ON THE CROSSOVER FROM ISING TO MEAN-FIELD BEHAVIOR IN COMPATIBLE BINARY-POLYMER BLENDS

Susceptibility data for various binary-polymer blends at critical comp osition far from and near to the critical point of phase separation ar e compared with a theoretical expression describing the crossover from critical mean field to a 3d-Ising behaviour close to the critical poi nt. The crossover function used is an explicit solution to first order in the perturbation parameter epsilon = 4 - d based upon a renormalis ation group analysis. The results obtained using the crossover functio n 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 fluctu ations in the Flory-Huggins model. The Ginzburg number Gi calculated f rom the crossover function can be related to the proposed universal co nstant c of the Ginzburg criterium which is a measure for the width of the Ising regime.