Abstract
Hydrogels derived from TEMPO-oxidised cellulose nanofibrils (TOCNs) are not robust and inherently water unstable if the TOCNs are not crosslinked or coated with a water-swellable polymer. Furthermore, the manufacturing of self-standing TOCN films is still a challenge due to the small TOCN diameter and the viscosifying effect of TOCNs. Here, we report the TEMPO-mediated oxidation of bacterial cellulose (BC) nanopaper as a route to produce robust and water stable TOCN hydrogels without the need of additional additives or crosslinking steps. Pristine BC pellicle was first press-dried into a dried and well-consolidated BC nanopaper, followed by TEMPO-oxidation at various NaClO concentrations. The oxidation reaction introduced carboxylate moieties onto the exposed BC nanofibrils within the nanopaper network structure. This then led to the expansion and swelling of the nanopaper into a hydrogel. A swelling ratio of up to 100 times the original thickness of the BC nanopaper was observed upon TEMPO-oxidation. The water retention value of the TEMPO-oxidised BC hydrogels was also found to increase with increasing carboxylate content. These TEMPO-oxidised BC hydrogels were found to be robust and water-stable, even under prolonged (>1 month) magnetic stirring in water. We further showed that high grammage self-standing TOCN films (100 g m−2) can be fabricated as simple as press-drying these water stable TEMPO-oxidised BC hydrogels without the need of vacuum-assisted filtration or slow-drying, which is typically the rate-limiting step in the manufacturing of TOCN films.
Highlights
TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] mediated oxidation of cellulose is a regioselective oxidation of the C6 primary hydroxyl groups of native cellulose using sodium bromide (NaBr) and sodium hypochlorite (NaClO) as the regenerating oxidants.[1]
When bacterial cellulose (BC) nanopaper was immersed in water, it did not rehydrate into its original pellicle form (Fig. 1c)
We demonstrated a simple method to fabricate robust and water stable TEMPO-oxidised cellulose nano brils (TOCNs) hydrogels based on the TEMPO-mediated oxidation of dried and well-consolidated BC nanopaper
Summary
TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] mediated oxidation of cellulose is a regioselective oxidation of the C6 primary hydroxyl groups of native cellulose using sodium bromide (NaBr) and sodium hypochlorite (NaClO) as the regenerating oxidants.[1]. We demonstrate that self-standing TOCN lms can be produced as simple as press-drying the TEMPO-oxidised BC hydrogels without the need of a challenging vacuum-assisted ltration or slow water evaporation step. The BC nanopaper disintegrated during the oxidation reaction instead of swelling into a water stable hydrogel (Fig. 1h), even at the lowest NaClO concentration used in this work.
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