Effect of pressure on phase behavior in polymer blends of poly(2,6‐dimethyl‐1,4‐phenylene oxide) and poly(styrene‐co‐ p ‐fluorostyrene) copolymers

The effect of pressure on the miscibility of blends of poly(2,6‐dimethyl‐l,4‐phenylene oxide) (PPO) with a random copolymer of styrene and para ‐fluorostyrene, P(S‐co‐p‐FS), has been studied by high pressure differential thermal analysis (HPDTA). P(S‐co‐p‐FS) copolymers less than 36 mole % p‐FS are miscible with PPO in all proportions irrespective of pressure up to 200 MPa, using the customary criterion of a single calorimetric glass relaxation. P(S‐co‐p‐FS) copolymers containing 40 to 50 mole % p‐FS undergo phase separation upon annealing at elevated temperatures, indicating the existence of a lower critical solution temperature (LCST). In these blends, pressure displaces the phase boundary associated with the LCST to higher temperatures causing an apparent increase in polymer miscibility. The phase diagram for the blend of PPO and P(S‐co‐p‐FS) containing 46 mole % p‐FS, shows that the critical composition at about 50 wt % PPO does not change with pressure, but the consolute temperature T c increases with increasing pressure. The pressure dependence of the LCST ( dT c / dP ) of this system is about 0.35°C/MPa.