Chain Connectivity and Conformational Variability of Polymers: Clues to an Adequate Thermodynamic Description of Their Solutions, 2

In Part 1 of this contribution we have reported how the Flory‐Huggins interaction parameter χ can be modeled as a function of chain length within the composition range of pair interaction between the macromolecules by means of the three parameters α , ζ , and λ . This contribution presents the extension of the approach to arbitrary volume fractions, φ , of the polymer and its application to published data on χ ( φ ). The resulting equation reads χ  =  α (1 −  νφ ) −2  −  ζ ( λ  + 2(1 −  λ ) φ ) and requires only the additional parameter ν to incorporate the composition dependence. Its employment to experimental data is very much facilitated by substituting for χ o (limiting value for φ  → 0); furthermore, the expression can in good approximation be simplified to χ  ≈ ( χ 0  +  ζλ )(1 −  ν φ ) −2  −  ζλ (1 + 2 φ ). That is: only two parameters, ν and the product of ζ and λ , need to be adjusted. This relation is capable of describing all types of composition dependencies reported in the literature, including the hitherto incomprehensible occurrence of pronounced minima in χ ( φ ). For a given system the evaluation of the chain length dependence of χ o , reported in Part 1, and the present evaluation of the composition dependence of χ yield the same data for the conformational response ζ . Similarly both types of measurements generate the same interdependence between ζ and α . The physical meaning of the different parameters and the reason for the observed correlations are discussed.Interrelation between ζλ and α for the evaluated systems.