Cellulose is the linear β-(1,4) linked homopolymer of glucose, and its native structure consists of micrometer-long nanofibrils with cross-sections in the nanometer range that are packed into larger aggregates and embedded in a matrix of other cell wall biopolymers. Despite the relatively simple chemical structure of its repeat unit, both naturally occurring and synthetic cellulose materials are highly heterogeneous, and that heterogeneity extends to the cellulose fibrils themselves. The cellulose structure simultaneously contains highly crystalline regions in the core of the nanofibrils, non-crystalline fibril surfaces, as well as less ordered domains occurring intermittently along the length of the fibril. Further, due to the formation of aggregates, cellulose also shows variation in the nature of interfaces, which may be located either within or on the outer surface of these larger structures. The experimental characterization of cellulose materials can be challenging, since, for instance, signals from non-crystalline regions commonly overlap with the one from the crystalline domain in 13C NMR chemical shifts, and broaden the scattering peaks in X-ray diffraction, respectively. Recently it was shown that the 13C T1 relaxation times of cellulose fibrils at different hydration levels all exhibited broad distributions ranging from 10−2 o 103 seconds. This indicates simultaneous fast and slow molecular motions at the nanosecond time scale. It is likely that this arises as a consequence of structural heterogeneity, but this is difficult to confirm experimentally. In this work, the heterogeneous molecular dynamics in cellulose is simulated. Distributions of NMR T1 relaxation times are calculated directly from MD simulation trajectories, and compared to available experimental data. Strong effects are observed from structural details, such as locations of individual molecules, surfaces and interfaces, and defects. In a long-term perspective, the present methodology will greatly improve our understanding of various types of cellulose (effects from plant source, processing etc). This will contribute to the development of improved cellulose nanofibrils, and high-performance cellulose nanomaterials.

Heterogeneous dynamics in cellulose from molecular dynamics simulations

2019 Spring National Meeting

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