The physicochemical effect of sugar alcohol plasticizers on oxidised nanocellulose gels and extruded filaments

Oxidised cellulose nanofibrils (OCNFs) have previously shown promise for the production of extruded filaments with high tensile strength properties. However, they also exhibit poor wet strength due to swelling upon immersion in water. This has resulted in the use of chemical cross-linkers, or co-extrusion with multivalent cations, or cationic polymers, to inhibit this. Interestingly, alcohols have also been reported to promote attraction between fibrils in primarily aqueous gels, which suggests that they could also be used to limit filament swelling. Sugar alcohols (SAs), in particular, are less volatile than other alcohols, are often used as ‘natural’ plasticizers, and many have FDA approval for use in food products.
Here, we report on the effect of incorporating SAs (glycerol, sorbitol and maltitol) in the OCNF gel before filament extrusion. Whilst their presence weakens the initial gel structure (Figure 1a), they enable the continuous wet spinning of OCNF filaments that are stable in aqueous media (Figure 1b). Analysis – including rheology, confocal fluorescent spectroscopy, and QCM-D – of the OCNF/SA systems lead us to conclude that the relative hydrophilicity of the SA and its ability to protonate surface carboxyl groups upon drying are key parameters regarding the physicochemical effects observed.

Figure 1. a) Rheological properties of OCNF-SA gels. Storage (closed symbols) and loss (open symbols) moduli of the pure OCNF gel (black diamonds), and OCNF gels with glycerol (green triangles), sorbitol (red circles) and maltitol (blue squares). b) Confocal fluorescent spectroscopy images of i) pure OCNF filament and OCNF filaments with ii) glycerol, iii) sorbitol, and iv) maltitol after immersion in water for 30 min.