Author: Nasser Alotaibi
Alotaibi, Nasser, 2017 Building Nanostructures Toward Improved RO Membrane Performance, Flinders University, School of Chemical and Physical Sciences
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In this thesis, single wall carbon nanotubes (SWNTs) were embedded into the
polyamide layer of RO membranes. Functionalisation of SWNTs was performed to
improve their distribution in the polyamide layer. To study the functionalisation
reactions of SWNTs, model reactions were undertaken. Anthracene-9-carboxylic
acid was the model substrate of the SWNTs because of the similarity of its aromatic
properties. The produced amine compounds from the model substrate reactions were
explored for formation of supramolecular structures. Benzene-1,3,5-tricarboxylic
acid chloride (TMC) was used to synthesise benzene-1,3,5-tricarboxamide (BTA)
derivatives as supramolecular motifs.
Four BTA molecules, with varingin aliphatic chain lengths, were synthesised and
characterised. Supramolecular structures were formed through the self-assembly
process in a chloroform solution. Due to the chain length differences between the
molecules, different self-assembled shapes were observed. Thus, this self-assembly
behaviour as a function of time was investigated for the shortest and longest aliphatic
chain molecules. This self-assembly was investigated by monitoring the change in
fluorescence, FT-IR and SEM as a function of time. It was found that the long-chain
molecule showed faster self-assembly and with greater overlap between the aromatic
rings, due to the flexibility of the long-chain molecule. Moreover, the molecule with
the long chain showed higher fluorescence and longer fibre formation on a substrate.
After investigating the self-assembly behaviour and the influence of chain length on
the model substrate, the functionalisation reaction progressed to the SWNTs in order
to embed them into the polyamide layer of the RO membrane. The influence of chain
length on the distribution of SWNTs into the polyamide layer was investigated as
well. Two molecules with different aliphatic chain lengths were synthesised and
separately grafted to SWNTs. Raman and FT-IR showed successful grafting of
SWNTs with the synthesised molecules.
Synthesis of the polyamide layer was performed by the interfacial polymerisation
method. TMC (0.1 wt%) in the organic phase was allowed to react with mphenylenediamine
(MPD) (2 wt%) in the aqueous phase on the top surface of the
polysulfone (PSf) layer, forming a thin polyamide film. To modify the polyamide
layer, the two grafted SWNTs were dispersed separately in the organic phase prior to
the polymerisation. Two modified polyamide layers, with a variety of mass loadings
of grafted SWNTs, were synthesised. Characterisation of the modified polymers was
obtained by Raman, FT-IR, AFM and SEM.
Membrane water flux and salt rejection were determined using 2000 ppm sodium
chloride as the feed solution. The modified membranes showed an improvement in
water flux, but there was a decrease in salt rejection. For example, an improvement
in flux from 24 to 32 Lm-2 h-1 for SWNTs-free membrane and modified membrane
with 5 wt% SWNTs respectively was observed, while for the same membranes the
salt rejection decreased from 73% to 67%.
Further investigation of the chain-length influence was obtained by the model
reaction. Anthracene-9-carboxylic acid was used to mimic the SWNTs and
synthesise the model reaction molecules. The model membranes were synthesised
using the model molecules as a mimic for grafted SWNTs. The flux and salt rejection
of the model membranes were determined under the same conditions as the modified
membranes.
Keywords: RO membrane , polyamide membrane , CNTs, SWNT, BTA
Subject: Chemistry thesis
Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Chemical and Physical Sciences
Supervisor: Prof. Joe Shapter