Chemical modification of P84 polyimide as anion-exchange membranes in a free-flow isoelectric focusing system for protein separation

By means of amination with diamine and methylation with methyl iodide, we have modified P84 microporous polyimide membranes with characteristics of highly charged anion-exchange membranes. FTIR (Fourier transform infrared-attenuated total reflection) and XPS (X-ray photoelectron spectroscopy) confirm the amination and methylation reactions on membrane surfaces. The intrinsic properties of the newly developed anion-exchange membranes have been fully characterized in terms of ζ-potentials, electrical resistances, PWPs (pure water permeation) and pore size distributions. By using the newly developed membranes, a free-flow isoelectric focusing (IEM-FFIEF) has been set up for the separation of myoglobin (Mb) and lysozyme (Lys) mixtures. Experimental data show that (1) the Mb flux via the highly charged P84 anion-exchange membrane can be 10 times higher than that of the original P84 membrane and (2) the high surface charge is the predominant factor for the enhanced Mb flux. HPLC (high performance liquid chromatography) results show that not only the Mb flux was high, but also its purity in the permeate side is extremely high. It is therefore concluded that the diamine and methyl iodide modifications can effectively modulate P84 nano-porous membranes with anion-exchange characteristics, which is suitable for the IEM-FFIEF application.