Cationic clays nanocomposites

Thermal Decomposition of Polymeric Nanocomposites Based on Anionic Clays The thermal decomposition of DGEBA nanocomposites cured with polyoxypropylene diamine (Jeffamine D230) and containing 4-toluenesul-fonate/LDH was investigated by simultaneous thermal analysis (STA) in air. The LDH nanocomposite (TS/LDH) is compared to the neat epoxy and to a bis(2-hydroxyethyl)ammonium montmorillonite nanocomposite (30B). The clay content was 5 wt% for both nanocomposites. In Figure 9.24, differential thermal analyses obtained by STA are shown. A main exothermic peak is observed at about 550° C for neat epoxy. In the LDH nanocomposite this peak is split in two parts, so the heat release rate is decreased and the heat evolution delayed, where as no relevant difference is observed between neat epoxy and the cationic clay nanocomposite. 

Clay-polymer nanocomposites have proven to be interesting candidates as gas barrier materials preventing permeation of volatile gases by creating a long path for diffusion and as flame-retardant materials. Previous work mainly involves the utihzation of cationic clays, although LDH materials... 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

It should be noted that although the quaternary ammonium is nominally chosen as the modifier to compatibilize the cationic clays with the polymer matrix, this does not refer to the processing aids or compatibilizers that help disperse the clay particle into the polymer matrix and set up the PN structure the compatibilizers may not necessarily be part of the interface between the polymer and the clay. For instance, the graft copolymer of ethylene or propylene with maleic anhydride (PE-g-MA or PP-g-MA) has proven to be an excellent compatibilizer/disperser for the PE/ or PP/ clay nanocomposite,47 but the graft copolymer is not part of the interface of the modified clay. 

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. 

Samakande A, Juodaityte JJ, Sanderson RD, et al. (2008) Novel cationic RAFT-mediated polystyrene/clay nanocomposites synthesis, characterization, and thermal stability. Macromol Mater Eng 293 428 37... 

In practice, the techniques of blending, compositing and nano-reinforcement are often used together. Thermoplastic starch/poly(vinyl alcohol) (PVOH)/clay nanocomposites exhibited the intercalated and exfoliated structures [260]. Mont-morillonite (MMT) with three types of cation or modifier (Na", alkyl ammonium ion, and citric acid) was examined. The prepared nanocomposites with modified montmorillonite indicated a mechanical improvement in the properties, in comparison with pristine MMT. 

Chitosan/day nanocomposites represent an innovative and promising class of materials. Potential biomedical applications of chitosan/clay nanocomposites include the intercalation of cationic chitosan in the expandable aluminosilicate structure of the clay is expected to affect the binding of cationic drugs by anionic clay the solubility of chitosan at the low pH of gastric fluid may decrease the premature release of drugs in the gastric environment ... 

More recently, the mechanistic change from ATRP to CROP was successfully applied for the synthesis of (PSt-b-PTHF)/clay nanocomposite. An ATRP initiator introduced onto clay surfaces by cation exchange was employed in the polymerization of St yielding St-clay nanocomposites with bromo functionality. Consequently, the CROP of THF was performed in the presence of this bromo-fimctional clay-polymer macroinitiator (Scheme 57). 

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]. 

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