ABSTRACT
The phase transitions of polymers include, first of all, phase transitions of the first kind – crystallization and melting under which the first derivatives of the Gibbs energy – enthalpy, entropy and volume – change abruptly. To clarify the role of relaxation phenomena during phase transformations of polymers, this chapter considers the general thermodynamic and kinetic aspects of the crystallization and melting of various compounds. For a semi-crystalline polymer, melting is observed in the temperature range, the middle of which is considered as the melting point. Unlike low-molecular-weight crystals for semi-crystalline polymers, the experimentally determined melting point is not a strict characteristic, since this parameter depends on the method and mode of measurement, primarily on the heating rate. From the point of view of kinetics, crystallization involves two stages – nucleation and growth of crystals. Processing of the experimental crystallization isotherms is based on the Kolmogorov–Avrami equation derived for low-molecular-weight substances.
