## 3 Results and Discussion  
### SEM of Natural Fibers  
SEM is an excellent technique for performing a macroscopic analysis of a fiber's morphology by examining the fiber's size and shape, the degree of its surface roughness, and the existence of any surface-lump materials (e.g., wax or other impurities [48]). The fibers' sizes and shapes constitute the most significant factor related to composite materials, with the effective surface area being inversely dependent on the fibers' dimensions. Thinner fibers expose more effective surface areas than thicker ones, providing greater surface area for contact with the polymer matrix and consequently positively influencing the mechanical properties [49]. Furthermore, the inverse correlation between fibers' diameters and their tensile strength is widely accepted, as a consequence of the fibers' heterogeneous microstructure [50; 51]. The main mechanism which is responsible for this correlation lies in the statistical distribution of irregularities throughout the fibers' three spatial dimensions. In the case of small diameters, the density of any defects, flows, or irregularities is smaller in comparison with bigger diameters, with this fact applying both to the surface and to the volume of the fiber. Consequently, thicker fibers tend to be less homogeneous than thinner ones, with a higher density of irregularities, leading to weaker points and stress raisers, capable of leading to premature failure in comparison with thinner fibers. Moreover, the surface morphology of the fibers plays a vital role in the performance of the final composites. The surface properties of each natural fiber influence the interactions between the polymer matrix and fibers, since wettability is greatly dependent on the morphology of the fibers' surface layer [52]. Roughness on the fiber surface can provide mechanical anchoring to the polymer composite matrix, leading to improved mechanical properties when compared to composites based on fibers with smoother surfaces [49]. The mechanism which leads to that superiority in terms of mechanical properties deals with the increase in specific surface area in the case of grid-like, rough surfaces. In that case, an increased number of hydroxyl groups in the cellulose molecules is exposed, which in turn creates the appropriate conditions for the enhancement of fiber-matrix adhesion through hydrogen bonding [53]. According to the fibers' surface topography, shown in Figure 2, it is evident that all the studied fibers differentiate in terms of all the aforementioned specific surface characteristics, implying the existence of differences on the level of interfacial adhesion that each fiber may exhibit in the matrix, when used as reinforcements. The micrographs of all fibers were taken with 350\(\times\) [(2a)--ramie, (2b)--hemp, (2c)--flaw, (2d)--kenaf], 900\(\times\) [(2e)--ramie, (2f)--hemp, (2g)--flaw, (2h)--kenaf], and 2000\(\times\) [(2i)--ramie, (2i)--hemp, (2k)--flaw, (2l)--kenaf] times magnifications. In the case of kenaf, its appearance is quite jagged and irregular, since its fibers are mostly composed of connected elementary fibers, forming bundles. Hemp partially exhibits the same behavior, since along with the individual fibers, some bundles are also observed. This may imply some kind of pretreatment to achieve hemp fibers' separation. Hemp fibers, also, exhibit a slightly segmented appearance, with less-grooved surfaces than those of kenaf. According to SEM microphotographs, flax is the fiber which exhibits the greater lump-material content, since a great number of deposited substances are observed on its surface (Figure 2i). On the other hand, ramie is distinguished for its increased level of smoothness, with a uniform and plain texture, implying a pretreatment process which has led to the removal of any substances on its surface, as has been already mentioned during the fibers' macroscopic examination (Figure 2c).