Morphology of segmented polyester thermoplastic elastomers

The morphology of a new family of copolyester thermoplastic elastomers is described and the physical properties of these polymers are interpreted in terms of their unusual two‐phase domain structure. The elastomers were synthesized by the equilibrium melt transesterification of dimethyl terephthalate with 1, 4‐butanediol and poly (tetramethylene ether) glycol, resulting in a molecule in which crystallizable tetramethylene terephthalate (4GT) segments and amorphous poly (tetramethylene ether) glycol terephthalate segments are randomly distributed along the backbone. The 4GT sequences were found to crystallize as lamellar domains measuring approximately 100 Å in width and up to several thousand angströms in length. These crystalline regions are not discrete as are the polystyrene domains in the typical polystyrene‐polybutadiene‐polystyrene (SBS) triblock polymers, but rather are continuous and highly interconnected. Like the glassy regions in the SBS polymers, however, they serve as intermolecular tiepoints for the amorphous portions of the molecules and lead to the formation of a three‐dimensional network structure. At temperatures below the crystallite melting point, the material exhibits many properties characteristic of conventionally cured elastomers, but at elevated temperatures the melting of these thermally labile noncovalent junctures allows the material to flow and be processed as a thermoplastic. The stress‐strain behavior of these polymers is discussed in terms of their morphology, and thermal analysis, electron microscopy, and x‐ray diffraction results are presented.