## 2 Polymer Nanocomposites for Membranes Applications
Polyethersulfone (PES) with the incorporation of inorganic fillers of different shapes (Na-MMT clays--layers with two silica tetrahedral sheets sandwiching a central alumina octahedral sheet, and TiO\({}_{2}\) nanoparticles-spherical) were prepared and tested for CO\({}_{2}\)/CH\({}_{4}\) separation [124]. The study shows that the permeability of PES/MMT MMM to both gases, CO\({}_{2}\) and CH\({}_{4}\), increased significantly  
Figure 3:7 Water flux of 10 wt % of water in water+isopropanol (white box) and water + 1,4-dioxane (black box) feed mixtures vs. PVA (M1), PVA/Na\({}^{+}\)MMT-5(M2) and PVA/Na\({}^{+}\)MMT-10 (M3) mixed matrix membranes at 30 \({}^{\circ}\)C for (**a**) thermally crosslinked membranes and (**b**) glutaraldehyde crosslinked membranes [120]. with increasing MMT content, that was accompanied by a dramatic decrease in selectivity. The main reason for this effect was called by authors as the appearance of voids and defects at the boundary of the polymer and inorganic phases. This phenomenon should be attributed to the big gas transport path created in the interface. Accordingly, the gas selectivity was far from the Robeson upper bounds (Figure 13). In particular, with a MMT content of 10% or more, the predominant mechanism of gas transport in these membranes is the Knudsen diffusion, so that CO\({}_{2}\) and CH\({}_{4}\) cannot actually be separated. The membranes with high MMT contents (more than 10 wt %) have some defects and, therefore, Knudsen diffusion with low separation ability became the predominant gas transport mechanism. In the case of PES/TiO\({}_{2}\) MMMs, the CO\({}_{2}\)/CH\({}_{4}\) separation factor increased from 24.5 for pure PES membrane to 38.5 for 4 wt % TiO\({}_{2}\) MMM and then decreased with a further increase in TiO\({}_{2}\) content (17.3 for 20 wt %) (Figures 14 and 15). A reason is the same like in the case of MMT additive. SEM and TEM images revealed the inorganic filler agglomeration in MMMs. The size of the agglomerate becomes larger with a higher filler content, and at the interspace of the filler and the polymer, voids formed. The conclusion was made that to improve the PES/MMT MMM performance, middle and micron size agglomerates as well as interface voids should be avoided. Some methods proposed in the literature may be useful for the production of MMMs with a more even distribution of MMT, less aggregation of particles and without defects at the polymer-inorganic interface, such as the ultrasonic treatment method for better distribution of the inorganic filler in the dispersion, the filler modification to improve its adhesion to the polymer, the use of organo-clays, melt processing in the membrane preparation, etc. Figure 3:8 Performance of MMMs compared to the Robeson upper bounds [124]. The essential role of the particle/polymer boundary region was discussed in papers wherein colloidal silica was used as a nano-filler. It has been experimentally proven that the introduction of nano-sized colloidal silicon dioxide into the matrix of a highly permeable polymer significantly increases (up to an order of magnitude) the material's permeability in most cases [125; 126; 127; 44; 128]. In the case of nano-sized silica, penetrant solubility is known to have been unchanged for filler concentrations of up to 40-50 wt % [126]. Therefore, the observed increase in rate of penetration after incorporation of these nanoparticles in the polymer matrix should be associated with an increased diffusion rate of penetrants. An explanation was proposed [125] that the introduction of impermeable nanoparticles disrupts the packing of polymer chains near the surface of the filler. This leads to an increase in the free volume in the polymer-filler interphase region that was detected by positron annihilation life time spectroscopy [127] and \({}^{129}\)Xe NMR spectroscopy [128] on these highly permeable nanocomposites. Increasing the available free volume leads to faster diffusion through the layers of the interfacial polymer.