PhD Thesis Presentation - Superstrong Ultrahigh Molecular Weight Polyethylene Membrane with Tunable Pore Structure for Separation Processes - Seawater Desalination and Air Purification

Polymer material has been recognized to take a vital role in separation processes. With the advent of commercial interests and activities in membrane separation, a tremendous amount of studies has been carried out on various polymer materials to investigate the polymeric structure-property relationship for high-performance separation. However, several limits still exist for these polymer membranes, especially the trade-off between permeability and selectivity. One of the specialty polymers receiving great attention in the past decade is Ultrahigh Molecular Weight Polyethylene (UHMWPE), primarily owing to its high-specific tensile properties, high toughness and chemical inertness. Nevertheless, current processing methods for manufacturing UHMWPE membranes remain two major problems, including the loss of mechanical strength and thick membrane thickness. This thesis used the low entanglements UHMWPE gel film and biaxially stretching method to develop super-strong thin membranes for desalination and air filtration. These successfully synthesized materials show significant impacts as follows:

1. The UHMWPE membranes fabricated for membrane distillation (MD) desalination exhibited an extraordinary high freshwater flux that is 2 times higher than the state-of-the-art MD membranes in research and an excellent salt rejection of higher than 99.95 %. In addition, the UHMWPE membrane had the capability to tackle high concentration brine water (> 15 wt.%), which is 3 times higher than the traditional reverse osmosis membrane.

2. The UHMWPE filter fabricated for air filtration is a 20 nm-thick monolayer nanofilm. It is composed of a highly interconnected triangular pore structure with a pore size of ~ 300 nm. The thinnest filter ribbon (fiber) only has a diameter of 7 nm, allowing the air molecules to permeate with minimized resistance called the slip effect. Therefore, the UHMWPE filter can reusably achieve a HEPA level filtration efficiency (> 99.97 %) with remarkably low airflow resistance ~ 70 Pa and high transparency. While, in comparison, the opaque single-use glass fiber HEPA filter had an airflow resistance larger than 250 Pa.