4.2.4.2 High surface pressure region (fr> 12mN/m)

 In this surface pressure region, similarly as for the PCL homopolymers and the star-shaped

block copolymers, the phase transition observed in Figures 4-20 and 4-21 at 13.5 mN/m arises

from collapse and crystallization of the PCL segments above the water surface. The

compression-expansion hysteresis experiment carried out on the linear block copolymer sample

with the longest PCL block (PEO60-b-PCL35) for a target pressure of 16 mN/m gives more

insight on the crystallization/melting behavior of the PCL segments. The n/~MMA curves and the

corresponding compressibility plots are shown in Figures 4-27 and 4-28, respectively. During

monolayer compression, the hydrophobic PCL segments collapse and crystallize on top of the

water surface as shown by the inflection points in the 2/MMA curves and the local maxima in

the compressibility plots at 12.5 and 13.5 mN/m. Similarly as for the PCL homopolymers and for

the star-shaped samples, the collapse/crystallization surface pressure in the first cycle is

approximately 1 mN/m higher than in the subsequent compressions (13.5 versus 12.5 mN/m),

and also, after the first compression, the subsequent curves are slightly shifted toward the low

mean molecular area region. After the first hysteresis cycle, the PCL crystals that did not melt

(i.e. did not readsorb at the A/W interface) act as nucleation sites to catalyze the crystallization of

other PCL segments during the subsequent cycles, therefore lowering the crystallization surface

pressure and shifting the compression isotherms toward lower mean molecular areas. During

monolayer expansion, melting of the PCL segments was characterized in the 2/MMA plots for

the star-shaped samples and the PCL homopolymers by an expansion pseudoplateau at a surface

pressure lower than for the crystallization. As shown in Figures 4-27 and 4-28, no pseudoplateau

or sharp local maximum in compressibility are observed during the first expansion, whereas for

the subsequent cycles the PCL segments clearly readsorb around 5 mN/m (sharp local maximum