Influence of morphology and dispersion stability of CNC modified with ethylene oxide derivatives on mechanical properties of PLA‐based nanocomposites

Abstract

The aggregation state of cellulose nanocrystals modified with surfactants (CNC/S) in water and CNC/S powder dispersion stability in chloroform were studied and correlated with mechanical properties of nanocomposites with poly(lactic acid) (PLA). Four types of nonionic surfactants were used: PEG 300, PEG 1000, poly(ethylene glycol) monooleate, and Pluronic® L44 (PL44). The CNC/PL44 showed the smallest size of CNC aggregates dispersed in water before freeze‐drying, as determined by atomic force microscopy. The respective powder also showed the lowest decay rate of relative turbidity among CNC/S dispersions in chloroform. This, together with the improved surface coverage of CNC by PL44, verified by zeta potential (large change in the zeta potential of CNC/S dispersion in chloroform) and thermogravimetric analysis (improved thermal stability of CNC/S powder), yielded the highest tensile strength at break (TS) among PLA/CNC/S nanocomposites. Despite none of surfactants produced nanocomposites with TS higher than PLA or PLA/CNC, the aggregation state of CNC/S in water is well correlated with mechanical properties of PLA/CNC/S nanocomposites prepared by solvent casting. This correlation indicates that hornification cannot be avoided for the conditions used to prepare CNC/S powder and that there are two opposite effects in preparation of PLA/CNC/S nanocomposites. First, the effectiveness of steric barrier of nonionic surfactants against aggregation during freeze‐drying depends on complete surface coverage of CNC, together with hydration of stabilizing chain. Since hydration depends on hydrophilicity of stabilizing chain and compatibility with PLA depends on its nonpolar nature, this strategy alone seems not to be efficient to produce high‐performance nanocomposites.