W. Richard Bowen and Nidal Hilal 4

234 8. ATOMIC FORCE MICROSCOPy ANd POLyMERS ON SURFACES
the repulsive excluded volume interactions between the polymer segments.
The stretching of the chains is moderated by their entropic elasticity.
Owing to the thermal agitation and fluctuations, a polymer chain adopts
an overall shape that allows it to maximise the number of possible conformations.
Stretching decreases this number, which leads to an entropy
loss, and for this reason a restoring force of entropic origin develops that
resists deformation. A polymer brush in good solvent conditions should
behave like a compliant/soft (non-linear) spring resisting compression,
protecting the surface and even exhibiting low adhesive and frictional
forces. An atomic force microscope allows us to probe directly the relevant
nanomechanical properties at the local level.
We prepared a polymer brush system by immersing gold substrates to
a dilute aqueous thiol-terminated poly(methacrylic acid), PMAA-SH, solution
for 24 h (M w� 66 200 g mol –1 , M w/M n� 2). We used AFM force spectroscopy
(forward and reverse force distance curves) in deionised water (good
solvent for PMAA) and acquired many force distance profiles. Two types of
force profiles were observed on these monolayers. Figure 8.8(a) shows an
example of the first type. During the forward movement of the tip towards
the surface (approach), we observe the gradual increasing of the loading
force owing to the non-linear compressive elasticity of the brush. No adhesive
force is observed because the brush steric repulsion force screens the
interaction of the tip with the substrate (in the same way that polymer
chains anchored on colloidal particles stabilise their suspensions). During
the reverse movement, there is no adhesion and the polymer brush is
decompressed in a largely reversible way. Figure 8.8(b) shows that a
simple exponential does describe the AFM data in the regime of intermediate
compression of the brush.
The Alexander–de Gennes theory of a polymer brush [29–31] predicts
a certain relationship for its compression that can be approximated by
the following exponential for intermediate compressions (0.2�D/L 0�0.9)
F (nN)
(a)
1.5
1.0
0.5
0.0
0 20 40 60
D (nm)
Forward
Reverse
80 100
F (nN)
(b)
10 0
10 –1
10 –2
0 20 40 60 80 100
D (nm)
FIgurE 8.8 (a) First type of force profile observed over a polymer brush. Notice the
absence of hysteresis. (b) Logarithmic plot of the forward force distance curve.