5.2.1 Addition of PEG to the Surface
PEG of various molecular weights, PEG-200, 400, 2000, 3350 and 4600, were used to evaluate the effects of PEG size on the grafting to the membrane surface. The chain transfer agent, mercaptoethanol, was used in the usual fashion to achieve chain termination. FTIR analyses of these modifications showed increases in the peak intensities with increasing molecular weights as shown in Figure 5.7.
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Figure 5.7: FTIR analyses of surface grafting with different molecular weights of PEG.
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Wave number, υ
Unmodified
PEG-400
PEG-2000
PEG-3350
PEG-4600
PEG-8000 OH peak Widening
CO Stretch
63 5.2.2 BSA Filtration
Variations of flux decline curves between membranes grafted with PEG-200, 2000, 4600 and unmodified membrane is shown in Figure 5.8. Results suggested that membranes grafted with PEG-200 and 4600 had the highest initial flux values during precompaction and filtration. On the other hand, membranes grafted with PEG-2000 showed no change in flux values as compared to unmodified membranes. Higher flux values for PEG-200 were theorized to be due to the flexibility of PEG-200, which was more conceivable at lower PEG molecular weights than at the higher PEG molecular weights of 2000 and 4600. Polymer chains formed through polymerization of PEG-200 were believed to be able to flow more freely with one end anchored to the membrane surface. This was possible due to the smaller length of monomer chains, which is consistent with observations by others [85]. In the case of PEG-2000 and 4600, grafted chains would experience lower flexibility owing to lengthy monomer chains containing 2000 and 4600 ethylene glycol units, respectively. The degree of chain entanglement increases as the number ethylene glycol units increase in the monomer side chain. As the entanglement increases, grafted chains lose their flexibility resulting in formation of dense grafted layer; thus, causing hindrance to the water flow through the membrane. This could explain the loss of flux with grafting PEG-2000 through polymerization to the membrane surface. As the number of monomer units increase further, as with PEG-4600, entanglement becomes so severe that, it completely covers some of the end functional groups present on the monomer chain. This results in non availability of those functional groups embedded in the chain entanglement leading to reduced degree of polymerization [85] which, in turn results in increased flexibility of the PEG chains on the surface, there
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by producing higher fluxes through the membrane. Again, these results agreed well with observations by other groups [85]. Therefore, optimal PEG molecular weight was determined to be 200. Other PEG grafting studies also indicate the decrease in maximal surface coverage when the length of the PEG chains increased as already attached chains induce a steric repulsion of the monomer remaining in the aqueous phase, thereby reducing the final surface coverage [86].
Figure 5.8: BSA filtration properties of membranes grafted with various molecular weights of PEG.
The membrane modified with PEG-200 and fouled with BSA (Figure 5.8) was analyzed using FTIR to determine if there were observable changes to the fouling layer
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0 100 200 300 400 500 600 700
Flux (L/(sqm-hr))
Time (min) Unmodified Membrane Precompaction Unmodified Membrane Filtration
Precompaction of Membrane grafted with PEG 2000 Filtartion: Membrane grafted with PEG 2000
Precompaction of Membrane grafted with PEG 4600 Filtration: Membrane grafted with PEG 4600
Precompaction of Membrane grafted with PEG 200 Filtration: Membrane grafted with PEG 200
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chemical make-up. Membranes fouled with BSA, both unmodified and modified, did not show any qualitative changes compared to unfouled membranes (Figure 5.9) indicating no irreversible fouling by BSA. This could be explained by the hydrophilicity of the unmodified and modified CA membranes and the large size of the negatively charged hydrophobic BSA protein (69.4 kDa). Unmodified membranes were hydrophilic enough to keep the BSA-membrane interactions to minimum. Furthermore, the negative membranes (the isoelectric point of cellulose acetate is approximately 4-5, while PEG is a neutral molecule) excluded the negative BSA. These along with the large size of BSA resulted in total rejection by the unmodified and modified membranes. Modified membranes with increased hydrophilicity showed the same non-fouling nature due to the already minimum level of BSA-membrane interactions and large size of BSA molecules.
The decline in flux, observed during BSA filtration (Figure 5.8), were likely due to the reversible accumulation of BSA on the membrane surface, which only increased the resistance to flux.
Figure 5.9: BSA fouling analysis of unmodified and modified membranes.
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Unmodified membrane prior to filtration Unmodified membrane after filtration Modified membrane after filtration