Comparison of Miscibility and Morphology in Two Ternary Blend Systems: Effects of Binary Interactions

Experimental and theoretical studies on two ternary blend systems comprised of polystyrene (PS), isotactic polystyrene (iPS) and poly(cyclohexy methacrylate) (PCHMA) or poly(2,6-dimethyl-p-phenylene oxide) (PPO) are described. The differences in phase behavior and morphology of PS/iPS/PCHMA and PS/iPS/PPO ternary blends were investigated and compared by using differential scanning calorimeter (DSC), polarized-light optical microscopy (POM), and scanning electron microscopy (SEM). These two ternary blend systems were found to be completely miscible within the entire composition range at ambient temperature. However, cloud-point transition and LCST behavior was observed for the PS/iPS/PCHMA blend system, but not for the PS/iPS/PPO blend system. Phase diagrams for ternary systems of PS/iPS/PCHMA vs. PS/iPS/PPO at elevated temperatures of cloud points were constructed with the experimental data. Theoretical calculations were found to agree well with the data. Interaction parameters in the PS/iPS/PCHMA vs. PS/iPS/PPO ternary blends were evaluated to assess possible Δχ among all the binary pairs that constitute these two ternary blends. The melting point depression was used to investigate interactions between pairs of PS/PCHMA vs. PS/PPO, and it was found that asymmetry in the binary interactions led to more ternary phase instability, which was manifested as lowering the LCST and/or cloud points.