Polymer nanocomposites solution processing

Fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in polyurethane matrix has been found to improve the properties of the nanocomposites. The dispersion extent of CNTs in the polyurethane matrix plays an important role in the properties of the polymer nanocomposites. Similar to the case of nanotube/solvent suspensions, pristine nanotubes have not yet been shown to be soluble in polymers, illustrating the extreme difficulty of overcoming the inherent thermodynamic drive of nanotubes to bundle. Therefore, CNTs need to be surface modified before the composite fabrication process to improve the load transfer from the polyurethane matrix to the nanotubes. Usually, the polyurethane/CNT nanocomposites can be fabricated by using four techniques melt-mixing (15), solution casting (16-18), in-situ polymerization (19-21), and sol gel process (22). 

A dispersion of nanoparticles of Au or other metals in a polymer matrix may also be obtained by a one-pot process of microemulsion polymerization. For instance, the UV-polymerization of a microemulsion of 35 wt% MMA, 35 wt% AUDMAA and 30 wt% of 0.1 M HAUCI4 aqueous solution would produce a Au-polymer nanocomposite, as shown in Fig. 12 [104]. This TEM micrograph shows a microtoned thin film of the sample. It is clearly apparent that Au particles of about 10-15 nm are well dispersed in the polymer matrix. 

A limited number of methods have been developed for the preparation of metal-polymer nanocomposites. Usually, such techniques consist of highly specific approaches, which can be classified as in situ and ex situ methods. In the in situ methods, two steps are needed First, the monomer is polymerized in solution, with metal ions introduced before or after polymerization. Then metal ions in the polymer matrix are reduced chemically, thermally, or by UV irradiation. In the ex situ processes, the metal nanoparticles are chemically synthesized, and their surface is organically passivated. The derivatized nanoparticles are dispersed into a polymer solution or liquid monomer that is then polymerized. 

The optical micrographs in Figure 5 show the effect of oxygen plasma exposure to pure nylon 6 and a nylon 6/7.5wt% layered silicate nanocomposite. Both are melt-processed samples recast from the 1,1,1,3,3,3-hexa-fluoro-2-propanol solution. The nylon 6 sample experiences almost complete deterioration after 8 hours (480 minutes) of continuous exposure. In contrast, deterioration of the nanocomposite is minimal, with no significant decrease in thickness. Buckling of the nanocomposite sample after exposure arises from differences in thermal expansivity of the self-generating ceramic surface and the bulk polymer nanocomposite. 

It is a simple coating of thin layer by dipping the suitable substrate into the nanocomposite solution. However, for this fabrication, the conjugated polymer or its nanocomposite must be soluble or at least it should form fine suspension in common organic solvent. This process involves five stages viz. immersion, start-up, deposition, drainage, and evaporation as shown schematically in Figure 12.11. 

Several techniques such as intercalation of polymer from solution, in-situ intercalative polymerization, melt intercalation, direct mixture of polymer and particulates, template synthesis, in-situ polymerization and solgel process, are being employed for the preparation of polmer-layered silicate nanocomposites. Among them the most common and important approaches are in-situ polymerization, solution-induced intercalation method, and melt processing method, which are briefly discussed below. 

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