Morphology, phase diagrams, and UCST behavior in blends of polystyrene with poly(4-methylstyrene)

The upper-critical-solution-temperature (UCST) phase behavior of blends composed of two styrenic polymers, poly(4-methylstyrene) (P4MS) and polystyrene (PS), and the morphology evolution during the demixing/mixing phase transition were examined. The blend system is immiscible at ambient temperature and remains so until it becomes miscible upon heating at high temperatures, with an upper critical solution temperature (UCST) at ≈ 235°C. The UCST-phase diagrams were constructed by experimental observations of blend morphology using both optical and scanning electron microscopy. Interesting phase-in-phase micro-domains existed in the immiscible PS/P4MS blend although it exhibited a single Tg. This apparent contradiction was resolved. For expanding the interpretation of the effects of differing molecular weights, a model based on a modified Flory-Huggins expression for binary interactions was constructed to describe the UCST-type phase behavior of the PS/P4MS blend. The model predicts that the UCST is below the ambient temperature for a PS/P4MS blend system if PS possesses Mw = 20 000 g/mol or lower, which essentially means that the blends of lower PS Mw are miscible at ambient or any temperatures above. This model was verified experimentally.