RESPONSIVE POLYMER-GRAFTED CELLULOSE NANOCRYSTALS FROM CERIC (IV) ION-INITIATED POLYMERIZATION

Abstract

<p>In recent years, cellulose nanocrystals (CNCs) isolated from natural cellulosic sources through an acid hydrolysis treatment have garnered significant interest in both academia and industry. CNCs have attracted attention not only because they stem from an abundant, renewable resource and are biodegradable, but also because of their low density, light weight, high aspect ratio, high tensile strength and a specific Young’s modulus comparable to steel and Kevlar. These properties make CNCs ideal for use as reinforcing agents in nanocomposites, as well as stabilizing agents in foams and gels. However, due to the high surface area and hydrophilic nature of CNCs, unmodified nanocrystals are difficult to disperse in non-polar materials. The lack of interfacial compatibility between components causes CNCs to agglomerate and thus their incorporation into conventional polymer matrices has been challenging at best, and unsatisfactory in most cases.</p> <p>In this work, CNCs have been rendered pH and temperature-responsive by surface-initiated graft polymerization of 4-vinylpyridine and <em>N</em>-isopropylacrylamide, respectively, using ceric (IV) ammonium nitrate as the initiator. The resultant suspensions of poly(4-vinylpyridine)-grafted cellulose nanocrystals (P4VP-<em>g</em>-CNCs) and poly(<em>N</em>-isopropylacrylamide)-grafted cellulose nanocrystals (PNIPAM-<em>g</em>-CNCs) show reversible hydrophilic to hydrophobic responses with changes in pH and temperature, respectively. The presence of grafted polymer and the tunable hydrophilic/hydrophobic properties were characterized via Fourier transform infrared spectroscopy, elemental analysis, electrophoretic mobility, mass spectrometry, transmittance spectroscopy, contact angle measurements, thermal analysis and various microscopies.</p> <p>The intention of this work has been to shift towards more industrially viable surface modification routes for CNCs by using a one-pot, water-based synthesis to produce a low cost and functional nanomaterial. Moreover, sonication is used throughout the polymerization reaction to avoid particle aggregation and ensure that individual CNCs are surface-functionalized. As a result, reproducible and uniform material properties have been measured in both suspensions and films of modified CNCs. The responsive nature of P4VP-<em>g</em>-CNCs and PNIPAM-<em>g</em>-CNCs may offer new applications for cellulose nanocrystals in hydrophobic nanocomposites, biomedical devices, as clarifying agents, and in industrial separation processes.</p>