Incorporation of POSS into PVDF water filtration membranes

Author: Jonathan Sierke

Sierke, Jonathan, 2018 Incorporation of POSS into PVDF water filtration membranes, Flinders University, College of Science and Engineering

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The use of pressure-driven water filtration using polymeric water filtration membranes is an important technology that has been adopted globally for the purification of water. One polymer, which has favourable properties for use as water filtration membranes is polyvinylidene difluoride (PVDF), due to the high chemical stability and good mechanical properties of PVDF. Two common issues faced with using polymeric water filtration membranes is compaction and biofouling, which reduce the efficiency of the process.

In this thesis, the research is focused on the incorporation of Polyhedral Oligomeric Silsesquioxane (POSS) into PVDF water filtration membranes to reduce the effects of membrane compaction. To combat biofouling, preliminary work has also been conducted on the incorporation of eugenyl methacrylate (EgMA) (an antibacterial agent) into PVDF water filtration membranes.

The first part of this thesis (Chapter 3) investigates PVDF water filtration membranes incorporating 3,3,3-trifluoropropyl POSS (TFP POSS). These TFP POSS/PVDF membranes were investigated by attenuated total reflectance – Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), tensile tests, and stirred cell experiments. The results of the analyses of these membranes indicated that TFP POSS was successfully incorporated into the PVDF membranes, but did not provide improvements in mechanical properties of the membranes, and did not improve the compaction resistance of the membranes either.

In order to provide compaction resistance to the PVDF membranes, a method of cross-linking PVDF with POSS was devised. This method uses thiol-ene addition of octa(3-mercpatopropyl) POSS (thiol POSS) with the alkenes on dehydrofluorinated PVDF (d-PVDF).

The next part of the thesis (Chapter 4) investigated the synthesis of thiol POSS using 3-mercaptopropyltrimethoxysilane (MPTMS). After examining different reaction conditions, a novel method of synthesising thiol POSS was developed, which afforded thiol POSS in a yield of 19.2 %. The structure of thiol POSS was confirmed with 1H, 13C, and 29Si nuclear magnetic resonance (NMR) spectroscopies, as well as ATR-FTIR spectroscopy. Following this, the reactivity of thiol POSS to thiol-ene addition reactions using different alkenes and different catalysis was examined. The results of the thiol-ene addition reactions indicated that thiol POSS reacted readily when using 2,2′-azobis(2-methylpropionitrile) (AIBN), or benzophenone/UV-irradiation as catalysts. On the other hand, using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a catalyst provided more selectivity, allowing an organic-solvent soluble octa(EgMA) POSS derivative to be produced.

Following the successful synthesis and modification of thiol POSS with thiol-ene addition, the dehydrofluorination of PVDF using DBU was investigated, as well the thiol-ene addition of thiols of d-PVDF (Chapter 5). Analysis of PVDF after treatment with DBU in solution (by ATR-FTIR spectroscopy) indicated that, under the conditions used, a treatment time of 4 h provided the largest quantity of alkenes on d-PVDF. Moreover, d-PVDF was found to react readily with thiols, either when the thiols were added together with DBU, or using benzophenone/UV-irradiation with thiols as a post-treatment.

Cross-linked PVDF membranes were then produced using benzophenone/UV-irradiation with thiol POSS and d-PVDF, with thiol POSS loadings of 0-10 wt% examined. Analysis of these membranes (Chapter 6) by tensile tests indicated that the Young’s modulus was highest for the membrane with 10 wt% of thiol POSS, and that this membrane also showed improved compaction resistance during pure water filtrations.

Finally, preliminary work on investigating the biofouling resistance of EgMA-POSS modified PVDF membranes, membranes incorporating thiol POSS and EgMA was conducted (Chapter 6). EgMA, a known antibacterial, was successfully incorporated into the membranes by benzophenone/UV-irradiation (as indicated by ATR-FTIR spectroscopy). However, bacterial adhesion tests performed on these membranes indicated that changes in the surface morphology of the membranes played a larger role in the extent of bacterial adhesion.

Keywords: PVDF, water filtration membranes, polymeric membranes, POSS, compaction, cross-linking, octa(3-mercaptopropyl) POSS, 3,3,3-trifluoropropyl POSS, thiol-ene click, thiol-ene addition, dehydrofluorination, MPTMS, eugenyl methacrylate

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Claire Lenehan