Hydrophobic polymer intercalation

This siloxane surface, to aU practical purposes, is identical to a talc surface in its behavior. The interaction energy between this surface and a number of polymers has been reported [9,10,11]. In the case of 

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It has been generally assessed that the mechanism of interaction among the components of PO/day nanocomposites involves the functionalities of the functional polymer used as matrix or compatibilizer. The most frequent explanation is related to the hydrophilic/ hydrophobic balance of components involving some kind of undefined polar interaction between the silicate layers and the functional polymer (Alexandre and Dubois 2000, Sinha Ray and Okamoto 2003, Pavlidou and Papaspyrides 2008). The direct intercalation/interac-tion of a wide numbers of MAH-grafted low-molecular-weight compounds with layered silicates has been studied suggesting that the anhydride can promote the intercalation even if this is not a modeling of polymer intercalation (Sibold et al. 2007). 

The layered silicate nanoparticles are usually hydrophilic and their interactions with nonpolar polymers are not favorable. Thus, whereas hydrophilic polymers are likely to intercalate within Na-activated montmorillonite clays [24-29], hydrophobic polymers can lead to intercalated [23,30-32] or exfoliated [33] structures only with organophilized clays, i.e., with materials where the hydrated Na+ within the galleries has been replaced by proper cationic surfactants (e.g., alkylammonium) by a cation exchange reaction. The thermodynamics of intercalation or exfoliation have been discussed [34-37] in terms of both enthalpic and entropic contributions to the free energy. It has been recognized that the entropy loss because of chain confinement is compensated by the entropy gain associated with the increased conformational freedom of the surfactant tails as the interlayer distance increases with polymer intercalation [34,38], whereas the favorable enthalpic interactions are extremely critical in determining the nanocomposite structure [39]. 

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]. 

PEO/OMMT mixture. Based on Flory-Huggins interaction parameters between polymers and silicate layers in a PEO/PMMA/OMMT mixture, the PMMA molecules have a larger affinity for the hydrophobic surfaces of OMMT than hydrophilic PEO. The larger layer thickness and broadness of the peak in the PMMA/ OMMT mixture are indicative of an improved intercalation for PMMA matrix over the OMMT surfaces. The relatively weak interaction between PEO and OMMT is because of the hydrophobic modification of the clay surface. 

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