Ex-situ Spectroscopic Characterization of Residual Effects of Thermomechanical Loading on Polyurea

The residual effect of thermally and mechanically loaded polyurea samples was investigated in this study using terahertz time-domain spectroscopy (THz-TDS), operating in the transmission mode. Samples of different thicknesses were submerged in liquid nitrogen and reached cryogenic isothermal condition before equilibrating at room temperature. Another set of samples were extracted from quasi-statically loaded strips. All samples were then interrogated using THz-TDS since terahertz waves exhibit nonionizing interactions with polymers, eliminating the need for any post-loading preparatory steps of the samples. The time-domain terahertz signals were used to extract the optical and electrical properties as a function of sample thickness and loading conditions. The residual effect was prominent in the mechanically loaded samples compared to a nearly negligible presence in thermally loaded ones. On average, the thermally loaded polyurea results were subtle compared to the results of the unloaded samples, whereas samples that were mechanically stretched showed a considerable difference. Spectral analysis reported the frequency-dependent, complex refractive index of virgin and loaded polyurea as a function of thickness and spectral peaks associated with fundamental vibrational modes of the polyurea structure. The spectral peaks were in good agreement with previous research while elucidating the residual effect via the disappearance of three peaks in the low terahertz regime for mechanically loaded samples. In general, the refractive index was dependent on the loading conditions. Terahertz spectroscopy was shown to be a promising tool for future in situ and in operando investigations of field-dependent polymer responses.