Thermodynamics and Phase Behavior of Block Copolymer/Homopolymer Blends with Attractive and Repulsive Interactions

Small-angle neutron scattering (SANS) experiments were conducted on a 25 vol % blend of a poly(ethylene- block -head-to-head propylene) copolymer (PE−PP) in polyisobutylene (PIB). PE/PIB and PE/PP chains are incompatible (the Flory−Huggins interaction parameter χ > 0), while PIB and PP are compatible (χ < 0) at low temperatures and incompatible (χ > 0) at high temperatures. These interactions lead to an order−disorder transition at 150 ± 5 °C and macrophase separation at 251 ± 5 °C. The nature of the concentration fluctuations in the homogeneous state (155 °C < T < 246 °C) was studied by comparing the experimentally determined SANS profiles with theoretical calculations based on the multicomponent random phase approximation (RPA). Order formation at low temperatures is driven by two parameters: χ PE/PIB and l PP (the statistical segment length of the PP block). The other temperature-dependent parameters drive the system away from the ordering transition. At high temperatures, the RPA predicts microphase separation, while experimental data (neutron and light scattering) indicate macrophase separation. This discrepancy is mainly due to the inability of the RPA to correctly account for the large length scale concentration fluctuations.