Microstructure effects on the lower critical solution temperature phase behavior of deuterated polybutadiene and protonated polyisoprene blends studied by small-angle neutron scattering

The miscibility of a blend of protonated polyisoprene (HPI) with the 3,4-linkage microstruc- ture in the range 7-15% and of deuterated polybutadiene (DPB) with the 1,Qlinkage microstructure in the range 12-28% was studied by small-angle neutron scattering (SANS). It was found that all blends studied here show lower critical solution temperature (LCST) type phase behaviors; i.e., the phase separation occurs by raising the temperature. It was also found that the miscibility is quite senaitive to the microstructures of the polydienes used. The effective thermodynamic interaction parameter xen per segment between two polymers was determined by fitting SANS data in the single-phase state with a theoretical scattering curve obtained on the basis of the random-phase approximation. The temperature dependence of xen showed a systematic change with the microstructure. For a given HPI, the xeff values decreased, and therefore, the blends became more miscible, with an increase in the vinyl content (Le., lI2-linkage content) in DPB. On the contrary, for a given DPB, the values increased, and therefore, the blends became more immiscible, with an increase in the vinyl content (Le., 3,4-linkage content) in HPI. We proposed an alternative explanation for the LCST phase behavior based on treatment for the random copolymer blends.