Thickness‐dependent swelling of molecular layer‐by‐layer polyamide nanomembranes

Abstract

The thickness‐dependent water vapor swelling of molecular layer‐by‐layer polyamide films is studied via specular X‐ray reflectivity. The maximum swelling ratio of these ultrathin films scale inversely with thickness but more importantly show a dual‐mode sorption behavior characterized by Langmuir‐like sorption at low relative humidity and network swelling at high relative humidity. The thickness‐dependent network parameters are extracted using a proposed model that builds on Painter‐Shenoy network swelling model while taking into account the glass‐like characteristic below a critical swelling ratio, which also scales inversely with thickness. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 412–417

Neutron Reflectivity and Performance of Polyamide Nanofilms for Water Desalination

“…Formally, the organic-soluble component (acid chloride) is insoluble into the polymer, whereas the water-soluble component (diamine) diffuses through the film to react with the acid chloride at the film/organic phase interface, where the film grows. [25,30,31] Toward this goal, considerable effort has been dedicated to the structural characterization of the skin layer of commercially available RO membranes, [26,[28][29][30][31][32][33][34][35][36][37] generally carried out under vacuum (away from operating conditions), molecular modeling, [38][39][40][41][42][43] or by developing more controlled synthetic pathways. [27] Despite being widely used on an industrial scale, membranes obtained via IP are generally inhomogeneous in terms of spatial variation, chemistry, and porosity at the micrometer to nanoscales.…”

“…[21,22] The resulting cross-linked polyamide (PA) film has an overall "apparent" film thickness of, typically, a few hundred nm, and is rough and crumpled, with individual PA film thickness of the order of 10 nm, as revealed by recent high-resolution imaging reports, [23][24][25][26] supported by a porous polysulfone layer with a heterogeneous nanoscale interface layer. [37,[44][45][46][47][48] These include spinassisted molecular layer-by-layer (mLbL) techniques, [37,44,45] and ultrathin films synthesized on Cd(OH) 2 nanowires [46] and carbon nanotube supports, [47] or a cellulose nanocrystal interlayer, [48] seeking to fabricate highly controlled PA thin films, and systematically correlate film thickness, roughness, and membrane performance. [28,29] Although the interplay between the top layer and porous support is important for the membrane properties and, for instance, the support layer undergoes compaction during operation [10] thus contributing to a reduction in flux, the actual separation process is governed by the PA skin layer.…”

“…[35][36][37]44,45] Combined with neutron reflectivity (NR) and isotopic contrast variation, we are able to resolve the membrane structure and selectively elucidate its various components, under dry and hydrated states, for various reactant stoichiometries and reaction times. X-ray reflectivity allows a robust quantification of the film thickness and roughness, as well as the scattering length density (SLD) profile with Å resolution.…”

Functional group quantification of polymer nanomembranes with soft x-rays

“…Characterizing the chemistry of a bulk polyamide sample is straightforward, but characterizing these chemical moieties for ultrathin polyamide films is nontrivial as they are ≈100 nm thick. Additionally, the degree of compositional homogeneity in these films is unclear, and it would be significantly beneficial to quantify the potential depth-dependent heterogeneity [6,7]. …”

“…We attribute the slight discrepancies to either the thickness-dependent crosslink density or the presence of network defects. Chan et al have recently reported the thickness-dependent crosslink density of P m PDTA thin films to show that the crosslink density increases with increasing film thickness up to ≈ 72 nm [7]. Network defects such as dangling bonds will affect the swelling behavior, and thus the pore size, of a polymer network.…”