## 3 Results
Several researchers have already reported that \(\alpha\)-monoclinic forms of PP are more brittle, while \(\beta\)-forms are ductile up to large strains [3; 38; 39]. Consequently, the further investigation was focused on the analysis of morphology changes of WF-PP and WF-R-PP composites determined with XRD measurements. In addition, X-ray diffraction was used to obtain more details about the nature of the crystallographic structure of WF-PP and WF-R-PP composites as a result of added WF. The X-ray diffraction results showed the influence of WF PP matrix addition on the polymorphism of the prepared WF-PP composites. The diffractograms of unrecycled PP and WF-PP composites with various amount of WF are presented in Figure 6. The diffraction peaks for PP are positioned at 20 angles of 13.9\({}^{\circ}\) (110), 16.8\({}^{\circ}\) (004), 18.5\({}^{\circ}\) (130), 21.4\({}^{\circ}\) (111) and 28.6\({}^{\circ}\) (200). All these peaks reflect the \(\alpha\)-crystalline phase of PP. On the other hand, the diffraction peaks of WF-PP composites were determined at 20 angles of 13.4\({}^{\circ}\) (110) and 18\({}^{\circ}\) (130), which revealed the \(\alpha\)-crystalline phase of PP. The diffraction peaks positioned at 20 angles of 16.3\({}^{\circ}\) (020), 21\({}^{\circ}\) (121), 28.1\({}^{\circ}\) (012) reflected the \(\beta\)-phase, and 14.9\({}^{\circ}\) (113) determined the \(\gamma\)-phase. By increasing the wt.% of WF the intensity of \(\beta\)-phase and \(\gamma\)-phase of PP increased, resulting in a polymorphic structure of the WF-PP composites. Figure 6: X-ray diffraction patterns of PP and WF-PP composites. Figure 7 shows the diffractograms of recycled PP (R-PP) and WF-R-PP composites. Comparing the results of R-PP matrix to the composites at all loading levels shows that there are no differences in diffraction peaks, and there is no evidence of any phase transformation. Diffraction peaks for the \(\alpha\)-crystalline phase of R-PP and all WF-R-PP composites were positioned at 20 angles of 13.9\({}^{\circ}\) (110), 16.8\({}^{\circ}\) (004), 18.5\({}^{\circ}\) (130), 21.4\({}^{\circ}\) (111) and 28.6\({}^{\circ}\) (200). The introduction of WF in the PP matrix resulted in the additional formation of the another two of the most prominent phases of PP, \(\beta\) and \(\gamma\)-phases. The formation of the well-known \(\beta\)-phase in PP composites has been in the literature explained with the first kinetic aspect of \(\beta\)-structure formation. Another reason for \(\beta\)-phase formation is related to the modification of wood filler. In detail, researchers who studied PP composites with sisal fibers, and another who studied PP composites with flax and Kevlar fibers, explained the formation of the \(\beta\)-phase through the occurring addition of any types of fillers. That means that fillers' increased roughness resulted in the presence of higher shear forces, which are responsible for the \(\beta\)-phase of PP formation [40; 41; 42; 43]. In contrast, for recycled based composites (WF-R-PP) only the main \(\alpha\)-crystalline phase of PP was detected, and the addition of wood fibers had no effect on the modification of the crystalline structure. From the presented results, it is clear that the interaction between the fibers and the polymeric matrix is different for pure PP and recycled PP (R-PP) materials. A handy tool for understanding the filler--polymer interaction is melt rheology. Figure 8 shows the results of the strain sweep experiments for both matrix materials (PP and R-PP) and the corresponding composites. All materials exhibited a linear behavior at low strains, followed by non-linear behavior, where storage modulus dropped considerably from constant values. As the shear strain increased, polymer molecules started to disentangle, which lead to the decrease of storage modulus as the strain was increased. For the unrecycled PP and recycled PP (R-PP) the change from linear to non-linear behavior occurred at shear strain around \(\gamma\sim 0.1\).