Protonation of Surface Carboxyls on Rice Straw Cellulose Nanofibrils: Effect on the Aerogel Structure, Modulus, Strength, and Wet Resiliency.

Rice straw cellulose nanofibrils from the optimal 2,2,6,6-tetramethylpiperidine-1-oxyl oxidation/blending process carrying 1.17 mmol/g surface carboxyls were protonated to varying charged (COO-Na+) and uncharged (COOH) surfaces. Reducing the electrostatic repulsion of surface charges by protonation with hydrochloric acid from 11 to 45 and 100% surface carboxylic acid most prominently reduced the aerogel densities from 8.0 to 6.6 and 5.2 mg/cm3 while increasing the mostly open cell pore volumes from 125 to 152 and 196 mL/g. Irrespective of charge levels, all aerogels were amphiphilic, super-absorptive, stable at pH 2 for up to 30 days, and resilient for up to 10 repetitive squeezing-absorption cycles. While these aerogels exhibited density-dependent dry [11.3 to 1.5 kPa/(mg/cm3)] and reduced wet [3.3 to 1.4 kPa/(mg/cm3)] moduli, the absorption of organic liquids stiffened the saturated aerogels. These data support protonation as a critical yet simple approach toward precise control of aerogels' dry and wet properties.

Tunable dialdehyde/dicarboxylate nanocelluloses by stoichiometrically optimized sequential periodate-chlorite oxidation for tough and wet shape recoverable aerogels.

Sequential periodate-chlorite (PC) oxidation has been optimized stoichiometrically according to the non-crystalline content in cellulose to generate a variety of versatile C2,C3 dialdehyde/dicarboxylate nanocelluloses (NCs) while economizing chemical and shear force inputs. The robust primary sodium periodate (NaIO4) oxidation not only regioselectively cleaved the C2-C3 carbon bond to oxidize the vicinal hydroxyls to aldehydes, but also governed the lengths of NCs, i.e., cellulose nanofibrils (PC-CNFs) at near-equal NaIO4 to non-crystalline anhydroglucose unit (AGU) stoichiometry and cellulose nanocrystals (PC-CNCs) at a doubled ratio. Secondary sodium chlorite (NaClO2) oxidation facilely converted C2,C3 dialdehydes to dicarboxylates and, upon deprotonation, facilitated defibrillation to NCs, irrespective of extents of carboxylation or charges. The optimal 0.5 : 1 NaIO4/AGU and 1 : 1 NaClO2/AGU oxidation produced highly uniform 1.26 nm thick, 3.28 nm wide, and ca. 1 µm long PC-CNFs with tunable surface aldehyde (0.71-0.0 mmol g-1) and carboxylate (0.64-1.35 mmol g-1) content at 94-98% yields. The C2-C3 glucosidic ring opening and oxidation along the 110 or 11̄0 crystalline surfaces increased the heterogeneity of the hydrophilic surfaces and flexibility of PC-CNFs to influence their self-assembling into fibrils and amphiphilic superabsorbent aerogels. The ultra-light (ρ = 10.3 mg cm-3) aerogels showed an ultra-high dry specific compression modulus (50.2 kPa mg-1 cm-3) and specific stress (8.2 kPa mg-1 cm-3 at 0.8 strain), cyclic wet compressive behavior, and excellent water-activated shape recovery following 0.8 strain dry compression.