Daichi Hayakawa

Molecular dynamics simulation of the dissolution process of a cellulose triacetate-II nano-sized crystal in DMSO.

An understanding of the dissolution process of cellulose derivatives is important not only for basic research but also for industrial purposes. We investigated the dissolution process of cellulose triacetate II (CTA II) nano-sized crystal in DMSO solvent using molecular dynamics simulations. The nano-sized crystal consists of 18 CTA chains. During the 9 ns simulation, it was observed that one chain (C01) located at the corner of the lozenge crystal was solvated by the DMSO molecules and moved away from the remaining cluster into the DMSO solvent. The analysis showed that the breakage of the interaction between the H1, H3, and H5 hydrogens of the pyranose ring and the acetyl carbonyl oxygen in the C01 and C02 adjacent chains would be crucial for the dissolution of CTA. The DMSO molecules solvating around these atoms would prevent the re-crystallization of the CTA molecules and facilitate further dissolution.

Computational study to evaluate the birefringence of uniaxially oriented film of cellulose triacetate

The intrinsic birefringence of a cellulose triacetate (CTA) film is evaluated using the polarizability of the monomer model of the CTA repeating unit, which is calculated using the density functional theory (DFT). Since the CTA monomer is known to have three rotational isomers, referred to as gg, gt, and tg, the intrinsic birefringence of these isomers is evaluated separately. The calculation indicates that the monomer CTA with gg and gt structures shows a negative intrinsic birefringence, whereas the monomer unit with a tg structure shows a positive intrinsic birefringence. By using these values, a model of the uniaxially elongated CTA film is constructed with a molecular dynamics simulation, and the orientation birefringence of the film model was evaluated. The result indicates that the film has negative orientation birefringence and that its value is in good agreement with experimental results.

Folded-chain structure of cellulose II suggested by molecular dynamics simulation.

We investigated the possibility of a folded-chain crystal of the cellulose II polymorph by molecular dynamics (MD) simulation. The molecular direction of cellulose chains in cellulose II is anti-parallel, which allows the crystal to have folded-chain packing. It is impossible for cellulose I to form such a structure due to its parallel up assembly. The folded-chain crystal of the cellulose II polymorph was suggested based on the following results: (1) the glucose residue with boat and skew boat ring conformations enabled the cellulose chain to form a hairpin turn; (2) the lattice parameters of the folded-chain crystal and original crystal were almost the same (deviations in the a, b, and γ parameters of both crystals were within 3%); (3) the folded-chain molecular sheet was as stable in a water medium as the extended-chain molecular sheet, and structural parameters such as the hydrogen bonding system and side chain conformation of both molecular sheets were almost the same, indicating that the folded-chain molecular sheet is an initial structure during crystallization of the folded-chain crystal.

Investigation of the structure and interaction of cellulose triacetate I crystal using ab initio calculations.

The crystal structure of cellulose triacetate I (CTA I) was investigated using first-principles density functional theory (DFT) calculations. The results are in good agreement with the experimental structure obtained by Sikorski et al. when performing the calculation with inclusion of the dispersion correction. However, the cell parameters calculated with inclusion of the dispersion correction are slightly smaller than those experimentally obtained, especially along the a-axis. This smaller cell parameter could be reasonably explained by considering thermal expansion effects, since optimization with the density functional calculation gives the structure without inclusion of thermal effects. The atoms-in-molecules (AIM) theory is also employed to identify and characterize interatomic interactions in the CTA I crystal. CH/O interactions sites are shown to exist in the crystal structure of CTA I. Moreover, CH/O interactions are considered the main interactions in operation to maintain the crystal structure of CTA I.

Evaluation of hydrogen bond networks in cellulose Iβ and II crystals using density functional theory and Car–Parrinello molecular dynamics

Crystal models of cellulose Iβ and II, which contain various hydrogen bonding (HB) networks, were analyzed using density functional theory and Car-Parrinello molecular dynamics (CPMD) simulations. From the CPMD trajectories, the power spectra of the velocity correlation functions of hydroxyl groups involved in hydrogen bonds were calculated. For the Iβ allomorph, HB network A, which is dominant according to the neutron diffraction data, was stable, and the power spectrum represented the essential features of the experimental IR spectra. In contrast, network B, which is a minor structure, was unstable because its hydroxymethyl groups reoriented during the CPMD simulation, yielding a different crystal structure to that determined by experiments. For the II allomorph, a HB network A is proposed based on diffraction data, whereas molecular modeling identifies an alternative network B. Our simulations showed that the interaction energies of the cellulose II (B) model are slightly more favorable than model II(A). However, the evaluation of the free energy should be waited for the accurate determination from the energy point of view. For the IR calculation, cellulose II (B) model reproduces the spectra better than model II (A).

Ab initio studies on the structure of and atomic interactions in cellulose III I crystals

The crystal structure of cellulose III(I)was analyzed using first-principles density functional theory (DFT). The geometry was optimized using variable-cell relaxation, as implemented in Quantum ESPRESSO. The Perdew-Burke-Ernzerhof (PBE) functional with a correction term for long-range van der Waals interactions (PBE-D) reproduced the experimental structure well. By using the optimized crystal structure, the interactions existed among the cellulose chains in the crystal were precisely investigated using the NBO analysis. The results showed that the weak bonding nature of CH/O and the hydrogen bonding occur among glucose molecules in the optimized crystal structure. To investigate the strength of interaction, dimeric and trimeric glucose units were extracted from the crystal, and analyzed using MP2 ab initio counterpoise methods with BSSE correction. The results estimated the strength of the interactions. That is, the packed chains along with a-axis interacts with weak bonding nature of CH/O and dispersion interactions by -7.50 kcal/mol, and two hydrogen bonds of O2HO2…O6 and O6HO6…O2 connect the neighboring packed chains with -11.9 kcal/mol. Moreover, FMO4 calculation was also applied to the optimized crystal structure to estimate the strength of the interactions. These methods can well estimate the interactions existed in the crystal structure of cellulose III(I).