In situ peak force quantitative nanomechanical mapping (PF-QNM) atomic force microscopy (AFM) was used to observe the onset of strain-induced crystallization (SIC) within polyurethane (PU) and its evolution under increasing uniaxial tensile strain. Notched and continuously tapered dogbone specimens were fabricated from a cast PU film, and subsequently annealed in a vacuum oven at 75 °C to remove any previous thermal history. Derjagin, Muller, Toropov (DMT) modulus maps of the surface ~150 μm in front of the crack tip exhibited a band-like structure perpendicular to the direction of applied load at ~10 % far-field strain, which suggests a change in the morphology of PU due to SIC. Furthermore, in the continuously tapered dogbone specimen, a significant change in the surface modulus of PU was observed at ~100% strain and continued to increase up to 300% far-field strain. To the best of our knowledge, this is direct nanoscale visualization of morphological and local mechanical property evolution as a result of SIC via in situ AFM. The phase transformation is further corroborated with differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) data, which show the formation of highly packed and ordered crystalline domains. A molecular-continuum model suggests that the local variation of Young’s modulus in PU is due to SIC. Strain energy density fields were created by parameterizing AFM DMT modulus maps using a finite element model (FEM), which allows for the estimation of macroscopic deformation ahead of a crack tip.

Monitoring the onset of strain induced crystallization in thermoplastic polyurethane via in situ peak force quantitative nanomechanical mapping atomic force microscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.