Anomalous scaling of the critical temperature of unmixing with chain length for two-dimensional polymer blends

The thermodynamics, structure and the chain configurations of symmetrical polymer mixtures confined into ultrathin films are studied by Monte Carlo simulations of the bond fluctuation model. It is shown that the Flory-Huggins–type scaling of the critical temperature ( T c ∼ N ) with chain length N in the bulk is replaced by a weaker increase, T c ∼ N 1/2 , in an ultrathin film, and this is interpreted in terms of geometric arguments. The pair-correlation function g ( r ) of monomers from different chains exhibits a pronounced correlation hole, and the density of intermolecular contacts z c decreases with N as z c ∼ N −1/2 .