4.2.3 Star-Shaped PEO-b-PCL Block Copolymers

 The isotherms of the PEO-b-PCL five-arm stars are presented in Figure 4-9. They exhibit

essentially three different regions that can be attributed to different conformations of the polymer

chains. In the high MMA region, the surface pressure slowly increases as the films are

compressed until a pseudoplateau is reached for intermediate mean molecular areas. As the

compression continues in the low MMA region, the surface pressure sharply increases, reaching

elevated surface pressure values and highly compressed films.

 40

 .Star#6
 30
 0 Star#5
 lit I* Star#4
 20 Star#3
 i'Star#2
 10~I -+Star#1



 0 3000 6000 9000 12000 15000
 MMA (Ajz)

Figure 4-9. Isotherms of the star-shaped PEO-b-PCL copolymers.

4.2.3.1 High MMA region

 The first step toward understanding the behavior of the stars in this region was to check

monolayer formation reversibility and stability. This was done by carrying out compression-

expansion hysteresis experiments with target pressures up to 9 mN/m. For Star#6, Star#5, Star#4,

and Star#3, all the compression and expansion curves are superimposable independent of the

target pressure, which is indicative of film formation reproducibility and stability. These four

stars have the largest PCL amounts and are therefore hydrophobic enough so the amount of