Glass transition heat capacity curve

Fig. 5. Schematic of the heat capacity on cooling and heating at largely different rates in the glass transition region. Top Curves Cooling at 5.0 K/min, heating at 0.25 K/min. Bottom Curves Cooling at 5.0 K/min, heating at 150 K/min. (Hypothetical polymer, time dependence as in polystyrene)...

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,... 

Schematic DTA curve of a typical polymer. Note glass transitions reflect a heat capacity change and thus they are seen as steps on the baseline rather than peaks.

According to this figure a crystalline polymer follows the curve for the solid state to the melting point. At Tm, the value of Cp increases to that of the liquid polymer. The molar heat capacity of an amorphous polymer follows the same curve for the solid up to the glass transition temperature, where the value increases to that of the liquid (rubbery) material. 

As an amorphous polymer, lignin undergoes chain segment motion upon heating. This motion, a glass transition, is characteristic of all amorphous polymers, and is indicated by an endothermic shift in the DTA or DSC curves. This glass transition is accompanied by abrupt changes in free volume, heat capacity, and thermal expansion coefficient. 

Figure 1-9 A DSC Curve for an Idealized Glass Transition in which Tg can be Taken as the Temperature at Which One-Half of the Change in Heat Capacity, ACp, has occurred.

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