Z ~- Star#6G
 E --Star#5
 0.21
 E L IStar#4
 Star#3
 S0.17
 -- Star#2
 Star#1







 2 4 6 8 10 12 14 16
 x; (mN/m)

Figure 4-14. Compressibility plots of the star-shaped PEO-b-PCL block copolymers versus
 surface pressure.

4.2.3.2 Intermediate MMA region

 This region is characterized by a pseudoplateau in the isotherms and by a maximum in the

compressibility plot (Figure 4-14). This phase transition occurs between approximately 12 and

15 mN/m, a similar pressure range as for the collapse pressure of PCL homopolymers. The

pseudoplateau length correlates with the PCL chain length in the stars, and this phase transition

is attributed to the PCL segments aggregating and crystallizing above the interface and the PEO

core, which is consistent with the work reported by Lee et al. on PCL-based block copolymers

with a dendritic hydrophilic head.120

 Crystallization of the PCL segments was also characterized by carrying out isobaric

experiments for Star#6 with target pressures below and within the pseudoplateau pressure range

(9, 11, and 13 mN/m). The results of MMA decrease versus time are presented in Figure 4-15,

and the y-axis has been normalized to facilitate comparison. No or very little MMA decrease

takes place over time for pressures below the pseudoplateau (9 and 11 mN/m), which is

indicative of thermodynamically stable monolayers. As the target pressure is increased (13