Morphology, Surface Structure, and Elastic Properties of PBT-Based Copolyesters with PEO-b-PEB-b-PEO Triblock Copolymer Soft Segments

Abstract

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.