Mechanics of Strong and Tough Cellulose Nanopaper

Cellulose nanopaper, which consists of a porous network of cellulose nanofibrils (CNFs), exhibits excellent mechanical properties with high strength and toughness. The physical mechanisms, including a realizable reduction of defect size in the nanopaper and facile formation/reformation of hydrogen bonds among CNFs, suggest a bottom-up material design strategy to address the conflict between strength and toughness. A thorough exploration of the rich potential of such a design strategy requires a fundamental understanding of its mechanical behavior. In this review, we supply a comprehensive perspective on advances in cellulose nanopaper mechanics over the most recent two decades from the three aspects of mechanical properties, structure-property relationship and microstructure-based mechanical modeling. We discuss the effects of size, orientation, polymerization degree, and isolate origins of CNFs; density or porosity and humidity of nanopaper; and hemicellulose and lignin on the mechanical properties of cellulose nanopaper. We also discuss the similarities and differences in the microstructure, mechanical properties, and toughening mechanisms between cellulose nanopaper and cellulose nanocrystal (CNC) nanopaper, chitin nanopaper, carbon nanotube (CNT) nanopaper, and graphene nanopaper. Finally, we present the ideas, status quo, and future trends in mechanical modeling of cellulose nanopaper, including atomistic- and microscale-level numerical modeling, and theoretical modeling. This review serves as a modest spur intended to induce scientists to present their valuable contributions and especially to design more advanced cellulose nanopapers and promote the development of their mechanics.