Effect of Final Monomer Deposition Steps on Molecular Layer-by-Layer Polyamide Surface Properties.

A current challenge to desalination membrane technology is the inability to precisely control the properties of the polyamide selective layer due to the complexity of interfacial polymerization. In this study, we investigate the ability of molecular layer-by-layer (mLbL) assembly, an alternative polyamide fabrication technique, to create polyamide surfaces with tunable chemistry. We explore the influence of terminating monomer, monomer deposition time, monomer size, and the presence of underlying ionizable functional groups on mLbL-derived polyamide surface properties. AFM colloidal probe measurements, contact angle titrations, QCM cesium adsorption experiments, and XPS data show that polyamide films terminated with m-phenylenediamine or trimesoyl chloride for 20-30 s are chemically similar. Increasing terminating monomer deposition time or using a smaller, more reactive monomer results in more distinct colloidal-probe adhesive interactions, contact angle titration curves, negative charge densities, and near surface atomic compositions. By optimizing the final monomer deposition steps, both amine-rich and carboxyl-rich polyamide surfaces can be fabricated, which has implications for the application of mLbL assembly to membrane-based desalination.

Thin Polymer Films with Continuous Vertically Aligned 1 nm Pores Fabricated by Soft Confinement.

Membrane separations are critically important in areas ranging from health care and analytical chemistry to bioprocessing and water purification. An ideal nanoporous membrane would consist of a thin film with physically continuous and vertically aligned nanopores and would display a narrow distribution of pore sizes. However, the current state of the art departs considerably from this ideal and is beset by intrinsic trade-offs between permeability and selectivity. We demonstrate an effective and scalable method to fabricate polymer films with ideal membrane morphologies consisting of submicron thickness films with physically continuous and vertically aligned 1 nm pores. The approach is based on soft confinement to control the orientation of a cross-linkable mesophase in which the pores are produced by self-assembly. The scalability, exceptional ease of fabrication, and potential to create a new class of nanofiltration membranes stand out as compelling aspects.

Experimental Evidence for Proposed Transformation Pathway from the Inverse Hexagonal to Inverse Diamond Cubic Phase from Oriented Lipid Samples.

A macroscopically oriented inverse hexagonal phase (HII) of the lipid phytantriol in water is converted to an oriented inverse double diamond bicontinuous cubic phase (QII(D)). The initial HII phase is uniaxially oriented about the long axis of a capillary with the cylinders parallel to the capillary axis. The HII phase is converted by cooling to a QII(D) phase which is also highly oriented, where the cylindrical axis of the former phase has been converted to a ⟨110⟩ axis in the latter, as demonstrated by small-angle X-ray scattering. This epitaxial relationship allows us to discriminate between two competing proposed geometric pathways to convert HII to QII(D). Our findings also suggest a new route to highly oriented cubic phase coatings, with applications as nanomaterial templates.

Scalable fabrication of polymer membranes with vertically aligned 1 nm pores by magnetic field directed self-assembly.

There is long-standing interest in developing membranes possessing uniform pores with dimensions in the range of 1 nm and physical continuity in the macroscopic transport direction to meet the needs of challenging small molecule and ionic separations. Here we report facile, scalabe fabrication of polymer membranes with vertically (i.e., along the through-plane direction) aligned 1 nm pores by magnetic-field alignment and subsequent cross-linking of a liquid crystalline mesophase. We utilize a wedge-shaped amphiphilic species as the building block of a thermotropic columnar mesophase with 1 nm ionic nanochannels, and leverage the magnetic anisotropy of the amphiphile to control the alignment of these pores with a magnetic field. In situ X-ray scattering and subsequent optical microscopy reveal the formation of highly ordered nanostructured mesophases and cross-linked polymer films with orientational order parameters of ca. 0.95. High-resolution transmission electron microscopy (TEM) imaging provides direct visualization of long-range persistence of vertically aligned, hexagonally packed nanopores in unprecedented detail, demonstrating high-fidelity retention of structure and alignment after photo-cross-linking. Ionic conductivity measurements on the aligned membranes show a remarkable 85-fold enhancement of conductivity over nonaligned samples. These results provide a path to achieving the large area control of morphology and related enhancement of properties required for high-performance membranes and other applications.

Aligned nanostructured polymers by magnetic-field-directed self-assembly of a polymerizable lyotropic mesophase.

Magnetic-field-directed assembly of lyotropic surfactant mesophases provides a scalable approach for the fabrication of aligned nanoporous polymers by templated polymerization. We develop and characterize a lyotropic liquid crystalline system containing hexagonally packed cylindrical micelles of a polymerizable surfactant in a polymerizable solvent. The system exhibits negative magnetic anisotropy, resulting in the degenerate alignment of cylindrical micelles perpendicular to the magnetic field. Sample rotation during field alignment is used to effectively break this degeneracy and enable the production of uniformly well-aligned mesophases. High-fidelity retentions of the hexagonal structure and alignment were successfully achieved in polymer films produced upon UV exposure of the reactive system. The success of this effort provides a route for the fabrication of aligned nanoporous membranes suitable for highly selective separations, sensing, and templated nanomaterial synthesis.

Tailoring elastic properties of silica aerogels cross-linked with polystyrene.

The effect of incorporating an organic linking group, 1,6-bis(trimethoxysilyl)hexane (BTMSH), into the underlying silica structure of a styrene cross-linked silica aerogel is examined. Vinyltrimethoxysilane (VTMS) is used to provide a reactive site on the silica backbone for styrene polymerization. Replacement of up to 88 mol % of the silicon from tetramethoxyorthosilicate with silicon derived from BTMSH and VTMS during the making of silica gels improves the elastic behavior in some formulations of the cross-linked aerogels, as evidenced by measurement of the recovered length after compression of samples to 25% strain. This is especially true for some higher density formulations, which recover nearly 100% of their length after compression to 25% strain twice. The compressive modulus of the more elastic monoliths ranged from 0.2 to 3 MPa. Although some of these monoliths had greatly reduced surface areas, changing the solvent used to produce the gels from methanol to ethanol increased the surface area in one instance from 6 to 220 m(2)/g with little affect on the modulus, elastic recovery, porosity, or density.