The elasticity of commonly known poly(butylene terephthalate)-poly(tetramethylene oxide) (PBT-PTMO)-based copoly(ether ester)s is increased by replacement of PTMO soft segments with poly-(ethylene oxide)-block-poly(ethylene-stat-butylene)-block-poly(ethylene oxide) (PEO-b-PEB-b-PEO) triblock copolymer soft segments containing a nonpolar middle block based on hydrogenated polybutadiene (PEB). The incorporation of this strongly incompatible PEB block resulted in the aimed increased phase separation between the PBT hard blocks and the soft segment phase, leading to a disperse PBT phase and hence to an increased elasticity. Dynamic shear experiments in combination with small-angle X-ray scattering revealed that crystallization of the PBT hard segments occurs from a microphase-separated melt. The resulting dispersed PBT hard phase in these materials is shown using transmission electron microscopy (TEM) and scanning force microscopy (SFM), whereas the increased elasticity is demonstrated using mechanical characterization. Hysteresis measurements reveal that the plastic deformation after recovery from 100% strain is only 1-6% (depending on composition) for the new PEB-containing copolyesters compared to 33% for a PBT-PTMO-based copoly(ether ester). The combination of results obtained with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) points toward a complex morphology for the PEB-containing copolyesters. Five different phases exist: a crystalline pure PBT phase, pure amorphous PEB, and PBT phases and a PEO-rich phase besides an amorphous mixed PEO/PBT phase.

The morphology of copoly(ether ester) elastomers, based on poly(butylene terephthalate) (PBT) hard blocks and poly(ethylene oxide)-block-poly(ethylene-stat-butylene)-block-poly(ethylene oxide) (PEO-b-PEB-b-PEO) soft blocks, has been investigated by various solid-state NMR methods. 13 C IRCP and 1 H T1F NMR experiments show a heterogeneity in molecular motions for the PEO and PBT segments, indicating the presence of a PEO-rich phase and a PEO/PBT mixed phase. In contrast, for the PEB segments a homogeneous NMR relaxation behavior is observed, indicating the presence of a separate pure PEB phase. Deuterium NMR spectra recorded of block copolymers with selectively deuterated PBT clearly show at least two distinct motional environments of PBT already at room temperature: a broad peak which is assigned to PBT segments in a crystalline phase and an extremely narrow peak which is assigned to highly mobile PBT segments embedded in an amorphous matrix (PBT/PEO mixed phase). For copoly(ether ester)s with a relatively high PBT content (45% (w/w)), 2 H T1 inversion-recovery experiments even reveal the presence of a "pure" amorphous PBT phase next to the PBT/PEO mixed phase. Hysteresis experiments show that copoly(ether ester)s based on PEO-b-PEB-b-PEO soft blocks have a significantly improved elastic behavior, i.e. lower plastic set, compared to that of PTMO-based copoly(ether ester)s.