In-operando spectroscopic interrogation of macromolecular conformational changes in polyurea elastomers under high strain rate loading

Abstract

Temporary and permanent macromolecular conformational changes can accompany the deformation of elastomers under high strain rate loading. Mechanical failure can occur as spallation, volumetric cracking, subsurface morphological changes, and plastic deformations. While high strain rate loading has been extensively reported using various loading mechanisms, where the current state-of-the-art relies on cascading failure and spectroscopic analyses after mechanical loading. In recent years, in-situ spectro-mechanical characterization, entailing concurrent spectroscopic interrogation and mechanical loading, has interested the scientific community in avoiding destructive evaluations in favor of noninvasive characterization, preferably during loading. To overcome the current limitations, this paper reported the first in-operando spectro-mechanical characterization of elastomeric polymers (polyurea is used as a representative material) loaded at high strain rate using bulk terahertz spectroscopy synchronized in real-time with laser-induced shock wave setup. Spectroscopic terahertz signals were collected concurrently with the imposition of shock waves based on the exfoliation of a sacrificial metallic layer using a high-energy laser pulse with nanosecond duration. The shock-loaded samples were also characterized using the scanning electron microscope, revealing signs of plastic deformations and morphological failure throughout the cross-section, including evidence of crazing and vitrification separately. Multifaceted time and frequency domain analyses elucidated the conformational changes, including spectral peak shifting, enhancement, manifestation, and concealment. The time domain analysis leveraged the dynamic time wrapping approach to quantify the temporal disparity between terahertz signals collected from unloaded, during shock, and loaded samples by calculating the Euclidean distances among signal pairs. Microscopy revealed morphological changes that corroborated the terahertz spectral differences at several energy fluences. Finite element analysis was performed to assess the levels of stresses and strains as a function of the energy fluence from focusing the high-energy laser illumination onto the sacrificial energy layer. The stresses at the depths of failure determined using electron microscopy, corresponded to the tensile strength of the material. The present results demonstrate the viability of the spectro-mechanical characterization of polymers using terahertz-based spectroscopy and laser-induced shock wave, contributing to a new experimental paradigm in polymer mechanics under shock loading.