Polymer grafting on cellulose nanocrystals initiated by ceric ammonium nitrate: is it feasible under acid-free conditions?

Abstract

It is of great significance to tailor the physicochemical properties of cellulose nanocrystals (CNCs) for broadening their application fields. Grafting polymerization initiated by cerium ammonium nitrate (CAN) in aqueous medium is a facile and efficient surface modification strategy for CNCs. However, CAN is generally used under strong acidic conditions to prevent hydrolysis; strong acids cause severe environmental problems and limit the monomer species of the candidate. Herein, an acid-free polymer grafting modification method initiated by CAN for carboxylated CNCs (CCNCs) was reported. For CCNCs, a high monomer conversion rate (>80%) and grafting yield (>200%) were obtained even at pH=7 when methyl methacrylate (MMA) was used as the monomer. It is worth noting that the time-consuming dialysis process for CNCs was not required before the grafting reaction, and the CNCs-g-PMMA can be “self-purified” due to its spontaneous precipitation from water. Moreover, the initiator's consumption in this system is very low (1.82 mmol L⁻¹) compared with previously reported values. By investigating the interaction between CAN and CCNCs and the effect of CCNCs on the hydrolysis of CAN, it is revealed that a CCNC complex with Ce⁴⁺ acts as the “macromolecular initiator” to initiate the grafting polymerization under acid-free conditions. This work overcomes the limitation of strong acidic conditions for the polymer grafting modification of CNCs initiated by CAN and is conducive to the industrial production of polymer-grafted CNCs under environmentally friendly conditions.