ABSTRACT:The miscibility in the pairs of homologous series of semicrystalline polyesters: poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET) and poly(trimethylene terephthalate) (PTT) and poly(butylene terephthalate) (PBT), respectively, were investigated as model systems. In this study, two binary blend systems each containing two semicrystalline aryl-polyesters differing by the number of methylene units, were found to be fully miscible (in absence of trans-esterifications) in accordance with criteria of microscopy morphology and thermal transition. In addition, only a single composition-dependent T cc was taken as a valid supportive evidence for miscibility in blends of two crystallizable polymers. The claimed miscibility applied to the quenched state of the blends, and not the crystallized domain involving the crystal cells in the crystalline regions. The crystal morphology evidence showed that the unit cell types in the miscible blends remained individually different, and co-existed in common spherulites.KEY WORDS Poly(trimethylene terephthalate) (PTT) / Poly(ethylene terephthalate) (PET) / Poly(butylene terephthalate) (PBT) / Miscibility / Miscibility is not always common in the blends of homologous polymers that differ in structures but possess the same functional groups. Studies of polymer miscibility have been more commonly focused on polymers of different structures, especially those possessing potential sites for specific interactions. By comparison, less has been reported on the phase behavior of blends comprising polymers with similar or homologous structures. Conventional notions have been that a slight offset in the chemical structures of the constituent polymers may lead from miscibility to phase separation, or vise versa, in the mixtures that comprise the polymers pairs whose structures are similar but offset by a simple chemical unit. Consequently, miscibility in polymers with similar structures has been an interesting subject of studies. It has been known that polymers of similar chemical structures (e.g., homologous series of polymers) are not always likely to form a miscible mixture. In general, polymers in homologue series differing by a methylene or methyl unit are usually not miscible, which is especially true for polymers that do not exhibit any polar forces or specific interactions. For examples, high-density polyethylene (HDPE) is generally not miscible with low-density polyethylene (LDPE), which differs from the former only by random and occasional pendant CH 3 groups in the main chains. It is also classically known that polyethylene (-[CH 2 -CH 2 ]-) and polypropylene (-[CH 2 -CH(CH 3 )]-), which differ from each other only by a pendant methyl group in the repeat unit, are not miscible. Further examples are abundant in recent papers in the literature. Blend systems comprising of polystyrene (PS) and poly(α-methyl styrene) (PαMS), whose repeat units differ only by a methyl group (-H vs. -CH 3 ) in the α-position, have been extensively studied. 1-6 PS and PαMS...

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A ternary miscible blend system comprising only crystallizable aryl polyesters [poly(ethylene terephthalate), poly(trimethylene terephthalate), and poly(butylene terephthalate)] was characterized with the criteria of thermal analyses, microscopy, and X‐ray characterizations. The reported ternary miscibility (in the quenched amorphous state of blends of the three aryl polyesters) was truly physical and under the condition of no chemical transesterifications; this justified that transesterification was not a necessary condition for miscibility in polyester blends in this case. This study further proposed and tested a novel concept of a new criterion for miscibility characterization for polymer blends of only crystallizable polymers. A single composition‐dependent cold‐crystallization‐temperature (Tcc) peak in blends of only semicrystalline polymers was taken as an indication of an intimate mixing state of miscibility. The theoretical background for establishing the single composition‐dependent Tcc peak as a valid miscibility criterion for crystallizable polymer blends was examined. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2394–2404, 2003