Blend with Layered Silicate Nanocomposites

The flow activation energy at constant shear stress and at constant shear rate E ) can be obtained from the slope of 

It can be observed from both tests at constant shear stress and constant shear rate that the melt viscosity is reciprocal of temperature. The melt viscosity is relatively related to the structure and free volume, whereby the increase in temperature might result in the enhancement of free volume and the improvement of chain mobility. Thus, viscosity gradually decreased exponentially with rising temperature. It is well known that the value of flow activation energy reflects the temperature-sensitivity of viscosity so, higher E or Ea leads to higher sensitivity of the blends to temperature. It can be seen from the values of E and Ea that E. increases with increasing 

Cooper-White, J.J., Mackay, M.E., 1999. Rheological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(l-lactic acid). J. Polym. Sci. Part BrPolym. Phys. 37, 1803-1814. 

Fukuda, K., 1993. Biodegradable plastics and polymers. In Doi, Y., Fukuda, K. (Eds.), Proceedings of the Third International Scientific Workshop on Biodegradable Plastics and Polymers. Elsevier Science, Amsterdam, pp. 464—469. Osaka, Japan, November 9—11, 1993. 

Dorgan, J.R., Lehermeier, H., Mang, M., 2000. Thermal and rheological properties of commercial-grade poly(lactic acid)s. J. Polym. Environ. Vol 8, 1—9. 

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Solution blending was found to produce a mixed immiscible intercalated nanocomposite with the clay causing a change in the degradation path (94). In situ polymerisation of polymer layered silicate nanocomposites has been investigated (36). 

Accordingly, a two-step method, named masterbatch process, has been approached for the preparation of PCL layered silicate nanocomposites by combining the in-situ intercalative polymerization and the melt blend intercalation process d. In such a process, a highly clay-filled (organo-modified) PCL is first prepared by in-situ intercalation pol)mierization of e-CL, followed by its addition as masterbatch, that is blended with the molten polyester matrix (commercial PCL CAPA 650). As it will be shown, this method permits to prepare PCL-based nanocomposites with a high degree of exfoliation, which cannot be achieved by directly mixing PCL and clay. 

Similarly to PCL layered silicate nanocomposites, melt intercalation of plasticized PLA with a constant amount of nanoclays (3 wt%) leads to an intercalated nanostructure , even for the unmodified natural montmorillonite (Cloisite Na. This particularity can be explained by the sole intercalation of the plasticizer (PEG chains) into the interlayer spacing of the filler, leading to an increase of the interlayer distance from 1.21 to 1.77 nm, as already observed for simple blends of natural montmorillonite with PEG alone. Selective PEG intercalation was further confirmed by the impossibility to form a nanocomposite by melt blending (non plasticized) PLA with Cloisite Na, only microcomposites could be recovered. XRD analysis performed on organoclay based blends (Figure 8) does not allow... 

Aliphatic polyester layered silicate nanocomposites based on poly(e-caprolactone) (PCL) and on plasticized poly(L-lactide) (PLA) have been prepared first by melt blending of the respective polymer matrix with different (organo-modified) montmorillonites. It has been demonstrated that melt blending with organo-modified clay such as Cloisite 20A, 25A or SOB, yields intercalated nanocomposites with the possibility of partial exfoliation. Even at low organoclay content, substantial improvement of thermal stability, gas barrier properties and physical-mechanical performances have been noticed. However, melt blending of natural montmorillonite with PCL or neat (non plasticized) PLA leads to microphase-separated compositions. 

Unlike melt intercalation, a layered silicate is mixed with monomer before polymerization takes place with in situ polymerization. This method was developed by Toyota researchers [27,28], in which electrostatically held 1-nm thick layers of layered alumina silicates were dispersed in a polyamide matrix on a nanometer level, which led to an exponential growth in the research endeavors, in layered silicate nanocomposites. These nanocomposites were based on the in situ synthesis approach in which a monomer or monomer solution was used to swell the filler interlayers, followed by polymerization. With this process, one can control the nanocomposite morphology through the combination of reaction conditions and clay surface modification. The in situ polymerization method is especially important for insoluble and thermally unstable polymers, which solution blending or melt blending technique cannot process. 

Even after organic modification of the clays, polypropylene does not wet the surface of clays because it is nonpolar. It is necessary to blend in a functionalized polymer such as maleated polypropylene (PP-g-MA) that wets the modified clay surface more readily and is also miscible with the bulk polymer. Okada and coworkers were the first to produce polypropylene layered silicate nanocomposites by melt compounding the modified elay with PP-g-MA and PP. The progress made since then in preparing and characterizing polypropylene layered silicate nanocomposites is reviewed in this chapter. We discuss advances in formulations, preparation methods and characterization then proceed to effects of the dispersion state (intercalated vs. exfoliated) and of silicate loading on crystallinity, mechanical performance and other properties, and end with a summary of progress to date with these composites. All the results presented in this chapter refer to isotactic polypropylene nanocomposites with layered or smectite clays. 

EVA/organic layered silicate nanocomposites can be also prepared by melt intercalation method. Alexandre and Dubois" found that nanocomposites were only formed when EVA was melt-blended at 1S0 C with nonfunctionalized organoclay, such as MMT exchanged with dimethyldioctadecyl ammonium. Zanetti et prepared EVA nanocomposites with fluorohectorite-like synthetic silicate exchanged with octadecylammonium and studied their thermal behaviors. 

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