On the crossover from ising to mean-field behaviour in compatible binary-polymer blends

Susceptibility data for various binary-polymer blends at critical composition far from and near to the critical point of phase separation are compared with a theoretical expression describing the crossover from critical mean field to a 3d-Ising behaviour close to the critical point. The crossover function used is an explicit solution to first order in the perturbation parameter ε = 4 - d based upon a renormalisation group analysis. The results obtained using the crossover function are compared with those obtained using the Flory-Huggins form of the bare free-energy density. The observed differences can be interpreted as being due to an underestimate of the perturbation caused by fluctuations in the Flory-Huggins model. The Ginzburg number Gi calculated from the crossover function can be related to the proposed universal constant c of the Ginzburg criterium which is a measure for the width of the Ising regime.