Microstructure effect on the phase behavior of blends of deuterated polybutadiene and protonated polyisoprene

Small-angle neutron scattering (SANS) was used to measure the miscibility of blends of protonated polyisoprene (HPI) with 3,4 microstructure in the range of 7-44%, and of deuterated polybutadiene (DPB) with 1,2 microstructure in the range of 9-70%. These data suggest that, for deuterated polybutadiene/protonated polyisoprene blends, overall miscibility is controlled by the 3,4 content of the polyisoprene. This isomeric group is strongly immiscible with the 1,4 butadiene group and the immiscibility decreases with temperature, while the 3,4 group is weakly miscible with the 1,4 isoprene group and miscibility increases with temperature. Conversely, the 3,4 group is strongly miscible with the 1,2 butadiene group and miscibility decreases with increasing temperature. The differing trends of the interaction parameter with temperature suggest that shifts in the LCST phase diagram can be controlled by judicious changes in microstructure. These theoretical predications were confirmed with experimental small-angle light scattering (SALS) results on all protonated blends. The statistical segment length, b, appears to be independent of temperature, but is dependent on total vinyl level. For the blends used in this study, b has a minimum value at approximately 30-35% total vinyl isomeric groups.