Polymer clay nanocomposites cation

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. 

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. 

As presented schematically in Figure 16, a second way to prepare polymer-clay nanocomposites via in situ polymerization consists of intercalation of the monomer (or a precursor of the monoma-) in the form of a cation and then later addition of an initiator to induce/polymaization. Thus, the direct exchange of the interlayer cations of smectites by anilinium cations, followed by oxidation with (NH4)2S20g, could be an alternative procedure to reach the formation of PANI/clay nanocomposites. In this case, the expaimental conditions allow the direct formation of PANl as a conducting ema-aldine salt (152). 

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... 

The model illustrated in Figure 2 summarizes the overall mechanism for formation of epoxy polymer - clay nanocomposites. Upon solvation of the organoclay by the epoxide monomers, the gallery cations reorient from their initial monolayer, lateral bilayer, or inclined paraffin structure to a perpendicular orientation with epoxy molecules inserted between the onium ions. A related reorientation of alkylammonium ions has been observed previously for e-caprolactam intercalated clay intermediates formed in the synthesis of Nylon-6 -exfoliated clay nanocomposites (9). Thus, the ability of the onium ion chains to reorient into a vertical position in order to optimize solvation interactions with the monomer may be a general prerequisite for pre-loading the clay galleries with sufficient monomer to achieve layer exfoliation upon intragallery polymerization. 

This problem is particularly acute for cationic surfactants attached to negatively charged surfaces, as in polymer-clay nanocomposites. In this situation, negatively charged stearates are poor tracers, as they are held in place by the neighboring cationic surfactants rather than by interaction with the surface charges. The... 

Photoinitiated cationic polymerization may also proceed by the AM mechanism if an HO-containing compound is present in the system. AM polymerization of cyclohexene oxide was used to prepare polymer/clay nanocomposite by photopoiymerization of cydohexene oxide in the presence of montmoryllonite modified with ammonium salts containing -CH2CH2OH groups and (Ph)2T PFs" as photoinitiator. Upon irradiation protic add was formed that catalyzed AM propagation on HO groups. As a result, polymer chains were attached to clay surfaces and exfoliated stmctures were obtained. ... 

In situ polymerization was the first method used to synthesize polymer-clay nanocomposites based on polyamide (PA) 6. In this technique, the modified layered silicate is swollen by a liquid monomer or a monomer solution. The monomer migrates into the galleries of the layered silicate, so that the polymerization reaction can occur between the intercalated sheets. The reaction can be initiated either by heat or radiation, by the diffusion of a suitable initiator or by an organic initiator or catalyst fixed through cationic exchange inside the interlayer before the swelling step by... 

Polymer-clay nanocomposites are the most commonly studied and applied nanocomposites. Natural clay is composed of oxide layers with cations between the layers. The thickness of the layers is about 1 nm. In practice, there exist three types of polymer-clay composite materials, as shown in... 

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. 

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