Flory-Huggins theta solvents

The deterioration of the solvent qnality, that is, the weakening of the attractive interactions between the polymer segments and solvent molecules, brings about the reduction in the coil size down to the state when the interaction between polymer segments and solvent molecules is the same as the mutual interaction between the polymer segments. This situation is called the theta state. Under theta conditions, the Flory-Huggins parameter % assumes a value of 0.5, the virial coefficient A2 is 0, and exponent a in the viscosity law is 0.5. Further deterioration of solvent quality leads to the collapse of coiled structure of macromolecules, to their aggregation and eventually to their precipitation, the phase separation. 

As z increases from 0 (theta conditions) to 2 (good solvent), the function also increases from 0 to about 0.2. For simplicity, dilute solutions are used to avoid the need for determination of higher-order virial coefficients. In this case, A2 provides a direct measure of the intermolecular interactions in polymer solutions and can be directly related to the respective Flory-Huggins parameter (x, vide infra, Sect. 2.6.1.1). However, since A3 oc A2M , Eq. 2.29 can be written as... 

Flory and Huggins developed an interaction parameter that may be used as a measure of the solvent power of solvents for amorphous polymers. Flory and Krigbaum introduced the idea of a theta temperature, which is the temperature at which an infinitely long polymer chain exists as a statistical coil in a solvent. 

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