Molecular dynamics simulations of bulk native crystalline and amorphous structures of cellulose

Molecular modeling has been performed on three cellulosic systems: the two native crystalline phases (Ialpha and Ibeta) and an amorphous phase, constituted by four independent microstructures. The goal of the study is to describe different organizations of the material and to emphasize how crystalline and amorphous celluloses differ. Besides, the study of the crystal structures for which many experimental data are available allows an estimation of the ability of the force field to model condensed phases of cellulose. For these organized structures the bulk parameters, such as unit cell dimensions, densities, Hildebrand solubility parameters, and hydrogen bonding, compare favorably well with the available experimental measures. The individual cellulose chains conformational behavior is as expected: torsion angles of the glycosidic bonds explore the g-values, hydroxymethyl groups are in the tg orientation, and the pyranoid ring puckering is in the C-4(1) chair form. On the contrary, all the conformational parameters of the amorphous models show large variations: preferred values of the glycosidic bond torsion angles reproduce, however, the potential energy surface of cellobiose model compound. Furthermore, the Phi torsion angle behavior is in good concordance with the exo-anomeric effect. Hydroxymethyl groups explore mostly the gg and gt orientations, and high-energy puckering of the pyran rings is stabilized within the amorphous solid. Interchain interactions on both amorphous and crystalline structures are analyzed by means of the hydrogen bonding network. Finally, estimation of the glass transition temperature of an amorphous microstructure is given.