Strengthening effect of nanofibrillated cellulose is dependent on enzymatically oxidized polysaccharide gel matrices

Galactose oxidase (GaO)-catalyzed oxidation of the terminal galactosyl groups of guar galactomannan (GM) and tamarind seed galactoxyloglucan (XG) results in cross-linking of these polysaccharides via hemiacetal bonds and formation of elastic hydrogels (GMox and XGox, respectively), and enables the formation of aerogels. Nanofibrillated cellulose (NFC) was added at varying quantities to the aqueous GM and XG solutions and was entrapped in the three-dimensional structure of polysaccharides by enzyme mediated gelation. Addition of NFC up to 25% did not hinder the enzyme activity. The reinforcing effect of NFC on hydrogels was measured with rheometer and texture analyzer. The GMox hydrogels with 25% NFC exhibited higher elastic modulus than corresponding XGox hydrogels, but the latter showed higher compressive modulus. On the other hand, the lyophilized GMox aerogels containing 25% NFC showed the highest compressive modulus, 167 kPa. The reinforcing effect of NFC depended on the type of polysaccharides (GM/XG) and state of the material (hydro- or aerogel). Viewing the aerogels with focused ion beam scanning electron microscopy showed that the pores were approximately 125–250 μm in diameter.