Carboxymethyl cellulose foams: fabrication, aqueous stability, and water capture

This paper describes the fabrication of biopolymer foams from aqueous solutions of carboxymethyl cellulose (CMC), as well as effective methods for stabilizing CMC foams in aqueous environments so as to prevent their conversion to gels that results from pore collapse. CMC foams were produced easily by lyophilization of aqueous solutions of high molecular weight CMC, but the considerable hydrophilicity of this anionic polyelectrolyte led to rapid pore collapse and gelation of the material upon its contact with liquid water. However, the porous structure of the CMC foam was stabilized in the presence of aqueous solutions of CaCl2. Mechanical measurements showed the foams to be stiffer in the presence of higher salt concentrations. In addition, CMC foams crosslinked with poly(acrylic acid) (PAA), via the residual hydroxyl groups on the CMC scaffold, were stable in liquid water. When covalent crosslinking was combined with the presence of CaCl2, and/or cellulose additives, the resultant CMC foams exhibited excellent aqueous stability for months or longer and withstood multiple compression cycles without loss of mechanical performance (i.e., by maintaining their porous foam structure). These hydrophilic foams absorbed many times their own weight of water (> 30 g of liquid water per gram of foam) and were amenable to ion exchange, absorption/desorption of organic substances and heavy metals, and water uptake at levels that make them attractive materials for applications in water recovery.