Flory-Huggins theory upper critical solution

The thermodynamic properties of concentrated polymer solutions were studied by Floryi and independently by Huggins. The Flory-Huggins theory of polymer solutions still forms the basis for much discussion of these solutions in industry and even in academic research. Understanding this model is important for making coimections to much of the literature. Flory also substantially improved this model to include compressible fluids. The Flory-OrwoU theory of polymer solutions is still transparent and easily applicable, predicting both upper and lower critical solution temperatures. More-empirically adequate theories of concentrated solutions do not lend themselves to simple lecture presentation and often require detailed computer calculations to obtain any results. Concentrated solutions also introduce the phenomenon of viscoelasticity. An extensive treatment of the full distribution of relaxation times necessary to imderstand the dynamic properties of polymers in concentrated solution is presented. 

Figure 3.7 shows an Upper Critical Solution Temperature (UCST) as is predicted by Flory-Huggins theory. This theory as presented carmot predict the Lower Critical Solution Temperature (LCST) sometimes observed. A LCST curve can only be predicted if one has temperature variation of c different from that described in the theory developed here. 

Even though the Flory-Huggins theory predicts an upper critical solution temperature and allows a qualitatively correct phase diagram to be calculated, it cannot predict the experimentally observed lower critical solution temperature observed for virtually all polymer solutions. The fundamentally incorrect assumption in the theory is that the volume of mixing of the solution is assumed to be zero. To remedy this problem and improve the predictive power of the theory of concentrated solutions, a full fteory for... 

The theta temperature 9 is the highest possible upper critical solution temperature (UCST) within the framework of the Flory-Huggins theory. 

Upper Critical Solution Temperature n The maximum temperature for phase separation of polymer-solvent solutions to occur (Flory-Huggins theory) also, phase separation occurs when the temperature is raised until a lower critical solution temperature is reached, the phenomenon is explained by the free-volume theories of polymer solutions (Kamide K, Dobashi T (2000) Physical chemistry of polymer solutions. Elsevier, New York). 

The coexistence of upper and lower critical solution temperatures, abbreviated UCST and LCST, is remarkable. This complex phase diagram could not be described by the modified Flory-Huggins theory with a temperature-dependent contact energy and a temperature-independent zAu hi2- However, it is possible to fit the Flory-Huggins equation to experimental data provided that the enthalpy part of the interaction parameter is given temperature dependence ... 

Although it is very useful in the description of phenomena such as those mentioned above in blends involving statistical copolymers, the Flory-Huggins theory is unable to account for the variation of the properties of polymer-polymer blends as functions of the temperature. Indeed, the coexistence of both a lower critical solution temperature and an upper critical solution temperature of mixing, which is a rather common phenomenon in polymer blends, could not be described by the Hory-Huggins theory. 

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