Boson peak analysis of sodium carboxymethyl starch glass by terahertz time-domain spectroscopy

For glass forming materials, universal excitation called boson peak (BP) is observed in the terahertz region. On the other hand, in polymer glass, there is fractal dynamics, so-called fracton, which is expected to appear above the BP frequency as a result of self-similarity of monomer molecules and anomalous diffusion process. In this study, we performed terahertz time-domain spectroscopy on microwave-treated sodium carboxymethyl starch (CM-Starch), to detect the BP and fracton. The BP of the CM-Starch appeared around 1.2 THz at lowest temperature in the α(ν)/ν² plot, where α(ν) is the absorption coefficient. The peak in the α(ν)/ν² plot disappeared due to the excess wing at high temperature. Therefore, we propose new determination of the BP frequency by inflection point of the extinction coefficient. In addition, the exponential behavior of the α(ν) above the BP frequency suggested the existence of a fracton, and the fractal dimension was estimated to be 2.17.