Molecular modeling has been performed on three cellulosic systems: the two native crystalline phases (Iα and Iβ) 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 4C1 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 Φ 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.

The influence of temperature on structure and properties of the cellulose Ibeta crystal was studied by molecular dynamics simulations with the GROMOS 45a4 force-field. At 300 K, the modeled crystal agreed reasonably with several sets of experimental data, including crystal density, corresponding packing and crystal unit cell dimensions, chain conformation parameters, hydrogen bonds, Young's modulus, and thermal expansion coefficient at room temperature. At high-temperature (500 K), the cellulose chains remained in sheets, despite differences in the fine details compared to the room-temperature structure. The density decreased while the a and b cell parameters expanded by 7.4% and 6%, respectively, and the c parameter (chain axis) slightly contracted by 0.5%. Cell angles alpha and beta divided into two populations. The hydroxymethyl groups mainly adopted the gt orientation, and the hydrogen-bonding pattern thereby changed. One intrachain hydrogen bond, O2'H2'...O6, disappeared and consequently the Young's modulus decreased by 25%. A transition pathway between the low- and high-temperature structures has been proposed, with an initial step being an increased intersheet separation, which allowed every second cellulose chain to rotate around its helix axis by about 30 degrees . Second, all hydroxymethyl groups changed their orientations, from tg to gg (rotated chains) and from tg to gt (non-rotated chains). When temperature was further increased, the rotated chains returned to their original orientation and their hydroxymethyl groups again changed their conformation, from gg to gt. A transition temperature of about 450 K was suggested; however, the transition seems to be more gradual than sudden. The simulated data on temperature-induced changes in crystal unit cell dimensions and the hydrogen-bonding pattern also compared well with experimental results.