Solid-state NMR to quantify surface coverage and chain length of lactic acid modified cellulose nanocrystals, used as fillers in biodegradable composites

Abstract

The surface of cellulose nanocrystals (CNC) was chemically modified with polylactic acid. The modification was characterized by infrared spectroscopy, by monitoring the colloidal behavior of CNC in different solvents, and by quantitative analysis using solid-state NMR (ss-NMR). The quantitative analysis showed that grafted PLA chains on average comprised two lactic acid units attached to 48% of all available hydroxyl groups on the surface of the CNC. Both modified and unmodified CNC were incorporated as fillers in three different biodegradable materials: polylactide acid (PLA), poly(lactide-co-glycolide) (PLGA) and poly(3-hydroxybutyrate) (PHB). Composite films with up to 20 wt% of unmodified and modified CNC were produced via solvent casting. Compared to unmodified CNC, the modified CNC showed less aggregation in organic solvents and hydrophobic polymer materials and increased interaction was seen between the polymer and filler after surface modification. Using ss-NMR, our study shows that even as little as two units of PLA assures stabile CNC dispersions in organic solvents and distribute CNC in hydrophobic polymers.