5.2.2.2 Cross-linking reaction at the A/W interface The A/W interfacial behavior of the (PB(Si(OEt)3)-b-PEO)3 three-arm star block copolymer was first studied through isotherm experiments for a subphase pH value of 3.0 (Figure 5-17). The (PB(Si(OEt)3)-b-PEO)3 Star was initially spread on the water surface, the cross-linking reaction (pH = 3.0) was carried out for 10 h in the liquid expanded region of the isotherm at zero pressure, and then the isotherm was recorded as shown in Figure 5-17 (bottom red curve). For comparison, the top blue curve illustrates the same sample spread and rapidly compressed (barrier compression speed = 100 mm/min) at pH = 3 before any significant triethoxysilane hydrolysis or condensation occurs. The isotherm of the non-hydrosilylated star is also included (middle black curve). The (PB(Si(OEt)3)-b-PEO)3 Star block copolymer before cross-linking occupies a larger interfacial area compared to the non-hydrosilylated star because of the molecular weight increase during reaction and also because of the increased affinity for the A/W interface of the hydrosilylated PB block that spreads better than the highly hydrophobic non-hydrosilylated PB block. Concerning the isotherm recorded after completion of the cross- linking reaction (bottom red curve), a significant shift toward the low mean molecular area region was observed, similarly as for the hydrosilylated PB homopolymer, because of the loss of ethanol and water molecules. Another interesting feature is that the pseudoplateau at 10 mN/m almost completely vanished for the unreacted hydrosilylated material, while it reappears upon cross-linking. This pseudoplateau corresponds to the desorption of the PEO chains from the interface into the aqueous subphase.223,224 Since the interfacial area occupied by the PB blocks after their hydrosilylation significantly increases, the fractional area occupied by the PEO segments therefore significantly decreases (even though the total area occupied by the PEO segments stays constant), which leads to a PEO phase transition (pseudoplateau) much less pronounced. As the cross-linking reaction proceeds, the area occupied by the cross-linked PB