| The conformation and the dilatational properties of three non-ionic triblock PEO-PPO-PEO (where PEO is polyethyleneoxide and PPO is polypropyleneoxide) copolymers of different hydrophobicity and molecular weight were investigated at the water-hexane interface. The interfacial behavior of the copolymers was studied by combining dilatational rheology using the oscillating drop method and ellipsometry. From the dilatational rheology measurements the limiting elasticity values, E_0, of the Pluronics as function of surface pressure, PI, and adsorption time were obtained, i.e. E_0(t) and E-0(PI). Here, it is shown that E_0(t) depends on the number of PEO units and on the bulk concentration, showing maximum and minimum surface elasticity values which indicate conformational changes in the interfacial layer. Furthermore, in the framework of the polymer scaling law theory, conformational transitions were discussed in E_0 vs. PI plots. In a dilute regime (PI < 14 mN m−1) at the water-hexane interface, E_0 = 2PI fits well all the data, which indicates a two-dimensional "stretched chain" conformation. Increasing PI, two other interfacial transitions could take place. The different behavior of Pluronic copolymers could be also described by the local minima of E_0, which depends on the hydrophobicity of the copolymers. Conformational transitions observed by interfacial rheology were compared to ellipsometric data. Experimental results were discussed and explained on the basis of two- and three-dimensional copolymer structure taking into account that PPO chains could be partially immersed in hexane and water. |
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