Microstructural study of aromatic polyamide membrane material

Thin Film Polyamide Nanocomposite Membrane Decorated by Polyphenol-Assisted Ti3C2Tx MXene Nanosheets for Reverse Osmosis

Transition-metal carbides (MXenes), multifunctional 2D materials, have caught the interest of researchers in the fabrication of high-performance nanocomposite membranes. However, several issues regarding MXenes still remain unresolved, including low ambient stability; facile restacking and agglomeration; and poor compatibility and processability. To address the aforementioned challenges, we proposed a facile, green, and cost-efficient approach for coating a stable layer of plant-derived polyphenol tannic acid (TA) on the surface of MXene (Ti₃C₂T_x) nanosheets. Then, high-performance reverse osmosis polyamide thin film nanocomposite (RO-PA-TFN) membranes were fabricated by the incorporation of modified MXene (Ti₃C₂T_x-TA) nanosheets in the polyamide selective layer through interfacial polymerization. The strong negative charge and hydrophilic multifunctional properties of TA not only boosted the chemical compatibility between Ti₃C₂T_x MXene nanosheets and the polyamide matrix to overcome the formation of nonselective voids but also generated a tight network with selective interfacial pathways for efficient monovalent salt rejection and water permeation. In comparison to the neat thin film composite membrane, the optimum TFN (Ti₃C₂T_x-TA) membrane with a loading of 0.008 wt % nanofiller revealed a 1.4-fold enhancement in water permeability, a well-maintained high NaCl rejection rate of 96% in a dead-end process, and enhanced anti-fouling tendency. This research offers a facile way for the development of modified MXene nanosheets to be successfully integrated into the polyamide-selective layer to improve the performance and fouling resistance of TFN membranes.

Nanodiamond-Enabled Thin-Film Nanocomposite Polyamide Membranes for High-Temperature Water Treatment

Despite growing demands for high-temperature wastewater treatment, most available polymeric membranes are limited to mild operating temperatures (<50 °C) and become less efficient at high temperatures. Herein we show how to make thermally stable reverse osmosis thin-film nanocomposite (TFN) membranes by embedding nanodiamond (ND) particles. Polyamide composite layers containing different loadings of surface-modified ND particles were synthesized through interfacial polymerization. The reactive functional groups and the hydrophilic surface of the NDs intensified the interactions of the nanoparticles with the polymer matrix and increased the surface wettability of the TFN membranes. Contact angle measurement showed a maximum decrease from 88.4° for the pristine membrane to 58.3° for the TFN membrane fabricated with 400 ppm ND particles. The addition of ND particles and ethyl acetate created larger surface features on the polyamide surface of TFN membranes. The average roughness of the membranes increased from 108.4 nm for the pristine membrane to 177.5 nm for the TFN membrane prepared with highest ND concentration. The ND-modified TFN membranes showed a higher pure water flux (up to 76.5 LMH) than the pristine membrane (17 LMH) at ambient temperature at 220 psi and room temperature. The TFN membrane with the highest loading of ND particles overcame the trade-off relation between the water flux and NaCl rejection with 76.5 LMH and 97.3% when 2000 ppm of NaCl solution was filtered at 220 psi. Furthermore, with increasing ND concentration, the TFN membrane showed a lower flux decline at high temperatures over time. The TFN400 prepared with 400 ppm of m-phenylene diamine functionalized ND particles had a 13% flux decline over a 9 h filtration test at 75 °C. This research provides a promising path to the development of high-performance TFN membranes with enhanced thermal stability for the treatment of wastewaters at high temperatures.

Robust fabrication of thin film polyamide-TiO2 nanocomposite membranes with enhanced thermal stability and anti-biofouling propensity

The development of nano-enabled composite materials has led to a paradigm shift in the manufacture of high-performance nanocomposite membranes with enhanced permeation, thermo-mechanical, and antibacterial properties. The major challenges to the successful incorporation of nanoparticles (NPs) to polymer films are the severe aggregation of the NPs and the weak compatibility of NPs with polymers. These two phenomena lead to the formation of non-selective voids at the interface of the polymer and NPs, which adversely affect the separation performance of the membrane. To overcome these challenges, we have developed a new method for the fabrication of robust TFN reverse osmosis membranes. This approach relies on the simultaneous synthesis and surface functionalization of TiO₂ NPs in an organic solvent (heptane) via biphasic solvothermal reaction. The resulting stable suspension of the TiO₂ NPs in heptane was then utilized in the interfacial (in-situ) polymerization reaction where the NPs were entrapped within the matrix of the polyamide (PA) membrane. TiO₂ NPs of 10 nm were effectively incorporated into the thin PA layer and improved the thermal stability and anti-biofouling properties of the resulting TFN membranes. These features make our synthesized membranes potential candidates for applications where the treatment of high-temperature streams containing biomaterials is desirable.

Deswelling of ultrathin molecular layer-by-layer polyamide water desalination membranes

The selective layer of pressure-induced water desalination membranes is an ultrathin and highly crosslinked aromatic polyamide (PA) film that separates salt from water based on differences in permeability, which is a product of diffusivity and solubility. Characterizing the transport properties of the selective layer is necessary in understanding its permselective performance. However, measuring transport of ultrathin films in general is nontrivial. Here, Poroelastic Relaxation Indentation (PRI) is employed as a simple deswelling technique for measuring the transport properties of these ultrathin selective layers.

Positron Chemistry in Polymers

Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singletpara-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived tripletortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.

Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane

In this study, positron annihilation lifetime spectroscopy (PALS) is applied to explain the flux-enhancement mechanism in thin-film-composite (TFC) membranes prepared by using dimethyl sulfoxide (DMSO) as an additive in the interfacial polymerization. The TFC membranes show a large increase in water flux, up to 5-fold, compared to nonadditive membrane. Atomic force microscopy (AFM) shows that surface roughness and surface area increase when DMSO in the aqueous phase solution phase works to increase miscibility of the aqueous and the organic phase by reducing the solubility difference of two immiscible solutions. X-ray photoelectron spectroscopy (XPS) reveals the variation of the chemical compositions to the extent that there is a considerable increase in the cross-linked amide linkages of the flux-enhanced TFC membranes. The effects of these structural changes on the molecular-size free volume properties are evaluated by PALS studies. The PALS results are the first to experimentally show that the thin films of cross-linked aromatic polyamide RO membranes are composed of two types of pores having radii of about 2.1-2.4 A from tau3, network pore, and 3.5-4.5 A from tau4, aggregate pore. The increase in the size and number of network pores by means of DMSO addition during interfacial polymerization enhances the water flux notably. The size of aggregate pores also increases and may contribute to enhance water flux, although their number inevitably decreases as the number of network pores becomes increased. Details on the correlations between RO performances and o-Ps lifetime parameters are clearly described based on the pore-flow model of reverse osmosis developed by Sourirajan et al.