Biobased Materials and Biocomposites (BMB)

Plant-derived fibers, such as flax, hemp, jute, sisal, coir, and bamboo, are being incorporated into polymer matrices at an increasing rate. This trend has revealed a gap in the analytical toolbox: very few techniques provide molecular specificity together with sub-micrometer spatial resolution at the level of an individual fiber, and even fewer can do so without destroying the sample. Raman spectroscopy offers both requirements in one non-destructive measurement. However, there is still no literature review that explicitly deals with its application in natural fiber reinforced composites (NFRCs); the corresponding results are scattered over studies addressing other questions. In this work, we bring together this scattered body of work and critically evaluate it. Five thematic strands are discussed: the spectral fingerprints of cellulose, lignin and hemicellulose; molecular tracking of surface treatments (mercerization, silane grafting, acetylation); confocal Raman imaging of fiber/matrix microstructure; micromechanical analysis based on stress-induced band shifts; and assessment of degradation and durability under ultraviolet, hydrothermal and oxidative ageing. The glycosidic C–O–C stretching band at about 1095 cm⁻¹ is a recurring motif: it behaves as a molecular strain gauge with a shift rate Δν/Δε between −0.5 and −4.3 cm⁻¹/% strain depending on fiber type and crystallinity, which opens the way to spatially resolved measurements of interfacial shear stress that bulk pull-out testing cannot provide. We also compile published band-shift data from across fiber species into one comparison and benchmark Raman directly against FTIR, XRD and SEM. When used in mapping mode, the technique can resolve degradation in its component processes, cellulose amorphization, lignin photo-oxidation and matrix chemi-crystallization phase by phase. As far as we are aware, no competing method combines chemical specificity, sub-micrometer resolution and mechanical sensitivity in a single non-destructive measurement. The limitations are set out with equal candour: an intrinsically weak effect, fluorescence from lignin-rich fibers, a mapping throughput still far below industrial requirements, and a sampling depth governed by excitation wavelength and objective numerical aperture. Further into the future, nanoscale tip-enhanced Raman spectroscopy, MCR-ALS chemometric decomposition and convolutional neural networks (already exceeding 95% classification accuracy) should help to bring the technique out of specialist laboratories and into routine industrial quality assurance, provided that an open-access, curated spectral database of natural fibers is assembled in parallel to these advances.