Polyurea, a thermoset elastomer consisting of inter-dispersed domains of hard and soft segments, is a technologically exciting material for many different implementations, particularly for impact mitigation in civilian and military applications. The novelty of this research is contingent on the exploitation of two unique experimental setups, namely, laser-induced shock waves (LSW) and terahertz time-domain spectroscopy (THz-TDS). A bulk spectroscopic technique, i.e., THz-TDS, is integrated with a laser-induced shock loading apparatus to investigate the molecular conformational changes in polyurea under ultrahigh strain rate loading. The laser-induced shock wave technique is used to simulate ballistic loading scenarios to uncover the mechanical response of polyurea under extreme loading conditions. LSW is capable of submitting the test material to strain rate exceeding 10<sup>6</sup> s<sup>-1</sup> at low strains (&lt;&lt;1%), thus, enabling the determination of the intrinsic failure modes and strength. The shock loading conditions using LSW result in ductile to brittle transition, usually causing failure within the elastic regime. Uniquely, we report plastic deformation of this elastomeric polymer, i.e., ductile failure despite the shock loading conditions. In turn, the deformation induced using the LSW changes the conformations of the polymeric molecules, which can be detected spectroscopically using terahertz waves. Therefore, the terahertz-based spectroscopy is incorporated into the shock loading experimental setup to detect the conformational changes in the molecular structure of polyurea undergoing shock loading. The spectral changes identified in the THz regime of the sample before and after shock-loading substantiate the validity of terahertz spectroscopy in explicating changes in the intermolecular conformations associated with the impact mitigating properties of dynamically loaded polyurea. Moreover, microscopic techniques, such as atomic force microscopy and scanning electron microscopy, are used to elucidate changes in the microscale properties, topography, and morphology. This research features a multiscale characterization experimental framework for the intrinsic mechanical and structural attributes of polymers, where the results can be used to accelerate and improve the development cycle of polymeric material systems through in-situ characterization.

Spectro-mechanical characterization of shock-loaded polyurea

ACS Spring 2022

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