Spatially Resolved Characterization of Cellulose Nanocrystal–Polypropylene Composite by Confocal Raman Microscopy

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC)–polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm⁻¹, respectively, obtained at 1 μm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.