W. Richard Bowen and Nidal Hilal 4

238 8. ATOMIC FORCE MICROSCOPy ANd POLyMERS ON SURFACES
Pl
PEO
(a)
(b)
PEO
Pl
FIgurE 8.11 Schematic drawing of the transition over time of the (a) initial brush-like
flat polymer island to (b) a surface micelle. In (a), the PEO blocks are swollen and stretched
away due to the presence of a water layer on mica surface and the resulting repulsive
excluded volume interactions. In (b), most of water has evaporated and parts of the PEO
blocks remain adsorbed on to the mica surface while other parts come together due to monomer-monomer
attractions. The PI blocks fuse together in a compact core.
amounts by employing several incubation times of the mica surface
within the solution.
Figure 8.12 shows three examples of topographies of samples prepared
using relatively short incubation times and consequently low adsorbed
amounts. In this case, the polymer islands correspond to single and isolated
polymer chains, as can be deduced by an analysis of their size from
the AFM images. We can clearly see the asymmetric/irregular shape of
the low-functionality star polymers and the gradual transition to more
circular shapes as the functionality increased. Furthermore, the height of
the individual star polymers increased with functionality; notice the flat
‘pancake’-like conformation taken by the low-functionality star polymers
and the increased height of the high-functionality ones. It is clear that the
higher the functionality, the closer is the star polymer behaviour to the
one expected for a (soft) colloidal particle; their shape was affected much
less strongly by the interactions with the surface.
Figure 8.13 shows an example of an image of sample prepared using
incubation times in the solution long enough for the 18-arm star polymers
to start interacting as they were swollen in good solvent conditions.
Owing to the increased adsorbed amount, i.e. increased number of chains
adsorbed on the surface, they compressed and confined each other,
decreasing their lateral extension and simultaneously increasing their
height. In this way, because of this confinement effect, they became uniformly
spaced, more symmetrical and circular in shape. At these moderate