Relationship between modulus and structure of annealed thermoplastic polyurethane

Relationship between elastic modulus and structure of an annealed thermoplastic polyurethane (TPU) has been investigated by small-angle X-ray scattering (SAXS), tensile testing, and dynamic mechanical thermal analysis (DMTA). With increasing annealing temperature (Ta) up to 125 °C, volume fraction of hard segment (HS)-rich domain increased, which is usually expected to increase elastic modulus. However, the elastic modulus at 25 °C decreased with increasing Ta. Due to the decrease in number density of the HS-rich domain acting as the cross-links, the glass transition temperature (Tg) of the soft segment (SS)-rich matrix decreased with increasing Ta. Nevertheless, the change in the Tg contributed only slightly on the change in the elastic modulus at 25 °C. On the other hand, the electron density difference between the HS-rich domain and the SS-rich matrix decreased with increasing Ta, suggesting that the densification in the HS-rich domain decreased with Ta. The modulus of the HS-rich domain calculated by the micromechanical approach with parallel model using the structural parameters obtained from SAXS analysis decreased with Ta. Thus the decrease of the modulus with increasing Ta is mainly ascribed to the decrease in the modulus of the HS-rich domain due to the decrease in the densification in the HS-rich domain.

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