Small angle neutron scattering studies of a polybutadiene/polystyrene blend with small additions of ortho-dichloro-benzene for varying temperatures and pressures. I. Mean field to 3D-Ising crossover behavior

Thermal composition fluctuations were studied in the blend of polybutadiene (PB) and polystyrene (PS) in temperature and pressure fields using the technique of small angle neutron scattering. The blends had a critical composition and were mixed with different amounts of the nonselective solvent ortho-dichloro-benzene. In this part we discuss the critical behavior as measured along the critical isochore and interpret the results in terms of a crossover function describing the transition between mean field and 3D-Ising behavior. In particular, we will discuss the Ginzburg number and the critical amplitudes of the susceptibility and correlation length. For the Ginzburg number Gi we found a much stronger increase with solvent content as predicted while the pressure fields always lead to a decrease of Gi as expected. In “pure” PB/PS blends we found a further crossover to a so-called renormalized Ising behavior; such a crossover proposes density fluctuations of the total polymer concentration near the critical point. This crossover was not observed in the blend–solvent mixtures.