First- and second-order phase transitions in asymmetric polymer mixtures

Publisher Website
Google Scholar
Abstract
The critical properties of dense asymmetric binary polymer mixtures are studied by grand canonical simulations within the framework of the three-dimensional bond fluctuation lattice model. The monomers interact with each other via a potential ranging over the entire first peak of the pair distribution. An asymmetry is realized by giving the ratio of interactions λ≡εAA/εBB between monomers of the A species and of the B species a value different from 1. Using multiple histogram extrapolation and finite size scaling techniques for the data analysis, the two-phase region, which is a line of first-order transitions driven by the chemical potential difference, and the critical point are determined for a mixture of chains with 32 monomers each and various asymmetries up to λ=5. At a critical potential difference Δμc unmixing occurs below a critical temperature Tc. It is found that the quantities Δμc/(1−λ)ε and 4kBTc/(3+λ)ε are both independent of the asymmetry, consistent with the prediction of the Flory theory. But the numerical values are overestimated by Flory theory by roughly a factor 1.5 for Δμc and 3.2 for Tc. For the first time a finite size scaling at first-order transitions and a way to distinguish first- and second-order transitions are presented for polymer mixtures.