Yang Kris Y, Wloch Daniela, Lee Koon-Yang
Department of Aeronautics, Imperial College London, South Kensington Campus London SW7 2AZ UK
Institute for Molecular Science and Engineering, Imperial College London SW7 2AZ UK.
RSC Adv. 2021 Aug 23;11(45):28352-28360. doi: 10.1039/d1ra04190h. eCollection 2021 Aug 16.
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) 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.
如果未对2,2,6,6-四甲基哌啶氮氧化物（TEMPO）氧化的纤维素纳米原纤（TOCN）进行交联或用水溶性聚合物包覆，由此得到的水凝胶并不坚固，且本身在水中不稳定。此外，由于TOCN直径小以及其增粘作用，制备自立式TOCN薄膜仍然是一个挑战。在此，我们报道了通过TEMPO介导的细菌纤维素（BC）纳米纸氧化，作为一种无需额外添加剂或交联步骤即可制备坚固且水稳定的TOCN水凝胶的方法。首先将原始BC薄膜压干成干燥且固结良好的BC纳米纸，然后在不同浓度的NaClO下进行TEMPO氧化。氧化反应将羧基部分引入纳米纸网络结构内暴露的BC纳米原纤上。这进而导致纳米纸膨胀并溶胀成水凝胶。TEMPO氧化后观察到溶胀比高达BC纳米纸原始厚度的100倍。还发现TEMPO氧化的BC水凝胶的保水值随羧基含量的增加而增加。发现这些TEMPO氧化的BC水凝胶即使在水中长时间（>1个月）磁力搅拌下也很坚固且水稳定。我们进一步表明，高克重的自立式TOCN薄膜（100 g/m²）可以像压干这些水稳定的TEMPO氧化的BC水凝胶一样简单地制备，无需真空辅助过滤或缓慢干燥，而这通常是TOCN薄膜制造中的限速步骤。
