Thermodynamic Quantities Governing Melting

Abstract

In this section a compilation and tabulation of the key thermodynamic parameters that govern the fusion of crys-talline homopolymers is presented. The key parameters of interest are the equilibrium melting temperature, T 0 m , defined as the melting temperature of a large perfect crystal comprising infinite molecular weight chains, the enthalpy of fusion per chain repeating unit, DH u , which is independent of the level of crystallinity, and the entropy of fusion, DS u , which is obtained from the above quantities. The data listed in Tables 11.1–11.3 are limited to those homopolymers for which a complete set of thermodynamic parameters are available. Copolymers will not be included since this would involve specifying the sequence distribution of the comonomer. An exception will be made, however, for stereoregular polymers which, from the point of view of crystallization behavior, should be treated as copolymers [1]. Most polymers in this category are not completely stereoregular and cognizance must be taken of this fact when using the data assigned. We shall not be tabulating melting temperatures by themselves, without the auxiliary thermodynamic parameters. Before examining and using the data listed it is important to understand the theoretical and experimental foundations for the quantities that are given and the limitations that are imposed. Although a collection of regularly structured, flexible chains will crystallize, they never do so completely. De-pending on the molecular constitution, the chemical nature of the chain and the crystallization conditions the level of crystallinity attained can range from 90% to just a few percentage. In order for the crystallization of polymers from the melt to be carried out at finite rates, it must be conducted at temperatures well below the equilibrium melt-ing temperature. Consequently, a polycrystalline, partially ordered, system is the one that actually develops. This crystalline system is well removed from equilibrium and can be considered to be in a metastable state. A structural and morphological complex system is thus developed that governs properties, including thermodynamic ones. The kinetic restraints that are placed on the crystallization of polymers make it difficult, if not impossible to directly determine their equilibrium melting temperatures. The directly observed melting temperatures are primarily a reflec-tion of the structure and morphology of the actual crystalline systems. The primary factors involved are the crystallite thickness, the interfacial free energy, and the influence, if any, of the noncrystalline region. There are, however, indirect methods by which to estimate the value of T