W. Richard Bowen and Nidal Hilal 4

148 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
sizes of studied membranes are of approximately the same size. The
topographical images give a clear perception of a notable difference in
the surface morphology of the membranes used for the modification.
A quantification of the surface parameters (Table 5.3) provides an insight
into morphological particularities of these membranes which influence
both the membrane separating properties and the process of modification
by graft copolymerisation.
The two membranes under study have notably different PSDs. It
can be noted here that PVDF has a narrower PSD with pore sizes from
0.336 to 0.68 �m compared with a PSD ranging from 0.219 to 0.948 �m in
PES membranes. Moreover, these membranes significantly differ in surface
roughness, with the PES membrane being smoother than the PVDF
membrane. Regarding the AFM images, one might notice that the
smoother surface allows for better contrast in pore observation, but more
importantly the surface roughness is expected to have an influence on
the graft copolymerisation.
The rate of membrane modification was higher in the case of PES
membrane than PVDF membrane [19]. It is impossible to associate the
difference only with the contribution of surface morphology. It is well
known that polysulphone and PES are intrinsically photo-active, undergoing
bond cleavage with UV irradiation to produce free radicals even
without the use of photo-initiators. PVDF is less photo-reactive than
PES and produces less surface free radicals than PES. However, higher
density of free radicals at the surface of more photo-reactive PES membranes
also results in a higher probability of termination of chain growth
and formation of cross-linked structures. These processes restricting an
increase in the DM are competitive with respect to the chain growth.
Since competitive processes, which enhance and decrease the amount of
grafted polymer, occur simultaneously in the case of the photo-reactive
polymer, the influence of surface morphology on graft copolymerisation
should not be discarded. For the relatively rough surfaces, such as
PVDF membrane, the decrease in UV-irradiation effectiveness and steric
hindrance for polymer growth in narrow valleys are possible effects that
may decrease the modification to some degree.
tAblE 5.3 Parameters of Pore size and PsD Obtained from AFM Images for Initial
PEs and PvDF Membranes.
Pore size (�m) PSD parameters
Membrane Mean Minimum Maximum X 0 (�m) %f max �
PES 0.470 � 0.188 0.219 0.948 0.353 � 0.028 19.8 � 2.2 0.56 � 0.08