The miscibility of blends of isotactic polypro- pylene and propylene-1-hexene (PH) copolymers with 11 and 21 mol% of 1-hexene (PH11 and PH21, respectively) has been studied theoretically and using DSC, DMA, and AFM techniques. Using experimental PVT data, the solu- bility parameter approach leads to a critical difference in 1-hexene content for melt miscibility of 17mass% (*11 mol%) at 200 °C and 0.1 MPa. The theoretical window for miscibility is in close agreement with thermal properties of the blends. The glass transition (Tg) of mis- cible blends (iPP/PH11 and PH11/PH21) decreases pro- portionally to the content of PH having the lowest Tg, while immiscible blends (iPP/PH21) display invariable Tg with blend composition. The same trend was extracted from the analysis of the b-relaxation by dynamic mechanical ana- lysis. Room temperature AFM images of blends quenched from 200 °C into liquid nitrogen confirm phase segregation of iPP/PH21 in domains of 1–5 microns, while the AFM images of iPP/PH11 and PH11/PH21 lack any obvious signature of phase separation prior to crystallization.

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