Synthesis of amine functionalized cellulose nanocrystals: optimization and characterization

• One-pot two-step synthesis of amine functionalized cellulose nanocrystals (CNC).
• High charge density amine functionalized CNC.
• Effect of reaction parameters on amine functionalization.
• pH-controlled charge reversal of amine functionalized CNC.
• pH-dependent viscosity of amine functionalized CNC suspension.

A simple protocol was used to prepare amine functionalized cellulose nanocrystals (CNC–NH2). In the first step, epichlorohydrin (EPH) was reacted with ammonium hydroxide to produce 2-hydroxy-3-chloro propylamine (HCPA). In the next step, HCPA was grafted to CNC using the etherification reaction in an organic solution media. Various reaction parameters, such as time, temperature, and reactant molar ratio were performed to determine the optimal reaction conditions. The final product (CNC–NH2(T)) was dialyzed for a week. Further purification via centrifugation yielded the sediment (CNC–NH2(P)) and supernatant (POLY–NH2). The presence of amine groups on the surface of modified CNC was confirmed by FTIR and the amine content was determined by potentiometric titration and elemental analysis. A high amine content of 2.2 and 0.6 mmol amine/g was achieved for CNC–NH2(T) and CNC–NH2(P), respectively. Zeta potential measurements confirmed the charge reversal of amine CNC from positive to negative when the pH was increased from 3 to 10. The flocculation of amine functionalized CNC due to its interactions with a negatively charged surfactant namely, sodium dodecyl sulfate (SDS) was investigated at pH 4. It showed promising results for applications, such as in flocculation of fine dispersions in water treatment. This simple and versatile synthetic method to produce high amine content CNC can be used for further conjugation as required for various applications.

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