20 -( *PCL1250

 `9 PCL2000
 ti15 -



 5 o

 I~ lo



 10 100 1000 10000 100000
 MMA (M)

Figure 4-6. Isotherms of the PCL homopolymers (compression speed = 100 mm/min).

 Compression-expansion hysteresis cycles were also carried out beyond the collapse point.

Figure 4-7 shows the compression-expansion hysteresis experiment carried out for PCL1250.

The first compression is similar to the isotherm reported in Figure 4-5. As the film is expanded,

the pressure suddenly drops until an expansion pseudoplateau appears at a pressure significantly

lower than the collapse pressure during compression. This expansion pseudoplateau corresponds

to the readsorption/melting of the PCL chains that had previously crystallized. This

pseudoplateau pressure is molecular weight-dependent, with the smaller PCL chains readsorbing

at higher pressure (10, 8, and 4 mN/m for PCL1250, PCL2000, and PCL10000, respectively).

The compression curve of the second cycle is slightly shifted toward smaller areas, and a

decrease is observed for the collapse pressure. This phenomenon is the result of residual PCL

crystals that remained on the water surface from the initial monolayer compression and that act

as nucleation sites for the crystallization of the other adsorbed PCL chainS.101,119 Further