Cationic clay mechanical properties

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

Kurokawa et al. [258-260] developed a novel but somewhat complex procedure for the preparation of PP/clay nanocomposites and studied some factors controlling mechanical properties of PP/clay mineral nanocomposites. This method consisted of the following three steps (1) a small amount of polymerizing polar monomer, diacetone acrylamide, was intercalated between clay mineral [hydrophobic hectorite (HC) and hydrophobic MMT clay] layers, surface of which was ion exchanged with quaternary ammonium cations, and then polymerized to expand the interlayer distance (2) polar maleic acid-grafted PP (m-PP), in addition was intercalated into the interlayer space to make a composite (master batch, MB) (3) the prepared MB was finally mixed with a conventional PP by melt twin-screw extrusion at 180°C and at a mixing rate of 160 rpm to prepare nanocomposite. Authors observed that the properties of the nanocomposite strongly dependent on the stiffness of clay mineral layer. Similar improvement of mechanical properties of the PP/clay/m-PP nanocomposites was observed by other researchers [50,261]. 

Bragaiifa et al. [97] showed the importance of the clay lamellar intercalation and exfoliation to define the material mechanical properties of nanocomposites. In this study, montmorillonite clay with different interlamellar cations (NaL Li+, K+, and Ca +) was mixed with styrene-acrylic latex and dried to produce nanocomposites. An ultra-thin cut of the sample was analyzed by ESI-TEM. The elemental maps of carbon, silicon, and calcium were used to identify the polymer, clay, and cation domains, as presented in Fig. 8.13. 

The clay and the calcium superimposed show that counterions are located at the interface between the polymer and lamellae. It was possible to elaborate an adhesion mechanism in which the cations form ionic bridges between the negative surface of the clay and the latex after water evaporation. Furthermore, it shows how the interface adhesion is important to the mechanical properties of the nanocomposites and how it is possible to formulate materials with the same polymer and clay, but with differentiated properties depending only on the counterions. The results of microscopy and mechanical tests lead to the conclusion that the lamellar separation and the interface adhesion of these lamellae at the polymer matrix play a crucial role in the properties of these systems. 

Following the appropriate pathway, in situ polymerization has allowed the preparation of a large variety of polymer-clay nanocomposites with interesting functional and/or mechanical properties. For instance, a doped PPy-synthetic hec-torite nanocomposite exhibits conductivity from about 10 to 10 S/cm (155). In gena-al, monomers showing affinity to be adsorbed by smectites, e.g., hydrophilic species, can produce infracrystalline homocondensations. Other monomers, such as acrylonitrile, are also easily intercalated in smectites, because such molecules are directly associated to the interlayer cations M" (M" = Li +, Na, etc.) through —C N—M"+ ion-dipole interactions (156-158). The action of "Y-irradiation (156) or thermal (158) treatments can induce the polymerization... 

Two polymerizable cationic surfactants were synthesized to produce thermally stable organoclays (ll-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (called hydroxyethyl surfmer), and (ll-acryloyloxyundecyl)dimethylethylammonium bromide (called ethyl surfmer (Table 3.5) [63]. PS nanocomposites were produced by bulk polymerization and by free radical polymerization using these organoclays. Exfoliated structures were obtained with the ethyl surfmer-modified clay, whereas a mixed exfoliated/ intercalated structure was obtained using the hydroxyethyl surfmer-modified clay. The nanocomposites exhibited enhanced thermal stability and an increase in the glass transition temperature, in addition to improved mechanical properties relative to polystyrene. However, intercalated structures were obtained when nanocomposites were prepared in solution, because of competition between the solvent molecules and monomer in penetrating the clay galleries. Enhanced thermal stability was also obtained in the solution polymerization case. 

Wilkie and co-workers [69, 70] synthesized two organically modified clays to produce nanocomposites of PS, HIPS, and ABS terpolymer. They used the following copolymers to modify clay vinylbenzyl chloride (COPS) and methyl methacrylate and vinylbenzyl chloride (MAPS). The cation head for clay modification with these compounds was ammonium. After melt-blending, styrene copolymer-modified clays yielded exfoliated nanocomposites, whereas the methacrylate copolymer clays yielded a mixture of immiscible and intercalated nanocomposites. In general, all nanocomposites exhibited improved thermal stability and mechanical properties, in addition to improvements in flame retardancy, depending on the quality of clay dispersion. 

Addition of several types of days as well as modified (organophiHzed clays) were found to improve mechanical properties and decrease water uptake of starch-based bionanocomposites viz., natural sodium montmoriUonite (MMT) [155], MMT, hectorite, hectorite modified with 2-methyl, 2-hydrogenated taUow quaternary ammonium chloride and kaolinite [150], natural sodium montmoriUonite (Na MMT, Cloisite Na+), and organically modified montmoriUonite (OMMT) with methyl taUow bis-2-hydroxyethyl ammonium cations located in the silicate gallery (Cloisite 30B) [151], MMT (hydrophilic Cloisite Na+ day and hydrophobic Qoisite 30B, lOA, and 15A) [152, 153). 

Gel nanocomposite polymer electrolytes based on PVDF-HFP were studied by NMR. In this type of electrolyte, BaTiOs and clay were used as nanoparticles and PC + LiCFsSOs as the liquid electrolyte. The filled gel electrolytes presented better mechanical properties than the gels without the filler. The fillers lowered the conductivity by a small amount. The diffusion coefficient measured by NMR indicated that both anion and cation mobilities were reduced by the presence of the filler, but the effect was generally greater for the anions. 

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