The Phase Diagram for Nearly Miscible Blends

Studies of polymer solutions and binary mixtures indicate that there exist two critical temperatures the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST). The LCST is the lowest temperature for which two phases can be observed and the USCT is the highest temperature at which two phases can be observed for a mixture. The bimodal condition is described by 

The spinodal is obtained by applying the stability condition. Note that two terms have been added to the equations that introduce the pressure dependence of the functions  

Differentiating eqn (8.28) with respect to (j)2 gives the spinodal condition  

These equations can be used to simulate the behaviour of various polymer mixtures. For the simulations the contact entropy Q,2 is the only adjustable parameter. X,2 and Q,2 are empirically interrelated  

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