Nanocomposites polymer-clay interface

In ternary nanocomposites, compatibilizers have been mostly used to improve the adhesion at the polymer/filler interface rather than to modify the polymer/elastomer interface. Mishra et al. [80] compounded PP/EPDM/organoclay (75/25/5 wt%) and added PP-g-MA (1 wt% MA) as a compatibilizer with a clay/PP- -MA ratio of 1/3. They characterized the interlayer spacing of the clay platelets by XRD and observed that it increased from 3.4 to 4.3 nm for systems without and with compatibilizer. This was attributed to a better diffusion of the PP-g-MA chains inside the interlayer spacing thanks to their functional groups. Numerous other authors prepared and characterized ternary composites with a compatibilizer. Examples include Lim et al. [81] and Lee et al. [5] on PP/PP- -MA/POE/ organoclay systems, Mehta et al. [23] on PP/PP-g-MA/EPR/organoclay systems, and Liu and Kontopoulou [24,45] on PP/PP-g-MA/ethylene-octene copolymer/silica composites. It should be noted that the compatibilizer itself may affect the properties of the matrix... 

In this contribution, we intend to introduce recent work dedicated to polymer-clay nanocomposites based on sepiolite and palygorskite fibrous silicates. We will consider as a priority the role of the interface between the mineral surface and the polymer matrix. In fact, this type of clay is markedly hydrophilic because their surface is covered by hydroxyl groups, mainly silanol groups (=Si-OH) [17, 22], and therefore they are compatible with many polar polymers. However, chemical modification of the silicate surface could be necessary for adjusting their... 

The increase of stress at break in thermoplastic-based nanocomposites is usually related to the nature of the interactions between the matrix and the filler. Table 5.1 shows the tensile stress values for different matrix day nanocomposites. Nanocomposites such as exfoliated nylon 6-based nanocomposites [78] or intercalated PMMA-based nanocomposites [80] exhibit an increase in the stress at break. This increase is usually due to the polar (PMMA) and even ionic interactions (nylon 6 grafted onto the layers) between the polymer and silicate layers. In polypropylene (PP) and PS nanocomposites, the interactions between polymer matrix and clay interface are weak, so no enhancements in tensile stress were observed. 

Stmctural data from XRD were combined with the ESR results in order to assess the extent and intensity of polymer-clay interactions at the interface. XRD measurements revealed that the silicate layers were exfoliated in the PMA matrix as the clay content was less than 15wt.%. ESR speara clearly indicated that the mobility of PMA chains in the nanocomposites is constrained due to the attractive interactions in the interface region, even though the DSC measurement showed little difference between the PMA homopolymer and PMA-clay nanocomposites. The restricted molecular motion is caused primarily by attachment of TMC moieties on the silicate platelet surface, and additionally by the polar interaction between the ester groups and the siloxane oxygen on the basal surfaces of the silicates, as seen in Figure 22(a). 

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. 

The pol5mier nanocomposite field has been studied heavily in the past decade. However, polymier nanocomposite technology has been around for quite some time in the form of latex paints, carbon-black filled tires, and other pol5mier systems filled with nanoscale particles. However, the nanoscale interface nature of these materials was not truly understood and elucidated until recently [2 7]. Today, there are excellent works that cover the entire field of polymer nanocomposite research, including applications, with a wide range of nanofillers such as layered silicates (clays), carbon nanotubes/nanofibers, colloidal oxides, double-layered hydroxides, quantum dots, nanocrystalline metals, and so on. The majority of the research conducted to date has been with organically treated, layered silicates or organoclays. 

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