Peculiar glass transition behavior and miscibility in a binary mixture comprising amorphous poly(ether imide) with semicrystalline poly(butylene terephthalate)

Miscibility and an asymmetric T_g-composition relationship have been demonstrated in the binary blends composed of amorphous pol(ether imide) (PEI) and semicrystalline poly(butylene terephthalate) (PBT). The peculiar T_g-composition behavior in the miscible PEI/PBT blends was analyzed using a free-volume approach, where uneven contributions to the mixtures free volumes were attributed to the vastly different crystallizing tendency of one constituent versus the other. A single T_g was observed in the PEI/PBT blends using differential scanning calorimetry (DSC) as well as dynamic mechanical analysis (DMA), which proved single relaxation and intimate mixing between these two components at the molecular scales. Additionally, homogeneous phase morphology in the blends was also substantiated using optical and scanning electron microscopy. Interactions between the polymers were investigated by analyzing the equilibrium melting point depression as well as the T_g dependence on composition.