## 2 Polymer Nanocomposites for Membranes Applications
In this case, the mechanical properties of the membrane approach to \"the difficult-to-process membrane\". After some reports of improved separation characteristics associated with the addition of zeolites to rubber polymers, the focus of attention shifted to mixed zeolites with glassy polymers [71]. Most glassy polymers can provide commercially acceptable minimum matrix phase characteristics, as compared to a rubber polymer. As mentioned above, the initial attempts to manufacture MMM using glassy polymers and zeolites resulted in voids in the interfacial region. This reduces the characteristics of the separation of the MMM relative to the polymer in its composition [72; 85]. Efforts to eliminate these non-selective sites were often focused on using binding agents to induce favorable interactions between the polymer and the zeolite, on adding a plasticizer to increase the flexibility of the polymer matrix, or on searching for ways of chemically linking two components [27; 86; 87]. As result of efforts to eliminate these voids, the resulting permeability of the MMM was often sacrificed. Thus, the MMM should ideally be a homogeneous mixture of polymer and molecular sieve, but this is rarely realizable [28]. Adding a filler material such as zeolite (KA, NaA, CaA, and NaX zeolites) to the polymer membrane network was reported to improve the PVA membrane performance [88]. It was shown that in zeolite-PVA membranes the separation factor was unchanged at low zeolite content and it decreases at high zeolite content because the more zeolite particles in the membrane, the more chances of creating a less selective leak around the zeolite particles. Most zeolite membranes are composites with a medium pore size of 0.55 nm. It is shown that ZSM-5 in the MMM is characterized by good contact between the nanoparticles and the polymer, since the polymer chains can penetrate the mesopores of ZSM-5. The pores of ZSM-5 crystals provide size and shape selectivity. The ideal H\({}_{2}\)/CH\({}_{4}\) separation factor increased from 83.3 to 169 at 20% loading [89]. In Reference [90], new polyimide (PI) films containing zeolite L and/or silica, having a thickness of tens of micrometers, and their gas transport properties were studied with respect to the film's structure. The gas permeability (O\({}_{2}\), N\({}_{2}\) and CO\({}_{2}\)) increased with an increase in the inorganic content in the composite film, while the selectivity was maintained constant in comparison with the PI film. For the 10% filled samples, there was not any observation of the significant enhancement in the gas transport characteristics. The results of the SEM cross section analysis of the film showed good compatibility between the polymer and the inorganic phase, and also indicated a dense membrane structure comparable to the structure of PI without nano-fillers (PI-0). The highest gas permeability was found for the sample filled with 20% of zeolite (PI-20). The permeability of N\({}_{2}\), O\({}_{2}\), and CO\({}_{2}\) through PI-20 film increased, compared to the reference PI film PI-0, while the selectivity for the O\({}_{2}\)/N\({}_{2}\) and CO\({}_{2}\)/N\({}_{2}\) gas pairs remained almost unchanged. Similar behavior was reported in the literature for hyperbranched PI-silica hybrid membranes, when O\({}_{2}\)/N\({}_{2}\) selectivities were maintained almost constant although their O\({}_{2}\) permeability increased with increasing silica content [91]. O\({}_{2}\)/N\({}_{2}\) and CO\({}_{2}\)/CH\({}_{4}\) selectivities of ODPA-TAPOB hyperbranched polyimide-silica hybrid membranes are discussed in [91; 92]. The O\({}_{2}\)/N\({}_{2}\) selectivity depends on the diffusivity selectivity rather than the solubility selectivity. The \(\alpha\) (O\({}_{2}\)/N\({}_{2}\)) values of the hybrids were maintained almost constant although their O\({}_{2}\) permeability increased with increasing silica content.