Abstract
The effect of pressure on miscibility and phase separation in blends of two kinds of random copolymers consisted of styrene and para-fluorostyrene, P(S-co-p-FS), and ortho-and para-fluorostyrene, P(o-FS-co-p-FS), and poly(2, 6-dimethyl-1, 4-phenylene oxide), PPO, has been studied bydifferential thermal analysis (DTA) at pressures up to 300MPa. P(S-co-p-FS) copolymers less than 36mole% p-FS are miscible with PPO in all proportions irrespective of pressures, using the customary criterion of a single calorimetric glass relaxation. P(S-co-p-FS) copolymers containing 40 to 50mol% p-FS undergo phase separation upon annealing at elevated temperatures, indicating the existence of a lower critical solution tempratures (LCST). In the PPO/P(S-co-p-FS) 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 PPO/P(S-co-46p-FS) containing 46mol% p-FS, shows that the critical composition at about 50wt% PPO does not change with pressure, but the consolute temperature Tc increases with increasing pressure. The pressure dependence of the LCST of this system is about 0.35°C/MPa of dTc/dP. On the other hand, the P(o-FS-co-p-FS) copolymers containing from 10 to 38mole% p-FS are miscible with PPO below 230°C. When the phase behaviors of the 50/50wt% blends are examined as a function of temperature and copolymer composition, a symmetric miscibility “window” can be observed in the resulting temperature-composition diagram with a maximum at about 22mol% p-FS. The pressure dependence of the phase diagram for the PPO/P(o-FS-co-29p-FS) blend containing 29mol% p-FS shows that the critical composition is at about 50wt% PPO and is independent of pressure. The Tc increases at about 0.10°C/MPa up to 200MPa and then becomes independent of pressure to reach an asymptotic value at around 270°C. Similar behavior is also observed for blends in which the copolymer composition contain either 16 or 23mol% p-FS. The decrease of dTc/dP at higher pressure may indicate that the negative volume of mixing approaches zero above 200MPa.
