Polyethylene/clay compatibility

As an inorganic mineral, most unmodified nanoadditives are strongly hydrophilic and are generally compatible and miscible only with a few hydrophilic polymers, for instance, clay can only be made into PNs with polyethylene oxide),27 poly(vinyl alcohol),28 and a few other water soluble polymers. Most polymers are hydrophobic and thus they are neither compatible nor miscible with the unmodified nanoadditives, leading to an inability to achieve a PN with a good nanodispersion in most cases. Therefore, for most nanoadditives that have been used to prepare the PNs, an important and necessary feature is their surface treatment that provides compatibility to the nanoadditives and enables them to be uniformly dispersed (and/or separated into single nanoparticles) in the polymer matrix. 

Natural, unmodified montmorillonite-Na (MMT-Na) has cation exchange capacity, typically 80-90 mequiv/100 g. Although some polymers, such as polyethylene oxide or polyvinylpyrrolidone, are of sufficient polarity to be able to directly exfoliate unmodified MMT-Na, organic modification of the layered clay is usually required to render the hydrophilic surface of the clay more hydrophobic and thus more compatible with most polymers, thereby improving the wettability and dispersibility of the clay in the polymer matrix. 

Ethylene polymerization catalysts based on late transition metals may be intrinsically more compatible with clay supports than early transition metal catalysts, because of their higher tolerance for water and other polar impurities. Two families of late transition metal catalysts have been particularly successful in producing high molecular weight polyethylene those based on bis(imino)pyridine ligands and those containing a-diimine or related ligands. 

In the case of polymer/clay nanocomposites, alkylammonium exchange species influence the affinity between the polymer and the clay surface. For example, it was reported that clays treated with dialkyl dimethylammonium halides, in particular with two chains of about 18 carbon atoms, have a surface energy similar to poly(olefins) such as polypropylene (PP) and polyethylene (PE) [27]. Polar polymers as polyamides (PA) have been recommended to get better interactions as reported by Toyota [8]. The alkyl chain length is related with increase in interlayer space required for the intercalation of polymer chains. Because of the nonpolar nature of their chains, they reduce the electrostatic interactions between the silicate layers and lower the surface energy of the layered silicates. As a consequence, an optimal diffusion of the polymer to dissociate the stacked clay layers, that is, an exfoliation process, can be obtained. Despite the compatibility of MMT modified by long alkyl chain quaternary ammonium with hydrophobic polymers (PE and PP), conventional alkylammonium ions show low thermal stability, that is, an onset decomposition temperature is close to 180°C.This poor thermal stability could limit their use in the preparation of PLS with matrices processed at high temperatures such as PA, poly(ethylene terephthalate) (PET), and poly(ether ether ketone) (PEEK) [30]. 

The type of quaternary alkylammonium salt influences the affinity between polymer chains and the clay mineral platelets. As an example, nonpolar polymers, such as polypropylene and polyethylene, are more compatible with organoclays prepared with dimethylammonium halides, while polar polymers present more affinity with the organoclays containing alkyl benzyl dimethylammonium halides and alkyl hydroxyethylammonium halides. 

Another variation of the ion-dipole surface treatment method utilizing bloek copolymers has been reported [8]. In this approach, one of the blocks of the copolymer can readily ion-dipole bond to the cations on the clay surface and the other block is compatible with the polymer of interest. Polyethylene oxide is very effective as the block that bonds to the clay surface cations and lies nearly parallel to the clay surface, and the other block is tailored to the polymer of interest and forms loops extending away from the clay surface, better to interact with the polymer. 

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