Phase Behavior of Polystyrene/Polybutadiene and Polystyrene/Hydrogenated Polybutadiene Mixtures:  Effect of the Microstructure of Polybutadiene

The effect of the microstructure on the phase behavior of mixtures of polybutadiene (PB) and poly(ethylene- co -1-butene) (PEB) with polystyrene (PS) has been investigated. A series of PBs with 1,2-addition content ranging from 7 to 93% were synthesized by anionic polymerization, and a portion of each was subsequently hydrogenated to yield PEB. Polymer pairs with blend compositions from 10 to 90 wt % were cast from toluene for each of the 16 PS/PB pairs and 7 PS/PEB pairs. Laser light scattering was used to obtain cloud point measurements, which were then used to construct phase diagrams. It was found that, for constituent components with equivalent degrees of polymerization, PS/PEB pairs give rise to higher upper critical solution temperatures than PS/PB pairs, indicating that PS/PEB pairs are less miscible than PS/PB pairs. Experimental phase diagrams were curve-fitted to theoretical phase diagrams predicted from the Flory−Huggins theory with the expression for the interaction parameter α: α = a + b / T + c φ PS / T, where α is related to the Flory−Huggins interaction parameter χ by χ = α V r, where V r is the molar reference volume, T is the absolute temperature, and φ PS is the volume fraction of PS in the mixture. α values for PS/PB mixtures increase with increasing 1,2-addition (miscibility decreases) while α values for PS/PEB mixtures decrease (miscibility increases) with increasing 1-butene content. Using these α values, the Helfand−Wasserman theory was applied to predict the order−disorder transition temperatures of PS- block -PB and PS- block -PEB copolymers with varying 1,2-addition and 1-butene content, respectively.