Evaluation of the mechanical properties of nanocomposites prepared by varying the degree of orientation of bio-nanofillers using a modified rule of mixtures

The mechanical properties of nanocomposites largely depend on the orientation of their fillers. However, fillers are often randomly dispersed, limiting the ability to maximize their effects. In this study, we used cellulose nanocrystals (CNCs), which have attracted attention as fillers, and employed a magnetic field processing technique to control their alignment, thereby fabricating alignment-controlled nanocomposites. The reinforcing effect of the CNC alignment was systematically evaluated, and a modified rule of mixtures was developed to quantitatively describe the alignment-dependent mechanical properties. Nanocomposites were prepared by dispersing CNCs derived from tunicate cellulose into a poly(vinyl alcohol) (PVA) solution, followed by casting under magnetic fields ranging from 0 to 6 T. X-ray diffraction analysis confirmed that higher magnetic field strength significantly enhanced the CNC alignment. Tensile testing revealed that incorporating 20 wt% randomly aligned CNCs increased the elastic modulus and tensile strength of the PVA films by 374% and 44.4%, respectively. Magnetic field-induced alignment of CNCs further boosted the nanocomposite film's elastic modulus by 74% and tensile strength by 88% compared to randomly oriented (0 T) samples. To rationalize these improvements, a modified rule of mixtures was proposed that incorporates the anisotropic mechanical properties of CNCs alongside stress transfer losses caused by changes in CNC contact points during alignment. The proposed model demonstrated good agreement with experimental results under all alignment conditions, demonstrating that magnetic field-induced alignment is effective in tuning the mechanical properties of CNC nanocomposites.