50

 40-

 30 impression

 S20-

 10- Expansion



 0 25000 50000 75000 100000

 MMA (a2)

Figure 3-6. Compression-expansion hysteresis plot of Dend1 (target pressure = 40 mN/m).


3.2.2 PS-b-PAA Dendrimer-Like Copolymer (Dend2)

 As a preliminary study, we investigated the behavior of a PAA homopolymer (PAA250K>,

since to our knowledge no information is available in the previous literature on the Langmuir and

LB monolayers of PAA homopolymers. PAA250K COuld not be dissolved in chloroform, so a

chloroform/ethanol mixture (1/2 in volume) was used instead as the spreading solvent. PAA is a

weak acid (pKa ~ 5.5),'0s and its degree of ionization and consequently its surface activity are

therefore strongly pH dependent. Under basic conditions (pH = 11i), the acid groups of PAA are

deprotonated, and stable Langmuir monolayers of PAA250K COuld not be prepared, with the

surface pressure remaining close to 0 mN/m during barrier compression. When ionized, PAA is

not surface activel09 and irreversibly dissolves into the aqueous subphase. Under acidic

conditions (pH = 2.5), the acid groups of PAA are protonated, the chains become less

hydrophilic and surface active, and therefore stable Langmuir monolayers of PAA250K COuld now

be prepared. The resulting isotherm is shown in Figure 3-7. The surface pressure slowly

increases in the high MMA region before a plateau is reached with the surface pressure leveling