Abstract
We show ionically cross-linked, 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), that is, PDEGMA-rnd-POEGMA-rnd-PNBOCAEMA. The PDEGMA and POEGMA repeating units lead to a lower critical solution temperature (LCST) behavior. Initially, homogeneous aqueous mixtures are obtained with CNCs, and no gelation is observed even upon heating to 60 °C. 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-POEGMA-rnd-PAEMA show gelation for sufficiently high polymer fractions upon heating to 60 °C due to the interplay of ionic interactions and LCST. For short heating times, the gelation is thermoreversible, whereas for long enough heating times, irreversible gels can be obtained, indicating importance of kinetic aspects. The ionic nature of the cross-linking is directly shown by adding NaCl, which leads to gel melting. In conclusion, the optical triggering of the polymer ionic interactions in combination with its LCST phase behavior allows a new way for ionic nanocellulose hydrogel assemblies.
Highlights
Nanocelluloses are colloidal fibrils that can be cleaved from plant or wood resources or produced bacterially.[1]
cellulose nanocrystals (CNCs) (1 mg/mL) and PDEGMA417-rnd-POEGMA109-rnd-PNBOCAEMA54 dispersion in Milli-Q H2O were used for both types of experiments, but an additional 1.0 mM of NaCl was used in the zeta potential samples
The CNCs were characterized via light scattering and ζ-potential measurements as well as electron microscopy
Summary
Nanocelluloses are colloidal fibrils that can be cleaved from plant or wood resources or produced bacterially.[1]. As the CNCs typically are negatively charged due to their sulfate ester groups arising from the sulfuric acid hydrolysis, one could ask whether CNC-based hydrogels could be fabricated by using ionic interactions as the network cross-links upon mixing the anionic CNCs with cationic polymers within the aqueous medium. Various routes for photoinduced and phototriggerable hydrogelations have been reported.[43−48] In the present work, a question can be posed whether a homogeneous aqueous mixture could be achieved by mixing an initially charge-neutral polymer with the anionic CNC dispersion followed by stimulus triggered by on-demand switching of cationic groups within the polymer. The resulting polycationic chains would force the formation of ionically interacting mixtures in situ, leading to aqueous gel networks In this case, the use of photocleavable chemical groups could be an interesting approach.[49] in the context of CNC, no previous literature exists to provide irradiation triggered hydrogelation. We show that depending on the thermal history and compositions, thermoreversible or irreversible gelation can be achieved
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