THz-TDS spectra study of polymer materials with different polarity

To exploring the effect of polarity on dielectric properties of polymers, the absorption and dispersion characteristics of 5 polymers (high density polyethylene (HDPE), polyethylene terephthalate (PET), polyurethane (PU), polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF)) in terahertz band were studied with terahertz time-domain spectroscopy (THz-TDS). Theoretical analysis was conducted by fitting the experimental results to Debye formula. It was demonstrated from the study that dipolar polarization occurred and was enhanced with the increase of the polymer polarity, which is responsible for the increment of absorption coefficient (alpha), imaginary part of dielectric constant (epsilon '') and relaxation strength (Delta epsilon). It was known that the relaxation of dipole following behind the periodical variety of THz electric field was due to the medium damping effect. As a result, abnormal dispersion behavior happened. It was found that both the refractive index (n) and the real part of dielectric constant (epsilon') was decreased with the increase of frequency. The relaxation time (tau) obtained from Debye fitting was also found to increase significantly with the dipole size and the rigidity of molecular.