Engineering Outstanding Water Retention and Toughness through Gradient Cellular Architecture and Delignification: A Case Study of Palm Fronds

Abstract

Delignification can engineer wood into high-performance multi-scaled materials, building up from its elementary fibrils of nanocellulose across its micro-scaled cellular architecture. However, the potential to engineer other, structurally diverse, non-woody biomass remains largely unexplored. Non-wood plants can add additional hierarchical structures at the mm- and cm-scale. Here, we explore such unique hierarchical architecture as found in date palm fronds, where the vasculature promotes the formation of near-mm scaled vasculature bundles. We also show that along the growth direction, the multi-scale structure varies substantially, i.e. considering the frond base, petiole, and rachis. These tissues are systematically investigated to establish correlations between cellular structure, delignification pathways, and the combination of water dynamics to mechanical performance. A range of property spaces are showcased, highlighting excellent water-storage capacity (~1212%) in the never-dried porous state and high toughness after air-dry densification. Alkaline, acidic-bleaching, and combined bleach-cold alkali treatments are employed to tailor chemical composition and microstructure selectively. The combined route facilitates nanocellulose liberation and redistribution, generating a hierarchical network that improves moisture retention. These findings establish a structure-processing-property relationship for unconventional plant ultra-structure biomass towards multi-scaled materials with unique features from the nano- to the cm-scales.