Third approximation of Flory-Huggins theory

I he development of experimental methods of determining the spinodal, the interaction parameter x (or g), and other critical parameters has promoted the appearance of the third approximation of h lory-IIuggins lattice theory where the dependence of the interaction parameter g on the polymer molecular weight (or MWD) and the peculiar features of dilute polymer solutions or of polymer-depleted phetse at phase separation arc taken into account. 

The dependence g = g(M), especially at low /V/ s, was revealed experiiiieiitally by means of equilibrium centrifugation (Scliolte, 1970b Rietveld et al., 1972). 

In view of g = g(ip,T, M), the expressions for the spinodal and critical state (Equations 3.2-2,-3) become more complicated (Koningsveld and Kleintjens, 1971 Kennedy et al., 1972). 

The features of dilute solutions arc considered by reprc.sentation of g with two summands, of which game has the form of the second-approximation function (eg. Equation 37) while the second summand contributes greatly at low concentrations and negligibly at high concentrations. 

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