W. Richard Bowen and Nidal Hilal 4

150 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
Pore density %
25
20
15
10
5
0
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6
(a) Pore size (µm)
(c)
Pore size (µm)
Pore density %
40
30
20
10
AFM data
Fitted data
AFM data
Fitted data
compared to smaller pores, it can be assumed that higher rates of polymer
growth initiated at the walls of larger pores. As mentioned earlier,
at the entrance of narrower pores, higher density of free radicals results
in chain termination and consequently lower rate of polymer grafting.
With time, when the PSD becomes more uniform, free radicals are eventually
distributed across the membrane surface. This leads to a gradual
decrease of all surface pores with PSD shifting to smaller sizes. With DM
higher than 202 �m cm �2 , slight fluctuation in the width of PSD (�) was
observed.
It can be seen from Table 5.5 that similar behaviour is observed regarding
changes in the surface morphology of the PVDF membrane as for the
PES membrane. However, the unmodified PVDF membrane has a more
uniform PSD than the PES membrane. With a mean pore size approximately
0.54 �m, PSD of this membrane is characterised by a low value
of �, �0.14 �m, compared with �0.56 �m for PES membranes. Although
modification of PVDF membrane with grafted qDMAEMA also led to
PSD shifting towards lower pore sizes, PSD was wider for the modified
membranes compared with initial membrane.
Pore density %
Pore density %
50
40
30
20
10
30
25
20
15
10
5
AFM data
Fitted data
AFM data
Fitted data
0
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
(b)
Pore size (µm)
(d)
Pore size (µm)
fIgurE 5.5 PSDs of initial (a) and modified with qDMAEMA (b)–(d) PES membranes;
(b) DM � 202 �g cm �2 , (c) DM � 367 �g cm �2 and (d) DM � 510 �g cm �2 .