W. Richard Bowen and Nidal Hilal 4

1000 nm
8.3 ENd-gRAFTEd POLyMER CHAINS 231
500 nm
0 nm 0 nm
500 nm
24.95 nm
1000 nm
FIgurE 8.4 AFM 3D-graph topography image of a gold substrate immersed in a toluene
solution of PS-SH (concentration: 0.1 mg ml –1 , molecular weight: 144 000 g mol –1 ) for 45 min.
The AFM imaging was conducted in water, which is a bad solvent for PS. The polymer chains
are in collapsed globular state and randomly distributed on the gold terraces.
including simple geometrical formulas in order to estimate the true volume
of the islands [18, 20]. The dimensions of polymer islands deduced
directly from AFM images show an average height and width of about
5.5 and 48 nm, respectively. As we have explained, although the measurement
of height can be reliable, the width is usually overestimated
owing to the convolution effect. Assuming that the shape of a polymer
island is a spherical cap and taking into account the convolution effect,
the average volume of the globules was estimated to be around 310 nm 3 .
This compares well with the volume of a single collapsed polymer chain,
which can be calculated assuming the usual bulk density for polystyrene
(�1 g cm –3 ). Therefore, we conclude that the polymer islands are isolated
single polymer chains chemisorbed and collapsed into dense globules
onto the gold substrate. This implies that for this incubation time the
polymer chains are sparsely distributed and the grafting density is low
enough for us to observe the ‘mushroom’ regime. The variation in the
size of the polymer globules reflects the polymer polydispersity (M w/
M n� 1.2, where M n is the number-average molecular weight) and possibly
small changes in the globular conformation, which can be magnified
by the convolution effect.
Figures 8.5 and 8.6 show two examples of gold substrates that
were immersed in a toluene solution of higher concentration of PS-SH
(2 mg ml –1 ) for much longer incubation times (2 days) and imaged under
water in contact AFM mode. We can clearly see that the gold substrates
are covered by a dense array of large polymer globules. Forty-eight hours
is adequate time for the adsorption to reach its maximum grafting density,
and we expect to have several thousands of chains within a square