GINZBURG CRITERION FOR THE MEAN-FIELD TO 3-DIMENSIONAL ISING CROSSOVER IN POLYMER BLENDS

Composition fluctuations within the mean-field and three-dimensional I sing range were measured in a homogeneous binary polymer blend by smal l angle neutron scattering as a function of temperature and pressure. The experimental data were analyzed in terms of the crossover function of Belyakov and Kiselev [Physica A 190, 75 (1992)]. It is shown that the reduced-crossover-temperature, the Ginzburg number Gi, decreases w ith pressure sensitively, in accordance with the prediction of Belyako v and Kiselev. On the other hand, de Gennes' crossover criterion for p olymer blends predicts an increase of Gi with pressure which contradic ts our results.