n = 60, m = 0, 11, 19, 27 and 35

Figure 4-2. The linear PEO-b-PCL block copolymers.

Table 4-2. Characteristic values of the linear PEO macroinitiator and of the linear diblock
 copolymers.
 PEO60-b- PEO60-b- PEO60-b- PEO60-b-
Name PEO2670
 PCL11 PCL19 PCL27 PCL35
M~n a (g/mol) 2,670 3,940 4,780 5,780 6,680
PDIa 1.05 1.10 1.13 1.11 1.24
Avg no. of ethylene
 60 60 60 60 60
 oxide units

 Av n. f -0 11 19 27 3 5
 caprolactone units
a Determined by GPC calibrated with linear poly(ethylene oxide) standards.

 4.2 Results and Discussion

4.2.1 PEO Homopolymers

 The isotherms of the five-arm PEO core for subphase pH values of 5.5 (Millipore filtered

water) and 13 (0.1 M NaOH) are presented in Figure 4-3. The isotherms of the linear PEO

homopolymers (PEO2000 and PEO2670) are also included. The inset shows the same isotherms

normalized with respect to the number of ethylene oxide repeat units. For PEO2000 and

PEO2670, the surface pressure increases until a pseudoplateau that corresponds to the

irreversible aqueous dissolution of the PEO chains is reached at 5 and 6.2 mN/m, respectively.

As previously described, Langmuir films of PEO homopolymers are thermodynamically stable

for low surface pressures."' Upon compression the monolayers collapse, and the water-soluble

PEO chains irreversibly dissolve in the water subphase for pressures that are molecular weight

dependent (maximum collapse pressure value ~ 10 mN/m for high molecular weight PEO).122