In nature, cellulose nanofibers form hierarchical structures across multiple length scales to achieve high-performance properties and different functionalities. Cellulose nanofibers, which are separated from plants or synthesized biologically, are being extensively investigated and processed into different materials owing to their good properties. The alignment of cellulose nanofibers is reported to significantly influence the performance of cellulose nanofiber-based materials. The alignment of cellulose nanofibers can bridge the nanoscale and macroscale, bringing enhanced nanoscale properties to high-performance macroscale materials. However, compared with extensive reviews on the alignment of cellulose nanocrystals, reviews focusing on cellulose nanofibers are seldom reported, possibly because of the challenge of aligning cellulose nanofibers. In this review, the alignment of cellulose nanofibers, including cellulose nanofibrils and bacterial cellulose, is extensively discussed from different aspects of the driving force, evaluation, strategies, properties, and applications. Future perspectives on challenges and opportunities in cellulose nanofiber alignment are also briefly highlighted.

Cellulose nanomaterials are promising materials for the polymer industry due to their abundance and renewability. In packaging applications, these materials may impart enhanced gas barrier performance due to their high crystallinity and polarity. In this work, low barrier to superior gas barrier pristine nanocellulose films were produced using a shear-coating technique to obtain a range of anisotropic films. Induction of anisotropy in a nanocellulose film can control the overall free volume of the system which effectively controls the gas diffusion path; hence, controlled anisotropy results in tunable barrier properties of the nanocellulose films. The highest anisotropy materials showed a maximum of 900-fold oxygen barrier improvement compared to the isotropic arrangement of nanocellulose film. The Bharadwaj model of nanocomposite permeability was modified for pure nanoparticles, and the CNC data were fitted with good agreement. Overall, the oxygen barrier performance of anisotropic nanocellulose films was 97 and 27 times better than traditional barrier materials such as biaxially oriented poly(ethylene terephthalate) (BoPET) and ethylene vinyl alcohol copolymer (EVOH), respectively, and thus could be utilized for oxygen-sensitive packaging applications.

Nanocellulose has potential as a reinforcing agent to improve stiffness and strength in polymer fiber; however, the inherent difference in hydrophilicity makes it challenging to incorporate it into nonhydrophilic polymers, and the composite properties are strongly anisotropic. In the present work, a dual approach was employed to incorporate cellulose nanofibrils (CNFs) into polylactic acid (PLA). Polyethylene glycol (PEG) acted as a compatibilizating agent to enable the melt spinning of CNF/PLA composite fibers without water/solvent, and CNFs were surface modified to improve compatibility, increase nanoparticle thermal stability, and increase CNF dispersion in PLA. While no significant difference was observed in strength, the stiffness improved up to 600% (1.3 wt % CNF, maximum draw) in the composite fibers. This improvement was correlated with the crystallinity and fiber orientation (Herman's order parameter) for as-spun and hot-drawn fibers.

Cellulose nanomaterials (CNMs) are a class of materials that have recently garnered attention in fields as varied as structural materials, biomaterials, rheology modifiers, construction, paper enhancement, and others. As the principal structural reinforcement of biomass giving wood its mechanical properties, CNM is strong and stiff, but also nontoxic, biodegradable, and sustainable with a very large (Gton yr−1) source. Unfortunately, due to the relatively young nature of the field and inherent incompatibility of CNM with most man‐made materials in use today, research has tended to be more basic‐science oriented rather than commercially applicable, so there are few CNM‐enabled products on the market today. Herein, efforts are presented for preparing and forming cellulose nanomaterial nanocomposites. The focus is on recent efforts attempting to mitigate common impediments to practical commercialization but is also placed in context with traditional efforts. The work is presented in terms of the progress made, and still to be made, on solving the most pressing challenges—getting properties that are competitive with currently used materials, removing organic solvent, solving the inherent incompatibility between CNM and polymers of interest, and incorporation into commonly used industrial processing techniques.

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