Anisotropic optical response of optically opaque elastomers with conductive fillers as revealed by terahertz polarization spectroscopy

Abstract

Elastomers are one of the most important materials in modern society because of the inherent viscoelastic properties due to their cross-linked polymer chains. Their vibration-absorbing and adhesive properties are especially useful and thus utilized in various applications, for example, tires in automobiles and bicycles, seismic dampers in buildings, and seals in a space shuttle. Thus, the nondestructive inspection of their internal states such as the internal deformation is essential in safety. Generally, industrial elastomers include various kinds of additives, such as carbon blacks for reinforcing them. The additives make most of them opaque in a wide spectral range from visible to mid-infrared, resulting in that the nondestructive inspection of the internal deformation is quite difficult. Here, we demonstrate transmission terahertz polarization spectroscopy as a powerful technique for investigating the internal optical anisotropy in optically opaque elastomers with conductive additives, which are transparent only in the terahertz frequency region. The internal deformation can be probed through the polarization changes inside the material due to the anisotropic dielectric response of the conductive additives. Our study about the polarization-dependent terahertz response of elastomers with conductive additives provides novel knowledge for in situ, nondestructive evaluation of their internal deformation.

Internal triaxial strain imaging of visibly opaque black rubbers with terahertz polarization spectroscopy

“…[14][15][16][17][18][19][22][23][24] Moreover, very recently, we found that the conductive carbon black aggregates embedded in fluoroelastomers show large optical anisotropy that is governed by the alignment of the anisotropically shaped carbon black aggregates. 24 It is interesting that this optical anisotropy can be strongly modulated by deformation of the fluoroelastomer-carbon-black composites. This experimental result suggests that the strain in black rubbers can be probed through the change in the optical anisotropy.…”

“…[13][14][15][16][17][18][19][20][21][22][23][24][25] Since terahertz waves have relatively low photon energies (∼meV), they can penetrate the visibly opaque materials without damage. Among the visibly opaque materials, black rubbers, such as styrene-butadiene rubbers and fluoroelastomers, are one of the most important materials for our modern civilization because they are widely used in tires, vibration absorbers, and also as sealing materials.…”

“…The PS THz-TDS experiment was performed using a commercially available high-speed THz-TDS system (T-ray 5000, Advanced Photonix, Inc.) in combination with a rotating wire-grid polarizer. 24,26,27 The schematic view of the experimental setup is shown in Fig. 1.…”

“…The frequency-dependent polarization is essential to determine the birefringent properties of the sample as described in the previous study. 24 The beam spot of the terahertz pulse was measured carefully by the knife-edge method and was estimated to be 2-3 mm at 0.2-0.4 THz. In our terahertz method, we measured the polarization of the terahertz pulse with and without the sample, and then we obtained the angle of the slow optic axis [θ(ω)] of the sample with regard to the x-axis and the phase retardation [∆(ω)] between the two optic axes.…”

“…The details of the experimental setup and numerical analysis to determine θ(ω) and ∆(ω) are described elsewhere. 24 In PS THz-TDS, both the thickness of the sample (t) and the refractive indices along the two optic axes were evaluated by using a modified total variation metric, 28 which measures the smoothness of the complex refractive-index functions along the two optic axes to identify which thickness and complex refractive index pair are more appropriate for the measured sample. 28 After calculating the degree of birefringence [∆n(ω)] by using the relation ∆n (ω) = ∆(ω)c tω , where c is the speed of light, we averaged θ and ∆n over the frequency range 0.2-0.4 THz.…”