Tuneable molecular-selective poly(vinylidene fluoride) supported boron nitride nanosheet composite lamellar membrane for enhanced solute exclusion properties
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Date
2020
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Universiti Teknologi Malaysia
Abstract
Two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheets being an isostructural to graphene, has received considerable interest in many emerging applications including water treatment due to its robust properties. However, experimental works on this novel material have not been extensively reported. Therefore, in this research, a fundamental investigation on molecular transport through h-BN nanosheet-based lamellar membrane was conducted. However, h-BN exhibits lower dispersibility in polar solvents which consequently limits its utilization for membrane application. A facile two-step process was started with liquid-phase sonication assisted exfoliation of h-BN with urea as reduction-stabilizing agent. This was followed by grafting different molecular weights of polyethylene glycol using 3-Aminopropyl triethoxysilane structure-directing agent. Lastly, lamellar membrane fabrication was done through vacuum filtration of PEGX-g-(f-BN) nanosheet dispersion. In the first step, the liquid-phase sonication assisted exfoliation process yielded 6 to- 14 layered 75±2 nm functionalized boron nitride (f-BN) nanosheets. In the second step, the deposited thickness was tuned through varying the concentration of the PEGX-g-(f-BN) nanosheet colloidal solution. The final polyethylene glycol grafting was intended to produce two-fold outcomes. Enhancing surface wettability through the transformation of superhydrophobic h-BN to superhydrophilic PEGX-g-(f-BN) was the aim of the first stage. In addition, varying molecular weight of polyethylene glycol aimed to alter the interplanar spacing of the nanosheet, implicating the nanosheet tunability in terms of permeability and solute exclusion. Improved wettability (small contact angle of ~36˚) depicted high trans-lamellar membrane permeance of ~1253 L m-2 h-1 bar-1. With the increasing deposition thickness of PEGX-g-(f-BN) nanosheet layer from ~3 μm to ~12 μm the permeance decreased from ~1253 L m-2 h-1 bar-1 to ~840 L m-2 h-1 bar-1 and the methylene blue (MB) exclusion improved to ~98% because of higher hydrodynamic resistance in trans-lamellar membrane. Moreover, with increasing interplanar spacing from ~0.336 nm to ~0.348 nm of the nanosheets the permeability increased from ~849 L m-2 h-1 bar-1 to a maximum of ~1253 L m-2 h-1 bar-1 and then started to decline to ~250 L m-2 h-1 bar-1, which was attributed to the long polyethylene glycol chain folding. This study indicated that the fabricated lamellar membranes are stable with interconnected nanopores with an average pore diameter of ~1.4 nm to ~3.0 nm. The high retention of ~99% for MB from aqueous solution through the PEGX-g-(f-BN) deposited membrane was achieved. This superior performance is attributed to the small nanosheet size and hydrophilic surface functionality.
Description
Thesis (Ph.D (Polymer Engineering))
Keywords
Boron nitride