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5.3.3.1 Tensile properties

The Young modulus, maximum tensile strength and elongation at break of the samples are compared to those of the neat PP in Figure 5.4. The Young modulus of PP is increased with the addition of CNCs and the compatibilizer (CO) by ca. 13 % and 26 %, respectively, as seen in Figure 5.4 a. A synergistic effect is observed in the nanocomposite samples containing both the CNCs and CO as the Young modulus increased by ca. 36 % when the masterbatch was prepared via melt mixing and ca. 47 % when the masterbatch was prepared via solution mixing. Here again

one can see another proof that the solution mixing method was more efficient for preparing the masterbatch, leading to a finer dispersion of the CNCs in the matrix.

In Figure 5.4b, the tensile strength of the nanocomposites does not show any enhancement compared to that of the neat PP, whereas by comparing the two compatibilized nanocomposite samples, i.e. PP/15SCO/5CNC and PP/15CO/5CNC, a 24 % larger tensile strength is observed for the sample in which the masterbatch was prepared via solution mixing. A similar tensile behavior, enhanced Young modulus but unchanged tensile strength, has also been observed for other nanocomposites [9, 22, 24]. The investigators ascribed this behavior to the lack of stress transfer from the matrix to the filler. Therefore, the composite is not able to show its potential. This happens usually when the polarity of the matrix and the filler is different and there is no strong bonds between the filler and the matrix, which would interfere with the stress distribution throughout the composite when a load is applied [28]. Finally, as shown by Figure 5.4c the elongation at break decreases significantly when CNCs are added to PP, which is generally the case of composite materials due to the reduction of chain mobility by the fillers.

Figure 5.4: Comparison of the tensile properties: Young modulus (a), tensile strength (b) and tensile strain at break (c) of the samples. The numbers above the bars represent the changes in percent with respect to the neat PP

5.3.3.2 DMTA properties

The variations of tan δ and the storage modulus obtained from DMTA for the samples over a wide range of temperature are displayed in Figure 5.5. Two characteristic peaks in tan δ are observed for all samples (Figure 5.5a). The peak observed at high temperature is the α-transition, which is related to the relaxation of trapped PP amorphous chains in the crystalline structure of the PP. It can also be ascribed to the local mobility in crystalline lamellae, which affects the trapped chains in the crystalline phase [37, 38]. It is worth mentioning that this transition in PP sometimes has been observed as a shoulder instead of a peak [39]. The first transition, the β- transition occurring at ca. 0 C, is related to the unrestricted PP amorphous chains and is considered as the glass transition temperature of PP (Tg) [37-39]; Tg of PP is not affected

significantly by the addition of the CNCs nor the compatibilizer. However, the area under the peak of Tg, which is strongly related to the extent of damping or energy dissipation due to the

segmental motion of the PP chains at Tg, is decreased by either the CNCs or the compatibilizer

within the PP phase. In case of the compatibilized nanocomposite samples this reduction is more significant especially when the masterbatch was prepared via solution mixing (PP/15SCO/5CNC). This motion will be reduced if there is any physical or chemical interaction between the polymer chains and the nanoparticles [38, 39]. A better dispersion of the filler leads to a larger interfacial area, which restricts more the polymer chains. Consequently, less energy is required for segmental motions and damping decreases at Tg.

Figure 5.5: DMTA data for the various samples over a large range of temperature, from −70 to 140 °C: (a) tan δ (the numbers in the legend of the figure represent the area under the Tg peak

observed for each sample divided by that of the neat PP); (b) storage modulus

Figure 5.5b shows that the storage modulus of PP is enhanced by the addition of the CNCs. This improvement is even better when the masterbatch containing both the nanoparticles and the compatibilizer were mixed to the neat PP, especially using the solution method. In fact, a better state of dispersion of the CNCs in PP/15SCO/5CNC made the material stiffer, with a higher modulus. The storage modulus of PP is enhanced up to ca. 60 % when a good state of dispersion is achieved using the compatibilizer. Similar results were obtained by Khoshkava and Kamal [34].

5.3.3.3 DSC properties

Figure 5.6 illustrates the effect of the CNCs and use of CO on the crystalline content of PP and onset temperature of crystallization during the cooling cycle. The crystalline content of the samples reported in Figure 5.6a was calculated from the enthalpy of melting, ∆Hm, in the first

(5.1)

where 𝑤_𝑃𝑃 is the weight fraction of the PP phase and ∆𝐻_𝑚0 _{is the enthalpy of the 100 %}

crystalline PP (207 J/g) [40].

Figure 5.6: Crystalline content of the samples (a), calculated based on the data obtained in the heating cycle using Eq. 1, and comparison of on-set temperature of the crystallization peak of the samples (b), occurring in the cooling cycle

In crystallization of polymers both the total amount of nuclei and the mobility of polymer chains affect the total crystalline content, the former increases whereas the latter decreases with the addition of nanoparticles. Therefore, sometimes no significant changes are observed with the addition of nanoparticles. Here, the addition of the CNCs slightly changed the total crystalline content of PP, up to ca. 13 % increase in PP/15SCO/5CNC. Almost half of this extent of increase in crystalline content was reported for polycaprolactone (PCL) containing 6 wt% unmodified Luffa cylindrica nanocrystals [14] and for polyethylene (PE) containing 5 wt% ramie cellulose whiskers modified with stearoyl chloride [24]. As the specimens for DSC measurements were cut from the dumbbell samples for tensile tests, we can conclude that the changes in the crystalline content of the samples had little effect on the enhancements of the mechanical properties. Figure 5.6b compares the onset temperature of crystallization of samples in cooling cycles, Tc, which is

gradually shifted to higher values compared to thatof the neat PP. This shift, from ca. 117 C for 0 m c pp m H X w H   

the neat PP up to ca. 131C for PP/15SCO/5CNC, can be definitely ascribed to the nucleation effect of the CNCs on the crystallization of PP, although a part of this change is due to the presence of CO. Therefore, better dispersed CNCs in PP/15SCO/5CNC compared to the other samples could act as nucleation sitesfor the crystal growth in PP.