ABSTRACT
We investigated the vibrational density of states of sodium carboxymethyl starch (CM-starch) by terahertz (THz) time-domain spectroscopy. The CM-starch showed a broad peak at â¼3 THz. The structure of the peak was similar to those corresponding to glucose-based polymer glasses possessing hydrogen bonds. The boson peak (BP) appeared at 1.16 THz at the lowest temperature and disappeared because of the existence of excess wing at higher temperatures. However, based on our novel BP frequency determination method using the inflection point of the extinction coefficient, the BP frequency showed almost no dependence on temperature. Further, the chain length dependence of the BP frequency of the glucose-based glasses showed that the BP frequency of the polymer glass was slightly lower than that of the monomer glass. The power law behaviour of the absorption coefficient suggested the existence of fractons, and the fractal dimension was estimated to be 2.33.
Subject(s)
Terahertz Spectroscopy , Hydrogen Bonding , Starch/analogs & derivatives , VibrationABSTRACT
Complex permittivity spectra were obtained herein by performing broadband terahertz (THz) spectroscopy on cellulose, paramylon, and paramylon ester. Absorption peaks observed for cellulose and paramylon at approximately 3 THz are attributed to hydrogen bonds. In addition, a broad absorption peak around 2 THz was observed for all the polymers, demonstrating a general feature of polymer glasses derived from weak interatomic van der Waals forces. The boson peak was observed for cellulose and paramylon ester. The boson peak frequency for cellulose nearly equaled that for glassy glucose-a unit structure of the cellulose polymer. Additionally, the insensitivity of cellulose to the polymerization degree was consistent with recent results obtained via molecular dynamics simulations. In contrast, the boson peak frequency of paramylon ester was markedly smaller than that of cellulose. These results demonstrate the importance of hydrogen bonds as determinants of the boson peak frequency.
ABSTRACT
Terahertz time-domain spectroscopy and low-frequency Raman scattering were performed on the natural polymer starch to investigate the boson peak (BP) dynamics. In the infrared spectrum, the BP was observed at 0.99THz at the lowest temperature. Compared to the result from a previous study for vitreous glucose, both the frequency of the BP and absorption coefficient show lower values than those of the vitreous glucose. These behaviors originate from the longer correlation length of the medium-range order and lower concentration of hydroxyl groups in the starch. In the Raman spectrum, the BP was observed at 1.1THz at room temperature, although the BP was not observed around room temperature due to the excess wing of the fast relaxation modes in the infrared spectrum. The temperature dependence of εâ³(ν) during the heating process and cooling process shows a hysteresis below 230K. During the heating process, kinks were observed at 140K and 230K. These kinks are attributed to the ß-relaxation and the ßwet-relaxation, respectively.
