Elsevier

Carbohydrate Polymers

Volume 232, 15 March 2020, 115789
Carbohydrate Polymers

Molecular vibration and Boson peak analysis of glucose polymers and ester via terahertz spectroscopy

https://doi.org/10.1016/j.carbpol.2019.115789Get rights and content

Highlights

  • Vibrational dynamics of three carbohydrate polymers in the THz region were studied.

  • Boson peak frequency of cellulose closes to that of glassy glucose.

  • Boson peak frequency of paramylon ester decreased compared to that of cellulose.

  • Hydrogen bond interaction is suggested as a determinant of the boson peak frequency.

Abstract

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.

Introduction

In recent years, increased efforts have been made to develop bioplastics based on polysaccharides, such as cellulose and paramylon, to replace petroleum-based polymers. Cellulose and paramylon are biopolymers having the same chemical notation (C6H10O5)n, but their glycosidic bonds and chain structures differ considerably. Cellulose comprises a linear structure with β-(1, 4) glucose units, whereas paramylon possesses a special triple-helical structure bonded by β-(1, 3) glycosidic bonds. Chemical structures of cellulose and paramylon are depicted in Fig. 1. Paramylon is stored within paramylon granules of the Euglena gracilis and constitutes nearly 25 % of the cell weight. Paramylon has been demonstrated to have the highest level of crystallinity (of the order of 90 %) amongst all known native cellulosic materials (Chuah, Sarko, Deslandes, & Marchessault, 1983; Marchessault & Deslandes, 1979). Further, paramylon derivatives have been reported to present excellent thermal properties such as a high thermal-degradation temperature, good thermoplastic features, visible-light transparency, and high thermal stability (Gan et al., 2017). Gan et al. (2017) also demonstrated that paramylon derivatives become amorphous when suitable side alkyl chains are introduced into the paramylon structure. This transformation from the crystalline form to the amorphous form provides a comprehensive model to analyze the glass-forming system, thereby obtaining insight into the interchain and intermolecular dynamics of polymers.

Several researchers have attempted to investigate the specific properties of paramylon and paramylon esters. Specifically, these properties have been detected and analyzed using various techniques, such as X-ray diffraction (XRD) spectroscopy (Gan et al., 2017), nuclear magnetic resonance (NMR) (Gan et al., 2017; Shibakami, Tsubouchi, & Hayashi, 2014), differential scanning calorimetry (DSC) (Gan et al., 2017), and Fourier transform infrared (FTIR) spectroscopy (Shibakami & Sohma, 2018). However, a paucity of investigations have been conducted concerning the use of terahertz (THz) spectroscopy to analyze paramylon and paramylon esters.

THz radiation refers to electromagnetic waves with frequencies in the range of 0.1–10 THz (with corresponding wavelengths between 30 μm and 3 mm). On the electromagnetic spectrum, the THz range is located between microwave and infrared (IR) radiation (Sirtori, 2002; Taday, 2003). Numerous studies have demonstrated that the THz spectrum provides unique information concerning intermolecular bonding, vibrations, and rotations (Ge, Liu, Ma, & Wang, 2009; Rungsawang, Ueno, Tomita, & Ajito, 2006; Takahashi, 2014). This technique facilitates the investigation of molecular structures and functions of different materials, especially the hydrogen-bonding network. (Fitzgerald, Cole, & Taday, 2005; Taday, Bradley, Arnone, & Pepper, 2003). Normally, materials in the solid phase are bonded by strong intramolecular forces, and their corresponding vibrational modes can be detected via IR spectroscopy. By contrast, weak intermolecular interactions are located in the lower-frequency region, the THz region. Furthermore, at a frequency of approximately 1 THz, the IR spectra of glass-forming substances demonstrate an excitation, called the universal boson peak (BP) (Kabeya et al., 2016; Mizuno, Shiba, & Ikeda, 2017; Nakayama, 2002).

In three-dimensional (3D) materials, the BP can be observed in the spectrum obtained by dividing the vibrational density of states (VDOS) by the square of the frequency g(ν)/ν2, thereby implying a deviation from the 3D Debye model for acoustic waves. Popular methods for BP detection include inelastic neutron or X-ray scattering and nuclear resonance scattering (Baldi, Giordano, Monaco, & Ruta, 2010; Buchenau et al., 1986; Chumakov et al., 2011; Rufflé, Parshin, Courtens, & Vacher, 2008), low-frequency Raman scattering (Hédoux, Paccou, Guinet, Willart, & Descamps, 2009; Kojima, 1993; Malinovsky & Sokolov, 1986; Surovtsev & Sokolov, 2002), and low-temperature specific-heat measurement (Carini, Carini, Cosio, D’Angelo, & Rossi, 2016; Crupi, D’Angelo, Wanderlingh, Conti Nibali, & Vasi, 2010). However, the BP properties of amorphous materials, especially organic compounds and polymers, are rarely investigated via IR spectroscopy (Kabeya et al., 2016; Naftaly & Miles, 2005; Shibata, Mori, & Kojima, 2015; Sibik & Zeitler, 2016). The interpretation of the broad THz spectrum is generally more difficult than the assignment of the crystal modes. It is not well known that BP detection can be accomplished via IR (i.e., THz) spectroscopy—a method complementary to Raman scattering (Kabeya et al., 2016; Terao et al., 2018).

For BP detection via THz spectroscopy, the relation α(ν) = CIR g(ν) can be deduced by transforming the linear-response theory for amorphous systems (Galeener & Sen, 1978; Kabeya et al., 2016; Shibata et al., 2015; Taraskin, Simdyankin, Elliott, Neilson, & Lo, 2006; Terao et al., 2018) into the following form whilst considering that for 3D materials, BP appears in the function g(ν)/ν2.α(ν)ν2=CIRg(ν)ν2

In Eq. (1), CIR denotes the IR light–vibration coupling coefficient. This implies that in THz spectroscopy, BP does not appear as a direct peak in the absorption coefficient. Instead, it takes the form α(ν)/ν2.

The occurrence of BP has previously been extensively studied via experiments (Kabeya et al., 2016; Terao et al., 2018; Violini, Orecchini, Paciaroni, Petrillo, & Sacchetti, 2012) as well as theoretically (Matic et al., 2004; Ruzicka et al., 2004). Recently, several studies involving molecular-dynamic (MD) simulations have been performed using coarse-grain models to understand BP manifestation in polymeric glasses (Giuntoli & Leporini, 2018; Milkus et al., 2018; Tomoshige, Mizuno, Mori, Kim, & Matubayasi, 2019). Therefore, to gain better insight into the origin of BP in polymer glass and determinants governing BP frequency, the experimental results of the THz spectrum of polymer glasses must be interpreted using the results of MD simulations.

In this study, THz spectroscopy was performed on three hydrocarbon polymers—cellulose, paramylon, and paramylon ester—to investigate the origin of the absorption features and the BP. Absorption spectra measured at room temperature using terahertz time-domain spectroscopy (THz-TDS) and FTIR were combined to determine the absolute value of the absorption coefficient over a frequency range of 0.2–10 THz. The broadband-absorption features of the three polymers were analyzed focusing on absorption peaks observed at approximately 2 THz and 3 THz. Additionally, the complex dielectric constant below 2 THz frequency was analyzed, and the BP behavior was examined. A large difference was observed between the BP-frequency values of cellulose and paramylon ester. Lastly, the origin of the BP-frequency determinant for these three polymers was also investigated.

Section snippets

Experimental section

Paramylon powder was extracted from Euglena gracilis cultured at the Algae Biomass and Energy System Research Development Center (ABES). Paramylon ester (purchased from NEC Global, Japan) is a chemical substance synthesized using a typical esterification method using propionyl chloride and long-chain acid chloride, as described in Fig. 2. The degree of substitution (DS) was used as a parameter to describe the substitution level, and the DS could be defined as the integration ratio of the number

Analysis of XRD patterns

XRD patterns for paramylon and paramylon-ester samples demonstrated similar diffraction spectra, as illustrated in Fig. 3. In the paramylon XRD spectrum, three distinguished and sharp peaks were observed at 2θ ≈ 7°, 19°, and 20°. In contrast, the XRD spectrum for paramylon ester showed two broad peaks at 2θ ≈ 8.7° and 2θ ≈ 20.4°. The occurrence of broad peaks can be attributed to the esterification of paramylon and transformation of its crystalline structure to a more disordered amorphous

Conclusions

In this study, by combining the THz-TDS and FTIR systems, spectral shapes and absolute values of the absorption or extinction coefficients for cellulose, paramylon, and paramylon-ester were determined at room temperature. It has been concluded that absorption bands observed for cellulose and paramylon at approximately 3 THz are caused by hydrogen bonds. The absolute value of the absorption coefficient for paramylon ester was much lower than that of the other polymers owing to its lack of a

CRediT authorship contribution statement

Junlan Zhong: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization, Software. Tatsuya Mori: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Funding acquisition. Yasuhiro Fujii: Conceptualization, Methodology. Takanari Kashiwagi: Validation. Wakana Terao: Data curation. Hidotoshi Minami: Validation. Manabu Tsujimoto: Validation. Makoto M. Watanabe: Supervision. Kazuo Kadowaki: Supervision, Writing - review & editing, Funding

Acknowledgments

We are grateful to Dr. M. Yoshida in ABES Center for stimulating discussions during this work. We also thank Prof. I. Suzuki and collaborators of his laboratory for many useful discussions. This work has been supported by JSPS KAKENHI Grand Number 15H01996, 17K14318, and 18H04476. This work was also partially supported by the Asahi Glass Foundation.

References (52)

  • M. Shibakami et al.

    Thermoplasticization of euglenoid β-1,3-glucans by mixed esterification

    Carbohydrate Polymers

    (2014)
  • M. Shibakami et al.

    Thermal, crystalline, and pressure-sensitive adhesive properties of paramylon monoesters derived from an euglenoid polysaccharide

    Carbohydrate Polymers

    (2018)
  • T. Shibata et al.

    Low-frequency vibrational properties of crystalline and glassy indomethacin probed by terahertz time-domain spectroscopy and low-frequency Raman scattering

    Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

    (2015)
  • P.F. Taday et al.

    Using terahertz pulse spectroscopy to study the crystalline structure of a drug: A case study of the polymorphs of ranitidine hydrochloride

    Journal of Pharmaceutical Sciences

    (2003)
  • W. Terao et al.

    Boson peak dynamics of natural polymer starch investigated by terahertz time-domain spectroscopy and low-frequency Raman scattering

    Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

    (2018)
  • G. Baldi et al.

    Sound attenuation at terahertz frequencies and the boson peak of vitreous silica

    Physical Review Letters

    (2010)
  • M. Bernier et al.

    Determining the complex refractive index of materials in the far-infrared from terahertz time-domain data

    Terahertz spectroscopy-cutting edge technology

    (2017)
  • U. Buchenau et al.

    Low-frequency modes in vitreous silica

    Physical Review B

    (1986)
  • G. Carini et al.

    Low temperature heat capacity of permanently densified SiO2 glasses

    Philosophical Magazine

    (2016)
  • C.T. Chuah et al.

    Packing analysis of carbohydrates and polysaccharides. Part 14. Triple-helical crystalline structure of curdlan and paramylon hydrates

    Macromolecules

    (1983)
  • A.I. Chumakov et al.

    Equivalence of the boson peak in glasses to the transverse acoustic Van Hove singularity in crystals

    Physical Review Letters

    (2011)
  • C. Crupi et al.

    Low frequency dynamics of lysozyme: A Raman scattering and low temperature specific heat study

    Journal of Spectroscopy

    (2010)
  • B. Fischer et al.

    Chemical recognition in terahertz time-domain spectroscopy and imaging

    Semiconductor Science and Technology

    (2005)
  • F.L. Galeener et al.

    Theory for the first-order vibrational spectra of disordered solids

    Physical Review B

    (1978)
  • M. Ge et al.

    Polymorphic forms of furosemide characterized by THz time domain spectroscopy

    Bulletin of the Korean Chemical Society

    (2009)
  • A. Giuntoli et al.

    Boson peak decouples from elasticity in glasses with low connectivity

    Physical Review Letters

    (2018)
  • Cited by (0)

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