Exfoliation polypropylene nanocomposites

Fig. 5. Schematic of the hierarchy of clay structures in polypropylene nanocomposites of mixed morphology. Clay tactoids and exfoliated platelets comprise the mesoscale morphology. The internal intercalation structure of clay tactoids is determined by the compatibilizer and compounding conditions. (View this art in color at www.dekker.co...

Kalaitzidou, K., Fukushima, H., and Drzal, L. T. (2007], A new compounding method for exfoliated graphite-polypropylene nanocomposites with enhanced flexural properties and lower percolation threshold. Compos. Sci. TechnoL, 10, pp. 2045-2051. 

The effect of stereochemistry and polydispersity on nanocomposite properties has yet to be explored fully. An intriguing result was obtained by Quijada et al. who performed melf mixing of PP-g-MA with octadecylamine-modified montmorillonite or hectorite, then blended these materials with different isotactic polypropylenes. Nanocomposites with better clay exfoliation were obtained using metallocene polypropylene compared to Ziegler-Natta polypropylene, presumably because of ifs lower polydispersity. While stereoselective metallocene catalysts have been used successfully for in situ propylene polymerization in combination with clay supports (see Section 5.2.2.2), the materials properties of these nanocomposites have thus far received insufficienf affention. 

The wide-angle x-ray diffraction patterns of the polypropylene nanocomposites with and without compatibilizer have been shown in Figure 10.11. The diffraction pattern of pure imidazolium-treated montmorillonite has also been plotted for comparison. The presence of basal peaks in all the composites containing different compatibilizers indicated that full exfoliation of the filler did not take place. Also, the increase in the basal plane spacing in the composites as compared to the modified filler or the composite without compatibilizer... 

SongP, CaoZ, Cai Y, Zhao L, Fang Z, Fu S (2011) Fabrication of exfoliated graphene-based polypropylene nanocomposites with enhanced mechanical and thermal properties. Polymer 52 4001 010... 

Lew C.Y., Murphy W.R., McNally G.M., Abe K., Yanai S., Metallocene-catalyzed polypropylene nanocomposite fibers the effect of melt spinning on the exfoliation of layered-silicates , Polymer Fibers 2004, Manchester 2004. 

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. 

Tjong et al. produced polypropylene nanocomposites with 2 wt.% exfoliated vermiculite after pre-treating the vermiculite with maleic anhydride for in-situ compatibilizer generation the vermiculite aspect ratio was at least twice that of the montmorillonite. The tensile modulus improved from 0.84 GPa for their neat PP to 1.3 GPa for the 2 wt.% nanocomposite. The tensile strength improvement was also more striking going up from 28 MPa to 37 MPa. 

Dolgovskij, M.K., Fasulo, P.D., Lortie, F., Macosko, C.W., Ottaviani, R.A., and Rodgers, W.R., Effect of Mixer Type on Exfoliation of Polypropylene Nanocomposites, Society of Plastics Engrs. Annual Tech. Conf, 61, 2255-2259 (2003)... 

There are two basic types of nanocomposites, in which particles are intercalated or exfoliated. In an intercalated composite the nanodispersed filler still consists of ordered structures of smaller individual particles, packed into intercalated structures. Exfoliated particles are those dispersed into practically individual units, randomly distributed in the composite. Layered silicates, such as montmorillonite clays or organoclays, can be used in nanocomposites. Because clays are hydrophilic and polyolefines are hydrophobic, it is not easy to make a nanocomposite based on polyethylene or polypropylene because of their natural incompatibility. 

The reinforcement of polypropylene and other thermoplastics with inorganic particles such as talc and glass is a common method of material property enhancement. Polymer clay nanocomposites extend this strategy to the nanoscale. The anisometric shape and approximately 1 nm width of the clay platelets dramatically increase the amount of interfacial contact between the clay and the polymer matrix. Thus the clay surface can mediate changes in matrix polymer conformation, crystal structure, and crystal morphology through interfacial mechanisms that are absent in classical polymer composite materials. For these reasons, it is believed that nanocomposite materials with the clay platelets dispersed as isolated, exfoliated platelets are optimal for end-use properties. 

Polypropylene (PP) is one of the most widely used plastics in large volume. To overcome the disadvantages of PP, such as low toughness and low service temperature, researchers have tried to improve the properties with the addition of nanoparticles that contains p>olar functional groups. An alkylammonium surfactant has been adequate to modify the clay surfaces and promote the formation of nanocomposite structure. Until now, two major methods, i.e., in-situ polymerization( Ma et al., 2001 Pirmavaia, 2000) and melt intercalation ( Manias et al.,2001) have been the techniques to prepare clay/PP nanocomposites. In the former method, the clay is used as a catalyst carrier, propylene monomer intercalates into the interlayer space of the clay and then polymerizes there. The macromolecule chains exfoliate the silicate layers and make them disperse in the polymer matrix evenly. In melt intercalation, PP and organoclay are compounded in the molten state to form nanocomposites. 

Depending on the degree of modification, intercalated or exfoliated nanocomposites can form. The degree of dispersion of clay platelets in thermoplastic polypropylene matrix also depends on the type of the com-patibilizer used. The compatibilizer as such may be compatible with the clay surfaces and hence easily intercalates the interlayer spacing of the clay... 

In the last decade, considerable progress was observed in the field of PO/compatibil-izer (predominantly on the base of PO-g-MA)/organo-surface-modified clay nanocomposites. Polyethylene (PE), polypropylene (PP), and ethylene-propylene (EP) rubber are one of the most widely used POs as matrix polymers in the preparation of nanocomposites [3,4,6,30-52]. The PO silicate/silica (other clay minerals, metal oxides, carbon nanotubes, or other nanoparticles) nanocomposite and nanohybrid materials, prepared using intercalation/exfoliation of functionalized polymers in situ processing and reactive extrusion systems, have attracted the interest of many academic and industrial researchers because they frequently exhibit unexpected hybrid properties synergisti-cally derived from the two components [9,12,38-43]. One of most promising composite systems are nanocomposites based on organic polymers (thermoplastics and thermosets). 

Ratnayake, U. N. and Haworth, B. 2006. Polypropylene-clay nanocomposites Influence of low molecular weight polar additives on intercalation and exfoliation behavior. Polymer Engineering and Science 46 1009-1015. 

He, A. H., Wang, L., Li, J., Dong, J.-Y, and Han, C. C. 2006. Preparation of exfoliated isotactic polypropylene/alkyl-triphenylphosphonium-modified montmorillonite nanocomposites via in situ intercalative polymerization. Polymer 47 1767-1771. 

Moncada, E., Quijada, R., and Retuert, J. 2007. Comparative effect of metallocene and Ziegler-Natta polypropylene on the exfoliation of montmorillonite and hectorite clays to obtain nanocomposites. Journal of Applied Polymer Science 103 698-706. 

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