Structural and thermodynamic properties of interfaces between coexisting phases in polymer blends: a Monte Carlo simulation

Abstract

A large-scale Monte Carlo investigation of the interface between the A-rich and B-rich phase of a partially incompatible binary (AB) symmetric polymer mixture is presented. The bond fluctuation model on the simple cubic lattice at a volume fraction ϕ= 0.5 of occupied lattice sites and chain lengths N_A=N_B=N= 32 is used, assuming an interaction ε_AA=ε_BB=–ε_AB=–ε between effective monomers, with an interaction range of √6 lattice spacings. The temperature range studied, 0.144 < T/T_c < 0.759, encompasses the whole range from the strongly segregated regime at low temperatures to the weak segregation regime near the critical temperature T_c(the latter has already been estimated very accurately from a finite-size scaling analysis in previous work). The temperature dependence of both interfacial Helmholtz energy and interfacial width, w, are presented, and it is shown that the former is compatible with current theory, while w exceeds the theoretical predictions by about a factor of two. Varying the linear dimension L parallel to the interface from L= 64 to L= 512 we can show that broadening of the concentration profile due to capillary waves cannot explain this discrepancy (note that the large size of our systems, up to ca. 16 × 10⁶ lattice sites, needed the use of a massively parallel CRAY-T3D system). We present evidence for the enrichment of chain ends and of ‘free volume’ at the interface and show that chains near the interface are oriented and slightly contracted in the strong segregation limit. We suggest that the ‘effective’ Flory–Huggins χ parameter is reduced near the interface by these effects.