Raman spectroscopic studies of load transfer in microfibrillated cellulose/poly(Lactic acid) composites.

Abstract

The objective of this research was to investigate the feasibility of using Raman-tensile tests to evaluatethe transfer of load between polylactic acid (PLA) and cellulose nanofibers. Microfibrillated cellulose (MFC) was modified with oligomeric lactic acid. The modified MFC was loaded into a PLA solution before being cast into a film. The nanocomposite films were subjected to tensile loading during which Raman spectroscopy was performed under a microscope. The Raman spectrum of MFC was isolated from the spectrum of its composites. The stress-sensitive Raman band of cellulose (1095 cm-1) was analyzed for wavenumber shift, which is an indication of load transfer from the PLA matrix to MFC. Compared to the control (untreated) sample, composites with lactide-treated MFC exhibited a higher Raman band shift as a response to both applied strain and stress, indicating an improved efficiency of load transfer across the compatibilized interphase between nanofibers and PLA. The effect of MFC addition ratio on the load transfer ability was also discussed. A higher MFC weight fraction (0.5 % versus 0.25 %) in the composite did not significantly affect the load transfer efficiency. This finding suggests that the efficiency of matrix/fiber load transfer at the local level is independent of the effects of macroscopic fiber-to-matrix ratio, at least at the (low) levels of MFC loading examined in this study. Overall, this research shows that it is feasible to use Raman spectroscopy to monitor load transfer in MFC/PLA composites for examining strategies aimed at improving adhesion and knowledge of the mechanism of PLA reinforcement by nano-cellulose.