Effect of added copolymer on the critical properties of polymer mixtures.

Monte Carlo simulation and finite-size analysis are used to determine the critical properties of a binary homopolymer blend and of a polymer blend that has been compatibilized with a random copolymer. Determination of nu, the critical exponent for the correlation length, for the mixture of two homopolymers shows that the binary blend exhibits properties that are consistent with Ising behavior (nu=0.63). Similar results for the compatibilized blend show that the added copolymer acts very much like an impurity; lowering the mixing transition temperature and increasing the value of nu to a value that is in qualitative agreement with Fisher renormalization (nu=0.69). These results are important as copolymers are often added to polymer blends to act as compatibilizers. These results, therefore, show that the analysis of the phase behavior of these mixtures must be analyzed with the understanding that its critical exponents differ from those of a binary polymer mixture.