(404a) Nanoporous Membranes with Narrowed Pore Size Distribution Via Initiated Chemical Vapor Deposition

Membrane technology represents one of the most energy-efficient, cost-effective, and environmentally friendly separation techniques developed to date. However, the performance of membranes is often limited by a tradeoff between the two key performance indicators, i.e., permeability and selectivity. For example, polymeric membranes with higher selectivity are bound to have lower permeability, but it is desirable to have membranes with both high selectivity and permeability for efficient separations.

This project aims to fabricate “isoporous” membranes, which provide the highest selectivity and permeability among membranes with the same average pore size. That is because selectivity is controlled by the largest pore in a membrane, while permeability is limited by the smallest ones. It is extremely hard to fabricate “isoporous” membranes commercially due to the high production cost of these highly ordered structures. Even though a broad pore size is almost inevitable during manufacturing in the industry, it is possible to narrow the pore size distribution via surface modification, which was demonstrated here using initiated Chemical Vapor Deposition (iCVD). iCVD was selected because it allows the fabrication of conformal coatings on any kind of substrate: 2D or 3D, planar or nano-/micro-structured, etc. We hypothesize that it is possible to fabricate “isoporous” membranes by iCVD, because the rate of coating growth is proportional to the diameter of a nano/micro-pore, thus narrowing the pore size distribution.

To test this hypothesis, Anodic Aluminum Oxide (AAO) membranes with pore sizes ranging from 80 nm to 160 nm, and depths of 5 and 10 μm were prepared using anodization and surface-modified using iCVD. Coatings of poly(4-aminostyrene) [PAS] were deposited at various values of fractional saturation pressure (P_m/P_sat, which represents monomer concentration at the surface). PAS has a primary amine group, allowing further improvements of membrane selectivity by secondary functionalization.

The shrinking of pore size distribution was only observed for membranes with pore sizes larger than 100 nm due to the dominance of the initiator-flux controlled regime, hence confirming our initial hypothesis. While for the membranes with the average pore size of 80 nm, broadening of pore size distribution was observed due to an effect of initiator amplification. Interestingly, for 80 nm membranes, the coating growth was in linear proportion with the total deposition time, while for membranes with pore size larger than 100 nm the coating thickness did not change with deposition time, indicating the existence of two kinetic regimes. That fundamental insight, along with the novel membranes with a narrowed pore size distribution, could transform the current understanding of CVD-based membrane modification/fabrication and improve many crucial separation processes including wastewater treatment and purifications in the pharmaceutical and manufacturing industries.