Protonation of surface carboxyls on rice straw cellulose nanofibrils: Effect on the aerogel structure, modulus, strength, and wet resiliency

Authors: Patterson, Gabriel; McManus, James; Orts, William; HSIEH, YOU-LO - University Of California, Davis

Submitted to: Biomacromolecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2023
Publication Date: 4/11/2023
Citation: Patterson, G.D., McManus, J.D., Orts, W.J., Hsieh, Y. 2023. Protonation of surface carboxyls on rice straw cellulose nanofibrils: Effect on the aerogel structure, modulus, strength, and wet resiliency. Biomacromolecules. 24(5):2052-2062. https://doi.org/10.1021/acs.biomac.2c01478.
DOI: https://doi.org/10.1021/acs.biomac.2c01478

Interpretive Summary: Crystalline cellulose was purified from rice straw, a major agricultural byproduct in California and worldwide. The cellulose was converted to nanocellulose, or nano-scale cellulose deritatives using coupled chemical and mechanical methods. The nanocelluloses in aqueous dispersion were characterized in terms of their surface properties and chemistries, particularly the effect of negatively charged surface groups on nanocellulose self-assembly behaviors. The negatively charged surface groups, in this case sodium carboxylates, were converted to the neutral form of carboxylic acid using protonation. This study analyzed how protonation affected the self-assembly of carboxylated nanocelluloses into three-dimensional porous structures called aerogels, which showed super-absorptive properties and tunable wet and pH stabilities.

Technical Abstract: 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.