# Thermo-Mechanical Properties of PLA/Short Flax Fiber Biocomposites
## 4 Results and Discussion
### Morphological Analysis  
SEM images of the chopped flax fibers and, as an example, the cross-sections of the composite with 10 wt.% flax fibers are reported in Figures 5 and 6, respectively. Further, Figure 7 shows the broken sections of the dog-bone specimens at different fiber loadings after doing tensile test.  
It can be noticed that, due to their very low aspect ratio, no twisting was observed; however, a slight damage, due to the extrusion process, can be observed as many fibers became thinner and weaker.  
As shown, the short flax fibers show a straight shape and appear like sticks. Figure 5(b) shows the typical polyhedron shape of flax fibers in which the elementary fibers are held pack together; the interphase is constituted by hemicellulose and pectin [5]. The adhesion between the fibers and the matrix is good; the fibers appear well adherent to the matrix and the very small space between the matrix and the fibers can be attributable to the different contraction/expansion coefficient of the fiber and matrix during the cryogenic fracture (Figure 5(b)). This supports the results of mechanical tests and analytical modelling. This good adhesion is confirmed also by the Figure 7 where there are many fibers that remain adhered to the matrix under the traction action and a moderate pull-out is observed. In addition, observing Figure 7, the short fibers are randomly oriented within the matrix and maintain their original straight shape without incurring evident deformation during the extrusion process.  
Figure 5: Chopped flax fibers used in this work.  
Figure 6: SEM images of PLA/flax fibers composites with 10 wt.% flax fibers at (**a**) \(\times\)100 and (**b**) \(\times\)750 magnification.  
Few works on the adhesion between PLA and flax fibers in biocomposites have been published. Most publications on the flax fiber composites concern the interaction between flax fibers and polymers including polypropylene (PP), polystyrene (PS), epoxy, bio-epoxy, and bio-phenolic resin, epoxy resins [47; 48]. It is known that natural fibers have a poor adhesion to hydrophobic matrices like PP, because of their hydrophilic nature; consequently, to improve the interaction between these hydrophobic matrices and the flax fibers several chemical treatments were proposed such as mercerization, silane treatment, acylation, peroxide treatment and coatings [47; 49; 50]. Whereas, because of the hydrophilic nature of PLA a better interaction between PLA and flax fibers than between PP or epoxy resins and flax fibers can be expected as assumed by [PERSON] and [PERSON] [51]. Nevertheless, [PERSON] et al. [30] and [PERSON] and [PERSON] [31] observed a poor adhesion between long un-treated flax fibers and the PLA  
Figure 7: SEM images, at different magnifications, of PLA/flax fibers composites: (**a,b**) 10 wt.% of flax fibers, (**c,d**) 20 wt.% of flax fibers and (**e,f**) 40 wt. % of flax fibers.  
matrix and they concluded that adhesion needs to be improved to optimize the mechanical properties of the PLA/flax composites. In this case, very short flax fibers were used and the good adhesion, evidenced by SEM analyses and the analytical modelling, between the short un-treated flax fibers and the PLA matrix can be attributable to the short length of the fibers (low aspect ratio) which facilitated the dispersion of the fibers within the matrix and reduced the fiber agglomeration favoring the wetting of all the fibers by the matrix.