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All-atom molecular simulation study of cellulose acetate: amorphous structure and the dissolution of small molecule

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Abstract

All-atom analysis was conducted for cellulose acetate (CA) using molecular dynamics simulation. The intermolecular interactions were elucidated at the amorphous state with degrees of acetyl substitution (DS) of 2, 2.5, and 3, and the energetics of dissolution was treated for H2O, CO2, and CH4. It was observed for the CA amorphous that DS strongly affects the hydrogen bonding among the hydroxy groups of CA and that the short-range packing of pyranose rings becomes tighter with acetylation. The free energy of dissolution was computed by the energy-representation method of solvation. The dissolution into CA was more favorable in the order of H2O > CO2 > CH4, and the DS dependence of the dissolution free energy was evident only for H2O between DS = 2 and 2.5. The roles of the intermolecular interaction components were further addressed. It was seen that the electrostatic component brings the DS dependence of the dissolution free energy for H2O as well as the difference in the affinity to CA between CO2 and CH4. The van der Waals component was still dominant for the nonpolar CO2 and CH4, and the summed contribution to it from the acetyl and main-chain groups of CA was weakly dependent on DS. The connection of the dissolution energetics with the underlying structures is also discussed.

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Data availability

The setup files for MD and free-energy calculation are available at https://doi.org/10.6084/m9.figshare.19534516.v2.

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Acknowledgments

We are grateful to Dr. Michinori Yokoi, Dr. Yoshiyuki Yamada, and Mr. Keisuke Towatari of Japan Tobacco Inc., and Dr. Hidekazu Kojima of Osaka University for valuable suggestions. The simulations were performed using OCTOPUS at the Cybermedia Center, Osaka University.

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The authors did not receive support from any organization for the submitted work and have no relevant financial or non-financial interests to disclose.

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All authors contributed to the study conception and design. Simulations were performed by RM. All authors interpreted and analyzed the data. The first draft of the manuscript was written by RM and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscription.

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Correspondence to Ryota Matsuba.

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Appendix: Molecular modeling

Appendix: Molecular modeling

The charges on the atomic sites were determined by the procedure of RESP (restrained electrostatic potential) through quantum-chemical calculations by Gaussian 16 (Frisch et al. 2016). In Methods section, we noted that 7 types of monomers were employed with varied degrees and sites of substitution. For each type of the monomers, a homo-tetramer was constructed at n = 4 with two antiparallel units in Fig. 1 and was treated at B3LYP using the 6-31G(d,p) basis set with geometry optimization (Lee et al. 1988; Becke 1993). The partial charges were then determined by RESP for all the atoms and were averaged over equivalent atoms.

To transfer the RESP charges to the MD force field, the following procedures were carried out. For the ether oxygen atoms connecting pyranose rings and those in the methoxy groups at the termini, the charges were averaged over all the ether sites in 7 types of tetramers. A single value of the charge was thus used in MD for the ether oxygen atoms between the pyranose rings and in the terminal methoxy groups. For the inner, two monomers in the tetramer treated by the quantum-chemical calculation, the charges on the same type of atoms were averaged and provided to the corresponding atoms in the inner parts of the CA molecules that were to be simulated with MD. This averaging was done not only for the atoms in the pyranose rings but also for those in the acetyl groups. The charges on the atoms in the terminal monomers and the methyl groups in the methoxy termini were transferred to the corresponding atoms in the termini of the CA molecules, furthermore. The CA molecules were not neutral right after the above and were made neutral by shifting the partial charge on each atom of CA by a uniform constant given by the excess charge divided by the number of atoms in the CA molecule.

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Matsuba, R., Kubota, H. & Matubayasi, N. All-atom molecular simulation study of cellulose acetate: amorphous structure and the dissolution of small molecule. Cellulose 29, 5463–5478 (2022). https://doi.org/10.1007/s10570-022-04616-4

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  • DOI: https://doi.org/10.1007/s10570-022-04616-4

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