Explanation for the inversion of an UCST phase diagram to a LCST diagram in binary polybutadiene blends

A simplified version of the random copolymer lattice cluster theory has bee n applied to explain the variation in the nature of the phase transition (u pper vs lower critical solution temperature phase diagram) with the microst ructure of isotopic polybutadiene (PB) blends that has been observed by Has himoto and co-workers (Macromolecules 1992, 25, 6078). This variation occur s within our theory when the temperature-dependent portion chi (h) of the e ffective interaction parameter chi (SANS) becomes negative (due to an incre asing difference \y-x\ in the compositions x and y of the two PB random cop olymers) and competes with a positive temperature-independent, "entropic" p ortion chi (s) of chi (SANS). The analysis is based on two important featur es. First is the calculation of chi (s) solely from the monomer united atom molecular structures, with no adjustable parameters. Second is the treatme nt of the van der Waals interaction energies in terms of united atom groups .