Spatial Correlation in Typical Binary Polycondensation Systems: An Essential Extension of the Kirkwood-Buff Theory

We present the isothermal susceptibility(chi(T)) for the typical binary polycondensation system of A(f)-B-g type, and relate chi(T) to the weight-average degree of polymerization in terms of the Kirkwood-Buff(KB) theory. The investigation is based on a new expression of chi(T) for mixtures, which is still expressed by the KB integrals(KBIs) but endowed with an explicit physical interpretation. For polymerization systems, it is proposed that the KBIs can be further decomposed according to whether there exists a bond between particles when conversions (extents of reaction) of functional groups are incorporated into the KBIs. In this way, chi(T) is directly decomposed into its relevant components as well. This is especially useful to reveal the relationship between local structures and average properties of various polymerization systems. As a consequence, the effect of polymerization on chi(T) is greatly simplified in comparison with the free energy route. Therefore, we have provided a very simple method to carry out some thermodynamic properties of polymerization systems.