Question

The thermal analysis of a polymer sample is the study of the change of the polymer properties with temperature. Draw a typical thermal analysis plot of a crystalline polymer showing the glass transition, melting endothermic and crystallization exotherm.

Answer 1

GLASS TRANSITION
If a polymer in its molten state is cooled it will at some point reach its glass transition temperature (Tg). At this point the mechanical properties of the polymer change from those of an elastic material to those of a brittle one due to changes in chain mobility. A typical example of a heat flow versus temperature plot at a glass transition temperature is shown in Figure. The heat capacity of the polymer is different before and after the glass transition temperature. The heat capacity Cp of polymers is usually higher above Tg. DSC is a valuable method to determine Tg. It is important to note that the transition does not occur suddenly at one unique temperature but rather over a range of temperatures. The temperature in the middle of the inclined region is taken as the Tg.

CRYSTALLIZATION

Above the glass transition temperature the polymer chains have high mobility. At some temperature above Tg the chains have enough energy to form ordered arrangements and undergo crystallization. Crystallization is an exothermic process, so heat is released to the surroundings. Less heat is needed to keep the heating rate of the sample pan the same as that of the reference pan. This results in a decrease in the recorded heat flow. If the convention of ‘exothermic - down’ is used then the result is a dip in the plot of heat flow versus temperature as seen in Figure. Such a crystallization peak can be used to confirm that crystallization occurs in the sample, find the crystallization temperature (Tc) and determine the latent heat of crystallization. The crystallization temperature is defined as the lowest point of the dip. The latent heat (enthalpy) of crystallization is determined from the area under the curve. Such a crystallization peak can be used to confirm that crystallization occurs in the sample, find the crystallization temperature (Tc) and determine the latent heat of crystallization. The crystallization temperature is defined as the lowest point of the dip. The latent heat (enthalpy) of crystallization is determined from the area under the curve.

MELTING
The polymer chains are able to move around freely at the melting temperature (Tm) and thus do not have ordered arrangements. Melting is an endothermic process, requiring the absorption of heat. The temperature remains constant during melting despite continued heating. The energy added during this time is used to melt the crystalline regions and does not increase the average kinetic energy of the chains that are already in the melt. In a plot of heat against temperature this appears as a jump discontinuity at the melting point as seen in Figure . The heat added to the system during the melting process is the latent heat of melting. It can be calculated from the area of a melting peak observed in a plot of heat flow against temperature, such as the one in Figure. The Tm is defined as the temperature at the peak apex. After melting the temperature again increases with heating. However, the heat capacity of a polymer in the melt is higher than that of a solid crystalline polymer. This means the temperature increases at a slower rate than before.

Combining Tg, Tc, and Tm