## 2 Polymer Nanocomposites for Membranes Applications
In Reference [78], the reason was discussed why the introduction of zeolites into a non-porous polymer matrix can really lead to an improvement in the MMM properties. In 1991, [PERSON] demonstrated that there is a correlation between permeability and selectivity that  
Figure 3: Performance of ideal Zeo TIPS membranes with an arrow indicating increasing polymer permeability (\(\bullet\)), non-ideal Zeo TIPS membranes with an arrow indicating increasing \(\beta\) (\(\square\)), and dense mixed matrix membranes (MMM) with an arrow indicating increasing polymer permeability (\(\blacktriangle\)), plotted with [PERSON]’s 1991 upper bound and region of commercial attractiveness [73]. All points correspond to 25 vol % zeolite loading and all Zeo TIPS membrane points correspond to a 3:1 ratio of void volume to polymer volume. results in an upper bound for polymer membranes [79]. For actual polymer membranes, the data points would appear below a so-called \"upper bound trade-off curve\" [80]. This suggests that even with remarkable developments in polymer chemistry it is unlikely there will be significant changes in the permeability/selectivity trade-off in the nearest future. Newly synthesized polymeric membranes seem to obey the trade-off line between permeability and selectivity. As is known, the problem encountered by the use of the gas separation membrane is the search for a higher selectivity and permeability of the material with a high resistance to aggressive conditions. The price of the material must be competitive, and this material can easily be made into a membrane [81]. Inorganic membranes [82; 83], for example, based on zeolites, may offer a solution to the permeability/selectivity trade-off problem. Their selectivity and permeability data are often in the upper right quadrant of a Robeson plot [78]. Thermally and chemically stable molecular sieve membranes with characteristics significantly exceeding the upper boundary of the interconnection curve, however, are difficult and expensive to manufacture. The development of new MMMs is considered to be the most practical approach to overcoming objective limitations in obtaining economical materials for a gas separation membrane [73; 84]. An obvious strategy to bridge the gap between polymer and molecular sieve membranes is to mix them in a single composite membrane. MMMs combine the advantages of flexible, processable polymer membranes with the selectivity of inorganic membranes [25]. It would seem that to achieve membrane performance above the upper bound of a Robeson plot [78] the content of the inorganic additive should be high. Unfortunately, MMMs become brittle as the concentration of molecular sieves in the membrane increases. Typical MMM loadings are \(<\)40 wt %. The study of MMM was mainly related to the optimization in the selection of different molecular sieves as dispersed particles in different polymers, both with high and low glass transition temperatures [77]. According to Reference [49], the first attempt to study MMM was undertaken by [PERSON] et al. in 1973. They used zeolite 5A in silicone rubber and observed delayed diffusional time lag but found no improvement in gas separation properties [49]. The authors observed good contact between the polymer and inorganic phases, linking this effect mainly with the soft and flexible structure of the rubber-like polymer [75]. However, some researchers have shown that the advantage of including a molecular sieves phase in a highly permeable rubbery polymer is in most cases minimal, since most of the penetrant diffusion will be passing through a polymer phase that exhibits less flow resistance than the molecular sieves. For example, poly (dimethylsiloxane) (PDMS) with high O\({}_{2}\) permeability (933 barrer) and low selectivity for O\({}_{2}\)/N\({}_{2}\) (2.1) can only give a minimal gain in the transport properties of a MMM, that by characteristics is much lower than the trade-off curve [75]. Large improvement could only be observed at very high loading inorganic fractions.