No abstract

Stimuli-responsive soft materials are a highly studied field due to their wide-ranging applications; however, only a small group of these materials display hysteretic responses to stimuli. Moreover, previous reports of this behavior have typically shown it to be short-lived. In this work, poly(acrylic acid) (PAA) chains at extremely high grafting densities and confined in nanoscale pores displayed a unique long-lived hysteretic behavior caused by their ability to form a metastable hydrogen bond network. Hydraulic permeability measurements demonstrated that the conformation of the PAA chains exhibited a hysteretic dependence on pH, where different effective pore diameters arose in a pH range of 3 to 8, as determined by the pH of the previous environment. Further studies using Fourier transform infrared (FTIR) spectroscopy demonstrated that the fraction of ionized PAA moieties depended on the thin film history; this was corroborated by metal adsorption capacity, which demonstrated the same pH dependence. This hysteresis was shown to be persistent, enduring for days, in a manner unlike most other systems. The hypothesis that hydrogen bonding among PAA units contributed to the hysteretic behavior was supported by experiments with a urea solution, which disrupted the metastable hydrogen bonded state of PAA toward its ionized state. The ability of PAA to hydrogen bond within these confined pores results in a stable and tunable hysteresis not previously observed in homopolymer materials. An enhanced understanding of the polymer chemistry and physics governing this hysteresis gives insight into the design and manipulation of next-generation sensors and gating materials in nanoscale applications.

Continued stresses on fresh water supplies necessitate the utilization of non-traditional resources to meet the growing global water demand. Desalination and hybrid membrane processes are capable of treating non-traditional water sources to the levels demanded by users. Specifically, desalination can produce potable water from seawater, and hybrid processes have the potential to recover valuable resources from wastewater while producing water of a sufficient quality for target applications. Despite the demonstrated successes of these processes, state-of-the-art membranes suffer from limitations that hinder the widespread adoption of these water treatment technologies. In this review, we discuss nanoporous membranes derived from self-assembled block polymer precursors for the purposes of water treatment. Due to their well-defined nanostructures, myriad chemical functionalities, and the ability to molecularly-engineer these properties rationally, block polymer membranes have the potential to advance water treatment technologies. We focus on block polymer-based efforts to: (1) nanomanufacture large areas of highperformance membranes; (2) reduce the characteristic pore size and push membranes into the reverse osmosis regime; and (3) design and implement multifunctional pore wall chemistries that enable solute-specific separations based on steric, electrostatic, and chemical affinity interactions. The use of molecular dynamics simulations to guide block polymer membrane design is also discussed because its ability to systematically examine the available design space is critical for rapidly translating fundamental understanding to water treatment applications. Thus, we offer a full review regarding the computational and experimental approaches taken in this arena to date while also providing insights into the future outlook of this emerging technology.

Membrane adsorbers are a proposed alternative to packed beds for chromatographic separations. To date, membrane adsorbers have suffered from low binding capacities and/or complex processing methodologies. In this work, a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) triblock polymer is cast into an asymmetric membrane that possesses a high density of nanopores (d ∼ 38 nm) at the upper surface of the membrane. Exposing the membrane to a 6 M aqueous hydrochloric acid solution converts the PDMA brushes that line the pore walls to poly(acrylic acid) (PAA) brushes, which are capable of binding metal ions (e.g., copper ions). Using mass transport tests and static binding experiments, the saturation capacity of the PI-PS-PAA membrane was determined to be 4.1 ± 0.3 mmol Cu(2+) g(-1). This experimental value is consistent with the theoretical binding capacity of the membranes, which is based on the initial PDMA content of the triblock polymer precursor and assumes a 1:1 stoichiometry for the binding interaction. The uniformly sized nanoscale pores provide a short diffusion length to the binding sites, resulting in a sharp breakthrough curve. Furthermore, the membrane is selective for copper ions over nickel ions, which permeate through the membrane over 10 times more rapidly than copper during the loading stage. This selectivity is present despite the fact that the sizes of these two ions are nearly identical and speaks to the chemical selectivity of the triblock polymer-based membrane. Furthermore, addition of a pH 1 solution releases the bound copper rapidly, allowing the membrane to be regenerated and reused with a negligible loss in binding capacity. Because of the high binding capacities, facile processing method implemented, and ability to tailor further the polymer brushes lining the pore walls using straightforward coupling reactions, these membrane adsorbers based on block polymer precursors have potential as a separation media that can be designed to a variety of specific applications.

Uranyl peroxide cluster species were produced in aqueous solution by the treatment of uranyl nitrate with hydrogen peroxide, lithium hydroxide, and potassium chloride. Ultrafiltration of these cluster species using commercial sheet membranes with molecular mass cutoffs of 3, 8, and 20 kDa (based on polyethylene glycol) resulted in U rejection values of 95, 85, and 67% by mass, respectively. Ultrafiltration of untreated uranyl nitrate solutions using these membranes resulted in virtually no rejection of U. These results demonstrate the ability to use the filtration of cluster species as a means for separating U from solutions on the basis of size. Small-angle X-ray scattering, Raman spectroscopy, and electrospray ionization mass spectrometry confirmed the presence of uranyl peroxide cluster species in solution and were used to characterize their size, shape, and dispersity.

Novel oppositely charged polyphosphazene polyelectrolytes containing grafted poly(ethylene glycol) (PEG) chains were synthesized as modular components for the assembly of biodegradable PEGylated protein delivery vehicles. These macromolecular counterparts, which contained either carboxylic acid or tertiary amino groups, were then formulated at near physiological conditions into supramolecular assemblies of nanoscale level, below 100 nm. Nanocomplexes with electroneutral surface charge, as assessed by zeta potential measurements, were stable in aqueous solutions, which suggests their compact polyelectrolyte complex "core"-hydrophilic PEG "shell" structure. Investigation of PEGylated polyphosphazene nanocomplexes as agents for noncovalent PEGylation of the therapeutic protein l-asparaginase (L-ASP) in vitro demonstrated their ability to dramatically reduce protein antigenicity, as measured by antibody binding using enzyme linked immunosorbent assay (ELISA). Encapsulation in nanocomplexes did not affect enzymatic activity of L-ASP, but improved its thermal stability and proteolytic resistance. Gel permeation chromatography (GPC) experiments revealed that all synthesized polyphosphazenes exhibited composition controlled hydrolytic degradability in aqueous solutions at neutral pH and showed greater stability at lower temperatures. Overall, novel hydrolytically degradable polyphosphazene polyelectrolytes capable of spontaneous self-assembly into PEGylated nanoparticulates in aqueous solutions can potentially enable a simple and effective approach to modifying therapeutic proteins without the need for their covalent modification.

Heavy metal contamination of water supplies poses a serious threat to public health, prompting the development of novel and sustainable treatment technologies. One promising approach is to molecularly engineer the chemical affinity of a material for the targeted removal of specific molecules from solution. In this work, nanoporous polymer thin films generated from tailor-made block polymers were functionalized with the bio-inspired moieties glutathione and cysteamine for the removal of heavy metal ions, including lead and cadmium, from aqueous solutions. In a single equilibrium stage, the films achieved removal rates of the ions in excess of 95%, which was consistent with predictions based on the engineered material properties. In a flow-through configuration, the thin films achieved an even greater removal rate of the metal ions. Furthermore, in mixed ion solutions the capacity of the thin films, and corresponding removal rates, did not demonstrate any reduction due to competitive adsorption effects. After such experiments the material was repeatedly regenerated quickly with no observed loss in capacity. Thus, these membranes provide a sustainable platform for the efficient purification of lead- and cadmium-contaminated water sources to safe levels. Moreover, their straightforward chemical modifications suggest that they could be engineered to treat sources containing other recalcitrant environmental contaminants as well.