Printable, castable, nanocrystalline cellulose-epoxy composites exhibiting hierarchical nacre-like toughening

TL;DR

This study develops printable, high-CNC-content epoxy composites with nacre-like microstructure, exhibiting high hardness and toughness, suitable for complex 3D shaping, advancing sustainable nanocomposite materials.

Contribution

Introduces a novel formulation and processing method for CNC-epoxy composites with over 50 wt.% CNC, demonstrating nacre-like toughening and printability.

Findings

CNC content exceeds 50 wt.% in composites.

Composites exhibit hardness comparable to aluminum alloys.

Fracture toughness similar to wood cell walls.

Abstract

Due to their exceptional mechanical and chemical properties and their natural abundance, cellulose nanocrystals (CNCs) are promising building blocks of sustainable polymer composites. However, the rapid gelation of CNC dispersions has generally limited CNC-based composites to low CNC fractions, in which polymer remains the dominant phase. Here we report on the formulation and processing of crosslinked CNC-epoxy composites with a CNC fraction exceeding 50 wt.%. The microstructure comprises sub-micrometer aggregates of CNCs crosslinked to polymer, which are analogous to the lamellar structure of nacre and promotes toughening mechanisms associated with bulk ductile behavior, despite the brittle behavior of the aggregates at the nanoscale. At 63 wt.% CNCs, the composites exhibit a hardness of 0.66 GPa and a fracture toughness of 5.2 MPa.m$^{1/2}$. The hardness of this all-organic material is comparable to aluminum alloys, and the fracture toughness at the centimeter scale is comparable to that of wood cell wall. We show that CNC-epoxy composite objects can be shaped from the gel precursors by direct-write printing and by casting, while the cured composites can be machined into complex 3D shapes. The formulation, processing route, and the insights on toughening mechanisms gained from our multiscale approach can be applied broadly to highly loaded nanocomposites.