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