Micromechanical modelling of cellulose hydrogel composites based on coarse-grained molecular dynamics

The mechanical response and structure of cellulose hydrogel composites (CHCs) are modelled as cellulose microfibrils using a bead-spring coarse-graining approach. Our results demonstrate that varying the adhesive contact energy between the flexible chains, as well as the number of contacts and structural anisotropy, significantly impacts the mechanical response of the network structure under tensile forces. Specifically, decreasing the contact energy and increasing its range is sufficient to increase the network's extensibility while decreasing its overall modulus. This key finding aligns qualitatively with experiments where the inclusion of polysaccharides in bacterial cellulose-based CHCs had an analogous effect. We hypothesise that polysaccharides, including hemicelluloses, facilitate alignment under strain by increasing the range ("softening") of the contact forces between microfibrils. Conversely, their absence results in "hard" contacts between microfibrils that are more energetic and can only act over short distances. In contrast to finite element models for CHCs, this coarse-grained approach incorporates non-permanent contacts between flexible microfibrils that permits structural rearrangement of the network in response to deformation, whereby controlling the density and proportion of long- and short- range contact forces suffice to qualitatively describe experiments. Controlling interactions between microfibrils thus provides a lever for designing CHCs with specific mechanical properties for various applications. Additionally, we suggest that plants naturally tune these variables in plant cell development to balance wall rigidity and extensibility.