Modeling Properties and Interaction Mechanisms of PMMA and Cellulose Nanocomposite Materials via Molecular Dynamics

Polymer nanocomposites are highly tunable materials with broad-reaching applications due to the diverse properties imbued through interactions between polymers and nanofillers. The vast combinatorial space of polymer composition, nanofiller types, sizes, mixture ratios, processing conditions, and use-cases necessitates improved modeling capability of the materials to help isolate the molecular mechanisms of interaction and predict the resultant potential properties. Poly methyl-methacrylate (PMMA) and cellulose nanocomposites are a potential combination of strength, biocompatibility, and availability. We link molecular dynamics results to interaction mechanisms and material properties as well as quantify trends with respect to structural features, including how the degree of polymerization and tacticity of PMMA affect the material structure and properties. The packing density, preferential alignment to the fibril, and end-to-end distance showed that the longer PMMA oligomers, while they generally align to the fibril's axis, tend to bundle and tangle more, expressing less linearity than the short-chain oligomers. This results in the formation of nanoscale free-volume cavities at the PMMA-cellulose nanofibril interface and weakens the toughness of the overall material. Within the tested range, the interaction energy between PMMA and the cellulose nanofibril is roughly an order of magnitude more sensitive to changes in polymer length than to changes in polymer tacticity. Trends in the material strength with respect to PMMA chain length are balanced out by the opposing effects of increased strength of the polymer matrix and the decreased interaction strength between the polymer and the cellulose nanofibril. While the addition of a cellulose nanofibril does increase the strength of PMMA material, applications using PMMA-cellulose nanocomposites should be selected with care due to the potential for increased formation of free volumes and resultant weaknesses.