Colloidal Behavior of Cellulose Nanocrystals Grafted with Poly(2-alkyl-2-oxazoline)s.

Abstract

Polymer grafting onto cellulose nanocrystals (CNCs) has been used as a tool to improve CNC dispersion in nonpolar solvents or polymeric matrixes. The grafting of flexible polymer chains onto rigid particle surfaces leads to significant modifications in colloidal behavior. Here, poly(2-alkyl-2-oxazoline)s of well-defined molar mass and narrow molar mass distribution were synthesized by cationic ring-opening polymerization and grafted onto CNC surfaces, where the coupling reaction was favored when partially hydrolyzed polymers were used (reaching 64% reaction yield). The particles grafted with polymer chains could be redispersed in water after freeze-drying, producing stable dispersions, and they were not cell-toxic up to 10 wt % aqueous dispersion. Colloidal stability, nanostructure organization, and rheological behavior of grafted CNC and CNC-grafted CNC mixtures were evaluated. The rheological behavior of grafted nanoparticles, meanwhile, showed new features when compared to original CNC dispersions. Aqueous CNC dispersions showed a liquid crystal nematic organization and rheological behavior characteristic of true gel (at 5 wt %) prior to drying. On the other hand, nanoparticle dispersions behaved as weak gels upon the addition of 10 wt % of CNC-g-(PEtOx95-s-Ei5) under the same conditions. Dispersions of CNC-g-P(PEtOx-s-Ei) particles obtained by redispersion of freeze-dried particles behaved as a fluid, without the presence of the nematic organization. Through oscillatory rheology and time-domain NMR results, it can be concluded that polymer–water interactions are dominant over CNC–water interactions, being responsible for CNC nematic phase disruption. By introducing polymer chains, the introduction of isotropic character modifies water organization, changing the flow behavior of CNC-grafted with poly(oxazoline)s.