Blends of Polyisoprene with Model Styrene-Olefin Copolymers: Mixing Energetics in Blends versus Block Copolymers

Most theoretical treatments of polymer mixing thermodynamics assume that the phase behavior of both polymer blends and block copolymers is controlled by the same Flory segmental interaction parameter, χ - a quantity which is proportional to the interaction energy density, X. However, there are few comparative experimental studies on this point. This work presents a systematic experimental comparison between the interaction energy densities measured on model polydiene-polyolefin blends (Xblend) and analogous block copolymers (XBC). Experimental cloud point temperature (Tcl) curves for blends of polyisoprene (PI) with selectively saturated poly(butadiene-r-styrene) (hSBR) random copolymers are fit to the Flory-Huggins theory, and the resultant values of Xblend are corrected for critical fluctuations and for end-group contributions, wherever significant. The temperature-dependent PI/hSBR Xblend(T) and previously reported PI-hSBR XBC values show satisfactory agreement at all levels of styrene incorporation, reinforcing the conclusion that random incorporation of styrene units into the saturated polybutadiene chain significantly enhances its compatibility with PI.