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 microstrucure in the range 7–15% and of deuterated polybutadiene (DPB) with the 1,2-linkage 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 sensitive to the microstructures of the polydienes used. The effective thermodynamic interaction parameter x_off 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 x_off showed a systematic change with the microstructure. For a given HPI, the x_off values decreased, and therefore, the blends became more miscible, with an increase in the vinyl content (i.e., 1,2-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 (i.e., 3,4-linkage content) in HPI. We proposed an alternative explanation for the LCST phase behavior based on treatment for the random copolymer blends.