Thermally Reversible Phase Separation in Polystyrene Poly(styrene-co-4-bromostyrene) Blends

Abstract

Mixtures of a narrow molecular weight distribution polystyrene with poly(styrene-co-4-bromostyrene) copolymers obtained by brominating 0.27 and 0.29 mole fraction (x) of its units exhibited reversible phase separation at and below their respective upper critical solution temperatures (UCST). An increase of only about 0.02 in x caused an increase of about 70 °C in the UCST of the polystyrene plus poly(styrene-co-4-bromostyrene) mixtures. Use of a Nomarski assembly in a polarizing microscope provided excellent contrast for phase detection and structural studies. The liquid-liquid phase boundary (binodal) for mixtures of the x = 0.27 copolymer with its parent polystyrene was examined within the framework of Koningsveld's formulation of the Flory-Huggins Gibbs free energy function. A concentration-dependent interaction parameter g = 4.02949 + (20/T) - 0.007411Φ₂ provided a good fit of the binodal data above the copolymer volume fraction (Φ_2c) at the critical point. An interaction parameter g = -0.00905 + (10/T) - 0.004691Φ₂ ovided a fit that was not as good in the higher & region but that accommodated the phase boundary data below Φ_2c in better fashion. Morphologies of the phase-separated blends varied in a way that was consistent with a nucleation/growth mechanism of separation at the extremes of composition and spinodal decomposition in intermediate ranges. Glass transition temperatures of quenched blends were examined by DSC. One-phase or two-phase character of annealed and quenched blends could be determined thereby, but the small temperature difference in the Tg values (102.5 and 115.6 °C) for the pure blend components made precise detection of incipient phase separation difficult.