# Effects of Matrix Modification on the Mechanical Properties of Wood-Polypropylene Composites
## 3 Results and Discussion
### Characteristics of the Composites
#### 3.2.2 Dynamic Mechanical Analysis  
The variation in storage modulus (G\({}^{\prime}\)) and tan\(\delta\) (damping factor) with temperature for different compositions of the composites is shown in Figure 8. Compared with G\({}^{\prime}\) value of WP at temperatures ranging from \(-50\) to \(30\)\({}^{\circ}\)C (Figure 8a), WPMA exhibited a higher G\({}^{\prime}\) value, and WMPD0 obtained a lower G\({}^{\prime}\) value. With a further increase in temperature above \(80\)\({}^{\circ}\)C, WMPD0 exhibited a higher value of G\({}^{\prime}\) than that of WPMA. This result suggested that effective coupling occurred between WF and matrix in WMPD0 [30; 31]. Figure 8b presents the corresponding tan\(\delta\) curve of the unmodified and modified composites. Compared with the unmodified composites, the WPMA and WMPD0 composites showed a significant decrease in the damping factor. The decrease can be attributed to the effective coupling between WF and the matrix, which tends to dissipate less viscous energy, thereby obtaining a lower value for tan\(\delta\)[32]. Moreover, WMPD0 exhibited a lower tan\(\delta\), compared with WPMA. The reason was that in higher WF content (60% in weight), the modified matrix underwent severe chain scission,  
Figure 6: Mechanical properties of the composites modified with various DVB concentrations. (**a**) Tensile strength; (**b**) tensile modulus; (**c**) flexural strength; and (**d**) flexural modulus. Figure 7: Changes in impact strength of WF/PP composites at various DVB concentrations. which showed a tendency to improve WF dispersion in the composites, and strengthened the interfacial coupling. The immobilized PP matrices surrounding the fibers in the strong fiber-matrix interface hindered the matrix chain motion during relaxation, and then reduced the deformation or friction between the fibers and the matrix, thereby decreasing tan8[33]. This trend was consistent with the aforementioned mechanical properties (Table 3). Figure 8c,d show the effects of matrix modification with different DVB concentrations on the G\({}^{\prime}\) and tan\(\delta\) of the resulting composites. The G\({}^{\prime}\) values of the composites were markedly increased by increasing the DVB concentration (Figure 8c). This increase in storage modulus may be attributed to the improved interfacial adhesion between WF and the modified matrix, as well as the decrease in degradation via the chain scission reactions of the modified matrix. However, with a further increase in the DVB concentration to 0.8% at a temperature above 30 \({}^{\circ}\)C, the lowest G\({}^{\prime}\) value was observed. This trend was consistent with the aforementioned mechanical properties of the resulting composites (Figures 6 and 7). This result indicated the critical concentration of DVB, beyond which the wetting ability of WF by the modified matrix and interfacial adhesion was decreased [34; 35; 36]. The damping factor, tan\(\delta\), was independent of the stiffness of the composites. The viscoelastic response of the composites was related to the nature of the individual composites, their fractions in the composites, and the fiber-matrix interface [31; 35]. When WF content was constant, and the DVB concentration was increased to 0.4%, the damping factor of the modified composites decreased, which could be attributed to the restriction of chain mobility due to strengthened interfacial adhesion (Figure 8d). This increased adhesion, reduced the viscoelastic lag between the stress and strain, and less viscous energy dissipated, thereby decreasing tan\(\delta\). With a further increase in DVB concentration, the MAH grafting level decreased, and was accompanied by a higher degree of branching and crosslinking (Figure 4), indicating that matrix chain motion in WMPD0.8 is more difficult than that in WMPD0.6 [33; 34]. Therefore, less viscous energy dissipated in WMPD0.8, and tan\(\delta\) was lower in WMPD0.8 than in WMPD0.6.