Synthetic Routes for Nanocomposite Formation

As for the thermodynamic consideration in Section 2.2.1, we attempt to highlight these challenges by describing in some detail the most common synthetic routes for nanocomposite formation employed for polymer/layered-inorganic hybrids. Most examples are drawn from layered-silicate fillers, but the conclusions are general across most nanofillers, and one should be able to envision similar strategies for nanocomposite formation based on other types of nanofillers. 

In all the cases above and in the absence of polymer cross-linking or favorable thermodynamics to retain the dispersion achieved (by solvent, mechanical shear/vibration, swelling agent, etc.), the fillers will reaggregate upon further 

1 Static Melt Intercalation This method involves the mechanical mixing of a polymer with an appropriately modified filler and subsequent annealing above the softening temperature of the polymer. This approach provides the best route to test with sensitivity the thermodynamic arguments detailed above and to yield well-defined systems for fundamental studies. However, due to the quiescent processing conditions (absence of external shear), which eliminate any mechanical contribution for the dispersion of fillers, and to the very slow interca-lation-exfohation kinetics, such methods are typically very slow, thus having very limited applicabihty in industry. 

The latter effect is particularly important for polymers that possess very high attractions for the filler surfaces and can be kinetically arrested under static melt intercalation. In many cases, end users of polymer nanocomposites are hesitant to incorporate nanofillers directly (in the form of ultrafine powders) in their current processing practices, and the concentrate or masterbatch two-step approach is preferred. In this case, first a polymer nanocomposite (concentrate) is formulated at relatively high filler loadings of about 25 wt%, which can be processed and palletized to look like a normal polymer resin. This concentrate is subsequently diluted (i.e., let down) to the desired filler loading by pure polymer resin (cf. below). 

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