Completely miscible blend of poly(trimethylene terephthalate) with poly(ether imide)

Miscibility in the amorphous region was discovered for the first time in a binary blend system composed of crystalline low-T_g poly(trimethylene terephthalate) (PTT) and amorphous high-T_g poly(ether imide) (PEI). The blends of all compositions exhibit a distinctly single T_g, whose temperature as well as broadening extent is dependent on the composition. Additionally, homogeneous phase morphology in the blends was also substantiated using optical and scanning electron microscopy results. Interestingly, discontinuity (a cusp near volume fraction of 0.5) and general asymmetry were noted in the T_g-composition relationship for this miscible blend system. This phenomenon is interesting but quite puzzling. The peculiar T_g-composition behavior in the miscible PTT/PEI blends was analyzed using a classical free-volume approach, where uneven contributions to the polymer mixtures' free volume were partially attributed to the vastly different T_g's and free volumes of these two constituents. Furthermore, it was found that varying degrees of blend's T_g broadening with respect to the composition might have also contributed to the noted discontinuity. The asymmetry was partially attributed to the PTT crystalline domain-induced variation in the intermolecular interactions.