1ST-ORDER AND 2ND-ORDER PHASE-TRANSITIONS IN ASYMMETRIC POLYMER MIXTURES

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 lambda = epsilon(AA)/epsilon(BB) 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 lambda = 5. At a critical potential difference DELTAmu(c) unmixing occurs below a critical temperature T(c). It is found that the quantities DELTAmu(c)/(1 - lambda)epsilon and 4k(B)T(c)/(3 + lambda)epsilon 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 DELTAmu(c) and 3.2 for T(c). 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.