Xueping Yao

Membranes with Highly Ordered Straight Nanopores by Selective Swelling of Fast Perpendicularly Aligned Block Copolymers

Membranes with uniform, straight nanopores have important applications in diverse fields, but their application is limited by the lack of efficient producing methods with high controllability. In this work, we reported on an extremely simple and efficient strategy to produce such well-defined membranes. We demonstrated that neutral solvents were capable of annealing amphiphilic block copolymer (BCP) films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) with thicknesses up to 600 nm to the perpendicular orientation within 1 min. Annealing in neutral solvents was also effective to the perpendicular alignment of block copolymers with very high molecular weights, e.g., 362 000 Da. Remarkably, simply by immersing the annealed BCP films in hot ethanol followed by drying in air, the originally dense BCP films were nondestructively converted into porous membranes containing highly ordered, straight nanopores traversing the entire thickness of the membrane (up to 1.1 μm). Grazing incident small-angle X-ray spectroscopy confirmed the hexagonal ordering of the nanopores over large areas. We found that the overflow of P2VP chains from their reservoir P2VP cylinders and the deformation of the PS matrix in the swelling process contributed to the transformation of the solid P2VP cylinders to empty straight pores. The pore diameters can be tuned by either changing the swelling temperatures or depositing thin layers of metal oxides on the preformed membranes via atomic layer deposition with a subnanometer accuracy. To demonstrate the application of the obtained porous membranes, we used them as templates and produced centimeter-scale arrays of aligned nanotubes of metal oxides with finely tunable wall thicknesses.

Highly Porous Metal Oxide Networks of Interconnected Nanotubes by Atomic Layer Deposition

Mesoporous metal oxide networks composed of interconnected nanotubes with ultrathin tube walls down to 3 nm and high porosity up to 90% were fabricated by atomic layer deposition (ALD) of alumina or titania onto templates of swelling-induced porous block copolymers. The nanotube networks possessed dual sets of interconnected pores separated by the tube wall whose thickness could be finely tuned by altering ALD cycles. Because of the excellent pore interconnectivity and high porosity, the alumina nanotube networks showed superior humidity-sensing performances.

Swelling-induced mesoporous block copolymer membranes with intrinsically active surfaces for size-selective separation

Block copolymers (BCPs) are receiving growing interest in the preparation of advanced membranes with regular pores due to their capability to form highly ordered, mesoscopic structures via microphase separation. We report on the fabrication of composite membranes with mesoporous amphiphilic BCPs as the size-selective layer and a macroporous membrane as the supporting layer by coating BCPs onto the supporting membrane. Mesopores were generated in the BCP layer by a selective swelling-induced pore-formation process. The composite membranes showed high pore regularity, strong mechanical robustness, and a separating property that can be tuned simply by changing the swelling time. Furthermore, the polyelectrolyte-natured blocks were exclusively relocated on the pore surface of the BCP layer during the swelling process, rendering an intrinsically active surface on the membrane. As a result, the hydrophilicity and fouling resistance of the membranes were significantly enhanced, and the membranes possessed a reversible pH-sensitive water flux. The membranes were used to separate nanoparticles of similar sizes and it was observed that the membrane subjected to 24 h of swelling was able to discriminate 10 nm gold particles from a mixture containing 2 nm gold particles with ∼100% yield, demonstrating its superior size selectivity.

Highly ordered TiO2 nanostructures by sequential vapour infiltration of block copolymer micellar films in an atomic layer deposition reactor

Sequential vapour infiltration operated in an atomic layer deposition (ALD) reactor was used to controllably deliver precursors of TiO2 into the cores of micellar films of the amphiphilic block copolymer, poly(styrene-block-4-vinylpyridine) (PS-b-P4VP). Vaporized precursors diffused through the PS corona and were exclusively enriched into the P4VP cores. Arrays of hexagonally arranged TiO2 nanoparticles were produced by burning off the polymeric components after UV-crosslinking the TiO2-incorporated micellar films. The size of the TiO2 particles was tunable simply by repeating the cycle numbers, and the interparticle distances were dictated by the distances between neighboring micelles of the original micellar films and could be changed by using block copolymers with different molecular weights. Compared to the extensively used solution impregnation method, the sequential vapour infiltration strategy was distinct in terms of the simplicity accompanying the “dry” process and precise control in particle sizes. More importantly, growth of TiO2 particles inside micellar cores was not limited by the available pyridyl groups as TiO2 continued to grow on preformed TiO2 particles after the pyridyl groups were consumed. Consequently, the particle sizes could be tuned in a much broader range compared to the solution impregnation method in which the particle size was limited by the saturation of impregnated precursors bound to the pyridyl groups. Furthermore, we demonstrated the versatility of this sequential vapour infiltration strategy in producing nanostructures with different morphologies and chemical compositions.

Filtration-Based Synthesis of Micelle-Derived Composite Membranes for High-Flux Ultrafiltration

Ideal membrane configurations for efficient separation at high flux rates consist of thin size-selective layers connected to macroporous supports for mechanical stabilization. We show that micelle-derived (MD) composite membranes combine efficient separation of similarly sized proteins and water flux 5-10 times higher than that of commercial membranes with similar retentions. MD composite membranes were obtained by filtration of solutions of amphiphilic block copolymer (BCP) micelles through commercially available macroporous supports covered by sacrificial nanostrand fabrics followed by annealing and removal of the nanostrand fabrics. Swelling-induced pore generation in the BCP films thus covering the macroporous supports yielded ∼210 nm thin nanoporous size-selective BCP layers with porosities in the 40% range tightly connected to the macroporous supports. Permselectivity and flux rates of the size-selective BCP layers were adjusted by the BCP mass deposited per membrane area and by proper selection of swelling times. The preparation methodology described here may pave the way for a modular assembly system allowing the design of tailored separation membranes.

Water-dispersible, uniform nanospheres by heating-enabled micellization of amphiphilic block copolymers in polar solvents.

Uniform nanospheres with tunable size down to 30 nm were prepared simply by heating amphiphilic block copolymers in polar solvents. Unlike reverse micelles prepared in nonpolar, oily solvents, these nanospheres have a hydrophilic surface, giving them good dispersibility in water. Furthermore, they are present as individual, separated, rigid particles upon casting from the solution other than continuous thin films of merged micelles cast from micellar solution in nonpolar solvents. These nanospheres were generated by a heating-enabled micellization process in which the affinity between the solvent and the polymer chains as well as the segmental mobility of both hydrophilic and hydrophobic blocks was enhanced, triggering the micellization of the glassy copolymers in polar solvents. This heating-enabled micellization produces purely well-defined nanospheres without interference of other morphologies. The micelle sizes and corona thickness are tunable mainly by changing the lengths of the hydrophobic and hydrophilic blocks, respectively. The heating-enabled micellization route for the preparation of polymeric nanospheres is extremely simple, and is particularly advantageous in producing rigid, micellar nanospheres from block copolymers with long glassy, hydrophobic blocks which are otherwise difficult to prepare with high efficiency and purity. Furthermore, encapsulation of hydrophobic molecules (e.g., dyes) into micelle cores could be integrated into the heating-enabled micellization, leading to a simple and effective process for dye-labeled nanoparticles and drug carriers.

Energy-saving, responsive membranes with sharp selectivity assembled from micellar nanofibers of amphiphilic block copolymers

Membrane technology contributes significantly in a large number of energy- and environment-related fields in a clean and efficient way. It remains a challenge to develop advanced membranes with simultaneous high flux and sharp selectivity. Such membranes require a thin selective layer, high porosity, and strong hydrophilicity. A promising strategy to fabricate such membranes is to build an integral selective layer of nanofibers on a macroporous support. We report an extremely simple method to produce uniform nanofibers with diameters of <30 nm by directly dissolving block copolymers of polystyrene-block-poly(4-pyridine) (PS-b-P4VP) with long, glassy PS blocks in a polar solvent. The fibers were cylindrical micelles with PS cores covered by P4VP coronae, formed through a heating-enabled micellization process. We deposited the fibers on the surface of macroporous supports by vacuum filtration to fabricate composite membranes. The gaps between the fibers served as mesh pores, with effective sizes down to several nanometers. Distinct from other types of nanofibers, the micellar fibers had a P4VP-covered surface, which not only enhanced the adhesion between fibers, but also endowed the membrane with a stimuli-responsive function. The fiber layers could be made very thin, with a thickness of ∼270 nm or even thinner, but were mechanically stable and exhibited a water flux as high as 940 L m−2 h−1 bar−1 at ∼100% rejection to bovine serum albumin. The fiber membrane displays an energy-saving characteristic as it provides high flux under low pressures compared with commercial ultrafiltration (UF) membranes. For example, it produces a flux over 10 times larger than that of commercial UF membranes. The membranes are promising for the removal of particulate contaminants from water as demonstrated by their excellent concentration capability for 5 nm gold colloidal nanoparticles.

Surface functionalization of carbon nanotubes by direct encapsulation with varying dosages of amphiphilic block copolymers

Encapsulation of carbon nanotubes (CNTs) by amphiphilic block copolymers is an efficient way to stabilize CNTs in solvents. However, the appropriate dosages of copolymers and the assembled structures are difficult to predict and control because of the insufficient understanding on the encapsulation process. We encapsulate multiwalled CNTs with polystyrene-block-poly (4-vinyl pyridine) (PS-b-P4VP) by directly mixing them in acetic acid under sonication. The copolymer forms a lamellar structure along the surface of CNTs with the PS blocks anchoring on the tube wall and the P4VP blocks exposed to the outside. The encapsulated CNTs achieve good dispersibility in polar solvents over long periods. To increase our understanding of the encapsulation process we investigate the assembled structures and stability of copolymer/CNTs mixtures with changing mass ratios. Stable dispersions are obtained at high mass ratios between the copolymer and CNTs, i.e. 2 or 3, with the presence of free spherical micelles. Transmission electron microscopy and thermal gravimetric analysis determine that the threshold for the complete coverage of CNTs by the copolymer occurs at the mass ratio of 1.5. The coated copolymer layer activates the surface of CNTs, enabling further functionalization of CNTs. For instance, atomic layer deposition of TiO(2) produces conformal thin layers on the encapsulated CNTs while isolated TiO(2) bumps are produced on the pristine, inert CNTs.