Cross-linking of dehydrofluorinated PVDF membranes with thiol modified polyhedral oligomeric silsesquioxane (POSS) and pure water flux analysis

Highlights

• Dehydrofluorination was used to add alkenes onto PVDF.

• Dehydrofluorinated PVDF/thiol modified POSS were cast into membranes using NIPS.

• Membranes were cross-linked via UV-catalyzed thiol-ene addition.

• Cross-linked PVDF/POSS membranes showed improved tensile mechanical properties.

• Cross-linked PVDF/POSS membranes showed improved compaction resistance.

Abstract

Porous poly(vinylidene difluoride) (PVDF) membranes often undergo compaction in pressure driven water treatment, contributing to a reduction in water flux. Here, compaction resistance was tested by cross-linking dehydrofluorinated PVDF (PVDF with alkene functionality) with a synthesized thiol modified polyhedral oligomeric silsesquioxane (thiol POSS) at 0–10 wt% via UV catalyzed thiol-ene addition. Analysis of these microfiltration membranes showed an increase in the Young's modulus from 36.60 ± 1.85 MPa (0 wt% thiol POSS) to 54.60 ± 1.97 MPa (10 wt% thiol POSS), correlating to improved mechanical properties. The pure water flux (PWF) of the membranes was found to increase with 1 wt% thiol POSS loading, then decreased with increasing thiol POSS loading up to 10 wt%, while BSA rejection decreased with 1 wt% loading of thiol POSS, then increased with thiol POSS loading thereafter. The cross-linked PVDF membrane with 10 wt% of thiol POSS was shown to possess the greatest compaction resistance, with the lowest pure water flux declines of 8.3 ± 2.04% (first pass, run 1) and 6.3 ± 2.37% (second pass, run 2) as compared to the membrane with no POSS additive at 26.3 ± 2.61 (first pass, run 1) and 12.8 ± 1.74 (second pass, run 2). The data showed that chemically cross-linking PVDF with POSS via thiol-ene addition has real potential to reduce compaction of PVDF water filtration membranes.

Introduction

The use of pressure driven water filtration membranes is a widely adopted technology, owing to its reasonable energy efficiency and robustness in removing contaminants [1,2]. However, often there is a decline in water permeance during water filtration over time [[3], [4], [5], [6], [7], [8], [9]]. This decline is due to a combination of various factors, including adhesion of substances to the membrane surface and/or clogging of the membranes pores due to fouling [3], growth of biofilms on the membrane (biofouling) [4], and membrane compaction [[5], [6], [7], [8], [9]].

Membrane compaction occurs due to compression of the porous structure of the membrane from the pressure applied during the filtration. This constricts the pores and can cause the microporous layer within membranes to collapse, which reduces the rate at which water flows through the membrane [10,11]. Membrane compaction is particularly a problem for polymeric water filtration membranes, which are more easily compressed compared to inorganic membranes [12].

Poly(vinylidene difluoride) (PVDF) is a polymer commonly used as a material for polymeric water filtration membranes, as it possesses strong chemical resistance [13,14] as well as good mechanical properties [13,15]. Despite these desirable properties, compaction of PVDF membranes during pressure-driven filtration is still an issue. A common strategy to improve the mechanical properties of polymeric membranes (including PVDF membranes), as well as the compaction resistance of these membranes, is to incorporate inorganic fillers, such as metal oxide particles [[16], [17], [18]]. For example, Jun et al. [19] incorporated TiO₂-carbon hybrid aerogel (TiO₂C) nanoparticles into flat sheet PVDF thus increasing the membranes Young's modulus from ∼29.7 MPa (0 wt% TiO₂C) to ∼46.1 MPa (0.5 wt% TiO₂C) and tensile strength from ∼1.47 MPa to ∼1.76 MPa with an increased % elongation at break from 10.55% to 20.83% [19]. Further, a decreased % flux decline of pure water was observed from ∼42% (0 wt% TiO₂C) to ∼14% (0.5 wt% TiO₂C).

Incorporation of SiO₂ particles into PVDF membranes has been accomplished by Liang et al. [20] who employed hydrolysis and condensation of tetraethoxysilane (TEOS) within membranes formed by thermally induced phase separation (TIPS). It was shown that with an increasing TEOS loading (between 0–20 wt%) in the casting solution an increased loading of silica particles was observed in the final membranes. This in turn increased the compaction resistance of the membranes [20]. The greatest compaction resistance was observed for the membrane with 20 wt% TEOS loading, changing the % flux decline from 83% (0 wt% SiO₂ particles) to 22% (20 wt% SiO₂).

There is, however, a large issue with incorporating inorganic fillers into polymeric membranes and this is poor dispersion, which results in large agglomerates forming and a reduction in membrane properties, such as reducing the mechanical stability of the membrane [[21], [22], [23], [24]]. To avoid poor dispersion, a class of organosilicon compounds called polyhedral oligomeric silsesquioxane (POSS), molecular formula [RSiO_3/2]_n have been used. A common form of POSS is a cage-like structure, where each silicon atom is bonded to other silicon atoms through SiOSi bridges (Fig. 1). Each silicon atom possesses an organic R-group which can be phenyl rings or alkyl chain of various lengths and functional groups, such as alcohols, amines, thiols, etc. [25]. As such the R-groups can allow for enhanced control of the dispersion of POSS in polymeric matrices [26,27], as well as allowing POSS to be covalently bound to polymers with the membrane matrix [28,29].

By adding POSS into polymers via blending [26,30] or covalently attaching as a pendant group [31] the polymer mechanical properties are greatly enhanced. Examples in literature on the incorporation of POSS into polymeric water filtration membranes have shown POSS to be capable of providing improvements to these membranes. Duan et al. [32] have used POSS bearing amine-groups to covalently bind into a polyamide (PA) layer on top of thin-film composite (TFC) PA reverse osmosis (RO) membranes. The amine-POSS additive was shown to effectively increase the hydrophilicity and water flux of TFC PA membranes, while still maintaining a similar salt rejection.

POSS-based coatings have also been applied to polysulfone (Psf) ultrafiltration membranes. These types of coatings are useful in improving antifouling resistance of the membranes. As an example, star polymer-POSS materials have been shown to reduce the % flux decline caused by fouling during the filtration of bovine serum albumin (BSA) solution and an oil-water emulsion [33,34].

The work of Worthley et al. [35] has demonstrated improvements in compaction resistance of cellulose acetate RO membranes. After blending POSS (0.5 wt%) into cellulose acetate RO membranes, examination of the membranes before and after filtration using scanning electron microscopy (SEM) indicated that the POSS modified membrane had a reduction in thickness of only 0.1%, whereas the membrane without POSS showed a reduction in thickness of 42%.

Here, we report for the first time PVDF water filtration membranes cross-linked using thiol modified POSS. This was accomplished by first using a non-nucleophilic base, 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU), to produce dehydrofluorinated PVDF (d-PVDF). This was designed to effectively introduce alkene functionality on the PVDF backbone. Dehydrofluorination of PVDF has previously been used to change the surface properties of PVDF materials, either using dehydrofluorination alone [36,37] or followed by subsequent functionalization with other compounds [38,39]. In the second step, octa (3-mercaptopropyl) POSS (referred to as thiol POSS) was synthesized and used to cross-link with the alkene modified d-PVDF via UV-catalyzed thiol-ene addition reaction. The membranes tensile and water flux properties where tested at thiol POSS loadings of 0–10 wt%.