Melting-transition temperature polymer heat capacity

The T of crystalline polymers may be determined by observing the first-order transition (change in heat capacity value) by DTA or by DSC (ASTM-D3418). Some comparative information on thermal properties of polyolefins may be obtained from the melt index. To determine the melt index, the weight of extrudate or strand under a specified load and at a specified temperature is measured. Melt index values are inversely related to the melt viscosity. 

The polymer heat capacity data bank is, as outlined in the Introduction, is only the first step towards the establishment of a comprehensive Thermal Properties Data Bank. Presently we are expanding our efforts to include glass transition temperatures, melting temperatures and heats of fusion. In the planning stage are specific volune, compressibility, and thermal conductivity data banks, as well as the expansion to non-equilibrium properties. 

Knowledge of the heat capacity of solids and melts is of importance not only for its own sake, but also for the discussion of the various latent heat effects in polymers which occur commonly between the glass transition temperature and the melting point and will be analysed in more detail in Section 4.3. Furthermore, in the glass transition region, the heat capacity may become time dependent without the presence of a latent heat effect. Both of these topics will be discussed in this section. 

Figure 4.55 shows a plot of various heat capacity data of PTT as shown in Figure 4.54 [65]. The fully amorphous point was calculated from the heat capacity of the glass and the melt, both extrapolated to the glass transition temperature. The heat of fusion of the 100% crystalline sample agrees also with a discussion of the entropies expected from similar polymers. The data points with somewhat lower ACp are most likely due to a small amount of RAF [64], frozen at the crystal interface, as indicated by the thin line.

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

The most common applications of DSC are to the melting process which, in principle, contains information on both the quality (temperature) and the quantity (peak area) of crystallinity in a polymer [3]. The property changes at Tm are often far more dramatic than those at Tg, particularly if the polymer is highly crystalline. These changes are characteristic of a thermodynamic first-order transition and include a heat of fusion and discontinuous changes in heat capacity, volume or density, refractive index, birefringence, and transparency [3,8], All of these may be used to determine Tm [8],... 

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. 

The formation of S o in sulfur melts is a slow reaction, and it takes about 1 h at 160 °C to establish the equilibrium concentration [24, 58]. From the temperature dependence of the polymer content, from the heat capacity Cp of the melt [29] as well as from calorimetric measurements [56, 58] it was concluded that the reaction Ss Sqo is endothermic with an estimated activation energy of ca. 120 kj mor (Ss) [58]. The same value was derived from DSC measurements of liquid sulfur [58]. In this context it was observed that the sudden viscosity increase of liquid sulfur takes place at exactly 159 only if the heating rate approaches zero. If the heating rate is varied between 1.25 and 40 K min higher transition temperatures are observed as the data in Table 1 show [58]. 

---