Polymer-clay composites Polymerization

Besides properties (which are usually enhanced by adding clay), another crucial factor of PCN materials elaborated through emulsion polymerization is their solid content. Although this point is less frequently addressed, most of the polymer/clay composite latexes reported in literature have solid contents below 20%. However, solid contents between 40% and 60% and sometimes higher are required for industrial applications. Using a seeded semi-batch emulsion polymerization process and a procedure otherwise very similar to that described above for Bentonite,... 

There have been many other reports on direct emulsion polymerizations involving nascent clays, either using clays as stabilizing agents or with the aim of preparing polymer-clay nanocomposites. In addition, organically modified clays have also been used in direct emulsion polymerization. Improved mechanical and thermal properties, reduced vapor permeability and improved flame retardancy for the prepared polymer-clay composites were reported. 

Three main types of structures, which are shown in Fig. 5.3, can be obtained when a clay is dispersed in a polymer matrix (1) phase-separated structure, where the polymer chains did not intercalate the clay layers, leading to a structure similar to those of a conventional composite, (2) intercalated structure, where the polymer chains are intercalated between clay layers, forming a well ordered multilayer structure, which has superior properties to those of a conventional composite, and (3) structure exfoliated, where the clay is completely and uniformly dispersed in a polymeric matrix, maximizing the interactions polymer-clay and leading to significant improvements in physical and mechanical properties [2, 50-52]. Production of nanocomposites based on polymer/clay can be done basically in three ways (a) in situ polymerization, (b) prepared in solution and (c) preparation of the melt or melt blending [53]. 

Funck, A. Kaminsky, W. (2007) Polypropylene carbon nanotube composites by in situ polymerization. Compos. Sci. Technol. Vol.67, No.5, pp.906-915 Garda-Leiner, M Lesser, A. (2003) Melt Intercalation in Polymer-Clay Nanocomposites Promoted by Supercritical Carbon Dioxide. Polym. Mater. Sci. Eng. VoL89, pp.649-650... 

Unlike polymer-clay nanocomposites, in rubber-clay nanocomposites complete exfoliation of clay layers results in disappearance of the diffraction maxima in their XRD patterns. However, this can also occur due to other reasons, like extremely low concentration of clay materials in the composites, crystal defects, etc. The majority of the reports on rubber-clay nanocomposites display the intercalated or swollen nature of the clay structures. The presence of the basal reflections in the XRD patterns of such type of nanocomposites indicates that the clay crystal structure is not destroyed completely. But, shifting of their positions to lower 26 values is interpreted as an expansion of the interlayer region by the macromolecular rubber chains. Besides, broadening of the characteristic reflections in nanocomposites is often related to the defects in the crystal layer stacking caused by the interlayer polymeric species. 

Figure 4.18 Suspected morphology of polymer/MMT composite materials produced through conventional emulsion polymerization without any pretreatment of the clay particles.

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

Intercalated polymer-clay nanocomposites have been synthesized by direct polymer intercalation (4-6), and in situ intercalative polymerization of monomers in the clay galleries (7, 8). Owing to the spatial confinement of the polymer between the dense clay layers, intercalated polymer-clay nanocomposites can exhibit impressive conductiviw (6) and barrier properties (7). The exfoliation of smectite clays provides 10 A-thick silicate layers with high in - plane bond strength and asjJect ratios comparable to those found for fiber reinforced polymer composites. 

An alternative route to the compatibilization of a filler such as clay with LDPE and HDPE, through the radical catalyzed polymerization of maleic anhydride (MAH) in the presence of the polymer and clay, has been shown to yield PE-g-MAH-clay composites having better mechanical properties than unfilled PE or PE-clay mixtures prepared in the absence of MAH and a radical catalyst, In the present paper, further improvements in the preparation and properties of HDPE-clay composites are described. These result from the use of high melt index HDPE as "coating PE" in the preparation of the PE/clay masterbatch and low melt index HDPE as "matrix PE" in the final HDPE-clay composite. The crosslinking which accompanies the graft polymerization of MAH onto PE also plays a significant role in the enhancement of the mechanical properties of the composite, ... 

Figure 16.24 shows the schematic representation of dispersed clay particles in a polymer matrix. Conventionally dispersed clay has aggregated layers in face-to-face form. Intercalated clay composites have one or more layers of polymer inserted into the clay host gallery. Exfoliated polymer/clay nanocomposites have low clay content (lower than intercalated clay composites which have clay content -50%). It was found that 1 wt% exfoliated clay such as hectorite, montmorillonite, or fluorohectorite increases the tensile modulus of epoxy resin by 50-65%. Montmorillonite was used in a two stage process of nanocomposite formation. In the first step, montmorillonite was intercalated with vinyl monomer and then used in the second step to insert polystyrene by in situ polymerization. 

Polymer/clay nanocomposites exhibit remarkable improvement in material properties relative to unfilled polymers or conventional composites. These improvements can include increased tensile modulus, mechanical strength, and heat resistance and reduced gas permeability and flammability [1], There are various methods of preparing polymer/clay nanocomposites (i) in situ polymerization, (ii) solution intercalation, (iii) melt intercalation, and (iv) in situ template synthesis [2],... 

These materials, unlike the other nanophase materials described in this chapter, are nano-sized in only one dimension and thereby act as nanoplatelets that sandwich polymer chains in composites. Mont-morillonite (MMT) is a well-characterized layered silicate that can be made hydrophobic through either ionic exchange or modification with organic surfactant molecules to aid in dispersion [5,23]. Polymer-layered silicates may be synthesized by exfoliation adsorption, in situ intercalative polymerization, and melt intercalation to yield three general types of polymer/clay nanocomposites. Intercalated structures are characterized as alternating polymer and siHcate layers in an ordered pattern with a periodic space between layers of a few nanometers [13], ExfoHated or delaminated structure occurs when silicate layers are uniformly distributed throughout the polymer matrix. In some cases, the polymer does not intercalate... 

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