A Study On The Gas Separation Performance Of Electric Field Aligned Multi-walled Carbon Nanotubes/Polystyrene Composite Membranes

The incorporation of particulate fillers in a membrane matrix has been proveneffective in improving the separation performance of the membrane to push up theupper bound between the permeability and selectivity of the membranes. Theinorganic fillers may disrupt the polymer chain packing and increase the free volumeand thus significantly enhance the gas diffusion. Besides, the inorganic fillersthemselves as well as the functional groups on their surface may interact with somegases to improve the solubility in the membranes. However, the inorganic fillers areusually randomly distributed in the matrix and tend to aggregate resulting in someinterfacial defects, which may cause the limited increase or even decrease of the gasseparation performance of the membranes. Therefore, the key to take full advantageof the inorganic-polymer interfacial interaction and improve the permselectivity of thecomposite membranes is to make the particulate fillers dispersed homogeneously inthe matrix and vertically aligned to the membrane surface.In this study composite membranes with varying contents of multi-walled carbonnanotubes (MWCNTs) dispersed in polystyrene (PS) were electrocast by exerting analternative electric field vertically on the pre-membrane solution until the solvent isevaporated completely. MWCNTs are fully polarized in the electric field and tend toalign along the direction of the field and exclude each other perpendicularly to thefield direction via the dipolar interactions among the nanotubes.The response of MWCNTs dispersed in solvents of different dielectric constantranging from0to40to the electric field was studied. The results show that MWCNTsdispersed in solvent with lower dielectric constant exhibit stronger response to thefield and tetrachloroethylene with dielectric constant of2.5was selected as thesolvent for membrane preparation.The alignment of MWCNTs dispersed in casting solution by electric field ofdifferent strength and frequency was also investigated. The results indicate thatMWCNTs can be aligned only in the electric field with field strength higher than thecritical strength, and the higher the field strength is, the better alignment will beachieved. Additionally, the higher frequency can contribute to the sooner formation ofthe stable alignment.The pure PS membrane, randomly dispersed and electrocast MWCNTs/PS composite membranes aligned by1Hz,2000V/cm and100Hz,2000V/cm electricfield were prepared respectively. The dispersion of MWCNTs in the compositemembranes, and the amorphous structure of the membranes were characterized. Thefluorescent and mechanical properties, the through-plane electric resistivity of themembranes, and the N2, O2, CH4, CO2permeability of the membranes were measured.The results show that the electrocasting achieves not only a vertical alignment butalso more uniform dispersion of MWCNTs in the membrane matrix; the gaspermeability increases with the MWCNTs content; the electrocast compositemembranes show higher permeability of all the gases and improved O2/N2andCO2/CH4selectivity; the membranes electrocast at higher frequency exhibit the higherincrement of both the permeability and the selectivity.