Carboxylated chitin nanofibers were successfully prepared by direct surface TEMPO-mediated oxidation of chitin micro/nano fibers, that is, chitin was pulverized into various sizes by aqueous counter collision (ACC) with different pass numbers prior to carboxylation. Meanwhile, chitin nanofibers (ACC-ChNs) were successfully obtained using ACC pulverization applied to crab shell α-chitin. Surface oxidation gave oxidized-ACC-ChNs superior pH-responsive sol-gel transition properties. The combination of aqueous counter collision and TEMPO-mediated oxidation led to double oxidation efficiency of chitin nanofibers compared to oxidation of original non-ACC pulverized chitin. Notably, the carboxylate content of oxidized-ACC-ChNs reached 0.67–0.79 mmol/g after the ACC defibrillation followed by TEMPO/NaBr/NaClO oxidation, comparing to ~ 0.40 mmol/g of oxidized original (without ACC defibrillation) chitin when the same dosage of NaClO (2.5 mmol/g) was applied. The results proved that the nanofibrillation of chitin by ACC process effectively improved the oxidation efficiency. Thus controllable moderate oxidation with slight amount of NaClO avoided severe degradation caused by excessive NaClO used. The chemical, crystalline structures and the morphological properties of ACC-ChNs and oxidized ACC-ChNs was evaluated. Furthermore, the oxidized ACC-ChNs based hydrogels possessing improved storage modulus were successfully formed by gas phase coagulation process.
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