Preparation and characterization of cellulose nanofibril aerogel cross-linked with maleic acid and sodium hypophosphite

Abstract

Cellulose nanofibril (CNF) is defined as a nano-scale fibrous material which can be obtained from cellulose fiber by means of a mechanical shearing action. Its diameter is in the range of 5 – 50 nm and its length is typically several micrometers. CNF is being studied by academia and industry for various applications

however, the most promising of these is considered to be as a starting material for the preparation of cellulose aerogel. An aqueous suspension of CNF produces a homogeneous hydrogel structure at a concentration of 1 wt % due to mechanical entanglement and interfibrillar hydrogen bonding. This unique capability of CNF to build up a self-assembled hydrogel structure allows for the preparation of a highly porous aerogel through direct water removal by means of freeze-drying. However, the network structure of CNF aerogels is built by interfibrillar hydrogen bonds between adjacent individual fibers. As a result, the network structure of CNF aerogel is easily destroyed by absorbed water. This weakness of the wet strength limits the wider application of the CNF aerogel. In this research, a cross-linked CNF aerogel was prepared. As cross-linking agents, maleic acid and hypophosphite were used. The cross-linking reaction was composed of esterification between maleic acid and cellulose in a suspension state and the formation of cross-linking by means of chemical bonds between cellulose-grafted maleic acid and hypophosphite in an aerogel state. Through this cross-linking reaction, the network stability of the CNF aerogel in a wet state was reinforced. Unlike typical CNF aerogels, the cross-linked CNF aerogel maintained its original shape after immersion in water under a mild shear condition. Moreover, the cross-linked CNF aerogel was rapidly able to absorb considerable amounts of water. The cross-linked CNF aerogel exhibited shape-recovery characteristics as well in a wet state. The shape-recovery characteristics of the wet cross-linked CNF aerogel were explained in terms of the interaction between the absorbed water and the amorphous region of the CNF. As potential applications, carrying media or a supporting matrix for precious materials are feasible. In order to evaluate the potential applicability of these suggestions, the ion-adsorption performance of the cross-linked CNF aerogel was investigated. The surface charge of CNF was made positive by means of a surface modification with glycidyltrimethylammonium chloride (GTMAC). Through an etherification process, GTMAC was grafted onto the surface of the CNF and the zeta potential of the cationic CNF was then increased to +39.5 mV. From the cationically modified CNF, a positively charged cross-linked CNF aerogel was prepared. The functional groups generating the surface charge of the positively charged cross-linked CNF aerogels were quaternary ammonium and carboxylic groups. For the negatively charged cross-linked CNF aerogel, only carboxyl groups contributed to the surface charge. As a result, the zeta potential of both cross-linked CNF aerogels was affected by the pH of the aqueous media. The pH also affected the ion-adsorption performance of the cross-linked CNF aerogels. An adsorption isotherm was carried out and the theoretical maximum adsorption performances of the cross-linked CNF aerogels were calculated using the Langmuir adsorption model. The ns value, representing the maximum ion-absorption capacity of the negatively charged cross-linked CNF aerogel, was 0.79 mmol/g for nickel ion, while the ns value of the positively charged aerogel was 0.62 mmol/g for permanganate ions. These values are low relative to previously reported performance levels of chemically modified micro-particular cellulose absorbent materials, but they are higher than those of commercially available strong acid ion-exchange resins.