Hydrophobic poly(vinylidene fluoride) / siloxene nanofiltration membranes

Jing Ji, Saeed Mazinani, Ejaz Ahmed, Y. M. John Chew, Davide Mattia

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Abstract

Hydrophobic, chemically resistant nanofiltration (NF) polymeric membranes could provide major improvements to a wide range of processes, from pharmaceutical manufacturing to hazardous waste treatment. Here, we report the fabrication of the first poly (vinylidene fluoride) (PVDF) NF membranes retaining their hydrophobicity and surface chemistry. This was achieved by incorporating in the polymer 2D siloxene, which induce a compaction of the PVDF chains, resulting in low free volume and a highly ordered microstructure. Siloxene nanosheets were obtained from deintercalation of Ca from CaSi2 using HCl, followed by exfoliation and size fractionation, with average lateral dimension of 1–2 μm and thickness of 3–4 nm. The resulting membranes, containing 0.075 wt% of siloxene, have a pure water permeance of 22 ± 2 L m-2 h-1 bar-1 and molecular weight cut-off (MWCO) of 530 Da. The same membrane also showed stable hexane permeance of 11 L m-2 h-1 bar-1 for 24 h with MWCO of around 535 Da. These results supersede the performance of commercial NF membranes, expanding the potential application of nanofiltration to processes requiring stable, chemically resistant and hydrophobic nanofiltration membranes.

Original languageEnglish
Article number119447
JournalJournal of Membrane Science
Volume635
Early online date1 Jun 2021
DOIs
Publication statusPublished - 1 Oct 2021

Bibliographical note

Funding Information:
Nanofiltration (NF) membranes are widely used in a range of separation processes such as water/wastewater treatment, food processing, chemical transformations, textile manufacturing, pharmaceutical production and others [1]. This success is due to a range of characteristics, including high permeance, molecular-scale molecular weight cut-off (MWCO), low energy consumption and operation costs [2]. Most commercial NF membranes are thin film composite (TFC) membranes composed of a thin cross-linked polyamide (PA) separation layer obtained via interfacial-polymerization deposited on a microporous polysulfone membrane support, which is cast on non-woven fabric [1,3]. PA TFC membranes, however, suffer from a number of limitations, particularly low stability to chlorine [ 4?6], and selectivity-permeability trade-off [7]. In contrast, polyvinylidene fluoride (PVDF), one of the most commonly used polymeric membrane materials with inherent hydrophobicity, possesses excellent chemical resistance, thermal stability, and mechanical strength [8,9]. Due to these properties, PVDF membranes now occupy a large market share of commercial microfiltration (MF) and ultrafiltration (UF) membranes, for applications such as drinking water production, pre-treatment for RO systems and wastewater treatment, gas-liquid absorption and membrane distillation [9,10]. In most cases, PVDF membranes are hydrophilized for better performance in water treatment. There is, however, no commercial PVDF NF membrane available in the market.Current research on fabrication of PVDF NF membranes can be grouped into two categories, i.e., surface modification and blending modification. In the former, TFC membranes are obtained by forming a NF separation layer on top of a PVDF UF support via techniques such as surface coating [3,11], grafting [12], electrospinning [13], interfacial polymerization [14], cross-linking [15], plasma treatment [16] or electret treatment [17]. For the latter, additives such as functionalized nanomaterials [18,19] and surfactants [20] have been employed to prepare PVDF NF mixed-matrix membranes (MMMs). In the former case, the PVDF support does not contribute to the NF rejection performance, whereas the majority of the resulting MMMs are more hydrophilic than the bare PVDF membranes. In both cases, the inherent inertness of PVDF is not fully utilized, limiting the potential applications of PVDF NF membranes in harsh conditions e.g., chlorine cleaning or organic solvent nanofiltration.Generally, size (steric) exclusion and Donnan (charge) exclusion are the main rejection mechanisms in NF [49,50]. As all the membranes have comparable negative surface zeta potential, regardless of siloxene loading, they will have similar electrostatic repulsions for the same dye. As such, while charge rejection is likely to occur, given the negative charge of the dyes tested, any difference between the membranes can be convincingly attributed to size rejection. The PVSi-075 membrane exhibits higher rejection for CR (Mw = 696.66 Da, R = 98 ? 2%) with lower molecular weight compared to RB (Mw = 1017.64 Da, R = 94 ? 3%). Careful considerations including the dye structures in relation to their shape, flexibility and compactness are required to explain the observed membrane rejections. To this aim, the hydrated radius (Rh), which reflects the apparent size (physical size) of the dye molecule [51], has been considered. Several methods have been used to estimate the hydrated radii of dyes using a correlation between more easily obtainable size parameters (e.g., Stokes and crystal radius) [26,52] and physical parameters (e.g., viscosity with hydration radius) [53,54]. Solute transport is strongly hindered when the pore size of a membrane is smaller than the hydrated radius [55]. Connolly Accessible Area (CAA) is defined as the locus of the center of the solvent molecule while it is rolled around the probe molecule's van der Waals surface [56,57]. A correlation (Fig. S6 in the Supporting Information) was established between CAA and the hydrated radii of various dye molecules (Table S3 in the Supporting Information) [51,58]. The hydrated radii of the dyes used in this work were then estimated from this correlation, as summarized in Table S4 in the Supporting Information. As shown in Fig. 6, compared to RB5, BB, DR23 and CR, Rose Bengal has a smaller hydrated radius although it has the largest molecular weight among all the dyes (insert table in Fig. 5b). Moreover, a closer look at the structure of the dyes reveals that the presence of centered/close conjugated rings reduces the flexibility of the molecules. This suggests that RB, AR1 and MO have almost spherical shape, whereas the other dyes are more elongated. According to Fig. 6, the cut-off of the PVSi-075 membrane corresponds to the hydrated radius of 5.89 ?.This work is supported by the Engineering and Physical Sciences Research Council (EPSRC) UK (grant EP/M01486X/1). The XRD data was analyzed using CrystalMaker?, CrystalMaker Software Ltd, http://www.crystalmaker.com. XPS data collection was performed at the EPSRC National Facility for XPS (?HarwellXPS?), operated by Cardiff University and UCL, under contract No. PR16195. All data produced during this research are available from the University of Bath open access data archive at https://doi.org/10.15125/BATH-01019.

Funding Information:
This work is supported by the Engineering and Physical Sciences Research Council (EPSRC) UK (grant EP/M01486X/1 ). The XRD data was analyzed using CrystalMaker®, CrystalMaker Software Ltd, http://www.crystalmaker.com . XPS data collection was performed at the EPSRC National Facility for XPS (“HarwellXPS”), operated by Cardiff University and UCL, under contract No. PR16195. All data produced during this research are available from the University of Bath open access data archive at https://doi.org/10.15125/BATH-01019 .

Keywords

  • Hydrophobic
  • Nanofiltration
  • Organic solvent
  • PVDF membranes
  • Siloxene

ASJC Scopus subject areas

  • Filtration and Separation
  • General Materials Science
  • Physical and Theoretical Chemistry

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