W. Richard Bowen and Nidal Hilal 4

236 8. ATOMIC FORCE MICROSCOPy ANd POLyMERS ON SURFACES
point (D � 0) and the minimum of each attractive force. We have assumed
that the Kuhn statistical segment is b � 0.6 nm. In Figure 8.9(b), we zoom
on an attractive peak (dashed rectangular in Figure 8.9a). For this plot, the
variables have been reversed (y � D and x � F) and the absolute value
of the force has been used. The theoretical formula of the stretching by
⎡
its ends of a freely jointed chain is
⎛ Fb⎞
kT ⎤
R � Lc
⋅ ⎢coth
⎜ �
⎝⎜
kT ⎠⎟
⎥ , where R is
⎣⎢
Fb ⎦⎥
the end-to-end chain distance when a restoring force F is exerted, Lc is
the contour length (full length of the stretched polymer chain), k is the
Boltzmann constant and T is the absolute temperature [32]. As we can see
in Figure 8.8, it fits the AFM data well.
8.4 DIBLoCk CoPoLymErS ADSorBED on SurFACES
Amphiphilic diblock copolymers have great potential in a variety of
applications, ranging from their use as responsive layers for the fabrication
of smart surfaces to supramolecular responsive carriers for drug/
gene delivery. We have recently studied the adsorption of poly(isopreneb-ethylene
oxide) block copolymer (PI-PEO) on mica [33]. The polymer
consists of a short and very flexible hydrophobic block (PI) (in melt state
at room temperature) and a long hydrophilic block (PEO). Tapping mode
AFM imaging was employed in dry state.
The deposition of a droplet of deionised water polymer solutions
onto freshly cleaved mica, followed by gentle rinsing and finally
drying, resulted in the initial formation of ultraflat polymeric islands
(Figure 8.10a). It is well known that mica is hydrophilic and an adsorbed
water layer in the form of semi-continuous water islands forms on its
fresh surface upon cleavage under normal ambient conditions [34, 35].
The polymer molecules organised in polymer brush-like flat nanometre-thick
islands with the PEO within the ultrathin water layer and the
PI block collapsed at the water/air interface (Figure 8.11a). As mica
became gradually less hydrophilic with time, most of the water evaporated
and the polymeric monolayer islands were laterally compressed
and increased in height (Figure 8.10b). Ultimately, parts of the PEO
blocks remained adsorbed on to mica (some strongly adsorbed water
molecules still remain on the mica surface) and other parts aggregated
owing to the monomer-monomer attractive interactions in the dry state
and dewetted the mica surface. The dewetting behaviour was aided further
by the aggregation and fusion of the flexible PI blocks; they came
together to create a hydrophobic core. The height of the polymer islands
was increased by a factor of about 5 as mica became less hydrophilic with
time. The final structure has the organisation of a surface micelle with