Spinodal Curve from B Values and EOS

Once the binary interaction parameters for the blend system are known, EOS theory can be used to predict phase separation behavior. Lower critical solution temperature (LCST) is the temperature above which a miscible system becomes immiscible. Upper critical solution temperature (UCST) is the temperature above which an immiscible polymer blend system becomes miscible. Some polymer-polymer systems exhibit either LCST or UCST or both or neither. Another set of phase separation can be obtained as shown in the copolymer-homopolymer example in Section 3.2 by varying the blend volume fraction. The Gibbs free energy of mixing per unit volume for a binary system of two polymers can be written as 

A spinodal condition is defined by Equation (3.19). A was obtained by Merfeld and Paul [14] by obtaining the third derivative of with respect to the composition, which is zero. This is where the boundary between the miscible and immiscible lies and is supposed to be a balance between the combinatorial entropy and component interactions  

Miscibility is expected when B is more favorable than This model can only predict the UCST-type phase boundaries. Although when B is allowed to vary with 

PVC/SAN homopolymer-copolymer blend systems have been reported to be miscible over some composition range of AN in the copolymer and blend composition. Zhang et al. [15] found that the value of LCST increases as the strength of interaction between the component polymer increases. The miscibility behavior of PVC/ AMS-AN blends was found to depend on the AMS content in AMS-AN copolymer using dynamical mechanical analysis (DMA) and scanning electron microscopy (SEM). The PVC was found to be immiscible with AMS-AN copolymer as the AMS content in AMS-AN is less than 15 wt% by melt mixing. 

However, the miscibility of PVC/AMS-AN blends is substantially improved with the increase of AMS content in AMS-AN copolymer containing identical 

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