Microstructure and Phase Behavior of Block Copoly(ether ester) Thermoplastic Elastomers

The microstructure of segmented block copoly(ether esters) composed of poly(tetramethylene oxide) (PTMO) “soft” blocks and poly(butylene terephthalate) (PBT) “hard” blocks was investigated. A variety of analytical techniques, including 13C solid-state NMR, infrared spectroscopy, dynamical mechanical analysis, dielectric spectroscopy, differential scanning calorimetry, and transmission electron microscopy, were applied. The samples vary in the amount (35−60 wt %) and block length (1000−2000 g/mol) of the soft component. It is generally assumed in the literature that copoly(ether esters) have a two-phase structure consisting of a crystalline PBT phase surrounded by an amorphous phase which is a homogeneous mixture of PTMO soft segments and amorphous PBT segments. Our experimental results reveal that the amorphous phase is not a homogeneous mixture of “hard” and “soft” segments but consists of a highly mobile “PTMO-rich phase” and a less mobile “PBT/PTMO mixed phase”. The extent of microphase separation in the amorphous phase appeared to be strongly dependent on the block length and composition. Those samples that revealed a strong microphase separation showed strain-induced crystallization of the soft segments upon mechanical deformation.