Sequence Distribution Effects on Miscibility

It can be noted that the observed miscible polymer systems may be due to intramolecular repulsion rather than intermolecular attraction as originally hypothesized. The miscible blends of random copolymer (AB) and a homopolymer (C) in AB/C type blends were found miscible over a range of compositions of comonomer 1 in copolymer AB. This indicates a possible effect of the chain sequence distribution on the miscibility. Even in cases where the homopolymers were not miscible, the copolymer and homopolymer were found to be miscible. The mean-field theory of random copolymer blends is an important development in the thermodynamics of polymer miscibility. The compositional window of miscibility has been found for systems where A, B, and C were found to be immiscible as homopolymers, but were found to be miscible as A-B copolymer and C homopolymer. 

Karasz et al. [20] considered AB-AB type blends. Per the first-order random copolymer blend theory as described in Section 3.9 the interaction parameter of the blend can be written as 

The critical value of the blend interaction is derived from the equation for the combinatorial entropy of the blend  

For a given composition, there is an optimal range of sequence distributions in which AB/C blends are miscible. This may be the reason for the lines in the phase diagram to intersect. It was assumed that 

c Xaab.c Xaab.c Xbab.c Xbbb.c Xbba.c Xbba.c Xaba.c. 

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