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Figure: Dialog box Thermodynamic date

The specific enthalpy is obtained from the integral of the specific heat capacity $c_p(T)$ between the limits T₁ and T₂:

$$\Delta h = \int_{T_1}^{T_2} c_p(T) \, dT$$

In this way the quantity of heat (per mass unit) is obtained which is needed to increase the temperature of the polymer from T1 to T2. In case of an amorphous material a steeper increase of the enthalpy is seen with the rising temperature up from the glass transition point Tg as for a semi-crystalline material (see figure).

Figure: Specific enthalpy as a function of temperature

In contrast semi-crystalline materials display a step-by-step increase conditional on the phase change. The additional quantity of heat is denoted as the melting enthalpy ΔhA. The figure shows the specific enthalpy as a function of temperature for both types of polymers, amorphous and semi-crystalline ones.

If an amorphous thermoplastic is specified, the entering sheet for the melting enthalpy is not available. In the case of semi-crystalline thermoplastics, the enthalpy increase Δh is made up of the enthalpy increase of the solid enthalpy ΔhF and the melting enthalpy ΔhA (see figure):

• semi-crystalline thermoplastics: $\Delta h = \Delta h_F + \Delta h_A$
• amorphous thermoplastics: $\Delta h = \Delta h_F$