Gas Permeability through Polymer Membranes

Abstract

Gas Permeability through Polymer membranes Yang Wang The primary objectives of this work were to (a) determine gas transport properties of polymer-based packaging films in conditions relevant to their use in food packaging; (b) investigate the relationship between polymer structure, membrane morphology and gas transport properties; (c) develop modified polymer membranes for gas separation. Gas transport properties of packaging films and polymer membranes were determined via measurement of gas permeability, using two permeation cells whose construction was based on published designs. In one cell the rate of increase of permeant gas concentration in a fixed-volume chamber on the downstream side of the membrane was measured, either continuously (for oxygen, with an in situ oxygen sensor), or at appropriate time intervals by sampling and gas chromatography (for gases other than oxygen). The other cell allowed detection and measurement of the rate of volume increase, on the downstream side of the membrane, resulting from gas transport through the membrane. The precision and accuracy of the cells was assessed, in part by measurements on an NBS certified permeability standard material. The use of polymer films for food packaging requires knowledge of their transport properties under varying conditions. The effects of several parameters (oxygen pressure, temperature, elongation, multiple layers and relative humidity) on the oxygen permeability of polymer films (particularly polyethylene) that are commonly used for packaging food were investigated. Since ethylene transport through packaging films is an important factor in relation to the shelf-life of fresh produce, and few data have been reported, the effect of a number of variable parameters (polymer density, film thickness, temperature, mixed permeant, etc) on ethylene permeability in polyethylene together with commercial zeolite-filled films was also investigated. The data for oxygen and ethylene permeability provide a basis for packaging material selection and package system design. Development of polymer membranes for gas separation proceeded in several directions, ethyl cellulose being used as the reference membrane material, and separation of oxygen and nitrogen as the model process. The effect of ethoxyl content, molar mass, casting solvent and temperature on gas transport properties of ethyl cellulose were investigated. Ethyl cellulose was physically modified via blending with poly(methyl methacrylate). The relationship between the permeability of the blend, its composition and the permeabilities of the component homopolymers was determined and analysed using series, parallel and logarithmic models. Interaction between iodine and ethyl cellulose was investigated by (a) immersing ethyl cellulose membranes in aqueous iodine/iodide solution (iodine doping); (b) incorporating iodine in the solution from which the membrane was cast. Oxygen and nitrogen permeabilities in iodine-doped ethyl cellulose decrease with increase in the concentration of iodine in the dopant solution up to 0.003 mol L-1, and increase sharply at higher iodine concentrations, and UV-VlS and far IR spectra indicate formation of a charge transfer complex. Differential scanning calorimetry of both types of membrane shows changes in the characteristic phase transitions of ethyl cellulose after iodine treatment, including the crystal-liquid crystal transition that has been reported to occur in ethyl cellulose. Further evidence for liquid crystal phases has been found from circular dichroism. X-ray photoelectron spectroscopy of iodine-doped ethyl cellulose films indicates that the iodine is present in two different chemical states, probably I3 and I2. A mixed ester of ethyl cellulose (EC) was prepared by reaction of trifluoroacetic anhydride with the residual hydroxy groups of ethyl cellulose. The mixed ester is soluble in tetrahydrofuran, dichloromethane, chloroform, benzene and pyridine. FTIR and NMR spectra show that hydroxy groups of ethyl cellulose were replaced by trifluoroacetoxy groups. The trifluoroacetyl ethyl cellulose (TFAEC) has higher selectivity for oxygen relative to nitrogen, in gas transport, than unmodified EC. Annealing at an elevated temperature further improves selectivity for oxygen, whilst ageing at ambient temperature partially reduces oxygen selectivity. The tensile strength of TFAEC is virtually the same as that of unmodified EC, but the elongation to break is 2007% higher than for EC.