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 n MA) 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 n MA 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.