Date of Award
Doctor of Philosophy (PhD)
Professor Ronald F. Childs
The fabrication, properties and performances of polyethylenimine pore filled and pore coated membranes are presented and discussed in this thesis. The membranes were prepared by an in-situ cross-linking process that involved filling the pores of a porous polypropylene host substrate with a solution of branched polyethylenimine and a diepoxy cross-linker of known concentration. The filled membrane was sandwiched between two polyethylene terephthalate sheets, to prevent solvent evaporation, and a cross-linked polymer network was formed within the porous microstructure of the host membrane.
Several cross-linkers were chosen based on their size, rigidity and hydrophobicity to cross-link polyethylenimine. A small, hydrophilic cross-linker- ethylene glycol diglycidyl ether- produced clear, space filling gels that had large swelling capacties by absorbing many thousand times its weight in water when immersed in aqueous solutions of varying pH. An oligomeric cross-linker- polypropylene glycol diglycidyl ether- produced colored space filling gels that exhibited moderate swelling abilities in aqueous solutions of varying pH. A rigid, hydrophobic cross-linker- bis-phenol A diglycidyl ether- produced gels that exhibited dimensionsal contraction (syneresis), scattered light and exhibited low swelling abilities.
Polyethylenimine cross-linked with ethylene glycol diglycidyl ether (PEI-EDGE) is hydrophilic and forms a hydrogel when swollen with water. The physical properties of hydrophobic polypropylene membranes were thus changed to hydrophilic upon the incorporation of the cross-linked polyethylenimine gel. The polyethylenimine gels in the membrane although formally neutral can acquire a charge by deprotonation of water, or addition of acid, due to the basicity of the aliphatic nitrogens. The membranes were very effective in the nanofiltration of ionic solutes because of the charge in the polyethylenimine gel. The rejection of the ionic feed exhibited Donnan exclusion behavior because the rejections of the ionic solutes were greatly dependent upon the valency of the respective co- and counter-ions and sucrose, a neutral solute, was not rejected. Co-ion rejection increased with an increase in ion valency and conversely, counter-ion rejection decreased with an increase in ion valency. Rejections of NaCl could be observed at high pH's, indicating the gels possessed a significant charge concentration over a large range of pH. The permeabilities of PEI-EDGE filled membranes could be fit to a modeled permeability curve calculated from a hydrodynamic gel permeability model developed by Alicja Mika and Ron Childs of McMaster University.
Membranes filled with polyethylenimine cross-linked with polypropyleneglycol diglycidyl ether (PEI-PODGE) exhibited a morphology distinctly different from the PEI-EDGE filled membranes. Laser fluorescence confocal and environmental scanning electron microscopy showed the presence of PEI-PODGE gel coatings on the outer walls of the hose substrate whereas a smooth, space filling gel morphology was seen in PEI-EDGE membranes. The permeability data suggested the existence of a gel plug in the central depths of the host substrate despite the microscopy evidence of coating. The PEI-PODGE membranes, however, exhibited fluxes that were higher than the EDGE cross-linked membranes due to the smaller thickness of the gel plug. The permeabilities of the PODGE cross-linked membranes, therefore, did not fit the modeled permeability curve. The low salt rejections observed in the PEI-PODGE membranes were attributed to concentration polarization of ionic solutes accumulated in the large cavities on the large membrane-feed interface.
Membranes filled with polyethylenimine cross-linked with bis phenol A diglycidyl ether (PEI-BADGE) exhibited unusually high fluxes and low salt rejection. The high fluxes and low salt rejections were consistent with the heterogeneity observed in the morphologies of the bulk PEI-BADGE gels and the effects heterogeneity has on flow through gels. As a result of the effects of heterogeneity, the permeability of PEI-BADGE filled membranes did not fit the modeled permeability curve.
A modification of the pore filling fabrication procedure resulted in the formation of pore-coated ultrafiltration membranes. Motivated by the coated morphology observed in the PEI-PODGE filled membranes, pore coated membranes were fabricated by a controlled evaporation of solvent from a solution onf polyethylenimine and cross-linker filled in the pores of a host membrane. The coating imparted a hydrophilic nature to the originally hydrophobic host substrate and the coating could not be removed even when extracted in boiling methanol for a few days. The membranes had very high fluxes typical of ultrafiltration membranes. The fluxes, however, gradually decreased as a function of both the composition and amount of feed passed through the membrane. Drying the membrane restored the flux to a higher value; however, the decrease in flux was observed upson subsequent testing suggesting the coated layer was swelling into the pores.
Kim, Marcus Young, "Polyethylenimine Filled Membranes" (2002). Open Access Dissertations and Theses. Paper 972.