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Effect of Soft‐to‐Hard Segment Ratio on Viscoelastic Behavior of Model Thermoplastic Polyurethanes during Phase Transitions
Author(s) -
Gadley Jesse L.,
Andrade Ricardo J.,
Maia João M.
Publication year - 2016
Publication title -
macromolecular materials and engineering
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.913
H-Index - 96
eISSN - 1439-2054
pISSN - 1438-7492
DOI - 10.1002/mame.201500450
Subject(s) - materials science , viscoelasticity , rheology , thermoplastic , crystallinity , composite material , isothermal process , strain hardening exponent , softening , dynamic mechanical analysis , phase transition , rheometry , hardening (computing) , thermodynamics , polymer , physics , layer (electronics)
Model thermoplastic polyurethanes (TPUs) are prepared with the aim of investigating the effect of soft‐to‐hard segment ratio on the phase transition and the resulting structure that forms upon isothermal exposure to temperatures near their phase transition temperature. The dynamic rheological properties of TPUs before exposure to these isothermal conditions show a phase transition at high temperatures that is directly related to the content of hard segments. The extensional viscosity data indicate a strain‐hardening behavior that becomes less pronounced with the increase of hard segments. After isothermal treatment, the DSC results show that the high‐temperature endotherm peak narrows and shifts to higher temperatures, suggesting a transition in structure. Small angle X‐ray scattering, wide angle X‐ray scattering, and atomic force microcopy results indicate a phase‐separated system in which the hard domain sizes and crystallinity change during this process. The rheological data collected after recrystallization show a significant increase in both moduli, transitioning from viscoelastic fluid‐like to glassy behavior. Concurrently, the uniaxial elongation viscosity presents a significant increase in absolute values, but with a shift from strain‐hardening to strain‐softening behavior for all strain rates. A transition from traditional phase separated viscoelastic melt behavior to more brittle rupture is also observed, marking a significant fundamental difference in properties before and after recrystallization.