The crystallization behavior and the mechanical properties of segmented poly(ether ester) thermoplastic elastomers

The isothermal melt crystallization of three segmented copoly(ether ester)s based on PBT as the hard and PTMO as the soft segments differing in their hard segment content was investigated. The kinetics of crystallization was studied using polarizing microscopy and DSC. Two regimes of crystallization were observed. Spherulitic structures of low crystallinity are formed via predominantly athermal nucleation in regime I at supercoolings ΔT_u < 30°C. Ill-defined aggregations of lamellae grow at ΔT_u > 30°C. The crystallization follows an apparent Avrami-equation; the Avrami-constants cannot be explained in terms of a simple model. The structure of the samples was investigated by SAXS and compared to the structure of samples crystallized by rapid quenching and subsequent annealing. Much higher long spacings are obtained on isothermal crystallization. The long spacing increases with decreasing ΔT_u. It increases at constant ΔT_u with decreasing average hard segment length contrary to the samples crystallized by subsequent annealing of the quenched melt. In this case polymers differing in composition do not differ with regard to the long spacing obtained at constant ΔT_u. The modulus in the Hookean-range as derived from stress-strain curves at T > T_g does not reflect the details of the morphology or chain architecture but it is found to depend logarithmically on the volume fraction of crystallinity in the sample. Data of pure PBT are described by the same relationship.