Precisely-Engineered Brush Active-Layer and Biomimetic Membranes for Aqueous Separations
Date of Award
Doctor of Philosophy (PhD)
Chemical and Environmental Engineering (ENAS)
Currently, a third of the world lives in areas with inadequate or unsafe water supply, with a projected shortage for at least half the population by 2025. With 97% of the Earth’s water contained in the ocean, improving desalination technology and other aqueous separations could be a solution. At present, the best polyamide thin film composite reverse osmosis (TFC-RO) membranes require 2 times the theoretical minimum energy to desalinate. Furthermore, these membranes cannot sufficiently reject in one pass certain solutes, like boron and chloride, to stringent regulatory levels. Additionally, other aqueous separations are important for the pursuit of environmentally sustainable technologies, water sources, and food and energy production that result in zero liquid discharge and a circular economy. Unconventional wastewaters and brine provide opportunities to augment water supply and recover valuable elements, like lithium for batteries and nitrate-rich nutrients for fertilizers. However, treatment of these waters requires separation of specific solutes from complex mixtures. Therefore, it is essential for membrane research to focus on improving water-salt and ion-ion selectivity. This dissertation focuses on three avenues toward this goal, 1) brush active-layer membranes (BAMs), 2) tethered electrolyte active-layer membranes (TEAMs), and 3) biomimetic membranes. First, this dissertation explores surface-initiated atom transfer radical polymerization (SI-ATRP) as a means to produce BAMs. SI-ATRP allows for the facile production of brush polymers—including distinct block copolymers—at a high density and low dispersity. The goal of producing an effective active layer comprising dense brush layers incentivized a fundamental study of SI-ATRP. In this work, cellulosic films and membranes were selected as appropriate support layers for modification through initiator bonding to the surface hydroxyl groups. Methods to control and verify brush density, polymer length, and growth location were developed and rigorously analyzed. These methods informed the production of BAMs of varied hydrophobicity that dictated the passive diffusion rates of solutes with different octanol−water partition coefficients. These findings suggest brush polymers can perform as active membrane layers. Next, this dissertation considers other polymer types within BAMs by discussing our development of tethered electrolyte active-layer membranes (TEAMs). Improvements in solute-solute selectivity by modifying TFC-RO membranes are difficult due to a limited number of reactive functional groups within aromatic polyamide. Highly-tunable polymeric membrane platforms are instrumental in harnessing both steric and charge-based exclusion mechanisms. These materials should be able to host a high variety of monomer types and/or tethered functional groups and have controllable pore sizes within the nanometer range. One such relevant platform is the polyelectrolyte multilayer membrane (PEM). This ionizable membrane is produced by the sequential, layer-by-layer deposition of polyelectrolytes on porous supports. Although PEMs have significantly rejected ions in dilute saline solutions, they are susceptible to swelling or detachment from the substrate in higher salinities and extreme pH conditions. Taking inspiration from PEMs, we developed TEAMs, where charged block copolymers are covalently grafted-from a porous support to reduce stimuli response. Neutral precursor brush polymers were grown through SI-ATRP and converted into polyelectrolytes. Ultrathin single-block TEAMs of either negative or positive charge rejected salt more than TEAMs with blocks of alternating charge. Single-block TEAMs exhibited 45–60% rejection of monovalent co-ions and ~75% rejection of divalent co-ions in dilute solutions and did not swell at high salinity. However, selectivity of monovalent over divalent co-ions with single-block TEAMs was 2–4, which is inferior to the selectivity of nanofiltration (NF) membranes and PEMs. Crosslinking of PEMs and self-assembled membranes of random zwitterionic amphiphilic copolymers has improved salt rejection, selectivity, and stability while reducing pore sizes. With this in mind, the dissertation next elucidates how crosslinking improves the performance of TEAMs by positioning polyelectrolytes overtop pores for fuller coverage and an effectively higher charge density. When relatively short crosslinkers were used, divalent co-ion rejection in dilute solutions was ~85−95%. NaCl was rejected ~80 and 55% by crosslinked positively- and negative-charged TEAMs, respectively. Anion monovalent selectivity, Cl-/SO42-, was as high as ~25 for negative TEAMs, while the maximum Na+/Ca2+ ratio achieved by positive TEAMs was ~9.5. By demonstrating this rejection and selectivity enhancement, we position brush active-layer membranes and TEAMs as powerful tools to understand fundamental transport of membranes and better control synthesis for targeted selectivity. Lastly, this research considers biomimetic desalination membranes, shifting away from dense polymeric membranes that exhibit transport described by solution-diffusion mechanism toward molecular sieving, which can theoretically achieve higher selectivity. By critically analyzing previous attempts to produce biomimetic membranes, we establish key strategies toward finally realizing their full potential after more than a decade of subpar performance. We prove that defects are the Achilles heel for biomimetic membranes and suggest that bottom-up synthesis of the channel-incorporating amphiphilic matrix may be beneficial. Overall, this dissertation discusses the new development of BAMs and TEAMs as highly-tunable polymeric membrane platforms and thoroughly analyzes the shortcomings of previous biomimetic membranes before suggesting new strategies. The novel membrane avenues presented in this dissertation may lead to unprecedented and tailor-able selectivity for water treatment and other aqueous separations.
Porter, Cassandra, "Precisely-Engineered Brush Active-Layer and Biomimetic Membranes for Aqueous Separations" (2022). Yale Graduate School of Arts and Sciences Dissertations. 513.