Nanoindentation Creep Behavior and Local Rheological Response of Biaxially Stretched TPU Films

ABSTRACT

Biaxial stretching alters the microphase‐separated structure of thermoplastic polyurethane (TPU) films, yet its effect on localized time‐dependent contact behavior remains unclear. This study examines nanoindentation creep in biaxially oriented TPU films using a flat‐ended punch while varying the stretching ratio, loading rate, and peak load. Two‐dimensional and one‐dimensional X‐ray diffraction (XRD) and differential scanning calorimetry provide structural context. Holding‐stage displacement is analyzed over a common 0–60 s interval using a rate‐based operational decomposition. A two‐term generalized Kelvin model describes the delayed response phenomenologically: pre‐stretching increases its fitted magnitude, whereas loading rate changes its temporal distribution. Model‐derived compliance, retardation spectra, and loss factors enable comparisons across conditions. Two‐dimensional XRD patterns retain diffuse rings without a new crystalline phase, while the broad maximum near 21°–22° becomes more pronounced with stretching, consistent with enhanced local segmental ordering. An effective activation‐volume parameter derived from indentation rate sensitivity varies with pre‐stretching and contact intensity. These results establish condition‐dependent links between biaxial pre‐orientation, structural ordering, and localized indentation creep, with molecular interpretations remaining hypotheses.