B.P. Frank, E.R. Caudill, R.S. Lankone, J.A. Pedersen, D.H. Fairbrother
Johns Hopkins University,
Keywords: nanocellulose, biodegradation, sustainable, nanocomposite, biopolymer, functionalization, nanofibrillated cellulose
Summary:Nanocellulose is a naturally abundant biomolecule characterized by impressive mechanical properties and rapid environmental degradability. By virtue of these properties, nanocellulose has been identified as an ideal sustainable option for enhancing material properties in biopolymers as a competitor to anthropogenic carbon-based nanomaterials (e.g. carbon nanotubes), which do not biodegrade. To fully realize its advantages as a polymer additive, nanocellulose often requires hydrophobic surface modification to improve its dispersion in the organic solvents used to create biopolymer composites. However, the effect that these changes in surface chemistry have on the biodegradability of nanocellulose is rarely considered. In this study, we have studied the biodegradability of a variety of surface functionalized/modified nanocellulose. These studies culminated in the identification of functionalization strategies which impart hydrophobic dispersibility while retaining the biodegradability of nanocellulose. Specifically, the effect of both hydrophobic coatings (silanization) and covalent surface functionalizations (esterification, etherification, and oxidation) on the dispersibility and biodegradability of nanocellulose was assessed. Modifications were confirmed via spectroscopic characterization (ATR-IR, XPS, NMR) and through observation of dispersion behavior in organic media and polymer nanocomposites. As a measure of their overall biodegradability, biogas production tests were run in the presence of a mixed culture of anaerobic microorganisms over the span of four months. These tests showed that although silane-modified cellulose nanofibrils (CNFs) dispersed well in polyhydroxyalkanoates (PHA) biopolymer nanocomposites, they exhibited decreased biogas evolution as a function of the extent and hydrophobicity of the silanization. This decrease in biodegradability was ascribed to the formation of an extensive siloxane coating on the surface of the CNFs. Covalent functionalization of CNFs with hydrophobic groups exhibited improved stability in chloroform compared to unmodified CNF. Amongst the various covalent functionalization strategies applied, esterification was shown to most fully retain the overall biodegradability of the nanocellulose while ether functionalized nanocellulose displayed negligible biodegradability. Experiments conducted with subtilisin Carlsberg esterase, an enzyme representative of those encountered in the mixed microbial culture used in our biogas studies, revealed that this behavior was a consequence of the ease with which ester groups were susceptible to enzymatic hydrolysis, effectively regenerating the native biodegradable CNFs. The effect that esterified nanocellulose additives have on the mechanical properties of biopolymer nanocomposites will also be presented. Our results underline the importance of identifying the biodegradability of commercially modified nanocellulose before widespread implementation as a green nanofiller. In this respect, esterified nanocellulose was shown to be an ideal nano-filler, combining stability in organic solvents with rapid biodegradability in aqueous environments.