## 2 Polymer Nanocomposites for Membranes Applications  
Recently, polymeric nanocomposite membranes have been widely used in various applications, such as gas separation, water purification, desalination, fuel cells, etc. [10; 32; 34; 40; 55; 56]. Most of the currently used membranes are polymeric, and their production is highly developed [57], but they are usually not suitable for critical technologies, in particular for very high temperature applications [58]. The trade-off between flow and selectivity underlies the optimization of the diffusion membrane. Improving the functional properties of polymer membranes and developing new composite membranes by structural modification of the polymer matrix plays an important role in membrane science and technology. The modified membrane can have excellent stability in a wide range of process conditions, high selectivity for the target chemicals, and can also have a large molecular flux of penetrants with a small driving force. Polymeric nanocomposite membranes with enhanced performances such as high selectivity, fluxes and favorable surface morphology have been prepared by incorporating nano-size fillers into a polymer matrix [59; 60; 61]. Rheology properties of solvents or melts of polymer nanocomposites were investigated [4; 16]. By including nano-size fillers into the polymer solution (melt) prior to the membrane fabrication, one can change its rheology (depending on the nature of the distribution of nanoparticles in casting dispersed mixtures). This makes it possible to form membranes of a different morphology, as follows:  
(i) The principle named 'particle-assisted wetting', based on the templating process with the introduction of nanoparticles into the acrylate-based reaction mixture, was the basis for a new method for preparing membranes with high porosity and uniform pore distribution [62; 63; 64]. A typical example of the preparation of membranes for the size-based separation is set forth below. A mixture of a hydrophobic monomer, monodisperse silica nanoparticles hydrophobized by a silanization, and a photoinitiator, formed a monolayer of particles on the water surface. After curing with UV irradiation and the subsequent removal of the particles with hydrofluoric acid, a thin porous membrane was obtained. Results of further experiments of authors [64] showed that these membranes on suitable support can be effectively used for the separation of substances in size. (ii) Multi-walled carbon nanotubes (MWCNTs)/polysulfone blend membranes were prepared by a phase inversion process, using N-methyl-2-pyrrolidone as a solvent and water as a coagulant. For the formation of blend membranes, MWCNTs were first treated with strong acid to make them well dispersed in organic solvents with the preparation of homogeneous MWCNTs/polysulfone dispersions. Because of the hydrophilic MWCNTs, the surface of the MWCNTs/polysulfone blend membranes appeared to be more hydrophilic than in the case of just a polysulfone membrane. The pore size of the blend membranes increased along with the contents of MWCNTs up to 1.5%, then decreased, and at 4.0% of MWCNTs it became even smaller than that of polysulfone membrane. The polysulfone membrane with 4.0% of MWCNTs showed higher flux and rejection than the polysulfone membrane without MWCNTs [65]. (iii) The organic-inorganic nanocomposites obtained on sol-gel chemistry can underlie both porous and non-porous diffusion membranes. For nanocomposite membranes based on glassy polymers with low free volume prepared by a sol-gel process in a polymer matrix, improvements in selectivity and permeability were reported [66; 67]. For other cases, an increase in selectivity but a decrease in permeability [68] and vice-versa [69] were observed. In these studies, the results were related to an improvement in the solubility of the nanocomposite membrane [67], the change in chain packing and segmental mobility of the polymer and the type of alkoxide used [66].