The reversibility of the two PEO phase transitions of the linear diblock copolymers was

investigated by carrying out compression-expansion hysteresis experimentS110,15 On the block

copolymer sample with the smaller PCL segment (PEO60-b-PCLll) for ai target pressure of 18

mN/m. The resulting x/1I1VA curves and the corresponding compressibility plots are shown in

Figures 4-25 and 4-26, respectively. After the first compression, the curves in Figure 4-25 shift

to the low IV1VA region, which is indicative of some irreversibility in the PEO phase transitions.

As shown in the compressibility plots, the local maximum at 6.5 mN/m disappears after the first

compression whereas the maximum at 10.5 mN/m is still present, independent of the number of

compression-expansion cycles. At 6.5 mN/m, we believe the PEO chains dissolve irreversibly in

the aqueous subphase and adopt a mushroom conformation. Nevertheless, because of the

anchoring effect of the PCL segments, the PEO chains stay in the vicinity of the interface. Upon

further monolayer compression, the PEO chains are compressed against each other and stretch

perpendicularly to the interface to form a compact brush at 10.5 mN/m.22,23,144,157-160 During

monolayer expansion, the PEO brush reversibly relaxes, but the PEO chains do not readsorb and

stay hydrated underneath the interface, which explains the complete absence of phase transition

at 6.5 mN/m after the first compression. The maximum in compressibility corresponding to the

transition at 10.5 mN/m is nevertheless slightly decreased after the first compression, which

suggests that the PEO chains do not completely relax during the subsequent expansions to their

original mushroom conformation of the first compression. It should be noticed that, contrary to

our linear PEO-b-PCL diblock copolymers, only one apparent PEO phase transition around 10

mN/m is usually observed in the isotherms of PEO-based block copolymers with high molecular

weight PEO blocks (Mn > 10,000 g/mol),22 prObably because PEO aqueous dissolution and brush

formation take place simultaneously.