Preparation and air-separation properties of membrane blends of low-molecular-weight liquid crystals with cellulose derivatives

Four kinds of low-molecular-weight liquid crystals (LCs) (cholesteryl oleyl carbonate (COC), the benzoate-containing mixture LC DYC, p-heptyl-p′-cyanobiphenyl (7CB), p-pentylphenol-p′-methoxybenzoate (5PMB) and four cellulose derivatives (ethyl cellulose (EC), cellulose diacetate, cellulose triacetate, cellulose nitrate) were used to prepare blend membranes of thickness 13–45 μm by a solution casting technique. The oxygen permeability, the oxygen/nitrogen separation factor, the permeation flux Q_OEA and the oxygen concentration Y_O2 of the oxygen-enriched air (OEA) through the cellulose derivative membranes with different LCs were studied by a constant pressure-variable volume method. The experimental results show that the membrane-forming ability, uniformity, toughness and air-separation properties of the cellulose derivatives have been improved by adding no more than 12 wt% LC. The air-separation properties of the resulting membranes vary significantly with membrane composition, temperature and pressure difference. Of all the membranes, the COC/EC(4/96) membrane of thickness 18 μm exhibits the highest Q_OEA value of 1.8 × 10⁻⁴ cm³(STP) s⁻¹ cm⁻² and maintains the Y_O2 value of 38.8% in a single operation.