Membrane separation is a field of both industrial and academic importance. Current technology is largely based on polymeric materials, and to a less extent other inorganic materials such as ceramics and metals. While developments in materials properties and membrane structures are constantly evolving, there are two challenges that need to be circumvented for better performance, i.e. the control over the pore structure and the chemical flexibility in modifying pore surface. “Bottom-up” approach to construct composite membranes using nanotubes in polymeric matrix is an effective route in fabricating membranes with well-defined architecture and tunable pore surface chemistry. This dissertation focuses on characterization and evaluation of cyclic peptide nanotubes (CPNs), a natural protein channel mimetic, in constructing sub-nanometer composite membranes with a cylinder-forming block copolymer (BCP) matrix in thin films. The fundamental understanding of the self-assembly of the CPNs from the building blocks establishes the foundation in utilizing the unique feature of CPNs to ensure precise structural control over the dimensions of the 1D nanotubes. The knowledge gained from the co-assembly of CPNs and BCP matrix in thin films allows further processing of the nanotubes to form well-aligned transport channels, establishing the guidelines in fabricating sub-nanometer porous membranes with and without surface chemistry modification. By identifying the key parameters in the membrane fabrication processes, design features for creating high-performance CPN based membranes can be determined and expanded. This indeed provides many exciting opportunities in developing new composite membranes with superior separation performances.
The self-assembly of cyclic peptide (CP) subunits forming high aspect ratio nanotubes is driven by strong intermolecular hydrogen bonding. To modulate and tune the growth of CPNs, polymers are conjugated to the exterior of the peptide subunits, resulting in the formation of polymer covered-CPNs (pc-CPNs). Due to the restriction of intermolecular hydrogen bonding, the conjugated polymer chains enter a confined space set by the hydrogen bonding distance. The entropic penalty associated with deforming the conjugated polymers serves as an opposing force destabilizes nanotube structure, while the enthalpic hydrogen bonding drives the nanotube formation. A delicate balance between the enthalpic driving force and the entropic destabilizing force enables one to modulate the growth of the nanotubes. Thus, the dimensions of the resultant pc-CPNs can be supervised simply by regulating the extent of the entropic penalty from the conjugated polymer chains.
In co-assembling CPNs and BCP matrix in thin films, both thermodynamic and kinetic parameters are critical to ensure homogeneous thin film morphology with well-aligned CPN channels at the center of the cylindrical microdomains of the BCP oriented normal to the substrate surface. The balance between the enthalpic interactions between the pc-CPNs and BCP and the entropic cost of polymer chain deformation gives rise to only one nanotube in the cylindrical microdomain. Due to the dynamic nature of CPN formation, preaggregation of the nanotubes causes defects of lay-down nanotubes at the membrane surface, hence compromising membrane quality and integrity. As a result, controlling the kinetic pathway of the co-assembly process is vital to fabricate high quality membranes for separation. Two simple approaches targeting two separate aggregation contributors have been developed to effectively prevent preaggregation of CPNs, resulting in high quality membranes suitable for molecular separation.
With the advancement in incorporating functional groups to the constituting peptide subunits, the interior surface of the CPNs can be further functionalized. Membranes have been fabricated using both the unmodified and modified CPNs, in which gas separation of CO2/CH4 mixture and hydronium ion transport were performed. In general, the incorporation of the CPNs improves the overall performance of the membranes, likely by providing additional pathways for the permeating molecules. Differences in the separation behaviors of the regular CPNs and the methyl-modified CPNs are observed for both gas separation and hydronium ion transport, where higher selectivity for CO2 over CH4 is seen for the methyl-modified CPNs. The local dipole interactions with CO2 molecules as well as the reduction in pore size are speculated to induce the differences in the performances of unmodified and unmodified CPNs.
These studies indeed establish the foundation in fabricating sub-nanometer porous membranes using self-assembled CPNs and BCP matrix in thin films. A delicate balance between the enthalpic and entropic contributions results in precise control over the structures of the nanotubes and the membranes. This unique “bottom-up” strategy demonstrates to be an effective platform in constructing new family of membranes for chemical separations.