Exploring the molecular properties of carbohydrate polymers under moist conditions is of fundamental relevance for improving the moisture stability of novel bio-composites in all their existing and potential applications. As a first step towards cellulose/hemicellulose composites, we start with xyloglucan hemicellulose itself. Here, we present a molecular dynamics (MD) simulation study of the 13C CP/MAS NMR longitudinal relaxation times (T1) observed in xyloglucan at increasing moisture content. Experimentally, the 13C T1 values of hemicellulose chains were successfully assigned to the different C-H vectors therein and correlated with nano-structural changes induced by hydration via the observed decrease of T1 upon increasing moisture content. Our MD simulations reproduce well the experimental observations and explain the observed decrease in T1 by the gradual replacement of polymer-polymer hydrogen bonds by polymer-water hydrogen bonds, which indeed accelerates the dynamics of polymer chains as water molecules move faster than polymer chains. Furthermore, the wide experimental distributions of T1 for each carbon site could be successfully decomposed, with the aid of simulations, into separate contributions from backbone, side chains and end groups of the polymer chain, in turn associated with differing levels of motional freedom. Our combined experimental and simulation methodology successfully captures the physical picture of the water-polymer interaction in xyloglucan and opens new possibilities to the molecular understanding of more complex systems, like cellulose/hemicellulose nanocomposites.

Molecular dynamics simulation study of moisture effects on chain mobility in hemicellulose-based bio-nanocomposites as observed by 13C CP/MAS NMR relaxometry

253rd National Meeting (2017)

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