Design of chemistry and morphology of polymer filtration membranes

Abstract

To improve membrane materials in water filtration, which currently display broad pore size distribution, hydrophobic chemistry, and fouling behavior, a novel design of chemistry and morphology of membranes is employed. First, through fundamental studies of morphological formation both in bulk by lattice-Boltzmann (LB) simulation methods and at the surface by observation of electron micrographs, it is illustrated that phase separation via spinodal decomposition is responsible for pore development. This understanding suggests the possibility to tailor a uniform and interconnected porous membrane by using the spinodal structure. Considering that the control of spinodal porous structure on a nanoscale can be challenging, an alternative approach to achieve a similar interconnected morphology by utilizing the self-assembled structure of a graft copolymer is presented. This graft copolymer permits not only the design of morphology through its architecture, but also the design of chemistrythrough its chemical components. Here, a comb-type structure of a copolymer is applied; this structure contains a hydrophobic poly(vinylidene fluoride) (PVDF) backbone for structural integrity and hydrophilic poly(ethylene oxide) (PEO) side chains for preferential water transport. A membrane with the microphase-separated structure of this copolymer at the surface is then utilized to clean oily water wastes where the membrane rejects more than 99.9% of the oil without fouling. This membrane can also perform molecular sieving, serve as a chromatography instrument, and isolate a product of a designated size distribution on a nanoscale via its tunability of channel sizes, as demonstrated in the uniform size dispersity of gold nanoparticles. Moreover, gold nanoparticles are introduced as a probe to study sieving characteristics of the membrane by tailoring their size and chemistry. The success in regulating transport across the membrane through the self-assembled platform leads to a new family of filtration membranes that could offer much broader applications for nanoscale separation.