Terahertz time-domain spectroscopy of boron nitride nanotube-reinforced PMMA composites

Polymethyl methacrylate (PMMA) reinforced with boron nitride nanotubes (BNNT) at weight fill factors f_w of 0.2 and 0.5 wt % are characterized by terahertz time-domain spectroscopy (THz-TDS). It is known that the introduction of BNNTs in polymer composites can provide thermal conduction paths and significantly reduce interface thermal resistance. Despite the low values of f_w, the presence of the BNNTs has a significant effect on the optical constants, either due to the intrinsic contribution of the BNNTs or due to extrinsic effects such as induced strain in the PMMA matrix. Negligible THz birefringence observed in the polarization-resolved THz transmission measurements indicates a high degree of isotropy of the BNNT dispersed in the PMMA. Assuming an intrinsic effect and the validity of modified Maxwell–Garnett theory regardless of the inclusion shape, the frequency-dependent optical constants of BNNTs are extracted. The refractive index of BNNTs varies roughly linearly from n(ν=500GHz)≈1.8 to n(ν=1.5THz)≈1.75, while the absorption coefficient increases from α(ν=500GHz)≈10 cm⁻¹ to α(ν=1.5THz)≈45 cm⁻¹. The extracted THz dielectric properties of the BNNTs trend toward reported values for bulk h-BNs.