Effective interaction parameter for homologous series of deuterated polystyrene-block-poly(n-alkyl methacrylate) copolymers

The effective interaction parameters, (χ _F) of the homologous series of deuterated polystyrene-block-poly(n-alkyl methacrylate) copolymers (dPS-b-PA _nMA) from methyl (n = 1) to hexyl (n = 6) side groups have been investigated by small-angle neutron scattering and a Hartree (fluctuation correction) analysis. While χ _F for dPS-b-PA _nMA with n = 1 and 6 is a typical decreasing function of temperature, it is shown that χ _F for n from 2 to 4 changes to a monotonic increase with temperature, revealing growing upward convexity. This action culminates in a maximum of χ _F for n = 5, which suggests an immiscibility loop-type behavior. Using the Hartree analysis for compressible diblock copolymers, the effect of directional interactions and the compressibility difference between the constituent blocks on χ _F is discussed. The ordering transition temperatures, their pressure dependences, and χ _F for the homologous block copolymer series are shown to be in good agreement with the experimental values.