UV-Triggered On-Demand Temperature Responsive Reversible and Irreversible Gelation of Cellulose Nanocrystals

We show ionically crosslinked, temperature responsive reversible or irreversible hydrogels of anionic cellulose nanocrystals (CNCs) and methacrylate terpolymers by mixing them homogeneously in the initially charge-neutral state of the polymer, which was subsequently switched to be cationic by cleaving side-groups by UV-irradiation. The polymer is a random terpolymer poly(di(ethylene glycol) methyl ether methacrylate)-rnd-poly(oligo(ethylene glycol) methyl ether methacrylate)-rnd-poly(2-((2-nitrobenzyl)oxycarbonyl)aminoethyl methacrylate), i.e., PDEGMA-rnd-POEGMA-rnd-PNBOCAEMA. The PDEGMA and POEGMA repeating units lead to Lower Critical Solution Temperature (LCST) behavior. Initially, homogeneous aqueous mixtures are obtained with CNCs and no gelation is observed even upon heating to 60 oC. However, upon UV-irradiation the NBOCAEMAs are transformed to cationic 2-aminoethyl methacrylate (AEMA) groups, as 2-nitrobenzaldehyde moieties are cleaved. The resulting mixtures of anionic CNC and cationic PDEGMA-rnd...