An Energy‐Efficient One‐Pot Swelling/Esterification Method to Prepare Cellulose Nanofibers with Uniform Diameter

Abstract

An energy-efficient method has been developed to prepare 3-5 nm-wide carboxyl-functionalized cellulose nanofibers (CNFs) from pulp fiber by a simple one-pot swelling followed by esterification process. Tetrabutylammonium acetate (TBAA)/dimethyl sulfoxide (DMSO) binary solvent is used as the swelling agent and the esterification medium admixed with maleic anhydride. All steps are performed at room temperature and no post-mechanical treatment is needed. The highly efficient defibrillation of pulp fiber to CNFs is thought to be due to two factors: 1) swelling in TBAA/DMSO effectively loosens the structure of cellulose supermolecules by breaking the intra- and intermolecular hydrogen bonds between cellulose chains; and 2) the carboxyl groups grafted onto the cellulose molecules by esterification prohibit the reformulation of hydrogen bonds between cellulose chains and therefore stabilize the disperse CNFs with uniform diameter in solution. Other than acid anhydride, no catalyst is added for the esterification, which facilitates the recycling and reuse of the binary solvent. This energy-efficient one-pot method could facilitate the large-scale manufacture of bio-based nanomaterials.

Pseudosolvent Intercalator of Chitin: Self‐Exfoliating into Sub‐1 nm Thick Nanofibrils for Multifunctional Chitinous Materials

“…Other surface ionization approaches, such as esterification, etherification, oxidation, and deacetylation, have also been applied to deconstruct natural materials into nanofibrils and served for plentiful state‐of‐the‐art applications. [ 2d,4 ] However, its high‐cost and low exfoliation efficiency obstruct the large‐scale production and wide application of biopolymer nanofibrils. Although the preionization is capable of reducing fibrillation energy consumption by 90%, the inevitable strong mechanical treatments are still energy intensive and time consuming.…”

“…The resultant ChNFs suspensions show a high light transmittance of 98% at 500 nm and excellent stability even for months (Figure S2, Supporting Information), suggesting a high level of individuality and colloidal stability of chitin nanofibrils. [ 4c ] Its viscosity, which relates to the aspect ratio and surface charge density of nanofibrils, depends strongly on the chitin source. As shown in Figure 1C, at a fixed concentration of 0.8 mg mL −1 , the viscosity of β‐ChNFs suspension exfoliated from loligo pen shows an unprecedent high value of 0.58 Pa sat 1 s −1 , which is not only 1–2 orders of magnitude higher than that of ChNFs originating from other chitin ingredients, but also dozen times higher than that of TEMPO‐oxidized cellulose nanofibrils with aspect ratio of 441.…”

“…The fully swelled but undissolved state of chitin in DMSO/KOH is one of the key factors for the self‐exfoliation of ChNFs with high yield. [ 4a ]…”

“…In spite of lacking postmechanical treatment, the exfoliation yield of this method is still higher than that of other exfoliation methods (30–80%). [ 3c,4c,22 ] To be noted, although a relatively short swelling time of 1 h is inadequate for the self‐exfoliation of ChNFs, the unexfoliated precipitation is capable of being completely dispersed in NaOH aqueous solution (pH = 11) after short sonication with ultralow power (e.g., 50 W, 30 s).…”

“…These lateral dimensions are comparable with that of β‐ChNFs exfoliated by other methods, and the corresponding nanofibrils can be recognized as the elementary fibrils in squid pen. [ 3c,4d ] With an increase of swelling time, the width of ChNFs barely changes while its thickness gradually decreases (Figure 3C; and Figures S13 and S14, Supporting Information). Typically, when the swelling time reaches 5 h, the thickness distribution of resulting ChNFs is determined to be in the range of 0.5–1.1 nm, which is related to the mono or bilayer chitin molecular sheets (0.5–1 nm) (Figure 3B–B).…”