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

Abstract

The residual effect of thermally and mechanically loaded polyurea samples was investigated in this study using terahertz time-domain spectroscopy (THz-TDS). Samples of different thicknesses were submerged in liquid nitrogen and allowed to reach cryogenic isothermal conditions while another set of samples were extracted from quasi-statically loaded strips. All samples were interrogated using THz-TDS since terahertz waves exhibit non-ionizing, nondestructive interactions with polymers. 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 polyurea. On average, the results of the thermally loaded samples were subtle when compared to the virgin samples, whereas samples that were mechanically stretched showed a considerable difference in the characteristics of the time-domain signals. For example, the peak amplitude in the time-domain signal of the stretched thick sample showed a 9% difference from that of the virgin sample, whereas the thermally loaded sample saw only a 4.9% 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 disappearance of three spectral peaks, 0.56 THz, 0.76 THz, and 0.95 THz, elucidated the residual effect of the mechanically loaded samples. In general, terahertz spectroscopy was shown to be a promising tool for future in situ and in operando investigations of field-dependent polymer responses.