Blends of polybutadiene with three different vinyl contents and polystyrene were studied by small-angle neutron scattering as a function of temperature and pressure. The data were analyzed with a crossover function combining the universality classes of the 3D-Ising and the mean field behavior and thus yield the spinodal temperature T-S, the Ginzburg number Gi, and the critical amplitude C-MF. From these parameters the mean field Flory-Huggins interaction parameter is calculated in terms of entropic and enthalpic contributions. The application of the theory of random copolymers delivers large enthalpic and entropic contributions of the intramolecular interaction between the 1,2- and 1,4-butadiene units. These numbers could not be confirmed from measurements of the corresponding intermolecular interaction contributions in an h-PB(1,2)/d-PB(1,4) blend, which thus leads to speculations about the equivalence of inter- and intramolecular interactions in random copolymers. The Ginzburg number proved to be specific to the vinyl content and thus deviated from the universal behavior Gi proportional to V-1 predicted by deGennes. By our pressure experiments we find that the Clausius-Clapeyron equation suggests the phase boundary to be influenced by the Flory-Huggins parameter and the Ginzburg number.