Polyisoprene
(PI) and hydrogenated medium-vinyl polybutadiene (hPB),
a polydiene–polyolefin pair, are expected to show limited compatibility
(with a high interaction energy density, X, proportional
to the Flory interaction parameter, χ). The regular mixing model
suggests that styrene (S) units can boost the interblock compatibility
when incorporated in small amounts into the hPB chain via random copolymerization.
The mixing thermodynamics in symmetric polydiene–polyolefin
“block–random” copolymers composed of PI and
a random copolymer of styrene and hydrogenated medium-vinyl butadiene
(hSBR) were investigated, through measurements of the order–disorder
transition temperature. Block and block–random copolymers were
prepared by anionic polymerization, followed by selective saturation
of the butadiene units. More than a 2-fold decrease in X was achieved on incorporating ∼20 wt % S in the random block,
with a further decrease observed at ∼30 wt % S incorporation,
indicating a strong enhancement in compatibility. At higher styrene
contents, X varied parabolically as qualitatively
predicted by the regular mixing model, but better quantitative agreement
was obtained with the copolymer equation model.