Layer-by-layer Membrane Research Articles

Synthesis and Characterization of Novel Forward Osmosis Membranes based on Layer-by-Layer Assembly

Forward osmosis (FO) has received considerable interest for water- and energy-related applications in recent years. FO does not require an applied pressure and is believed to have a low fouling tendency. However, a major challenge in FO is the lack of high performance FO membranes. In the current work, novel nanofiltration (NF)-like FO membranes with good magnesium chloride retention were synthesized using layer-by-layer (LbL) assembly. The membrane substrate was tailored (high porosity, finger-like pores, thin cross-section, and high hydrophilicity) to achieve a small structural parameter of 0.5 mm. Increasing the number of polyelectrolyte layers improved the selectivity of the LbL membranes while reducing their water permeability. The more selective membrane 6#LbL (with 6 polyelectrolyte layers) had much lower reverse solute transport compared to 3#LbL and 1#LbL. Meanwhile, the FO water flux was found to be strongly affected by both membrane water permeability and solute reverse transport. Severe solute reverse transport was observed for the active-layer-facing-draw-solution membrane orientation, likely due to the suppression of Donnan exclusion as a result of the high ionic strength of the draw solution. In contrast, the active-layer-facing-feed-solution orientation showed remarkable FO performance (15, 20, and 28 L/m².h at 0.1, 0.5, and 1.0 M MgCl₂, respectively, for membrane 3#LbL using distilled water as feed solution), superior to other NF-like FO membranes reported in the literature. To the best of the knowledge of the authors, this is the first work on the synthesis and characterization of LbL based FO membranes.

Graphene Oxide−Polyelectrolyte Nanomembranes

Owing to its remarkable electrical, thermal, and mechanical properties, graphene, an atomic layer of carbon, is considered to be an excellent two-dimensional filler for polymer nanocomposites with outstanding mechanical strength along with the potential for excellent electrical and thermal properties. One of the critical limitations with conventional fillers is that the loading fraction required for achieving significant improvement in mechanical properties is relatively high, frequently reaching 50% for maximum strength. Here, we demonstrate that the mechanical properties of ultrathin laminated nanocomposites can be significantly enhanced by the incorporation of small amounts of a dense monolayer of planar graphene oxide (GO) flakes. Negatively charged functionalized graphene oxide layers were incorporated into polyelectrolyte multilayers (PEMs) fabricated in a layer-by-layer (LbL) assembly via Langmuir-Blodgett (LB) deposition. These LbL-LB graphene oxide nanocomposite films were released as robust freely standing membranes with large lateral dimensions (centimeters) and a thickness of around 50 nm. Micromechanical measurements showed enhancement of the elastic modulus by an order of magnitude, from 1.5 GPa for pure LbL membranes to about 20 GPa for only 8.0 vol % graphene oxide encapsulated LbL membranes. These tough nanocomposite PEMs can be freely suspended over large (few millimeters) apertures and sustain large mechanical deformations.

PH-Controlled Assembly and Properties of LbL Membranes from Branched Conjugated Poly(alkoxythiophene sulfonate) and Various Polycations

We report on multilayer layer-by-layer (LbL) films of the conjugated polymer sodium poly[2-(3-thienyl)ethyloxy-4-butylsulfonate] (PTH) assembled with polycations: poly(diallyldimethylammonium chloride) (PDDA), 20% quaternized poly(N-ethyl-4-vinylpyridinium bromide) (Q20), poly(ethylene imine) (PEI), and poly(allylamine hydrochloride) (PAH). These films were prepared through spin-assisted LbL assembly under various pH conditions. We demonstrated a crucial role of the deposition pH in formation of PTH/polycation films and showed that decrease in the deposition pH from 7.5 to 2.5 limits the PTH multilayer formation to Q20/PTH and PDDA/PTH films due to reduced charge density in the poly(thiophene) chains. We show that optical and surface properties of the resulting PTH/polycation films can be tuned by varying a polycation component and/or by varying the deposition pH. The fluorescence properties of the Q20/PTH, PEI/PTH, and PDDA/PTH films are pH-dependent, and the films exhibit the drastic changes in photoluminescent intensity when transferred into solutions with different pH values, which may find useful in optical sensing applications.

Assembly of multilayer PSS/PAH membrane on coherent alginate/PLO microcapsule for long‐term graft transplantation

Conventional alginate/poly-L-ornithine (AP) membranes used to immunoisolate foreign tissue transplants fail in long-term transplantations of immortal cell lines. We have developed a novel layer-by-layer (LbL) membrane using polystyrene sulfonate and polyallylamine hydrochloride (PSS/PAH) on top of the coherent AP membrane. Assembly of the LbL membrane was followed by electrophoresis, and the surface morphologies and structure were characterized and examined by cryo-scanning electron microscope and transmission electron microscopy. Unlike the standard AP membrane, the LbL membrane withstood the internal pressure generated by continuous cell proliferation of microencapsulated HEK-293 and Min-6 cells. The new membrane did not affect insulin secretion or diffusion by Min-6 cells.

Layer-by-layer coating of degradable microgels for pulsed drug delivery

Recently, we reported on “self-rupturing” microcapsules which consist of a biodegradable dextran-based microgel surrounded by a polyelectrolyte membrane. Degradation of the microgel increases the swelling pressure in the microcapsules which, when sufficiently high, ruptures the surrounding polyelectrolyte membrane. The membrane surrounding the microgels is deposited using the layer-by-layer (LbL) technique, which is based on the alternate adsorption of oppositely charged polyelectrolytes onto a charged substrate. In this paper, we characterize with confocal microscopy, electrophoretic mobility, scanning electron microscopy and atomic force microscopy in detail the deposition and the properties of the LbL coatings on the dextran microgels. We show that by fine-tuning the properties of both the microgel core and the LbL membrane the swelling pressure which is evoked by the degradation of the microgel is indeed able to rupture the surrounding LbL membrane. Further, we show that the application of an LbL coating on the surface of the microgels dramatically lowers the burst release from the microcapsules and results in massive release at the time the microcapsules rupture.