Molecular Dynamics Simulations of Bulk Native Crystalline and Amorphous Structures of Cellulose

Abstract

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.

Conversion of lignocellulose into renewable chemicals by heterogeneous catalysis

“…18 By milling cellulose in a ceramic pot with ZrO 2 balls for 2 days, the crystallinity index of cellulose decreased from 81% to 22%, calculated from the X-ray 35 diffraction (XRD) patterns, 7 resulting in the reduction of number of hydrogen bonds in cellulose. 19 The median diameter for the secondary particles of cellulose in water, determined by laser diffraction, also shrunk from 67 m to 42 m. Thus, we expected the improvement of the reactivity and used the ball-milled 40 cellulose hereafter.…”

Empirical and theoretical models of equilibrium and non-equilibrium transition temperatures of supplemented phase diagrams in aqueous systems (IUPAC Technical Report)

“…The same criteria to determine T g , but using NPTMD was reported for maltodecaose (w 2 = 0.84-0.99) aqueous solutions [148]; glucose and isomaltodecaose (in the pure form and hydrated with one molecule of water) [149]; pure cellulose [150]; pure myo-and neo-inositol [151]; pure glucose, sucrose, and trehalose [152]; trehalose (w 2 = 0.95-1.00) aqueous mixtures [153]; and glycerol [154]. Figure 7 shows the experimental and calculated T g of concentrated aqueous trehalose solutions as a function of concentration on the very concentrated region (w 2 > 0.92) where MD simulations are available.…”

Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

“…The following force fields, which have been parameterized to simulate the properties of organic networks, were assessed for their ability to accurately predict the structure of crystalline cellulose I b : Compass (Sun 1998;Sun et al 1998;Rigby et al 1997), Dreiding (Mayo et al 1990), Universal (Rappé et al 1992), CVFF (Hagler et al 1979a) and PCFF (polymer consistent force field) (Sun et al 1994). These force fields have all been previously used to model cellulose, see for example Eichhorn and Davies (2006), Bazooyar et al (2012), Dri et al (2015), Asensio et al (1995), and Mazeau and Heux (2003). The force field which gave the best overall prediction of lattice parameters and bond parameters (lengths and angles) was then selected for a more detailed investigation into the prediction of the elastic constants of single-crystal cellulose I b before and after unit-cell swelling.…”