Structure and Hydration of Asymmetric Polyelectrolyte Multilayers as Studied by Neutron Reflectometry: Connecting Multilayer Structure to Superior Membrane Performance

Abstract

Porous membranes coated with so-called asymmetric polyelectrolyte multilayers (PEMs) have recently been shown to outperform commercial membranes for micropollutant removal. They consist of open support layers of poly­(styrene sulfonate) (PSS)/poly­(allylamine) (PAH) capped by denser and more selective layers of either PAH/poly­(acrylic acid) (PAA) or PAH/Nafion. Unfortunately, the structure of these asymmetric PEMs, and thus their superior membrane performance, is poorly understood. In this work, neutron reflectometry (NR) is employed to elucidate the multilayered structure and hydration of these asymmetric PEMs. NR reveals that the multilayers are indeed asymmetric in structure, with distinct bottom and top multilayers when air-dried and when solvated. The low hydration of the top [PAH/Nafion] multilayer, together with the low water permeance of comparable [PAH/Nafion]-capped PEM membranes, demonstrate that it is a reduction in hydration that makes these separation layers denser and more selective. In contrast, the [PAH/PAA] capping multilayers are more hydrated than the support [PSS/PAH] layers, signifying that, here, densification of the separation layer occurs through a decrease in the mesh size (or effective pore size) of the top layer due to the higher charge density of the PAH/PAA couple compared to the PSS/PAH couple. The [PAH/PAA] and [PAH/Nafion] separation layers are extremely thin (∼4.5 and ∼7 nm, respectively), confirming that these asymmetric PEM membranes have some of the thinnest separation layers ever achieved.

Effects of Feed Solution pH on Polyelectrolyte Multilayer Nanofiltration Membranes

“…For the asymmetric system, these characterization methods suggest rather distinct layers of PAH/PAA and PAH/ PSS, which has been indicated in the literature. 59 While zeta potential is dominated by the top layer of PAH/PAA (2 bilayers), the overall swelling behavior of the film is dominated by PAH/PSS (8 bilayers), due to the higher relative layer number. Direct Membrane Characterization.…”

“…We mainly attribute the minor increase in swelling compared to the pure PAH/PAA system to the dominant amount of PAH/PSS layers (8 bilayers). While it was shown that these kind of asymmetric films indeed form rather distinct layers, 59 slight intermixing of the systems could influence the overall swelling degree.…”

Laser‐Directed Foaming of Hydroplastic Polyelectrolyte Films toward Tunable Structures and Programmable Routes

“…As a solvent with a high dielectric constant (78.36 F m −1 , 25 °C), water tends to enhance the screening of charges along the main chains of polyelectrolytes and thus increase the possibility of splitting the electrostatic interaction between positively charged groups on polycation and negatively charged groups on polyanion. [ 44–46 ] With the existence of the screening effect, the polarity of both the NH 3 + groups and the COO − groups within a PEI/PAA‐N 3 film is expected to be attenuated under water. This can be verified by the shift of the vibrating peaks of NH 3 + groups and COO − groups to higher wavenumbers on the attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectra of a wet (PEI/PAA‐5.4N 3 ) 10 film, compared to that of a dry one (Figure 5C).…”

Virtual Special Issue: Polymeric Membranes for Advanced Separations

“…In another work, neutron reflectometry (NR) was used to distinguish the support and separation layers of membranes composed of polyelectrolyte multilayers. 24 NR reveals that the membranes have an asymmetric structure with distinct bottom and top multilayers in either the dried or hydrated state. It has also been demonstrated that reduced hydration leads to denser and consequently more selective separation layers.…”

“…It is revealed that on the nanometer length scale water diffuses through the desalination membrane at a rate similar to bulk water, while water diffuses through the AEM 2 times slower than bulk water. In another work, neutron reflectometry (NR) was used to distinguish the support and separation layers of membranes composed of polyelectrolyte multilayers . NR reveals that the membranes have an asymmetric structure with distinct bottom and top multilayers in either the dried or hydrated state.…”