Polymer-Grafted Cellulose Nanocrystals as pH-Responsive\nReversible Flocculants

Abstract

Cellulose nanocrystals (CNCs) are\na sustainable nanomaterial with\napplications spanning composites, coatings, gels, and foams. Surface\nmodification routes to optimize CNC interfacial compatibility and\nfunctionality are required to exploit the full potential of this material\nin the design of new products. In this work, CNCs have been rendered\npH-responsive by surface-initiated graft polymerization of 4-vinylpyridine\nwith the initiator ceric­(IV) ammonium nitrate. The polymerization\nis a one-pot, water-based synthesis carried out under sonication,\nwhich ensures even dispersion of the cellulose nanocrystals during\nthe reaction. The resultant suspensions of poly­(4-vinylpyridine)-grafted\ncellulose nanocrystals (P4VP-<i>g</i>-CNCs) show reversible\nflocculation and sedimentation with changes in pH; the loss of colloidal\nstability is visible by eye even at concentrations as low as 0.004\nwt %. The presence of grafted polymer and the ability to tune the\nhydrophilic/hydrophobic properties of P4VP-<i>g</i>-CNCs\nwere characterized by Fourier transform infrared spectroscopy, elemental\nanalysis, electrophoretic mobility, mass spectrometry, transmittance\nspectroscopy, contact-angle measurements, thermal analysis, and various\nmicroscopies. Atomic force microscopy showed no observable changes\nin the CNC dimensions or degree of aggregation after polymer grafting,\nand a liquid crystalline nematic phase of the modified CNCs was detected\nby polarized light microscopy. Controlled stability and wettability\nof P4VP-<i>g</i>-CNCs is advantageous both in composite\ndesign, where cellulose nanocrystals generally have limited dispersibility\nin nonpolar matrices, and as biodegradable flocculants. The responsive\nnature of these novel nanoparticles may offer new applications for\nCNCs in biomedical devices, as clarifying agents, and in industrial\nseparation processes.