential scanning calorimeter (DSC), a convenient and widely available instrument. 

Here, we will focus on a specific high-resolution calorimetric technique, i.e. nonadiabatic scanning calorimetry. This technique implies a calorimeter apparatus capable of operating in the ac as well as in the relaxation mode. The combination of these modes makes it possible to distinguish between the continuous and discontinuous transitions. 

In order to determine precisely the latent-heat values, the calorimeter has to be operated in the relaxation mode. In the most recent version of this mode, the bath is accurately stabilized and then a linearly ramped power is applied to the sample [31], This mode of operation is sensitive not only to the continuous but also to the discontinuous part of the enthalpy change, i.e. the latent heat L. Hence, L can be quantitatively determined by subtracting the integrals of the resulting Cp and Cp ff anomalies of the ac and relaxation modes, respectively ... 

The thermal relaxation effects already outlined must be kept in mind in any analysis of the characteristics of the data output of calorimeters regardless of the particular type of instrument involved. 

The calorimeter vessel (sample container), in which the reaction being studied takes place, is connected by means of two separate thermopiles with a thermostat kept constant vith a precision of 0.01 K. Heat could flow through the thermopiles between the vessel and the thermostatic surroundings. Because the thermopiles have a relatively high thermal resistance (thin wires), the relaxation of temperature would take rather a long time (large time constant). The surroundings serve as a reference for the measurement of temperature of the vessel, and the temperature... 

The essentia] point for classification of this type of calorimeter is that the local temperature difference, which invariably exists wherever a heat exchange takes place, is measured. The heat released or consumed in the calorimeter (measuring system) initially causes a change of temperature with regard to the surroundings. This causes a relaxation process the heat exchange with the surroundings continues until the reestablishment of isothermal or steady-state conditions. The possible mechanisms of heat transport - thermal conduction, convection, and radiation - are discussed in Chapter 4. 

Remark Calorimeters that use the quantitative analysis of such AT(t) functions for the determination of the sample heat capacity are called relaxation-type calorimeters. The method became important in particular at low temperatures because it works sufficiently well even with very small sample masses (Collocott, 1999). 

To sum up, the Calvet calorimeter has a very simple exponential apparatus function based on the above assumptions (namely, a very small Rthi and a rapid heat relaxation inside the sample and sample container (calorimeter vessel)). The measured curve can then be desmeared in a simple and accurate maimer according to Eq. (7.9). The assumptions are complied with to some extent if Rth2 is made suitably large by an appropriate design (see Figure 7.16). The large time... 

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