Impact of Silanization on the Structure, Dispersion Properties, and Biodegradability of Nanocellulose as a Nanocomposite Filler

For nanocellulose to function effectively as a nanofiller in polymers, its interfacial properties are often modified to enhance the dispersion of nanocellulose in the polymer matrix. However, the effect of different surface modification strategies on the persistence of nanocellulose in the environment is unclear. In this study, we examined the effect of three different hydrophobic silanization reagents on the structure, dispersion properties, and biodegradability of cellulose nanofibrils (CNFs). Specifically, we modified CNFs with hydrophobic alkoxysilanes containing methyl, propyl, or aminopropyl functional groups to form silane-modified CNFs (Si-CNFs). Using a combination of analytical techniques that included ATR-IR, XPS, and solid-state NMR, we demonstrated that silanization coated the CNFs with a nanometer-scale siloxane layer, and the extent of the siloxane coating could be controlled by varying the amount of silane added to the CNFs. The stability of Si-CNFs in chloroform-based casting solutions improved compared to untreated CNFs, and scaled with extent and hydrophobicity of the siloxane coating as quantified via a mass recovery settling test. Improvements in stability in casting solutions translated into improved Si-CNF dispersion in solution-blended polyhydroxyalkanoates composites as determined with optical microscopy and SEM. Conversely, the biodegradability of Si-CNFs, assessed by sample mineralization in a mixed microbial culture from an anaerobic sludge digester, was inversely related to both the degree and hydrophobicity of CNF surface modification. As mineralization of nanocellulose is rapid and complete, tracking biogas production served as a proportional measure of overall biodegradability. In the most extensively silanized samples, no mineralization of Si-CNFs was observed, demonstrating that a <2-nm-thick siloxane coating was sufficiently dense and uniform to prevent microbial access to the easily mineralized nanocellulose substrate. This study highlights the important and contrasting effects that changes to surface chemistry can have on the material and environmentally relevant properties of nanocellulose.