## 4 Conclusions  
A standardized procedure is proposed in the present study, which will be able to provide comparative estimations of the performance of lignocellulosic-based polymer composites based on a thorough study of individual plant fibers. SEM microphotographs are employed in order to study the fibers' morphology, size, surface roughness, and the existence of any surface-lump materials. Moreover, XRD is employed for the estimation of the crystalline index, which is an indirect way of comparatively estimating the amount of cellulose in each fiber. Furthermore, the TGA of fibers is mostly applied for the comparative estimation of moisture content and for the study of the fibers' behavior at the processing temperature of respective composites, since the integrity of the natural composites is highly dependent on it.  
Kenaf demonstrates the highest average fiber diameter, followed by hemp, then ramie, and finally flax, according to the SEM microphotographs. Flax stands out for having the highest lump-material content due to the numerous deposited substances observed on its surface, whereas ramie is notable for its enhanced smoothness, characterized by a uniform and plain texture. For kenaf, its visual appearance appears jagged and irregular due to its fibers primarily consisting of interconnected elementary fibers that form bundles. Correspondingly, hemp displays a partially similar characteristic, with fibers also exhibiting a slightly segmented appearance and surfaces that are less grooved compared to those of kenaf. According to XRD, ramie and kenaf demonstrate higher levels of crystallinity, implying greater cellulose contents, while kenaf possesses the lowest crystallinity index among the compared fibers. Moreover, according to the TGA, ramie has the lowest moisture content, followed by hemp, then flax, and finally kenaf, which has the highest moisture content among the studied fibers. Finally, ramie exhibits the lowest mass loss at the processing temperature of the corresponding composites, followed by hemp, flax, and kenaf, in increasing order of mass loss.  
The subsequent study of the respective polymer composites of various filler contents (10, 20, and 30 wt.%) discusses aspects which are affected by the outcomes of the aforementioned characterization techniques. The thermal stability of polymer composites is studied by means of TGA, where composites whose fibers exhibit higher crystallinity indexes and less moisture absorption, namely ramie and hemp, present greater thermal stability. Moreover, the SEM of polymer composites allows for the study of the extent of interfacial adhesion, which at present study is poor for all fiber-based composites, leading to the low wettability of fibers and to impaired mechanical performance, because no chemical modification is performed in the materials, since the crystallinity of fibers should remain intact and with no crystallinity alterations that extra chemical steps could grant. Furthermore, each composite's mechanical performance is multivariate, but it mainly depends on the particle size, volume fraction, filler-matrix adherence, and degree of filler dispersion. Fibers with smaller diameters and greater crystalline indexes, namely ramie-hemp-flax, present greater Young's modulus values of the corresponding composites, while the interplay between the aforementioned factors leads to various trends when referring to the effect of filler content on the strength of composites.  
Conceptualization, D.N.B. and K.C.; methodology D.P., E.P., D.N.B. and K.C.; investigation D.P., E.P., D.N.B. and K.C.; writing--original draft preparation, D.P.; writing--review and editing, D.P., E.P. E.T., D.N.B. and K.C.; supervision, E.P., D.N.B. and K.C. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.