W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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236 8. ATOMIC FORCE MICROSCOPy ANd POLyMERS ON SURFACES<br />
point (D � 0) <strong>and</strong> the minimum of each attractive force. We have assumed<br />
that the Kuhn statistical segment is b � 0.6 nm. In Figure 8.9(b), we zoom<br />
on an attractive peak (dashed rectangular in Figure 8.9a). For this plot, the<br />
variables have been reversed (y � D <strong>and</strong> x � F) <strong>and</strong> the absolute value<br />
of the force has been used. The theoretical formula of the stretching by<br />
⎡<br />
its ends of a freely jointed chain is<br />
⎛ Fb⎞<br />
kT ⎤<br />
R � Lc<br />
⋅ ⎢coth<br />
⎜ �<br />
⎝⎜<br />
kT ⎠⎟<br />
⎥ , where R is<br />
⎣⎢<br />
Fb ⎦⎥<br />
the end-to-end chain distance when a restoring force F is exerted, Lc is<br />
the contour length (full length of the stretched polymer chain), k is the<br />
Boltzmann constant <strong>and</strong> T is the absolute temperature [32]. As we can see<br />
in Figure 8.8, it fits the AFM data well.<br />
8.4 DIBLoCk CoPoLymErS ADSorBED on SurFACES<br />
Amphiphilic diblock copolymers have great potential in a variety of<br />
applications, ranging from their use as responsive layers for the fabrication<br />
of smart surfaces to supramolecular responsive carriers for drug/<br />
gene delivery. We have recently studied the adsorption of poly(isopreneb-ethylene<br />
oxide) block copolymer (PI-PEO) on mica [33]. The polymer<br />
consists of a short <strong>and</strong> very flexible hydrophobic block (PI) (in melt state<br />
at room temperature) <strong>and</strong> a long hydrophilic block (PEO). Tapping mode<br />
AFM imaging was employed in dry state.<br />
The deposition of a droplet of deionised water polymer solutions<br />
onto freshly cleaved mica, followed by gentle rinsing <strong>and</strong> finally<br />
drying, resulted in the initial formation of ultraflat polymeric isl<strong>and</strong>s<br />
(Figure 8.10a). It is well known that mica is hydrophilic <strong>and</strong> an adsorbed<br />
water layer in the form of semi-continuous water isl<strong>and</strong>s forms on its<br />
fresh surface upon cleavage under normal ambient conditions [34, 35].<br />
The polymer molecules organised in polymer brush-like flat nanometre-thick<br />
isl<strong>and</strong>s with the PEO within the ultrathin water layer <strong>and</strong> the<br />
PI block collapsed at the water/air interface (Figure 8.11a). As mica<br />
became gradually less hydrophilic with time, most of the water evaporated<br />
<strong>and</strong> the polymeric monolayer isl<strong>and</strong>s were laterally compressed<br />
<strong>and</strong> increased in height (Figure 8.10b). Ultimately, parts of the PEO<br />
blocks remained adsorbed on to mica (some strongly adsorbed water<br />
molecules still remain on the mica surface) <strong>and</strong> other parts aggregated<br />
owing to the monomer-monomer attractive interactions in the dry state<br />
<strong>and</strong> dewetted the mica surface. The dewetting behaviour was aided further<br />
by the aggregation <strong>and</strong> fusion of the flexible PI blocks; they came<br />
together to create a hydrophobic core. The height of the polymer isl<strong>and</strong>s<br />
was increased by a factor of about 5 as mica became less hydrophilic with<br />
time. The final structure has the organisation of a surface micelle with