N IEVE Y SUBLIMACIÓN

Although the significant amount of research done on different aspects of PPNCs, new topics would contribute to better understanding of the structure– property relationships of PPNCs. Many studies have investigated the enhancement of undrawn PPNCs in tensile properties. However, it is well known that additional considerable enhancement of some PP properties such as stiffness and strength can be produced by solid-state molecular orientation [6]. This important area was not investigated for PPNCs. Hence, the present study focuses on the tensile and fracture behaviours of different loading die-drawn PPNCs at various speeds. Orientation can also have a dawn side in strength of the directions other than the draw direction. For this reason, a combined of fracture behaviour is particular interest in studying tensile properties of drawn PPNCs.

From the several studies that have been presented in different areas of PPNCs, the following points are concluded and employed to direct this research: 1. Although the great progress in enhancement of PPNCs tensile modulus that

has been reported, this enhancement is offset by reduction of elongation at break and, in some publications, the impact strength. In most studies, too high fraction of PPMA would be detrimental to the mechanical properties and too low fraction of PPMA could not reach the desirable degree of clay

dispersion. There would be an optimum composition for individual PPMA to be incorporated into the PPNCs.

2. Dispersion of the nanoclay particles can influence the mechanical properties of PP as a result of the high modulus clay inclusions of anisotropic and thus becomes important to mechanical property enhancement; however (as it was suggested in the review), the nanometric scale of clay may play no direct role. On the other hand, non-homogeneous dispersion that produces from the clay aggregation can cause a serious problem for the mechanical properties of PPNCs. Thus homogeneous dispersion is suggested to be desirable to get the benefit of the addition of organoclay to the polymer matrix.

3. For the effect of PPMA-MW and PPMA content, it can be said that PPMA can interact to a lower extent with clay due to non-sufficient grafting content leading to some limited intercalation. However, its higher MW and better miscibility with PP may allow some larger level of intercalation to partial exfoliation to be achieved.

4. There are several techniques that are used to characterize the nanostructure of PPNCs, including SEM, NMR, and neutron-scattering methods (SAXS and WAXS); however, WXRD and TEM analyses should be more regularly employed to afford better assessment in evaluation of different compounding and processing routes. Care should be taken when XRD data alone is used, which only describe the relationship between clay layers in the polymer, not the relationship of the clay to the polymer. Therefore, TEM should be combined as a very powerful tool for the analysis of PPNCs.

5. Most publications have been reported that the PPNCs degree of crystallinity is kept constant or decreases modestly with increasing of clay loading. The average characteristic size of spherulitic structure in crystalline morphology of PPNCs has been found to decrease significantly with increase of clay loading because of a role of nanoparticles in nucleation which leads to a difference in the number of point nuclei in PPNCs. Thus, nucleation of PP crystals on nano-particles represents a phenomenon to be further investigated for the solid phase PPNCs. If this conducting to a crystallinity improvement can be found, it will be possible to better understand the link between nano- dispersion and enhanced tensile properties.

6. PPNCs consently exhibit a remarkable increase in thermal stability. However, an attention should be considered in future work whenever heat and shear are involved.

7. It was shown that the rheological behaviour of PPNCs at lower frequencies is completely independent on the fine structure of the nanocomposites, i.e. whether it is end-tethered or stacked intercalated, but it is depends primarily upon the amount of clay loading in the nanocomposites. However, the viscosity of the PPNCs even at low loading level (less than 5%) can be increased at low shear rates. But at high shear rates, which is similar to what is usually used in practical application, the viscosity is reduced to be in the normal range of PP. It was also shown that the typical rheological response in PPNCs arises from frictional interactions of the silicate layers and not due to the confined polymer molecules in the clay galleries.

8. In melt state, Layered silicate based nanocomposites, like other anisotropic materials, exhibit the ability to orient the silicate layers in response to

externally applied flow. This orientation can lead to improve some properties of PPNCs systems in presence of the compatibilizer. The orientation, which may direct the clay particles and lamellae of the crystal structure in the direction of the applied shear, are suggested to depend on the clay content, crystallinity, the interaction with PP, as well as PP molecular weight, in addition to the introduced shear or rate of deformation.