Nanoporous Polymers for Membrane Applications

Membrane technology has been realized as a useful tool in a variety of applications, such as health sector, food industry, sustainable water treatment and energy conversion and storage. However, the widespread use of this technology has been impeded by many issues including cost, performance, durability and etc. These limitations can be directly related to the membrane used. In particular, advances in the design and fabrication of nanoporous materials are expected to open up new opportunities for the development of membrane technology. Nanoporous polymer membranes derived from self-assembling block copolymers are the focus of the thesis work. Block copolymers consist of macromolecules composed of two or more chemically different blocks. Block copolymers can self-organize into different morphologies with characteristic sizes in the nanometer scale. Self-assembled block copolymer is evolving as a powerful yet affordable tool to fabricate nanoporous materials with well defined morphology, pore size and distribution, porosity, and surface chemistry. This type of nanoporous materials is therefore attractive for the regulation and detection of transport at the molecular level. We have used 1,2-polybutadiene-b-polydimethylsiloxane (1,2-PB-b-PDMS) block copolymer for the production of nanoporous membranes. Nanoporous 1,2-PB membrane with bulk gyroid morphology was obtained via selective and quantitative removal of PDMS block in 1,2-PB-b-PDMS. The gyroid structure shows isotropic percolation without the need for structure pre-alignment. The structure of the membrane outer surface can be controlled from being closed (compact) to open (porous) by
adjusting the interface energy between polymer and different substrates used in membrane formation process. Surface chemistry of nanopores can be changed using photochemistry for a specific need. The work presented in this thesis focuses on exploration of three relevant aspects.
• We studied the effect of surface morphology, membrane thickness and active porosity on permeation of glucose in a pure diffusion mode. The glucose permeability could be tuned over an extending range with different structural/physical-chemical properties of the membranes. Membrane selectivity was assessed by comparing the effective diffusion 5 iv coefficients for a range of antibiotics, proteins and other biomolecules. Efficient selectivity is facilitated by a high degree of control on pore size. A desired selectivity could be achieved by involving other interactions of solute-solute or solute-membrane. The nanoporous membranes were finally tested as the outer membranes for amperometric glucose sensors.
• We have also tested the nanoporous 1,2-PB membranes in convection mode
(ultrafiltration). A number of polyethylene glycol (PEG) molecules with different
molecular weights dissolved in water or in ethanol-water were used to explore the effect of membrane fouling on flux and selectivity for the nanoporous membranes. The flux decline could be significantly diminished by changing the solvent property (i.e. the presence of ethanol) or surface property (i.e. hydrophilization). The experimental PEG rejection profiles were measured for the different systems and compared with BungayBrenner model based on molecule-pore size ratios. The nanoporous membranes showed distinct rejection properties based on different separation mechanisms, due to the adjustment of solvent property and surface property.
• Finally, we examined the loading and release of SDS detergent (sodium dodecyl sulfate) in nanoporous 1,2-PB membranes. We show that the SDS adsorption isotherm is well described by Langmuir model, and is consistent with the formation of a monolayer at the pore interface. We investigated the release process of the SDS out of nanopores in water and in methanol. Initial tests of the SDS-loaded nanoporous 1,2-PB membranes as antibiofilm surfaces, showed promising results.