Cellulose being the most widely available biopolymer on Earth is attracting significant interest from the industry and research communities. While molecular simulations can be used to understand fundamental aspects of cellulose nanocrystal selfassembly, a model that can perform on the experimental scale is currently missing. In our study we develop a supra coarse-grained (sCG) model of cellulose nanocrystal which aims to bridge the gap between molecular simulations and experiments. The sCG model is based on atomistic molecular dynamics simulations and it is developed with the forcematching coarse-graining procedure. The validity of the model is shown through comparison with experimental and simulation results of the elastic modulus, self-diffusion coefficients and cellulose fiber twisting angle. We also present two representative case studies, self-assembly of nanocrystal during solvent evaporation and simulation of a chiral nematic phase ordering. Finally, we discuss possible future applications for our model.
We show that anisotropic foams based on aligned cellulose nanofibrils are superinsulating also at high relative humidity (RH). Thermal conductivity measurements and non-equilibrium molecular dynamic simulations show that the moistureinduced swelling and increase of the inter-fibrillar distance results in a reduction of the thermal boundary conductance that exceeds the thermal conductivity increase due to water uptake up to 75% RH. Phonon engineering by moisture could be used to tailor the heat transfer properties of hygroscopic nanofibrillar materials.
The hierarchical self-assembly of cellulose nanocrystals (CNCs) is an important phenomenon occurring naturally in plant cell walls. Utilization of this assembly for advanced applications requires a fundamental theoretical understanding of interactions between the CNCs, which is still incomplete. Hence, in this work, we used molecular dynamics simulations to study the effect of surface modification on the interactions between the CNCs and the resulting bundling process. We consider two types of common surface modifications of native CNCs, sulfated CNCs (SCNCs) and TEMPO-oxidized CNCs (TCNCs), in the presence of two types of counterions, Na + and Ca 2+ , in solution. We used the umbrella sampling method to calculate the potential of the mean force (PMF), and we found that the strength of interaction between the modified CNCs decreases, compared with the native CNCs. The strength of interaction for TCNCs is almost similar to that for SCNCs at the same level of surface substitution, whereas the type of counterion has a strong effect on the PMF with a higher interaction energy between the CNCs in the presence of a divalent counterion as compared to a monovalent counterion. Finally, we studied the self-assembly of CNCs into a hexagonal bundle for the native CNCs and sulfated CNCs focusing on the twist of the bundle, bound water inside the bundle, inter-CNC gap, and interaction energy between the CNCs in the bundle, and the effect of the counterions on the morphology of the bundle. The equilibrium spacing of the CNCs within the bundle is found to be consistent with the results of PMF calculations for the minimum separation distance between the respective crystal surfaces.
Foams made from cellulose nanomaterials are highly porous and possess excellent mechanical and thermal insulation properties. However, the moisture uptake and hygroscopic properties of these materials need to be better understood for their use in biomedical and bioelectronics applications, in humidity sensing and thermal insulation. In this work, we present a combination of hybrid Grand Canonical Monte Carlo and Molecular Dynamics simulations and experimental measurements to investigate the moisture uptake within nanocellulose foams. To explore the effect of surface modification on moisture uptake we used two types of celluloses, namely TEMPO-oxidized cellulose nanofibrils and carboxymethylated cellulose nanofibrils. We find that the moisture uptake in both the cellulose nanomaterials increases with increasing relative humidity (RH) and decreases with increasing temperature, which is explained using the basic thermodynamic principles. The measured and calculated moisture uptake in amorphous cellulose (for a given RH or temperature) is higher as compared to crystalline cellulose with TEMPO- and CM-modified surfaces. The high water uptake of amorphous cellulose films is related to the formation of water-filled pores with increasing RH. The microscopic insight of water uptake in nanocellulose provided in this study can assist the design and fabrication of high-performance cellulose materials with improved properties for thermal insulation in humid climates or packaging of water sensitive goods. Graphic abstract
Self-assembly is ubiquitous in nature and underlies the formation of many complex systems from the molecular to the macroscopic scale. Kern-Frenkel like patchy particles are powerful models to investigate this phenomenon by computational methods such as Monte-Carlo or Molecular Dynamics simulations. However, in these models the interactions are mediated by circular patches at the particles surface, which can be hardly mapped to realistic systems, containing for instance faceted particles with rectangular 1 surfaces. In this paper we extend the model to take into account such geometries, and we use it to build a supra Coarse-Grained model of Cellulose Nanocrystal where the interactions are parametrized against All-Atomistic Molecular Dynamics simulations.The formation of cholesteric ribbons and defects in cholesteric droplets of Cellulose Nanocrystal are investigated and confirm experimental behavior reported in the literature. The flexibility of this new patchy particle model makes it a powerful tool to develop supra Coarse-Grained models of self-assembly for large space and time scales and should find a broad range of applications for self-assembly in chemical and biological systems.
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