EXPRESS: Percolation-Governed Transitions in Recycled Epoxy Composites: A Terahertz Spectroscopy Framework for
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This work utilized terahertz time-domain spectroscopy (THz-TDS) and Fourier Transform Infrared (FTIR) spectroscopy to characterize epoxy blends. The epoxy blends were prepared by blending a virgin DER-332/LC-100 epoxy system with Recovered Epoxy Polymer (REP) at 1-10 wt%, and were compared with references of virgin epoxy and REP. The REP was obtained via peracetic acid digestion of Carbon Fibre Reinforced Polymer (CFRP), followed by purification. Using THz-TDS amplitude-phase analysis, five optical parameters were obtained: refractive index (n), extinction coefficient (κ), absorption coefficient (α), and the real and imaginary of the dielectric constant (ε' and ε″). Under ambient conditions, these parameters extracted from THz-TDS were unable to distinguish low REP content 0-10% REP but were able to clearly differentiate 100% REP from all other epoxy blends. Compared to the other blends, 100% REP shows markedly increased n, κ, α, ε', and ε″, attributed to oxidation, formation of polar groups, chain scission and microvoid-induced scattering. To distinguish low REP-content blends using THz-TDS, external perturbation was applied. Under perturbation, linear concentration-dependent trends in Δn, Δκ, Δα, Δε' and Δε'' were observed. The external perturbation enhanced the interaction between the samples and the THz wave, likely due to amplification of dipole relaxation. Additionally, the transmission spectra (3500-7500 cm⁻1) obtained from FTIR increased as the REP content increased from 0-10%. This concentration dependent trend can be attributed to partial network dilution and reduced overtone absorption. However, the transmission of 100% REP showed strong attenuation due to structural disorder and enhanced scattering.
