*5.2.2 Polyurethane rubbery polymers*

Polyurethanes have found commercial applications using in a diversity of products in industry, from coatings and adhesives for automotive, shoe soles, mattresses, and foam insulation. Polyurethanes are rubbery polymers which are introduced for different gas pair separations vastly, such as CO2/N2, CO2/CH4, O2/ N2. The solubility and diffusivity of gases indicate solubility domination of gas transport in these membranes. Varies researchers have shown that the permeability of polyurethane membranes increases, while the selectivity decreases with the decrease the hard segment content (urethane or urea), or the increase the soft segment (polyether or polyester) molecular weight [111–115].

Poly(urethane–urea) (PUU) membranes display moderate C3+/CH4 selectivity for removal of C3+ hydrocarbons from natural gas. Sadeghi and coworkers [91] evaluated the separation performance of a series of PUU membranes synthesized with different type of diisocyanates and polyols. It was found that PUU membranes with polypropylene glycol (PPG) polyol had the maximum C3H8 permeability of 200 Barrer and C3H8/CH4 selectivity of 5.5 because of its higher rubbery property and higher phase separation. In their recent paper [116], two series of PUU membranes containing aromatic and aliphatic side chains with different sizes were developed for separating of C2H6 and C3H8 from CH4. The longer aliphatic side chain based PUU provides the best hydrocarbon separation performance with C3H8 permeability of 186 Barrer and C3H8/CH4 idea selectivity of 6.5.
