Deformation and fracture mechanisms in nanocellulose reinforced composites

Cellulose is the main constituent of plants. In the cell wall of plants, cellulose nanofibrils act as a reinforcing agent embedded in a matrix of hemicelluloses and lignin, forming a nanocomposite material. Manmade nanocellulose reinforced composites began to receive attention approximately two decades ago when isolation methods for nanocellulose were developed. However, studies on the deformation of these novel materials have been limited. The effect of the composites' preparation method on the mechanical properties was investigated and compared with theoretical models. Deformation mechanisms in composites reinforced with low weight fractions of different types of nanocellulose were investigated along with the effects of acetylation. Then the stress-transfer and micromechanics of composites reinforced with higher weight fractions of nanocellulose were studied using Raman spectroscopy. Finally, the effect of nanocellulose on thermomechanical properties of the composites and their behaviour in moist environment were addressed. The results show that the preparation method has an influence on the final mechanical properties of composites. Degassing of the nanocellulose/polymer mixture showed a positive effect on the Young's modulus and tensile strength at lower weight fractions of nanocellulose due to the lower viscosities of the mixtures. However, degassing had no effect on the density of the composites. Chemical modification significantly improved the dispersion of nanocellulose in non-polar media as Raman imaging revealed. In turn, the mechanical properties and deformation of the composites was different with respect to the degree of substitution. The toughening of poly(lactic) acid by the addition of low weight fractions of nanocellulose was attributed to extensive polymer crazing which was also dependent on the morphology and degree of substitution of the nanocellulose. Using Raman spectroscopy it was shown that the deformation micromechanics at high weight fractions of nanocellulose are network dominated. This leads to a stress transfer mechanisms similar to a composite within a composite, where composite strength is dependent on stress transfer within the dense network. The mechanical properties of the composites were improved as well as the glass transition temperature. The crystallization behaviour and, in turn, crystallinity of the composites was observed to be impeded at large weight fractions of nanocellulose. Furthermore, the composites had better mechanical properties in humid environments compared to the pure PLA matrix and the pure nanocellulose film. Thus embedding of hydrophilic fibrils in a hydrophobic matrix improves the performance of these materials in humid environments.