Polylactic acid nanocomposites toughened with nanofibrillated cellulose: microstructure, thermal, and mechanical properties

Structure-property interdependency is of vital importance in developing advanced polymer nanocomposites as well as enhancing their ultimate properties. In this research study, toughening of polylactic acid (PLA) with nanofibrillated cellulose (NFC) was studied and comparison was made between the thermal and mechanical properties of systems containing pristine and modified NFC. NFC was modified through two different methods: acetylation of hydroxyl groups and grafting of poly(ethylene glycol) (PEG) onto cellulose chains. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and differential scanning calorimetry were employed to probe into the surface characteristics, thermal properties, and crystallinity of NFC/modified NFC, respectively; moreover, SEM imaging was utilized for surface morphology of the samples. Subsequently, PLA with modified and unmodified NFC was prepared to evaluate the filler addition effect on toughness. Acetylated NFC has changed the PLA crystallinity degree and rate, which affected the modulus of PLA, as signaled by the changed NFC surface. Particularly, mechanical and toughening behaviors of the prepared nanocomposites were analyzed based on tensile measurements which showed an eightfold rise in toughening along with 18% decrease in modulus of the samples comprising 1 wt% of acetylated NFC compared to a blank PLA. However, a sevenfold increase in toughening was observed upon introduction of both modified NFC to the PLA matrix. SEM observations divulged proper dispersion of NFC-g-AC in the PLA matrix which reduced stress concentration, but enhanced toughness in comparison with NFC-g-PEG with agglomeration that caused stress concentration leading to brittle behavior. In the light of the obtained results, it can be inferred that brittle PLA can be toughened by the surface-modified NFC. This research study can illuminate the way for future works on the modifications of nano-scale fillers/additives to achieve improved mechanical and thermal properties of PLA.