Two equilibrium melting temperatures and physical meaning of DSC melting peaks in poly(ethylene terephthalate)

For poly(ethylene terephthalate), two equilibrium melting temperatures, 262 °C and 276 °C, were found with DSC. They should be assigned to two forms of crystals with the infinite crystal length, respectively. This finding made it possible every crystal form to derive the end surface free energy per unit area of crystals and the transition enthalpy of the ordered parts in the amorphous regions and using these thermodynamic quantities to convert DSC melting curves into the crystal length distribution. Through the derivation of them, the participation of the crystallization temperature on cooling in the thermal analysis of DSC melting curves was revealed thermodynamically.