Tunable microstructures and mechanical deformation in transparent poly(urethane urea)s

Transparent poly(urethane urea) (TPUU) materials offer an avenue to enable material designs with potential to achieve simultaneous enhancements in both physical and mechanical properties. To optimize the performance required for each application, the molecular features that influence the microstructure, the glass transition temperature (Tg), the deformation mechanisms, and the mechanical deformation behavior must be understood and exploited. In this work, a comprehensive materials characterization of select model PUUs with tunable microstructures is addressed. Increasing the hard segment (HS) content increases the stiffness and flow stress levels, whereas altering the soft segment (SS) molecular weight from 2000 to 1000 g/mol leads to an enhanced phase mixing with a SS Tg shifted ∼17 °K toward higher temperatures as well as broadening of the SS relaxation closer to room temperature. As a result, the 1K TPUU materials display greater rate‐dependent stiffening and strain hardening on mechanical deformation over the broad range of strain rates covered in this work (10⁻³ to 10⁴ s⁻¹). In such case of similar urea‐based HS content, the molar content of the urethane linkages, per stoichiometric requirements, is much higher in the 1K TPUUs than the 2K TPUUs. These additional urethane moieties lead to an increase in the extent of intermolecular interactions, via hydrogen bonding between the HS and the SS, providing not only further phase mixing and stronger rate sensitivity but also provide 1K TPUUs with drastically improved barrier properties.