Nonuniformly modifying high-aspect-ratio rigid cellulose nanocrystals to enhance percolation advantage in weakly compatible biomass polymer systems

The percolation network of low-content rigid particles can effectively reinforce biomass polymers, which hopefully can solve the problem of renewable materials with low mechanical properties, such as the low strength and Young’s modulus, and bad elongation at break. However, it hardly works in weakly compatible polymer/particle systems due to its poor dispersion and even if chemical modification can increase the compatibility so that nanoparticles can be well-dispersed, as a side effect it decreases the percolation reinforcement effect. Herein, we took the advantage of cellulose nanocrystals (CNCs) with high aspect ratio and realized a nonuniform modification on their chemical surface structure, where the modification focused on their middle part. The modified part could enhance the compatibility between CNCs and hydrophobic biomass polymers (eucommia ulmoide gum, EUG, in this work), when the unmodified ends still maintained the percolation properties. Increasing the aspect ratio of cellulose nanocrystals (CNCs) from 18 to 80, we found that the strength of the nanocomposites increased from 8.7 to 11.2 MPa (EUG/CNC-5). Importantly, the nonuniform modification, which was confirmed by AFM, further allowed the EUG to chemically cross-link with the middle part of CNCs, such that the crosslink network could be separated from the percolation network. The molecular movement of EUG in crosslink network was thus free of the percolation network, leading to a 3.2 times increase in elongation at break. The nonuniform modification strategy can thus control the multiple networks in weakly compatible composite systems and offer EUG-based materials, which usually suffer hard molecular movement, mechanical properties greater than the common petroleum-based rubbers and elastomers.