Tailoring cellulose nanocrystals rheological behavior in aqueous suspensions through surface functionalization with polyethyleneimine

The present paper reports the surface modification of commercially available cellulose nanocrystals (CNCs) using polyethyleneimine (PEI) by means of non-covalent electrostatic interaction between the negatively charged sulfate groups of CNCs and positively charged amine functionalities of PEI. The modification, carried out in an aqueous medium, results in a stable CNC-PEI suspension with no phase separation that exhibit interesting rheological behavior due to bridging-type inter-particle interactions. The Newtonian 3% (w/w) CNC suspension evolves into a non-Newtonian gel system after modification with PEI with a consequent increase of almost three decades in complex viscosity. Preshearing of the 3% (w/w) CNC-PEI suspension resulted in the loss of the linear viscoelastic properties with increasing shear rate, as would be expected from the breaking of the inter-particle network. However, the system gradually re-established the inter-particle network in less than an hour to give the original rheological parameters. The effect of PEI on the rheological properties was attributed to the physical adsorption of PEI chains on the CNC particles, examined by dynamic light scattering (DLS), zeta potential, X-ray photoelectron spectroscopy (XPS), elemental analyses, and isothermal adsorption studies. The modified CNC-PEI particles did not show any significant change in the particle morphology compared to the unmodified CNCs, as observed from transmission electron microscope (TEM) images.