off around 3.5 mN/m. In the high MMA region, the surface pressure increases with a behavior

characteristic of liquid expanded and liquid condensed phases, and PAA250K is likely adsorbed in

a pancake conformation. The AFM image shown in Figure 3-8a (7<= 1.5 mN/m) confirmed the

presence of a pretty homogeneous PAA250K LB film with the randomly distributed darker holes

possibly forming after transfer as the PAA chains un-swell (dehydrate) during the drying step.

PAA in its protonated form is surface active but it is still highly water soluble, and it has been

previously shown for aqueous solutions of protonated PAA that the surface pressure reaches a

maximum, for instance around 3 mN/m for a molecular weight similar to PAA250K.109a This value

correlates well with the plateau surface pressure, which indicates that, at this pressure, PAA250K

desorbs from the interface and aggregates inside the water subphase. The AFM images obtained

for the LB films transferred at 3 and 3.5 mN/m are shown in Figures 3-8b and 3-8c, respectively.

As the films are compressed within the plateau region, more and more desorbed aggregates (~ 3-

5 nm thick) are observed. This indicates that when the PAA chains collapse by dissolving into

the water subphase, they stay in the vicinity of the interface, probably remaining anchored by

some adsorbed acrylic acid repeat units that have not collapsed yet.














 0 50000 100000 150000 200000
 MMA (A~2)

Figure 3-7. Isotherm of PAA250K (pH = 2.5).